Lead-based halide perovskite solar cells (PSCs) have gained significant attention due to their rapid performance advancements, with certified power conversion efficiencies (PCE) now reaching up to 26.7%, making them comparable to silicon solar cells. Recently, a perovskite/silicon tandem solar cell efficiency of 34.9% was reported, which exceeds the theoretical efficiency limit of 29.5% for a silicon single-junction cell [1]. Buckminsterfullerene (C₆₀) is the standard electron-transporting layer (ETL) used in high-efficiency p-i-n perovskite single-junction and perovskite/silicon tandem solar cells, owing to its ability to rapidly extract electrons from the perovskite absorber [2, 3], its relatively high electron mobility [4], and compatibility with thermal evaporation allowing for the deposition of thin, conformal layers over large substrate areas [5]. However, C₆₀ has several disadvantages such as high cost, interfacial non-radiative recombination [6], high parasitic absorption, and poor mechanical stability at the interface with the perovskite absorber as well as the SnO_x layer on top leading to interfacial delamination [7,8]. Organic electron-transporting materials (ETMs) have recently attracted much attention as an alternative to fullerene C₆₀ due to their low cost, straightforward synthesis as well as better adhesion with perovskite [9, 10].

In this work, fluorenylidenemalononitrile-substituted carborane derivatives were synthesized and investigated as non-fullerene ETM for perovskite-based solar cells. Charge mobility of the layers based on o,m,p-series carborane derivatives was determined by xerographic time-of-flight (XTOF) measurements. The samples for the measurements were prepared by evaporating carborane compounds on polyester films with conductive Al layer. As a result, the electron mobility of oCB-FMN, mCB-FMN and pCB-FMN was found to be 1×10^-5, 5×10^-5, and 5×10^-6 cm²/Vs at zero-electric field. The new materials were tested as ETMs and successfully applied in single-junction PSCs. During device fabrication all ETMs were deposited by thermal evaporation. Furthermore, m-carborane derivative demonstrated best performance exhibiting a PCE >20% and revealing an efficiency potential that surpasses control devices with C₆₀. The new carborane derivatives demonstrate lower interfacial non-radiative recombination losses and reduced parasitic absorption in tandem devices for wavelength shorter than 650 nm, which highlights their potential as alternatives to C₆₀.