Davide Regaldo

Tin perovskites are emerging as a sustainable alternative to lead‐based photovoltaics, yet their efficiency remains limited by energy‐level mismatch and intrinsic instability from tin oxidation. Progress is further hindered by inconsistent electrical behavior, obscuring the true bottlenecks of device performance. Here, we deliver a comprehensive, layer‐resolved analysis of FASnI ₃ solar cells, combining Kelvin probe and photoelectron yield spectroscopy to directly map the band structure and quantify interfacial losses. We reveal that the commonly overlooked Bathocuproine buffer layer plays a decisive role in boosting open‐circuit voltage by forming a hybrid energy level with silver, enabling efficient electron extraction.Using time‐resolved surface photovoltage, we uncover the ultrafast charge‐transfer dynamics governing device operation. These insights expose a critical limitation in the conventional p‐p‐n, where severe recombination at the perovskite and hole transport interface suppresses performance. A transition to an n‐p‐p configuration significantly enhances charge extraction and minimizes recombination losses.By integrating these findings into a predictive digital twin, we establish a clear, experimentally validated roadmap toward tin perovskite solar cells exceeding 25% efficiency. This work provides both fundamental understanding and actionable design rules, accelerating the development of high‐performance, lead‐free photovoltaics.

Etude d'interfaces dans des dispositifs opto-electroniques à base de matériaux perovskites: Les cellules solaires à base de matériaux pérovskites (PSK) ont récemment suscité l'engouement en raison d'une augmentation rapide de l'efficacité de conversion de l'énergie (PCE) pour les cellules les plus performantes, leur permettant de concurrencer la technologie établie des cellules solaires au silicium. Toutefois, le peu de connaissance acquise sur les nouveaux matériaux PSK hybrides organiques-inorganiques, ainsi que l'emploi de couches d'extraction sélective des porteurs (SCTL) réactives ou instables constituent un frein à leur commercialisation rapide. Une grande partie des améliorations en matière de PCE et de stabilité des cellules provient d'une meilleure compréhension des propriétés optoélectroniques de leurs matériaux constitutifs.Dans ce contexte, cette thèse de doctorat s'est concentrée sur le couplage de techniques de caractérisation avancées, telles que la sonde de Kelvin (KP), la microscopie à force atomique à sonde de Kelvin (KPFM), la spectroscopie électronique XPS et la mesure de photo-tension de surface en régime transitoire (TrSPV), avec des simulations 2D reposant sur la résolution numérique des équations de dérive-diffusion (DD) par différences finies, utilisées pour interpréter les résultats des expériences et obtenir des informations sur les propriétés optoélectroniques des matériaux dans les cellules solaires.Deux dispositifs à hétérojonction latérale (LHJ) ont été produits. Il s'agit en fait de cellules photovoltaïques munies de deux contacts en face arrière, laissant la surface de la pérovskite accessible pour différents types de caractérisation. Nous avons étudié ces structures LHJ à l'aide de la spectroscopie électronique XPS. L'analyse nous a permis d'évaluer une densité de dopage effective maximale dans la fine couche de PSK d'environ 10^11 cm^-3, cette valeur très faible attestant la qualité et le caractère intrinsèque de cette couche.Nous avons aussi étudié des structures où le matériau PSK était déposé sur une couche d'extraction sélective d'électrons (ETL), à savoir du TiOx. Les techniques KP et KPFM ont été utilisées pour étudier la réponse des structures PSK/TiOx à l'illumination continue et leur relaxation après coupure de l'illumination sur une gamme temporelle allant de la seconde à plusieurs milliers de secondes. Le signal de SPV obtenu présente une forte intensité d'environ 300 mV en régime stationnaire et un temps de décroissance après coupure de la lumière très long (t=700 s). L'analyse de cette réponse par modélisation DD nous a conduit à postuler la présence de pièges donneurs profonds dans la couche d'extraction d'électrons de TiOx comme la seule cause électronique possible de cette réponse.Enfin, nous avons utilisé les mesures de TrSPV dans la gamme [1 ns,0.5 s] pour étudier la réponse des structures PSK/TiOx à des impulsions laser de quelques nanosecondes. La modélisation DD a montré que la réponse transitoire de SPV pouvait être bien reproduite en intégrant les défauts donneurs profonds dans TiOx. L'ajustement des transitoires calculés par ce modèle aux résultats expérimentaux a permis de préciser les valeurs de divers paramètres des matériaux TiOx et PSK, et a suggéré l'existence de pièges peu profonds accepteurs d'électrons dans le matériau PSK.

Nickel oxide (NiO_x) is widely utilized as an inorganic hole transport layer (HTL) in inverted metal halide perovskite (MHP) solar cells due to its high bandgap, transparency, stability, and scalability. However, its high surface reactivity and the presence of interfacial defects at the NiO_x/MHP interface negatively impact the device performance. To address these issues, the community has explored ultraviolet ozone (UVO) post-treatment of NiO_x and the use of organic molecules for surface passivation. Nevertheless, the individual effects of these processes and their influence on the bulk and surface characteristics of NiO_x, as well as the NiO_x/MHP interface, have not been thoroughly investigated and understood. This study based on photoemission analyses reveals that the UVO process increases the NiO_x reactivity and introduces defects. We identify the nature of defect states at the interface of pristine and UVO-treated NiO_x with MHP and demonstrate that the implementation of MeO-2PACz (M2P) as an organic interlayer mitigates this issue. Additionally, we find that neither UVO treatment nor M2P molecule anchoring significantly impacts the bulk properties of NiO_x.

Inorganic perovskite CsPbI₃ solar cells hold great potential for improving the operational stability of perovskite photovoltaics. However, electron extraction is limited by the low conductivity of TiO₂, representing a bottleneck for achieving stable performance. In this study, a co‐doping strategy for TiO₂ using Nb(V) and Sn(IV), which reduces the material's work function by 80 meV compared to Nb(V) mono‐doped TiO₂, is introduced. To gain fundamental understanding of the processes at the interfaces between the perovskite and charge‐selective layer, transient surface photovoltage measurements are applied, revealing the beneficial effect of the energetic and structural modification on electron extraction across the CsPbI₃/TiO₂ interface. Using 2D drift‐diffusion simulations, it is found that co‐doping reduces the interface hole recombination velocity by two orders of magnitude, increasing the concentration of extracted electrons by 20%. When integrated into n–i–p solar cells, co‐doped TiO₂ enhances the projected T_S80 lifetimes under continuous AM1.5G illumination by a factor of 25 compared to mono‐doped TiO₂. This study provides fundamental insights into interfacial charge extraction and its correlation with operational stability of perovskite solar cells, offering potential applications for other charge‐selective contacts.

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 900 to 600 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 band gap 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. Drift-diffusion modeling allowed us to analyze the shape of the wavelength dependence of the SPV. It was also observed that the intense illumination in the KPFM measurements induces slow SPV transients which were explained by the creation and migration of negative ions and their trapping at the PVK surface. Finally, aging effects were studied by measuring again SPV spectra after one-year storage in air, and an increase in the concentration of shallow defect states at the PVK/n-Si interface was found.

Lead halide perovskites are semiconductor materials which are employed as nonintentionally doped absorbers inserted between two selective carrier transport layers (SCTL), realizing a p‐i‐n or n‐i‐p heterojunction. In our study, we have developed and investigated a lateral device, based on methylammonium lead iodide (MAPbI₃) in which the p‐i‐n heterojunction develops in the horizontal direction. Our research suggests that the effective doping level in the MAPbI₃ film should be very low, below 10¹² cm⁻³. Along the vertical direction, this doping level is not enough to screen the electric field of the buried heterojunction with the SCTL. The perovskite work function is therefore affected by the work function of the SCTL underneath. From drift‐diffusion simulations, we show that intrinsic perovskite‐SCTL structures develop mV range surface photovoltages (SPVs) under continuous illumination. However, perovskite‐SCTL structures can develop SPVs of hundreds of mV, as confirmed by our measurements. We therefore analyzed the compatibility between low doping and low defect densities in the perovskite layer and such high SPV values using numerical modeling. It is shown that these high SPV values could originate from electronic processes due to large band offsets in the buried perovskite‐SCTL heterojunctions, or at the SCTL‐transparent conductive oxide (TCO) buried heterojunction. However, such electronic processes can hardly explain the long SPV persistence after switching off the illumination.