Publications
1) "Surface photovoltage characterisation of metal halide perovskite on crystalline silicon using Kelvin probe force microscopy and metal-insulator-semiconductor configuration", Aleksandra Bojar, Davide Regaldo, José Alvarez , David Alamarguy, Vesselin Donchev, Stefan Georgiev, Philip Schulz and Jean-Paul Kleider.
EPJ Photovoltaics 13, 18 (2022) https://doi.org/10.1051/epjpv/2022016
In this study we analysed halide perovskite films deposited directly on crystalline silicon by means of two set-ups using different operating modes of the surface photovoltage (SPV) methods, i.e., the Kelvin probe force microscopy (KPFM) and the metal-insulator-semiconductor (MIS) technique. The KPFM allowed to visualize surface potential distribution on a microscale while MIS technique allowed to study SPV spectral dependence. We studied wavelength dependent SPV of these samples, which allowed us to effectively vary the probe depth in the sample and discern the contribution from each interface to the overall effect measured under white light illumination. Depending on where the photocarriers are generated, different SPV signals are observed: at the perovskite/Si interface, the signal depends on Si doping type, while at the surface the SPV is always negative indicating downward surface band bending. This is confirmed by analysing SPV phase measured in the AC MIS mode. In addition, distinction between slow and fast processes contributing to measured SPV was possible. It has been observed, that with decreasing the illumination wavelength, the processes causing SPV become slower, which can indicate that high energy photons not only generate electronic photocarriers but can also induce chemical changes with creation of defects or ionic species that also modify the measured SPV.
2) "Simulating experimental techniques: Kelvin Probe Force Microscopy", J.P. Connolly, J. Alvarez, J.-P. Kleider, M. Da Lisca, V. Donchev, Ahmed Nejim, invited presentation, 19th International Conference on Nanosciences & Nanotechnologies (NN22), 5-8 July 2022, Thessaloniki, Greece (invited presentation)
In this presentation we review the kelvin probe force microscopy technique which consists of scanning a atomic force micrope tip across a surface to measure the surface potential, including instrumentation and applications, and the challenge of this method which is to deduce bulk properties from a surface technique dominated by surface states. We then report on the extent to which analytical theory can bring a solution to analysing KPFM measurements with surface and bulk defects under illumination and in the dark. We then detail numerical methods simulating the experimental methods including motion of the tip and presence of surface, concluding with simulation of modified band structure in a multilayer solar cell structure.
3) "Surface photovoltage study of metal halide perovskites deposited directly on crystalline silicon", Vesselin Donchev, Stefan Georgiev, Aleksandra Bojar, Davide Regaldo, Mattia da Lisca, José Alvarez, Jean-Paul Kleider, 20th ISCMP School and Conference, 29 August -2 September 2022, Varna, Bulgaria (poster), ACS Omega 2023, 8, 9, 8125–8133, February 24, 2023, https://doi.org/10.1021/acsomega.2c07664
In this presentation, perovskite (PVK) films deposited directly on n-type crystalline Si substrates were investigated by two operating modes of the surface photovoltage (SPV) method: i) the metal-insulator-semiconductor (MIS) mode and ii) the Kelvin probe force microscopy (KPFM). By scanning from 1300 to 500 nm in the MIS mode we consecutively studied the relatively fast processes of carrier generation, transport and recombination first in Si, then on both sides of the PVK/Si interface and finally in the PVK layer and its surface. The PVK optical absorption edge was observed in the range of 1.61-1.65 eV in good agreement with the bandgap of 1.63 eV found from photoluminescence spectra. Both SPV methods evidenced an upward energy band bending at the PVK/n-Si interface generating positive SPV and a downward bending at the PVK surface generating negative SPV. The interface component dominates the SPV spectra down to 500 nm. However, the intense illumination with 488 nm in the KPFM measurements induces the creation and migration of negative ions and their trapping at the surface leading eventually to an overall negative SPV transient. Ageing effects were studied by measuring SPV spectra after one year and an increase in the concentration of shallow defect states at the PVK/n-Si interface was found.
4) "Modelling investigations of surface photovoltage including surface defects", James P. Connolly, Mattia Da Lisca, Davide Regaldo, José Alvarez, Stefan Georgiev, Vesselin Donchev, Jean-Paul Kleider, Journées Nationales du Photovoltaïque (JNPV) 2022, 29 novembre - 2 December 2022, Dourdan, France (poster presentation https://hal.science/hal-03878613)
In this contribution we describe analysis of KPFM measurements by solving semiconductor transport and continuity equations numerically for the KPFM measurement geometry including the nanometric atomic probe and its position on the cross section of multilayer samples. The numerical methods allow a description of surface defect properties in terms of distributions in the gap, as well as descriptions of defect species in terms of their energy distributions and capture cross sections. We investigate the physical modification of band profiles across this section by distributions of donor and acceptor surface defects in the bandgap of the materials. We discuss the modification of net charge densities as a result of charged and neutral donor and acceptor populations, and the resulting band bending affecting the KPFM and SPV signals.
Applications of these methods are considered with a focus on photovoltaic applications, for the dominant materials in the field including group IV, III-V and concluding with considerations for emerging materials such as perovskites.
5) "Frequency dependence of modulated SPV to characterize surface defects", Davide Regaldo, Vesselin Donchev, Stefan Georgiev, Kiril Kirilov, James P. Connolly, Jean-Baptiste Puel, Philip Schulz and Jean-Paul Kleider, 40th European Photovoltaic Solar Energy Conference and Exhibition (EU-PVSEC), 18-22 September 2023, Lisbon, Portugal (poster). (preprint : https://hal.science/hal-04286402)
This work investigates the impact of the light modulation frequency on the response of untreated p-type silicon, used as a model system. It highlights the capabilities of coupling modulated SPV in MIS configuration with full drift – diffusion modelling to unveil key electronic properties of photovoltaic materials.