Promoting the stability of organic photovoltaics by planar heterojunction optimization

Abstract

In organic photovoltaics (OPVs) with a bulk heterojunction (BHJ) active layer, the donor and acceptor materials have a metastable nanoscale phase mixture, where the uncontrollable morphology greatly reduces the stability of the device. However, OPVs with planar heterojunction (PHJ) structures show obvious vertical phase separation, which can provide separately regulated donor and acceptor layers, achieving significantly higher stability when compared to BHJ based devices. In addition, there is relatively little attention paid to understand the internal attenuation mechanism of OPV devices and how to promote their stability for commercial application. In this work, we investigated the stability of OPV devices with both BHJ and PHJ structures, and further discussed the internal mechanisms and why PHJ structures could help enhance device stability compared to BHJ structures. The results showed that after being placed in a nitrogen environment for 240 hours, the power conversion efficiency (PCE) of PBDB-T:ITIC BHJ devices decreased by 92.52%, while the PCE of PBDB-T/ITIC PHJ devices only decreased by 68.79%. When combined with interface reaction and carrier characteristics analyses, it was found that during the aging process of PBDB-T:ITIC BHJ devices, the active layer formed a top surface rich in PBDB-T and a bottom surface rich in ITIC. The adverse interface reaction between the PEDOT:PSS hole transport layer and ITIC acceptor accelerated the performance deterioration, while the PBDB-T/ITIC PHJ structure can effectively suppress such a reaction, resulting in improved device stability. In addition, we also prepared BHJ and PHJ devices with a PM6:Y6 system for verification, and the results also demonstrated that PHJ devices exhibited better device stability.