A novel alcohol-soluble squaraine dye as an interfacial layer for efficient polymer solar cells

Abstract Interfacial engineering is an important study region of polymer solar cells(PSCs). Among all kinds of cathode interfacial layers (CIL), alcohol-soluble materials have attracted considerable attention and exhibited comparable performance in PSCs. Here, a series of novel alcohol-soluble ionic small-molecule squaraine dyes (BBSQNEt, BBSQNEtOH, TBuSQNEtOH, TBuSQNEtOH1) were synthesized for PTB7:PC 71 BM(PTB7: poly[[4,8-bis[(2-ethyl- hexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]; PC 71 BM: [6,6]-phenyl-C71-butyric acid methyl ester) based PSCs. A high power conversion efficiency (PCE) of 7.47% was obtained when BBSQNEt was inserted between the active layer and the Al cathode in conventional PSCs. It is observed that the open voltage ( V oc ) increases from 0.67V to 0.75V, the short circuit current ( J sc ) increases from 11.09 mA cm −2 to 14.98 mA cm −2 and the fill factor (FF) increases from 56.8% to 66.1%, resulting in a 78% improvement in PCE relative to the devices without CIL. Scanning Kelvin probe microscopy (SKPM) indicated that the BBSQNEt layer lifts the surface potential of the active layer, strengthens the built-in electrical field, reduces electron barrier and facilitates the electron transportation and collection, thus resulting in the excellent performance of PSCs. Our work demonstrated for the first time that the alcohol-soluble squaraine dye is a promising material as CIL in PSCs.