Elucidating the interplay between dark current coupling and open circuit voltage in organic photovoltaics

A short series of alkyl substituted perylenediimides (PDIs) with varying steric bulk are used to demonstrate the relationship between molecular structure, materials properties, and performance characteristics in organic photovoltaics. Devices were made with the structure indium tin oxide/copper phthalocyanine (200 Å)/PDI (200 Å)/bathocuproine (100 Å)/aluminum (1000 Å). We found that PDIs with larger substituents produced higher open circuit voltages (⁠$VOC$’s) despite the donor acceptor interface gap $(ΔEDA)$ remaining unchanged. Additionally, series resistance was increased simultaneously with $VOC$ the effect of reducing short circuit current, making the addition of steric bulk a tradeoff that needs to be balanced to optimize power conversion efficiency.