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 angstrom)/PDI (200 angstrom)/bathocuproine (100 angstrom)/aluminum (1000 angstrom). We found that PDIs with larger substituents produced higher open circuit voltages (V-OC's) despite the donor acceptor interface gap (Delta E-DA) remaining unchanged. Additionally, series resistance was increased simultaneously with V-OC 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. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3595679]