2, ZAE Bayern, Erlangen, , Germany
Organic semiconductors are in general known to have lower mobility compared to their inorganic counterparts. As such, the bimolecular recombination rate of holes and electrons, usually referred to as Langevin recombination, is typically an important loss mechanism. Here, we elucidate the photophysics of BHJ solar cells based on a non-fullerene acceptor (IDTBR) in combination with various polymers showing an unprecedented low bimolecular recombination rate despite the unbalanced charge carrier mobility. The high FF observed (above 65%) is attributed to the non-Langevin behaviour with a beta/betaL ratio of 1x10-4. We calculated high charge carrier lifetimes without parasitic recombination in P3HT:IDTBR solar cells, leading to an almost perfect bimolecular recombination. Most interestingly, light intensity mobility measurements reflect a strongly non-thermalized carrier transport, indicating the origin of this unusual slow recombination kinetics. As a consequence of this peculiar recombination kinetics, devices between 80 nm and 450 nm and found to show a thickness- independent PCE.
The second part investigates the radiative and non-radiative voltage losses related with NFAs. Voc values of up to 1.15 are found for semiconductor composites with a bandgap below 1.75, and record Voc values of up to 1.25 are observed for semiconductors with slightly larger bandgaps. Non-radiative Voc losses as low as 0.2 V in combination with low radiative losses due to the absence of CT states open a technical venue to construct OPV devices with efficiencies beyond 15 %.