Self‐Aggregation‐Controlled Rapid Chemical Bath Deposition of SnO₂ Layers and Stable Dark Depolarization Process for Highly Efficient Planar Perovskite Solar Cells

Abstract

Planar perovskite solar cells (PSCs) incorporating n‐type SnO2 have attracted significant interest because of their excellent photovoltaic performance. However, the film fabrication of SnO2 is limited by self‐aggregation and inhomogeneous growth of the intermediate phase, which produces poor morphology and properties. In this study, a self‐controlled SnO2 layer is fabricated directly on a fluorine‐doped tin oxide (FTO) surface through simple and rapid chemical bath deposition. The PSCs based on this hydrolyzed SnO2 layer exhibit an excellent power conversion efficiency of 20.21 % with negligible hysteresis. Analysis of the electrochemical impedance spectroscopy on the charge transport dynamics indicates that the bias voltage influences both interfacial charge transportation and the ionic double layer under illumination. The hydrolyzed SnO2‐based PSCs demonstrate a faster ionic charge response time of 2.5 ms in comparison with 100.5 ms for the hydrolyzed TiO2‐based hysteretic PSCs. The results of quasi‐steady‐state carrier transportation indicate that a dynamic hysteresis in the J–V curves can be explained by complex ionic‐electronic kinetics owing to the slow ionic charge redistribution and hole accumulation caused by electrode polarization, which causes an increase in charge recombination. This study reveals that SnO2‐based PSCs lead to a stabilized dark depolarization process compared with TiO2‐based PSCs, which is relevant to the charge transport dynamics in the high‐performing planar SnO2‐based PSCs.

Hydrolysis-Regulated Chemical Bath Deposition of Tin-Oxide-Based Electron Transport Layers for Efficient Perovskite Solar Cells with a Reduced Potential Loss

“…42 Hydrolysis is the primary mechanism involved in the CBD of SnO 2 films and is significantly affected by transitional activation energy and equilibrium of the chemical species. 9,42 Moreover, these physicochemical properties affect the quality, microstructure, and chemical state of the fabricated film. 42 Therefore, the partial oxidation of the Sn ion to the Sn 2+ state is likely to occur during CBD, and the morphological quality of the corresponding film is consequently inferior (Figure 4b).…”

“…From the viewpoint of energy, the barriers responsible for hysteresis in the J−V curves do not exist at the interface with the perovskite in all of the samples. 9 The driving force determined using the difference between the conduction band minimum values of the ETLs and that of the perovskite appears to be a minimum in the Glyc−SnO 2 film at 0.06 eV, potentially benefitting the extraction of electrons from the photoactive perovskite layer with minimum potential loss. 27 PSCs employing the developed SnO 2 film were subsequently configured and characterized to investigate the influence of the fabricated films on device performance (Figure 5e).…”

“…Metal-halide-based semiconducting perovskite materials have attracted considerable attention because of their potential to exceed the contribution of crystalline Si solar cells in the photovoltaic (PV) industry. − The power conversion efficiency (PCE) of perovskite photovoltaics, which was first reported by Miyasaka et al to be 3.8%, has been improved to beyond 25% primarily by strengthening the engineering of fine perovskite films. − Investigations on reinforcing the stability of these devices have indicated the pivotal role of charge selective layers in the vicinity of the perovskite absorber in enhancing long-term stability and performance credibility, based on properties such as current density–voltage ( J – V ) hysteresis. − For example, TiO 2 , which is extensively used as an electron transport layer (ETL), is known to cause perovskite degradation and efficiency overestimation, primarily due to its photocatalytic reaction with perovskite absorbers. − Consequently, chemically stable metal–oxide materials that form an electrical ohmic contact with the perovskite layer are critical for suppressing the parasitic ionic reactions occurring at the interface. , …”

“…Low-temperature solution-processed SnO 2 films fabricated via spin-coating are known to significantly enhance the durability of the perovskite layer under UV irradiation and realize high-performance devices. , Spin-coating methods based on solution processing are currently preferred for fabricating films using SnO 2 for ETL-based applications. ,, However, studies on solution-processed SnO 2 ETLs have uncovered that SnO 2 films prepared via spin-coating are highly susceptible to environmental conditions during fabrication. , The demanding conditions used to deposit SnO 2 on substrates tend to produce films with poor morphologies, optical haziness, and unreliable reproducibility. , Notably, the scalability of spin-coating-based approaches is known to be significantly affected by the characteristics of the substrate surface at various spin rates, leading to an imperfect hole-blocking ability with several pinholes and cracks. , These surface defects can generate a leakage current and carrier recombination losses by localized defect states at the conduction band edge when connecting to the perovskite absorber . Recently, techniques based on chemical bath deposition (CBD), which covers the entire activated surface, have been employed to resolve these issues while facilitating high selectivity for charge carriers with suppressed recombination. ,,,− CBD is a straightforward and cost-effective method that can be efficiently utilized on an industrial scale. However, the intermediate phase of the SnO 2 precursor used in CBD is prone to producing large agglomerated particles that can lead to an inferior film morphology; , therefore, the deposition of a SnO 2 film during CBD typically takes a few hours and involves low concentrations to suppress aggregate formation. ,,, The very time-consuming approach for CBD counters its methodological advantages and limits its versatile application on an industrial scale, and elaboration on the aggregation behavior is still lacking.…”

“…Recently, techniques based on chemical bath deposition (CBD), which covers the entire activated surface, have been employed to resolve these issues while facilitating high selectivity for charge carriers with suppressed recombination. ,,,− CBD is a straightforward and cost-effective method that can be efficiently utilized on an industrial scale. However, the intermediate phase of the SnO 2 precursor used in CBD is prone to producing large agglomerated particles that can lead to an inferior film morphology; , therefore, the deposition of a SnO 2 film during CBD typically takes a few hours and involves low concentrations to suppress aggregate formation. ,,, The very time-consuming approach for CBD counters its methodological advantages and limits its versatile application on an industrial scale, and elaboration on the aggregation behavior is still lacking. Therefore, the development and elucidation of aggregation-controlled SnO 2 films via CBD are crucial for advances in the perovskite solar cells and large-scale commercialization for the PV industry.…”