Charge Carrier Lifetime and Recombination in Bulk Heterojunction Solar Cells

In this paper, the main photocurrent density and power conversion efficiency limiting mechanisms in bulk heterojunction solar cells are discussed with the emphasis on recombination processes. Charge extraction by linearly increasing voltage, time of flight, and other methods that allow the carrier lifetime and recombination to be studied experimentally in operating solar cells are discussed. It is shown that non-Langevin recombination is required for high-performance organic photovoltaic devices, which typically have low charge carrier mobility. Long charge carrier lifetime, exceeding carrier transit time through the film, can be achieved when non-Langevin recombination is observed. Langevin-type recombination dominates in most low-efficiency solar cells, whereas non-Langevin recombination is present in high efficient, e.g., annealed poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester blend devices. The film nanomorphology plays a crucial role governing the charge transport and the carrier lifetime. Double injection current with non-Langevin carrier recombination is demonstrated in high-efficiency devices, which strongly exceeds the injection current with Langevin recombination, due to the high carrier concentration attainable under non-Langevin recombination. Several different models explaining the non-Langevin recombination in organic solar cells are reviewed. Requirements for charge carrier mobility and recombination to maximize power conversion efficiency in organic photovoltaic devices are outlined.