Enhanced charge extraction in carbon-based all-inorganic CsPbBr3 perovskite solar cells by dual-function interface engineering

Novel Materials in Perovskite Solar Cells: Efficiency, Stability, and Future Perspectives

Solar energy is regarded as the finest clean and green energy generation method to replace fossil fuel-based energy and repair environmental harm. The more expensive manufacturing processes and procedures required to extract the silicon utilized in silicon solar cells may limit their production and general use. To overcome the barriers of silicon, a new energy-harvesting solar cell called perovskite has been gaining widespread attention around the world. The perovskites are scalable, flexible, cost-efficient, environmentally benign, and easy to fabricate. Through this review, readers may obtain an idea about the different generations of solar cells and their comparative advantages and disadvantages, working mechanisms, energy alignment of the various materials, and stability achieved by applying variable temperature, passivation, and deposition methods. Furthermore, it also provides information on novel materials such as carbonaceous, polymeric, and nanomaterials that have been employed in perovskite solar in terms of the different ratios of doping and composite and their optical, electrical, plasmonic, morphological, and crystallinity properties in terms of comparative solar parameters. In addition, information on current trends and future commercialization possibilities of perovskite solar have been briefly discussed based on reported data by other researchers.

97.3% Pb-Reduced CsPb1-xGexBr3 Perovskite with Enhanced Phase Stability and Photovoltaic Performance through Surface Cu Doping

Ge doping has been regarded as an effective way to explore the low-toxicity inorganic halide perovskite. However, Ge²⁺ ions are easy to oxidize because the Ge dopant raises the valence band maximum (VBM) over the water oxidization (H₂O/O₂) potential. Here we find that surface Cu doping can bend down the band levels and decline the VBM of the CsPb_1-xGe_xBr₃ surface below the H₂O/O₂ potential, then prevent the Ge²⁺ from being oxidized into Ge⁴⁺ by water because the Cu dopant reduces the perovskite surface electron accumulation. Note that the Cu dopant prefers to locate at the perovskite surface rather than the interior, and it reduces the surface energy and enhances the stability. Consequently, the largest Pb reduction increases to 97.3% for the Cu-doped CsPb_1-xGe_xBr₃ surface. Moreover, the exciton binding energy and optical absorption of CsPb_1-xGe_xBr₃ could be further improved by the surface Cu dopant. This work provides guidance for finding low-toxicity stable inorganic perovskites.

Enhanced Efficiency of Air-Stable CsPbBr3 Perovskite Solar Cells by Defect Dual Passivation and Grain Size Enlargement with a Multifunctional Additive

The perovskite solar cells (PSCs) based on cesium lead bromide (CsPbBr₃) with outstanding environmental stability and low preparation cost are regarded as one of the most promising photovoltaic devices for commercial applications. However, the performance of CsPbBr₃ PSCs can be badly deteriorated by the intense charge recombination arising from the ionic defects at the grain boundaries of perovskite film. To cope with this issue, we adopt an amino acid of l-lysine with two amino and one carboxyl groups as a chemical additive to incorporate into perovskite film to simultaneously anchor the uncoordinated Pb²⁺ (Cs⁺) and halogen ion defects. Further, the grain size of CsPbBr₃ perovskite is boosted from 688 to over 1000 nm after l-lysine incorporation as a result of the decreased nucleation rate and the sufficient growth of perovskite, which effectively reduce the grain boundaries for load defects. As expected, the optimized device achieves a best power conversion efficiency of 9.69% attributed to the remarkably reduced charge recombination and enhanced charge extraction arising from the efficient defects dual-passivation and enlarged grain size of perovskite film as well as the improved energy level alignment at the device interface after the introduction of l-lysine, which is elevated by 61.23% in comparison to 6.01% efficiency of the pristine one. Moreover, the unencapsulated device with l-lysine incorporation exhibits remarkable long-term stability in air with 80% RH at 25 °C and 0% RH at 80 °C as well as under continuous illumination conditions. This work provides an effective multifunctional additive for imperfection passivation and grain size enlargement of perovskite to build PSCs with high efficiency and stability.

Grain Enlargement and Defect Passivation with Melamine Additives for High Efficiency and Stable CsPbBr3 Perovskite Solar Cells

The preparation of high-quality perovskite films with low grain boundaries and defect states is a prerequisite for achieving high-efficiency perovskite solar cells (PSCs) with good environmental stability. An effective additive engineering strategy has been developed for simultaneous defect passivation and crystal growth of CsPbBr₃ perovskite films by introducing 1,3,5-triazine-2,4,6-triamine (melamine) into the PbBr₂ precursor solution. The resultant melamine-PbBr₂ film has a loose, large-grained structure and decreased crystallinity, which has a positive effect on the crystallization process of the perovskite as it retards the crystallization rate as a result of the interaction between melamine and lead ions. Additionally, the passivation by melamine gives a high-quality CsPbBr₃ perovskite film with fewer grain boundaries, lower defect densities, and better energy level matching is achieved by multistep liquid-phase spin-coating, which greatly suppresses the nonradiative recombination resulting from the defects and promotes charge extraction at the interface. A champion power conversion efficiency as high as 9.65 % with a promising open-circuit voltage of 1.584 V is achieved for PSCs with an architecture of fluorine-doped tin oxide/c-TiO₂ /m-TiO₂ /melamine-added CsPbBr₃ /carbon-based hole-transporting layer. Furthermore, the unencapsulated melamine-added CsPbBr₃ PSC shows superior thermal and humidity stability in ambient air at 85 °C or 85 % relative humidity over 720 h.

Interface Engineering of Imidazolium Ionic Liquids toward Efficient and Stable CsPbBr3 Perovskite Solar Cells

The defect passivation of perovskite films is an efficacious way to further boost the power conversion efficiency (PCE) and long-term stability of perovskite solar cells (PSCs). In this work, ionic liquids (ILs) of 1-butyl-2,3-dimethylimidazolium chloride ([BMMIm]Cl) are used as a modification layer in perovskite films in carbon-based CsPbBr₃ PSCs without a hole-transporting material (HTM) for passivating the surface defects. The preliminary results demonstrate that the [BMMIm]Cl modifier passivates the surface defects of the perovskite film and reduces the valence band of perovskite close to the work function of the carbon electrode, which causes a remarkably inhibited nonradiative and radiative charge recombination, improved energy-level matching, and decreased energy loss. After optimization, a champion efficiency of 9.92% with a V_oc as high as 1.61 V is achieved for the [BMMIm]Cl tailored carbon-based CsPbBr₃ PSC without HTM, which is improved by 61.3% in comparison with 6.15% for the control device. Furthermore, the encapsulation-free PSC presents good long-term stability after storage in an air atmosphere with 70% RH at 20 °C or 0% RH at 80 °C as well as under continuous illumination conditions for 30 days. The significantly improved PCE and stability in high humidity or temperature suggest that the perovskite passivation by ILs is an effective strategy for fabricating high-PCE and stable PSCs.