Synthesis and characterization of PbS nanocrystals

in MDMO-PPV semiconducting polymer

for photovoltaic applications

Juan Carlos Ferrer∗, Alfonso Salinas-Castillo†, Jose Luis Alonso∗

Susana Fernandez de Avila∗ and Ricardo Mallavia†

∗Area de Electronica, Universidad Miguel Hernandez,

Av. Universidad, s/n, Ed. Torrepinet, 03202, Elche, Spain

Email: jc.ferrer@umh.es†Instituto de Biologa Molecular y Celular, Universidad Miguel Hernandez,

Av. Universidad, s/n, Ed. Torregaitan, 03202, Elche, Spain

Abstract—We report on a simple method to synthe-size PbS quantum dots directly in poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene] conducting polymer.Optical absorption, photoluminescence and transmission electronmicroscopy measurements have been performed in order tocharacterize the colloidal suspension with different proportionsof PbS nanocrystals. Results show that as the PbS amount rises,the optical density of the compound increases slightly and theluminescence is partially quenched. Microscopy results show thatincreasing the amount of quantum dot precursors is followed by ahigher density of nanocrystals, although the particle size remainsnearly unchanged.

I. INTRODUCTION

Research on composites of organic polymers and

semiconductor nanocrystals, or quantum dots, has evolved

into an important interdisciplinary field of materials science.

Optoelectronic devices and solar cells based on organic

polymers offer promise for fabrication of low-cost, large-area

devices [1], [2]. Likewise, the unusual properties conferred

by quantum confinement [3], such as the process of multiple

exciton generation by impact ionization reported recently

[4] presents a possible route to improve the photovoltaic

conversion efficiency in solar cells. Besides, the rate of

electron relaxation through electron-phonon interactions,

which is a very fast process in bulk semiconductors, could be

significantly reduced because of the discrete character of the

electron-hole spectra [5]. Several reports on the fabrication of

photovoltaic devices relying on quantum dots can be found

in the literature [6], [7].

Quantum dots in colloidal solution are very attractive from

the point of view of the device fabrication processes since

they can be readily incorporated into devices based on organic

polymeric films by the addition of the nanocrystals when the

polymer is still in solution. Besides, the processes employed

for the fabrication of polymer-based devices, such as spin

casting, dip coating, ink jet printing, etc., usually remain

operative for the polymer/nanoparticle blend. Two approaches

are commonly used to synthesize the nanocomposite: first,

methods based on the separate formation of the nanoparticles

followed by the mechanical mixing with the polymer [8], [9].

Second, direct synthesis of the quantum dots in the hosting

polymer [10].

The former approach has several drawbacks: the use of a

surfactant is required for the synthesis of the nanoparticles

which also prevents from crystal aggregation. Direct mixing

of colloid, including the surfactant, and polymer could

result in degradation of the electrical properties of the blend

since the charge transfer between the quantum dots and the

hosting polymer strongly depends on the surface ligands

[9], [11]. Conversely, separation of the surfactant from the

colloidal suspension could lead to particle agglomeration.

Another shortcoming arises from the fact that the mixing of

co-solvents could adversely affect nanocrystal and polymer

solubility.

The direct synthesis of nanocrystals in polymer avoids the

disadvantages explained above. Nevertheless, the nanocrystal

size control becomes a complex issue with this approach

and, although water-soluble isolating polymers have been

used to synthesize and stabilize nanocrystals since the early

1980s [12], this method has been scarcely used in the case of

electroactive polymers.

Several organic semiconductors are commonly used in

photovoltaic cells. The conjugated polymers poly(3-

hexylthiophene (P3HT), poly[2-methoxy-5-(3’,7’-

dimethyloctyloxy)-p-phenylenevinylene] (OC1C10-PPV) and

poly[2-methoxy-5-(2’-ethylhexoxy)-1,4-phenylenevinylene]

(MEH-PPV) contain side chains that make them soluble

in common organic solvents. This allows these polymers

to be cast from solution using wet-processing techniques.

Recently, a new class of polymer, poly[2-methoxy-5-(3’,7’-

dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV)

has been reported to exhibit very good solubility and

electroluminescence performance [13]. However, to the

best of our knowledge, no study about the suitability of

this polymer as a matrix for nanocomposites has been

published. Thus, the aim of this work is the direct synthesis

of quantum dots in (MDMO-PPV) semiconducting polymer.

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PbS semiconductor was selected as nanocrytal material

because it has a broad absorption band, and a large excitonic

radius, which allows to modulate de absorption edge from

3200 nm for the bulk, to 530 nm for very small clusters [14].

II. EXPERIMENTAL

Chemicals were purchased from Sigma-Aldrich and were

used without further purification. The method of systhesis

is similar to that employed by Watt and coworkers [10].

Typically, a solution consisting on 55 mg of MDMO-PPV

semiconducting polymer in 44 ml of toluene and 22 ml of di-

methylsulfoxide (DMSO) was used as nanocrystal stabilizer.

An aliquot of 6 ml of this solution was kept for refer-

ence purposes. A solution of Na2S·9H2O 0.1%wt in DMSO

and Pb(NO3)2 salt were used as sulfur and lead precursors

respectively. 69.2 mg of Pb(NO3)2 were dissolved in the

polymer solution and increasing volumes of the Na2S solution

were added in order to obtain solutions of colloidal PbS

nanocrystals in MDMO-PPV with PbS weight ratios of 2.4%,

5.2%, 8.4% and 12.4% relative to the polymer. Reaction took

place immediately at 110C under magnetic stirring in a round

bottom flask. Attempts to increase the PbS fraction result in a

black solid precipitate.

Optical absorption measurements were performed with a Shi-

madzu UV-1603 spectrometer to obtain the absorption edge.

Photoluminescence (PL) measurements were recorded with a

Photon Technology International fluorimeter using a excitation

wavelength of 370 nm. Transmission electron microscopy

(TEM) images were obtained with a Philips CM-30 micro-

scope operating at 300KV. The samples for TEM analysis were

prepared by deposition of a single drop of the quantum dot

dispersions on a 300 mesh copper grid with a carbon film over

a filter paper which absorbed excess solution. The copper grid

was allowed to dry at room temperature.

III. RESULTS

Optical absorption, photoluminescence and transmission

electron microscopy (TEM) measurements of the samples

were performed in order to characterize their optical and

structural properties.

A. Optical absorption

Absorbance measurements were recorded immediately after

the synthesis of the nanocrystals. An 1 ml aliquot of each

colloidal suspension was dissolved in 10 ml toluene for the

measurements. Toluene was used as reference sample. The

absorbance of the reference (0%), 5.2% and 12.4% samples are

presented in fig. 1. The rest of the samples have been omitted

for the sake of clarity. The spectra have been normalized to

the height of the peak located at 2.51 eV (494 nm) which

corresponds to the maximum absorption of MDMO-PPV.

Although the spectra are very similar, a slight increment of the

absorption can be noticed for higher energies, feature which is

consistent with the presence of PbS quantum dots in colloidal

solution. However, useful information about the absorption

edge of PbS nanoparticles is difficult to obtain. In order to

Fig. 1. Optical absorption of reference solution (0% PbS), and samplescontaining 5.2% and 12.4% Pbs. A slight increment of absorbance is foundfor higher energies as the PbS contents is raised.

Fig. 2. Absorbance of the samples 2.4%, 5.2%, 8.4% and 12.4% aftersubstraction of the reference spectrum. The absorption increases with the PbScontents.

have a better view of the contribution of PbS nanocrystals to

the absorbance, the reference spectrum has been numerically

substracted from the samples which contain PbS nanocrystals.

The result is shown in fig. 2, where it can be observed a higher

absorbance as the PbS content is increased in the solutions. As

far as the absorption edge is concerned, we can roughly judge

that the energy edges are similar for samples with higher PbS

concentrations and far below the bulk energy that for PbS is

0.41 eV, which indicates that strong quantum confinement has

been achieved.

B. Photoluminescence

The concentration of PbS nanocrystals strongly influences

the photoluminescence of the colloidal solution as it is demon-

strated in fig. 3. The reference (0%) sample has an emission

peak located around 2.13 eV (580 nm) which losses intensity

as the PbS concentration increases in the solution. Following

the works of Greenham et al. [9], the partial quenching of

the photoluminescence may be related to the charge transfer

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Fig. 3. Photoluminescence of the whole series. The intensity of the MDMO-PPV polymer becomes reduced by the presence of PbS nanocrystals.

beetween the polymer and the quantum dots, which is essential

for the composite material to be useful for optoelectronic and

photovoltaic applications. Further investigations to elucidate

the type of recombination of the electron-hole pair in the

nanocrystal should be performed: the absence of an emission

peak related to PbS in the measured range could be due

to the emission in the infrared range or to a non-radiative

recombination.

Regarding the peak location, no influence of quantum dot

concentration on the maximum emission wavelength has been

detected.

C. Transmission electron microscopy

TEM allowed to verify the formation of nanometer sized

PbS crystals in the hosting polymer solution. Fig. 4 shows a

representative picture of a nanocrystal ensemble corresponding

to sample 2.4%. The quantum dots tend to develop dendritic

structures, which makes difficult the precise measurement of

the particle size. The mean particle diameter has been roughly

estimated to be 6 nm. The fact that the size is similar for

the samples with higher PbS concentrations suggests that the

addition of further sulfur and lead precursors is followed by

the formation of a higher number of quantum dots instead of

increasing their size.

This type of agglomeration has been previously reported

in nanoparticles hosted in polymers such as PVA [15] and

polyacrylamide [16]. A likely explanation is that several

polymer chains could be bridged by connecting to the same

nanoparticle, as long as neighbor nanoparticles could share a

polymer chain. Thus, a multiplicity of such bridged chains and

particles could lead to particle clustering.

IV. CONCLUSION

The synthesis of PbS quantum dots in MDMO-PPV polymer

using a simple method has been demonstrated. The partial

quenching of the photoluminescence and the slight absorbance

increment as the PbS concentration is augmented suggests that

charge transfer between polymer and quantum dots occurs.

Fig. 4. Transmission electron microscopy of sample 2.4% (a) and 12.4%(b).

Modification of the amount of nanocrystal precursors is fol-

lowed by changes in particle concentration rather than size

variations using this method.

ACKNOWLEDGMENT

This work has been partially supported by grants

UMH-Bancaja 2007, GV/2007/32 (Generalitat Valenciana),

MAT2006-04057 (Ministerio de Educacin y Ciencia), FEDER

and program Juan de la Cierva for A. Salinas-Castillo. TEM

measurements were performed in Serveis Cientificotecnics,

Universitat de Barcelona.

REFERENCES

[1] H. Hoppe and N.S. Sariciftci, “Organic solar cells: an overview,” J.

Mater. Res., vol. 19, pp. 1924-1945, Jul. 2004.

[2] C.J. Brabec, “Organic photovoltaics: technology and market,” Solar

Energy Mater. Solar Cells, vol. 83, pp. 273-292, Jun. 2004.

[3] A.D. Yoffe, “Semiconductor quantum dots and related systems: elec-tronic, optical, luminescence and related properties of low dimensionalsystems,” Adv. Phys., vol. 50, pp. 1-208, Jan. 2001.

[4] R.J. Ellingson, M.C. Beard, J.C. Johnson, P.R. Yu, O.I. Micic, A.J.Nozik, A. Shabaev and A.L. Elfros, “Highly efficient multiple excitongeneration in colloidal PbSe and PbS quantum dots,” Nano Lett., vol. 5,pp. 865-871, May 2005.

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1ΣΠ∆ΦΦΕϑΟΗΤ ΠΓ ΥΙΦ 2009 4ΘΒΟϑΤΙ ∃ΠΟΓΦΣΦΟ∆Φ ΠΟ &ΜΦ∆ΥΣΠΟ %ΦΩϑ∆ΦΤ - ∋ΦΧ 11-13, 2009. 4ΒΟΥϑΒΗΠ ΕΦ ∃ΠΝΘΠΤΥΦΜΒ, 4ΘΒϑΟ.

[5] A.J. Nozik,“Quantum dot solar cells,” Physica E, vol. 14, pp. 115-120,Apr. 2002.

[6] A. Maria, P.W. Cyr, E.J.D. Klem, L. Levina and E.H.Sargent, “Solution-processed infrared photovoltaic devices with > 10% monochromaticinternal quantum efficiency,” Appl. Phys. Lett., vol. 87, pp. 213112-3,Nov. 2005.

[7] K.P. Fritz, S. Guenes, J. Luther, S. Kumar, N.S. Sariciftci andG.D.Scholes, “IV-VI nanocrystal-polymer solar cells,” J. Photochem.

Photobiol. A-Chem., vol. 195, pp.39-46, Mar. 2008.[8] Y. Wang and N. Herron “Semiconductor nanocrystals in carrier-

transporting polymers. Charge generation and charge transport,” J.

Lumin., vol. 70, pp. 48-59, 1996.[9] N.C. Greenham, X.Peng and A.P. Alivisatos, “Charge separation and

transport in conjugated-polymer/semiconductor-nanocrystal compositesstudied by photoluminescence quenching and photoconductivity,” Phys.

Rev. B, vol. 54, pp. 17628-17637, Dec. 1996.[10] A. Watt, H. Rubinsztein-Dunlop and P. Meredith, “Growing semicon-

ductor nanocrytals directly in a conducting polymer,” Mater. Lett., vol.59, pp. 3033-3036, Oct. 2005.

[11] T.W.F. Chang, S. Musikhin, L. Bakueva, L. Levina, M.A. Hines, P.W.Cyr and E.H. Sargent, “Efficient excitation transfer from polymer tonanocrystals,” Appl. Phys. Lett., vol. 84, pp. 4295-4297, May 2004.

[12] R. Rossetti and L. Brus, “Electron-hole recombination emission as aprobe of surface chemistry in aqueous cadmium sulfide colloids,” J.

Phys. Chem., vol. 86, pp. 4470-4472, Nov. 1982.[13] H. Spreitzer, H. Becker, E. Kluge, W. Kreuder, H, Schenk, R. Demandt

and H. Schoo, “Soluble phenyl-substituted PPVs - New materials forhighly efficient polymer LEDs,” Adv. Mater., vol. 10, pp. 1340-1343,Nov. 1998.

[14] Y. Wang, A. Suna, W. Mahler, and R. Kasowski, “PbS in polymers. Frommolecules to bulk solides,” J. Chem. Phys., vol. 87, pp. 7315-7322, Dec.1987.

[15] J.C. Ferrer, A. Salinas-Castillo, J.L. Alonso, S. Fernandez de Avilaand R. Mallavia “Synthesis and characterization of CdS nanocrystalsstabilized in polyvinyl alcohol-sodium polyphosphate (Accepted forpublication) Mater. Lett. DOI: 10.1016/j.matlet.2008.12.007.

[16] M. Mukherjee, A. Datta and D. Chakravorty, “Electrical-resistivity ofnanocrystaline PbS grown in a polymer matrix,” Appl. Phys. Lett., vol.64, pp. 1159-1161, Feb. 1994.

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