Analyzing local exciton generation profiles as a means to extract transport lengths in organic solar cells

In this work, we determine the carrier-transport lengths of electrons and holes (L_e,h) for bulk heterojunction (BHJ) organic solar cells using a method applicable to functional devices. By linking the local exciton generation profile [G (x)] in the photoactive layer to photocurrent losses, we are able to determine the onset of bimolecular recombination, which is the dominate loss process of free carrier transport. Even though many factors affect photocurrent generation, we single out bimolecular recombination by measuring the scaling of photocurrent with light intensity as a function of applied voltage. For the common BHJ system, annealed poly-3-hexylthiophene:[6,6]-phenyl- C61-butyric acid methyl ester (P3HT:PCBM), a minimum for L_e in PCBM is found to be 340 nm while L_h is estimated to be 90 nm for P3HT. The relationship between G (x) and carrier transport is further exemplified by demonstrating a scaling exponent below that for traditional space-charge-limited photocurrent. Likewise, by incorporating a drift/diffusion model, an intuitive link between G (x) and charge transport is established where recombination is shown to occur in regions of the photoactive layer far from the electrode of the slowest carrier species. Finally, the consequences of L_e,h on device design for operation under 1 Sun conditions are described.