Innovative MXenes Carrier Selective Contact for Sb₂Se₃ & CZTSe Hybrid Heterostructure Devices

MXenes are a recently discovered family of inorganic 2D materials combining high conductivity, transparency in ultrathin layers, work function tunability via surface termination and ease of fabrication in large areas, which has found widespread possible applications in energy storage and conversion technologies. As carrier selective contacts become ubiquitous in state-of-the-art photovoltaic research, the use of MXene electrodes is a promising pathway to further improve thin film devices beyond the classic p-n junction architecture. Studies remain scarce and <b>a fundamental understanding of the absorber/MXene interface</b>, with a fine tuning of the 2D electrode, is of high interest for the community.<br/>Through this work, we realize photovoltaic devices combining Ti₃C₂Tx as a charge (electron/hole) selective contact layer with Sb₂Se₃ and Cu₂ZnSnSe₄, two of the most promising emergent thin film PV absorbers. MXenes sheets are synthetized by selectively etching a Ti₃AlC₂ (MAX) phase using the minimum intensive layer delamination (MILD) method, forming in-situ HF allowing the selective removal of aluminum from the bulk MAX phase. Secondly, following centrifugation and washing processes, the obtained Ti₃C₂T_x solution is deposited on FTO, Mo and ITO substrates with different MXenes concentrations and thicknesses by spray-coating at temperatures ranging from 100 ^oC up to 200 ^oC, which permits to obtain <b>an excellent reproducibility and homogeneity over areas up to 100 cm²</b>. It is to our knowledge is the first time that spray-coating deposition of MXene is assessed for energy applications.<br/>Subsequently, surface modifications with selenium Se are performed on the as-deposited Ti₃C₂T_x to <b>modify the surface termination and consequently the material’s work function</b>, allowing to finely tune the electrode and adapt it to the desired extraction energy level. Se deposition is carried out by thermal evaporation of the element followed by a reactive annealing phase in Se atmosphere. The resulting selenized MXene sheets are used as a substrate for the fabrication of Sb₂Se₃ and CZTSe hybrid heterostructures. An extensive Raman spectroscopy analysis reveals that following the selenization process, the <b>Se crystallographic structure deposited on the MXenes surface can be modified to be either an equilibrium between Se₈ rings and Se</b><b>_µ</b><b> chains or strictly Se₈ rings</b>. For the sake of comparison, other surface modifications involving organic compounds such as citric acid or perfluorinated ionomer (PFI) and UV-Ozone treatment are performed, with the similar objectives to tune the surface termination of as-deposited MXenes for selective carrier extraction. The devices are completed following the standard CdS/TCO front contact architecture for solar cells and are characterized using room temperature and temperature-dependent optoelectronic analyses methods, shedding light on the mechanisms involved in the photocarrier extraction at the absorber/MXenes interface.<br/>Results of optoelectronic characterizations in the form of J-V, EQE and C-V for the devices will be presented to elaborate on the impact of MXenes concentrations and thickness on PV devices performance, and the <b>selenization of MXene is found to markedly improve the voltage, current, and conversion efficiency</b> of the resulting PV devices. Additionally, material characterizations by means of SEM, XRD, XPS and Raman spectroscopy are carried out to assess the physico-chemical properties of the as-deposited layers (both MXenes and absorber) along with the MXenes surface termination modifications. The results will be discussed in the frame of the state of the art and compared with reference devices fabricated on crystalline Silicon. Finally, improvement pathways will be proposed to further tailor the properties of MXenes film to the needs of thin film photovoltaic devices.