In order to improve the efficiency of perovskite solar cells (PSCs), careful device design and tailored interface engineering are needed to enhance optoelectronic properties and the charge extraction process at the selective electrodes. Here, we use two-dimensional transition metal carbides (the MXene Ti3C2TX) with various termination groups (TX) to tune the work function (WF) of the perovskite absorber and the TiO2 electron transport layer (ETL), and to engineer the perovskite/ETL interface. Ultraviolet photoemission spectroscopy measurements and Density Functional Theory calculations show that the addition of Ti3C2TX to halide perovskite and TiO2 layers permits to tune the materials' WFs, without affecting other electronic properties. Moreover, the dipole induced by the Ti3C2TX at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified PSCs, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without Mxene.

MXenes, two-dimensional transition metal carbides or nitrides, have recently shown great promise for gas sensing applications. We demonstrate that the sensitivity of intrinsically metallic Ti3C2T x MXene can be considerably improved via its partial oxidation in air at 350 °C. The annealed films of MXene sheets remain electrically conductive, while their decoration with semiconducting TiO2 considerably improves their chemiresistive response to organic analytes at low-ppm concentrations in dry air, which was used to emulate practical sensing environments. We demonstrate that partially oxidized MXene has a faster and a qualitatively different sensor response to volatile analytes compared to pristine Ti3C2T x . We fabricated multisensor arrays of partially oxidized Ti3C2T x MXene devices and demonstrate that in addition to their high sensitivity they enable a selective recognition of analytes of nearly the same chemical nature, such as low molecular weight alcohols. We investigated the oxidation behavior of Ti3C2T x in air in a wide temperature range and discuss the mechanism of sensor response of partially oxidized MXene films, which is qualitatively different from that of pristine Ti3C2T x .

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