Improved photovoltaic properties of pentacene/N,N 0 -Dioctyl-3,4,9, 10-perylenedicarboximide-based organic heterojunctions with thermal annealing S. Karak, S.K. Ray, A. Dhar n Department of Physics & Meteorology, IIT Kharagpur, Kharagpur, West Bengal-721302, India article info Article history: Received 18 September 2009 Received in revised form 29 December 2009 Accepted 2 January 2010 Available online 27 January 2010 Keywords: Organic solar cell Pentacene PTCDI-C 8 Annealing temperature abstract We have studied the effect of thermal annealing on the photovoltaic properties of an organic solar cell based on an efficient donor/acceptor combination of pentacene / N,N 0 -Dioctyl-3,4,9,10-perylenedicar- boximide (PTCDI-C 8 ) discrete heterojunctions. As deposited samples showed a maximum power conversion efficiency about 1.12% under standard A.M 1.5 illumination, whereas 1.60% power conversion efficiency has been achieved after annealing the samples at 100 1C for 5 min. External quantum efficiency (EQE) is also found to be increased from60% to  70% after heat treatment. Photocurrent action spectra and the absorption profile of this donor/acceptor combination confirm the efficient light harvesting capability of the device throughout the visible region of the solar spectrum. Morphology and crystallinity of the active layers improved significantly after annealing the samples, resulting in an almost two times enhancement in the short-circuit current density and improved overall cell performances. & 2010 Elsevier B.V. All rights reserved. 1. Introduction In the last few decades, continuously increasing demand of alternative renewable energy sources has stimulated new scien- tific researches in the field of photovoltaic devices. Organic solar cells (OSCs) have attracted much attention as a promising choice for unconventional energy source because of their low-cost potential, light weight, mechanical flexibility and ease of proces- sing [1–5]. In the last ten years, the power conversion efficiency of organic photovoltaic (OPVs) cells has significantly improved due to the use of new materials and precise device architectures. To date, both bilayer and bulk-heterojunction OPVs have achieved power conversion efficiencies up to  5–6% [6–10] and efficiency about 6.5% has been successfully realized in tandem OPV devices [11–12]. In addition to these, one of the major important factors for the successful realization of OPVs in real life application is the long operational lifetimes [13–17]. However the power conver- sion efficiency (Z) and the stability of OSCs are still lower than inorganic or dye-sensitized solar cells. The performance of these solar cells has been limited due to the relatively short-exciton diffusion lengths, low charge carrier mobility and poor morphol- ogy of the active layers. For further improvement of OPVs, the short-circuit current density (J SH ) of the devices has to be optimized accurately. The performance of OPVs has been found to be strongly depending on the microstructures of the active layers. Several researchers have shown that the device perfor- mance can be significantly enhanced by annealing the cells at different temperatures at different conditions [8], [18–21]. The improvement was attributed to the change in active layer crystallinity and morphology. Though thermal treatments have been proved to be very effective for bulk-heterojunction OPVs, surprisingly, similar effects have been rarely investigated for discrete heterojunction OPVs [22]. In this paper we have investigated the effect of annealing temperatures on the photovoltaic properties of pentacene/N,N 0 - Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C 8 )-based organic heterojunctions. We select pentacene and PTCDI-C 8 as donor and acceptor materials because of their high hole ( 41 cm 2 V 1 s 1 for pentacene) and electron (0.6 cm 2 V 1 s 1 for PTCDI-C 8 ) mobility [23–26] and better spectral coverage throughout the visible spectrum [27–28]. The device showed improved perfor- mance after annealing at 100 1C for 5 min. 2. Experimental procedures 2.1. Materials In our study, we have used pentacene (99.9%) as an electron donor material and N,N 0 -Dioctyl-3,4,9,10-perylenedicarboximide ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.01.002 n Corresponding author. Tel.: + 91 3222 283830; fax: + 91 3222 255303. E-mail address: adhar@phy.iitkgp.ernet.in (A. Dhar). Solar Energy Materials & Solar Cells 94 (2010) 836–841