KEYWORDS: Photovoltaics, Solar energy, Solar cells, Multijunction solar cells, Energy efficiency, Standards development, Silver, Silicon solar cells, Silicon, Renewable energy
We present a critical review on optical research in photovoltaics guided by the question: Which topics do we regard as most relevant to accelerate the large-scale implementation of photovoltaics? The following areas were identified: 1) The development of high-performance earth-abundant solar cell materials and the reduction of indium and silver in the device. 2) Color concepts for an appealing optical appearance of solar modules as photovoltaic modules enter urban environments as well as agricultural areas on a large scale. 3) Optical optimization of multijunction solar cells based on perovskite, III-V semiconductors and silicon to overcome the single-junction efficiency limit. 4) Accurate energy yield predictions considering the full complex illumination conditions particularly regarding bifacial and multijunction solar cells. 5) Advanced concepts with strong momentum such as radiative cooling and spectral conversion.
KEYWORDS: Perovskite, Refractive index, Interpolation, Solar energy, Solar cells, Modeling, Data modeling, Spectroscopic ellipsometry, Mathematical optimization, Linear regression
Optimizing the bandgap energy for a perovskite cell typically requires the complex refractive index,
which are obtained from spectroscopic ellipsometry measurements. While these measurements are
usually available for a few discrete bandgap energies, it is desired to have them in continuous range to
find the optimal bandgap energy. We propose a new method for generating the complex refractive
index, based on the interpolation of a few measured data sets. The Forouhi-Bloomer model is used to fit
the measured data. Then a linear regression is applied to the fit parameters with respect to the bandgap
energy. From the interpolated parameters, the refractive index curve of perovskite with any desired
bandgap energy is finally reconstructed. We validated our method by simulating the absorptance of a
single junction perovskite cell and a perovskite/silicon tandem cell, showing that our method can
accurately model the refractive index.
As silicon solar cells are getting close to their fundamental efficiency limit of 29.4%, perovskite/silicon tandem solar cells are the most promising candidate for a technology with higher efficiency potential. It is project to enter mass production within two years. Many companies looking to introduce perovskite/silicon tandems aim for an application in the residential rooftop market. For a successful market entry, this technology does not just need to demonstrate high efficiencies under Standard Testing Conditions (STC: 25°C, 1 kW/m², AM1.5g), but also high energy yields in different climates to be bankable to investors.
However, the classical energy yield prediction tools (PVSyst, SAM, etc.) currently do not cover tandem devices. The research community has done several yield prediction studies for perovskite/silicon tandems. However, these studies are done with plane of array irradiance models, which assign homogenous irradiation to the whole module and cannot solve the irradiation situation in complex geometries typically surrounding modules on residential roofs tops with objects like chimneys and trees leading to partial shading.
In this work, we introduce a new comprehensive model called “PVMD Toolbox”, which can simulate tandem PV systems as well as integrated PV Systems (IPV) in complex shading scenarios such as residential buildings. We combine wave optics-based generation profiles and a semiconductor software to simulate the optoelectrical properties of the tandem device. On a system level, we use ray tracing to determine irradiance and photon flux onto every solar cell in the PV system. Based on this we determine the temperature and IV curve of each solar cell for every hour of the year, which are then used in a lumped element model to calculate the yield of each module and factoring the inverter the AC yield is determined for two tandem systems in four different climates.
This paper studies two different approaches for evanescent wave optical sensing: an horizontal one and a vertical one. In horizontal waveguides, the evanescent wave is distributed on the upper cladding. While in a vertical configuration, the evanescent wave is distributed on the left and right sides of the waveguide. In an horizontal configuration the evanescent wave can be also on both sides of the waveguide in order to increase the optical energy for sensing if the substrate under the waveguide is locally removed. However, in this configuration to achieve sensitive devices, the layers have to be freestanding and thin [1] limiting practical implementations of such approaches. Furthermore, very few materials can be defined as tall and thin in the case of a vertical configuration, as the deposition techniques often used (PECVD/LPCVD) are meant for films in the couple of micron range. In the following we will investigate the properties of the materials used but also the fabrication feasibility for both configurations.
KEYWORDS: Perovskite, Solar cells, Thin films, Thin film solar cells, Multijunction solar cells, 3D modeling, Modeling, Optical properties, Crystals, Silicon, Absorption, Photovoltaics, Thin film devices, Reflectivity
Barium di-silicide (BaSi2) is an abundant and inexpensive semiconductor with appealing opto-electrical properties. In this work we show that a 2-μm thick BaSi2-based thin-film solar cell can exhibit an implied photo-current density equal to 41.1 mA/cm2, which is higher than that of a state-of-the-art wafer-based c-Si hetero-junction solar cell. This performance makes BaSi2 an attractive absorber for high-performing thin-film and multi-junction solar cells. In particular, to assess the potential of barium di-silicide, we propose a thin-film double-junction solar cell based on organometallic halide perovskite (CH3NH3PbI3) as top absorber and BaSi2 as bottom absorber. The resulting modelled ultra-thin double-junction CH3NH3PbI3 / BaSi2 (< 2 μm) exhibits an implied total photo-current density equal to 38.65 mA/cm2 (19.84 mA/cm2 top cell, 18.81 mA/cm2 bottom cell) and conversion efficiencies up to 28%.
KEYWORDS: Solar cells, Interfaces, Light scattering, Scattering, Absorption, Silicon solar cells, Tandem solar cells, Electromagnetic scattering theory, Transparent conductors, Silicon
Thin-film solar cells contain nano-textured interfaces that scatter the incident light, leading to increased absorption and hence increased current densities in the solar cell. In this manuscript we systematically study optimized random nano-textured morphologies for three different cases: amorphous hydrogenated silicon solar cells (a-Si:H, bandgap 1.7 eV), nano-crystalline silicon solar cells (nc-Si:H, bandgap 1.1 eV) and tandem solar cells consisting of an a-Si:H and a nc-Si:H junction. For the optimization we use the Perlin texture algorithm, the scalar scattering theory, and a semi-coherent optical device simulator.
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