Molecular Layers Push Perovskite Tandems to 29.1%

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Molecular layers helped two perovskite tandem solar-cell designs combine high efficiency with improved resistance to heat, light and moisture, according to researchers at The Hong Kong University of Science and Technology (HKUST).

The studies, published in Joule and Nature Communications, target different tandem architectures but share the same strategy: engineering interfaces to control perovskite crystallization, reduce defects, improve charge transport and slow degradation.

“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.”

PEDOT:PSS-free tandem cells reach 29.1% efficiency

The Joule paper focuses on two-terminal monolithic all-perovskite tandem solar cells, which stack two perovskite absorbers with complementary band gaps in a single structure.

A key problem is the buried interface of the narrow-bandgap tin-lead perovskite subcell. High-performance devices commonly use PEDOT:PSS as a hole-transport material, but the polymer can absorb moisture, react unfavorably with perovskite precursors and encourage phase segregation during crystallization.

Using in situ characterization, the team found that PEDOT:PSS triggers an unstable crystallization pathway in mixed tin-lead perovskite films. It replaced the polymer with a phenothiazine-functionalized self-assembled monolayer called 4PAPT. The molecular layer promoted direct phase transition, improved crystal orientation and suppressed nonradiative recombination losses.

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That approach produced a narrow-bandgap single-junction perovskite cell with 23.2% efficiency. The researchers then developed a hybrid self-assembled monolayer interconnecting layer combining thiol and phosphonic acid anchoring groups on SnO2/Au surfaces.

The resulting PEDOT:PSS-free all-perovskite tandem achieved a reported 29.1% power conversion efficiency, described as the highest reported efficiency to date for PEDOT:PSS-free all-perovskite tandem configurations. Encapsulated devices retained 90% of their initial efficiency after more than 800 hours at around 40°C (104°F) under simulated one-sun illumination and maximum power point tracking.

“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.”

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All-inorganic tandems withstand extended heat testing

The Nature Communications study examined four-terminal all-inorganic perovskite tandem solar cells. Although all-inorganic perovskites may offer thermal and photostability, their surfaces remain vulnerable to moisture and defect-related energy losses.

The team used tetrabutylammonium trifluoromethanesulfonate (TTFS) to form a self-assembled one-dimensional/three-dimensional perovskite heterojunction on the absorber surface. Its cationic component created a hydrophobic barrier, while its anionic component passivated surface defects and supported electron extraction.

The resulting semitransparent wide-bandgap all-inorganic perovskite top cell achieved a certified 17.10% power conversion efficiency. Combined with a narrow-bandgap all-inorganic perovskite bottom cell in a four-terminal tandem, it reached a certified 21.54% efficiency, the highest certified efficiency reported for this type of tandem solar cell.

The devices retained 80% of their initial efficiency after 1,210 hours at 65°C (149°F) and after 650 hours at 85°C (185°F) under continuous one-sun maximum power point tracking.

Photoluminescence mapping, photoluminescence quantum yield mapping and quasi-Fermi-level splitting mapping helped the researchers connect interface structure with energy loss and carrier movement.

“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.”

“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.”

Publication details: Fengzhu Li et al., “Interface-mediated crystallization enables PEDOT:PSS-free all-perovskite tandems with 29.1% efficiency and enhanced durability,” Joule (2026), DOI: 10.1016/j.joule.2026.102501. Hao Zhang et al., “Self-assembled 1D/3D heterojunction enables all-inorganic perovskite 4-terminal tandem solar cells with 21.54% certified efficiency,” Nature Communications (2026), DOI: 10.1038/s41467-026-72099-z.