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.

Thin film solar cells are one of the important candidates utilized to reduce the cost of photovoltaic production by minimizing the usage of active materials. However, low light absorption due to low absorption coefficient and/or insufficient active layer thickness can limit the performance of thin film solar cells. Increasing the absorption of light that can be converted into electrical current in thin film solar cells is crucial for enhancing the overall efficiency and in reducing the cost. Therefore, light trapping strategies play a significant role in achieving this goal. The main objectives of light trapping techniques are to decrease incident light reflection, increase the light absorption, and modify the optical response of the device for use in different applications. Nanostructures utilize key sets of approaches to achieve these objectives, including gradual refractive index matching, and coupling incident light into guided modes and localized plasmon resonances, as well as surface plasmon polariton modes. In this review, we discuss some of the recent developments in the design and implementation of nanostructures for light trapping in solar cells. These include the development of solar cells containing photonic and plasmonic nanostructures. The distinct benefits and challenges of these schemes are also explained and discussed.

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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.

High-quality crystalline large grains with uniform morphologies of the perovskite films are particularly important for achieving stable, high-performance perovskite solar cells. Herein, an effective strategy to control the growth of large grains in the CH3NH3PbI3 perovskite films is demonstrated by modifying the perovskite film deposition process through forming an intermediate CH3NH3PbI3·methylammonium chloride (MACl)·xCH3NH2 liquid phase induced by CH3NH2 gas treatment in combination with a MACl additive without an antisolvent. By tuning the incorporation of the MACl additive to the perovskite precursor solution, this intermediate liquid phase enables the well-controlled growth of large grains up to 3 μm, highly uniform morphology, and higher crystallinity in the final CH3NH3PbI3 perovskite films. The high-quality CH3NH3PbI3 film derived from the CH3NH3PbI3·MACl·xCH3NH2 phase leads to enhanced carrier lifetime and reduced charge-trap density and nonradiative recombination of the perovskite films. In addition, the defect healing and reduced grain boundaries also greatly improve the environmental stability in ambient air. The perovskite solar cells made via the CH3NH3PbI3·MACl·xCH3NH2 phase exhibit high power conversion efficiency of 18.4%, much higher than that of the perovskite solar cells made without MACl (15.8%).

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In this work, a periodic upright nanopyramid structure was developed for light harvesting applications suitable for thin film solar cells. The periodic inverted nanopyramid structure was fabricated on Si substrate by laser interference lithography (LIL) and subsequent pattern transfer by combined reactive ion etching and KOH wet etching. The silicon substrate was used as a master mould in the replication process utilising ultraviolet curable nanoimprint lithography (UV-NIL) process. The inverted nanopyramid patterns were transferred onto OrmoStamp resist layer to form upright pyramids on glass substrates by UV-NIL. The replicated periodic upright nanopyramid structures can be used as light trapping structures in thin film solar cells and as soft mould in the 3D imprint process.

The morphological bulk defects and trap sites between TiO2 and the perovskite layer are critical issues in charge separation and electron transport. In this study, an effective method for improving the mesoporous TiO2/perovskite interfacial characteristics in perovskite solar cells (PSCs) is demonstrated by modifying mesoporous TiO2 with potassium (K) treatment. It is found that the modification of mesoporous TiO2 with the K treatment enhances the perovskite crystallization process, producing a perovskite film with higher crystallinity and larger grain sizes. It is also found that the K treatment not only effectively passivates the trap sites in mesoporous TiO2 but also reduces the sub-band-gap deep defect states at the interface of the perovskite, as revealed by photothermal deflection spectroscopy, thereby suppressing the nonradiative recombination and improving the V oc of the PSCs. Moreover, stronger photoluminescence quenching and shorter carrier lifetime are observed for the perovskite on K-treated mesoporous TiO2, indicating more efficient charge collection across the interface. As a result of these advancements, the PSC based on K-treated mesoporous TiO2 shows a high power conversion efficiency of 20.60% compared to the PSC without the K treatment (18.17%) and also significantly improves the environmental stability in ambient air.