Researchers from Soochow University, National Taiwan University and Chang Gung University have developed a co-assembled self-assembled monolayer (SAM) strategy that tackles long-standing buried-interface limitations in inverted perovskite solar cells by combining the widely used Me-4PACz (Me4) with two newly designed donor–acceptor molecules, LYS-H and its fluorinated analogue LYS-F.
Image credit: NTU
In conventional inverted architectures, Me-4PACz SAMs serve as hole-selective layers but often suffer from solution aggregation, poor perovskite precursor wettability, insufficient interfacial contact and suboptimal crystallinity at the buried perovskite interface, all of which contribute to non-radiative recombination losses and limit device performance. To overcome these issues, the team designed LYS-H and LYS-F with a donor-acceptor backbone that strengthens the interfacial dipole, thereby facilitating more efficient hole extraction while introducing functional groups that can effectively passivate defects at the buried perovskite surface.
A key feature of LYS-H and LYS-F is the triphenylamine (TPA) group, whose multi-directional phenyl rings can engage in π–π stacking with the carbazole units of Me4, suppressing Me4 aggregation and yielding a more uniform mixed SAM at the transparent electrode. This co-assembly not only improves the morphological uniformity of the interfacial layer but also significantly enhances wettability toward the perovskite precursor solution, enabling more controlled spreading and coverage during film deposition. As a result, the overlying perovskite layer grows with improved crystallinity and fewer buried interfacial defects, which supports more efficient charge transport and reduces non-radiative losses inside the device stack.
Between the two donor–acceptor SAMs, fluorination in LYS-F plays a central electronic role: it deepens the highest occupied molecular orbital (HOMO) level and increases the effective work function of the SAM-modified electrode, giving better energetic alignment with the perovskite valence band. This more favorable interfacial energy-level alignment in the Me4+LYS-F system helps to suppress interfacial recombination and promote selective hole extraction, while the defect passivation from the tailored functional groups further reduces trap-assisted losses at the buried interface.
Device measurements highlight the benefits of this molecular-engineering approach. Perovskite solar cells incorporating the Me4+LYS-F co-assembled SAM achieve a power conversion efficiency of 25.02% and a high fill factor of 83.64%, outperforming devices based on pristine Me4 and demonstrating the strongest performance among the tested SAM configurations. Importantly, both Me4+LYS-H and Me4+LYS-F devices show substantially improved operational stability compared to Me4-only controls, with unencapsulated cells stored in air at around 30% relative humidity and 25 °C under dark conditions maintaining performance significantly better over time.
By demonstrating that carefully designed donor–acceptor SAMs can co-assemble with Me-4PACz to reduce aggregation, enhance perovskite film growth at the buried interface, and simultaneously boost both efficiency and stability, this work outlines a promising interface-engineering route toward more practical and commercially relevant perovskite photovoltaics.
“This work shows how molecular design can overcome buried-interface limitations and unlock more efficient, stable perovskite solar cells,” said co-corresponding author Chu-Chen Chueh, Ph.D., professor of chemical engineering at National Taiwan University.
