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

The charge carrier transport and recombination in two types of thermally treated bulk-heterojunction solar cells is reviewed: in regioregularpoly(3- hexylthiophene) (RRP3HT) mixed with 1-(3-methoxycarbonyl)propyll-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-ofFlight (ToF), Charge Extraction by Linearly Increasing Voltage (CELTV), 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 CELTV 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.