Molecular Dipole Assisted Tuning of Work Function in Ti₃C₂T_x/n-Si and Ti₃C₂T_x/p-Sb₂Se₃ Heterojunctions

MXene-class materials have emerged as promising candidates in areas currently dominated by graphene, transition metal dichalcogenide (TMD) and related 2D materials, and hold several assets for applications to energy related fields including storage and conversion. Since the first report of Ti₃C₂T_x synthesis in 2011 [1], this ever growing family of materials has garnered significant attention from the research community owing to their remarkably tunable physicochemical properties and ease for large area synthesis using low-cost methods. Ti₃C₂T_x MXene has been utilized as a functional layer/additive in various opto-electronic devices such as solar cells, photodetectors etc. [2]. Ti₃C₂T_x has particularly shown remarkably promising results when used as charge selective contacts in conventional c-Si and Perovskite solar cells [3]. The high conductivity, fully tunable work function, and thickness dependent transparency coupled with a high chemical stability make Ti₃C₂T_x an ideal material to partner with Si and possibly other emerging chalcogenide thin film absorbers as a replacement of the standard p-n junction.<br/>In the present work, we aim at investigating the interplay of a model MXene/c-Si interface with different organic molecular dipole interlayers in terms of carrier transport, carrier selectivity, and chemical compatibility. Initially, a screening of different organic dipoles is performed to identify the most suitable candidates out of PEDOT, PEI, PAMAM, APTES. Multilayer MXene is synthesized from the bulk Ti₃AlC₂ MAX phase through Minimum Intensive Layer Delamination (MILD) technique and 2D sheets are subsequently deposited by drop casting and spray coating on the n-Si/dipole stack. Optimization routes are investigated to tune the band alignment at the MXene/n-Si interface by using the self-assembled mono layers of different organic dipoles. An in-depth characterization of the as prepared MXene sheets by means of XPS, Raman Spectroscopy, AFM and other methods is presented to assess the chemical and structural properties of the heterojunctions, along with the complete electrical characterization of the MXene/n-Si Schottky diodes where the fundamental properties of the interface formed between the 2D charge selective contact and the 3D semiconductor are investigated. Additionally, optoelectronic properties of first proof of concept devices, both solar cells as well as photodetectors, are reported and the complete set of results is presented in regard to the state of the art. Finally, the use of MXene in combination with emerging Sb₂Se₃ photovoltaic absorbers is investigated using the c-Si as reference framework.<br/><b>References:</b><br/>[1] Naguib, Michael, et al. "Two Dimensional Nanocrystals: Two Dimensional Nanocrystals Produced by Exfoliation of Ti₃AlC₂ (Adv. Mater. 37/2011)." <i>Advanced Materials</i> 23.37 (2011): 4207-4207.<br/>[2] Wang, Yizhou, et al. "MXenes for energy harvesting." <i>Advanced Materials</i> (2022): 2108560.<br/>[3] Aydin, Erkan, et al. "Scaled Deposition of Ti₃C₂T_x MXene on Complex Surfaces: Application Assessment as Rear Electrodes for Silicon Heterojunction Solar Cells." <i>ACS nano</i> 16.2 (2022): 2419-2428.