Effect of Band Bending and Energy Level Alignment at the Donor/Acceptor Interface on Open-Circuit Voltage in Organic Solar Cells

Open-circuit voltage (V-OC) is a key parameter governing the power conversion efficiency of organic solar cells. We clarified the effect of band bending in organic semiconductor films on the V-OC of phthalocyanine (H2Pc)/fullerene (C-60) planar heterojunction (PHJ) OSCs. The V-OC was significantly affected by the H2Pc layer thickness, varying from 0.20 V at 2 nm to 0.49 V at 100 nm thickness. The large Voc change is due to the band bending in the H2Pc layer, where the difference in H2Pc layer thickness led to significant changes in the degree of the vacuum-level shift in the C-60 layer because of the Fermi level (E-F) difference between the H2Pc and Co layers. This result demonstrated that a new influential factor affecting the V-OC is the position of the EF that is dependent on band bending in organic semiconductor films.