A review of charge transport and recombination in polymer/fullerene organic solar cells

Abstract

The charge carrier transport and recombination in two types of thermally treated bulk‐heterojunction solar cells is reviewed: in regioregular poly(3‐hexylthiophene) (RRP3HT) mixed with 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐methanofullerene (PCBM) and in the blend of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)‐phenylene vinylene] (MDMO‐PPV) mixed with PCBM. The charge carrier mobility and bimolecular recombination coefficient have been comparatively studied by using various techniques including Time‐of‐Flight (ToF), Charge Extraction by Linearly Increasing Voltage (CELIV), Double Injection (DI) transients, Current–Voltage (I–V) technique. It was found that the carrier mobility is at least an order of magnitude higher in RRP3HT/PCBM blends compared to MDMO‐PPV/PCBM. Moreover, all used techniques demonstrate a heavily reduced charge carrier recombination in RRP3HT/PCBM films compared to Langevin‐type carrier bimolecular recombination in MDMO‐PPV/PCBM blends. As a result of long carrier lifetimes the formation of high carrier concentration plasma in RRP3HT/PCBM blends is demonstrated and plasma extraction methods were used to directly estimate the charge carrier mobility and bimolecular recombination coefficients simultaneously. A weak dependence of bimolecular recombination coefficient on the applied electric field and temperature demonstrates that carrier recombination is not dominated by charge carrier mobility (Langevin‐type recombination) in RRP3HT/PCBM blends. Furthermore, we found from CELIV techniques that electron mobility in RRP3HT/PCBM blends is independent on relaxation time in the experimental time window (approx. hundreds of microseconds to tens of milliseconds). This reduced carrier bimolecular recombination in RRP3HT/PCBM blends implies that the much longer carrier lifetimes can be reached at the same concentrations which finally results in higher photocurrent and larger power conversion efficiency of RRP3HT/PCBM solar cells. Copyright © 2007 John Wiley & Sons, Ltd.