Inorganic Hole Transport Materials for Advancing n-i-p Perovskite Solar Cells: A Comprehensive Review

The rapid advancement of perovskite solar cells (PSCs) through high-power conversion efficiencies (PCEs) and low fabrication costs made them a potential candidate for the next generation of photovoltaic technology. Although, the inverted (p-i-n) configuration of the PSCs has recently gained attention due to low temperature production, the regular (n-i-p) architecture remains a benchmark model for inorganic HTLs due to high PCEs and well characterized interfacial energy levels. Hole transport layer plays a vital role in extracting photogenerated holes and minimizing charge recombination and energy losses in n-i-p architecture. While organic materials like spiro-OMeTAD have dominated HTL research, their limitations in stability, cost, environmental sustainability, and scalability have steered interest in inorganic alternatives. This comprehensive review systematically explores recently progress in inorganic HTLs for n-i-p PSCs, focusing on metal oxides, metal chalcogenides and emerging inorganic compounds. Important aspects of the HTLs required for enhancing the PSCs, including optical properties, energy gap, band alignment, deposition techniques and interfacial engineering strategies with emphasis on their influence on PCEs, stability and commercial viability is carried out. By consolidating recent advancements and identifying remaining key challenges, this review offers a critical foundation for advancing the design and optimization of efficient, stable and scalable n-i-p PSCs.