Novel benzo[1,2-b:4,5-b']dithiophene-benzothiadiazole derivatives with variable side chains for high-performance solar cells.

Polymer solar cells (PSCs) have attracted considerable attention because of their advantages of low cost, light weight, and fl exibility in large area applications by ink-jet printing and rollto-roll solution processes. [ 1 ] The concept of bulk heterojunction (BHJ) has improved the power conversion effi ciencies (PCEs) of the PSCs signifi cantly by blending electron-donating conjugated polymers and electron-accepting fullerene derivatives (such as [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM)). [ 2 ] In the past few years, regioregular poly(3-hexylthiophene) (P3HT) and PC 61 BM based PSCs were taken as standard BHJ devices with high effi ciencies of 4–5%. [ 3 ] However, the improvement of PCEs was limited in these devices by their low open circuit voltages ( V oc ) and narrow absorption spectra. Aiming at achieving broader absorption, higher hole mobility, and more suitable energy levels, low bandgap (LBG) conjugated polymers were developed rapidly to be the alternatives of P3HT with effi ciency values over 7%. [ 1c , 1d , 4 ]