We simulated the optical absorptance of BaSi₂-based heterojunction solar cells with transition metal oxides as hole transport layer (HTL) using GenPro4 software and optimized the device structures. The complex refractive index of each layer was used as an input in the optical simulations. We adopted ITO (80 nm) / HTL / a-Si (3 nm) / n-BaSi₂(500 nm) / TiN (250 nm) / glass substrates (200µm) structures. First, the implied photocurrent density (Jph) loss caused by parasitic absorption in 20-nm-thick p⁺-BaSi₂ layer was calculated to be 7.9 mA cm^-2, The Jph increased to 29.1 mA cm^-2 by substituting p⁺-BaSi₂ with 2-nm-thick MoO₃. To figure out the optimal HTL materials and the structures for BaSi₂ solar cells, we simulated the absorption spectra as function of materials such as NiO, Cu₂O, MoO₃, V₂O₅, and WO₃, which have already demonstrated the HTL functionality, and their thicknesses. The highest Jph was obtained with MoO₃, V₂O₅, or WO₃, meaning that these oxides are optically suitable HTL materials. By increasing the n-BaSi₂ absorber layer thickness to 2 µm and importing 3D random pyramidal texture structure with the height of 4 µm, the Jph reached a maximum of 33.1mA cm^-2, This is the largest value of all BaS_i2 solar cells ever reported.

The tandem PV technology can potentially increase the efficiency of PV modules over 30%. To design efficient modules, a quantification of the different losses is important. Herein, a model for quantifying the energy loss mechanisms in PV systems under real-world operating conditions with a level of detail back to the components and their fundamental properties is presented. Totally, 17 losses are defined and divided into four categories (fundamental, optical, electrical, and system losses). As example, a system based on a > 29% two-terminal perovskite/silicon tandem cell is considered. The loss distribution at standard test conditions is compared to four geographical locations. The results show that the thermalization, reflection, and inverter losses increase by 1.2%, 1.1%, and 1.4%, respectively, when operating outdoors. Additionally, it is quantified how fill factor gains partly compensate the current mismatch losses. For example, a mismatch of 7.0% in photocurrent leads to a power mismatch of 1.2%. Therefore, the power mismatch should be used as indicator for mismatch losses instead of a current mismatch. Finally, herein, it is shown that solar tracking increases not only the in-plane irradiance but also the efficiency of the tandem module.

We introduce a novel simulation tool capable of calculating the energy yield of a PV system based on its fundamental material properties and using self-consistent models. Thus, our simulation model can operate without measurements of a PV device. It combines wave and ray optics and a dedicated semiconductor simulation to model the optoelectronic PV device properties resulting in the IV-curve. The system surroundings are described via spectrally resolved ray tracing resulting in a cell resolved irradiance distribution, and via the fluid dynamics-based thermal model, in the individual cell temperatures. A lumped-element model is used to calculate the IV-curves of each solar cell for every hour of the year. These are combined factoring in the interconnection to obtain the PV module IV-curves, which connect to the inverter for calculating the AC energy yield. In our case study, we compare two types of 2 terminal perovskite/silicon tandem modules with STC PV module efficiencies of 27.7% and 28.6% with a reference c-Si module with STC PV module efficiency of 20.9%. In four different climates, we show that tandem PV modules operate at 1–1.9 °C lower yearly irradiance weighted average temperatures compared to c-Si. We find that the effect of current mismatch is significantly overestimated in pure optical studies, as they do not account for fill factor gains. The specific yields in kWh/kWp of the tandem PV systems are between −2.7% and +0.4% compared to the reference c-Si system in all four simulated climates. Thus, we find that the lab performance of the simulated tandem PV system translates from the laboratory to outdoors comparable to c-Si systems.

We performed advanced opto-electrical simulations on thin-film BaSi₂ solar cells. First, absorption spectra of BaSi₂-pn homojunction solar cells on Si substrate were calculated based on flat and/or pyramidally-textured surfaces, wherein 20-nm-thick n⁺-BaSi₂ was the topmost electron transport layer. By changing the front surface structure from flat to texture, the reflectance decreased in the wavelength (λ) range 700–1200 nm and the photocurrent density (J_ph) delivered by the photogenerated carriers in the 500-nm thick p-BaSi₂ layer increased by 1.2 mA/cm². Simulations revealed that the key factor inhibiting light absorption in the p-BaSi₂ layer was parasitic absorption in the n⁺-BaSi₂ and in the c-Si substrate. To solve these optical issues, we propose a new device structure, Al-doped n⁺-ZnO (AZO, 50 nm)/i-ZnO (20 nm)/p-BaSi₂ (500 nm) heterojunction solar cell (HJSC). In this device structure, the parasitic absorption reduced drastically, and J_ph reached 30.23 mA/cm². Furthermore, by replacing the Si substrate with a glass substrate, the light trapping worked more effectively, and the absorber layer thickness required for J_ph to saturate was reduced to 1 μm, yielding 32.06 mA/cm². Based on these simulation results, we manufactured n⁺-AZO/p-BaSi₂ HJSC. The internal quantum efficiency exceeded 30% at λ = 600 nm, meaning that we demonstrated the operation of n⁺-AZO/p-BaSi₂ HJSC for the first time. We investigated origins of small efficiencies compared to those simulated, and found that the passivation of defects in the p-BaSi₂ layer and the reduction of carrier recombination at the i-ZnO/p-BaSi₂ interface would significantly improve the solar cell performance.

The contact resistivity is a key parameter to reach high conversion efficiency in solar cells, especially in architectures based on the so-called carrier-selective contacts. The importance of contact resistivity relies on the evaluation of the quality of charge collection from the absorber bulk through adjacent electrodes. The electrode usually consists of a stack of layers entailing complex charge transport processes. This is especially the case of silicon heterojunction (SHJ) contacts. Although it is known that in thin-film silicon, the transport is based on subgap energy states, the mechanisms of charge collection in SHJ systems is not fully understood yet. Here, we analyse the physical mechanisms driving the exchange of charge among SHJ layers with the support of rigorous numerical simulations that reasonably replicate experimental results. We observe a connection between recombination and collection of carriers. Simulation results reveal that charge transport depends on the alignment and the nature of energy states at heterointerfaces. Our results demonstrate that transport based on direct energy transitions is more efficient than transport based on subgap energy states. Particularly, for positive charge collection, energy states associated to dangling bonds support the charge exchange more efficiently than tail states. The conditions for optimal carrier collection rely on the Fermi energy of the layers, in terms of activation energy of doped layers and carrier concentration of transparent conductive oxide. We observe that fill factor (FF) above 86% concurrently with 750-mV open circuit voltage can be attained in SHJ solar cells with ρ_c lower than 45 mΩ·cm² for p-contact and 20 mΩ·cm² for the n-contact. Furthermore, for achieving optimal contact resistivity, we provide engineering guidelines that are valid for a wide range of silicon materials from amorphous to nanocrystalline layers.