Aïcha Hessler-Wyser

The growing integration of photovoltaic (PV) systems into complex environments—such as rooftops, façades, and vehicles—has introduced new shading patterns and the need for accurate performance modeling under these partial shading conditions. In particular, building-integrated photovoltaics (BIPV) are often subject to thin shadows from nearby building components, vegetation, or infrastructure. These shadings can impact only portions of individual cells, leading to inhomogeneous irradiances that are difficult to capture with conventional simulation tools. A few commercial tools consider the impact of near shading losses on photovoltaic arrays, a well-documented example being PVsyst. PVsyst incorporates a more sophisticated approach using four empirically derived I-V curve templates based on the number of shaded corners per sub-module, but the core logic remains: if at least one corner of a sub-module intersects a shadow, it is treated as electrically shaded. This assumption is valid for large open-rack systems with large shadows. The choice of points to check for shadow intersection could however be improved for thin shadows by considering partial shading at the cell level. This work presents a shadow modeling approach based on vertex projection, and the shadow positions were experimentally validated against photographs of shadows cast on an outdoor BIPV module in Neuchâtel, Switzerland. The near shading simulation is done at the cell level, and several strategies for selecting points to check for shadow intersection are compared to determine the shaded fraction and shading-adjusted plane-of-array irradiance of each cell. This irradiance is calculated by summing the diffuse plane-of-array irradiance and the direct plane-of-array irradiance adjusted for the shaded fraction. Compared with a conservative full-shading baseline, the cell-level model which captures partial cell shading may predict about 2% lower annual irradiance losses for a 25 cm shadow and even more for thinner shadows. At sufficient resolution, the model avoids missing thin shadows entirely—a key limitation of the submodule-level approach. The algorithm scales efficiently at low resolutions, simulating a full year of hourly cell irradiance data in about 17 s for a 40-cell module at 2 × 2 to 5 × 5 points per cell, and about 25 s for 1200 cells at 3 × 3 resolution.

For several classes of crystalline silicon (c-Si) photovoltaic (PV) cells, such as silicon heterojunction (SHJ) or tunnel oxide passivated contact (TOPCon), an obstacle to producing them at the terawatt scale is the use of silver in their contacts. One promising alternative is copper, which is often implemented in combination with a barrier material to prevent degradation via copper diffusion. But because there may be cases where the barrier is locally absent between copper and Si (caused by fabrication imperfections such as dust), it may be desirable to design silicon-based cell layers that are themselves resistant to copper-induced degradation. Here, we study how the crystallinity of n-type polysilicon layers (like those used at the rear of a TOPCon cell) affects their susceptibility to copper-induced degradation. We study simple, symmetric samples consisting of sputtered copper thin films on flat n-type polysilicon / silicon oxide (poly-Si(n) / SiOx) passivating contact stacks. After annealing the samples at 200°C for 30 minutes in nitrogen, the passivation is ruined for samples whose poly-Si crystalline fractions are high, while more amorphous poly-Si(n) show little to no change in passivation. The passivation is also ruined for a sample with highly crystalline poly-Si(n) that is >1.5x thicker than the more amorphous poly-Si(n). This study may aid in the development of poly-Si thin films that, combined with a diffusion barrier layer, provide the highest protection against copper-induced degradation in scalable, copper-metallized PV cells.

Perovskite–silicon triple-junction photovoltaics offer efficiency gains beyond dual-junction devices but at the expense of added complexity ¹ . Here we address two key bottlenecks in perovskite–silicon-based triple-junction solar cells: reduced open-circuit voltage ( V _OC ) in the wide-bandgap (WBG) top cell and limited photocurrent generation in the middle cell ^1,2 . A non-volatile additive, 4-hydroxybenzylamine (HBA), regulates WBG perovskite crystallization and passivates defects, promoting oriented growth and suppressing non-radiative recombination. Together with improved energy-level alignment, this yields V _OC s of up to 1.405 V and enhanced stability. To overcome the current limitations in the middle cell, a three-step deposition strategy enables the formation of thick, low-bandgap perovskite absorbers while preserving microstructural integrity and enhancing electron extraction. Also, low-refractive-index SiO _x -nanoparticles (SiO _x -np) that accumulate in the front valleys of the textured silicon bottom cell act as an optical middle reflector, enhancing light absorption in the middle cell. These advances are then combined in 1-cm ² perovskite–perovskite–silicon devices, achieving a certified efficiency of 30.02%.

Wide-bandgap (WBG) perovskite absorbers play a pivotal role in enabling high-efficiency tandem solar cells; yet, their long-term operational stability remains a significant hurdle to commercialization. Although interface engineering has led to promising progress, these improvements have not yet translated into the level of stability required for market readiness. Recent studies increasingly highlight the intrinsic instability of the perovskite bulk as a key limiting factor. This review examines the underlying mechanisms that compromise bulk stability in WBG perovskites (1.65–1.8 eV), covering both mixed-cation mixed-halide absorbers and fully inorganic systems such as CsPbI₃. Particular attention is given to the intrinsic factors that compromise the long-term stability of WBG perovskites, including compositional inhomogeneities, crystallization-driven disorder, insufficient crystallinity and texture, nanoscale phase impurities, and intrinsic phase-instability phenomena. Mixed-cation mixed-halide formulations, widely used to access tandem-relevant bandgaps, frequently exhibit spatially uneven elemental distributions and light- or thermally induced halide segregation, both of which introduce structural and electronic disorder. In parallel, inorganic WBG absorbers such as CsPbI₃ are predominantly limited by their strong propensity for converting to non-perovskite phases. In both material families, the crystallization pathway critically dictates the spatial distribution of components and the incorporation of defects. The resulting heterogeneities, together with nanoscale impurities and secondary phases, serve as initiation sites for absorber degradation under operational conditions. This review discusses emerging strategies aimed at overcoming these challenges, including compositional engineering, crystallization control, and targeted passivation. By addressing the root causes of bulk instability, this work outlines guidance toward achieving the long-term stability required for WBG perovskites in tandem photovoltaic technologies.

Inverted perovskite solar cells face performance limitations due to non-radiative recombination at the perovskite surfaces in devices, including functional layers. Advanced characterization and density functional theory reveal that phosphonic acids passivate perovskite surface defects, while piperazinium chloride mitigates interface recombination by improving energy level alignment, introducing a field effect, and homogenizing the surface. Together, the quasi-Fermi level splitting of the perovskite is homogeneously increased by ca. 100 mV. This enables two-terminal perovskite-on-silicon tandems to achieve a certified open-circuit voltage of 2 V for a 1 cm² device and high performance in excess of 31%. The scalability of the passivation is furthermore demonstrated with homogeneously passivated devices reaching certified efficiencies of 28.9% for an active area of 60 cm².

Perovskite–silicon triple-junction photovoltaics offer efficiency gains beyond dual-junction devices, but at the expense of added complexity. Here, we address two main challenges in perovskite-silicon-based triple-junction solar cells: reduced open-circuit voltage in the wide-bandgap top-cell and limited current density in the middle-cell. Additive engineering with 4-hydroxybenzylamine modulates the crystallization dynamics of the top-cell absorber and suppresses bulk recombination, enabling open-circuit voltages up to 1.405 V. The three-step-manufactured lead-halide middle-cell absorbers result in an increasing thickness and a reduced bandgap, thereby boosting the current density. Low-refractive-index SiOx nanoparticles accumulated in the front valleys of the textured silicon bottom-cell act as a middle-reflector, enhancing light absorption in the middle-cell. These innovations are then combined in 1-cm2 perovskite-perovskite-silicon devices, reaching a steady-state efficiency of 30.5%.

The impact of the composition of the A-site cations on the intra-domain defect density in cesium-formamidinium perovskite formulations is modulated via alloying.