Jean Paul Kleider

Atomic layer deposition (ALD) is a versatile approach for depositing nickel oxide (NiO _x ) hole‐selective layers in perovskite solar cells (PSCs), particularly for integration into multijunction devices where low processing temperatures are required. However, deposition temperature can strongly influence the chemical and electronic properties of NiO _x films and their interfaces, affecting device performance. Here, we investigate ALD‐NiO _x layers deposited at temperatures ranging from 75 to 250 °C and correlate their properties with photovoltaic performance. X‐ray photoelectron spectroscopy reveals an increase in stoichiometric NiO content from 76% to 96% with increasing deposition temperature, while only minor variations in work function are observed. PSCs fabricated using these ALD‐NiO _x layers show only marginal performance improvements above 150 °C, with efficiency gains below 1% absolute, mainly due to changes in fill factor and short‐circuit current. As an application‐oriented demonstration, ALD‐NiO _x deposited at 100 °C was implemented in a monolithic CIGS/perovskite tandem device, yielding a power conversion efficiency above 23%, with an open‐circuit voltage of 1651 mV and a fill factor of 76%. This result demonstrates the compatibility of the investigated low‐temperature ALD process with the fabrication of temperature‐sensitive multijunction device architectures.

Perovskite solar cells (PSCs) are among the most promising photovoltaic technologies, offering high efficiencies and low fabrication costs. However, their widespread deployment is hindered by fundamental challenges limiting performance stability.

The reported record efficiencies of silicon heterojunction solar cells using hydrogenated nanocrystalline silicon (nc-Si:H) as carrier-collector layers have increased interest in this material due to low activation energy. Band-bending in silicon creates a difference in work function (Φ) between the bulk and the surface of the film. However, Φ values of nc-Si:H have only been reported as estimates, with few direct measurements performed. Given the lack of reported data on the Φ of nc-Si:H films, we compared various measurement techniques to establish reference values and evaluate the reliability of each method. Using Kelvin probe (KP), ultraviolet photoelectron spectroscopy, and electron beam repulsion (EBR), we measured the Φ of intrinsic, n-type and p-type nc-Si:H layers. We discuss the effect of surface treatments based on ozone and hydrofluoric acid on the Φ. EBR revealed that the Φ of p-type films between 10 and 45 nm thick decreases with increasing thickness, dropping from 5.4 to 4.5 eV. This trend was not evident in KP measurements of thinner films. A similar plateau was observed for films thicker than 45 nm by both techniques. We also compared Φ values measured using these techniques with those estimated from the activation energy obtained via temperature-dependent conductivity measurements.

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.

Studying defects in semiconductors is, in practice, a very important topic for opto-electronic applications. It involves advanced characterization tools able to quantify and qualify the defect densities present in the materials. In the present article we focus on the use of a contactless frequency domain technique: modulated photoluminescence (MPL), and show its potential to detect defects. MPL has been used for the measurement of differential lifetime for several decades in silicon wafers. By extending it to low lifetime/highly defective materials we discovered its potential to become a defect spectroscopy method, measuring time constants close to the ones governing impedance spectroscopy measurements. Proofs of concept and an analytical model for doped materials have been presented already. Here, we reformulate the analytical model more explicitly and check its applicability by extensive numerical simulations for the case of a low illumination for a thin layer with a single defect. We present a parametric numerical study simulating the response of a single Shockley-Read-Hall center, showing the appearance of so-called V-Shapes in the MPL phase patterns as predicted by the analytical model, and valid beyond small-signal approximation. We discuss the difference between these two approaches and extend the analytical model and numerical investigations to intrinsic materials.

Boron phosphide (BP) thin films are potential selective contact layers for photovoltaic (PV) devices. However, lower deposition temperatures are preferred in the fabrication of many PV devices, such as solar cells, to reduce their cost. Here, boron phosphide layers were grown on silicon (100) substrates using the standard plasma-enhanced chemical vapor deposition (PECVD) and time-modulated PECVD methods with trimethylboron [B(CH3)3, TMB] and phosphine precursors. The effect of plasma power and Ar addition on structural properties and chemical composition is investigated, while material properties are analyzed by transmission electron microscopy. Chemical characterization by the electron diffraction x-ray spectroscopy method showed high carbon content in the BP layer. Electron energy loss spectroscopy demonstrated almost stoichiometric B and P (1:1) content. Raman spectroscopy of annealed samples showed an increase in carbon-related peaks, therefore indicating that annealing does not lead to the crystallization of boron phosphide. Thus, using TMB as a precursor of boron leads to carbon contamination in both standard and time-modulated modes. Optical emission spectroscopy showed that the low-temperature growth of BP without plasma (Ar or phosphine) assistance using TMB is impossible. We conclude that there is a need to investigate other boron precursors for boron phosphide low-temperature growth to avoid carbon contamination in BP films.

Multilayer III–V-based solar cells are complex devices consisting of many layers and interfaces. The study and the comprehension of the mechanisms that take place at the interfaces is crucial for efficiency improvement. In this work, we apply frequency-modulated Kelvin probe force microscopy under ambient conditions to investigate the capability of this technique for the analysis of an InP/GaInAs(P) multilayer stack. KPFM reveals a strong dependence on the local doping concentration, allowing for the detection of the surface potential of layers with a resolution as low as 20 nm. The analysis of the surface potential allowed for the identification of space charge regions and, thus, the presence of several junctions along the stack. Furthermore, a contrast enhancement in the surface potential image was observed when KPFM was performed under illumination, which is analysed in terms of the reduction of surface band bending induced by surface defects by photogenerated carrier distributions. The analysis of the KPFM data was assisted by means of theoretical modelling simulating the energy bands profile and KPFM measurements.