Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectroscopy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect.

TiO 2 is most commonly employed as an electron transport layer (ETL) in mesoscopic n−i−p perovskite solar cells (PSCs). However, the low electron mobility, low electrical conductivity, and high electronic trap states of TiO 2 may have negative impacts on further enhancement of PSC performance. Metal doping is an efficient way to improve the electronic properties of TiO 2 films. In this work, we investigate the concentration-dependent impact of alkali lithium metal doping of the mesoporous TiO 2 ETL on the performance of mesoscopic CH 3 NH 3 PbI 3 PSCs. It was found that Li doping results in remarkable improvement in electrical conductivity and electron mobility and reduces the number of electronic trap states arising due to the oxygen vacancies within TiO 2 lattice. Such enhancements led to an enhanced charge extraction and transport and reduced charge recombination rate at the perovskite/ mesoporous TiO 2 interface as revealed by steady-state photoluminescence (PL) and time-resolved PL (TRPL) spectra, and resulted in an increase in the V OC , J SC , and FF of the PSCs. Moreover, the J−V curve hysteresis behavior after Li doping was effectively suppressed due to the reduced charge accumulation and recombination at the TiO 2 /perovskite interface. Consequently, the device performance relies on the concentration of alkali lithium metal doping, and the power conversion efficiency (PCE) of the PSC was significantly improved from 13.64% to 17.59% with reduced the J−V curve hysteresis behavior for a Li doped mesoporous TiO 2 layer with an optimized concentration of 30 mg/mL.

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