Are we there yet? Design of better conjugated polymers for polymer solar cells

Over the last ten years, polymer solar cells have been developed as an attractive alternative to the traditional silicon photovoltaic devices. Remarkable progress has been made in polymer-fullerene solar cells and several polymers have shown power conversion efficiencies (PCEs) greater than 7%. The most important recent development has been the synthesis of new low bandgap polymers with optimal properties for the solar cells. Herein we provide an overview of the key strategies of optimization of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels in the design of new low bandgap conjugated polymers. Detailed examples of representative donor polymers, the current state of the art in their solar cell performances and possible future directions for development of new polymers with improved efficiencies are discussed.