Progress and Challenges of SnO₂ Electron Transport Layer for Perovskite Solar Cells: A Critical Review

Abstract

Organic inorganic halide perovskites have drawn great attention in the past decade, due to their superior photovoltaic performance with an efficiency over 25%. For planar heterojunction structure perovskite solar cells (PSCs) tin oxide based electron transport layers (ETLs) have become one of the most suitable candidates to replace titanium oxide to make flexible devices because of their low‐temperature processing. The deposition techniques of SnO2 can be categorized into chemical deposition, such as sol‐gel, chemical bathing or atomic layer deposition, and physical deposition, such as thermal evaporation or sputtering. Depending on the deposition technique, defects, and morphology in the SnO2 layer may vary drastically, leading to poor performance of PSCs. In this review, we have provided a comprehensive picture of the recent progress and challenges of different SnO2 ETL deposition techniques and the performance of PSC devices. The additional modifications on SnO2 mentioned in this article are also effective ways to eliminate the intrinsic defects in the film. The drawbacks and benefits of SnO2 ETLs and the corresponding actions for this are also discussed. We hope this review will help with the comprehensive understanding of the relationship between the property of SnO2 and its structure of ETLs to enhance PSCs' performance.

Recent Criterion on Stability Enhancement of Perovskite Solar Cells

“…Moreover, in heterojunction planar-structured devices, they act as hole-blocking layers (HBLs) that suppress the unwanted transitions of photo-generated holes from the active perovskite layer to the FTO-substrate [179]. In addition, ETLs also act like a scaffold in the mesoporous structured PSCs to support the formation of the active perovskite layer [180][181][182]. However, several groups of researchers have demonstrated some ETL-free devices but ultimately, they could not outstrip the ETL-incorporated PSCs in terms of stability and efficiency [183].…”

“…A number of inorganic materials such as metal oxides (e.g., TiO 2 [13,14,184], ZnO [185,186], SnO 2 [181,182,187], selenides (e.g., CdSe [188]), sulfides (e.g., CdS [143,189]), mp-Al 2 O 3 [190], fullerene [191], PCBM (i.e., phenyl-C 61 -butyric acid methyl ester) [192], as well as various ternary metal oxides (e.g., SrTiO 3 [193], BaSnO 3 [194,195], Zn 2 SO 4 [196,197]) have been introduced by many researchers. However, the majority of the research to date has focused on TiO 2 -based ETLs, from the very initial stage of its journey due to its low-cost availability and high photo-conversion performance under different experimental conditions.…”

Molecular design for perovskite solar cells

“…229 SnO 2 possesses a wider bandgap and a deeper conduction band than TiO 2 , allowing the formation of the ohmic contact with the halide perovskite layer and improved interfacial electron charge process that can be synthesized at low temperature. 230 Incorporating the organic materials into SnO 2 is also common to improve the photovoltaic performance. However, the molecular passivation strategy for the SnO 2 / perovskite interface and that for the TiO 2 /perovskite is different, because of the different energy level positions of the two materials with respect to the perovskite layer, while alternative functional groups such as triphenylphosphine, 231 tetraacetic acid, [232][233][234] and ethoxylated polyethyleimine [235][236][237] have been identified to be the more effective organic moieties for SnO 2 due to the different undercoordinated surface species between TiO 2 and SnO 2 .…”

“…Different hierarchical structures of polymers are studied, including the amino‐functionalized copolymer 228 and the metal oxide‐in‐polymer matrix that helps modify the ETL layer 229 . SnO 2 possesses a wider bandgap and a deeper conduction band than TiO 2 , allowing the formation of the ohmic contact with the halide perovskite layer and improved interfacial electron charge process that can be synthesized at low temperature 230 . Incorporating the organic materials into SnO 2 is also common to improve the photovoltaic performance.…”

Metal-Doped TiO2 Thin Film as an Electron Transfer Layer for Perovskite Solar Cells: A Review

“…Various metal oxides, such as TiO2 [16][17][18], ZnO2 [19], and SnO2 [20,21], are used as ETL materials. Among these, TiO2 is commonly used due to its simple device structure, high thermal stability, low cost, and high compatibility with flexible substrates [22][23][24].…”