Mechanically Robust and Electrically Conductive C60-SnO₂ Electron Transporting Bi-Layer for Perovskite Solar Cells

Inverted perovskite solar cells (i-PSCs) have demonstrated superior power conversion efficiencies (PCEs). However, they encounter considerable challenges regarding long-term stability due to their fragile interfaces, such as C60 and SnO₂. These interfaces are vulnerable to degradation from thermal cycling. Utilizing robust C60-SnO₂ bilayers presents a highly effective approach in order to overcome challenges like delamination at the interface of C60 and SnO₂. One key strategy is the application of atomic layer deposition (ALD) combined with an ozone treatment of C60, which can enhance mechanical robustness and electrical conductivity. The importance of C60-SnO₂ bilayers lies in their ability to improve charge transport and stability, which contributes to the overall efficiency and durability of i-PSCs. The present study focuses on optimizing the ALD process of SnO₂ on ozone-treated C60 surfaces as electron transport bi-layers (ETLs). The deposition of SnO₂ by ALD on ozone-treated C60 surfaces led to a reduced incubation time, as evidenced by an increase in growth per cycle (GPC) with a constant value. Furthermore, the conductive ohmic contact achieved by the bi-layer of C60-SnO₂ was proven by current–voltage characteristics of Schottky devices. It is expected to be reduced in the charge recombination demonstrated by steady-state photoluminescence (PL) measurements. Additionally, the ozone treatment decreases the series resistance between C60 and SnO₂, as shown by electrochemical impedance spectroscopy (EIS). The treatment also significantly enhanced the interface adhesion between C60 and SnO₂, confirmed through mechanical delamination tests using 3M tape and further analyzed via X-ray photoelectron spectroscopy (XPS) to identify the weakest interface. Consequently, the PCE of i-PSCs were 22.3% for untreated samples and 22.6% for those with ozone treatment. The i-PSCs retained 80.3% of their initial performance without ozone treatment and 96.0% with ozone treatment under operating conditions at 85°C. This study highlights the effectiveness of surface modification at the C60-SnO₂ interface as ETLs in i-PSCs, leading to enhanced efficiency and stability. Furthermore, the use of low-temperature ALD processes can mitigate perovskite degradation, contributing to improved device performance.