Ternary Solar Cells Employing Thieno[3,4-b]thiophene-Based Copolymer Offer High Performance with Large Current Density and Fine-Tuned Morphology

A novel thieno[3,4-b]thiophene-based donor polymer PTBTz-2 was used to construct ternary organic solar cells with two classical acceptors 3,9-bis(2-methylene-(3-(1,1-dicyano-methylene)-indanone)-5,5,11,11-tetrakis-(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene) (ITIC) and PC71BM. After the optimization of the donor-to acceptor weight ratio, a better match of the absorption spectrum and a high short circuit current density of 20.75 mA cm(-2) are obtained. The power conversion efficiency is improved to 11.26 from 10.43% after 35% PC71BM was added. The crystal structure data obtained by grazing-incidence wide-angle X-ray scattering gives a clearer picture that increase in the crystal correlation length of a donor polymer on the out-of-plane direction while decrease in the pi-pi stacking distances as the third phase of PC71BM was added. The added acceptor not only affects the structure character of the donor but also enhances the crystallization of the second acceptor ITIC on the in-plane direction. Homogeneous phase separation with a periodic distribution of donor and acceptor was observed by grazing incidence small-angle X-ray scattering as 35% PC71BM was added. Atomic force microscopy and transmission electron microscopy clearly demonstrated that a bicontinuous interpenetrating network between donors and acceptors was formatted. Our work demonstrates how to improve the photovoltaic device performance in the blend films by the added third phase from the nanomorphology and microscopic mechanism.