Unveiling the Role of tBP–LiTFSI Complexes in Perovskite Solar Cells

Abstract

Lead halide-based perovskite materials have been applied as an intrinsic layer for next-generation photovoltaic devices. However, the stability and performance reproducibility of perovskite solar cells (PSCs) needs to be further improved to match that of silicon photovoltaic devices before they can be commercialized. One of the major bottlenecks that hinders the improvement of device stability/reproducibility is the additives in the hole-transport layer, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP). Despite the positive effects of these hole-transport layer additives, LiTFSI is hygroscopic and can adsorb moisture to accelerate the perovskite decomposition. On the other hand, tBP, the only liquid component in PSCs, which evaporates easily, is corrosive to perovskite materials. Since 2012, the empirical molar ratio 6:1 tBP:LiTFSI has been wildly applied in PSCs without further concerns. In this study, the formation of tBP−LiTFSI complexes at various molar ratios has been discovered and investigated thoroughly. These complexes in PSCs can alleviate the negative effects (decomposition and corrosion) of individual components tBP and LiTFSI while maintaining their positive effects on perovskite materials. Consequently, a minor change in tBP:LiTFSI ratio results in huge influences on the stability of perovskite. Due to the existence of uncomplexed tBP in the 6:1 tBP:LiTFSI mixture, this empirical tBP−LiTFSI molar ratio has been demonstrated not as the ideal ratio in PSCs. Instead, the 4:1 tBP:LiTFSI mixture, in which all components are complexed, shows all positive effects of the hole-transport layer components with dramatically reduced negative effects. It minimizes the hygroscopicity of LiTFSI, while lowering the evaporation speed and corrosive effect of tBP. As a result, the PSCs fabricated with this tBP:LiTFSI ratio have the highest average device efficiency and obviously decreased efficiency variation with enhanced device stability, which is proposed as the golden ratio in PSCs. Our understanding of interactions between hole-transport layer additives and perovskite on a molecular level shows the pathway to further improve the PSCs' stability and performance reproducibility to make them a step closer to large-scale manufacturing.

Influence of Lithium and Lanthanum Treatment on TiO2 Nanofibers and Their Application in n‐i‐p Solar Cells

“…We would like to note that the lithium salt, Bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSI), is widely employed as a beneficial additive in the PSCs in both the ETL and HTL . Specifically in TiO 2 , LiTFSI has been shown to downshift the conduction band, resulting in higher photocurrents and improved electron transport within the mesoporous TiO 2 by partial reduction of Ti 4+ to Ti 3+ .…”

“…Despite this, the benefits of Li have been most commonly associated with increased conductivity and charge densities, both of which are more favorable in the p ‐type dopant LiTFSI . While it is known that LiTFSI is a vital additive in the HTL for PSCs, its benefit to the TiO 2 layer have only been observed using LiTFSI. This suggests that the TFSI − anion is playing a crucial role in performance enhancement and can likely be associated with its conductivity .…”

1000 h Operational Lifetime Perovskite Solar Cells by Ambient Melting Encapsulation

“…Literatures have shown that oxygen and moisture trigger phase segregation and decomposition of absorber layer . Meanwhile, the Spiro‐OMeTAD is degraded faster with the assistance of moisture . In addition, the thermal stability tracked at 85 °C is shown in Figure S12 (Supporting Information), and the UVCA with paraffin encapsulated device also present much better stability than that without paraffin‐encapsulated devices.…”

Accurately Stoichiometric Regulating Oxidation States in Hole Transporting Material to Enhance the Hole Mobility of Perovskite Solar Cells

“…Pristine 2,2′,7,7′‐Tetrakis( N,N ‐ di‐p ‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD), the most commonly used HTM in PSCs, suffers from low hole mobility and thus exhibits a relatively poor efficiency . To achieve high‐efficiency PSCs, chemical dopants such as Li‐TFSI and Co(III) salts are necessary for spiro‐OMeTAD to increase the conductivity . Actually, Li‐TFSI is a chemical mediator that can promote the reaction between spiro‐OMeTAD and O 2 in atmosphere .…”