Deciphering the Role of Halide Engineering in Enhancing Outdoor and Indoor Photovoltaic Performance of DMSO-Free Tin Halide Perovskites

With the exponential growth of the Internet of Things (IoTs) ecosystem, the urgency to explore sustainable energy solutions has become paramount. At this juncture, indoor photovoltaics (IPV) emerge as a promising candidate to power IoT devices. Lead halide perovskites have demonstrated power conversion efficiencies (PCE) exceeding 42% under standard indoor illumination (1000 lx),¹ but their inherent toxicity and high recycling costs render them unsuitable for IoT applications. Thus, the need to explore lead-free perovskites has paved the way for development of tin-perovskite solar cells (tin-PSCs). Despite their potential for high efficiency and stability, tin-PSCs face the critical challenge of minimizing defect density, primarily caused by the oxidation of Sn(II) to Sn(IV). Recent studies have revealed that dimethyl sulfoxide (DMSO),² the widely used solvent in tin PSC fabrication, is a major contributor to tin oxidation. To overcome these detrimental effects, we employed DMSO-free solvent mixture for tin PSC fabrication. Further, the bandgap of the active material should be precisely tuned to match with the light spectrum as the optimal bandgap should be ~1.1-1.4 eV, and ~1.8-2.0 eV for OPV and IPV, respectively.³ In this context, we employ halide engineering strategy to enhance the PCE of DMSO-free tin perovskites. We fabricated the device with the p-i-n configuration by replacing iodide with bromide. All the composition crystallizes in orthorhombic Amm2 crystal structure without any impurities. The bandgap of the compound increase from 1.4 eV to 1.7 eV by increasing the bromide content in the perovskite framework. Steady-state emission spectra showed an increase in emission intensity at low Br concentration followed by a decline at higher Br levels, suggesting suppression of trap-assisted non-radiative recombination at lower Br levels. This trend was reflected in the PCE obtained from J-V characterizations under 1-sun where the V_OC increases and J_SC decreases with bandgap widening. Under 1-sun, the control device achieved a maximum PCE of 7.4%, which was enhanced to 8.0% for low Br content. However, further increases in Br ratio led to a decrease in PCE due to a sharp decline in J_SC. In contrast, under standard indoor illumination (1000 lx), the champion PCE of 11% was observed for high Br content, while the control device showed a PCE of 8.2%. Unlike the 1-sun condition, the PCE increases with increasing the bandgap of the perovskite composition under 1000 lx illumination. In this talk, we shall describe the the interplay between bandgap, defect density, hole mobility, energy alignment, and trap density for wide band gap tin perovskites. These findings establish a new benchmark for DMSO-free tin perovskites, particularly in the context of indoor photovoltaics.

References:
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