Realizing Highly Efficient Inverted Photovoltaic Cells by Combination of Nonconjugated Small-Molecule Zwitterions with Polyethylene Glycol

Organic ionic materials have been reported to be efficient cathode interlayer (CIL) materials in polymer solar cells (PSCs); however, most of them are employed in conventional PSCs. For an inverted structural device which has better stability, the efficiency is still far from expectation and the report is also limited. In this study, by using nonconjugated zwitterions as the CIL and inverted structure, the power conversion efficiency (PCE) is ∼6%, though the PCE can reach 9.14% in the conventional device. By introducing polyethylene glycol (PEG) into the zwitterions, the PCE of the inverted PSCs was improved ∼33% and reached ∼8% mainly because of the enhancement of the open-circuit voltage (V_oc) and fill factor (FF). Further research on the device parameters, work functions, morphology of indium tin oxide (ITO) with various CILs, and recombination resistance of the devices indicated that PEG + zwitterion induced not only a lower work function of ITO but also a more uniform morphology of CILs with less contact of the photoactive layer with ITO, which induced suppressed charge recombination and a higher V_oc and FF. Enhanced ability in interface modification of PEG + zwitterion CILs displayed a simple and feasible approach to elevate the performance of inverted PSCs with ionic CILs.