Understanding charge transport and recombination losses in high performance polymer solar cells with non-fullerene acceptors

The photovoltaic performance of organic solar cells can be enhanced by achieving a fundamental understanding of the key processes that govern the device behaviour. In this work, we comprehensively investigate temperature (T)-dependent charge transport, non-geminate recombination losses and intermolecular stacking based on three representative organic bulk heterojunction (BHJ) solar cells comprising the polymeric donor of PBDB-T blended with non-fullerene small molecule ITIC and polymeric P(NDI2OD-T2) alongside PC71BM acceptors. Surprisingly, the champion solar cell based on PBDB-T: ITIC, even though exhibiting the most imbalanced transport, produces the highest PCE approaching 10%. We find that such an imbalance is in association with the decrease in the recombination reduction factor with respect to the Langevin rate constant. This beneficially leads to mitigated non-geminate recombination and gains in photoconductivity. In contrast, the all-polymer solar cell using the P(NDI2OD-T2) acceptor displays an excellent balance in mobility while suffering from a more substantial recombination, which causes severe carrier losses and reduced photocurrent. T-dependent mobility measurements indicate that the activation energy for the transport in these BHJ films is low (50-150 meV) which is rationalized by the preferential out-of-plane intermolecular pi-pi stacking mainly adopted by the donor molecules. The combined results point to an indication that the electron mobility in non-fullerene acceptors may not be a severe restraint while charge recombination losses play a critical role in ultimate photovoltaic characteristics based on these emerging materials.