Solar Energy Materials & Solar Cells 95 (2011) 2314–2317

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Bulk and contact resistance in P3HT:PCBM heterojunction solar cells

Yang Shen, Kejia Li, Nabanita Majumdar, Joe C. Campbell, Mool C. Gupta n

Department of Electrical and Computer Engineering, University of Virginia, 351 McCormick Road, Charlottesville, VA 22904, USA

a r t i c l e i n f o

Article history:

Received 9 December 2010

Received in revised form

28 March 2011

Accepted 30 March 2011Available online 6 May 2011

Keywords:

Organic solar cells

P3HT

PCBM

Series resistance

Contact resistance

Thermal annealing

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.03.046

esponding author. Tel.: þ1 757 325 6850; fax

ail address: mgupta@virginia.edu (M.C. Gupta

a b s t r a c t

In this paper, the series resistance of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl

C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells (OSC) has been

studied. The series resistance of thermal annealed and un-annealed devices with different active layer

thicknesses was measured. The series resistance of the organic solar cells consists of the bulk resistance

of the active layer itself and the specific contact resistance between the active layer and the electrode.

The bulk resistance and contact resistance were extracted from the measured series resistance using

the vertical transmission line model (TLM) method. By fabricating solar cell devices with different

active layer thicknesses, a relationship of the series resistance with thickness was established from

which bulk and contact resistances were derived. We have also found that thermal annealing helps

reduce both contact resistance and bulk resistance significantly; the contact resistance dropped by a

factor of 2, while the bulk resistance decreased by a factor of 8. Results have shown that for an annealed

P3HT:PCBM device that has an active layer thickness of 85 nm (optimum thickness for high efficiency),

17% of the total series resistance was due to the contact resistance, and bulk resistance contributed the

rest 83%. The bulk resistance value for thermal annealed organic solar cell device with an active area

of 0.1 cm2 was found to be 150 O, and the measured specific contact resistance was 3.1 O cm2.

The measured bulk and contact resistance values are much higher as compared to the high efficiency

silicon solar cells. Bulk resistance and contact resistance need to be further decreased in order to

achieve higher organic solar cell efficiency.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The organic photovoltaics have been receiving increasingattention over the last few decades. Comparing with conventionalphotovoltaic materials such as Si, organic polymers offer manypractical advantages due to the solution processing techniquesthat offer low fabrication cost and the potential to deposit onflexible substrates [1]. Organic polymers also have relativelyhigh absorption coefficients in the visible spectrum (usually4105 cm�1), yielding high absorption in devices thinner than0.1 mm [2–4]. However, the promise of organic solar cells islimited by the low efficiencies that have been reported. To date,the maximum demonstrated efficiency of an organic BHJ solar cellis only �8.3%, as reported by Konarka Technologies, Inc. [5]. Onereason is that most of the organic polymers have an optical bandgap near 2 eV, e.g. 1.9 eV [6] for P3HT, which is much higher thanthat of Si (1.1 eV). This limits the ability of organics to harvest thefull solar spectrum. Compared to inorganic materials, the lowercarrier mobility and shorter lifetime of organic polymers [7–10]result in lower efficiency.

ll rights reserved.

: þ1 757 325 6988.

).

Series resistance is another important parameter for solar cells. Itis well known that a high series resistance reduces the fill factor,thus reducing cell efficiency [11–13]. A typical multi-crystallinesilicon solar cell has a series resistances less than 1 O cm2 [14],while a P3HT PCBM organic solar cell has a series resistance�20 O cm2 [15], and the loss due to I2R is �30% of the total outputpower. In order to reduce series resistances to enhance efficiency,the series resistance of organic solar cells needs to be studied andthe origin of series resistances needs to be better understood.Rakhshani [16] has developed a method of fitting dark I–V char-acteristics of CdTe/CdS solar cells for series resistance. The TLMmodel is a well-established method to characterize contact resis-tance in semiconductor devices [17,18]. By varying the active layerthickness, Aernouts et al. [19] have extracted the bulk and contactresistances from series resistance on MDMO-PPV:PCBM solar cellsunder illuminated (AM 1.5) conditions. It has been shown thatthermal annealing can be used to reduce series resistance, which canlead to the enhanced performance of organic solar cells [20–22].

In this paper, we study the bulk and contact resistances forboth annealed and un-annealed organic solar cell devices underthe dark condition. The results show that bulk resistance con-tributes 83% to the total series resistance and that thermalannealing helps reduce bulk resistance more than it helps reducecontact resistance.

Fig. 1. Structure of P3HT:PCBM solar cell. Fig. 2. Equivalent circuit model of solar cells.

Fig. 3. I� I(dV/dI) characteristic of an organic solar cell.

Y. Shen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2314–2317 2315

2. Experiments

P3HT:PCBM solar cells (Fig. 1) were prepared according to thefollowing procedure: P3HT and PCBM were dissolved in chlor-obenzene for 24 h before fabrication. The blend solution was1 wt% in total, composed of a ratio of 45:55 of P3HT (provided bySigma-Aldrich) to PCBM (provided by Nano-C). The indium tinoxide (ITO)-coated glass substrate (provided by Delta Technolo-gies) was first cleaned with acetone and isopropyl alcohol andsubsequently dried by air. Highly conducting PEDOT:PSS(poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) pro-vided by H.C. Starck was spin-cast (4000 rpm) from aqueoussolution. The substrate was baked for 15 min at 110 1C in air.The blend solution was then spin-cast at 500 rpm on PEDOT layer.Then the sample was baked in a vacuum oven at 60 1C for 30 min.An 80 nm Al layer was then deposited as the top electrode usingelectron beam vapor deposition. Finally, the sample was annealedat 130 1C for 2 min in air. We also used epoxy resin (provided byDELO Industrial Adhesives) to encapsulate the organic solar cellsto enhance their stability.

In order to study the series resistance, we measured thecurrent –voltage (I–V) characteristics using a Keithley 2611source meter. Series resistances were then extracted from thedark I–V measurements. The methodology of extracting seriesresistance from dark I–V measurements will be discussed later.

To extract the contact resistance and bulk resistance fromseries resistance, we fabricated devices with different active layerthicknesses using different spin speeds. The contact resistanceand bulk resistance were then obtained by linear fitting to thedata of series resistances versus active layer thickness.

To study the effect of annealing on contact resistance and bulkresistance, we measured the series resistance for both the un-annealed and the annealed devices. Current–voltage characteris-tics were measured for the un-annealed devices. The un-annealeddevices were then annealed at 130 1C for 2 min. Current–voltagecharacteristics were then measured for these annealed devices.The purpose of using the same devices for both the un-annealedand the annealed conditions was to eliminate other variables for amore precise study of the annealing effect on contact resistanceand bulk resistance.

3. Results and discussion

3.1. Extraction of series resistance from I–V measurements under dark

Typically, the current–voltage characteristics of solar cells canbe described by

I¼ IL�I0 eqðV�IRsÞ=nkT�1h i

�V�IRs

Rshð1Þ

The corresponding circuit model is shown in Fig. 2. In dark, thephotocurrent is zero and we assume that shunt resistance is

sufficiently high to eliminate the last term in Eq. (1). Therefore,the dark I�V characteristics can be expressed as

I¼ I0½eqðV�IRs=nkTÞ�1� ð2Þ

The voltage can be expressed as a function of current as

V ¼nkT

qln

I

I0þ1

� �þ IRs ð3Þ

From this expression the current derivative of voltage can bewritten as

V ¼nkT

q

1

ðI=I0Þþ1

1

I0þRs ð4Þ

In the high current region, the total current is much higherthan the reverse saturation current (Ib I0). Then, Eq. (4) becomes

IdV

dI¼ RsIþ

nkT

qð5Þ

Under these assumptions I(dV/dI) is linearly dependent on I.Therefore, the series resistance can be easily extracted from theslope by a linear fit in the high current region. A typical darkI� I(dV/dI) characteristic is shown in Fig. 3.

3.2. Extraction of contact and bulk resistances from the series

resistance

The series resistance between two contacts can be expressedby the following equation:

Rs ¼ RbulkþRcontact ¼ rbulk � dþRcontact ð6Þ

Fig. 4. Rs of P3HT:PCBM organic solar cells with different active layer thicknesses

of 45, 58, 68, 85 and 100 nm; the measurements were conducted in dark.

Fig. 5. Linear fitting of Rs versus active layer thickness (45, 58 and 68 nm).

Y. Shen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2314–23172316

where Rbulk (O cm2) is the bulk resistance of the active layer itself,Rcontact (O cm2) is the specific contact resistance between theactive layer and the electrode, rbulk (O cm) is the resistivity ofthe active layer and d is the thickness of the active layer. From theabove equation, we can see that by fabricating devices withdifferent active layer thicknesses, a relationship of the seriesresitance with thickness can be obtained and the contact resis-tance can be derived.

Samples with different active layer thicknesses were fabri-cated, each sample had 8 individul devices. The thickness of theactive layer was measured by an atomic force mircoscope (AFM).

From Fig. 4, we can see that the Rs values decreased afterannealing for all active layer thicknesses. The thicker the activelayer, the larger the reduction. For both the un-annealed and theannealed devices, Rs increases rapidly for thickness 480 nm. Thiscan be explained by the diffusion length of P3HT:PCBM solar cells.When the active layer is thicker than the typical diffusion lengthof carrier in P3HT:PCBM solar cells, the probability of recombina-tion will increase rapidly. This effect will be manifested as lesscurrent in the devices, which will result in higher Rs.

To extract the contact and bulk resistances from the totalseries resistance, devices with active layer thinner than 80 nmwere chosen. The results are shown in Fig. 5.

For the annealed devices, the specific contact resistance wascalculated as 3.1 O cm2 and for the un-annealed devices, thespecific contact resistance was 6.6 O cm2; this shows that anneal-ing helps decrease the contact resistance but the magnitude of thechange is only a factor of 2. For an annealed P3HT:PCBM device,which has an active layer thickness of 85 nm chosen for thehighest efficiency, the average series resistance is 18 O cm2.Therefore, contact resistance only contributes �17% to the totalseries resistance while bulk resistance is the primary componentof series resistance. As the series resistance is 126 O cm2 for un-annealed 85 nm thick devices, by subtracting the contact resis-tance, we find that annealing causes the bulk part of the seriesresistance to drop from 120 to 15 O cm2, which is a factor of 8, amuch larger relative change than that observed for the contactresistance.

3.3. Discussion

The effect of thermal annealing on P3HT:PCBM solar cells is acomplex process. Both bulk and contact resistances decreased dueto thermal annealing. As to bulk resistance, the decrease can belinked to the improvement of nanoscale morphology, which ismainly due to the reordering of P3HT molecular chains [23–25].This is due to increased crystallinity of P3HT, which will help inimproving the charge carrier mobility of the P3HT [26], thusproviding lower bulk resistance. Thermal annealing has also beenshown to decrease contact resistance for organic solar cells.Ma et al. [23] have reported that solar cell device results obtainedby annealing prior to top electrode deposition shown a cleardecrease in efficiency comparing to the device results obtained byannealing after the deposition of top electrode. Using the ‘‘stickytape’’ technique, their AFM image showed a rougher surface of theinterface of bulk material and Al electrode, which implies stron-ger interfacial adhesion, and stronger adhesion provided a lowercontact resistance. Studies have also shown that the formation ofmetal–polymer bonds (C–Al or C–O–Al bonds) [27,28] and Aldiffusion [29] could also contribute to the decrease in contactresistance due to thermal annealing.

The current–voltage characteristics of organic solar cells aregoverned by two basic processes: carrier injection in the bulkmaterial by the electrodes and charge transport in the bulkmaterial [30,31]. In general, the injection process itself includestwo processes: one is the Schottky emission process due tothermionic emission and the other is the Poole–Frenkel emissiondue to field-enhanced thermal excitation of trapped charges andcharge tunneling (Fowle–Nordheim tunneling) from the metalcontact into the HOMO and LUMO levels of the polymer. Carriersinjected by the electrode have to overcome or tunnel through thebarrier of the Schottky junctions formed at metal-organic inter-face. In our case, these barriers are formed at the PEDOT–P3HTinterface and the PCBM–Al interface, which are all small barriers(less than 0.3 eV) and when operating at room temperature, theI–V characteristics will not be dominated. This is consistent withour reported low contact resistance values compared to seriesresistance value.

Once the carriers are injected, their transport through thepolymer layer toward the opposite electrode is determined by theconduction properties of the material itself. The transport in bulkis mainly explained using Ohmic conduction at low voltages dueto thermally excited carriers hopping from one isolated state tothe next and the space-charge-limited current (SCLC) due tocarriers injected into the polymer from the contacts [32]. In theP3HT:PCBM solar cell, P3HT and PCBM form the percolated paths,and the charge carrier transport through these paths. The holesare supposed to flow through P3HT channels and electronsthrough the PCBM channels [33]. The transport of carriers in

Y. Shen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2314–2317 2317

these separated channels is limited by their own mobility and thetotal current will be limited by the one with lower mobilityvalues, which is P3HT in our case. Generally the charge carriermobility in the organic polymers is very low. Low charge carriermobility is related to structural disorder. In fact, as with inorganicamorphous semiconductors, conduction in such relatively disor-dered materials is mostly a function of ‘‘mobility gaps’’ [34] withphonon-assisted hopping, polaron-assisted tunneling, etc.between localized states. However, the interaction betweenP3HT and PCBM molecules cannot be simply neglected. Theelectrons and holes recombination is very likely to happen duringthe separated transport process. If the bulk material were toothick, most carriers will end up recombining before they getto the other electrode. As shown in the devices with active layerthicker than 80 nm, the total resistance went up exponentially,which is due to the recombination effect.

4. Conclusion

We have extracted the contact resistance and bulk resistancefrom series resistance measurements by fabricating devices withdifferent active layer thicknesses. Compared with the bulk resis-tance, the contact resistance is relatively smaller but still notnegligible. We then studied the effect of thermal annealing.Thermal annealing reduces the series resistance, especially fordevices with an active layer thickness greater than 80 nm. This isbecause the diffusion length of carriers in P3HT:PCBM solar cellsis around 80 nm. For devices thicker than 80 nm, recombinationcan be expected to increase rapidly, which will lead to a rapidincrease in series resistance. Thermal annealing of the devicesreduces both contact resistance and bulk resistance: the contactresistance dropped by a factor of 2, while the bulk resistance by afactor of 8. This factor depends on the active layer thickness.

Acknowledgment

We would like to thank the NASA Langley Professor Program,NSF I/UCRC program, the University of Virginia Energy Initiative,for the financial support. Li, Majumdar and Campbell weresupported as part of the Center for Energy Nanoscience, an EnergyFrontier Research Center funded by the U.S. Department ofEnergy, Office of Science, Office of Basic Energy Sciences underAward number DE-SC0001013.

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