Understanding the loss mechanisms in high-performance solution-processed small molecule bulk heterojunction solar cells doped with a PFN impurity

Contamination of the active layer with an impurity could result in significant degradation in the performance of bulk heterojunction (BHJ) solar cells as a result of enhancing the loss of the charge carriers via a trap-assisted recombination. In this study, PFN as an impurity was intentionally introduced to a BHJ solar cell composed of a high-performance solution-processed small molecule (p-DTS(FBTTh2)2 as a donor and PC60BM as an acceptor. The power conversion efficiency (PCE) of PFN doped devices degrades owing to the reduction of short-circuit current (Jsc) and fill factor (FF). At a low concentration, PFN mostly reduces the generation of charge carriers, whereas doubling the PFN concentration conversely affects both generation and collection of charge carriers. Charge carrier dynamics of devices has also been probed using photovoltage decay, time-resolved charge extraction (TRCE) and photoinduced charge extraction by linearly increasing voltage (photo-CELIV) before and after incorporation of PFN. The results reveal that traps introduced by PFN reduce the decay of charge carriers via bimolecular recombination, leading to a higher charge carrier density and photovoltage at long times under an open-circuit potential (Voc). However, under short-circuit (Jsc) conditions, traps considerably impede the collection of charge carriers causing the appearance of an S-shaped current density-voltage curve.