Hybrid solar cells based on nanocellulose and titania

Hybrid solar cells based on nanocellulose

and titania nanoparticles

Final Degree Thesis

Physics Engineering

Alejandro López Vernet

Universitat Politècnica de Catalunya

Technische Universität München

Physik-Department

July 2017

Director: Prof. Peter Müller-Buschbaum

Supervisor: Dr. Volker Körstgens

Co-Director: Dr. Carlos Alemán Llansó

Contents

Contents

ABSTRACT ..................................................................................................................... 2

1. INTRODUCTION ...................................................................................................... 1

2. THEORETICAL BACKGROUND ........................................................................... 3

2.1 Physic principle of hybrid solar cell .................................................................. 3

2.1.1 Light absorption and exciton generation ................................................... 4

2.1.2 Exciton diffusion and dissociation .............................................................. 6

2.1.3 Possible morphologies of the active layer ................................................. 7

2.2 Cellulose extraction ............................................................................................. 8

2.2.1 Mechanical size reduction ............................................................................ 9

2.2.2 Purification ..................................................................................................... 9

2.2.3 Acid hydrolysis ............................................................................................ 10

2.2.4 Drying ........................................................................................................... 11

3. METHODOLOGY AND INSTRUMENTATION ................................................ 12

3.1 Preparation methods ......................................................................................... 12

3.1.1 Blade-coating ................................................................................................ 12

3.1.2 Spray-coating ............................................................................................... 12

3.1.3 Vacuum Filtering ......................................................................................... 13

3.1.4 Laser ablation ............................................................................................... 14

3.2 Characterization methods ................................................................................. 17

3.2.1 Optical microscope ...................................................................................... 17

3.2.2 UV-visible spectroscopy ............................................................................. 17

3.2.3 Current-voltage measurements ................................................................. 18

3.2.4 Wide Angle X-ray Scattering (WAXS)...................................................... 19

3.3 Materials .............................................................................................................. 20

3.3.1 Cellulose nanocrystals (CNC) ................................................................... 20

3.3.2 Titania ........................................................................................................... 21

3.3.3 Poly[3-(potassium-6-hexanoate)thiophene-2,5-diyl] (P3P6T) ............... 22

Hybrid solar cells based on nanocellulose and titania nanoparticles

1

3.3.4 Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)

................................................................................................................................. 22

4. EXPERIMENTAL SECTION .................................................................................. 23

4.1 Experiments performed with blade-coating method ................................... 23

4.1.1 Cellulose films ............................................................................................. 23

4.1.2 Cellulose with non-ablated titania films .................................................. 24

4.1.3 P3P6T on top of nanocellulose .................................................................. 25

4.1.4 PEDOT:PSS on top of nanocellulose......................................................... 27

4.2 Experiments performed with spray-coating technique................................ 30

4.2.1 Cellulose films with non-ablated titania .................................................. 30

4.2.2 Cellulose films with ablated titania .......................................................... 32

4.3 Experiments performed with vacuum filtering technique .......................... 40

4.3.1 Cellulose films ............................................................................................. 40

4.3.2 Cellulose films with non-ablated titania .................................................. 41

4.3.3 Cellulose films with ablated titania .......................................................... 43

5. CONCLUSIONS ...................................................................................................... 50

6. BIBLIOGRAPHY ...................................................................................................... 52

Acknowledgements ..................................................................................................... 54

Abstract

ABSTRACT

In this thesis, the use of cellulose nanocrystals and titanium dioxide to make the

active layer of hybrid solar cells is going to be considered.

The interactions between the two materials will be deeply studied, as well as the

influence of each production method on the properties of the layer. All the steps

made in the production of these layers will be done in an environmental friendly

was, that is to say, avoiding toxic materials and organic solvents to be used as

well as high temperature processes.

The effects of a laser ablation process on the titania and cellulose will also be

observed and discussed, exposing its advantages over other procedures.

The thesis structure is divided in three main parts. First of all, the physical

background about solar cells and cellulose extraction is going to be explained in

order to have a general view about the research topic. Afterwards, all the

materials used as well as the different preparation and characterization methods

are going to be accurately explained. Finally, the experiments done and the

information that can be extracted about them will be discussed. The experimental

section has been divided into three subsections, one for each of the three

preparation methods used.

At the end, a general overview of the results will be done to discuss the most

remarkable aspects of the thesis.


1

1. INTRODUCTION

It is widely known that the society needs every year more energy to satisfy its

needs. Until now, this energy has been extracted mostly from the fossil fuels

combustion. This fact has two main problems. First of all, it is contaminating the

Earth and producing some irreversible effects on it that can endanger its

ecosystem. Secondly, these fossil fuels are a limited resource, so soon or later it is

going to be finished. Hence, science has to evolve and develop new methods of

transforming the energy from the natural resources that the environment

provides to us, trying to contaminate it as less as possible.

In this context, some years ago, the renewable energies began being investigated

and developed until today, when they are one of the main investigation lines in

science over the world. The most investigated renewable energy is, without any

doubt, the Sun. The main reason is that it is a very constant power source, which

can be accessed everywhere on the Earth surface during several hours per day.

There are a lot of different ways to take profit of the sun radiation, but the most

common one, which this thesis talks about, are solar cells.

Solar cells use the photoelectric effect induced by Sun light to create electric

current. Nowadays, most of them are made with Silicon or some other

semiconductor (or alloy) with a very well characterized band gap. The efficiency

of solar cells made with crystalline Silicon is around 24% [1].

However, there exist other types of solar cells, as organic and hybrid solar cells.

The difference between them and the inorganic solar cells is that the light energy

is absorbed by an organic material instead of a semiconductor. More concretely,

the organic solar cells are made with two organic materials, while hybrid solar

cells combine an organic material with an inorganic one. Of course, each type has

its advantages and disadvantages, that are going to be discussed in the following

lines.

First of all, the main disadvantage of solar cells using organic materials is the

efficiency. It is still far away from the inorganic ones, and at the moment is not

even enough to use them for industrial applications. Actually, the efficiency of

the most efficient organic solar cells is around 10%.

Another important disadvantage is that their life time is really short. This is

because of the photochemical degradation of the organic components forming

the active layer, and by the high temperatures that Sun radiation induce. For

hybrid solar cells, this problem is not as worrying as for the organic ones, because

the inorganic material prevents morphological changes in the active layer,

1. Introduction

2

endowing it with more stability and therefore maintaining the efficiency more

constant over time. The short lifetime combined with a low efficiency makes

them not economically profitable yet, even though they are cheap to produce.

However, these alternative types of solar cells are becoming a very important

field of investigation because they have some advantages that can easily surpass

the limitations of conventional solar cells.

Beginning with the mechanical properties, the organic materials present in

organic and hybrid solar cells, makes them flexible and lightweight. These two

properties suppose a very important advantage in comparison with inorganic

solar cells, which are heavy and completely rigid. This fact breaks all the frontiers

in solar cells applications, opening hundreds of new fields and situations where

they can be used.

Another important factor where organic and hybrid solar cells overcome

inorganic ones is the fabrication price. These solar cells are much cheaper to

produce mainly for two reasons exposed below. On one hand, the organic

materials used for these alternative solar cells are really cheap compared with

semiconductors. The reason is that organic materials are very abundant and easy

to achieve. On the other hand, the fabrication process of these solar cells is much

easier and fast than the conventional type one, leading to a cheapest price. This

is because they don’t need high temperature processes to be produced, and can

be created in very simple ways as for example coating methods. Furthermore,

avoiding this high temperature processes a lot of energy is saved, making these

solar cells more environmentally friendly.

In this thesis I am going to focus in the interaction between titanium dioxide and

cellulose for the active layer of hybrid solar cells. The reasons why these materials

are interesting is, above all, that they are very cheap and can be obtained very

easily. The fact that cellulose is a natural polymer and it can be found abundantly

in the nature has a lot to do with its low price. This polymer also possesses the

great mechanical properties stated before, which make it suitable to use it as a

flexible substrate for electronic devices. It also can be recycled in a very easy way,

fact that would make the solar cells even more environmentally friendly.

Moreover, in this thesis the use of organic solvents as well as toxic polymers is

going to be avoided in order to accomplish one of the objectives of this work,

which is keeping the fabrication process as much environmentally friendly as

possible.


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2. THEORETICAL BACKGROUND

2.1 Physic principle of hybrid solar cell

The fundament of current generation in a conventional Silicon solar cell is the

photoelectric effect. Through it, the electromagnetic energy can be transformed

into electric energy. Thus, the radiation coming from the Sun is absorbed and an

electron-hole pair or an exciton are created. These free charge carriers are

separated and collected in their respective electrodes, creating electric current.

The main part of the solar cells is the active layer. Is there where the light is

absorbed and where the exciton is created. In conventional solar cells, active

layers with one materials can be built. In contrast, the organic and hybrid solar

cells need at least two materials to work, fact that is going to be explained later.

In this thesis, the active layer of hybrid solar cells will be investigated. The

difference between the active layer of an organic solar cell and a hybrid solar cell

is that while the first one is made with two organic materials, the second one

consists in an organic material (donor), and an inorganic material (acceptor). This

mixture tries to take profit of the best characteristics of each one: the inorganic

material provides thermal stability and high conductivity to the layer, and the

organic one provides high light absorption, flexibility and low weight.

Although the active layer can be considered the most important part of the solar

cell, a more complex structure is needed to extract the charge carriers efficiently.

There are different types of structures that can be made depending on the

position of the different layers, but the general idea is always the same. The active

layer is sandwiched between two electrodes, where the electrons and holes are

collected. The one that collects the electrons is typically made with fluorine-

doped tin oxide (FTO), while the one collecting holes use to be made with some

metallic material like gold.

To improve the efficiency of the solar cell and avoid the carriers to go the wrong

way and recombine in the wrong electrodes, electron and hole blocking layers

are placed in between the active layers and the corresponding electrodes. The

hole and electron blocking layers are selected taking in account their work

function. Compact titanium oxide is very used as a hole blocking layer due to its

low valence band and PEDOT:PSS as an electron blocking layer due to its high

LUMO. All this structure is usually built on a glass, that supports all the layers

and as it is transparent, allows the light to pass through it.

2. Theoretical Background

4

As said before, depending on the order of this layers a normal structure or an

inverted structure can be made (Figure 2.1). It has been proved that the inverted

one is more thermally stable than the normal one [2].

In order to select the proper materials to build the layers, the energy levels have

to be kept in mind. An overall view of the process can be seen in the images

(Figure 2.2) [3]. In first place, a photon is going to be absorbed in the active layer,

and it is going to excite the electron from the Highest Occupied Molecular Orbital

(HOMO) of the polymer to the Lowest Unoccupied Molecular Orbital (LUMO).

Then, by diffusion, the electron-hole pair created (or exciton) is going to move,

trying to reach a donor-acceptor interface, which in hybrid solar cells is between

an organic and inorganic material. The inorganic semiconductor has a Valence

Band (VB) and a Conduction Band (CB). The latter is going to attract the electron

and dissociate the exciton. Therefore, the Conduction Band of the inorganic

material has to have lower energy level than the LUMO. Once the exciton is

dissociated, each carrier is going to go to their respective electrodes, but first

having to pass through the blocking layers.

2.1.1 Light absorption and exciton generation

As explained above, the first step in the generation of electric current using solar

radiation energy is the light absorption in the active layer. To improve the light

absorption of this layer and therefore the efficiency of the solar cell is very

important to analyze first the solar irradiance spectrum arriving to the earth

surface and try to make it fit with the absorption spectrum of our layer. As

Figure 2.1: Image

showing the normal

structure (left) and

inverted structure (right)

of a hybrid solar cell. The

active layer can be seen in

the center.

Figure 2.2: General scheme

showing the production current in

a hybrid solar cell from the point of

view of the energy levels (a), and

the location (b). Image extracted

from an article [3]. The blocking

layers are omitted.


5

explained before, in hybrid solar cells most of the excitons will be created in the

polymer. This can be understood now, knowing that the inorganic materials

used, like titania, have the absorption spectrum in the ultraviolet range of the

spectrum, where the solar irradiance is low.

The absorption in the layer can be described by the Beer-Lambert law:

𝐼𝑇(𝜆) = 𝐼0(𝜆) · 𝑒−𝛼(𝜆)·𝑑

Where 𝐼𝑇 and 𝐼0 are the transmitted and incident light intensity respectively; 𝛼 is

the absorption coefficient, depending on the material and the wavelength; and

finally d is the thickness of the layer. In resume, this equation says that the

absorbed light will decrease exponentially with the layer thickness.

One of the main benefits of using organic materials to do the solar cells is that

their absorption coefficient is very high, fact that allows to create very thin and

low weight layers which can absorb almost all the light coming from the Sun.

However, the absorption rate is not the only parameter to take in account to

produce energy. To excite the electron from one molecular orbital to the superior

one, a certain amount of energy has to be provided by the incident photon. If this

energy is not overcome, the electron will return to its initial molecular orbital and

the photon energy will dissipate in the form of heat or radiation. Therefore, to

have a correct performance of the solar cell, the next condition has to be fulfilled:

𝐸𝑝ℎ𝑜𝑡𝑜𝑛 = ℎ · 𝜈 ≥ 𝐸𝐿𝑈𝑀𝑂 − 𝐸𝐻𝑂𝑀𝑂 = 𝐸𝑔𝑎𝑝

Where ℎ is Planck’s constant, and 𝜈 is the frequency of the incoming photon.

Talking about organic materials, it is important to notice that most of these

composites have a very high gap energy, what means that a great quantity of the

photons heating the polymer will just pass through it or be converted in heat,

without exciting an electron. That means that not with all the polymers can a

Figure 2.3: Image showing the solar

radiation spectrum at the top of the

atmosphere (yellow) and at the sea level (red)

compared with the irradiance of a blackbody

at 5250ºC [4].


6

solar cell be created, organic materials with lower enough gap energy have to be

chosen.

When the electron is excited, it leaves a hole behind. Thus, two free charge

carriers are created, one with negative charge and the other with the same

amount of positive charge. These two particles will attract each other due to the

Coulomb forces, because they have opposite signs, creating what is known as an

exciton.

The energy of the "bond" between them is inversely proportional to the square of

the dielectric constant. To have a general idea, Silicon has a dielectric constant

near 12. That makes the exciton have a very low bond energy, approximately one

half of the K·T in normal room temperature, which means that this exciton will

be dissociated very easily.

On the other hand, organic materials use to have a dielectric constant around 3.

That makes the exciton "bond" much higher than the previous one,

approximately 6·K·T at room temperature. That will make the exciton impossible

to dissociate in normal conditions, thus no current will be generated.

2.1.2 Exciton diffusion and dissociation

The solution for this problem is to add another material to create what is known

as a heterojunction. One of them will be the donor, where the exciton is created,

and the other one will be the acceptor. The first thing to keep in mind is that the

exciton as a whole is a neutral system. Then, non-electric field will be able to

move it. Thus, the movement of the exciton will be only due to the diffusion. A

very important parameter in the active layer construction is the average length

that an exciton can travel within the polymer before being recombined again, the

so-called diffusion length:

𝑙𝑑𝑖𝑓𝑓 = √𝐷 · 𝜏𝑒𝑥𝑐

Where 𝐷 is the diffusion coefficient of the material and 𝜏𝑒𝑥𝑐 is the exciton lifetime.

This length is of vital importance and will have an important role when choosing

the morphology, as is going to be showed later.

Due to the fact that the electron will look for lower energy states, when the

exciton arrives to the junction, this separation will actuate as a knife, dissociating

the exciton in an electron, that will go to the acceptor; and a hole, that will remain

in the donor. The difference between the lowest unoccupied molecular orbital

(LUMO) of the donor, and the conduction band of the acceptor must be higher

than 0.3 eV, that is the energy needed for exciton dissociation.


7

Once separated, the applied electric field will make the electron move to its

contact through the conduction band of the inorganic material, while the hole

will move to the other contact through the HOMO of the polymer via a hopping

mechanism.

2.1.3 Possible morphologies of the active layer

One of the main problems of this system is the recombination. The exciton is very

unstable, and if no junction is found in a certain amount of time, the electron will

just decay filling the hole that it left behind before. Even when the exciton is

dissociated, if the electron is near the junction, it can recombine with a hole near

it. Once the electron is far from the junction, the recombination problems

disappear, because there are not holes in the acceptor material (and no electrons

on the donor). If the exciton recombines, the photon energy will be lost, and thus

the efficiency of the solar cell will decrease. Then, this is a problem to avoid in

organic solar cells.

To be sure that the exciton arrives to the junctions to dissociate, the only thing

that can be done is to change the morphology of the active layer in order to try

that the distance between the junction of the two materials and the point where

the exciton is created is shorter than the diffusion length. The different structures

that can be created to reduce this problem are explained below.

As it has been said before, a voltage is applied between the two contacts to attract

the electrons and holes to the correct place. This voltage induces an electric field

in the junction in the opposite direction, which is going to retain the electrons

more time near the junction. This have to be avoided because the junction is an

area where the electrons still can recombine. One way to reduce this problem is

to use as an acceptor material with high electron mobility.

There are different structures that help us to deal with this problem and to reach

a high enough efficiency for our solar cell (see Figure 2.4):

Figure 2.4: Image showing three possible morphologies for the active layer [5]. a) Bilayer structure, b) Bulk

Heterojunction (BHJ), c) Ordered BHJ.


8

Bilayer structure: This one is the simplest one. It simply consists in 2 layers

thin enough to ensure a high amount of dissociations. This model is

theoretically useless, because the same amount of excitons that dissociate

in the junction will also dissociate in the wrong contact, making the total

current equal to zero.

Bulk Heterojunction (BHJ): This is by far the most used structure, due to

its high efficiency and its cheap production method. It simply consists in

doing a mixture between the two materials, to achieve a very entangled

and mixed structure. When creating this type of structures, it exists a trade

off with the degree of mixture that is wanted to be achieved in order to get

the highest efficiency possible. On one hand, if the mixture between the

materials is very high, the structure will provide a very large contact

surface between them, making very easy for the exciton to find a junction

to dissociate. On the other hand, the electrons and holes have to find a way

to arrive to their respective contacts in order to produce an effective

current. If the solution is too mixed, once the exciton in dissociated the

carriers won't be able to find a path to its contacts, otherwise they will get

trapped into "islands" of the material. Then, a structure with a high

entanglement has to be achieved, in order to maximize the surface of the

junction, but always trying to maintain all the parts of material connected

like a network.

Ordered BHJ (Checkerboard): This one is one of the most efficient models.

It consists in alternate vertical layers between the two contacts, with a

width thin enough to ensure the arrival of the exciton to the junction

(shorter than the diffusion length). Wherever the exciton is created, it will

always have a junction near it. Once dissociated, the electric field will

carry the electrons and holes to the correct contact, creating a "highway"

of electrons in the acceptor and of holes in the donor. Notice that, with this

structure, almost all the excitons will dissociate and the electrons will be

collected in the correct place. Although this structure is more efficient than

the previous ones, its disadvantage is that it is very expensive to produce

it in the nanoscale, because it needs a very accurate process to be

produced.

2.2 Cellulose extraction

Cellulose is almost a renewable resource, because it is present in huge quantities

in the nature. It can be obtained from plants and trees, because it constitutes the

cellular walls of their cells. This natural polymer has attracted the attention of

several researchers because it is obtained from the nature, which means that the


9

procedure needed to have it in its final form its much cheaper than the other

artificial polymers. Furthermore, it is biodegradable, fact that is very important

if the environment is taken in account. In this section, the most general procedure

to obtain cellulose from the nature will be briefly exposed. An image of where is

the cellulose placed inside a plant cell can be seen below (Figure 2.5).

The process is usually divided in four main steps [6][7][8]:

2.2.1 Mechanical size reduction

In this first step, the objective is to reduce the size of the cellulose fibers. This

simply consists in milling, grinding or cutting the plants or trees, in order to

obtain cellulosic fibers with a length of the order of millimeters. After that, a first

washing and filtering are done to remove the largest impurities.

2.2.2 Purification

This step begins with a mixture of cellulose fibers altogether with some

impurities like lignin and hemicellulose. These impurities have a very negative

effect on the colloidal stability, the thermal stability and the mechanical

performance of the future cellulose nanocrystals, so they should be removed as

far as possible. That is the main objective of this step.

The most common way of purification of the cellulose nanofibers is through a

two-step treatment called Alkali-Bleaching. At the end of this process almost all

the impurities will be removed. Furthermore, this process also helps to decrease

the length of the fibers.

The first step of the purification treatment, Alkaline treatment (also called

mercerization), consists in mixing the fibers with strong base solutions, like

potassium hydroxide (KOH) or sodium hydroxide (NaOH). These solutions

induce a reaction with the impurities, removing them and breaking some of the

OH bonds present in the fibers, what causes its division into smaller ones.

All these things together produce the crystallinity of the fibers to increase.

However, this treatment is not environmentally friendly, because some toxic

radicals are created. Thus, other similar procedures with less consequences for

the environment are being investigated. One example is the Electron-Beam

Irradiation (EBI), that has even better results than the treatment explained above.

After this first purification step, a second one called Bleaching is needed. This is

because of the fact that lignin is very difficult to remove completely, and even

after the alkaline treatment, some residues of this substance will remain within

the fibers. These residues cannot be undervalued, because they have a great


10

influence on the result of the acid hydrolysis step explained in the following lines

and the future surface wettability of the fibers.

This process it is usually done boiling the cellulose fibers in sodium chloride

(NaClO2). These procedure is repeated several times in order to remove the

maximum quantity of lignin possible. The number of bleaching steps affect some

properties of future cellulose nanocrystals, for example its thermal stability and

their crystallinity. As more bleaching steps are done, more stability and

crystallinity are going to be achieved. This is due to the fact that the residual

lignin impedes the acid used in the next step to reach the amorphous parts of the

fibers, making the crystallinity decrease. This step is also environmentally

unfriendly.

2.2.3 Acid hydrolysis

In this step, the objective is to separate the crystalline part of the cellulose from

the amorphous part. At the beginning cellulose is in the form of elementary

fibrils, that basically are crystalline structures in the form of compact

nanocrystals among adjacent disordered amorphous segments which bond them

together. The amorphous parts can be removed in two different ways, either with

enzymes or with acid hydrolysis. The most common one is the acid hydrolysis.

This treatment takes advantage of the acid solubility of the amorphous regions,

against the insolubility of the crystalline regions. Thus, if the fibrils are mixed

with an acid, almost all the amorphous regions between the crystals will dissolve,

leading to a high degree of crystallinity and to smaller crystals (nanometer size).

There are a lot of acids that can be used (H2SO4, HCl, oxalic acid, etc.), and

depending on which one is chosen, different properties of the nanocrystals are

going to be obtained. The most used one is sulfuric acid (H2SO4). When this acid

is used, some sulfate groups (SO42-) remain attached on the surface of the

nanocrystals. Therefore, negatively charged nanocrystals will be created. This

fact is really important because it induces an electric repulsion between the

nanocrystals, avoiding the creations of aggregates. Hence, the colloidal stability

of the solution is greatly improved. However, this process worsens the thermal

stability. The time duration of this process is also a critical issue, because if it is

done too much time, the crystals can also be affected by the acid, producing the

opposite effect and decreasing the crystallinity.

Afterwards, a centrifugation is done to the nanocrystals to get rid of the acid.

Subsequently, a homogenizer or microfluidizer can be used to equalize the length

of the nanocrystals through mechanical shearing effects.


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2.2.4 Drying

At this point, with the nanocrystals created, they are processed as aqueous

suspensions. The reason why water is used is that the strong hydrogen bonds

between the cellulose and the water itself make the solution very thermally and

kinetically stable. Nonetheless, the water adds an additional weight to the

crystals, that have repercussion in the price of their transport. For this reason, a

lot of producers prefer to commercialize them dried.

The drying step is critical, because aggregates can be formed easily. The most

used techniques to dry the cellulose nanocrystals are the oven drying, the freeze

drying, the supercritical drying and the spray drying.

Figure 2.5: Scheme of where to find nanocellulose from the plant cell. Impurities like hemicellulose or lignin are also

shown [8].

Secondary wall

3. Methodology and Instrumentation

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3. METHODOLOGY AND INSTRUMENTATION

In this chapter, the different methodologies and materials used to prepare the

samples, as well as the characterization methods are going to be explained.

3.1 Preparation methods

To prepare the samples, three different methods were used in order to compare

the results and analyze the main advantages and disadvantages of each one.

3.1.1 Blade-coating

The blade-coating method can only be used with a gel substance. It simply

consists in spreading the gel over a glass using another microscope glass as a

blade (Carl Roth). The thickness of the layers made by this method could be

roughly controlled using adhesive tape (TESA) of approximately 120 µm thick.

Thus, depending on the number of tapes used, thicker layers could be made. The

main advantages of using this technique are the easy, fast and cheap preparation

of the setup needed. On the other hand, it has some drawbacks as a rough control

of the layer thickness and the time needed to let the samples dry. An example of

a sample made by this method can be seen (Figure 3.1).

3.1.2 Spray-coating

The spraying technique needs a liquid solution to be performed. The setup can

be seen in the picture (Figure 3.2). It consists in a structure that holds a spray-gun

over the samples that have to be sprayed, being possible to change the height.

The spray gun has two parameters that can be controlled, the flux of solution

sprayed per second, and the amount of time that each spay lasts. For a correct

performance of this technique, the time between two consecutive spays has to be

enough to let the sample completely dry. To accelerate the drying process, a

heating plate was used.

The spray gun used for the experiment was purchased to Harder & Steenbeck

Airbrush (see Figure 3.2).

Figure 3.1: Cellulose nanocrystals layer made

with blade-coating method. The microscope

glass is attached to a paper sheet using 3

adhesive tape on each side. Two drops of

polymer are poured over it.


13

The main advantages of this method are highlighted in the following lines. It

produces films with high homogeneity due to the fact that they are created from

a liquid solution and are sprayed nearly homogeneously over the glass. It also

has a very easy and accurate way of controlling layer thickness, since it can be

controlled just knowing the number of sprays done. The sample spraying can be

done in a few minutes thanks to the heating plate used. Nevertheless, a previous

extra time is needed to prepare the glasses used in the experiments. They are cut

in a squared shape and cleaned with an acid bath in order to have a totally clean

surface to spray on.

3.1.3 Vacuum Filtering

This method also needs a liquid solution to be done. The setup consists on a glass

container connected to a tap where the vacuum is done to enhance the filtering.

It has an aperture in the superior part, where the filter has to be placed with the

help of a ceramic piece (see Figure 3.3). In order to prevent the film of having the

Figure 3.2: a) Image of the spray-coating setup used to create

the films. Extracted from a master thesis [9]. b) Spray-gun

used to do the spray coating experiments [10].

a) b)

Figure 3.3: a) Image of the glass used to do

the vacuum filtration. The filtering was

done connecting a plastic tube in the blue

nozzle, that was indirectly connected to a

tap. b) SEM image of the nitrocellulose

filter provided by the supplier.


14

shape of the holes of the ceramic piece, a filter paper is placed under the main

filter. Once this filter is placed, the solution has to be poured over it with a pipette,

so that the solution its spread over all the area of the filter. After some minutes,

when the solution has been totally filtered, the sample can be removed.

MicronSep nitrocellulose filters used were provided by Osmonics Inc., and are

composed of a mixture of inert cellulose nitrate and cellulose acetate polymers.

Their pore size is 0.22 µm, they have a diameter of 47 mm and a plain surface.

The advantage of this method is that it is a very easy way to do thin homogeneous

layers. However, the layer thickness is not easy to control and some films are

very attached to the filter at the ending of the experiments, making them difficult

to peel off.

3.1.4 Laser ablation

In this section the laser ablation procedure is going to be described. This step is

fundamental for the proper performance of the active layer. It functionalizes

titania nanoparticles, ablating and melting them in order to achieve a suitable

material for constituting the active layer.

The ablation of the titania has to be performed under the water in order to keep

in a controlled environment all the products of the ablation. The titania ablation

can be done in two ways, that lead to titania nanoparticles with very different

characteristics. These two options consist in ablating a solid titania target under

the water and ablating a titania powder solution [11].

The first of the options, the solid target ablation produce perfect spherical shaped

nanoparticles. However, previous investigations found some drawbacks for this

method that make them not suitable to be used for constituting the active layer

of a solar cell. On the first hand, this titania target has to be constantly moved in

order to reach different parts of the surface of the target. Another important

disadvantage is the low production rate achieved, since only 2 mg of ablated

product could be achieved per hour. The last and most important disadvantage

found is that the nanoparticles produced with this technique are mostly

amorphous, which makes them low conductive and then not recommended to

build a solar cell.

The second option consisted in using as a target titania powder dissolved in a

water solution. This technique solved all the problems commented in the

preceding paragraph. First of all, a magnetic stirring could be placed in the

solution to have it in constant movement, which will allow a very large number

of nanoparticles to be focused and ablated by the laser. The production time is


15

also shortened, being that for a 3 ml solution containing 30 mg of titania powder,

only 30 minutes are needed. To finish the titania powder was purchased in a

crystalline form, and during the ablation process almost all the titania remained

in that form. This fact makes them conductive and then recommendable to be

used in the active layers of hybrid solar cells.

Experimental setup

The experiments were done at the chair of Laser and X-ray Physics of TUM. The

lab is kept at a constant temperature and humidity to ensure a smooth operation

of the laser. The setup can be seen in the images (Figure 3.4).

The titania dispersion is constantly stirred with the help of a magnetic stirrer

during all the experiment. The velocity of stirring of the solution will have

repercussions on the final ablated nanoparticles. Is important to keep the velocity

constant and low during all the experiment in order to achieve a smooth surface.

If the stirring velocity is too high, turbulences can lead to undesired reflections

and a decrease in the efficiency of the ablation process.

The solution is placed on a stirring plate, and is covered by an acrylic glass box

to prevent from splashes and to avoid titania dust, product of the ablation, to

leave the setup and damage some sensitive equipment in the lab. On the top side

of the box there is a hole that allows the laser beam to enter the acrylic box and

focus into the solution. A clean microscope slide is placed above the hole during

the ablation process to keep the solution totally closed inside the box. This glass

is changed for a new one after every ablation process, to be sure that any

undesired dust particles change the conditions of the experiment.

The laser beam is directed to the solution by a mirror system and focused by a

100 mm focal length lens.

Figure 3.4: a) Setup

used to do the laser

ablation process, where

the acrylic box can be

seen. The laser path is

depicted in red. b)

Scheme of the laser

ablation process

performed on a titania

powder solution [9].


16

Laser characteristics

The solutions are always ablated with a pulsed titanium:sapphire laser (CPA 2010

from Clark-MXR, Inc). Its properties are shown in the following table:

Type Average

Power

Pulse

duration

Wavelength Repetition

rate

Pulse

Energy

Peak

power

Ti:sapphire 1 W 150 fs

(FWHM)

779 nm 993 Hz 1 mJ 6.67 GW

To perform successfully the ablation process, the power density of the laser has

to exceed the ablation threshold for the titanium dioxide. It has been measured

that the average diameter of the beam in the focus is 27.4 µm. Taking this in

account and using the data in Table 3.1, the power density per pulse in the focus

plane can be computed:

𝑃𝑑𝑒𝑛 =𝐸𝑝𝑢𝑙𝑠𝑒

𝑡𝑝𝑢𝑙𝑠𝑒 · 𝐴=

1 𝑚𝐽

150 𝑓𝑠 · 𝜋 · (27.4 µm 2)⁄ 2 = 1.13 · 1015𝑊

𝑐𝑚2

Which it can be seen in the literature that is higher than the titania ablation

threshold [12]. The success of ablation process can be checked in a very easy way

looking at the color of the titania dispersion (Figure 3.5). Before the ablation it is

white, and after the ablation it becomes blueish.

It can be seen in the SEM images below (Figure 3.5) how before the ablation small

and edged particles are present in the sample (green circles). After the ablation,

some particles have melted, becoming larger and round-shaped (red circles).

Also smaller sized particles can be found after the process (yellow circles).

These results confirm the success of the ablation. The morphology of titania after

this process is suitable to be part of the active layer of a solar cell, because it

provides an interconnected matrix that will allow the electrons to find a path to

their corresponding electrode.

Table 3.1: Characteristics of the laser used for the ablation process.

Figure 3.5: a,b) SEM images showing the titania P-25 powder before (a) and after (b) the ablation process. Green circles

indicate small edged particles, while red and yellow ones indicate big and small round-shaped particles respectively. c,d)

Images of the titania dispersion before (c) and after (d) the ablation process.


17

3.2 Characterization methods

In this section, all the characterization methods used in this thesis to analyze the

samples are going to be explained.

3.2.1 Optical microscope

In this thesis, the Optical Microscope is used for all the layers done in order to

have an idea about their homogeneity and morphology in small distance scales.

Titania aggregates can be easily seen with this method, so a very valuable

information can be extracted from its images. They will be mostly used to

compare the samples between them. The achievable resolution R with visible

light is limited be the Rayleigh criterion:

𝑅 =1.22 · 𝜆

2 · 𝑁𝐴

Where λ is the wavelength microscope light source and NA the numerical

aperture of the different objectives.

The microscope used in this thesis is the Axiolab A by Carl Zeiss. The images could

be seen and saved in the computer through a PixeLink USB Capture BE 2.6 charge

coupled device (CCD) camera, with a resolution of 1280 x 1024 pixels. It was

equipped with a halogen light source (Hal 100) emitting at 700 nm wavelength.

Using this data, the resolution for each objective can be computed:

Magnification NA Resolution (µm) Pixel size (µm)

1.25x 0.035 12.2 6.26

2.5x 0.075 5.7 3.11

10x 0.20 2.1 0.82

50x 0.70 0.61 0.17

100x 0.75 0.57 0.082

3.2.2 UV-visible spectroscopy

UV-Vis measurements are crucial in the research field of solar cell. These

measurements provide information about the amount of light absorbed by the

sample, which is always tried to maximize for solar cells.

The basic scheme of a UV-Vis spectrometer is depicted in the image (Figure 3.6).

The light of the lamps first passes through a filter and a monochromator, which

select the desired wavelength in each moment. Then, the light is split in two

Table 3.2: Resolution and pixel size of the images taken with the different lenses.


18

beams, one of them will go to one photo diode detector through a reference, and

the other will go to a different detector passing through the sample.

The used equipment was Perkin Elmer Lambda 650S UV-Vis spectrometer together

with the software UV-Winlab. This device can also measure the transmitted

scattered light thanks to its integrating sphere (Spectralon R) which is highly

reflective. It is equipped with two light sources: a deuterium lamp, which cover

a wavelength range from 190 nm to 326 nm; and a tungsten-halogen lamp, whose

wavelength range cover from 326 nm up to 900 nm. Hence, with the combination

of both lamps, transmittance measurements between 190 nm and 900 nm can be

done. The scan speed is around 265 nm/min and for all the measurements the slit

size was set to 2 nm.

3.2.3 Current-voltage measurements

Some current-voltage measurements were performed on the films during this

thesis in order to know the resistance and conductivity of the samples and do

comparisons between them. To do them, the 4-point probe setup was used. This

setup consists in 4 collinear equally spaced tips. The main advantage of this

method is that, when aligned in this geometry, the voltage and resistance effects

from the contacts are negligible, leading to very accurate measurements.

To do the measurement, all the 4 tips have to be in contact with the layer. The

two probes on the outside supply a controlled current which induces a voltage

on the sheet. The inner probes measure this voltage, avoiding in that way to take

in account the parasite resistances (Figure 3.7). From this data, the resistance can

be extracted.

Using this technique, the sheet resistance of the layers can be also computed:

𝜌𝑆 =𝜋

𝑙𝑛(2)·

𝑉

𝐼

Where 𝑉

𝐼 is the resistance, that is modified by a factor

𝜋

𝑙𝑛(2) that belongs to the sheet

shape correction.

Figure 3.6: Scheme of a UV-Vis

spectrometer [13].


19

This result combined with the layer thickness (d) can be used to measure the

conductivity of the film:

𝜎 =1

𝜌𝑆 · 𝑑

A CascadeMicrotech C4S Resistivity 4-point Probe Head was used to make the

measurements (Figure 3.7). The distance between 2 consecutive tips in this device

is 1.25 mm. To supply the current, a Keithley 2400 sourcemeter was used. The

current was switched from -5·10-8 A to 5·10-8 A in 200 steps (stepwidth of 5·10-10

A).

3.2.4 Wide Angle X-ray Scattering (WAXS)

Wide angle X-ray diffraction is technique that allows to determine morphological

structure of polymeric and metal crystals as well as its orientation among others.

It consists in detecting the X-rays scattered with a wide angle from the sample,

approximately more than 10º. Thus, the detector has to be placed near the sample.

There is an analogous technique called SAXS that consists in the same setup but

analyses the lower scattering angles, where the only change is the increase of the

detector distance.

In this thesis, thin films are analyzed using this technique. Due to the low

intensity diffuse scattering of polymers, a very intense X-ray beam is needed to

properly analyze the results. For this reason, the measurements took place in P03

Nanofocus X-ray Scattering Beamline at the PETRA III storage ring at DESY

(Deutsches Elektronen-Synchrotron) in Hamburg [14]. It is a 3rd generation

source [15], which means that it is optimized for the generation of synchrotron

light. With this aim it incorporates wigglers and undulators, to create a very

intense and coherent light at a selected wavelength. The storage ring is 2304 m

long and is operating at an electron energy of 6 GeV with beam current of 100

mA.

Figure 3.7: a) Sketch of 4-point probe working principle. b) Image of the 4-point probe head used on a sample.


20

To record the scattered X-ray intensity, the CCD detector Pilatus 300K was

employed. It has a resolution of 487 x 689 pixels and a pixel size of 172 x 172 µm2.

The counting rate per pixel is >2·106 X-rays per second. One beam stop was placed

in the direct beam spot in order to preserve the detector. It is made up of three

different modules, each with an active area of 83.8 x 33.5 mm2. For the

experiments of this thesis, the detector was placed 244.3 mm after the sample.

The beam is directed perpendicular to the sample surface and has a very small

size (250 nm x 250 nm), property that allow to measure smaller sample areas.

This small size, keeping a high intensity is very difficult to achieve, hence only

the last technology synchrotrons can provide this type of beam. The wavelength

used was 0.97243 Å, which corresponds to an energy of 12.759 keV.

3.3 Materials

3.3.1 Cellulose nanocrystals (CNC)

Three different types of cellulose were used in this thesis in order to compare

them and evaluate which one fitted more with the needed characteristics.

UltraTM

This type of cellulose nanocrystals was supplied by Blue Goose Biorefineries Inc.

Under the name BGB UltraTM Cellulose Nanocrystals Suspension. Its molecular

formula is (C6H10O5)n. It is an 8.0% w/w suspension in reverse osmosis water. It

is produced with a transition metal catalyzed oxidative process. Its crystallinity

index is 80%, and the crystals are between 100-150 nm length and 9-14 nm of

diameter. Its hydrodynamic diameter is also 150nm, which means that they are

not overlapped. They have a zeta potential around –35mV. The TEM image

provided by the supplier can be seen below (Figure 3.9).

NaturalTM

This type of cellulose nanocrystals was supplied by Blue Goose Biorefineries Inc.

Under the name BGB NaturalTM Cellulose Nanocrystals Suspension. Its molecular

formula is, as the previous one, (C6H10O5)n. It is an 7.4% w/w suspension in

reverse osmosis water. It is produced with an oxidation process in nature,

Figure 3.8: Scheme of the setup of WAXS

measurements.


21

without involving any kind of acid hydrolysis. All the values are very similar to

the previous type of cellulose, its crystallinity index is 83%, and the crystals are

between 100-150 nm length and 9-14 nm of diameter. Its hydrodynamic diameter

is higher than the other one, around 400nm. They have a zeta potential around

–35mV.

The difference between these two first types of cellulose is that in this second

type, non-sulfate half esters could be found. This is due to the fact that no acid

hydrolysis was used to make the crystals. Instead, a natural oxidative process

was performed. The sulfate half ester groups are negatively charged, and have a

high influence in the colloidal stability of the nanocrystals, the self-assembly

properties, electrostatic and rheological (viscosity) behavior, and also in the

optical properties (DOI 10.1007/s10570-014-0513-y). The TEM image provided by

the supplier can be seen above (Figure 3.9).

Microfibrillated cellulose obtained by an enzymatic process

This cellulose was the less used in the experiments. Although it is not the

commercial name, in the thesis it will be referred as microfibrillated enzymatic

cellulose. To extract this type of cellulose, the acid hydrolysis step in it

production has been substituted by enzymes that can do the same function. These

enzymes are able to separate the amorphous part from the crystalline part as the

acid hydrolysis do. Analogously, this step is named enzymatic hydrolysis. These

cellulose nanocrystals are also provided in a gel consistence, with a concentration

of 1.2%.

3.3.2 Titania

The titanium dioxide, also known as titania (TiO2), is a very common material in

the active layer of solar cells. In the nature, it can be found in four different

structures: anatase, rutile and brookite, which are crystalline phases; and

amorphous structure. For hybrid solar cells the desired structures are the

crystalline ones, since they are semiconductive. On the other hand, amorphous

structure is an electrical isolator.

Figure 3.9: TEM images

provided by the supplier

showing UltraTM (a) and

NaturalTM (b) cellulose

nanocrystals.


22

Titania used for the experiments was provided by Evonic Industries under the

name AEROXIDE® TiO2 P-25. It’s a powder, with an average particle size of 21

nm, and a purity of 99.5 %. Its crystal structure is a mixture between anatase and

rutile phases, with anatase phase being majority. A TEM image of the titania

powder can be seen above (Figure 3.10).

3.3.3 Poly[3-(potassium-6-hexanoate)thiophene-2,5-diyl] (P3P6T)

The polymer P3P6T was also used in some experiments to observe its behavior

together with cellulose nanocrystals. The P3P6T used was distributed by Rieke

metals. Its molecular formula is (C10H11KO2S)n. This polymer was used instead of

poly(3-hexyl-thiophene)(P3HT), which has better performance. The reason is that

it is water soluble, thus the building procedure is more environmentally friendly.

The drawback is that its hole mobility decreases around two orders of magnitude

with respect to P3HT.

Before using this polymer, a sand bath at 60ºC has to be done for 3 hours in order

to dissolve it properly and eliminate all the aggregates present in the solution.

3.3.4 Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS)

This polymer is widely used in the fabrication of solar cells because it actuates as

a hole blocking layer. For this reason, the behavior together with the cellulose

nanocrystals was important to be studied. The polymer is supplied by Ossila Ltd,

with the name Clevios PH1000. It has a PEDOT:PSS weight ratio of 1:1.25 and it is

dispersed in water, which leads to a density of approximately 1 g/ml.

Before the application, the PEDOT:PSS is kept for 10 minutes in the ultrasonic

bath to eliminate aggregates.

Figure 3.10: TEM image showing titanium dioxide P-25 as

purchased from the supplier.


23

4. EXPERIMENTAL SECTION

In this section, all the experiments done in the lab are going to be explained. More

than fifty samples were done in total for all the experiments. The experiments are

divided by the production method used. As these types of cellulose were never

used before in the lab, the first experiment done for each preparation method was

dedicated to learn about their behavior and stability. After that, and analyzing

the capacity of each technique to produce mechanically consistent layers,

mixtures with titania and polymers were done.

4.1 Experiments performed with blade-coating method

Blade-coating has the advantage of being a very easy technique to do, which

don’t require any complex setup. However, not liquid solutions can be used to

create layers with this method, fact that is going to bring some problems with the

addition of titania. The control of the thickness is also a little bit rough, because

it is controlled with adhesive tape (TESA) which approximately has 120 µm thick.

4.1.1 Cellulose films

The first step, as said above, was to know the behavior of the cellulose, which is

the material that gives mechanical consistency to the active layer. To make these

samples UltraTM and NaturalTM cellulose nanocrystals were used. Six different

samples were done, with different thickness of the layers (1, 2 and 3 tapes thick)

and with both types of nanocellulose. After letting them dry during one day, a

different performance was observed for each type of cellulose.

Beginning with the color, the films made with the UltraTM type were almost

transparent while the ones made with the NaturalTM were translucent.

The other significant difference at first sight was about the mechanical resistance

of the layers. Only the thicker layer of the UltraTM type was able to be peeled off.

The other two layers were too fragile to do it. On the other side, the NaturalTM

type proved much higher mechanical consistency in the three films done.

4. Experimental section

24

Analyzing the microscope images (Figure 4.1), it can be observed that the UltraTM

type is more homogeneous than the NaturalTM one, since no particles can be seen

with that magnification in the first one, while for the second one, particles are big

enough to see them with low magnifications.

On the other hand, comparing pictures a and b, which belong to the border of the

films, it can be observed how for the NaturalTM type the film seems to be more

consistent, even that the thickness of the layer is one third of the UltraTM one.

In view of the results and the difficulty of peeling off the films, thicker layers

were done. This time films up to six tape layers were made for each type of

cellulose.

It was found that for UltraTM nanocellulose the optimal thickness to achieve

consistent layers easy to peel off was 5 and 6 tape thick. On the other hand, the

layers made out of NaturalTM cellulose with more than three layers thick resulted

to crack and peel off the glass even before it was intended to do so. Thus, the

optimal thick for that type of cellulose was found to be 2 and 3 tape thick.

4.1.2 Cellulose with non-ablated titania films

As commented before, this method has the disadvantage of needing a gel

substance to be performed. As the laser ablation of titania has to be done in an

aqueous dispersion, the mixture of cellulose and ablated titania using this

method is not possible to do.

However, as the titania comes as a powder, the mixture between the two

components was just performed directly adding the powder to the gel and

Figure 4.1: Optical microscope images with a

magnification of x2.5 (a and b), and x10 (c and

d). The images show a comparison between

UltraTM nanocellulose film with 3 tape thick (a

and c) and NaturalTM nanocellulose film with

1 tape thick (b and d).


25

stirring the substance long enough to achieve an almost homogeneous gel. This

was done with both UltraTM and NaturalTM gels.

The amount of titania added was the necessary to achieve a 1:1 proportion in the

gel, taking into account that the gel has only an 8% of concentration of cellulose

nanocrystals. Different thicknesses were also tried. For each type of cellulose,

layers from one to three tape thick were made.

In the microscope images (Figure 4.2) it can be seen how in both UltraTM and

NaturalTM nanocellulose, huge titania aggregates were formed. As a result, these

large particles impede to create a consistent layer, creating cracks in the films that

were impossible to peel off (see Figure 4.3).

This time, the difference between both types of cellulose could not be observed

easily, because both of them created very inhomogeneous films.

4.1.3 P3P6T on top of nanocellulose

As explained in the introduction, the active layer of a hybrid solar cell is

composed of two different materials, one organic (polymer) and the other one

Figure 4.2: Optical microscope images with a

magnification of x2.5 (a and b), and x10 (c and d).

The images show a comparison between UltraTM

nanocellulose-titania film (1:1) with 2 tape thick (a

and c) and NaturalTM nanocellulose-titania film

(1:1) with 2 tape thick (b and d).

Figure 4.3: Optical microscope images showing

the inconsistency of the films and the huge

titania aggregates created.


26

inorganic. Nanocellulose has a lot of good properties in order to be the organic

component, for example its low weight, its easy and cheap extraction and

production, and its biodegradability among others.

Nonetheless, it lacks one of the most essential characteristics for an active layer

of a solar cell: the conductivity. Nanocellulose has a low conductivity, which will

lead to a poor efficiency of the solar cell. To enhance this property, preserving the

good characteristics of nanocellulose, it is suitable to try to mix it with a

conducting polymer, as P3P6T, to see how do they behave together.

Remember that P3P6T is a non-toxic and water soluble conducting polymer. This

means that with this polymer, the environmental friendliness of the solar cell will

be preserved. It is a derivative of P3HT, a polymer that despite it is more

conductive, it is not water soluble.

As it has been commented before, blade-coating can’t be performed with a liquid

solution. Thereby, to mix both materials, the nanocellulose film will be created

first from the gel. Afterwards, some drops of a liquid P3P6T solution, with a

concentration of 10 mg/ml, are going to be poured over the film.

To do this experiments, UltraTM and NaturalTM nanocellulose were used. The

UltraTM ones had 6 tape thickness and the NaturalTM ones had 3 tape thickness, as

these were found to be the optimal thicknesses.

In order to compare, the solution was poured in two different moments for both

types of nanocellulose. For some samples it was poured over the fresh layer, that

is to say, some seconds after the blade-coating; and for the other ones it was

poured once the layer was completely dry.

Figure 4.4: Optical microscope images showing the comparison between UltraTM (top row) and NaturalTM (bottom

row) cellulose nanocrystals films, when a drop of P3P6T is poured over them once they are dried. Different

magnifications are shown in each column. The yellowish part corresponds to P3P6T, while the rest corresponds to

cellulose nanocrystals.


27

It can be observed from the microscope images (Figure 4.4) that for the layers

where the drop was poured once they were dried, the border between P3P6T and

the cellulose nanocrystals is very well-defined in both cases. It can be

distinguished the two materials because of the color, P3P6T looks yellow in the

microscope (although at naked eye it is purple), and the cellulose is close to

brown. The characteristic particles of the cellulose can be seen behind the drop of

the polymer, which are more noticeable in NaturalTM type.

Something very different happened for the drops that were poured with fresh

layers. For the UltraTM nanocellulose, it seems that the P3P6T has created

aggregates. This is due to the fact that when the gel is fresh, it contains water (the

composition of the gel is 8% cellulose and 92% water). In consequence, P3P6T

mixed together with water, spreading himself over all the sample and creating

an inhomogeneous film with non-defined regions.

For the NaturalTM nanocellulose, it can be seen that the border is really diffuse.

However, unlike the UltraTM case, now the two regions can be very well

distinguished. This fact shows that P3P6T has been absorbed by the NaturalTM

cellulose creating a good mixture, not as in the previous case.

At the end, none of these layers with P3P6T on top could be peeled off.

4.1.4 PEDOT:PSS on top of nanocellulose

In the overall structure of a solar cell, the active layer is sandwiched between two

blocking layers, one in each face, in order to prevent the electrons and holes to

arrive to the wrong contact resulting in a loss of efficiency in the system.

Figure 4.5: Optical microscope images showing the comparison between UltraTM (top row) and NaturalTM (bottom

row) cellulose nanocrystals films, when a drop of P3P6T is poured over them when they are fresh. Different

magnifications are shown in each column. In this case the darker colors correspond to P3P6T and brighter ones to

nanocellulose.


28

PEDOT:PSS is widely used for the researchers to constitute the electron blocking

layer in solar cells. Hence, the behavior of these two materials together has to be

studied.

As in the previous case, the fact that no liquid solution can be used in blade-

coating entails that the cellulose nanocrystal layer has to be created first with the

gel, and then a drop of the polymer solution has to be added.

To do this experiments, both UltraTM and NaturalTM nanocellulose were used. As

with P3P6T, the thickness of the UltraTM films was 6 tape and the NaturalTM ones

was 3 tape, as these are the optimal thicknesses for each gel.

In order to compare the results, the procedure was exactly as the experiment with

the P3P6T. The solution was poured in two different moments for both types of

nanocellulose. For some samples it was poured over the fresh layer, that is to say,

some seconds after creating them; and for the other ones it was poured once the

layer was completely dry.

From the optical microscope images (Figure 4.6), it can be seen that the border

between the two regions is very clear, the same that happened with dried layers

and P3P6T. The greenish part corresponds to PEDOT:PSS, while the brown

regions correspond to the cellulose nanocrystals. The characteristic particles of

the cellulose can also be seen behind the green drop of the polymer, more

noticeable in NaturalTM type.

Observing the microscope images of the fresh layer (Figure 4.7), for the UltraTM

cellulose nanocrystals it can be seen that the border is more diffuse than in the

dried layer, although it is defined. It doesn’t have a regular form. As it can be also

observed in image a), the two regions can’t be distinguished very well, so the

PEDOT:PSS has not been uniformly spread. For the NaturalTM type, the border

between the two materials is very diffuse, and it seems that the polymer has been

absorbed.

Figure 4.6: Optical microscope images

showing the comparison between

UltraTM (top row) and NaturalTM

(bottom row) cellulose nanocrystals

films, when a drop of PEDOT:PSS is

poured over them once they are dried.

Different magnifications are shown in

each column. The greenish part

corresponds to PEDOT:PSS, while the

rest corresponds to cellulose

nanocrystals.


29

These films could be easily peeled off the glass, unlike the P3P6T ones.

Comparing the results of this experiment with the previous one made with

P3P6T, and taking and overall view, it can be said that polymer drops on dried

cellulose layers always have a well-defined border, while if the drops are poured

on a fresh layer, the water of the gel makes the polymers spread in a non-

uniformly way, creating strange shapes and more than one different region

containing the polymer. That is to say, it won’t be possible to make a uniform

polymer layer over a fresh nanocellulose layer.

Comparing now the two types of cellulose used, it can be observed that they have

a very similar behavior when it refers to dry layers. However, the behavior on

fresh layers is different. While the UltraTM cellulose seemed to form aggregates of

the polymer, the NaturalTM one seemed to absorb it.

To finish with the comparisons, the fact that PEDOT:PSS could be peeled off

easily while with P3P6T was impossible to peel off is also interesting. That

property of PEDOT:PSS could allow in the future to build the electron blocking

layer of a solar cell in a very easy and cheap way.

Figure 4.7: Optical microscope images showing the comparison between UltraTM (top row) and NaturalTM (bottom row)

cellulose nanocrystals films, when a drop of PEDOT:PSS is poured over them when they are fresh. Different

magnifications are shown in each column. The greenish part corresponds to PEDOT:PSS, while the rest corresponds to

cellulose nanocrystals. In image a), the center of the drop on the Ultra cellulose can be seen with a magnification of x2.5.

Figure 4.8: Image of the

polymer drops on cellulose

layers when the film was dry

(a), and fresh (b). P3P6T is on

the top and PEDOT:PSS is on

the bottom.


30

4.2 Experiments performed with spray-coating technique

This method is very interesting to be used because beside it needs a liquid

solution, the layers done with this method are very homogeneous and the thick

is also very controllable. However, before doing it some preparation time was

needed to build the setup and to clean the glasses with an acid bath.

For all of the experiments in this section, the same setup was used. The spray-

gun was placed 16 cm above the sample. The flow of the spray-gun was set to 0.1

ml/s and the spray duration was 0.5 seconds. So, approximately 0.05 ml of

solution were sprayed in each spray. The time left between two consecutive

sprays was enough to left the sample dry completely. This last fact is important

in order to achieve a homogeneous sample. To accelerate the drying time

between the sprays, the sample was placed on a heating plate at 85ºC.

4.2.1 Cellulose films with non-ablated titania

In this first experiment, both UltraTM and NaturalTM cellulose types are going to be

used. Two solutions were prepared, one with each type of cellulose nanocrystals.

The concentration of the solution was 10%, and afterwards non ablated titania

(titania P25) was mixed with the solution with a ratio of 1:1 with the cellulose.

To see the effects of the number of sprays on the layers, three different samples

with a different amount of sprayed solution were made for each cellulose type.

In the images for the UltraTM sprayed layers (Figure 4.9), it can be seen, thanks to

the image brightness, how the thickness of the layer increases with the amount

Figure 4.9: Optical

microscope images showing

sprayed UltraTM cellulose

nanocrystal layers. The

corresponding magnifications

can be seen in each column.

The amount of solution

sprayed can be seen in each

row.


31

of solution. It can also be easily seen in the x50 magnification images how the

titania forms aggregates. The abundance and size of these aggregates also

increase with the amount of solution sprayed.

The images of the NaturalTM cellulose nanocrystals are omitted because they are

very similar to the UltraTM ones. A comparison between both types can be seen

above (Figure 4.10).

Some current measurements were also done to this samples. The equipment used

was the 4 tip probe. Five measurements were done for each sample in five

different areas in order to average them and extract a general behavior of the

sample.

It was found for both types of cellulose that for higher amounts of solution

sprayed, the samples were less conductive and therefore had a higher sheet

resistance.

However, the sheet resistances belong to a very high order of magnitude (x109),

which shows us that the layers are not conductive at all.

To finish, UV-vis measurements were also done to this samples to know their

absorbance in the ultraviolet and visible wavelength range. For the active layers

of the solar cells this property is as important as the conductivity, because it will

define the efficiency of the solar cell as well as its thickness. Then, high

absorbance is needed to be able to absorb the sun light in the minimum thickness

possible.

The UV-vis transmittance spectra for the different samples are shown below

(Figure 4.11).

From plot a, the transmittance spectra for the three different amount of solution

of UltraTM sprayed cellulose nanocrystals can be seen (as well as the transmittance

of the glass. As it is logic, the more solution sprayed, the lower transmittance

(higher absorbance) the layer will have. The transmittance of the three layers

Figure 4.10: Optical

microscope images showing

the comparison between

sprayed UltraTM (a) and

NaturalTM (b) cellulose when

mixed with non-ablated

titania. Magnification x10.


32

decrease drastically under 350 nm wavelength approximately. Above that

number, that is to say for the visible range, it can be observed how the layers

absorb less amount of light, increasing significantly their absorptions for lower

wavelengths. This means that even the thickness of these films are in the

nanometer order, they can absorb a significant amount of light. Thus, for slightly

thicker films, this absorbance will increase clearly, allowing the layer to absorb

almost all the light. This is due to the organic material in the solar cell (cellulose),

that have a great absorption rate.

In plot b (Figure 4.11), it is shown the comparison between both types of cellulose

used. It can be clearly seen how the NaturalTM layer has always a lower

transmittance, hence a higher absorption. This could also be guessed from the

colors of the samples, because the ones made with UltraTM one were slightly more

transparent.

4.2.2 Cellulose films with ablated titania

In the following experiment, microfibrillated enzymatic cellulose was used. This

section will be divided in three parts. The objective of the first part will be to

compare spray-coating samples with different proportions of cellulose and

ablated titania. In the second part, the effect of the ablation process in the

cellulose will be seen. To finish, the difference between doing the ablation with

the mixture already done will also be seen.

To begin, two different types of solution were prepared. On one hand, 3 ml titania

P-25 solution were done, with a concentration of 10 mg/ml. These solutions were

ablated following the process explained in section 3.1.4. On the other hand, a

Figure 4.11: a) Transmittance comparison of titania and Ultra cellulose sprayed films with different volumes of sprayed

solution. b) Transmittance comparison of titania-UltraTM and titania-NaturalTM cellulose sprayed films with 1 ml of

sprayed solution.


33

solution of microfibrillated enzymatic cellulose was done, with a proportion of

1:8.

The idea was to mix these two solutions in different proportions and spray them

over the glasses to compare their properties. The mixtures were done in order to

keep constant the amount of sprayed titania. Hence, the amount of ablated titania

solution used in each mixture was 0.5 ml. This fact is important to have a correct

comparison between the samples, even if the total amount of solution sprayed

had to be different. The proportions tried for the solutions and their total amount

are resumed in the following table:

Proportion

(cellulose:titania)

1:0 0:1 1:2 1:1 2:1 3:1 4:1 5:1

Solution sprayed

(ml)

2 0.5 0.75 1 1.5 2 2.5 3

Figure 4.12: Optical microscope images comparing cellulose with ablated titania films with different proportions (columns).

Two magnifications are shown for each sample (rows).

Table 4.1: Total sprayed volume for each solution.


34

All the samples were made in the same conditions, with the spray duration set in

0.5 seconds and a flow rate of 0.1 ml/s. The distance from the spray-gun to the

samples was 16 cm, and they were placed over a heating plate at 85ºC to

accelerate the drying process between consecutive sprays.

In the optical microscope images (Figure 4.12), the comparison between all the

samples can be seen. First of all is important to notice how, for the layer made

only with cellulose, the sample is very homogeneous and the surface seems to be

very flat (image x50). On the other hand, the film made only with ablated titania

also seems to be very homogeneous, although in this case the surface is quite

rough.

For all the others samples first the x10 images will be commented. It can be seen

that for the 1:2 sample, the crated film is not homogeneous, because it has several

bright spots on its surface. This fact would probably also cause mechanical

instability of the film. As more cellulose is added to the samples, more

homogeneous become the layers and less bright spots can be seen. For the two

last ones, with the higher proportion of cellulose, the layers become similar to the

layer made only with cellulose, and only a few spots can be seen.

Very interesting is also the analysis of the x50 magnification images. To compare

the pictures, the image of the layer made out only with ablated titania has to be

kept in mind. It can be observed how for this layer the titania creates kind of a

matrix with a lot of free space in between. This is a very good new, because that

is exactly what is needed for creating the bulk heterojunction active layer, an

interconnected material that cover all the space. As more cellulose is added in the

samples, this free space inside the titania matrix is progressively filled. In the

samples with less amount of cellulose, some free space in the surface can be easily

seen. However, as the amount of polymer increases, the surface begins to get

Figure 4.13: Current-Voltage plot, showing

the comparison between different sprayed

cellulose-ablated titania films. Two

resistances were calculated, the first one

belonging to the initial slope, and the second

one belonging to the second slope, as it can be

seen in the image.


35

flatter and more homogenous, until the point where no free space can be seen on

the surface.

Some current measurements of the samples were also taken. For each sample,

five measurements were done in different areas, and they were later averaged in

order to have the general behavior of the samples. Two different sheet resistances

were computed, the first one belonging to the initial slope and the second one

belonging to the second part, as it is shown in the plot (Figure 4.13).

It can be observed that the R1 slightly changes for the different layers, while R2 is

more or less the same always. The highest resistance belongs to the laser ablated

titania without cellulose, and the second one to the sample with a proportion 1:2.

Therefore, the two layers that contain less cellulose have higher resistances.

However, all of the samples have too high resistance (order of 109 Ω), which

means that none of them is conductive at all. To calculate the sheet resistance, the

slopes of the plot have to be multiplied by a form factor 𝜋

𝑙𝑛(2).

So for each sample 𝜌𝑆1 and 𝜌𝑆2 are going to be calculated. The results, computed

with MATLAB can be seen in the following table:

UV-vis measurements were performed on the samples as well. As it is seen in the

plot (Figure 4.14), the samples don’t behave in any predictable way. The less

absorbent spectrum by far corresponds to the sample only containing cellulose.

It is followed by the film with ablated titania. This fact supports the idea of using

Figure 4.14: UV-vis plot

comparing the different sprayed

films containing cellulose and

ablated titania.

Table 4.2: Sheet resistances of the different sprayed solutions.


36

a mixture between cellulose and titania to increase the absorbance of the active

layer, because all the mixtures have higher absorbances than the materials alone.

The most absorbent one belongs to the film with a proportion 3:1. Comparing

these films with the ones obtained before without the ablated titania and with the

other types of cellulose, it can be seen that these ones can absorb significantly

more light, although the shape of the plot is similar in both cases, with the

transmittance increasing with the wavelength.

Now, the ablation process effect on cellulose will be checked. The aim is to know

if when doing this process only to cellulose it can change any of its properties, or

if it has structural changes. To do this part of the experiment, a 2 ml solution of

microfibrillated enzymatic cellulose in proportion 1:8 is done. The solution is

ablated as the section 3.1.4 explains. The spray-coating is done in the same

conditions mentioned above, with 2 ml solution for each sample.

In the microscope images (Figure 4.15), it can be observed that the differences

between both films are not very important. On a first sight, a slightly different

color can be noticed in the films. The typical large particles in nanocellulose can

be seen in the non-ablated cellulose layer. However, if x2.5 and x10

magnifications are observed, it can be noticed that these particles do not appear

in the ablated cellulose. The third remarkable difference can be seen in the x50

images. While the non-ablated seems to be pretty flat, the ablated one appear a

little bit rough.

Also UV-vis measurements were performed on this samples. As it can be noticed

in the plot (Figure 4.16), the transmittance spectrum is almost the same for both

samples, being the ablated one slightly more absorbent.

Figure 4.15: Optical microscope images belonging to ablated and non-ablated cellulose sprayed films.

Different magnifications can be seen in each column.


37

To finish with the experiment, a new ablated solution was needed. The aim of

this last part was to see if ablating the titania altogether with the cellulose would

have any effect on the layer properties. To prepare the solution, 30 mg of powder

titania were added to a 3 ml solution of microfibrillated enzymatic cellulose with

proportion 1:8. This solution was laser ablated following the normal procedure

and then it was sprayed under the same conditions than the previous ones. Only

1 ml of solution was sprayed in this case.

The comparison will be done with the sample prepared before with a proportion

2:1. The reason is that a part from the same amount of titania, they both have the

same amount of cellulose (3 ml 1:8 solution), although the total volume of the

solution was different in the two cases.

Figure 4.16: UV-vis measurement

comparing ablated and non-

ablated cellulose layers.

Figure 4.17: Comparison between sprayed layers. In the top row images, the solution was made mixing the cellulose with

the titania after the ablation. In the bottom row, the solution was mixed before the ablation. Different magnifications can be

seen in each column.


38

From the optical microscope images (Figure 4.17) it can be seen that the layer

prepared in this last experiment is much more inhomogeneous than the previous

one (see magnification x10). In the other two pictures, a lot of similarities between

the samples can be detected. They both have a rough surface, with some free

space in between the titania matrix that the cellulose didn’t fill up.

Talking about the conductivity measurements (Figure 4.18), it can be clearly seen

how the ablated cellulose and titania sample has a higher sheet resistance than

the cellulose with ablated titania one. This can be related with the microscope

images, because the layer made with ablated cellulose is more inhomogeneous

than the other as commented above. This inhomogeneity probably causes a larger

electrical resistance in the layer. However, the fact that both resistances are still

really high has to be kept in mind. The sheet resistances, computed with

MATLAB, can be seen in the following table:

Cell. with abl. titania Abl. cell. and titania

𝝆𝑺𝟏 (x109 Ω) 4.53 ± 0.2 7.47 ± 0.8

𝝆𝑺𝟐 (x109 Ω) 32.18 ± 0.5 32.68 ± 0.3

To finish with the comparison between these two samples, UV-vis measurements

were made. The result in this case was totally unexpected. As it can be seen in

the plot (Figure 4.19), the absorption of the ablated cellulose and titania was

much higher than the one of cellulose and ablated titania. On average, the

absorption in the visible range of the new layer is near 50%. Despite the fact that

Figure 4.18: Current-Voltage plot

showing the comparison between the

cellulose and ablated titania layer

and the ablated cellulose and titania

layer.

Table 4.3: Sheet resistances of cellulose with ablated titania and ablated cellulose and titania layers.


39

the layer is very thin, it can absorb approximately one half of the light received.

This means that with a material like this, very thin active layers, and in

consequence solar cells, can be done.

This fact was unexpected, because from the previous cellulose samples without

titania (Figure 4.16), it can be seen how the UV-vis spectra before and after the

ablation is almost the same, so the laser ablation didn’t have any effect on the

absorption at all.

In conclusion, from all the microscope images seen in this section, it can be said

that with the laser ablation of the titania, a matrix is created. This is a good point,

because is just what it is needed, an interconnected matrix capable of transporting

the electrons from one place to the other. On the other hand, current

measurements show that the conductivity of the layers is not good enough, to

this property still have to be improved. And to finish, the conclusion that can be

extracted from the UV-vis measurements is that mixing titania with an organic

material leaves a more absorbent layer. Furthermore, if this organic material is

also laser ablated altogether with the titania, the absorption will increase

considerably.

Figure 4.19: UV-vis transmittance

spectra showing the comparison

between cellulose with ablated titania

layer and ablated cellulose and titania

layer.


40

4.3 Experiments performed with vacuum filtering technique

This method provided a very easy way to do films. Differently from the previous

one, solutions can be used to make the films. This will suppose an enhancement

in the homogeneity of the films and will allow aqueous ablated titania

nanoparticles to be used. The layer’s thicknesses can be pretty controlled with

this method.

Three different types of cellulose are going to be used in the next experiments.

UltraTM, NaturalTM and microfibrillated enzymatic cellulose.

4.3.1 Cellulose films

To begin, as with blade-coating experiments, cellulose layers will be done in

order to know the behavior of the different types of cellulose using this method.

First of all, two solutions with UltraTM cellulose were prepared, with different

concentrations of the polymer in order to know how can this parameter affect the

layers. Both solutions had a volume of 10 ml, the first of them with 10%

concentration, and the second one with 20% concentration. After 5 minutes of

vacuum filtering, the layers were removed together with the filter and were left

one entire day drying. One day later, these nanocellulose layers were beginning

to peel off and fold over themselves.

Two more 10 ml solutions were prepared for NaturalTM cellulose, this time with

concentrations of 6% and 10%. After 5 minutes of vacuum filtering, the layers

were removed and left drying for a whole day. The resulting layers were much

more folded and auto-peeled that the ones prepared with the UltraTM solution.

From the microscope images (Figure 4.20) it can be seen how the particles in the

NaturalTM film are much bigger that the ones in UltraTM film. However, it can be

seen in the UltraTM images how some straight lines appear on the surface of the

layer. This is probably due to the fact that this images are taken with the layers

together with the nitrocellulose filters. Then this lines are probably an effect of

the relief of the filter. They can’t be observed in the NaturalTM cellulose because it

has larger crystals that cover them.


41

Once created, it was tried to peel off the layers. It was completely impossible to

take out the complete layer without breaking it into smaller pieces. Taking in

account that this method was done to achieve layers in an easy way, this fact had

to be solved. Acetone was tried as a solvent to dissolve the filter, although it is an

organic solvent that it has to be avoided to have a more environmentally friendly

process. The acetone resulted to work, dissolving completely the nitrocellulose

filter and leaving intact the cellulose nanocrystal layer (Figure 4.21). However,

this way of releasing the layer was finally rejected because of the environmental

issue stated above.

4.3.2 Cellulose films with non-ablated titania

The next step is to add titania to the solutions. Non-ablated titania is going to be

added first. Three different types of cellulose are going to be used to build the

layers, UltraTM, NaturalTM, and a gel of 50 µm cellulose microcrystals. Four

solutions were prepared for this experiment, all of them with a volume of 10 ml.

The two first solutions that are going to be compared are the UltraTM and

NaturalTM ones. In this case, a concentration of 8% was used to make the solution.

Then, the same amount of titania powder P-25 was added, taking in account that

the concentration of cellulose nanocrystals inside the gel is approximately 8%.

The vacuum filtering was done, and then the samples were left during one day

to dry. In this case, a metal ring was placed on the layers while drying in order

to prevent the auto-folding of the films and achieve flatter layers.

The microscopic images of the samples (Figure 4.22) show how the titania has

not formed big aggregates in both cases, especially compared with the ones

created with blade-coating method. This is definitely due to the fact that now the

Figure 4.21: Optical microscope image of a

NaturalTM cellulose layer released from the

filter through acetone solvent. Magnification

x2.5.

kjjk

Figure 4.20: Optical microscope images showing the

comparison between Ultra and NaturalTM cellulose layers

attached with the filter. Different magnifications are shown

in each column.


42

titania and the cellulose are being mixed in an aqueous solution, which

distributes the titania uniformly in all the sample.

These images look very different from the images without the titania seen before

(Figure 4.20). Surprisingly, the particles of NaturalTM cellulose, that have always

been very visible can’t be seen in this case. This should be because of the addition

of titania, that is changing the surface of the layer impeding them to be seen.

The other two solutions used were made using cellulose microcrystals. Two

different concentrations were tried in order to compare them, one solution had

10% and the other one 20%. Then, titania P-25 was added until getting a

proportion of 1:1, as in the previous cases. The filtering of these two layers was

much faster than with the others.

It can be observed from the images (Figure 4.23) how for the solution with higher

concentrations (images a and b), no consistent layer has been created. The layer

has a lot of cracks that make it impossible to be peeled off. For the solution with

lower concentration, no film was made. Instead, it can be seen how some

aggregates were created, making the sample seem like dust.

Hence, this type of cellulose is not useful to build layers using the vacuum

filtering method.

Figure 4.23: Optical microscope images of

microfibrillated enzymatic cellulose layers with

titania. Images a and b belong to the same layer

(20% concentration), with a magnification of

x2.5 and x50 respectively. Images c and d belong

to the other layer (10% concentration) with a

magnification of x2.5 and x10 respectively.

Figure 4.22: Optical

microscope images with a

magnification of x10. In the

image a, Ultra cellulose layer

with titania can be seen. In the

image b, NaturalTM cellulose

layer with titania can be seen.


43

4.3.3 Cellulose films with ablated titania

Hereunder, the experiment done with laser ablated titania is going to be

explained. The only type of cellulose used for this experiment was

microfibrillated enzymatic cellulose. Also a layer with only ablated titania was

done to know its consistency without the cellulose.

To ablate the titania P-25, the procedure explained in section 3.1.4 was followed.

In this case, two solutions were made. The first of them simply consisted in 3 ml

of solution with a titania concentration of 10 mg/ml. The second one was a

solution containing microfibrillated cellulose obtained by an enzymatic process

in proportion 1:8 (11%), and a concentration of 10 mg/ml of titania.

After the ablation, the first solution was mixed with water until reaching 10 ml

in order to guarantee homogeneity in the layer. Filtering technique was then

done. The resulting layer can be seen in the microscope images (Figure 4.24).

From the images, it can be observed how the titania alone did not form any layer.

Instead, a lot of rigid little plaques were formed, with cracks between them.

The comparison between two different parts of the layers was also done. The

layer was not completely homogeneous because of the holes of the ceramic piece

used to hold the layer (see Figure 4.25). Then the layer showed parts with more

titania than others. This is the case of images b and c in Figure 4.24, they are taken

with the same magnification but the first one in an area with high concentration

of titania and the second one in an area with much lower concentration. The layer

was impossible to peel off, because it behaved as dust.

Figure 4.25: Images of a piece of the

titania layer before (a), and after (b)

trying to peel it off with adhesive tape.

It can be seen that it behaved as a non-

coherent film.

Figure 4.24: Optical microscope images showing different magnifications of the titania layer made with vacuum filtering

technique. The images a, b, c and d have magnifications of x2.5, x10, x10 and x50 respectively


44

The second solution contained, as said before, the cellulose and the titania ablated

altogether. After the ablation, as with the first solution, water was added until

reaching 10 ml of volume, and after that filtering was performed normally.

Some pictures of the layer can be seen below (Figure 4.26). In the first one, it can

be observed that the layer is homogeneous, that is to say, the titania and the

cellulose have mixed together correctly, without forming aggregates or

aggregates. In the second picture the holes of the ceramic piece commented above

can be clearly seen. It can be observed also how in the borders of the filter; the

layer is beginning to peel off alone. The layer was really easy to remove with an

adhesive tape, and resulted to be very mechanically consistent. A hole in the

middle of the tape (and in the paper were it was adhered) was done in order to

have an area where the cellulose layer was the only material, so that the

measurements could be correctly performed. The final sample is shown in

picture c.

This sample was brought to Hamburg Electron Synchrotron, DESY [14]. Some

WAXS experiments were performed on the film with the same parameters

exposed in section 3.2.4, with the last technology small beam (250nm x 250nm)

Figure 4.26: a) Optical microscope image showing the microfibrillated enzymatic cellulose with ablated titania

layer. Magnification x10. b) Picture of the layer attached to the nitrocellulose filter. c) Picture of the final sample

ready to be measured. The hole made in the tape, where the free-standing layer is located, is depicted in red.

Figure 4.27: Example of an image obtained in

WAXS measurement for the cellulose-titania layer

obtained with vacuum filtering method.


45

that will allow to know the morphology of the sample in the molecular scale. The

main objective of this measurements was to have a first insight of the morphology

of the laser ablated titania over the sample, as well as its phase behavior and its

crystal orientation.

An 11x11 squared scanning was done over an area of 5 µm x 5µm. Thus, 121

different intensity measurements were obtained, one for each step done.

From the images obtained (Figure 4.27), an analysis of the crystallographic planes

and the orientation of the crystals can be made. In this kind of images, it can be

observed both the diffraction of the polymer and the metal.

Polymers use to be low scatterers, and cellulose is not an exception. For this

reason, the scattering of the polymer can be seen close to the center of the image,

which means that it has a small scattering angle. In this case, the corresponding

scattered light diffused by the polymer can be seen in green, producing a circular

shape with its center over the beam stopper. From it, the orientation of the

cellulose nanocrystals can be determined. In this case, as the intensity is the same

in all the directions, it can be deduced that the polymer crystals are randomly

oriented.

On the other hand, metals like titanium dioxide are great scatterers. It can be

observed how the scattering of the X-rays on titania crystals produce ring-shaped

intensity patterns. This means that these crystals have not any specific

orientation, thus they are isotropically oriented in the sample. In total, four rings

can be seen in the image. Each of the rings can be related with a crystallographic

plane present in different titania phases. Therefore, knowing the intensity of

these rings, the amount of each titania phase in the sample can be resolute.

Figure 4.28: At the left, WAXS image showing the four diffraction rings of titania crystals. At the right, the radial

integration intensity of the image between the two red concentric circles depicted, where the rings can be seen quantitatively.


46

To observe quantitatively this rings, a radial integration will be done using the

software DPDAK. The integration will be done between the two concentric circles

depicted in red in the image (Figure 4.28). The four peaks belonging to the rings

can be clearly seen in the plot. The first and more intense ring corresponds to the

plane 101 of the anatase phase of titania (1.785 Å-1). The third and fourth rings

also correspond to the anatase phase, concretely to the planes 103 (2.634 Å-1) and

200 (3.303 Å-1) respectively. The second ring, hardly visible in this case, belongs

to the plane 110 of rutile phase (1.931 Å-1) [16].

Therefore, it can be guessed qualitatively that the amount of anatase phase in the

sample will be much greater than the amount of rutile. This fact perfectly agrees

with the titania P-25 powder used, which according to the provider is composed

by these two phases, with anatase as the predominant one.

A plot with the intensity data of all the row can also be done (Figure 4.29). There

it can be noticed how the rings remain in the same place for all the measurements

done within the scanned surface.

To know the orientation of the bigger titania crystals, individual bright spots

belonging to any of the rings have to be found. This can be done doing an

azimuthal integration of the intensity, that will show the angle in which this spots

can be found. The azimuthal integration can be done for each ring separately, as

shown in the example (Figure 4.30), where the integrated area is depicted in red.

For the integration of all the rings blind areas can be observed, where no data has

been collected (out of the image). In this plots, a part from the peaks, some

depressions can be seen. This lack of intensity belongs to the black horizontal

straight lines where there is no data acquisition, due to the detector building

process. These areas are called intermodule gaps.

Consequently, knowing the position of a bright spot will give information about

the titania crystal phase an orientation. These two parameters will be nearly

Figure 4.29: Radial integration

intensity for all the WAXS images of an

entire row, where the different rings of

titania crystals can be seen.


47

always different for each crystal in the sample. Thus, the same crystal can be

identified in different WAXS images only looking at the position of the bright

spot.

Figure 4.30: At the left, the same image repeated three times, with the azimuthal integration region depicted in red.

For each image, a different ring is analyzed. At the right, the corresponding azimuthal integrated intensity, showing

the peaks that correspond to the bright spots in the images. The shadowed depressions correspond to the intermodule

gaps, were no data is available.

Rutile 110

Anatase 103

Anatase 200


48

Keeping this in mind, a map of the topography of the scanned area can be done

just comparing the bright spots of a measurement with the ones of the

measurements done in its neighborhood. If a bright spot remains in the same

position in two measurements will mean that the crystal is big enough to cover

the distance between the two points. This fact can be clearly seen in the images

belonging to the row 4 columns 7 and 8 (Figure 4.31), where five different

repeated crystals can be found.

Following this procedure, the topography of the entire scanned area has been

done, taking only in account the spots brighter than 20 belonging to the rings 2,

3 and 4. The map obtained is shown in Figure 4.32, where the bars cover all the

squares where the same spot could be found. The different colors used mark the

ring where the spot belongs.

As it can be seen (Figure 4.32), most of the repeated spots belong to the same row,

explanations have been found to understand it. The first one would be that due

to the method used to make the layer, the water flux in the filtering could have

oriented the crystals in a particular direction. The other possible explanation

would be a non-perpendicular incidence of the X-rays into the sample surface,

Figure 4.31: At the top, WAXS images corresponding to neighbor measurements, with the points that coincide in both

images circled. The intensity scale has been changed in order to properly see the bright spots. At the bottom, the

comparison of their azimuthal integrated intensity, showing that the peaks are in the same positions.


49

that would lead to a higher scanned surface in one of the directions. However,

this last option is highly improbable.

Another thing that can be extracted from the crystal distribution image is that

almost all the bright spot coincidences belong to rutile 110. The second most

common, far away from the first one are the ones belonging to ring anatase 103

and the less common coincidences are produced in anatase 200.

This last experiment has been a first look into the characterization technique, and

it has been seen how powerful can it be. In a near future, SEM images are going

to be taken to the sample in order to compare them with the topographic map in

Figure 4.32. For a matter of time, this images cannot be included in this thesis.

However, the results obtained can be the starting point for a new investigation

line.

Figure 4.32: Distribution of the bigger crystals in the measured region. At the left, the horizontal coincidences. At the

right, the vertical coincidences.

5. Conclusions

50

5. CONCLUSIONS

The focus of this thesis was to analyze the interaction between different types of

cellulose and the titania through three particular production methods. From the

results, the goal was to discuss their effects in the properties of the samples and

see if they were suitable to be used to integrate the active layer of a hybrid solar

cell.

The second objective was to do all the procedures always thinking about being

respectful with the environment, ergo avoiding the use of toxic materials and

organic solvents among others.

In first place, blade-coating technique was discussed. It was seen that NaturalTM

cellulose could create more consistent layers than the UltraTM one. However, this

method could not be combined with the ablated titania particles, which are

necessary for a correct performance of the solar cells. Therefore it is not a good

method to build the active layer. The useful knowledge that can be extracted from

this section is the easy way to create mechanically consistent cellulose

nanocrystals layers, that can be useful for creating the substrate of some flexible-

electronic device.

In second place, spray-coating method was performed. This method

demonstrated to create homogeneous and thickness controllable layers, which

facilitates the reproducibility of the experiments. It has been seen in conductivity

measurements, nonetheless, that layers made out with this combination of

cellulose and titania were not conductive at all, which make them not suitable to

constitute the active layer of a solar cell. However, some profitable information

has been extracted from this method. The necessity of a laser ablation for titania

nanoparticles have been proved. Even if the particles were in a water dispersion,

if the ablation was not done, huge aggregates were formed within the layer,

which lowered their conductivity and mechanical properties. The other

important fact discovered was about the absorption spectra of the layers. It was

found that for mixed cellulose-titania solutions, the absorption spectra were

much higher than for each of the components separately. Furthermore, if the

mixed solution was ablated all together, that is to say the cellulose nanocrystals

and the titania, the absorption spectrum was considerably higher than the one

obtained without ablating the cellulose. These information is suitable to be used

in a future for other water soluble polymer based active layers.

To finish vacuum filtering was done. From it, self-freestanding layers can be

obtained in a very easy way. The ablation of titania, as with the previous method,

has also been proved to be necessary to obtain mechanically coherent layers. The


51

layers done with non-ablated titania were extremely delicate, so it was

impossible to achieve a large consistent layer. On the other hand, if the titania

was ablated, very nice and easy to peel off layers were obtained.

It was from this last sample done with a mixed solution of ablated titania and

microfibrillated enzymatic cellulose where it was tried to go further in the

investigation. The idea was to have a first insight about the nanometric size

morphology of this layer and to see if there was any dominating crystal

orientation in the layer. For this reason, the sample was brought to DESY

synchrotron in Hamburg, were very intense and small X-ray beams can be

achieved. From the measurements taken, it was seen that both the

microfibrillated cellulose and the titania were randomly oriented within the

layer. Furthermore, the different phases of the titania could be seen and matched

with the information supplied by the producer. A topographic map containing

the larger crystals of the sample was created. They were found to be mostly all

placed in parallel, thing that could be done to the processing method of the

sample. Sadly, there was no time to do the SEM images and include them in the

thesis to compare them with topographic map, but they are going to be done

shortly after the end of this thesis. The information achieved with this method

can be very interesting to learn more about the titania morphology in the active

layers of hybrid solar cells, therefore this will be a starting point for future

investigations on the topic.

In conclusion, this thesis has deepened in cellulose and ablated titania properties.

Even though it has been found that cellulose is not conductive enough to be part

of the active layer of a solar cell, their properties were found to be interesting for

other kind of uses, for example as a flexible substrate. A lot of valuable

information has also been extracted from the interaction of ablated titania and

the organic polymer, for example the interconnected titania matrix created with

the laser ablation, that is very useful for a Bulk Heterojunction active layer of

hybrid solar cells. It is also remarkable that all this research has been done

agreeing with the objective of being respectful with the environment.

For all these facts, a lot of investigation still has to be done in order to enhance

hybrid solar cells efficiency and find more environmentally friendly ways to

produce them.

6. Bibliography

52

6. BIBLIOGRAPHY

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Acknowledgements

In first place, I would like to thanks Prof. Peter Müller-Buschbaum to give me the

opportunity to elaborate my thesis in his group, Lehrstuhl für Funktionelle

Materialen (E13), and researching in such an interesting field.

I want to express my gratitude also to Dr. Volker Körstgens for being my

supervisor and guide during these five months, as well as for introducing me in

lab work and teaching me all necessary methodology for creating and analyzing

the samples.

My thanks also to Dr. Carlos Alemán for accepting to be my co-director at the

distance.

Many thanks also to Adrian Haussmann and Florian Buschek to have the

patience to teach me how to do some of the techniques, as well as for the time

spent doing the laser ablations.

For the X-ray measurements at the synchrotron DESY, in Hamburg, I would like

to thank all the beamline team Volker Körstgens, Johannes Schlipf, Kiran Mathew

John and Shambhavi Pratap.

To finish, thanks to my family and friends for their support and for making this

experience possible.

Documents

Hybrid solar cells based on nanocellulose and titania