ARTICLE

Compositional dependence of the performance of bulk hetrojunctionsolar cells based on PTOPT and PCBMNewayemedhin Abera and Genene Tessema

Abstract: The compositional dependence of the performance of the blends of [3-(4-octylphenol)-2,2=-bithiophene] (PTOPT) and6,6-phenyl-C61-butric acid methyl ester (PCBM) sandwiched between ITO/PEDOT:PSS and Al was studied. The observed darkcurrent–voltage curves showed that the current (J) is space charge limited except at low voltages (V). The best power conversionefficiency (�) and short circuit current (JSC) were found at 72% PCBM loading. Moreover, we have observed significant reductionon the fill factor with increasing PCBM concentration due to high recombination of charge carriers. The impedances across theelectrodes were discussed based on low frequency impedance analyzer measurements.

PACS Nos.: 73.61.ph, 73.61.Wp.

Résumé : Nous avons étudié la dépendance selon la composition de la performance des mélanges de [3-(4-octylphenol)-2,2=-bithiophene] (PTOPT) et -PCMB ((6,6) phényl-C61-butanoate de méthyle), stratifié entre ITO/PEDOT:PSS et Al. Les courbes carac-téristiques de courant–voltage dans le noir montrent que le courant est limité par les charges d'espace a bas voltage. Noustrouvons que les meilleure efficacité de conversion de puissance (�) et de courant en court-circuit (JSC) sont observés avec 72% dePCBM. Nous avons observé une réduction significative du facteur de remplissage avec l'augmentation de la concentration enPCBM, causée par un haut taux de recombinaison des porteurs de charge. Nous analysons les impédances entre les électrodes, enutilisant un analyseur d'impédance a basse fréquence. [Traduit par la Rédaction]

1. IntroductionThe search for renewable source of energy has been the focus of

immense scientific research, in the last few decades, with the viewto avert possible global energy crises. In the face of dwindlingfossil fuel reserves at present there is an urgent need to diversifyenergy source options. Solar energy conversion by way of conven-tional silicon-based solar cells is one of the solutions, however, thecost of the solar panel is still high and unaffordable for manyaround the globe. Recently, conducting polymers have emergedas a possible substitute to silicon in photovoltaic (PV) cell re-search. The ease of processability, light weight, flexibility, and lowcost of production are some of the attractive features of thesepolymers.

Although conducting polymers offer a wide range of applica-tion potential in photonic devices, there are still several unre-solved problems in the area of charge transport, powerconversion efficiency, stability, etc. [1–7]. Low band gap polymershave received special attention at present because of their highphoton-induced charge generation in the visible and infrared re-gions as well as better charge carrier mobilities. The most widelyused device architecture, in the preparation of the active layer oforganic PV cells, is the bulk hetrojunction (BHJ) structure. The BHJphotoactive layer is composed from the blend of electron-donating polymer and electron acceptor (fullerene). The photo-conductivity of this layer is the result of ultrafast electron transferfrom the donor to the fullerene. However, there are numerousfactors affecting the performance of BHJ solar cells; among othersthe stoichiometric ratio of the donor polymer to the fullereneblend plays a significant role in the efficiency of the devices. Themonomer group of the polymer is another determining factoraffecting the charge transport and stability of the polymer me-

dium. The polythiophenes group (e.g., P3HT, PTOPT), for example,showbetter photo stability than PPV's polymers (e.g., MDMO-PPV).That is why polythiophenes take comparative advantages overothers to be used as an active layer of organic PV cells. This articlediscusses the compositional dependence of the organic PV cells,whose active layers are composed of PTOPT and PCBM at variouscompositions. The transport properties of the charge carriers insingle-layer devices are also presented.

2. ExperimentalThe devices under test were prepared on ITO-coated glass sub-

strate in a sandwich-type structure. To tune the work function ofITO with PTOPT a thin layer of hole donating polymer PEDOT:PSSwas spin coated at 3000 rpm which corresponds to an approxi-mately 11 nm film thickness. Following the PEDOT:PSS the activelayer was spin coated from chloroform-based solution containingPTOPT and PCBM blend. The solution was prepared at a concen-tration of 5 mg/ml.

Finally, a lower work function electrode Al was deposited in avacuum at �2.5 × 10–5 mbar using Edward Auto 306 vacuum de-position. The current density – voltage characteristics were mea-sured with computer interfaced HP4140B pico-ammeter with a DCvoltage source both under dark and illumination conditions. Thelight illuminationwas carried out using a solar simulator workingat air mass 1.5 AM. Furthermore, the impedance spectroscopy wasmeasured using a computer interfaced HP4192A low frequencyimpedance analyzer. The data was taken at a frequency rangebetween 1 kHz and 1 MHz with a step of 5 kHz, and the sampleswere scanned from –2 to 2 V bias voltages, which oscillate sinu-soidally with Vrms = 10 mV.

Received 11 August 2012. Accepted 30 October 2012.

N. Abera. Department of Physics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia.G. Tessema. School of Chemisty & Physics, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01, Scottsville 3209, South Africa.

Corresponding author: Genene Tessema (e-mail: mola@ukzn.ac.za).

89

Can. J. Phys. 91: 89–92 (2013) dx.doi.org/10.1139/cjp-2012-0340 Published at www.nrcresearchpress.com/cjp on 1 November 2012.

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3. Results and discussion

3.1. Absorption spectraThe optical absorption of PTOPT was measured for different

composition of PCBM as shown in Fig. 2. The band gap of PTOPTwas estimated from the onset of absorption and found to be Eg =hc/� = 1.98 eV, which favorably places the polymer in the semicon-ductor category.

The absorption spectrum of PCBM, in the visible range, is verypoor, which is why PCBM alone cannot be used as an active layerof a PV cell. In fact, the addition of PCBM into the PTOPT solutionhas significantly reduced the absorbance peak of PTOPT while atthe same time enhanced the absorbance of PCBM at various com-positions (see Fig. 2). The role of the fullerene in the blend ismainly to serve as an electron acceptor and electron transportingmedium. There are two absorbance peaks at 335 and 520 nm inthe mixture, which are associated with the presence of PCBM andPTOPT molecules, respectively. Therefore, the variations of theabsorbance peaks in the blend are due to the superposition ofabsorption lines from both molecules. However, increasing theconcentration of PCBMwill further decrease the absorption inten-sity of the blend as well as generation of excitons. Moreover, theabsorption spectrum is another indication for the fact that theexcitons are indeed generated by the absorption of photons inthe PTOPT rather than in the PCBM.

4. Compositional dependence of thecurrent–voltage characteristics

Figure 3 shows the J–V characteristics of the PV cell produced inthe current experiment under illumination. It shows clearly thatthe performance of the cells heavily depends on the composi-tional stoichiometric ratio of PCBM and PTOPT in the active layer.In this section, we discuss the effect of PCBM content on themajorparameters of the cells.

4.1. Open Circuit VoltageThe maximum open circuit voltage (VOC) obtained in this study

was from pristine PTOPT-based PV cells.However, the value of the (VOC) drastically decreases at 50%

PCBM, and eventually starts to recover some of its loses up until72% PCBM concentration. Ideally, the open circuit voltage of thedevice can be estimated by the net electric field in the cell whenthe resultant current is zero. Therefore, based on the energy banddiagramof the polymers and thework functions of the electrodes,it is possible to estimate the expected value of the open circuitvoltage. For instance, for pure PTOPT active layer (0% PCBMcontent) Voc = e(�PEDOT:PSS/ITO − �Al) = 0.9 V, where e is an elec-tron charge. When the active layer is composed of PTOPT andPCBM blend, the expected value becomes Voc = e(LUMOPCBM −HOMOPTOPT) = 1.4 V. The experimental value obtained for 0%PCBM content is in agreement with the prediction. However, theexperimental values of the Voc are well below the prediction forthose devices whose active layers are composed of PTOPT and

PCBM blend. For example, the open circuit voltages for 50% and72% PCBM loading found were Voc = 0.59 and 0.69 V, respectively.This is due to the reduction potential of the fullerene and themorphology of the layer. This is in agreement with the previousreport on MEH-PPV:PCBM [5]. The overall fluctuation of the opencircuit voltage in PCBM-loaded samples was small, which has nosignificant influence on the power conversion efficiency com-pared to the current at various PCBM compositions.

4.2. Short Circuit Current (Jsc)The short circuit current (Jsc) increases rapidly with PCBM com-

position. This is due to the fact that the dissociation efficiency hasbeen enhanced with higher PCBM concentration in the activelayer. The high PCBM concentration increases the donor–acceptorinterfaces, which results in high dissociation probability of thephoto-generated exciton. This does notmean that the dissociationprobability always increases with high levels of PCBM concentra-tion, instead, it must be noted that the current output from thedevices decreases significantly for PCBM loading in excess of 72%.

Fig. 1. Chemical structure of PTOPT. Fig. 2. The optical absorption spectra of PTOPT and PTOPT:PCBMblend at 1:2.

300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

0.6

Abs

orba

nce

(abs

.uni

t)

Wavvelenght (nm)

PTOPT:PCBM PTOPT

Fig. 3. Compositional dependance of the J–V characteristics ofpolymer based solar cell under illumination.

-0.2 0.0 0.2 0.4 0.6 0.8

-0.8

-0.6

-0.4

-0.2

0.0

FF=0.20

Voltage(V)

J(m

A/c

m2 )

72% 50% PCBM

PCBM

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This is due to the formation of a large separate domain of PCBMembedded in the homogeneous PTOPT:PCBM photoactive layer.The existence of such domains could hinder the charge transportalong the percolation pathways resulting in low current yield. Thecell parameters' dependence on PCBM concentration are given inTable 1.

The low power conversion efficiency in Table 1 is attributed tothe low polymer concentration in the solution used for the prep-aration of the devices and the inherent photoconductive proper-ties of the PTOPT.

5. Charge transport propertiesThe current density – voltage (JD–V) characteristics under dark

conditions allows us to directly determine the most importantparameters of the devices as well as the investigation of the trans-port properties of the charge carriers in themedium [5–14]. All themeasured JD–V curves are asymmetric around the origin, whichshows the non-ohmic nature of the current in the devices. Therectification ratios (n) found from the deviceswere n = 1220 for 50%PCBM loading and 10.25 × 103 for pure PTOPT at ±2 V. These valuesindicate that the active layers can be considered good candidatesfor solar cell application given that other cell parameters are fa-vorable. The log–log plots of the dark current for each composi-tion are space charge limited at high field regions. The spacecharge limited currents are found to increase with increasingPCBM concentration, and are strongly dependent on the appliedelectric field. The SCL current in organic PV cells can often bedescribed by the well-knownMott–Gurney law [9–14]. The lawwasdesigned under the assumptions that there is ohmic injectionbetween the electrode and semiconductor contacts as well as atrap-free medium. It also assumes a constant charge mobility inthemedium. The current density (J) under these conditions can beexpressed by

J �9

8��0�

V2

L3(1)

where �0 and � are the relative dielectric permittivity of free spaceand the polymer medium respectively. In most charge transportstudies in polymers the value � is often taken to be between 3 and5. The quantity � is the mobility of the charge in the medium,which is often expressed as � = �0 exp(��E).

The parameters L and V are the thickness of the active layer andthe voltage drop across the sample, respectively. In the currentexperiment, single-layer devices were fabricated from PTOPT andPCBM to study the transport of charge carriers in the mediums.Those single-layer devices based on PTOPT are whole only whilethose PCBM-based are electron-only devices. The measured J–Vdata show that the space charge limited current in PCBM is largerthan in PTOPT, which suggests that the electronmobility in PCBMis considerably higher than the hole mobility in PTOPT. The holeand electron mobilities reported from both PTOPT and PCBM are�h = 2.23 × 10–10 and 2.0 × 10–7 m2/Vs, respectively [5, 11]. In thisstudy, we found four orders of magnitude difference on the mo-bilities of the hole and the electron in the same materials. This

however could be due to the low concentrations of the solutionsused in the preparation of the devices. By blending 50% PCBM inPTOPT the device still behaved as hole-dominated and its mobilityimproved by one order of magnitude compared to the purePTOPT. The measured space charge currents are well described by(1) as depicted in Fig. 4.

We employed impedance spectroscopy to measure the equiva-lent resistance of each device under testing. It is often convenientto discuss the impedance of devices in terms of the equivalentcircuit of a solar cell given in Fig. 5. The diode (D) is the voltage-dependent resistor that accounts for the asymmetry of the J–Vcurves. Shunt resistance (Rsh) is due to the recombination ofcharge carriers and the leakage current in the device due to traps.The series resistance (Rs) is simply the sum of all the series resis-tances in the device.

This series resistance is responsible for the conductivity of thematerial, which is mainly determined by the mobility of chargecarriers in the device. Finding the values of the shunt and seriesresistances gives us information about the shape of the J–V curve.

At nearly zero bias voltage (V ≈ 0), the diode is not conductingand the flow of current is largely controlled by (Rs + Rsh). Becausethe shunt resistance ismuch higher than the series resistance, it isonly Rsh that determines the J–V characteristics. Hence, the shuntresistance is given by Rsh � (J/V)–1 for V ≈ 0.

In the case of high voltages (V > Voc), the diode becomes moreconducting than the shunt resistance. Then, the series resistancedominates the shape of the J–V curve. Hence, the value of thisseries resistance is given by Rs � (J/V)–1 for V > Voc. Based on thepreceding analysis we have calculated the values of Rsh and Rs forsome of the devices studied in this work. The calculated values arein good agreement with the impedance measurements on thesame samples. For example, the shunt resistance of the devicewhose active layer contains 50% PCBM is found to agree very wellwith the prediction. Moreover, the values of the series resistanceis almost similar to the values shown in Table 2.

For a large fill factor a high shunt resistance is needed to pre-vent the leakage current in the device. The shunt resistance de-creased with increasing PCBM concentration by a factor of around380. Consequently, even if the device with higher PCBM concen-tration generates a larger photocurrent much of it is lost as aleakage current due to a very low shunt resistance. This was thereason for the reduction of the fill factor with high PCBM loading.The series resistance decreased with increasing PCBM concentra-tion. This is evident from the fact that the series resistance isresponsible for the conductivity of the device. Therefore, for largefill factor it is required to have a very high shunt resistance toprevent the leakage current as well as a low series resistance sothat all the generated charge carriers can contribute to the con-ductivity of the device.

6. ConclusionWe have studied the performance of a BHJ solar cell based on

PTOPT and PCBM for various compositions. The optical absorp-tionmeasurement suggest that PTOPT has a good semiconductingproperties which can be utilized as an active layer of organic PVcells. The dark J–V characteristics of PTOPT:PCBM based BHJ solarcells show that the space charge limited current is enhanced withincreasing PCBM content due to large electron mobility in thePCBM. The electronmobility in PCBM is found 10 000 times higherthan the hole mobility in pristine PTOPT. The most efficient PVcell found in this study was the blend of PTOPT and PCBM at 72%PCBM loading, which results in 16 timesmore efficiency than purePTOPT. The fill factor decreased monotonically with increasingPCBM due to the decrease in shunt resistance at high PCBM con-tent. However, further works need to be done to decrease recom-bination in the blend for a better efficiency.

Table 1. Compositional dependance ofshort circuit current, open circuit volt-age, Fill factor and efficiency of PTOPTbased PV cells.

PCBM (%) Jsc (m) Voc (V) FF � (%)

0 0.02 0.89 0.20 0.00425 0.07 0.76 0.20 0.01150 0.45 0.55 0.17 0.04372 0.75 0.65 0.13 0.064

Abera and Tessema 91

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AcknowledgmentsThe authors are grateful to the International Program in the

Physical Sciences (IPPS) of Sweden for the financial support theyreceived for this work, and W. Mammo for providing one of thepolymers.

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Fig. 4. Transport properties of the space charge limited currents.

1.0 1.1 1.2 1.3 1.4 1.5-5

-4

-3

-2

-1

0

1

2

3

1.0 1.1 1.2 1.3 1.4 1.5

50%PCBM

ln

J D(A

/m2 )

Volts

PTOPT

Fig. 5. Equivalent circuit diagram of a PV cell diode.

IL

Id

Ish I

D

Rs

RshRLV

Table 2. Compositional dependance Rsh and Rs in organic PVcells.

PCBM (%)

Prediction (�mm–2) Experiment (�mm–2)

Rsh Rs Rsh Rs

0 3.8×106 2.3×105 2.8×106 4.1×105

25 9.5×105 9.1×104 8.7×105 9.6×104

50 2.8×105 1.6×104 4.0×105 3.8×104

72 1×104 830×103 8.2×104 2.1×103

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