Enhanced carrier extraction efficiency in organic solar cells with reduced space-charge limited transport by engineered vertical compositional profile

A multi-layered polymer solar cell (PSC) with a graded compositional profile to reduce space-charge limited (SCL) transport was designed and analyzed in details. We found that by increasing the effective carrier lifetime for the slower photo-carriers, SCL effects that degrade the fill factor and the power conversion efficiency can be alleviated in PSCs with a thick active layer. The reduction of SCL effects was evidenced by a diminished dependence of the incident photon-to-electron conversion efficiency (IPCE) on light bias, which also indicates the importance of light bias in characterizing the operating IPCE of PSCs with thick active layers which operate in SCL region. Based on our results, device architecture to enhance FF was proposed.