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.