en.Wedoany.com Reported - A research team from the Hong Kong University of Science and Technology has designed a fully PEDOT:PSS-free all-perovskite tandem solar cell, achieving a conversion efficiency of 29.1% by using a phenothiazine-functionalized phosphonic acid monolayer as the hole transport layer. PEDOT:PSS, a conductive polymer mixture, is widely used as a hole transport layer due to its high transparency, good hole conductivity, and suitable work function. However, its acidity and hygroscopicity degrade the sensitive perovskite layers in tandem cells, affecting long-term efficiency and operational stability.
To overcome this challenge, the research team used in-situ characterization to reveal the unstable crystallization pathway induced by PEDOT:PSS in tin-lead mixed perovskite films and replaced PEDOT:PSS with a self-assembled phenothiazine-functionalized monolayer (abbreviated as 4PAPT). Compared to PEDOT:PSS, 4PAPT enables faster, more direct, and more uniform perovskite crystallization, suppresses the formation of the tin iodide-dimethyl sulfoxide (SnI₂–DMSO) intermediate phase, improves crystal orientation and interface stability, thereby reducing defect density and enhancing carrier transport.
The team developed a two-terminal monolithic all-perovskite tandem solar cell with a device architecture stacked on an indium tin oxide (ITO) transparent electrode. The bottom cell includes a wide-bandgap (WBG) perovskite absorber layer, interconnected via a carbazole-based naphthalene derivative (CbzNaph) as the hole-selective layer, followed by a fullerene (C60) electron transport layer, atomic layer deposited tin dioxide (ALD-SnO₂) as the recombination layer, and a gold (Au) electrode. The top cell employs 4PAPT as the hole transport interface, combined with a narrow-bandgap (NBG) perovskite absorber layer, a C60 electron transport layer, a bathocuproine (BCP) exciton blocking layer, and a silver (Ag) back electrode. The 4PAPT-based narrow-bandgap single-junction cell achieved an efficiency of 23.2%, while the PEDOT:PSS-free all-perovskite tandem cell achieved a conversion efficiency of 29.1%, the highest reported for this type. Encapsulated devices retained 90% of their initial efficiency after 800 hours of maximum power point tracking under simulated one-sun illumination at approximately 40°C.
Corresponding author Fengzhu Li noted that the molecular interface strategy enabled efficiency improvements in narrow-bandgap single-junction cells and was extended to tandem cells. Co-author Yen-Hung Lin emphasized that the instability of PEDOT:PSS is not only a material issue but also affects the formation of perovskite films at the buried interface; replacing it with a molecularly designed self-assembled monolayer can control crystallization from the outset. The study was published in the journal Joule under the title "Interface-mediated crystallization enables PEDOT:PSS-free all-perovskite tandems with 29.1% efficiency and enhanced durability."
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