Abstract: In this paper, two novel D-π-A structured small molecular triphenylamine derivatives, were synthesized with classical reactions such as Stille coupling and Suzuki coupling. The structure of H457 is based on triarylamine scaffold, where the nitrogen atom of the aniline core is substituted by two p-methoxyphenyl groups, while the para-position of the aniline is linked to a 5-(4-pyridyl)-2-thienyl conjugated unit, forming a D-π-A (donor-π-acceptor) molecular framework. In this system, the p-methoxyphenyl groups act as electron donors (D), and the thiophene-pyridine moiety serves as the π-bridge and electron acceptor (A). The structure of H459 is based on a triarylamine scaffold, wherein the amino nitrogen is substituted by two 4-methoxyphenyl groups, while the para-position of the aniline is conjugated to a 2,3-dihydro-7-(4-pyridyl) thiophene moiety, constituting a D-π-A (donor-π-acceptor) molecular framework. In this system, the 4-methoxyphenyl groups serve as electron donors (D), the dihydrothiophene ring provides a partially conjugated π-bridge, and the 4-pyridyl group acts as the electron acceptor (A). The small molecular derivatives were deposited onto FTO/c-TiO2/m-TiO2/CsPbI3 composite films with crystallization modification and surface post-treatment modification to fabricate CsPbI3 perovskite solar cells. The champion power conversion efficiency of modified CsPbI3 perovskite solar cells reached 15.82%. The devices were characterized with SEM, XRD, J-V curves. SEM analysis revealed a significant reduction in intergranular gaps among the modified CsPbI3 crystallites, accompanied by improved film flatness and complete elimination of structural defects, XRD results confirm the successful synthesis of black-phase CsPbI3, while demonstrating that modification with the triphenylamine-based small molecule does not alter the crystalline structure of CsPbI3. The mechanisms for the improvement of the modified CsPbI3 perovskite solar cell were investigated with electrochemical impedance spectroscopy, cyclic voltammetry, UV-Vis absorption spectrum, steady-state fluorescence spectrum. Cyclic voltammetry measurements determined the HOMO and LUMO energy levels of the triphenylamine-based small molecule derivatives. Electrochemical impedance spectroscopy revealed reduced charge transfer resistance and increased recombination resistance in devices modified with these triphenylamine derivatives. Steady-state photoluminescence spectroscopy demonstrated significantly lower emission intensity for H459 modified CsPbI3 films compared to H457 modified counterparts. Moreover, the stability and operational lifespan of the modified devices were also improved.

Key words: Power conversion efficiency, D-π-A structured, Small molecular triphenylamine derivative, CsPbI₃ perovskite solar cell