Researchers from Nanjing University, JA Solar, Chongqing University, Jiangsu New Energy Development Co. and Jiangsu Guoxin Research Institute have designed a new self-assembled monolayer (SAM) additive molecule that fixes a longstanding defect problem at the buried hole-transport interface of inverted perovskite solar cells, enabling a wide-bandgap perovskite cell with a champion power conversion efficiency (PCE) of 23.7% and a perovskite/silicon tandem device that reaches 33.3% (certified at 33.1%), with no measurable PCE loss after 30 days of outdoor operation.
Device configuration illustration and chemical structures of Me and TTA. Image from: Science Advances
In inverted perovskite solar cells, the SAM sitting beneath the perovskite layer governs how well the perovskite crystallizes and how efficiently charge is extracted at that buried interface. The carbazole-based SAM molecule Me-4PACz is widely used for its strong hole-extraction properties, but it tends to self-aggregate when processed from alcoholic solvents, producing uneven coverage with island-like clusters and pinholes. That patchy coverage weakens binding to both the substrate below and the perovskite above, raises interface defect density, and impairs charge transport - a problem that has proven difficult to solve without sacrificing some other aspect of performance.
To address this, the team designed a new molecule, 5,5′,5″-(nitrilotri-4,1-phenylene)tris[2-thiophenecarboxylic acid], abbreviated TTA, built around three complementary features. Its noncoplanar molecular configuration - the molecule's arms are not held flat in the same plane - suppresses the π-π stacking between adjacent molecules that normally drives Me-4PACz's self-aggregation, allowing more regulated molecular packing instead. TTA also carries tridentate carboxylic acid (-COOH) anchoring groups, which, aided by the noncoplanar structure's freedom of rotation, can bind at multiple sites to both the nickel oxide (NiOx) substrate and the perovskite layer above it, helping passivate defects and improve wetting. Finally, TTA's thiophene units extend its delocalized π-electron system, giving the researchers a further way to tune π-π interactions and interfacial binding strength. Density functional theory (DFT) calculations confirmed that these three features work together as intended.
Blending TTA with Me-4PACz produces a hybrid SAM layer that is more homogeneous and more strongly bonded than Me-4PACz alone, which in turn supports better perovskite crystallinity and lower trap density in the film grown on top of it. The wide-bandgap perovskite absorber used in the study (about 1.67 eV) is the kind used in the top cell of a perovskite/silicon tandem, and devices built with the Me-4PACz+TTA hybrid SAM showed higher open-circuit voltage (VOC) and fill factor (FF) alongside the efficiency gains, as well as improved resistance to thermal cycling and light-soaking stress.
Built into a full perovskite/silicon tandem device, the same interface strategy delivered a PCE of 33.3%, certified at 33.1%, and the devices showed no observed PCE loss after 30 days of outdoor operation - a real-world stability result the researchers highlight alongside the efficiency figures. The team frames the work as a demonstration that a single, rationally designed hybrid-SAM molecule can simultaneously improve molecular packing, interfacial bonding and perovskite crystallization, offering a route toward wide-bandgap perovskite cells and perovskite/silicon tandems that are both efficient and durable.
