Solar cells' efficiency and lifespan are often determined by what happens at interfaces — the microscopic boundaries where different materials meet. Researchers from The Hong Kong University of Science and Technology (HKUST) have contributed to two recently published studies that show molecular interface engineering can unlock major gains in the efficiency and durability of next-generation perovskite tandem solar cells.
The two studies, published in academic journals Joule (impact factor: 37.1) and Nature Communications , focus on different tandem architectures but convey one common message: carefully designed molecular layers can do far more than connect materials. They can guide how perovskite films crystallize, suppress energy-wasting defects, promote charge transport and protect solar cells against degradation.
The two studies were jointly led by Prof. LIN Yen-Hung, Assistant Professor of the Department of Electronic and Computer Engineering at HKUST, and Dr. Fion YEUNG Sze-Yan, Senior Manager at the State Key Laboratory of Displays and Opto-Electronics , leveraging HKUST’s expertise in perovskite interface design, optical characterization and tandem device physics. Dr. LI Fengzhu, Research Assistant Professor of the Department of Electronic and Computer Engineering at HKUST, played a leading role in the Joule study, while Ms. ZHANG Qingqing, a PhD student of the Department of Electronic and Computer Engineering at HKUST , was a key HKUST contributor to the Nature Communications study.
Prof. Lin said, "Perovskite tandem solar cells have reached a stage where every interface matters. These two studies highlight a shared principle: molecular interfaces can be designed as active platforms to control crystallization, reduce energy loss, facilitate charge transport, and improve long-term stability across different tandem architectures."
The study published in Joule , titled " Interface-mediated crystallization enables PEDOT:PSS-free all-perovskite tandems with 29.1% efficiency and enhanced durability ", lists Dr. Li as co-first author and one of the corresponding authors. It focuses on two-terminal monolithic all-perovskite tandem solar cells. These devices stack two perovskite absorbers with complementary band gaps in one structure, offering a promising route to surpass the efficiency limits of single-junction solar cells while retaining the advantages of lightweight and potentially low-cost manufacturing.
A major challenge lies at the buried interface of the narrow-bandgap tin-lead perovskite subcell. Many high-performance devices rely on PEDOT:PSS as a hole-transport material, but this polymer can absorb moisture, interact unfavorably with perovskite precursors and promote phase segregation during crystallization. These issues can undermine both device performance and stability.
The research team used in-situ characterization to reveal that PEDOT:PSS induces an unstable crystallization pathway in mixed tin-lead perovskite films. To address this issue, the team then replaced PEDOT:PSS with a phenothiazine-functionalized self-assembled monolayer, known as 4PAPT, which promotes direct phase transition, improves crystal orientation, and suppresses non-radiative recombination losses.
This molecular interface strategy enabled a narrow-bandgap single-junction perovskite cell with 23.2% efficiency. The team further applied the strategy to monolithic all-perovskite tandem solar cells by developing a hybrid self-assembled monolayer interconnecting layer that combines thiol and phosphonic acid anchoring groups on SnO 2 /Au surfaces. The resulting dense molecular interlayer maintained efficient charge transport while avoiding the instability associated with PEDOT:PSS.
The PEDOT:PSS-free all-perovskite tandem solar cell achieved a reported power conversion efficiency of 29.1%, representing the highest reported efficiency to date for PEDOT:PSS-free all-perovskite tandem configurations. Encapsulated devices retained 90% of their initial efficiency after continuous operation for more than 800 hours at around 40°C under simulated one-sun illumination and maximum power point tracking conditions.
Dr. Li noted, "The instability of PEDOT:PSS is not only an issue with the material itself; it also affects how the perovskite film forms at the buried interface. By replacing this polymer with molecularly designed self-assembled monolayers, we were able to control crystallization from the start and carry that benefit into high-efficiency tandem devices."
The second study, published in Nature Communications and titled " Self-assembled 1D/3D heterojunction enables all-inorganic perovskite 4-terminal tandem solar cells with 21.54% certified efficiency ", casts a spotlight on a complementary route: four-terminal all-inorganic perovskite tandem solar cells. All-inorganic perovskites have attracted considerable attention due to their potential thermal and photo-stability, but their surfaces are vulnerable to moisture-induced degradation and defect-related efficiency losses.
To address these challenges, the research team developed an in-situ self-assembly strategy using tetrabutylammonium trifluoromethanesulfonate (TTFS) to form a one-dimensional/three-dimensional perovskite heterojunction on the surface of the all-inorganic perovskite absorber. The molecule performs two complementary functions: its cationic component forms a hydrophobic barrier that helps block moisture, while its anionic component passivates surface defects and supports efficient electron extraction.
Using this strategy, the team achieved a certified power conversion efficiency of 17.10% for a semi-transparent wide-bandgap all-inorganic perovskite top cell. When paired with a narrow-bandgap all-inorganic perovskite bottom cell in a four-terminal tandem configuration, the device reached a certified efficiency of 21.54%, representing the highest certified efficiency for this type of tandem solar cell. The devices also demonstrated strong operational stability, maintaining 80% of their initial efficiency after 1,210 hours at 65°C and 650 hours at 85°C under continuous one-sun maximum power point tracking conditions.
Ms. Zhang contributed to the study together with Dr. Yeung, providing guidance on photoluminescence mapping, photoluminescence quantum yield mapping, and quasi-Fermi-level splitting mapping. These optical mapping techniques helped reveal how the engineered interface reduced energy losses and improved carrier dynamics across the perovskite film.
Dr. Yeung noted, "Across the two studies, our shared focus was to understand what happens at the interface before losses show up in device performance. Optical and optoelectronic characterization allows us to connect molecular design with how charges move, recombine, and ultimately determine solar-cell efficiency."
Ms. Zhang said, "Through spatial optical mapping, we could visualize how the engineered 1D/3D interface reduces energy losses across the film. This provided important evidence that molecular interface design can improve both the performance and stability of all-inorganic perovskite solar cells."
These two papers demonstrate how molecular interface engineering can drive perovskite tandem photovoltaics toward high efficiency and long-term stability. The Joule study advances the two-terminal monolithic all-perovskite route by removing an unstable polymer interface and controlling buried-interface crystallization. The Nature Communications study advances the four-terminal all-inorganic route by constructing a protective and charge-selective heterojunction. Both point to a common conclusion: molecularly engineered interfaces are key to moving perovskite solar cells closer to long-term practical operation.
The Joule study was conducted by HKUST in collaboration with the City University of Hong Kong, Southern University of Science and Technology, the University of Oxford and other institutions. The Nature Communications study was carried out in collaboration with researchers from Peking University Shenzhen Graduate School, Ludong University, Shenzhen Polytechnic University and other partners.
Joule
Experimental study
Not applicable
Interface-mediated crystallization enables PEDOT:PSS-free all-perovskite tandems with 29.1% efficiency and enhanced durability
10-Jun-2026