Thieno[3,2- <i>b</i> ]thiophene-Based Organic Cation Forming Semitransparent Thin Film: Synthesis, Photophysical Properties, and Inverted Perovskite Solar Cells

Design and synthesis of small molecules for perovskite-based solar cells to improve the stability and power conversion efficiency are important issues. Organic cations have proven to be good solutions for this type of engineering. Herein, we present the design and easy synthesis of thieno[3,2-b]thiophene-based organic cation, TT-PhCH2NH3I, to be used as an additive for perovskite solar cell application. The salt forms an excellent smooth and semitransparent thin film, which was characterized by UV–vis, CV, XRD, SEM, and contact angle (CA) measurements. TT-PhCH2NH3I showed remarkable film properties of a uniform and smooth surface morphology with an RMS roughness of 65 nm and a superior hydrophilic surface of a 55.1° water droplet. It had a good adhesive surface. Its salt morphology was clarified in micrometer-sized layers of very compact sheets. Unlike general organic cations available in the literature, superior molecular engineering provided TT-PhCH2NH3I with an ammonium iodide group at its 3-position, leaving the active 2- and 5-positions available for further functionalization. TT-PhCH2NH3I was applied as an additive material to MAPbI3 in an inverted perovskite solar cell application. The solution-processed device demonstrated an excellent long-term cell stability, maintaining 97% of its initial PCE efficiency for 5 weeks (>800 h). These characteristics show that TT-PhCH2NH3I is a promising organic semiconductor to achieve a high performance in photovoltaic applications.