pH Modulation for Self-Assembly-Monolayer Type Hole Transport Layer for Efficient and Stable Perovskite-Silicon Double-Junction Solar Cells

Runmin Tao^a^,^b, Guoliang Wang^a^,^b, Zhihao Li^c, Nan Sun^d, Jueming Bing^a^,^b, Tik Lun Leung^a^,^b, Fraser J. Angus^e, Jianbo Tang^f^,^g, Chwenhaw Liao^a^,^b, Jianpeng Yi^a^,^b, Christopher Bailey^a^,^b, Li Liu^h, Yu Wangⁱ, Gaosheng Huang^d, Andreas Lambertz^d, Songyan Yin^j, Bin Gong^j, Alex-Anthony Cavallaro^k, Drew Evans^k, Matthew Griffith^k, Kourosh Kalantar-Zadeh^f^,^h, Jianghui Zheng^a^,^b, Pablo Docampo^e, David R. McKenzie^a^,^b, Md Arafat Mahmud^a^,^b, Kaining Ding^d, Anita W.Y. Hobaillie^a^,^b

^aSchool of Physics, The University of Sydney, Sydney, NSW 2006, Australia

^bSydney Nano, The University of Sydney, Sydney, NSW 2006, Australia

^cSchool of Nanoscience and Materials Engineering, Henan University, Kaifeng, 475004 P. R. China

^dIMD-3 Photovoltaics, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany

^eDepartment of Chemistry, University of Glasgow, University Avenue, Glasgow G12 8QQ, U.K.

^fSchool of Chemical Engineering, University of New South Wales (UNSW), Kensington, Sydney, NSW 2052, Australia

^gSchool of Engineering and Research Centre for Industries of the Future, Westlake University, Hangzhou 310030, China

^hFaculty of Engineering, School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, NSW 2006, Australia

ⁱX-ray Diffraction Lab, SSEAU, Mark Wainwright Analytical Centre, University of New South Wales (UNSW), Kensington, Sydney, NSW 2052, Australia

^jSurface Analysis Laboratory, Mark Wainwright Analytical Centre, University of New South Wales (UNSW), Kensington, Sydney, NSW 2052, Australia

^kFuture Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia

^lAustralian Centre for Advanced Photovoltaics (ACAP), University of New South Wales, Sydney, NSW, 2052 Australia

Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP26)

Nagoya, Japan, 2026 January 13th - 15th

Organizers: Pablo P. Boix and Seigo Ito

Oral, Runmin Tao, presentation 017
Publication date: 5th November 2025

The performance and stability of perovskite solar cells are susceptible to the quality of the charge transport layer, particularly when self-assembled monolayers (SAMs) are used as hole transport layer. It has been reported that SAM molecules tend to aggregate into colloidal because of concentrations and solvent effects. However, the underlying mechanism behind this remains unclear. In this study, we regulated the pH of SAM solutions via water-containing and water-free methods to elucidate the effect of pH on SAM colloidal aggregation and associated film quality. We then designed and synthesized a novel material, 6-aminohexylphosphonic acid hydrochloride (6AHPACl), to be added to the (4-(3,6-dimethyl-9H-carbazol-9-yl)butyl)phosphonic acid) Me-4PACz solution. Apart from the anticipated colloid aggregation suppressing for better film coverage, this co-SAM strategy served multiple functions that could not be achieved by pH modulation alone. They include better anchoring of the 6AHPACl to the underlying NiOX layer via covalent bonding; improved energetics for the SAM/perovskite interface due to the dipole moment offered by the 6AHPA+; and better wettability and therefore better quality of the overlaying perovskite layer. Compared to conventional Me-4PACz approach, this co-SAM strategy enabled a champion efficiency of 22.8% to be achieved by a wide-bandgap (1.67 eV) perovskite solar cell with a high open-circuit voltage (VOC) of 1.25 V and a fill factor of 84.9%. When applied to a 1cm2 monolithic perovskite-silicon double junction, the champion device produced a certified efficiency of 29.1% and a high VOC of 1.95V. Outstanding stabilities were also achieved by encapsulated devices. One retained 95% of its efficiency after 1010 Thermal Cycles (-40℃ to 85℃), more than five times the number required by the International Electrotechnical Commission (IEC) 61215 standard. Another encapsulated double junction device successfully passed the IEC 61215 Humidity Freeze test by an additional 16 Humidity Freeze cycles for the first time to date, marking a significant milestone in the reliability of perovskite-silicon double junction solar cells. These findings will guide future designs of SAM for efficient and durable perovskite single junction and multi-junction photovoltaics.

Acknowledgements:

The authors acknowledge the facilities and the scientific and technical assistance of Sydney Analytical, a core research facility at The University of Sydney. The authors also acknowledge the technical and scientific assistance provided by i) Research & Prototype Foundry Core Research Facility at the University of Sydney, part of the Australian National Fabrication Facility, ii) Sydney Microscopy & Microanalysis, the University of Sydney node of Microscopy Australia, iii) X-ray Diffraction Lab and iv) Surface Analysis Laboratory as part of the Solid State & Elemental Analysis Unit at Mark Wainwright Analytical Centre at UNSW, and finally iv) Microscopy Australia (ROR: 042mm0k03) at the University of South Australia, enabled by National Collaborative Research Infrastructure Strategy (NCRIS).

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