Efficient Perovskite Solar Cells with Improved Carrier Transport via Composition-Graded Titanium Oxynitride Electron Transport Layer

State-of-the-art perovskite solar cells (PSCs) have achieved power conversion efficiencies (PCEs) exceeding 25% by enhancing the charge transport layers to facilitate efficient carrier transport while minimizing non-radiative recombination. The effective transport of charge carriers through the perovskite and the charge transport layers can improve the fill factor and open-circuit voltage. An ideal electron transport layer (ETL) should possess complete and conformal coverage, as well as optimal band alignment that facilitates efficient extraction of electrons. Additionally, the minimal defect density in charge transport layers is essential to prevent detrimental interface recombination. While TiO₂ has been widely used as an ETL, it exhibits limitations as an ideal ETL due to an energy level mismatch and relatively poor electrical characteristics. Specifically, the conduction band minimum (CBM) of TiO₂ is slightly higher than that of MAPbI₃, leading to a significant energy barrier for electron extraction. Additionally, the relatively low conductivity and mobility of TiO₂ hinder efficient charge transfer. To address the limitations of TiO₂, metal doping such as Nb, Al, and Li has been employed to modify its electronic band structures and improve its electrical properties.<br/>In this study, nitrogen is incorporated into TiO₂ using the pulsed laser deposition (PLD) method to form titanium oxynitride (TiO_xN_y), resulting in enhanced electron transport characteristics including minimal defect density. Controlled substrate temperature and oxygen partial pressure during the deposition facilitated the achievement of optimal thin-film characteristics, including high transparency typical of TiO₂ semiconductors and high conductivity typical of nitrogen-rich TiN. Furthermore, TiO_xN_y thin films with graded atomic ratio nitrogen to oxygen (N/O) are achieved by sequentially changing the oxygen partial pressure during the process. The graded TiO_xN_y thin film exhibits characteristics that enable the achievement of superior functionality as an ETL compared to TiO_xN_y thin films without grading. By increasing the oxygen partial pressure during the deposition process, the CBM was raised, leading to a reduced energy offset with the mesoporous TiO_x layer directly above it. Furthermore, TiO_xN_y fabricated in a multilayer structure can provide a continuous variation of energy levels within the ETL, optimizing the electron pathway and improving charge transport, ultimately enhancing charge extraction capabilities. This alignment facilitates the selective transfer of electrons to the respective electrodes while inhibiting hole transport. The accurate composition control of TiO_xN_y films is characterized using such as atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Hall measurement to uncover the mechanism behind the enhanced properties. The conformally deposited films with high crystallinity and tailored optoelectrical properties exhibit effective suppression of interfacial recombination as an ETL, resulting in solar cell devices and resulting in high PCEs.