Effect of active layer morphology on recombination mechanism in polymer: fullerene organic bulk heterojunction solar cel
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Effect of active layer morphology on recombination mechanism in polymer: fullerene organic bulk heterojunction solar cells Pavel Dutta1, Mukesh Kumar1, Monika Rathi2, Phil Ahrenkiel2, David Galipeau1, Venkat Bommisetty1 1 Department of Electrical Engineering, South Dakota State University, Brookings, SD 57007, USA 2 Nanoscience and Nanoengineering Department, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA ABSTRACT The effect of phase separation of the donor-acceptor (DA) blend on the dominant recombination mechanism in polymer-fullerene [(poly(3-hexylthiophene) (P3HT) and phenylC61-butyric acid methyl ester (PCBM)] based bulk heterojunction (BHJ) cells has been investigated. Coarse (70-150 nm) and fine (20-25 nm) phase separated blends and corresponding devices were prepared using chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB) as spin casting solvents respectively. Nanoscale mobility measurements indicated highly unbalanced charge transport in coarse morphology based (CB cast) devices. Linear dependence of short circuit current (Jsc) vs. light intensity (I) suggested first order monomolecular (MR) recombination in the fine phase separated devices (1,2-DCB cast) whereas sub-linearity suggested dominant role of bimolecular (BR) recombination in coarse phase separated devices (CB cast). Improved device efficiency of 1,2-DCB based devices (η ≈ 2.54 %) compared to CB (η ≈ 0.9 %) may be attributed to reduced BR recombination as a result of finer phase separation. INTRODUCTION Organic bulk heterojunction (BHJ) solar cells have received considerable attention in recent years due to their potential for cost-effective energy conversion and because of their light weight and compatibility with flexible plastic substrates [1]. However, organic BHJ cells suffer from low efficiency compared to existing inorganic photovoltaic technologies and therefore further improvements are required for broader adaptation of organic solar cells into commercial market. Low efficiency of polymer: fullerene solar cells may arise from various reasons like incomplete absorption of photons, insufficient exciton generation, low mobility of carriers, leakage of current and recombination of carriers to name a few. Out of these, carrier recombination may play an important role in limiting the performance of these devices. Recombination is mainly divided into two types: monomolecular (MR) recombination which is a first order recombination process and bimolecular (BR) recombination which is a second order recombination process [2]. Though studies have been conducted to determine the underlying mechanisms of MR and BR recombinations, their role on the performance of BHJ devices is still not clear. MR recombination has often been identified as the primary efficiency limiting factor while several other reports emphasized dominant role of BR recombination [3-5]. Moreover, the connection between morphology and dominant recombination mechanisms is also not clearly understood yet [6]. Therefore, the study of the recombination proces
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