Solar Energy Materials & Solar Cells 95 (2011) 3114–3118

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

Roles of thermal annealing in cyclopentadithiophene/thienopyrroledione:fullerene blended film and performance of organic photovoltaics

Qiao Zheng a, Guojia Fang a,n, Robert C. Coffin b, Christopher M. MacNeill b, Yuan Li b, Nanhai Sun a,Pingli Qin a, Wanyi Nie b, Eric D. Peterson b, Xi Fan a, Fei Cheng a, Huihui Huang a, Mingjun Wang a,Xingzhong Zhao a, David Carroll b,n

a Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, Department of Electronic Science and Technology, School of Physics and Technology,

Wuhan University, Wuhan, Hubei 430072, PR Chinab Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest University, Winston-Salem, NC 27109, United States

a r t i c l e i n f o

Article history:

Received 29 April 2011

Received in revised form

23 June 2011

Accepted 24 June 2011Available online 23 July 2011

Keywords:

PCPDTTPD: PCBM blended film

High open-circuit voltage

Thermal annealing

Carrier transport

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

016/j.solmat.2011.06.045

esponding authors.

ail addresses: gjfang@whu.edu.cn (G. Fang), c

a b s t r a c t

The authors have attentively investigated the roles of the thermal annealing in the thin blended film of

a novel low band-gap polymer poly[(4,4-bis(2-ethyl)cyclopenta- [2,1-b:3,4-b’]dithiophene)-2,6-diyl-

alt-(5-octylthieo[3,4-b]pyrrole-4,6-dione)-1,3-diyl] (PCPDTTPD) and [6,6]-phenyl C60 butyric acid

methyl ester (PCBM). The surface morphology, optical absorption spectra and carrier mobility of the

PCPDTTPD:PCBM film as a function of annealing temperature were investigated. Open-circuit voltage as

high as 0.86 V was obtained in the PCPDTTPD:PCBM active layer based organic photovoltaics. The post-

deposition annealing effects on the performance of solar cells also have been discussed. The results

illustrated that the maximum power conversion efficiency of 2.01% was achieved in the as-cast devices.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Organic photovoltaics (OPVs) are promising candidates for thefuture low-cost, light-weight and flexible source of renewableelectrical power generation. Power conversion efficiencies (PCEs)as high as 6–8% have been reported for bulk heterojunction (BHJ)polymer solar cells [1–3]. However, improvement in PCEs, stabi-lity and lifetime of the devices are necessary before organic solarcells become commercially valuable [4,5]. Over the past decade,research has focused on regio-regular poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl C60 butyric acid methyl ester (PCBM)as the standard donor and acceptor materials in the polymerBHJ solar cells, respectively [6,7]. Open-circuit voltage (VOC) ofP3HT:PCBM cells is limited around 0.6 V due to the relativelysmall energy difference between the highest occupied molecularorbital (HOMO) of P3HT and the lowest unoccupied molecularorbital (LUMO) of PCBM [8]. With a large band gap of 1.9 eV ofP3HT, this system is still limited by the mismatch of the absorp-tion to the terrestrial solar spectrum [1]. A band gap of 1.3–1.5 eVis regarded as ideal for polymer–fullerene BHJ solar cells [9].Several classes of low band-gap polymers have been developed tobetter harvest the solar spectrum with deeper HOMO energies

ll rights reserved.

arroldl@wfu.edu (D. Carroll).

that can successfully exhibit larger VOC [10,11]. PCEs approaching7% have been reported for thieno [3,4-c]pyrrole-4,6-dione-based copolymer solar cells [12]. High VOC, up to 0.92 V in BHJsolar cells, was attained using a novel low band-gap poly-mer poly[(4,4-bis(2-ethyl)cyclopenta-[2,1-b:3,4-b0]dithiophene)-2,6-diyl-alt-(5-octylthieno[3,4–b] pyrrole-4,6-dione)-1,3-diyl](PCPDTTPD, P) to replace P3HT as the donor material. But PECsof OPVs based on P:PCBM blended layer without additive wereonly around 2% [13]. The low short-circuit current density (JSC)and fill factor (FF) result in the unsatisfied PCEs directly.

P3HT and PCBM are shown to be highly miscible, rapidlyinterdiffusioning within seconds of annealing at 150 1C. The ultimatemorphology is related to PCEs of the device. [14] Kim et al. [15] haveexamined the formation of nanodomains within the matrix inP3HT:PCBM active layer by thermal annealing close to the glasstransition temperature. These domains modify charge transportpathways leading to PCEs exceeding 6%. Annealing of P3HT:PCBMactive layer causes PCBM molecules to diffuse to the surface andchanges the surface properties. Such a surface effectively shields theactive layer from the negative effects when the aluminum electrodesare deposited [16]. To further understand the photovoltaic mechan-ism of the novel narrow band-gap polymer P blended with PCBMBHJ solar cells, the roles of the thermal annealing in P:PCBM filmsand the performance of OPVs need to be investigated.

In this paper, the morphology of polymer P became more regionregular after the P:PCBM blended film was annealed at 200 1C for

Q. Zheng et al. / Solar Energy Materials & Solar Cells 95 (2011) 3114–3118 3115

10 min. PCBM agglomerates could be formed in the as-cast blendedfilm immediately. However, PCBM did not diffuse into agglomeratesto grow up while the heat treatment temperature increased. Theblended film showed a broad optical absorption enhancement in theUV–vis range after 200 1C thermal annealing. Carrier mobility alsoincreased mildly as the array of P molecule became more orderly. Theunbalanced charge transfer between hole and electron turned to belarger after the OPVs were annealed and this might lead to decreasein values of JSC and FF . High VOC of 0.86 V was achieved in the devicewithout heat treatment. The electrical leakage increased with the riseof the annealing temperature, which led to a decline of VOC gradually.Comparing as-cast cells with the annealed ones, the principal para-meters were decreased in the BHJ OPVs with annealing. As a result, amaximum PCE of 2.01% was achieved in the as-cast devices.

Fig. 1. Microscope images of P:PCBM (1:2) from CB-cast blended films:(a) as-cast and

(b), (c), (d) and (e) heat-treated at 60,150, 200 and 300 1C for 10 min, respectively.

2. Experiments

The P:PCBM blended polymer were prepared by dissolving PCBM(Nano-C) in chlorobenzene (CB), and then P was blended into thePCBM solutions (PCBM:P¼ 2:1 in weight). Dichlorobenzene (DCB)also was taken as the solution to compare the solubility of thepolymer and its effect on cell performances. The substrates offluorin–tin oxide (FTO) glass (Rs¼14 O square�1) were thoroughlycleaned in an ultrasonic bath with methanol, acetone successivelyfor 10 min, and dried in a dry nitrogen stream. The solvent-cleanedsubstrates were further cleaned in a UV–ozone cleaner for 30 minunder ambient atmosphere. PEDOT:PSS (Baytron P) was spin coatedon FTO glass substrates at 2000 rpm and then dried at 100 1C for20 min in atmosphere. Subsequently, the blend solution wasdropped onto the PEDOT:PSS layer at 800 rpm. Devices were leftto dry in the nitrogen glove box overnight (at least 12 h). Then,approximately 150 nm of Al was evaporated on at a base vacuumpressure of 10–6 Torr through a shadow mask as top electrode. Theactive electrode area of the cell was 0.03 cm2. Before takingperformance measurements some of the devices were thermallyannealed on a digitally controlled hot plate at 60, 150, 200 and300 1C for 10 min in a dry nitrogen glove box . For measurement ofsingle-carrier mobility, the hole carrier device was constructed witha diode configuration of FTO/PEDOT:PSS/P:PCBM/Au. Electron mobi-lity of the device was measured using electron-only devices fabri-cated with a diode configuration of Al/P:PCBM/Al.

Using a surface profilometer (FTS2-S4C-3D, Taylor Hobson, UK)the thicknesses of the films were checked. Optical microscope imagesof the blend films were measured by a Leica MPS 30 microscope. Filmcrystal structure was characterized by X-ray diffraction (XRD, BrukerAxs, D8Advance) using CuKa radiation at 40 kV and 40 mA. Theabsorption spectra of the P:PCBM layers were recorded using a UV–visible (vis)–near IR (NIR) spectrophotometer in the 300–800 nmwavelength range at room temperature. The current-voltage (I–V)characteristics were measured using a 2400 source meter (Keithley,USA). The solar simulator used was a 1000 W xenon lamp(Newport, USA) in which a light intensity of 100 mW/cm2 wascalibrated by a Si photodiode. From this, the fill factor and thepower conversion efficiency of the device were calculated.

3. Results and discussion

Optical microscopy was used to monitor the surface morphologyof the blended films spin-coated from CB. Microscope images werecollected using a CCD camera. Fig. 1 shows the microscope images ofP:PCBM (1:2) films spin-coated before and after annealing at 60, 150,200 and 300 1C for 10 min. Agglomerations were observed in all thefilms at the micrometer scale. The agglomerations were assigned toPCBM-rich domains that had been reported [17]. This phenomenon

was very different in P3HT:PCBM blended films, where it was knownthat PCBM molecules diffuse to form aggregates or needle-likecrystals eventually during the film annealing [18,19]. Fig. 1a showsthe aggregates presented immediately in the as-cast spin coated film.The colored ‘halos’ around the agglomerations were attributed tolight interference originating from changes in film thickness andwere due to PCBM-depleted regions.[7,19] From the Fig. 1a–c, thesize of PCBM agglomerations was almost unchanged as the heat –treatment temperature increased. These phenomena illustrated thatincreasing annealing temperature did not cause PCBM to diffuse intogrowing PCBM agglomerates, which act as a localized sink. However,the amount of agglomerations decreased acutely when the annealingtemperature was 200 and 300 1C. The melting temperature ofP3HT:PCBM blended films was close to 200 1C [20]. That PCBMmay melt at 200 1C could explain the low density of the PCBMagglomerates in Fig. 1d and e. The hollow circles in Fig. 1e could beregarded as PCBM-depleted regions. Fig. 1d shows that the morphol-ogy of polymer P changed completely after thermal annealing at200 1C for 10 min. The fibrils matrix was observed clearly at themicrometer scale. However, the polymer P turned back to amor-phous morphology after heat treatment at 300 1C. From the inset ofFig. 1e, it was found that the P:PCBM film was not very uniform butsegmentation into local area network was formed.

Identical information was also revealed in the detailed X-raydiffraction (XRD) measurements of thin P:PCBM films spin-coatedfrom CB after heat treatment at different temperatures for 10 min. Asa common view, the region-regular polythiophene self-organizes intoa crystalline structure owing to the p–p stacking direction. The peakposition corresponding to the p–p stacking was absent in Fig. 2,which illustrated that the polymer P didnot crystallize after thermalannealing. Fig. 2 also shows the changes of the relative XRDintensities at the specific position (2y¼5.71), which correspond tointermolecular distance between polymer chains [13]. The peakintensity of polymer P increased gradually as the full width at the

Fig. 2. XRD results of P:PCBM from CB-cast films without annealing and heat

treatment with different temperatures.

Fig. 3. UV–visible absorption spectra of the P:PCBM from CB-cast active layers

without annealing and annealing at various temperatures.

Fig. 4. Dark JD–V characteristics of (a) electron single-carrier and (b) hole single-

carrier device without annealing and annealing at different temperatures. The

inset shows J1/2–V characteristics for SCLC single-carrier mobility.

Q. Zheng et al. / Solar Energy Materials & Solar Cells 95 (2011) 3114–31183116

half maximum of the peak decreased when the heat temperatureincreased from 150 to 200 1C, suggesting that more ordered regions ofpolymer P were achieved. There was no peak present in the filmsannealed at 60 or 300 1C for 10 min, which indicated the polymerfilms were amorphous.

The UV–vis–NIR absorption spectra of the blended P:PCBM aspristine and annealed films spin-coated from CB with varyingtemperature are shown in Fig. 3. The blended film showed a broadabsorption enhancement in the UV–vis range after thermal annealingat 200 1C for 10 min. Two essential factors leading to optical absorp-tion increase. One is a change in the state of the polymer fromamorphous to crystalline, and the other one is the interactionbetween the polymer molecules becoming stronger. [21] The opticalmicrographic images are shown in Fig. 1d. The results of XRDmeasurements in 200 1C heat-treated film illustrated that the absorp-tion enhancement due to the polymer P underwent a change of statefrom amorphous to regular, as a result of the interaction betweenintermolecular P becoming stronger. It is also worth noting that theportion of the PCBM absorption spectra around 330 nm faded awaywhen the film was annealed at 300 1C, which meant the PCBM hadbeen melted. Such situations are consistent with that of Fig. 1e.

The charge transport from the dark current in a single-carrierdevice was measured. The single-carrier mobility can be calculatedfrom the dark current density–voltage (JD–V) curve by the trap-freespace charge limited current (SCLC) model using the Theott–Gurneysquare law: J¼ 9=8ðere0mhðeÞÞV

2=L3, where e0 is the permittivity offree space, er is the dielectric constant (e0 is ca. 3 for polymer and ca.3.9 for PCBM) of the polymer, mh(e) is the zero-field mobility of holes(electrons), V is the effective applied voltage and L is the thickness ofthe active layer. [22] Fig. 4 shows that single-carrier mobilityperformed on the completed device without annealing and annealedat different temperatures. For clarity, only the devices as-cast andpost-deposition thermal annealing at 60, 150 and 200 1C are shown.The values of the carrier mobilities are listed in Table 1. mh and me

increased mildly after heat treatment at 60 and 150 1C compared tothose of devices without annealing. mh and me was increased from1.33�10–6, 3.00�10–6 to 3.70�10–6, 2.5�10–5 cm2 V�1 s�1,respectively. However, mh and me decreased sharply after the deviceswere annealed at 200 1C. This degradation might be because theamount of PCBM aggregates reduced considerably in the 200 1Cheat-treated blended film (see the insert of Fig. 1d).

After the investigation of the morphology, optical absorption andcharge transport characteristics upon thermal annealing of P:PCBMfilms, we proceeded with the study in the role of the post-depositionannealing in the solar cell based on P:PCBM layer. Fig. 5 shows the I–V

Table 1Summary of the parameters of OPVs based on the blended P:PCBM layer without and with annealing at different temperatures.

Temperature (1C) JSC (mA/cm2) VOC (V) FF (%) PCEs (%) mh (cm2 V�1 s�1 me (cm2 V-1 s�1)

not annealed 5.97 0.86 0.39 2.01 1.33�10–6 3.00�10–6

60 3.42 0.84 0.35 1.00 2.37�10–6 8.33�10–6

150 4.28 0.74 0.38 1.22 3.70�10–6 2.50�10–5

200 4.08 0.61 0.34 0.85 1.48�10–7 9.26�10–7

Fig. 5. Current density–voltage performances under illumination for (a) CB-cast and

(b) DCB-cast device without annealing and annealing at different temperatures.

Fig. 6. (a) Energy level diagrams of the device components referenced to the

vacuum level (b) Dark J–V characteristics of the devices without annealing and

annealing at different temperatures.

Q. Zheng et al. / Solar Energy Materials & Solar Cells 95 (2011) 3114–3118 3117

performance of the OPVs derived from different solvents withoutannealing and annealed at different temperatures under illumination.PCEs of the CB-cast device were found to be higher than that ofDCB-cast device when the devices were fabricated under the sameconditions. These phenomena could be attributed to the solubility ofthe active layer in CB being better than that in DCB. The graininessstuff was visible to the naked eye in the DCB-cast film. The principalparameters of the OPVs also are listed in Table 1. The best perfor-mance was obtained in the device without annealing. The value of JSC,VOC and FF was 5.97 mA/cm2, 0.86 V and 39%, respectively, resultingin the maximum PCEs of 2.01%. Compared to these parameters of theas-cast cells, the values of the parameters in the annealed devicedecreased. The efficiency of photocurrent generation depends on thebalance between charge carrier generation, recombination and trans-port. [23] The difference in electron and hole mobility is too large,

resulting in external photocurrent densities strongly limited by thebuildup of space charge [24]. It has generally been accepted that FF isaffected by the morphology of the active layer, the balance betweenhole and electron mobilities, and the interface of layers in OPVs. [22]Considering the above principles, the balance between electron andhole mobilities of the blended film was focused in this study. Theratio of electron-to-hole mobility (me /mh) of the as-cast device was2.3, while me /mh was increased to 6.8 and 6.3 after thermal annealingat 150 and 200 1C, respectively. The unbalanced charge transferbetween electron and hole mobilities increased after thermal anneal-ing. The mobility mismatch between electron and hole transportbecame larger and thus the space charge effect became large. Thevalues of JSC and FF of the cells decreased as a result of theenhancement and unbalanced charge conduction in the thermalannealed P:PCBM layer.

The cell structure employs P as the donor and PCBM as theacceptor material, with the relevant energy level was shown inFig. 6a. The P HOMO lies at –5.2 eV, [13] while the PCBM LUMO liesat –3.9 eV. According to the general understanding of BHJ OPVs of VOC

directly determines the energy level offset between the HOMO ofdonor and the LUMO of the acceptor if both electrodes contact the

Q. Zheng et al. / Solar Energy Materials & Solar Cells 95 (2011) 3114–31183118

photoactive layer ohmically [25]. Thus, the VOC for this type ofP:PCBM OPVs without annealing was as high as 0.86 V. However,VOC of the devices decreased as the annealing temperature increased.The J–V characteristics of OPVs in dark are shown in Fig. 6b. Theelectrical leakage of the annealed devices was increased system-atically, in the order of 60, 150 and 200 1C, compared with that of as-cast devices. The electrical leakage for the devices is a significantfactor that contributes to the VOC decline. [26] As a result, VOC reducedfrom 0.86 to 0.61 V gradually.

4. Conclusions

We have analyzed the roles of thermal annealing in P:PCBMblended films. The polymer P became more region regular afterheat treatment, which led to improvement of optical absorptionand carrier mobility. However the imbalance between the elec-tron and hole mobilities and the electrical leakage was alsoenhanced by the post-deposition annealing, which resulted inthe degradation of the performance of the P:PCBM based BHJOPVs. As a result, the maximum power conversion efficiency of2.01% was achieved in the as-cast devices. These results arevaluable for the understanding of the photoelectrical propertiesof a novel low band-gap polymer and further improving theperformance of OPVs.

Acknowledgments

This work was supported by the National High TechnologyResearch and Development Program of China (2009AA03Z219),the National Natural Science Foundation of China (11074194), theNatural Science Foundation of Jiangsu Province (BK2009143) andthe National Basic Research Program (no. 2011CB933300)of China.

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