Energy yield analysis of textured perovskite silicon tandem solar cells and modules

Complex refractive indices of Spiro-TTB and C60 for optical analysis of perovskite silicon tandem solar cells

Evaporated charge extraction layers from organic molecular materials are vital in perovskite-based solar cells. For opto-electronic device optimization their complex refractive indices must be known for the visible and near infrared wavelength regime; however, accurate determination from thin organic films below 50 nm can be challenging. By combining spectrophotometry, variable angle spectroscopic ellipsometry, and X-ray reflectivity with an algorithm that simultaneously fits all available spectra, the complex refractive index of evaporated Spiro-TTB and C₆₀ layers is determined with high accuracy. Based on that, an optical losses analysis for perovskite silicon solar cells shows that 15 nm of Spiro-TTB in the front of a n-i-p device reduces current by only 0.1 mA/cm², compared to a substantial loss of 0.5 mA/cm² due to 15 nm of C₆₀ in a p-i-n device. Optical device simulation predicts high optical generation current densities of 19.7 and 20.1 mA/cm² for the fully-textured, module-integrated p-i-n and n-i-p devices, respectively.

Ray-optics study of gentle non-conformal texture morphologies for perovskite/silicon tandems

We investigate gentle front side textures for perovskite/silicon tandem solar cells. These textures enhance the absorption of sunlight, yet are sufficiently gentle to allow deposition of an efficient perovskite top cell. We present a tandem solar cell with such gentle texture, fabricated by Kaneka corporation, with an efficiency as high as 28.6%. We perform an extensive ray-optics study, exploring non-conformal textures at the front and rear side of the perovskite layer. Our results reveal that a gentle texture with steepness of only 23° can be more optically efficient than conventional textures with more than double that steepness. We also show that the observed anti-reflective effect of such gentle textures is not based a double bounce, but on light trapping by total internal reflection. As a result, the optical effects of the encapsulation layers play an important role, and have to be accounted for when evaluating the texture design for perovskite/silicon tandems.

Annual energy yield of mono- and bifacial silicon heterojunction solar modules with high-index dielectric nanodisk arrays as anti-reflective and light trapping structures

While various nanophotonic structures applicable to relatively thin crystalline silicon-based solar cells were proposed to ensure effective light in-coupling and light trapping in the absorber, it is of great importance to evaluate their performance on the solar module level under realistic irradiation conditions. Here, we analyze the annual energy yield of relatively thin (crystalline silicon (c-Si) wafer thickness between 5 μm and 80 μm) heterojunction (HJT) solar module architectures when optimized anti-reflective and light trapping titanium dioxide (TiO₂) nanodisk square arrays are applied on the front and rear cell interfaces, respectively. Our numerical study shows that upon reducing c-Si wafer thickness down to 5 μm, the relative increase of the annual energy yield can go up to 23.3 %_rel and 43.0 %_rel for mono- and bifacial solar modules, respectively, when compared to the reference modules with flat optimized anti-reflective coatings of HJT solar cells.

Multimodal Microscale Imaging of Textured Perovskite-Silicon Tandem Solar Cells

Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.

Backside light management of 4-terminal bifacial perovskite/silicon tandem PV modules evaluated under realistic conditions

Perovskite/silicon tandem modules have recently attracted growing interest as a potential candidate for new generations of solar modules. Combined with a bifacial configuration it can lead to considerable energy yield improvement in comparison to conventional monofacial tandem solar modules. Optical modeling is crucial to analyze the optical losses of perovskite/silicon solar modules and achieve efficient light management. In this article we study the optical properties of four-terminal bifacial tandem modules, using metal-halide perovskite top solar cell and a conventional industrial crystalline silicon PERC bottom solar cell. We propose a method to analyze bifacial gains, improve back side light management and challenge it under realistic spectral conditions at several locations with various albedos. We show that both optimized designs for the back side show comparable advantages at all locations. These results are a good sign for the standardization of bifacial four-terminal perovskite/silicon modules.

Numerical study on the angular light trapping of the energy yield of organic solar cells with an optical cavity

A limiting factor in organic solar cells (OSCs) is the incomplete absorption in the thin absorber layer. One concept to enhance absorption is to apply an optical cavity design. In this study, the performance of an OSC with cavity is evaluated. By means of a comprehensive energy yield (EY) model, the improvement is demonstrated by applying realistic sky irradiance, covering a wide range of incidence angles. The relative enhancement in EY for different locations is found to be 11-14% compared to the reference device with an indium tin oxide front electrode. The study highlights the improved angular light absorption as well as the angular robustness of an OSC with cavity.

Vapor-Phase Formation of a Hole-Transporting Thiophene Polymer Layer for Evaporated Perovskite Solar Cells

Homogeneous layer formation on textured silicon substrates is essential for the fabrication of highly efficient monolithic perovskite silicon tandem solar cells. From all well-known techniques for the fabrication of perovskite solar cells (PSCs), the evaporation method offers the highest degree of freedom for layer-by-layer deposition independent of the substrate's roughness or texturing. Hole-transporting polymers with high hole mobility and structural stability have been used as effective hole-transporting materials (HTMs) of PSCs. However, the strong intermolecular interactions of the polymers do not allow for a layer formation via the evaporation method, which is a big challenge for the perovskite community. Herein, we first applied a hole-transporting terthiophene polymer (PTTh) as an HTM for evaporated PSCs via an in situ vapor-phase polymerization using iodine (I₂) as a sublimable oxidative agent. PTTh showed high hole mobility of 1.2 × 10^-3 cm²/(V s) and appropriate energy levels as HTM in PSCs (E_HOMO = -5.3 eV and E_LUMO = -3.3 eV). The PSCs with the in situ vapor-phase polymerized PTTh hole-transporting layer and a co-evaporated perovskite layer exhibited a photovoltaic conversion efficiency of 5.9%, as a proof of concept, and high cell stability over time. Additionally, the polymer layer could fully cover the pyramidal structure of textured silicon substrates and was identified as an effective hole-transporting material for perovskite silicon tandem solar cells by optical simulation.