Efficiency limitations in organic bulk heterojunction solar cells

This paper presents a systematic study of key physical mechanisms in organic polymer/fullerene solar cells that determine the power conversion efficiency. The carrier loss at each step is calculated or measured. We compare poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene)-thiophene-thienopyrazine (P3HTT-TP)/PCBM solar cells. The latter polymer, P3HTT-TP, is a novel low-bandgap “semi-random” alkythiophene-based copolymer with a broader absorption spectrum than P3HT. Absorption, a significant loss factor for all polymer/fullerene solar cells, was studied using external quantum efficiency and absorption spectroscopy measurements and modeled with a multi-layer transfer-matrix theory. In addition, hole traps inside the polymers play an important role in polaron pair dissociation and free charge transport. It was found that this is particularly deleterious for P3HTT-TP/PCBM.