Highly efficient polymer solar cells with PTB7-based narrow band-gap conjugated polyelectrolytes as cathode interlayers: Device performance dependence on the ionic pendants

Narrow band-gap conjugated polyelectrolytes (NBGCPs) combine the advantages of narrow band-gap conjugated polymers and polyelectrolytes. However, they are limited reported and seldom used in polymer solar cells (PSCs). Herein, we design and synthesized two PTB7-based NBGCPs, cationic PTB7-NBr and zwitterionic PTB7-NSO₃, as cathode interlayers (CILs) in conventional PSCs. Compared to poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-ethylhexyl-3-fuorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7), both PTB7-NBr and PTB7-NSO₃ have similar absorption while PTB7-NBr has a higher HOMO level than PTB7 and PTB7-NSO₃. With PTB7-NBr and PTB7-NSO₃ as CIL and Al as cathode, the devices both exhibited high efficiencies. The optimal power conversion efficiencies (PCEs) of PTB7-NBr device and PTB7-NSO₃ device are about 9%. However, PTB7-NBr devices are less sensitive to the CIL film thicknesses and the PCEs of PTB7-NSO₃ device decrease sharply when the thickness of PTB7-NSO₃ over 3 nm. What's more, when the devices with an Ag cathode PTB7-NBr exhibits a much better interfacial modification than PTB7-NSO₃. Ultraviolet photoelectron spectroscopy (UPS) indicate that the work functions of PTB7-NBr and PTB7-NSO₃ film on Al are identical while on Ag they are different. We suggest the insensitivity to CIL thickness and good performance for Ag device of PTB7-NBr devices are attributed to the bromide anions in PTB7-NBr. The report on PTB7-based NBGCPs broads the types of organic cathode interfacial materials and is beneficial to the deep insight of ionic effect in electrolyte materials.