Titanium-carbide MXenes for hole transporting layer in high-performance perovskite solar cells

Abstract

Over the past decade, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been increased to over 25% for the single junction. The remaining challenges for commercialization include scale production of large-area devices with good long-term stability and flexible devices for multiple applications. Among different device structures, the inverted (p-i-n) structure is beneficial for flexible devices due to the low hysteresis and simple fabrication. Particularly, the anode interfacial layer is of vital significance to the efficiency and stability of inverted PSCs. The hole transporting layer (HTL) as a significant component of perovskite solar cells has a substantial influence on light-harvesting, carrier extraction and transportation, perovskite crystallization, stability, and cost. Various HTLs have been investigated, such as polymers like poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and nanocrystals including NiOX, CuSCN, and MoO3, etc. MXenes materials encounter many glaring merits including high conductivity and tuneable work function (WF), which facilitates its application for optoelectronic devices. The first use of Ti3C2TX MXene in PSCs is as the additive of the perovskite absorber layer. Incorporating MXene with nanoparticles like SnO2 and TiO2 was reported to be a useful approach as the ETL of PSCs. Besides the use in ETL and HTL, Ti3C2 MXenes have also been reported to use in PSCs as an electrode in perovskite solar cells. However, similar to other two-dimensional materials, MXenes tend to stack and aggregate due to the strong van der Waals forces, which will lead to penetration and current leakage, which needs to be addressed for further use in devices. In this work, we demonstrate a self-promoted vertical-gradient Ti3C2Tx MXene structure-based PEDOT:PSS HTL. The unique gradient HTL is realized via a glucose-assisted Ti3C2Tx MXene self-dispersion strategy in which the addition of glucose and its half-caramelization process make the aggregated MXene nanosheets spontaneously exfoliate and redistribute on the top surface. Our results show that the self-organized MXene/glucose-rich composite concurrently tailors electrical properties of PEDOT:PSS, and promotes the quality and crystallization of perovskite film deposited on top of it. With the optimized gradient MXene HTL, we have fabricated different size of flexible PSCs based on MAPbI3 and Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3 (FA = formamidinium) perovskites. The best performed devices show PCE of 20.25% and 18.34% with an active area of 0.04 cm2 and 1.00 cm2, respectively. Finally, we assemble the flexible PSCs into a module with 15 cm2 in size and obtain a PCE of 16.28%. Meanwhile, the unencapsulated PSCs show remarkable long-term thermal stability at 85°C in ambient air (~90% efficiency retention after 100 hours) and extended operational lifetime (~90% efficiency retention after 220 hours). The work shows a promising future with the possibility of the self-assembled HTL for applications in photovoltaic devices.