All-solution-processed inorganic CsPbBr3 solar cells and their bifacial-irradiation functions†

Solution-processed technologies for (semi)transparent top electrodes remain suboptimal, although see-through perovskite solar cells (s-PSCs) are required in realizing window-integrated photovoltaics. Herein, we choose an inorganic perovskite, CsPbBr₃, offering the best matching example with wavelength-selective transparency and weatherability, and present the simplest s-PSC excluding organic components, fluorine-doped tin oxide (FTO)/TiO₂/CsPbBr₃/single-walled carbon nanotube (SWNT). The semitransparent electrode is realized by solution-processed filter-transferred SWNT thin films with different transmittances of 60–80%T at 550 nm. The diode ideal factors range between 1 and 2, suggesting high heterojunction qualities as a single-diode model with hole-transfer-layer-free CsPbBr₃/SWNT. Under monofacial pseudo-sunlight irradiation through FTO, the increased electrical conductivities (densities) of SWNT improve power-conversion efficiencies/short-circuit currents, PCEs (FTO)/J_sc = 8.68/7.49 (60) > 8.18/7.33 (70) > 7.30%/6.91 mA cm⁻² (80%T). Through SWNT, the increased transparency improves PCEs (SWNT)/J_sc inversely as 4.21/3.79 (60) < 4.45/4.13 (70) < 4.56%/4.56 mA cm⁻² (80%T). Open-circuit voltages/fill factors are 1.48/0.79 through FTO and 1.33 V/0.84 through SWNT (60%T). A tradeoff exists between the conductivities and transparency to achieve high performance. Bifacial irradiation using light-emitting diodes shows close values of PCEs (bifacial) = 3.67 (60), 3.86 (70), and 3.71% (80%T) based on 32–35% of pseudo-sunlight power (100 mW cm⁻²), equivalent to the sums of the monofacial-irradiation PCEs (FTO) and PCEs (SWNT). Enhancement ratios of PCEs (bifacial) to PCEs (SWNT)/to PCEs (FTO) are 3.19/1.36 (60), 3.04/1.46 (70), and 2.81/1.53 (80%T). The bifacial function solves the monofacial tradeoff. The black color of SWNT is not a serious obstacle visually under exterior environments.