Researchers from Henan University, The University of Sydney, Forschungszentrum Jülich (IMD-3 Photovoltaics), University of Glasgow, University of New South Wales (UNSW), Westlake University and University of South Australia have developed a pH-modulated co-SAM (self-assembled monolayer) strategy that improves the performance and robustness of wide-band-gap perovskite and perovskite-silicon tandem solar cells. Their approach centers on suppressing aggregation in SAM precursor solutions, a common bottleneck that limits film quality and device stability.
The team discovered that aggregation in standard SAM precursor molecules can be effectively mitigated by controlling the solution’s pH. To achieve this, they synthesized a new compound - 6-aminohexylphosphonic acid hydrochloride (6AHPACl) - and introduced it to the widely used (4-(3,6-dimethyl-9H-carbazol-9-yl)butyl)phosphonic acid (Me-4PACz) solution. The resulting co-SAM formulation not only stabilizes the pH but also enhances surface chemistry at the SAM/perovskite interface.
The acidic nature of 6AHPACl improves anchoring to the underlying substrate (NiOₓ/ITO/glass), while its strong polarity supports halogen defect passivation via halogen bonding. Moreover, the improved wettability encourages uniform perovskite film formation. Together, these effects reduce recombination losses and enhance charge extraction efficiency.
Using this co-SAM strategy, the researchers achieved:
- A wide-band-gap (1.67 eV) perovskite single-junction cell with a champion efficiency of 22.8%, an open-circuit voltage (Voc) of 1.25 V, and a fill factor (FF) of 84.9% - among the highest values reported for this bandgap range.
- A monolithic 1 cm² perovskite-silicon tandem cell delivering a certified 29.1% efficiency.
The devices also exhibited exceptional long-term stability. An encapsulated double-junction retained 93% efficiency after 1,000 hours of continuous maximum power point tracking under 1‑sun illumination. Under rigorous environmental stress:
- A tandem device maintained 95% efficiency after 1,010 thermal cycles (−40°C to 85°C) - over five times the IEC 61215 standard requirement.
- Another passed 26 humidity freeze cycles, retaining 97% of its initial efficiency, exceeding the IEC minimum of 10 cycles - marking a first for perovskite-Si tandems.
This work establishes pH modulation of SAM precursor solutions as a new lever in optimizing interfacial chemistry and long-term durability in perovskite photovoltaics. The researchers note that the mechanistic understanding gained here can guide future co-SAM formulations for both single- and multi-junction solar cells, helping drive perovskite PV closer to industrial viability.
