A

(doti©

P

K

1

adifefors

mfhe[[

0d

Available online at www.sciencedirect.com

Synthetic Metals 158 (2008) 190–193

Thermal treatment under reverse bias: Effective tool for polymer/fullerenebulk heterojunction solar cells

Yan Li, Yanbing Hou ∗, Yan Wang, Zhihui Feng, Bin Feng, Lifang Qin, Feng TengKey Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology,

Beijing Jiaotong University, Beijing 100044, China

Received 12 October 2007; received in revised form 29 November 2007; accepted 27 December 2007Available online 20 March 2008

bstract

The effect of post-thermal treatment under reverse bias on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-20-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and fullerene (C60) composites is investigated. The experimental results show that treatedevices deliver increased short-circuit current density (Jsc) and conversion efficiency (ηp) values. Especially, for the device annealed under the biasf −6 V, the short-circuit current density (Jsc) and the conversion efficiency (ηp) are enhanced by a factor of 6 and 5.4, respectively. It is concluded

hat the enhancement of charge mobility induced by polymer chains modified orientation along the direction of electric field leads to a significantncrease in the photovoltaic parameters.

2008 Published by Elsevier B.V.

ACS: 72.40.+w; 72.80.Le

aotvmPgafib

tpcId

eywords: A. Polymer; D. Photovoltaic

. Introduction

Organic photovoltaic devices with active layer comprised ofconjugated polymer and fullerene blend are promising candi-ates for solar energy conversion [1]. Recent rapid improvementn bulk heterojunction polymer/fullerene plastic solar cell per-ormance has currently distinguished this technology as anffective alternative to silicon-based solar cells. The mainocus in the polymer photovoltaic (PV) devices research liesn increasing the power conversion efficiency (PCE). As iseported, a promising PCE over 2.5% in polymer/fullerene PVystems have been obtained [2].

Despite the great progress in the development of poly-er/fullerene devices, low charge mobility still challenges the

urther improvement and limits the PCE. Several research groupsave tried to enhance charge mobility to increase PCE by ori-

nting polar molecules [3,4], or by thermal annealing treatment5,6]. A recent breakthrough was achieved by Padinger et al.7], who showed a significant increase in the short-circuit current

∗ Corresponding author. Tel.: +86 10 51688605; fax: +86 10 51688018.E-mail address: ybhou@njtu.edu.cn (Y. Hou).

2

wo

379-6779/$ – see front matter © 2008 Published by Elsevier B.V.oi:10.1016/j.synthmet.2007.12.015

nd an improvement in open-circuit voltage of the devices basedn PCBM:P3HT by a post-production treatment consisting ofhermal annealing and simultaneous application of an externaloltage. It was suggested that the enhancement of charge carrierobility induced by crystallization, and the resulting order of3HT led to better charge carrier transport [5–7]. Recently, ourroup reported that both the electric-field-induced orientationnd post-thermal annealing of the polymer/fullerene compositelm can enhance photovoltaic properties of polymer/fullereneulk heterojunction solar cells [8,9].

In the present work, the influences of thermal treatment inhe presence of a reverse bias (the ITO anode was at negativeotential with respect to metal cathode.) on the photovoltaicharacteristics of MEH-PPV:C60 composites were investigated.t was found that the reverse bias plays an important role inetermining the PV efficiency of the composites.

. Experimental

To prepare MEH-PPV:C60 solutions, MEH-PPV and C60ere dissolved in xylene (the weight of MEH-PPV:the weightf C60 = 4:1). Glass substrates with an indium–tin oxide

Y. Li et al. / Synthetic Metals 158 (2008) 190–193 191

(wew8sfiiPatbdicabib

fe2ni

3

bpaca

TP1

B

0−−−−−−

Fu

sotcP−biistwa

ttwtimproved short-circuit current density (Jsc) and conversion effi-ciency (ηp). Especially, the Jsc of the devices treated at −6 V(E = 6 × 105 V cm−1) is enhanced by a factor of ∼6 and up to

Fig. 1. The structure of devices and thermal treatment bias.

ITO) electrode were sonicated consecutively with distilledater, acetone and ethanol. After blow-drying, poly(3,4-

thylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)as deposited by spin coating at 1000 rpm and then dried at0 ◦C for 10 min and polymerizing at 150 ◦C for 10 min tomooth the substrate surface. Thin MEH-PPV/C60 compositelms were prepared under ambient conditions by spin cast-

ng from the xylene solution of the blended materials onto theEDOT:PSS substrates. The thickness of a photoactive layer wasround 100 nm. The Al electrode was thermally deposited ontohe surface of the MEH-PPV:C60 film inside a vacuum cham-er (10−6 Torr). The active area of the composite photocells,efined by shadow mask, was 2 mm2. Subsequently, the result-ng samples were transferred to a dry N2-filled glove box (theoncentrations of both O2 and H2O were lower than 5 ppm) andnnealed for 10 min at 120 ◦C. During the annealing a reverseias was applied to the devices. The direction of the bias is shownn Fig. 1. Finally, all samples were encapsulated in the glove boxefore measurement.

All devices were illuminated through ITO electrodes, andorward bias was defined as positive voltage applied to the ITOlectrode. Current–voltage curves were recorded by a Keithley410 Source Measure Unit both in the dark and under illumi-ation. The incident light came from a white diode with thentensity of 45 mW/cm2.

. Result and discussion

The devices were treated at 120 ◦C under the different reverseias. Table 1 lists the PV parameters for MEH-PPV:C60 com-

osite devices as a function of reverse bias. The devices treatedt 120 ◦C under various reverse bias have better performanceomparing with those as spin devices. For all the devices treatedt different reverse bias, the device treated at the bias of −6 V

able 1hotovoltaic parameters of MEH-PPV:C60 composite devices annealed at20 ◦C under various reverse bias

ias (V) Voc (V) Jsc (mA/cm2) FF η (%)

0.69 0.25 0.27 0.1042 0.65 0.28 0.27 0.2264 0.625 0.7 0.26 0.2536 0.6 1.5 0.28 0.568 0.55 1.3 0.24 0.38110 0.49 0.68 0.24 0.17812 0.39 0.66 0.25 0.143 F

u

ig. 2. Dark J–V characteristics of devices annealed at T = 120 ◦C for 10 minnder different bias.

hows the best performance. It is implied that −6 V is theptimum reverse bias for the devices. It is also observed thathe larger bias degrades the PV parameters. Fig. 2 shows theurrent–voltage curves of four photodiodes based on MEH-PV:C60 blends under different bias treatment (0, −2, −4 and6 V) in the dark. It is observed that the dark current increased in

oth the reverse and low forward bias regions (0–0.6 V) with thencreasing of treating bias. At those regions, the current is dom-nated by leakage through the device and determined by the celleries resistance (Rs) and shunt resistance (Rp) [10]. Thermalreatment under the presence of a bias may reduce Rs and Rp,hich cause the corresponding dark current in both the reverse

nd low forward bias region to increase.Figs. 3 and 4 compare the light current–voltage curves of

he mentioned above devices based on MEH-PPV:C60 blendsreated under different bias. The devices are illuminated withhite light at an illumination intensity of 45 mW/cm2. It is clear

hat the devices thermally treated under reverse bias exhibit

ig. 3. Light J–V characteristics of devices annealed at T = 120 ◦C for 10 minnder different bias.

192 Y. Li et al. / Synthetic Metals

Fu

1∼

ppttlclbpfianctcbeaopu

TPud

B

0−−−−−−U

bpmGciadMftoe

mcmilcdist[sHtsschnotti

4

ig. 4. Light J–V characteristics of devices annealed at T = 120 ◦C for 10 minnder different biases.

.5 mA/cm2, the corresponding ηp is enhanced by a factor of5.4 and up to 0.56%.The low mobility of the charges inside the photoactive layer of

olymer solar cells is one of the main factors limiting the deviceerformance. According to our previous research, it is knownhat when MEH-PPV:C60 film is treated at a temperature higherhan its glass transition temperature (Tg), an enhanced crystal-ization of the polymer takes place [8]. Due to this enhancedrystallization of polymer, the hole conductivity of the activeayer increases dramatically [11]. During the heat treatmenteyond the glass transition temperature of the polymer, theolymer chains are more mobile. An applied external electriceld greater than the open-circuit voltage of the device injectsdditional charges into the polymer bulk and therefore exter-al electric field leads to a modified orientation of the polymerhains inside the photoactive layer along the direction of elec-ric field [12]. This leads to an enhancement in the mobility ofharge carriers in the polymer matrix as seen in other polymer-ased light emitting diodes, which were subjected to an externallectric field and temperature simultaneously [13]. Interestingly,lthough the external electric field also can lead to a modified

rientation of the undoped MEH-PPV films, the improved PVarameters were not found. Table 2 lists the PV parameters forndoped MEH-PPV devices annealed under different reverse

able 2hotovoltaic parameters of undoped MEH-PPV devices annealed at 120 ◦Cnder various static field reverse bias, compared with the parameters of theevices without applied voltage

ias (V) Voc (V) Jsc (�A/cm2) FF η (%)

1.2 12.6 0.27 0.0092 0.92 6.7 0.25 0.0034 0.63 6.2 0.23 0.0026 0.59 5.3 0.23 0.0018 0.50 1.3 0.19 <0.00110 – – – –12 – – – –

nder −10 and −12 V, the device does not show J–V characteristics.

Pbtifitopib

A

FE

158 (2008) 190–193

ias at 120 ◦C. According to the report by Inigo et al. [14], inristine polymer film, those ordered nanometer-sized domainay act as deep traps which are not good “contact” with theaussian density of the states of the amorphous regions. Thus,

harge mobility may be hindered. However, the doping of C60nto MEH-PPV offers a large contact area for the photogeneratednd dissociated charge carriers. Also, for the device with C60oping concentration of 25%, interpenetrated interfaces betweenEH-PPV and C60 have formed efficient percolation pathways

or charge transportation to the respective electrodes. Therefore,he combination of phase separation and modified orientationf the polymer chains and C60 causes the enhancement of PVfficiency by annealed under static field reverse bias.

Besides, in our previous work, the polarization of the poly-er molecules under high electric filed without heating hardly

hanges the open-circuit (Voc) [9,15]. But thermodynamicolecule arrangement under external electric field creates mod-

fied orientation, which leads to an ordered structure and aengthened conjugated segment [8]. The gradual drop of Vocan provide the evidence for the ordered structure. It has beenemonstrated that the improvement of three-dimensional ordernduces lower oxidation potential [16]. In polymer-fullereneolar cells, the Voc may scale as the polymer oxidation poten-ial which also affects the Coulombic barrier for recombination17]. Therefore, the decline of Voc can indicate the increase oftructural order and reduction of polymer oxidation potential.owever, the key for the enhancement in the Jsc and ηp is not

he simple increasing of the order, but the combination of thistructural order with the percolation pathways. Unlike the chargeeparation at the electrode in the undoped MEH-PPV film, bulkharge generation occurs in the composite devices due to bulketerojunction based on MEH-PPV and C60. Those orderedanometer-sized domains improve the transport of charge carrierver short distance, which may be sufficient for charge separa-ion and transport in the bulk heterojunction film. Therefore, forhe MEH-PPV:C60 composite device, reverse biased annealingnduces a significant enhancement in PV efficiency.

. Conclusions

In summary, the effect of annealing under reverse bias on theV characteristics of MEH-PPV:C60 composite devices haveeen investigated. An applied external electric field greater thanhe open-circuit voltage of the device injects additional chargesnto the polymer bulk and therefore may be external electriceld leads to a modified orientation of the polymer chains inside

he photoactive layer along the direction of electric field. Therdered structure may induced by the modified orientation of theolymer chains combine with the efficient percolation pathwaysncreases the mobility of charge carriers. So the Jsc and ηp cane improved significantly.

cknowledgements

This work is supported by Trans-Century Training Programoundation for the Talents of Natural Science by the Stateducation Commission, Key Project of Chinese Ministry of

etals

Ed1(

R [[[

[[

[15] Q.M. Shi, Y.B. Hou, J. Lu, H. Jin, Y.B. Li, Y. Li, X. Sun, J. Liu, Chem.

Y. Li et al. / Synthetic M

ducation (No. 105041), National Natural Science & Foun-ation Committee of China (NSFC) (project No. 90401006,0434030 and 90301004) and State key project of basic research2003CB314707).

eferences

[1] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992)1474.

[2] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C.Hummelen, Appl. Phys. Lett. 78 (2001) 841.

[3] G.M. Wang, J. Swensen, D. Moses, A.J. Heeger, J. Appl. Phys. 93 (2003)6137.

[4] J.A. DeAro, D. Moses, S.K. Buratto, Appl. Phys. Lett. 75 (1999) 3814.[5] T.J. Savenije, J.E. Kroeze, X.N. Yang, J. Loos, Thin Solid Films 511–512

(2006) 2.[6] P. Vanlaeke, G. Vanhoyland, T. Aernouts, D. Cheyns, C. Deibel, J. Manca,

P. Heremans, J. Poortmans, Thin Solid Films 511–512 (2006) 358.

[

[

158 (2008) 190–193 193

[7] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003)85.

[8] H. Jin, Y.B. Hou, X.G. Meng, Y. Li, Q.M. Shi, F. Teng, Solid State Commun.142 (2007) 181.

[9] H. Jin, Y.B. Hou, Q.M. Shi, X.G. Meng, F. Teng, Solid State Commun. 140(2006) 555.

10] J.H. Yang, A. Garcia, T.-Q. Nguyen, Appl. Phys. Lett. 90 (2007) 103514.11] J.J. Dittmer, E.A. Marseglia, R.H. Friend, Adv. Mater. 12 (2000) 1270.12] G.D. Sharma, V.S. Choudhary, M.S. Roy, Sol. Energy Mater. Sol. Cells 91

(2007) 275–284.13] T. Lee, O.O. Park, Appl. Phys. Lett. 77 (2000) 3334.14] A.R. Inigo, C.C. Chang, W. Fann, J.D. White, Y.-S. Huang, U.-S. Jeng,

H.S. Sheu, K.-Y. Peng, S.-A. Chen, Adv. Mater. 17 (2005) 1835.

Phys. Lett. 425 (2006) 353.16] M. Catellani, C. Arbizzani, M. Mastragostino, A. Zanelli, Synth. Metal 69

(1995) 373.17] C.J. Brabec, Sol. Energy Mater. Sol. Cells 83 (2004) 273.