Effects of annealing and degradation on regioregular polythiophene-based bulk heterojunction organic photovoltaic devices Soheil Ebadian a , Bobak Gholamkhass b , Shabnam Shambayati a , Steven Holdcroft b , Peyman Servati a,n a Department of Electrical and Computer Engineering, University of British Columbia, 2332 Main Mall, Vancouver, BC, Canada V6Z 1T4 b Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6 article info Article history: Received 29 March 2010 Received in revised form 22 June 2010 Accepted 22 July 2010 Keywords: Solar cells Organic photovoltaic devices P3HT:PCBM Regioregularity Hole mobility Space charge limited current abstract This work investigates the effect of annealing and aging on the performance of devices made from blends of poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C 61 -butyric acid methyl ester (PCBM) with different regioregularity (RR) for P3HT (98% and 94%). We compare different parameters of fabricated photovoltaic devices, including power conversion efficiency, extracted charge mobilites, external quantum efficiency and optical density as well as current–voltage characteristics, before and after annealing. We find that devices based on higher RR P3HT initially have a better performance but are more prone to degradation due to tendency for micor-phase segregation of donor and acceptor domains, as confirmed by electron and optical microscopy. The lower RR devices, on the other hand, demonstrate a minor increase in hole mobility and PCE in post annealed conditions, since annealing facilitates better p –p stacking when there is a lower tendency for segregation. It was found that the hole mobility is almost unchanged by RR and annealing time, and its minor changes cannot explain the main cause of the performance degradation. The electron mobility, however, decreases with the annealing time for both RR values. The mobility drop for the 98% RR devices was greater than that of the 94% RR ones. This is attributed to extensive phase segregation in blends of 98% RR P3HT and PCBM resulting in discontinuity of the electron transporting phase, i.e, PCBM. As such, the lower RR devices are found to be more stable during a four month degradation experiment, while the efficiency of higher RR devices decreases in the same period of time. These results show that electron transport plays a critical role in the degradation of effective charge mobility of P3HT:PCBM solar cells, and that the selection of RR of the polymer is critical in determining not only the initial performance and efficiency of an organic photovoltaic device but also its durability. & 2010 Published by Elsevier B.V. 1. Introduction The advent of conjugated polymers [1] has led to the emergence of organic photovoltaic (OPV) devices, with the potential for low cost roll-to-roll manufacturing and direct integration of solar energy devices onto unconventional substrates such as plastic, paper and fabric [2,3]. The introduction of more stable conjugated polymers and better device structures in the past decade has resulted in a significant improvement in power conversion efficiency (PCE) of OPV devices from 0.1% [4,5] to 5.5% [6,7] and most recently to a record 7.4% for a low band gap polymer and PCBM-C 71 [8]. The use of donor and acceptor constituents and fabrication of bi-layer heterojunction devices [6–9] instead of single layer devices [4] played an important role in improving the PCE of OPV devices. Unlike conventional inorganic photovoltaic devices, blends of a donor polymer and acceptor molecule form a bulk composite with nanoscale heterojunction interfaces that facilitate exciton dissociation over the entire volume of the film. The exciton dissociation is limited to a thin interfacial layer between donor and acceptor domains of bulk heterojunction (BHJ). The low carrier mobility (in particular, low hole mobility) of the conjugated polymers requires a balanced dispersion of donor and acceptor molecules for enhanced charge transport toward electrodes. Through drastic improvement of molecular structures, blends of several acceptor and donor conjugated polymers have been examined for fabrication of more efficient and stable bulk heterojunction devices [10–12]. Effective blending and formation of well packed donor and acceptor domains not only enhance exciton dissociation, but it is believed to significantly influence transport of electrons and holes by hopping between these domains [13–15] and control the injection of electrons [16]. Different solvents and co-solvents (e.g., 1,2-dichlorobenzene (DCB), chlorobenzene (CB) and chloroform) and different anneal- ing routines and temperatures have been used to enhance morphology and performance [17–18]. The devices made at room Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.solmat.2010.07.021 n Corresponding author. E-mail address: peymans@ece.ubc.ca (P. Servati). Solar Energy Materials & Solar Cells 94 (2010) 2258–2264