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