Bandgap Engineering for Efficient Perovskite Solar Cells Under Multiple Color Temperature Indoor Lighting

Perovskite indoor photovoltaics (PIPVs) are emerging as a transformative technology for low-light intensity energy harvesting, owing to their high power conversion efficiencies (PCEs), low-cost fabrication, solution-processability, and compositionally tunable band gaps. In this work, methylammonium-free perovskite absorbers were compositionally engineered to achieve band gaps of 1.55, 1.72, and 1.88 eV, enabling matching the spectral photoresponse with the indoor lighting. Devices based on a scalable mesoscopic n-i-p architecture were systematically evaluated under white LED illumination across correlated color temperatures (3000-5500 K) and light intensities from 250 to 1000 lux with active area of 1 cm2. The 1.72 eV composition exhibited the most promising performance across different light intensities and colors, achieving PCEs of 35.04 % at 1000 lux and 36.6 % at 250 lux, with a stable device operation of over 2000 hours. On the other hand, the 1.88 eV band-gap variant reached a peak PCE of 37.4 % under 250 lux (5500 K), however performance trade-offs were observed across the different color lights LEDs. Our combined experimental and theoretical optical-electrical simulations suggest that decreasing trap-assisted recombination in wide-bandgap compositions may further improve PIPV performance across the different illumination conditions. In contrast, devices with 1.55 eV band gap underperformed in such conditions due to suboptimal spectral overlap and utilization. These findings establish bandgap optimization and device architecture as key design principles for high-efficiency, stable PIPVs, advancing their integration into self-powered electronic systems and innovative indoor environments.

💡 Research Summary

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This paper addresses the challenge of achieving high‑efficiency, stable perovskite indoor photovoltaics (PIPVs) under the diverse spectral conditions of indoor lighting. The authors avoid the volatile methylammonium (MA) cation by employing a methylammonium‑free composition based on cesium (Cs) and formamidinium (FA) mixed on the A‑site of the perovskite lattice (CsₓFA₁₋ₓPb(I₁₋ᵧBrᵧ)₃). By varying the iodide‑to‑bromide ratio, three band‑gap values—1.55 eV, 1.72 eV, and 1.88 eV—are obtained, each tailored to match the emission spectra of white LEDs with correlated color temperatures (CTs) of 3000 K, 4000 K, and 5500 K.

The study uses a scalable mesoscopic n‑i‑p architecture (FTO/c‑TiO₂/m‑TiO₂/Perovskite/Spiro‑OMeTAD/Au) with an active area of 1 cm², thereby moving beyond the small‑area devices (≤0.1 cm²) that dominate the literature. Perovskite films are deposited from a 1.3 M precursor solution, yielding thicknesses of 300–400 nm, which the authors identify as optimal through both experimental variation and optical‑electrical simulations.

Comprehensive material characterization confirms successful band‑gap tuning: photoluminescence peaks shift from ~800 nm (1.55 eV) to 662 nm (1.88 eV); X‑ray diffraction shows systematic lattice contraction with increasing Br content; scanning electron microscopy reveals compact, pinhole‑free films, while grain size decreases and surface roughness increases with higher bromide fractions. These morphological trends correlate with simulated reductions in carrier lifetime at high Br concentrations, suggesting a trade‑off between band‑gap widening and recombination losses.

Device performance is evaluated under three CTs and three lux levels (1000, 500, 250 lux). The 1.55 eV cells deliver open‑circuit voltages (Voc) of 0.82–0.90 V and short‑circuit currents (Jsc) that scale linearly with light intensity, but their power conversion efficiencies (PCEs) remain limited to 28–31 % because the lower band gap yields insufficient voltage under indoor spectra. The 1.72 eV devices achieve the most balanced results: Voc of 0.92–1.01 V, Jsc up to 132 µA cm⁻², fill factors (FF) of 74–78 %, and PCEs ranging from 32 % to a peak of 36.6 % (5500 K, 250 lux). This band gap aligns with theoretical Shockley‑Queisser analyses that predict optimal indoor efficiencies for Eg≈1.7–1.8 eV.

The 1.88 eV cells exhibit the highest Voc (0.96–1.04 V) but suffer from reduced Jsc (<126 µA cm⁻²) due to limited absorption of red photons, especially under warm (3000 K) illumination. Nevertheless, under cool white light (5500 K) at low intensity (250 lux) they reach a record indoor PCE of 37.4 % for devices with ≥1 cm² active area.

To rationalize these observations, the authors perform full‑wave optical simulations that generate spatially resolved carrier generation profiles for each LED spectrum and intensity. These profiles feed into drift‑diffusion electrical models, reproducing the experimental J‑V curves. The modeling highlights that trap‑assisted recombination becomes the dominant loss mechanism in the wide‑bandgap (1.88 eV) compositions; reducing trap density would simultaneously raise Voc and FF, closing the gap to the theoretical limit.

Long‑term stability is demonstrated for the 1.72 eV devices, which retain performance over 2000 hours of continuous operation at 250 lux (5500 K). The MA‑free composition together with the robust n‑i‑p stack (FTO/TiO₂) underpins this durability, addressing a key obstacle for commercial indoor PV deployment.

Overall, the work establishes three design principles for high‑performance indoor perovskite photovoltaics: (1) band‑gap engineering to match indoor spectra, (2) avoidance of volatile A‑site cations for stability, and (3) adoption of scalable n‑i‑p mesoscopic architectures. The authors suggest future directions including surface/interface passivation to suppress trap‑mediated recombination, optimization of electron‑transport layers for high‑CT illumination, and integration with real‑world IoT devices to validate practical power delivery. This study moves PIPVs from laboratory curiosities toward viable, large‑area, self‑powered indoor energy solutions.