Real reason for high ideality factor in organic solar cells: Energy disorder

Abstract

Organic semiconductor has recently emerged as a promising and versatile material for solar cells. This paper shows that the structural order of the electron transport layers has a significant influence on the ideality factor and the power conversion efficiency of organic solar cells. By using Fermi-Dirac distribution, we demonstrate that both ideality factor and degeneracy will up to 3, even 6, when the energy disorder of the transport layer is large enough. By comparing the numerical values of 15 materials with experimental data, our results show that the ideality factor is almost the same value as the degeneracy when the light intensity gets near to 1 mW/cm2. Additionally, both degeneracy and ideality factor have a positive correlation with the energy disorder. However, the power conversion efficiency will decrease as the energy disorder increasing. These results shed light on the real reason for high ideality factor and the origin of the power conversion efficiency loss, and provide a direction to increase the power conversion efficiency of the organic solar cells.