Single-crystalline ReS2 nanosheets with a few layers were vertically grown on a SiO2/Si wafer by a gas phase reaction process at ambient pressure.

Organic solar cells (OSCs) demonstrating high power conversion efficiencies have been mostly fabricated using halogenated solvents, which are highly toxic and harmful to humans and the environment. Recently, non-halogenated solvents have emerged as a potential alternative. However, there has been limited success in attaining an optimal morphology when non-halogenated solvents (typically o-xylene (XY)) were used. To address this issue, we studied the dependence of the photovoltaic properties of all-polymer solar cells (APSCs) on various high-boiling-point non-halogenated additives. We synthesized PTB7-Th and PNDI2HD-T polymers that are soluble in XY and fabricated PTB7-Th:PNDI2HD-T-based APSCs using XY with five additives: 1,2,4-trimethylbenzene (TMB), indane (IN), tetralin (TN), diphenyl ether (DPE), and dibenzyl ether (DBE). The photovoltaic performance was determined in the following order: XY + IN < XY + TMB < XY + DBE ≤ XY only < XY + DPE < XY + TN. Interestingly, all APSCs processed with an XY solvent system had better photovoltaic properties than APSCs processed with chloroform solution containing 1,8-diiodooctane (CF + DIO). The key reasons for these differences were unraveled using transient photovoltage and two-dimensional grazing incidence X-ray diffraction experiments. The charge lifetimes of APSCs based on XY + TN and XY + DPE were the longest, and their long lifetime was strongly associated with the polymer blend film morphology; the polymer domain sizes were in the nanoscale range, and the blend film surfaces were smoother, as the PTB7-Th polymer domains assumed an untangled, evenly distributed, and internetworked morphology. Our results demonstrate that the use of an additive with an optimal boiling point facilitates the development of polymer blends with a favorable morphology and can contribute to the widespread use of eco-friendly APSCs.

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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:PC71BM 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:PC71BM). This work conclusively demonstrates that our new H-PNIPAM is a promising surface modifier of the electron-transporting ZnO layer.