Improving Transparency and Efficiency in Semi-Transparent PSCs through ALD-TiO2 ETL

The growing interest in integrating semi-transparent (ST) perovskite solar cells (PSCs) into urban and agricultural infrastructures draws attention to the challenging trade-off between light transmittance and power conversion efficiency (PCE) in ST devices [1-2]. The use of transparent and conductive charge transport layers is essential for achieving high electrical and optical performance in ST-PSCs [3]. For this reason, a charge transport layer with controlled thickness and surface roughness is necessary to reduce parasitic absorption and reflection losses [4]. Indeed, the atomic layer deposition (ALD) has emerged as an effective technique for controlling both surface morphology and thickness [5]. Additionally, ALD produces compact and pinhole-free films with optimal substrate coverage. This prevents charge recombination at the interface and enhances the films' optical transparency across most of the visible spectrum [6]. In this context, the effect of TiO2 electron transport layer (ETL) deposited by ALD in ST-PSCs is here investigated. An accurate analysis has been conducted to optimise the thickness, ranging from 5 to 20 nm, and the ALD deposition methodology, plasma or thermal, of the TiO2 layer. The improved TiO2 film, 20 nm thick by thermal ALD, was combined with SnO2 thin film creating an ETL bilayer to enhance the charge extraction. In addition, different perovskite formulations were investigated to enhance both average visible transmittance (AVT) and PCE of the ST-PSCs. Comprehensive films and devices' characterization, including UV-Vis spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction, helped in identifying the optimal layers for ST-PSCs. The ALD-TiO2/SnO2 ETL bilayer demonstrated enhanced AVT and PCE, and a superior light utilisation efficiency (LUE=AVT×PCE) when compared, through external quantum efficiency (EQE) and current-voltage measurements, to the most common TiO2 by spray pyrolysis (SP), also employed in a bilayer, and to the SnO2 stand-alone ETL. Indeed, the ALD-based ST-PSC showed reduced light loss in the photon balance check (EQE+Reflectance+Transmittance≤1) and increased perovskite grain size in the SEM imaging. These results suggest that the ALD-TiO2/SnO2 bilayer effectively improves, at the same time, both optical transmittance and electrical performance, paving the way for ST-PSCs with balanced AVT and PCE and thus enhanced LUE values. References: [1] Giuliano, G.; Bonasera, A.; Arrabito, G.; Pignataro, B. Semitransparent Perovskite Solar Cells for Building Integration and Tandem Photovoltaics: Design Strategies and Challenges. Solar RRL 2021, 5 (12), 2100702. [2] Gorjian, S.; Bousi, E.; Özdemir, Ö. E.; Trommsdorff, M.; Kumar, N. M.; Anand, A.; Kant, K.; Chopra, S. S. Progress and challenges of crop production and electricity generation in agrivoltaic systems using semi-transparent photovoltaic technology. Renewable and Sustainable Energy Reviews 2022, 158, 112126. [3] Bing, J.; Caro, L. G.; Talathi, H. P.; Chang, N. L.; McKenzie, D. R.; Ho-Baillie, A. W. Y. Perovskite solar cells for building integrated photovoltaics-glazing applications. Joule 2022, 6 (7), 1446-1474. [4] Ma, L.; Xu, X.; Cui, C.; Li, T.; Lou, S.; Scott, P. J.; Jiang, X.; Zeng, W. A Review of Measurement and Characterization of Film Layers of Perovskite Solar Cells by Spectroscopic Ellipsometry. Nanomaterials 2025, 15 (4), 282. [5] Johnson, R. W.; Hultqvist, A.; Bent, S. F. A brief review of atomic layer deposition: from fundamentals to applications. Materials today 2014, 17 (5), 236-246. [6] Wang, C.; Zhao, D.; Grice, C. R.; Liao, W.; Yu, Y.; Cimaroli, A.; Shrestha, N.; Roland, P. J.; Chen, J.; Yu, Z. Low-temperature plasma-enhanced atomic layer deposition of tin oxide electron selective layers for highly efficient planar perovskite solar cells. Journal of Materials Chemistry A 2016, 4 (31), 12080-12087.