UV- and NIR-Protective Semitransparent Smart Windows Based on Metal Halide Solar Cells

In this study, a solution-processable lead iodide semiconductor having a wide band gap was investigated as a light absorbing material for various organic electron transport materials, in a search for low-cost semiconductor materials allowing the facile fabrication of efficient photovoltaic devices. A Tauc plot suggested a wide intrinsic optical band gap of 2.4 eV for a thin film of PbI₂, while X-ray diffraction revealed that the spin-coated PbI₂ thin film had a hexagonal crystalline structure with preferable orientation along the (001) plane. The effect of the light intensity on the values of V_oc and J_sc was studied to investigate the charge recombination mechanism of fabricated devices. An efficient bifacial solar cell was prepared featuring a thin Ag film sandwiched between BCP and MoO₃ layers as a transparent rear electrode. The whole device featuring the BCP/Ag/MoO₃ electrode exhibited a maximum transmittance of approximately 60% in the visible region, less than 15% in the UV region, and less than 25% in the NIR region. A power conversion efficiency of 2.19% was achieved for a device featuring an opaque electrode (Ca/Al), while the corresponding device featuring the transparent electrode (BCP/Ag/MoO₃) provided values of 0.75% and 0.67% when illuminated from the front and rear, respectively. Thus, wide band gap metal halide materials potentially open up a new path for fabricating efficient and transparent photovoltaic devices having applications as building-integrated smart windows. It also effectively prevents the penetration of UV and NIR light, which is harmful for human health, into the building.