Insights into the energy-level alignment at interfaces between blended self-assembled monolayers based HTLs and donor polymers.

Biswajit Pal^a^,^b, Nimer Wehbe^b, Nisreen Alshehri^b^,^c, Vijay K. Singh^a, Ambesh Dixit^a, Frédéric Laquai^b^,^d, Christopher E. Petoukhoff^b

^aAdvanced Materials and Devices (A-MAD) Laboratory, Department of Physics, Indian Institute of Technology Jodhpur, Rajasthan, 342030, India

^bKing Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division (PSE), KAUST Solar Platform (KSP), Thuwal, 23955-6900, Kingdom of Saudi Arabia

^cPhysics and Astronomy Department, College of Sciences, King Saud University, Riyadh 12372, Kingdom of Saudi Arabia

^dLudwig-Maximilians-Universität München, Department of Chemistry, Faculty of Chemistry and Pharmacy, Butenandtstr. 5-13, Haus E, 81377 Munich, Deutschland

Proceedings of MATSUS Fall 2025 Conference (MATSUSFall25)

A2 Molecular Interfaces for Emerging Photovoltaics - #InterPero

València, Spain, 2025 October 20th - 24th

Organizers: Vincent M. Le Corre and Esma Ugur

Oral, Biswajit Pal, presentation 003
Publication date: 21st July 2025

Identification and development of various novel charge transport and photoactive materials, additive engineering, and controlling the absorber’s morphology mainly helped to achieve efficient perovskite and organic solar cell (PSC and OSC) devices. However, one of the dominant factors in improving cell Power Conversion Efficiency (PCE) is the charge-carrier extraction from the perovskite or organic photo-absorber material. It has been observed that the improvement in PCE of the PSCs and OSCs depends very much on the hole-transport layer (HTL) of the device. Recently, self-assembled monolayers (SAMs), notably [2-(9H-carbazol-9-yl)ethyl] phosphonic acid (2PACz) and its derivatives have gained attention as interfacial modifiers for tuning the work function (WF) of indium tin oxide (ITO) substrate in both inverted PSC and conventional OSC configurations.

This work explores the interfacial behaviour of several donor polymers- PFO, P3HT, PM6, PCE10, and PCE13 used as hole-injection layers, in combination with blended SAMs of varying dipole strengths. Through ultraviolet photoelectron spectroscopy (UPS), we examined that the strategy of blending SAMs altered the ITO work function by more than 1 eV. Our results, however, demonstrate that the optimal behaviour predicted by the existing integer charge transfer (ICT) model [1,2] is not always shown by polymer/SAM/ITO interfaces. Rather, large interfacial dipoles predominate, which hinders alignment of the vacuum level and reduces the impact of the underlying SAM/ITO WF on the polymer WF. When donor polymer generations are compared, clear patterns emerge that the older materials, such PFO and P3HT, have greater SAM compatibility and are less sensitive to variations in ITO WF. The surface potential of the electrode is more important for newer donor polymers (such PM6, PCE10, and PCE13), hence SAMs with higher intrinsic dipole moments are required to minimize energy losses. This knowledge is particularly important for perovskite devices, as the interface between the HTL and the perovskite layer has a big impact on the effectiveness of hole extraction, determining V_OC, [3,4] and the overall functionality of the device.

Overall, this study underscores the need for deliberate interface design in both OSCs and PSCs. By understanding the interfacial energetics shaped by blended SAMs and donor materials, we can more effectively engineer contacts to enhance charge transfer, reduce recombination, and support the advancement of efficient next-generation solar technologies.

References:

[1] Carl Tengstedt; Wojciech Osikowicz; William R. Salaneck; Ian D. Parker; Che-H. Hsu; Mats Fahlman. Fermi-level pinning at conjugated polymer interfaces. Appl. Phys. Lett. 2006, 88(5), 053502.

[2] Slawomir Braun; William R. Salaneck; Mats Fahlman. Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces. Advanced Materials. 2009, 21(14-15), 1450-1472.

[3] Christopher E. Petoukhoff; Divya K. Vijapurapu; Deirdre M. O’Carroll. Computational comparison of conventional and inverted organic photovoltaic performance parameters with varying metal electrode surface workfunction. Sol. Energy Mater. Sol. Cells. 2014, 120(B), 572-583.

[4] C. J. Brabec; A. Cravino; D. Meissner; N. S. Sariciftci; T. Fromherz; M. T. Rispens; L. Sanchez; J. C. Hummelen. Origin of the Open Circuit Voltage of Plastic Solar Cells. Advanced Functional Materials. 2001, 11(5), 374-380.

[5] Luis Victor Torres Merino et al. Impact of the valence band energy alignment at the hole-collecting interface on the photostability of wide band-gap perovskite solar cells. Joule. 2024, 8(9), 2585-2606.

Acknowledgements:

The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). B.P. acknowledges the DST-INSPIRE Fellowship Programme, Government of India for the financial assistant through IIT Jodhpur under the Sanction Order No. DST/INSPIRE Fellowship/2020/IF200238. C.E.P. acknowledges support from KAUST Global Fellowship Program under Award No. ORA-2022-5002.

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