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DOI: 10.1007/s00339-003-2496-7

Appl. Phys. A 79, 2125 (2004)

Materials Science & Processing

Applied Physics A

v. dyakonov Mechanisms controlling the efficiencyof polymer solar cells

Energy and Semiconductor Research Laboratory, Institute of Physics,University of Oldenburg, Carl-von-Ossietzky Str. 911, 26129 Oldenburg, Germany

Received: 4 August 2003/Accepted: 9 December 2003

Published online: 5 March 2004 Springer-Verlag 2004

ABSTRACT To improve the efficiency of polymer solar cells, itis vital to understand which mechanisms control the currentvoltage characteristics of a given device. Temperature and light

intensity dependence of the main solar cell parameters are veryinformative for analyzing losses. We report on the currentvoltage characteristics and the external photogeneration quan-tum yield of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al aswell as of ITO/PEDOT:PSS/P3HT:PCBM/Al devices investi-gated in the broad temperature range 120325 K under variableillumination, between 0.02 and 100 mW/cm2. We discuss therecombination on traps and the low mobility of charge carrierscaused by poor morphology of active layers as possible mech-anisms limiting the efficiency of these devices.

PACS 73.50.P; 73.61.P; 72.80.R

1 Introduction

In recent years, the development of thin film plas-tic solar cells, using polymer-fullerene bulk heterojunctionsas an absorber, has made significant progress [1]. Efficienciesbetween 2.5% and 3.5% for laboratory cells under AM1.5 il-lumination conditions have been reported [24]. The typicalstructure of these devices consists of a composite of two ma-terials with donor and acceptor properties, respectively, sand-wiched between two electrodes having different work func-tions. The most investigated combination to date is the conju-gated polymer poly (2-methoxy-5-(3-,7-dimethyl-octyloxy)-1,4-phenylene vinylene (OC1C10-PPV) and the soluble ful-

lerene derivative [6,6]-phenyl-C61-butyric acid methyl ester(PCBM; a methanofullerene). One advantage of this type ofdevices is the easy processability of the absorber layer aswell as of the optional interface layer(s). They all are solutionprocessed by using spin-casting or doctor blade techniques.A significant breakthroughhas been achieved by realizing thatthe morphology of the composites plays an important role forthe device performance. In the case of OC1C10-PPV:PCBMsolar cells, the improved morphology has been achieved byusing chlorobenzene as solvent [2]. However, a direct trans-

u Fax: +49-441/7983-326, E-mail: [email protected] work is supported by European Commission (Project HPRN-CT-2000-00127) and BMBF (Project 01SF0026)

fer of this finding to other systems is not straightforward;therefore, a morphology optimization, in which the choice ofsolvent is only one parameter, should be performed for anycombination of materials.

The internal quantum yield of photogenerated charge car-

riers in these composites is expected to be as high as 100%in contrast to a very low photogeneration yield in pure con-jugated polymers. This is due to ultrafast photoinduced elec-tron transfer between donor and acceptor [5]. The chargetransfer occurs within 4050fs [6], and the separated stateis metastable [7]. Therefore, a much higher development po-tential can be anticipated taking the high quantum yield ofcharge carrier generation in these composites into account.In spite of significant advances in the understanding of thequalitative behavior of polymer solar cells, a fully quantita-tive description of charge injection at the electrodes, chargetransport in the bulk under solar cell operation conditions, andelectron-hole recombination has yet to be done. To optimizethe efficiency, the electronic processes limiting the deviceperformance should be identified. Studies of current densityvoltage (JV) characteristics of the polymer solar cells asa function of the temperature and the illumination intensity aswell as of the external quantum yield may provide valuableinformation on the charge transport properties, the injectionmechanisms, and the presence of defects both in the bulk andat the interface.

2 Experimental

We studied the current densityvoltage ( JV) be-havior of the ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al.

For this, a 1 : 4 (weight) blend of poly [2-methoxy-5-(3, 7-dimethyl-octyloxy-)1,4-phenylene-vinylene] (OC1C10-PPV) [8], as an electron donor, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), as the electron acceptingmoiety, was prepared in chlorobenzene. A 100 nm activelayer of OC1C10-PPV:PCBM was spin cast from solutionunder inert atmosphere in the glove-box. As a back elec-trode, we used aluminum, which was thermally evaporatedin a high vacuum. As a hole extracting electrode, the 80nmlayer of poly-[ethylene dioxy-thiophene] doped with poly-styrene sulfonate (PEDOT: PSS), (BAYTRON P, Bayer AG,Germany) was spin coated onto the substrate. Another set ofdevices consisted of poly(3-hexylthiophene-2,5-diyl (P3HT)purchased at Rieke Met. Inc. in the absorber blend. The active


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22 Applied Physics A Materials Science & Processing

layer was made by casting a 1 : 2 toluene-chloroform solutionof regioregular P3HT and PCBM onto dried PEDOT:PSS.The P3HT based devices were annealed in a manner de-scribed in [4]. The novel fullerene acceptor, the diphenyl-methanofullerene, DPM-12, was used instead of PCBM inblends with OC1C10-PPV and P3HT.

Monochromatic external quantum efficiency (EQE) spec-

tra were measured using a home made lock-in-based set-upequipped with a calibrated Si/Ge photodiode, as reference.The light from the halogen and Xenon lamps was spectrallydispersed using a monochromator with a set of gratings tocover the range of wavelength from 250 to 1000 nm. Thecurrentvoltage profiling was automatically carried out witha source-meter unit (Avantest TR-6143) in a variable tem-perature cryostat equipped with a liquid nitrogen supply anda temperature controller (Lakeshore 330). The devices wereilluminated through a sapphire window by white light froma Xenon arc lampwhose intensityis controlled bya set of neu-tral density filters with transmission coefficients from 104

to 1. The maximum intensity was calibrated to 100 mW/cm2

inside the cryostat.

3 Results and discussion

3.1 Polyphenylenevinylene-based solar cells

3.1.1 Currentvoltage characteristics. Figure 1 shows thedark current densityvoltage characteristic of a ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Aldevice at room temperature ina semi-logarithmic representation. A good diode rectificationbehavior can be seen. The JV curve can be separated intothree regions: i) at reverse bias and at forward bias up to thevoltages of approximately of0.7 V, thecharacteristic is nearlysymmetrical and is governed by a leakage current throughthe parallel resistance; ii) at bias corresponding to flat-bandconditions, the injection starts and the exponential region canbe distinguished; iii) the saturation due to the series resistance

FIGURE 1 Dark JV for a ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Aldiode. T= 293 K. The solid line represents the fit with the Shockley equa-

tion (Eq. (1)) with the ideality factor n = 2. The inset shows the same cellunder white light illumination P = 100 mW/cm2. The solid line representsthe expected JV behavior under illumination with the dark fit parame-ters. The deviations in the fill factor and the open circuit voltage are mostly

pronounced

is observed at higher bias. The dark JV can be fitted withShockley equation with ideality factor n = 2:

J= J0

exp

q(V JRS)

nkBT

1

+

V JRS

RP, (1)

where RS, RP denote series and parallel resistances, respec-tively, V the applied voltage, n the ideality factor, kB the Boltz-manns constant, T the absolute temperature, J0 the saturationcurrent

J0 = NV NCkBT exp

Eg

kBT

1

L NA, (2)

where NV, NC are the effective densities of states in valenceand conduction bands, respectively, is mobility, Eg the bandgap, L the mean-free path, NA the acceptor density. In a firstapproximation, we treat the absorber composite as a semi-conductor with an effective band gap Eg = 1.11.2 eV (seediscussion below).

The situation changes significantly, when the device is il-

luminated. Deviations of the fit (solid line) from experimentaldata (open squares) in the fourth quadrant are clearly seen, asshown in the inset to Fig. 1. The illuminated p-n solar cell isdescribed by the following equation:

J= J0

exp

q(V JRS)

nkBT

1

JL , (3)

where JL is photogenerated current density, J= JL JSC atV= 0 V, J0 is saturation current density. The fit curve wasgenerated with the parameters derived from the dark JV an-alysis and JL = JSC.

The illuminateddevice has a lowerfill factor (FF) than wasexpected. The FF depends of both R

Sand R

Pin a complex

way. Both of them may vary with the light intensity. We foundthat most dramatic is thevariation ofRP with light intensity, asshown in Fig. 2. The RP, which is in the order of100 kOhmcm2 at a light intensity of0.02mW/cm2, decreases by nearlythree orders of magnitude at 100 mW/cm2. In contrast, theRS (not shown) varies very little from 1.2 Ohm cm

2 at

FIGURE 2 Parallel resistance RP as a function of light intensity in loglog

scale at temperatures between 120 K and 325 K


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DYAKONOV Mechanisms controlling the efficiency of polymer solar cells 23

ILight = 0.02mW/cm2 to 1.01 Ohm cm2 at 100 mW/cm2,

both at T= 325 K. This presents evidence that the efficiencyof this type of solar cell is limited by a light dependent shuntresistance.

3.1.2 Incident light intensity dependence. The charge carriersin the solar cells under investigation are efficiently photo-

generated via ultrafast electron transfer between donor andacceptor counterparts of the composite, usually having a verylow charge carrier concentration in the dark. The fate ofthe photogenerated electrons and holes is crucial for thedevice efficiency; therefore, the influence of light intensityon the JV characteristics is important. The illuminationintensity was varied from 0.7 mW/cm2 to 100 mW/cm2.Figure 3 shows the short-circuit current density JSC ofan ITO/PEDOT:PSS/OC1C10-PPV:PCBM/ Al device asa function of incident light intensity at several temperaturesfrom 120 K to 325 K. The short-circuit current density in-creases with the illumination intensity ILight. The behavior is

close to linear, but not exactly a linear, i.e., JSC I0.92Light at

T=

325 K. The power exponent decreases further at lowertemperatures ( JSC I0.85Light at T= 120 K). The nearly lin-

ear current densitylight intensity relationship indicates thatcharge carrier losses in the absorberbulk are dominated by themonomolecular recombination on defects or impurities. Thisimplies that some part of charge carriers is trapped on defectsand further recombines with the charges of the opposite sign.The bimolecular eh recombination is not significant even atthe highest illumination intensity. This is in agreement withthe picture of photogeneration and transport of charge carri-ers in a donor-acceptor bulk heterojunction: an ultrafast ehseparation is followed by the transport of negatively and pos-itively charged carriers within spatially separated networks:electrons along percolated fullerenes travel towards the Alcontact, holes within the polymer matrix travel towards thePEDOT:PSS contact. As both contacts may be consideredohmic for the respective charge carriers (i.e., the potentialbarriers are below 0.5 eV), no space charge limited condi-tions under short circuit conditions, even at highest generationrates, exist. A deviation from the linear current densitylight

FIGURE 3 Short circuit current density as a function of light intensity fordifferent temperatures. The scaling exponents (slopes) between 0.85 and 0.92

are obtained for T= 120 K and T= 325 K, respectively

intensity dependence at low temperatures is attributed to anincreased bimolecular recombination at lower mobility.

3.1.3 Temperature dependence. The dependence of the de-vice characteristics on temperature in the range from 120 Kto 325 K was measured. Figure 4 shows the short-circuit cur-

rent density as a function of temperature. JSC grows for allillumination intensities used. Such a strong temperature de-pendence of the photocurrent is not typical for inorganic solarcells. In these devices, the main contribution to a positivetemperature coefficient of the JSC is due to thermally ex-cited intrinsic charge carriers along with the narrowing ofthe semiconductors band gap with increasing temperature,resulting in a red shift in the optical absorption. The strongtemperature effect we observed is not understood in detail todate, but definitely due to electronic transport properties oforganic absorber materials. Charge carrier transport in dis-ordered conjugated polymers and methanofullerenes is thehopping type and, therefore, thermally assisted [9, 10]. (The

dotted lines in Fig. 4 illustrate a thermally activated behav-ior.) Furthermore, it is negatively influenced by the captureof charge carriers by traps. In both cases, one expects that anincrease in temperature will promote the current through thedevice. A detailed discussion on the quantitative analysis ofthe temperature-dependent short-circuit current density canbe found elsewhere [11, 12].

The origin of the open-circuit voltage in bulk-heterojunc-tion solar cells is a controversial issue. It was found to cor-relate directly with the acceptor strength (LUMO), i.e., it isdetermined by the HOMOLUMO energy gap of donor andacceptor, respectively, being nearly independent of electrodematerials in the investigated range of work functions [13].If both electrodes form Ohmic contacts, the HOMOLUMOgap indeed gives an upper limit for the VOC [17]. In a metal-insulator-metal (MIM) picture [16], the VOC is limited by thework function difference between electrodes, i.e., is equal tothe built-in voltage. In [18, 19], the VOC is expected to be de-termined by the electrochemical potential gradient across theabsorber layer. An increase of the open-circuit voltage from

FIGURE 4 Short circuit current density JSC as a function of temperature at

light intensities indicated in the legend


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24 Applied Physics A Materials Science & Processing

850 mV to 940 mV is observed when cooling down the de-vice from room temperature to 100 K, as described in [14, 15].Qualitatively, such behavior is indeed expected, and is typicalfor inorganic solar cells [20]. The temperature dependence ofVOC can be easilyderived fromthe Shockleyequationdescrib-ing the illuminated solar cell:

VOC =kT

q ln Jph

J0+1

Eg

q nkBT

q lnNV NCkBT

JSC L NA

.

(4)

The overall behavior expected is such that VOC will in-crease at lower temperatures and, in the limit T 0 K, willreach the value EG/q. In our heterojunctions, Eg is not the en-ergy gap of the donor or acceptor materials in the blend, butrather the energy gap between the LUMO of the acceptor (thefullerene derivative) and the HOMO of the donor (conjugatedpolymer). The blend is treated as a semiconductor with an ef-fective energy gap, which is approximately 1.11.2 eV. Thismeans that the open-circuit voltage of 0.9 eV is close to thethermodynamic limit given by absorber materials redox po-

tentials. In contrast, the difference in the work function of theelectrode materials, expected to be in the range of 0.70.8 eVfor PEDOT:PSS and Al, would give somewhat smaller opencircuit voltage.

3.2 Polythiophene-based solar cells

The poly(3-hexylthiophene-2,5-diyl) (P3HT) rep-resents at the moment one of the most promising donor mate-rials for polymer photovoltaics. Compared to OC1C10-PPV,following features are advantageous: i) red-shifted optical ab-sorption; ii) higher charge carriers mobility; iii) thermal sta-bility; iv) large scale commercial availability.

3.2.1 Currentvoltage characteristics. Figure 5 shows thecurrent densityvoltage characteristic for a ITO/PEDOT:PSS/P3HT:PCBM/Al device under white light illuminationat room temperature in a linear representation. Two featuresare to be mentioned. The JSC of8.2 mA/cm

2 represents a rea-sonably high value for the short-circuit current. Note: noadditional interface layer were deposited between absorberand the top contact. In contrast, the open-circuit voltage of0.55 V is much lower than in OC1C10-PPV devices. Thisvalue fits neither the HOMOLUMOgap picture, nor the con-tact potential difference of electrodes, as a limit for VOC inbulk-heterojunction solar cells. This issue needs to be fur-ther investigated. The device shown in Fig. 5 has the powerconversion efficiency of2.2%.

3.2.2 External quantum efficiency. Efficient charge carriergeneration with a quantum efficiency close to unity is ex-pected in our devices. Figure 6 shows spectra of the exter-nal quantum efficiency (EQE) in ITO/PEDOT:PSS/P3HT:PCBM/Al device: as fabricated (open squares) and ther-mally treated after deposition of the Al electrode (closedsquares). The EQE spectrum of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al device is shown as reference (open circles).This figure demonstrates an important feature of the P3HT-based solar cells, namely, the broadening of the optical ab-sorption towards low energy part of the visible spectrum with

FIGURE 5 JV for a ITO/PEDOT:PSS/P3HT:PCBM/Al diode in dark(closed squares) and under white light illumination (100 mW/cm2) (opensquares). The cell parameters are: JSC = 8.2 mA/cm

2, VOC = 0.55 V,FF = 50%, = 2.24

FIGURE 6 External quantum efficiency spectra for the ITO/PEDOT:PSS/

P3HT:PCBM/Al device: as fabricated (open squares) and annealed for 3 minat 353 K) (closed squares). The EQE spectrum for a ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al diode is shown for comparison (open circles).T= 293 K

the better match to the solar AM1.5 spectrum. The influenceof the annealing on the polythiophene based light-emittingdiodes and solar cells was reported recently in [4, 21, 22].However, the detailed mechanisms behind the treatment is notclear. Modification of the morphology improving the mobil-ity of charge carriers, and/or the improvement of injectionproperties of polymer/Al contact are under discussion. Thebroadening of the photocurrent action spectrum is noticeablein the heat-treated devices only, and is due to the change ofpolymer absorption.

3.2.3 Increasing the open circuit-voltage. Alternative accep-tor materials are possible candidates to improve the opticalabsorption and/or the open-circuit voltage of the polymer-fullerene solar cells. (Note: the photoinduced hole transfertakes place in blends, too, but its contribution to the photocur-rent is weak due to poor absorption of PCBM in the visiblepart of spectrum.)


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DYAKONOV Mechanisms controlling the efficiency of polymer solar cells 25

FIGURE 7 Open circuit voltage VOC for a ITO/PEDOT:PSS/OC1C10-PPV:DPM-12/Al diode as a function of temperature under different illumi-nation intensities (white light) as indicated in the legend in units of mW/cm2

We studied a novel fullerene acceptor, DPM-12, in blendswith both OC1C10-PPV and P3HT. Though DPM-12 has theidentical redox potential as PCBM, surprisingly high open-circuit voltages in the range VOC = 0.94 V at room tempera-ture and nearly 1 V at T= 220 K were measured for OC1C10-PPV:DPM-12 based devices. Figure 7 shows the temperaturedependence of the open-circuit voltage VOC for a OC1C10-PPV:DPM -based diode as a function of temperature underdifferent illumination intensities (white light). The VOC is wellabove the difference in the work functions of the electrodes al-ready at room temperature. However, the efficiency of thesedevices is limited by a low photocurrent and high series resis-tance, which arises from poor absorber morphology and lowelectron mobility within the fullerene subnetwork.

A straightforward suggestion is to blend the DPM-12 withP3HT to benefit from the better electrical transport proper-ties of the latter. Significantly higher JSC (4.4 mA/cm

2) wasobtained for annealed ITO/PEDOT:PSS/P3HT:DPM-12/Aldevices [23]. However, the open circuit voltage was found tobe much lower, being in the range of0.6 V, i.e., similar to de-vices based on thePCBM acceptor. The P3HT:DPM-12 baseddevices are, therefore, controlled by the holetransport throughthe polymer network.

4 Conclusions

We have examined the electrical transport prop-erties of novel polymer-fullerene solar cells by means ofcurrentvoltage characteristics in the temperature range120325 K and the monochromatic external quantum ef-ficiency technique. The short circuit current density inOC1C10-PPV based devices grows monotonically with tem-perature until 320K. This is indicative for a thermally acti-vated transport of photogenerated charge carriers, influencedby recombination on shallow traps. We present evidence thatthe efficiency of this type of solar cell is limited by a lightdependent shunt resistance. Hence, the electronic transportproperties of the absorber materials, e.g., low effective charge

carrier mobility with a strong temperature dependence, limitthe photogenerated current due to a high series resistance.Therefore, the active layer thickness must be kept low, whichresults in low absorption for this particular composite ab-sorber. High monochromatic external quantum yield in therange of 60% (at absorption maximum) was measured in allinvestigated devices. These values were not corrected for the

transmission and reflexion and are, therefore, in accord withexpected internal quantum efficiency close to 100%. Ther-mal post-treatment significantly improves the parameters ofP3HT-based cells; however, the microscopic mechanisms forthis remain unclear. The overall solar cell power efficienciesof the devices investigated in this work were in the range of1.8% and 2.2% under white light illumination.

ACKNOWLEDGEMENTS Author thanks I. Ridel, E. von Hauff,

D. Chirvase, M. Pientka, Z. Chiguvare, V. Mertens and J. Parisi (U. of Old-

enburg, Germany), C.J. Brabec, P. Schilinsky, Ch. Waldauf (Siemens AG,

Germany), J.C. Hummelen (U. of Groningen, The Netherlands), D. Van-

derzande, L. Lutsen (L.U. C, Diepenbeek, Belgium), N. Martin, F. Giacalone

(UCM, Madrid, Spain) for stimulating ideas, material supply, experimental

support and fruitful discussions.

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