The future photovoltaic technologies based on perovskite materials are aimed to build low tech, truly economical, easily fabricated, broadly deployable, and trustworthy solar cells. Hole transport material (HTM) free perovskite solar cells (PSCs) are among the most likely architectures which hold a distinctive design and provide a simple way to produce large-area and cost-effective manufacture of PSCs. Notably, in the monolithic scheme of the HTM-free PSCs, all layers can be printed using highly reproducible and morphology-controlled methods, and this design has successfully been demonstrated for industrial-scale fabrication. In this review article, we comprehensively describe the recent advancements in the different types of mesoporous (nanostructured) and planar HTM-free PSCs. In addition, the effect of various nanostructures and mesoporous layers on their performance is discussed using the electrochemical impedance spectroscopy (EIS) technique. We bring together the different perspectives that researchers have developed to interpret and analyze the EIS data of the HTM-free PSCs. Their analysis using the EIS tool, the limitations of these studies, and the future work directions to overcome these limitations to enhance the performance of HTM-free PSCs are comprehensively considered.

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Perovskite solar cells (PSCs) expressed great potentials for offering a feasible alternative to conventional photovoltaic technologies. 2D/3D hybrid PSCs, where a 2D capping layer is used over the 3D film to avoid the instability issues associated with perovskite film, have been reported with improved stabilities and high power conversion efficiencies (PCE). However, the profound analysis of the PSCs with prolonged operational lifetime still needs to be described further. Heading towards efficient and long-life PSCs, in-depth insight into the complicated degradation processes and charge dynamics occurring at PSCs' interfaces is vital. In particular, the Au/HTM/perovskite interface got a substantial consideration due to the quest for better charge transfer; and this interface is debatably the trickiest to explain and analyze. In this study, multiple characterization techniques were put together to understand thoroughly the processes that occur at the Au/HTM/perovskite interface. Inquest analysis using current–voltage (I–V), electric field induced second harmonic generation (EFISHG), and impedance spectroscopy (IS) was performed. These techniques showed that the degradation at the Au/HTM/perovskite interface significantly contribute to the increase of charge accumulation and change in impedance value of the PSCs, hence resulting in efficiency fading. The 3D and 2D/3D hybrid cells, with PCEs of 18.87% and 20.21%, respectively, were used in this study, and the analysis was performed over the aging time of 5000 h. Our findings propose that the Au/HTM/perovskite interface engineering is exclusively essential for attaining a reliable performance of the PSCs and provides a new perspective towards the stability enhancement for the perovskite-based future emerging photovoltaic technology.

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Despite the remarkable progress in perovskite solar cells (PSCs), their instability and rapid degradation over time still restrict their commercialization. A 2D capping layer has been proved to overcome the stability issues; however, an in-depth understanding of the complex degradation processes over a prolonged time at PSC interfaces is crucial for improving their stability. In the current work, we investigated the stability of a triple cation 3D ([(FA0.83MA0.17)Cs0.05]Pb(I0.83Br0.17)3) and 2D/3D PSC fabricated by a layer-by-layer deposition technique (PEAI-based 2D layer over triple cation 3D perovskite) using a state-of-art characterization technique: electrochemical impedance spectroscopy (EIS). A long-term stability test over 24 months was performed on the 3D and 2D/3D PSCs with an initial PCE of 18.87% and 20.21%, respectively, to suggest a more practical scenario. The current-voltage (J-V) and EIS results showed degradation in both the solar cell types; however, a slower degradation rate was observed in 2D/3D PSCs. Finally, the quantitative analysis of the key EIS parameters affected by the degradation in 3D and 2D/3D PSCs were discussed.