Semi-Transparent Perovskite Solar Cells Design and Fabrication

Abstract

Semi-transparent solar cells represent a transformative opportunity for energy harvesting in both urban and farming environments. They enable the integration of photovoltaic technologies into buildings and greenhouses without compromising natural lighting. Metal halide perovskites continue to be highly promising thanks to their tunable bandgaps, and strong absorption coefficients. Perovskite solar cells (PSCs) exhibit a record transmittance of 84.6% 1 in the visible range, and can be fabricated on transparent glass, and thanks to the low-temperature fabrication processes polymer substrates are also possible. We focus on formamidinium halide perovskites (FASnI 3 ), which enable tailoring of optical and electronic properties through halide composition tuning. Recent studies have shown that halide mixing not only adjusts optical behavior but also improves film morphology and stability 2 . Additionally, the ZnO electron transport layer (ETL) provides high transparency and favorable energy level alignment 3 , while Spiro-OMeTAD remains one of the most effective hole transport layers (HTLs) for maintaining efficiency in semi-transparent perovskite solar cell (ST-PSC) configurations. In our research, we are developing ST-PSCs based on the FTO/ZnO/FASnI 3 /Spiro-OMeT/Au architecture, designed to balance solar absorption with visible light transmission. The ultimate goal is to control the optical bandgap and transmission spectrum to selectively harvest light in the UV to yellow spectral regions. Furthermore, in a tandem cell structure, the NIR portion of the solar spectrum can be harvested by a bottom cell, for instance silicon cell. To achieve the required transparency, we use density functional theory (DFT) and hybrid functionals (i.e., HSE06) in Quantum ESPRESSO to model the electronic structure of the FASnI (3-x) Br x system and optimize optical properties. By varying the bromine concentration, x, we adjust the halide composition to modulate the bandgap and absorption onset. Full atomic relaxation of the lattice for the perovskite prototype atomistic model is conducted to ensure the ground-state configuration and thermodynamic stability, minimizing formation energy at both cationic and anionic sites. This is followed by energy band structure computation and analysis to evaluate band alignment. The optical absorption is then derived from the calculated dielectric functions ε(ω)= ε 1 (ω)+ i ε 2 (ω), enabling the identification of material compositions that offer optimal trade-offs between solar energy absorption and visible transparency. The fabricated absorber, guided by theoretical optimization, is characterized using scanning confocal Raman spectroscopy and conductive atomic force microscopy (c-AFM), alongside measurements of transmission and reflection. The 3D Raman capability of our nanoprobe system allowed detecting gradients in composition and strains across the absorber. Carrier recombination at grain boundaries is investigated using high resolution Light Beam Induced Current (HR-LBIC) setup, added to a WITec Near field Scanning Optical Microscope (NSOM) with a true 200 nm or 80 nm optical aperture. Furthermore, both scanning photoluminescence mapping and 3D Raman mapping are performed and spatially correlated with the recombination current to analyze the composition and spatial distribution of chemical elements forming the recombination centers. The solar cell device is optimized using SCAPS-1D software, utilizing both geometric parameters and optical properties, to balance transparency and efficiency. This enables better solar spectrum utilization and ensures the required semi-transparency. We investigated the effect of the absorber composition on the absorption spectrum, noting that the absorber thickness should be lower than the diffusion length to achieve optimal performance. The energy band structure calculated using DFT/GGA-PBE shows a band gap energy of 1.36 eV. The deduced absorption coefficient indicates that the absorption of FASnI3 can be tuned to extend into the high-energy part of the spectrum, reducing the absorption above 500 nm, which reflects the intended transparency of the perovskite absorber. Acknowledgements: NSF EiR (Award# DMR 2401243), NSAM-ML Project (Award# DE-NA0004112, DOE-NNSA), and CINT User Grant (UNPUA No. 1409-07_2024). [1] Tianran Liu et al, Adv. Energy Mater. 2023, 13, 2200402. [2] Junjie Ma, et al, Matter, Volume 4, Issue 1, 2021, Pages 313-327 [3] Youssef El Arfaoui, et al, Eur. Phys. J. Appl. Phys. Volume 98, 2023. . Figure 1