Bithiophene\nImide-Based Self-Assembled Monolayers\n(SAMs) on NiOx for High-Performance Tin Perovskite Solar Cells Fabricated\nUsing a Two-Step Approach

Three new bithiophene\nimide (BTI)-based organic small molecules, <b>BTI-MN-b4</b> (<b>1</b>), <b>BTI-MN-b8</b> (<b>2</b>), and <b>BTI-MN-b16</b> (<b>3</b>), with varied\nalkyl side chains, were developed and employed as self-assembled monolayers\n(SAMs) applied to NiOx films in tin perovskite solar cells (TPSCs).\nThe NiOx layer has the effect of modifying the hydrophilicity and\nthe surface roughness of ITO for SAM to uniformly deposit on it. The\nside chains of the SAM molecules play a vital role in the formation\nof a high-quality perovskite layer in TPSCs. The single crystal structure\nof <b>BTI-MN-b8</b> (<b>2</b>) was successfully obtained,\nindicating that a uniform SAM can be formed on the NiOx/ITO substrate\nwith an appropriate size of the alkyl side chain. By combining <b>BTI-MN-b8</b> (<b>2</b>) with NiOx, a maximum PCE of 8.6%\nwas achieved. The TPSC devices utilizing the NiOx/<b>BTI-MN-b8</b> configuration demonstrated outstanding long-term stability, retaining\n∼80% of their initial efficiency after 3600 h. Comprehensive\ncharacterizations, including thermal, optical, electrochemical, and\nmorphological analyses, alongside photovoltaic evaluation, were carried\nout thoroughly. This study presents a pioneering strategy for improving\nTPSC performance, highlighting the efficacy of combining organic SAMs\nwith NiOx as the HTM and offering a promising pathway for future advances\nin TPSC technology using a two-step fabrication approach.