Design of terpolymers as electron donors for highly efficient polymer solar cells

Remarkable progress has been realized in improving the power conversion efficiency (PCE) of polymer solar cells (PSCs) over the past few years, raising the possibility of their commercialization as an alternative counterpart to conventional inorganic solar cells. The development of conjugated polymers that contain various alternately bonded electron-rich (D) and electron-deficient (A) units, called D-A alternating copolymers, plays a crucial role in improving the PCE of PSCs, which has now exceeded 8-9%. However, further development of conjugated polymers is essential to produce highly efficient and stable PSCs with a PCE of more than 10% for commercialization. In this regard, terpolymers, which consist of three different units in the polymer backbone, are promising candidates because they can show synergetic effects of each unit by tuning the composition ratio, thereby enabling full-range absorption, high charge mobility, and good solubility. Substantial progress in the development of terpolymers has been achieved with regard to PCE. Herein, we review the development of terpolymers for PSCs and discuss the major factors that need to be considered in the design of new terpolymers.