Graphene oxide hole transport layers for large area, high ...epubs.surrey.ac.uk/805887/1/Smith 2014 - Graphene... · Graphene oxide hole transport layers for large area, high efficiency

Accepted for publication in Applied Physics Letters

1

Graphene oxide hole transport layers for large area, high efficiency organic

solar cells

Chris T. G. Smith1, Rhys W. Rhodes

1, Michail J. Beliatis

1, K. D. G. Imalka Jayawardena

1, Lynn J.

Rozanski1, Chris A. Mills

1, and S. Ravi P. Silva

1,a)

1Advanced Technology Institute University of Surrey Guildford GU2 7XH, UK

Graphene oxide (GO) is becoming increasingly popular for organic electronic applications. We present

large active area (0.64 cm2), solution processable, poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-

thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:[6,6]-Phenyl C71 butyric acid methyl

ester (PCDTBT:PC70BM) organic photovoltaic (OPV) solar cells, incorporating GO hole transport layers

(HTL). The power conversion efficiency (PCE) of ~5% is the highest reported for OPV using this

architecture. A comparative study of solution-processable devices has been undertaken to benchmark GO

OPV performance with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) HTL

devices, confirming the viability of GO devices, with comparable PCEs, suitable as high chemical and

thermal stability replacements for PEDOT:PSS in OPV.


2

Considerable recent interest in Organic photovoltaics (OPV) can be attributed to their potential for

solution-processing, attractive for low-cost, high throughput production. To become price-competitive

with the existing solar cell market, novel polymer development has increased device efficiencies, while

lowering the manufacturing cost. One recent polymer of interest is Poly[[9-(1-octylnonyl)-9H-carbazole-

2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT), which

combined with the small molecule acceptor [6,6]-Phenyl C71 butyric acid methyl ester (PC70BM), has

produced ~6.8% device power conversion efficiencies (PCEs).1 However, PCEs and lifetimes remain

limited due to the use of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), (PEDOT:PSS) - the

most commonly utilised hole-transport layer (HTL) material to date. PEDOT:PSS, while an excellent

material for hole injection, shortens device lifetimes due to its high acidity and hygroscopic nature.2 As a

result, replacement air-stable, long lifetime, solution processable and inert HTL materials are being

investigated.3

Many exciting carbon-based materials have been recently developed applicable to large area electronics.

Graphene is a highly conducting material that has attracted considerable interest due to its physical,

chemical and electrical properties,4 and its potential to be used as a transparent conducting material.

Recently, lab-scale graphene devices have been demonstrated via transfer subsequent to chemical vapor

deposition,5 or by solution processing, e.g. inkjet printing.

6 However, depositing high quality graphene

over large areas without conductivity-limiting defects is difficult and requires high temperatures and

vacuum conditions for production.5

In contrast, graphene oxide (GO), a graphene derivative, can be solubilized in water and

combined in solution with a variety of organic and inorganic materials.7 It retains the high transparency of

graphene, and has a work-function similar to PEDOT:PSS, making it a suitable HTL replacement.8

Chemical modification of GO or combination with different materials also allows for tuning the work-

function, broadening the applicability.9 Here we evaluate the performance of large area (0.64 cm

2)

PCDTBT:PC70BM solar cells incorporating a GO HTL, and compare the performance with OPV

incorporating PEDOT-PSS.


3

ITO on glass substrates (15 Ω-1

), and BCP were purchased from Luminescence Technology

Corp (Taiwan). PEDOT:PSS (Clevios P VP AI 4083, Heraeus GmbH, Germany) was filtered (0.2 µm).

GO was produced by the Hummers and Offeman process,10

and exfoliated by sonication. TiOx was

produced using a previously reported method.11

Acetone and propan-2-ol (IPA) were purchased from

Sigma Aldrich (UK).

Solar cells were constructed with architectures of ITO / HTL (GO or PEDOT:PSS) /

PCDTBT:PC70BM / ETL (BCP or TiOx) / Al, where ETL = electron transport layer. Aqueous GO was

spincast onto ITO substrates (1000 rpm, 1 min) to give 2-3 nm thick films. PEDOT:PSS was sonicated for

5 min. before spincoating (5000 rpm, 1 min) giving a 40 nm film. Both GO and PEDOT:PSS films were

annealed (200°C) for 10 min. PCDTBT:PC70BM (1:4, 7mg/mL PCDTBT concentration) were dispersed

in dichlorobenzene (DCB) and chlorobenzene (CB) (1:3). This was spincoated (5500 rpm) for a ~70 nm

film before annealing (80°C) for 10 min. prior to depositing electron transport layers. For TiOx devices,

the TiOx precursor was spincoated on top of the PCDTBT:PC70BM layer, giving a 10 nm film, then

annealed in air (80°C) for 10 min. For BCP devices, BCP was evaporated onto to the PCDTBT:PC70BM

giving a 5 nm layer. Finally, Al electrodes (100 nm thick) were deposited by evaporation, producing a

0.90 cm2 active area.

The GO was examined using UV-visible and Raman spectroscopy (not reported), FTIR (See

supplemental material)12

and compared to the literature,13, 14

to confirm material quality and purity. The

electrical properties of the cells were characterized by a four-probe voltage source/ammeter (Keithley

2425). Current-voltage responses were taken using a solar simulator (Model 10500, Abet Technologies)

at 1000 Wm-2

. During measurements a mask with a 0.64 cm2

aperture was used in order to remove errors

associated with optical wave-guiding, shadow effects, etc.

SEM images (Figure 1a) show a complete surface coverage of GO flakes on the ITO. AFM

imaging of individual flakes (Figure 1b) confirm the presence of single layers of ~1 nm tall. Given the

long axis dimensions of the GO flakes (up to ~1 µm), they may provide short-circuits through the active


4

layer if not fully orientated parallel to the substrate. However, good GO coverage is assumed from the

AFM and SEM images, and supported by similar Rsh and Voc values for GO and PEDOT:PSS. Shorts in

the device would cause significantly lower these values.

The majority charge carriers in PCDTBT are holes, as with most semiconducting polymers,15

with PC70BM typically being the electron conductor. The dark current has been examined with respect to

electrode limited- (space charge limited current (SCLC),16

Schottky17

) and bulk limited- (Fowler

Nordheim (FN),18,19

Poole Frenkel (PF)20,21

) conduction mechanisms to show whether the GO has

comparable charge injection ability to PEDOT:PSS, and to elucidate the limiting charge transport

mechanism(s). Figures S2 and S3 (See supplemental material)12

show plots of conduction mechanisms fit

to the current-voltage (I-V) data for the ITO / HTL / PCDTBT:PC70BM / BCP / Al and the ITO / HTL /

PCDTBT:PC70BM / TiOx / Al OPV cells respectively. The characteristic parameters produced for each

conduction mechanism are given in Table 1 and are calculated using the following equations.

Schottky: ( (

)

) where, (

)

(1)

Poole Frenkel: ( (

)

) where, (

)

(2)

Fowler Nordheim: (

) where,

( )

(3)

where, I, J and E are the current, current density and electric field respectively; A* is the modified

Richardson constant; h, k, e, and m* are Plank’s constant (= 6.626x10-34

C), the Boltzmann constant (=

1.38x10-23

JK-1

), the elementary charge (= 1.602x10-19

Js) and the effective mass of a hole (assumed to be

equivalent to the mass of an electron = 9.11x10-31

kg); ε0 and εr are the vacuum permittivity (= 8.85x10-12

Fm-1

) and relative permittivity; κ is the slope of the Fowler Nordheim plot; and ϕ is the respective barrier

height for each conduction model. For Schottky and Poole-Frenkel calculations of εr and φ, data points

were only used at sufficiently high fields (<105 Vm

-1, calculated using a high relative permittivity for

conjugated polymers, εr = 12) to render the pre-factors (A*T

2 and E, respectively) insignificant.


5

The relative permittivity of conducting polymers typically ranges between 2 and 12,22

and the

relative barrier heights between materials can be estimated using the band diagram. The calculated FN

barrier heights suggest that FN barrier tunneling is not occurring. On the other hand, the SCLC plots obey

Ohm’s law, J V, at low voltages. At higher voltages, the slope of the graph increases to J Vn where n

> 8 after the trap filled limit, VTFL. If trap-modified SCLC is occurring, the curve will continue on this

slope until all the traps are filled, and the current returns to a trap-free situation, i.e. a J V2

relationship.17

Although there is no evidence of significant portions of the graph displaying a J V2

character, a trap-modified SCLC mechanism may occur in our diodes, as a number of polymer films,

including thiophene based polymers, have been found to have SCLC using ITO substrates as hole

injectors.25

Although the barrier heights (ϕS = 0.4-1.0 eV) of the Schottky plots are in the expected range, the

relative permittivities (εr = 0.01-0.08) are low, suggesting this mechanism does not accurately represent

the measured data. However, the barrier heights (ϕPF = 0.2-0.8 eV) and the relative permittivities (εr = 3-

9) of the majority of the PF plots are in the expected range, suggesting that a PF conduction mechanism is

occurring in our devices. Taken together, it is possible that the conduction mechanism is SCLC with a PF-

field dependency typical of a hopping mechanism, which is supported by similar PF conduction identified

in other polymer blends.26, 27

The presence of trap states in the polymer can also be supported by examining the open circuit

voltage, Voc, of the devices. The Voc for PCDTBT:PC70BM solar cells is typically in the region of 0.88 –

0.91 V,28

but the Voc for the GO devices (0.81-0.83 V) is lower. A low Voc indicates a reduction of the

flow of charge carriers through the diode (such as would occur with the presence traps), consequently

necessitating a higher solar cell operating voltage. However, as the conduction mechanism remains

similar for both PEDOT:PSS and GO, the GO is a comparable HTL, and does not hinder OPV charge

carrier extraction.


6

When illuminated with AM1.5 G solar radiation, (Figure 2) the short circuit current (Jsc) is

similar when switching from PEDOT-PSS to GO (Table 2). The fill factor increased for GO devices

using TiOx ETLs (from 53.3% to 56.0%), but dropped in devices using BCP ETLs (58.7% to 53.7%),

possibly due to incomplete coverage of the BCP over the PCDTBT:PC70BM film. As a result of these

differences, there is a negligible decrease in power conversion efficiency (PCE) between PEDOT-PSS

and GO for TiOx devices (5.3% vs. 5.1%), but a larger difference for BCP devices (6.0% vs. 5.0%). The

reproducibility of device performance can be illustrated by the BCP devices (0.64 cm2), fabricated on

different days, which display an average PCE of 4.9 (0.2) % (n=3).

Improvements are consistently observed when comparing GO-incorporated devices to those with

no HTL, with almost double the PCE with GO. Analysis of the series (Rse) and shunt (Rsh) resistances for

all devices (Table I) shows that in all cases Rsh is lower for the GO devices suggesting higher charge

carrier losses occurring with GO. These losses can be due to misalignment of the band levels between GO

and PCDTBT:PC70BM, short-circuit pathways or charge / mobility imbalances in the device, inducing

carrier recombination. Interestingly, the Rse of GO inclusive devices, versus PEDOT-PSS, increases with

a BCP ETL, but decreases with TiOx, directly related to the differences observed in the FF and PCE.

Analysis of the device physics and morphology will give an insight to the origin of these different values.

It should be highlighted that the PCEs seen here (η ~5%) for our GO based devices are the

highest reported for this size (0.64 cm2). Comparing with literature, our GO based devices fare well (See

supplemental material)12

. Typically, comparative devices using different active layers produce a similar

relationship between the efficiencies of GO and reference PEDOT:PSS cells. As device efficiencies are

known to decrease with increasing active area,30

and reported efficiencies for other OPV incorporating

GO were for comparatively smaller active areas, our PCEs are a record for their size, and a milestone in

the development of large area OPV.

The external quantum efficiency (EQE) of the GO cell (Figure 2) mirrors the shape of the HTL-

free device with a maximum at approx. 400 nm and a plateau extending from approx. 440 to 545-580 nm.

In comparison, EQE for devices containing PEDOT-PSS show a decrease in EQE to approx. 450 nm for


7

BCP as the ETL, and to about 400 nm for TiOx as the ETL, attributed to the absorption maximum for

PEDOT-PSS, and remain relatively constant for the rest of the wavelength range. Regardless of the ETL

used, EQE is increased for GO devices over those without an HTL, although less enhancement is seen for

devices with TiOx. The difference in device efficiencies between PEDOT-PSS and GO incorporated

devices can partially be explained by the difference in their EQE maxima- with the decrease seen for GO

in the green to red region relative to PEDOT-PSS, equivalent to the maximum absorption range for

PCDTBT:PC70BM. In all cases however, the peak EQE approaches 65-72%, with the wavelength range of

EQE > 50 % lies typically between 345-620 nm.

We have demonstrated that GO is an efficient HTL, producing device performances (PCE ~5%)

for PCDTBT:PC70BM solar cells comparable to PEDOT:PSS over large active areas (0.64 cm2). Based on

data fitting, a trap modified SCLC conduction mechanism, with PF field dependence, typical of charge

hopping can be proposed for both types of devices, indicating that GO is an comparable HTL. Large area,

high efficiency GO-based devices were achieved with both BCP and TiOx ETLs with minimal changes in

the device performance. Given the comparable performance of a GO HTL, allied with its increased

operational stability when compared to PEDOT:PSS,31

it should improve the performance of all solution-

processable, large area PCDTBT:PC70BM solar cells.

ACKNOWLEDGMENTS

(This work was supported by the Defense Science and Technology Laboratory (DSTL), via the DSTL UK

PhD program (CTGS), the Engineering and Physical Sciences Research Council (EPSRC), through

postdoctoral fellowships (to MJB and KDGIJ), and via funds provided through the University of Surrey

Knowledge Transfer Account. The authors would like to thank Mr. Ranga Prabhath and Mr. Keyur

Gandhi, (ATI, University of Surrey) for their assistance.


8

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FIG. 1. (a) SEM (bar = 500 nm) and (b) AFM (25 μm2, z-scale = 20 nm) images of GO deposited on

ITO. Inset in (a) is a SEM (bar = 20 µm) showing the edge of the GO film deposited on ITO. The line

scan (Inset in b), corresponding to the white line in (b), shows the height (α) of a single GO flake, ~1 nm.


11

FIG. 2. J-V plots (illuminated), band diagrams and EQE spectra for ITO/HTL/

PCDTBT:PC70BM/ETL/Al OPV with BCP (a-c) and TiOx (d-f) as the ETL. Data is given for OPV with

no HTL (Black Squares), with PEDOT:PSS (Red Circles) and GO (Blue Triangles). The energy level

diagrams of the materials used are shown with data taken from [8, 23, 24].


12

Table I. Approximate relationship between SCLC current and voltage, and slope (-κ), relative

permittivity (εr) and corresponding barrier height (ɸ) for Schottky†, Fowler Nordheim and Poole Frenkel

‡

conduction mechanisms, calculated from the plots in Figure S2 and S3.

SCLC Schottky† Fowler Nordheim Poole Frenkel

‡

<0.5V >0.5V εr ɸ (eV) Slope (=-κ) ɸ (eV) εr ɸ (eV)

BCP/GO IV IV10

0.0700 0.448 -14.82 0.0054 2.85 0.281

BCP/PP IV IV16

0.0112 0.985 -34.56 2817.3 21.5 0.843

BCP/No HTL IV IV12

0.0245 0.797 -23.51 1.780 8.82 0.622

TiOx/GO IV IV8 0.0716 0.431 -12.43 0.0011 2.65 0.258

TiOx/PP IV IV8 0.0547 0.459 -13.00 0.0016 3.49 0.285

TiOx/No HTL IV IV10

0.0335 0.617 -17.61 0.0349 5.95 0.428

† for data points after the increase in slope at high fields, and above E > 5.50x10

6 Vm

-1 [calculated

when εr=12]

‡ for data points after the increase in slope at high fields, and above E > 1.38x10

6 Vm

-1 [calculated

when εr=12]


13

Table II. Characteristic performance values for ITO/HTL/PCDTBT:PC70BM/ETL/Al OPV.

ETL HTL Voc

(V)

Jsc

(mA.cm-2

)

FF

(%)

PCE

(%)

Rsh

(Ω.cm-2

)

Rse

(Ω.cm-2

)

BCP None 0.45 11.11 51.6 2.6 250.8 5.9

PEDOT:PSS 0.88 11.74 58.7 6.0 342.4 9.3

GO 0.81 11.48 53.7 5.0 245.0 11.8

TiOx None 0.46 11.57 50.9 2.7 280.1 6.0

PEDOT:PSS 0.88 11.25 53.3 5.3 348.6 18.0

GO 0.83 11.04 56.0 5.1 288.5 9.9

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Graphene oxide hole transport layers for large area, high ...epubs.surrey.ac.uk/805887/1/Smith 2014 - Graphene... · Graphene oxide hole transport layers for large area, high efficiency