Tetrabutylammonium\nHydroxide-Functionalized Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXene for Significantly\nImproving the Photovoltaic Performance of Perovskite Solar Cells

An appropriate electron transport layer (ETL) or cathode\nbuffer\nlayer (CBL) is critical for high-performance perovskite solar cells\n(PVSCs). In this work, tetrabutylammonium hydroxide (TBAOH)-functionalized\nTi<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXene (TBAOH-Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub>) is developed\nto improve the photovoltaic performance of PVSCs. TBAOH-Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> is synthesized by HF\netching and then TBAOH intercalation, and TBAOH can effectively attach\nto the Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> surface\nduring the intercalation process. In hole transport material (HTM)-free\ncarbon-based PVSCs with the structure of ITO/ETL/MAPbI<sub>3</sub>/carbon, the SnO<sub>2</sub> doped by TBAOH-Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> (SnO<sub>2</sub>:TBAOH-Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub>) as ETL shows decreased\nWF and increased conductivity and improves the growth of the perovskite\nfilm with a larger grain and significantly reduced defects, which\nsynergistically facilitate charge transport and extraction and reduce\ncharge recombination. The HTM-free carbon-based PVSC with SnO<sub>2</sub>:TBAOH-Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> ETL exhibits a significantly higher PCE of 14.93% with enhanced\ndevice stability compared to the control device with pristine SnO<sub>2</sub> ETL (11.95%) and also outperforms most of the HTM-free carbon-based\nPVSCs with MAPbI<sub>3</sub> perovskite reported so far. In traditional\ninverted PVSCs with the structure of ITO/PTAA/MAPbI<sub>3</sub>/PCBM/CBL/Ag,\nthe TBAOH-Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> is utilized as a CBL to significantly enhance device performance\nwith a high PCE of 21.16%, which is obviously superior than that (16.26%)\nof the control device without CBL. The impressive results indicate\nthat tetrabutylammonium hydroxide-functionalized Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXene possesses great application\npotential in different functional layers for high-performance PVSCs.