Optimal film thickness and Sn oxidation state of sputter-deposited SnO2 electron transport layers for efficient perovskite solar cells†

Abstract

Ultra-thin SnO₂ films, fabricated at low temperatures, exhibit outstanding performance as electron transport layers (ETLs) in perovskite solar cells (PSCs). To better understand the electron transport characteristics of SnO₂ films, we investigated photovoltaic (PV) properties in relation to the film thickness and oxidation state of Sn. Herein, SnO₂ films were prepared by a novel two-step process: metallic Sn films were deposited using a sputtering technique, followed by heat treatment at various temperatures. This method offers facile control of the Sn oxidation state and prevents pinhole formation in the resulting SnO₂ films. We found that a SnO₂ ETL with a thickness of 15 nm provided the optimal power conversion efficiency (PCE), while increasing the thickness beyond 20 nm significantly decreased the PCE. Heat treatment temperatures were also varied during the conversion from Sn to SnO₂ films to control the oxidation states of Sn. An optimal PCE of 21.30% on average was achieved from the SnO₂ films heat-treated at 420 °C, whereas annealing at 470 and 520 °C resulted in relatively lower PCEs. X-ray photoelectron spectroscopy (XPS) analysis revealed that SnO₂ films heat-treated at 320, 370, 420, 470, and 520 °C contained 28%, 20%, 14%, 7%, and negligible levels of Sn²⁺, respectively. Hence, the presence of small amounts of Sn²⁺ and oxygen vacancies in ultra-thin SnO₂ films seems to have beneficial effects on PV performance, although they can also induce charge recombination. We also applied various photoelectrochemical analysis tools to analyze the electron transport and charge recombination properties of SnO₂ films prepared under different conditions.