Mechanisms of device degradation in organic solar cells: Influence of charge injection at the metal/organic contacts

Abstract

Organic solar cells suffer from device degradation under various undesirable ambient conditions, which limits their efficiencies and applications a lot. The frequently appeared S-shaped current density–voltage (J–V) characteristics just reflect this effect and make its underlying mechanisms even obscurer for experimentalists. By performing device model simulation, we investigate the J–V curves of a organic bulk heterojunction solar cell for different interfacial charge injection conditions. We find the S-shaped kinks appear when both electron and hole injection barriers exceed 0.3eV, which is attributed to the shortage of dark injection carriers. For the nearly Ohmic contacts, the open circuit voltage is significantly reduced. The quantitative study suggests it arising from the majority carrier accumulation and the induced local field variation at the vicinity of the contacts. S-shaped kink is gradually eliminated with increasing carrier mobilities. Finally, we find the decreasing of fill factor under enhanced light absorption, which results from the drastic bimolecular recombination and thus signifies remarkable internal losses.