Researchers from India's Indian Institute of Technology (IIT) Mandi have developed semi-transparent perovskite solar cells (ST-PSCs) based on compositionally tuned MAPbI₂.₆Br₀.₄ films, demonstrating a well-balanced trade-off between transparency and efficiency for building-integrated photovoltaics (BIPVs) and indoor energy harvesting.
The team engineered the perovskite absorber by systematically substituting methylammonium bromide (MABr) with methylammonium iodide (MAI) in MAPbI₃ precursors, while simultaneously varying precursor concentrations from 1 M down to 0.15 M. This dual strategy enabled precise control over both halide composition and film thickness - two key parameters governing optical transmission and photovoltaic performance.
Reducing precursor concentration led to thinner films, directly increasing transparency. The optimized films achieved average visible transmittance (AVT) values of 26.69% for ~100 nm thickness and 33.11% for ~49 nm thickness, surpassing the typical ~25% benchmark required for practical semi-transparent applications. However, excessive thinning can introduce defects and trap states, increasing recombination losses. The study addresses this by combining thickness control with bromine incorporation, which stabilizes the crystal lattice, suppresses ion migration, and improves charge transport.
From a device perspective, the MAPbI₂.₆Br₀.₄ composition benefits from partial substitution of iodide with bromide ions, which widens the bandgap and enhances transmittance while maintaining favorable photovoltaic properties. This compositional tuning also contributes to improved open-circuit voltage, reduced non-radiative recombination, and enhanced overall device stability.
The researchers evaluated device performance under both outdoor one-sun illumination and indoor LED lighting, highlighting the dual-use potential of these ST-PSCs. Devices fabricated with precursor concentrations between 1 M and 0.30 M retained over 90% of their initial efficiency over time, indicating strong operational stability.
Among all tested conditions, the 0.30 M device emerged as the optimal configuration. It delivered a power conversion efficiency (PCE) of 9.83% under outdoor conditions and 16.08% under indoor lighting, alongside an AVT of 26.69%. Importantly, this device also achieved light utilization efficiency (LUE) values of 2.62% (outdoor) and 4.29% (indoor). Since LUE - defined as the product of AVT and PCE - is a critical metric for BIPV applications, these values fall within or above the desirable 2-5% range for window-integrated photovoltaics.
The performance gains are said to stem from the interplay between bandgap tuning and thickness optimization. Bromine incorporation increases the bandgap, allowing more visible light to pass through, while reduced precursor concentration yields thinner films that further enhance transparency. At the same time, maintaining sufficient film quality and crystallinity ensures efficient charge generation and extraction, preventing the typical efficiency losses associated with ultra-thin absorbers.
Overall, this work establishes a practical pathway for designing ST-PSCs that simultaneously meet key BIPV requirements - high transparency, competitive efficiency, and strong light utilization - under both indoor and outdoor conditions. The integrated evaluation framework and material optimization strategy provide valuable guidance for future development of semi-transparent photovoltaic technologies targeting energy-generating windows and self-powered indoor devices.
