One of the key components in inverted organic solar cells is a zinc oxide (ZnO) layer as an electron-extraction layer. However, this layer contains electron traps that decrease the electron-extraction efficiency and reduce the photovoltaic performance. In this work, we report the photovoltaic property improvement of inverted PTB7-Th:PC 71 BM solar cells by coating high-molecular-weight poly(N-isopropylacrylamide-co-methacrylic acid) (H-PNIPAM) on top of the ZnO layer. The H-PNIPAM film thicknesses were carefully controlled by spin-coating different concentrations of H-PNIPAM solutions to generate an optimal thickness (3-5 nm). Atomic force microscopy and X-ray photoelectron spectroscopy revealed a uniformly coated H-PNIPAM layer. The photoluminescence spectra showed that the layer reduced the number of ZnO trap states. Contact angle measurements indicated that the layer modified the ZnO surface to become more hydrophobic, resulting in good contact with photoactive films. At the same time, the treatment decreased the work function of the ZnO layer from 4.12 to 3.82 eV. Moreover, electron mobility measurements indicated that the use of the H-PNIPAM layer increased the electron mobility in the photoactive layer. Furthermore, the use of the H-PNIPAM layer maintained the initial performance over a long period of time (>3000 h) and improved the photovoltaic performances of other devices based on the photoactive layer (PBDB-T:ITIC and PV-D4610:PC 71 BM). This work conclusively demonstrates that our new H-PNIPAM is a promising surface modifier of the electron-transporting ZnO layer.
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