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This study addresses the effects of atmospheric-pressure plasma on the properties of methylammonium lead halide perovskite thin films. Plasma-treated films then were used to fabricate an inverted p-i-n perovskite solar cell. The plasma treatment employed a large-area plasma of extremely high-volume power density, up to 100 W/cm3, capable of generating diffuse, homogeneous and cool plasma (<70 °C) in ambient air, as well as in other technical-grade gases including nitrogen, argon, methane, hydrogen, carbon dioxide and pure water vapour. Although the temperature of the plasma remained very low, the population of energetic states proved sufficient to induce both physical and chemical changes on the surfaces of perovskite films. The results demonstrate that plasma treatment leads to perovskite films with homogeneous surfaces. Photoluminescence measurement revealed that plasma treatment enhanced the intensity of peaks, a property that may be attributed to improved crystallinity. Most importantly, plasma treatment of perovskite films improved the performance of the perovskite solar cells.IntroductionExperimentalMethylammonium lead halide perovskite precursor was purchased from Ossila (UK). In order to study the effect of plasma treatment on the properties of perovskite, the precursor was spin-coated onto quartz glass (1×1 cm2) and then annealed on a hotplate at a temperature of 80 °C for 2 hours. Plasma treatment of the perovskite films was carried out by exposure to DCSBD plasma (RPS40, Roplass s.r.o., Czech Republic) in a nitrogen glove-box. A range of plasma treatment times were tested: 1 s, 2 s, 4 s and 8 s; it was observed that plasma treatment for 2 s resulted in the most homogeneous surface and the best PCE. Plasma treatment for longer than 8 s led to deformation of the surface. Optimum treatment time was therefore considered to be 2 s.Steady-state photoluminescence (PL) spectra in the visible wavelength were recorded using a RM1000 system (Renishaw, UK). Scanning electron microscopy (MIRA3 Tescan, Czech Republic) was employed to evaluate the surface morphology of the perovskite samples. The performance of the PSC devices was evaluated by using a system of AAA-certified Abet Sun solar simulator with an air mass (AM) 1.5G filter that simulated light intensity, adjusted to 1000 W/m2 with an NREL-calibrated Si solar cell. REsults and discussionFigure 1 SEM images of perovskite films before and after plasma treatment.Figure 2 Steady-state photoluminescence spectra of perovskite films.Figure 3 demonstrates the J-V characteristics of the PSC device with plasma-treated film in comparison with the reference device with untreated perovskite film. The relevant parameters are listed in Table 1. The device with untreated perovskite film exhibited VOC 721.57 V and JSC 19.87 mA/cm2, which resulted in an FF of 42.36 % and PCE of 6.08 %. Plasma treatment of the perovskite film removed surface contamination, which led to better electrical conductivity and consequently Jsc should therefore have improved. However, and surprisingly, the short current density fell (Jsc 18.23 mA/cm2) after plasma treatment. However, plasma treatment of the perovskite film increased VOC to 795.12 V and FF to 53.16 %. The increase in VOC and FF which could be attributed to reduction of carrier recombination after plasma treatment, was observed from PL measurement (above). This may suggest that plasma treatment of the perovskite film led to a reduction in charge recombination at the interface between the perovskite layer and the PCBM. The compensation between decreased Jsc and increased VOC and FF resulted in an improved PCE of the PSC device from 6.08 % to 7.76 %.ConclusionThe effects of low-temperature plasma treatment on the surface properties of lead halide perovskite films were investigated. Perovskite films became more homogeneous with fewer pinholes in the surface after short plasma treatment. It was observed that plasma treatment increased the photoluminescence intensity of the perovskite films. Plasma treatment of the halide perovskite films is revealed as a promising post-treatment in the quest for high-efficiency perovskite solar cells. The results demonstrate that plasma treatment of the perovskite films led to improvement in the open-circuit voltage and fill factor of the PSCs, which could be attributed to improved charge transfer at the perovskite interface treated by plasma. 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