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Abstract The building sector accounts for 36% of energy consumption and 39% of energy-related greenhouse-gas emissions. Integrating bifacial photovoltaic solar cells in buildings could significantly reduce energy consumption and related greenhouse gas emissions. Bifacial solar cells should be flexible, bifacially balanced for electricity production, and perform reasonably well under weak-light conditions. Using rigorous optoelectronic simulation software and the differential evolution algorithm, we optimized symmetric/asymmetric bifacial CIGS solar cells with either (i) homogeneous or (ii) graded-bandgap photon-absorbing layers and a flexible central contact layer of aluminum-doped zinc oxide to harvest light outdoors as well as indoors. Indoor light was modeled as a fraction of the standard sunlight. Also, we computed the weak-light responses of the CIGS solar cells using LED illumination of different light intensities. The optimal bifacial CIGS solar cell with graded-bandgap photon-absorbing layers is predicted to perform with 18%–29% efficiency under 0.01–1.0-Sun illumination; furthermore, efficiencies of 26.08% and 28.30% under weak LED light illumination of 0.0964 mW cm⁻²and 0.22 mW cm⁻²intensities, respectively, are predicted. 

Antireflection coatings are vital for reducing loss due to optical reflection in photovoltaic solar cells. A single-layer magnesium fluoride (MgF2) antireflection coating is usually used in thin- film CIGS solar cells. According to optics, this coating can be effective only for a narrow spec- tral regime. Further reduction of reflection loss may require an optimal single-layer or multi-layer coating. Hence, we optimized the refractive indices and thicknesses of single- and double-layer an- tireflection coatings for CIGS solar cells containing a CIGS absorber layer with: (i) homogeneous bandgap, (ii) linearly graded bandgap, or (iii) nonlinearly graded bandgap. A relative enhancement of up to 1.83% is predicted with an optimal double-layer antireflection coating compared to the efficiency with a single-layer antireflection coating. 

In Part I [Appl. Opt.58,6067(2019)APOPAI003-693510.1364/AO.58.006067], we used a coupled optoelectronic model to optimize a thin-film ${\mathrm{C}\mathrm{u}\mathrm{I}\mathrm{n}}_{1-<\#comment/>\mathrm{\xi <\#comment/>}}{\mathrm{G}\mathrm{a}}_{\mathrm{\xi <\#comment/>}}{\mathrm{S}\mathrm{e}}_2$(CIGS) solar cell with a graded-bandgap photon-absorbing layer and a periodically corrugated backreflector. The increase in efficiency due to the periodic corrugation was found to be tiny and that, too, only for very thin CIGS layers. Also, it was predicted that linear bandgap-grading enhances the efficiency of the CIGS solar cells. However, a significant improvement in solar cell efficiency was found using a nonlinearly (sinusoidally) graded-bandgap CIGS photon-absorbing layer. The optoelectronic model comprised two submodels: optical and electrical. The electrical submodel applied the hybridizable discontinuous Galerkin (HDG) scheme directly to equations for the drift and diffusion of charge carriers. As our HDG scheme sometimes fails due to negative carrier densities arising during the solution process, we devised a new, to the best of our knowledge, computational scheme using the finite-difference method, which also reduces the overall computational cost of optimization. An unfortunate normalization error in the electrical submodel in Part I came to light. This normalization error did not change the overall conclusions reported in Part I; however, some specifics did change. The new algorithm for the electrical submodel is reported here along with updated numerical results. We re-optimized the solar cells containing a CIGS photon-absorbing layer with either (i) a homogeneous bandgap, (ii) a linearly graded bandgap, or (iii) a nonlinearly graded bandgap. Considering the meager increase in efficiency with the periodic corrugation and additional complexity in the fabrication process, we opted for a flat backreflector. The new algorithm is significantly faster than the previous algorithm. Our new results confirm efficiency enhancement of 84% (resp. 63%) when the thickness of the CIGS layer is 600 nm (resp. 2200 nm), similarly to Part I. A hundredfold concentration of sunlight can increase the efficiency by an additional 27%. Finally, the currently used 110-nm-thick layer of ${\mathrm{M}\mathrm{g}\mathrm{F}}_2$performs almost as well as optimal single- and double-layer antireflection coatings. 

We model the e ect of concentrated sunlight on CIGS thin- lm graded-bandgap solar cells using an optoelectronic numerical model. For this purpose it is necessary  first to solve the time-harmonic Maxwell equations to compute the electric  eld in the device due to sunlight and so obtain the electron-hole-pair generation rate. The generation rate is then used as input to a drift-diffusion model governing the flow of electrons and holes in the semiconductor components that predicts the current generated. The optical submodel is linear; however, the electrical submodel is nonlinear. Because the Shockley{Read{Hall contribution to the electron-hole recombination rate increases almost linearly at high electron/hole densities, the effciency of the solar cell can improve with sunlight concentration. This is illustrated via a numerical study. 

A systematic study was performed with a coupled optoelectronic model to examine the effect of the concentration of sunlight on the efficiencies of CIGS, CZTSSe and AlGaAs thin-film solar cells with a graded-bandgap absorber layer. Efficiencies of 34.6% for CIGS thin-film solar cells and 29.9% for CZTSSe thin-film solar cells are predicted with a concentration of 100 suns, the respective one-sun efficiencies being 27.7% and 21.7%. An efficiency of 36.7% is predicted for AlGaAs thin-film solar cells with a concentration of 60 suns, in comparison to 34.5% one-sun efficiency. Sunlight concentration does not affect the per-sun electron–hole-pair (EHP) generation rate but reduces the per-sun EHP recombination rate either near the front and back faces or in the graded-bandgap regions of the absorber layer, depending upon the semiconductor used for that layer, and this is the primary reason for the improvement in efficiency. Other effects include the enhancement of open-circuit voltage, which can be positively correlated to the higher short-circuit current density. Sunlight concentration can therefore play a significant role in enhancing the efficiency of thin-film solar cells. 

We have heavily relied for a few centuries on fossil fuels, which are basically dead plant material that was sequestered and converted millions of years ago, but the rapidly increasing energy demand combined with climatic challenges means we need to develop a large-scale supply of energy from sources without climatic impact. An obvious choice is to use solar energy directly when possible, and a complete global transition to solar energy by 2050 is realistic and cost effective. However, in order to find space for the large areas needed for harvesting solar energy by photovoltaic means, it would be advantageous if solar panels could be incorporated into urban buildings and free land for other uses. We undertook an analysis of the needs and requirements from the building industry that will allow for a more widespread use of solar panels on buildings, also referred to as Building Integrated Photo-Voltaics. Specifications and options for the visual incorporation of the solar panels in the building envelope were identified. Special attention was paid to (i) the role of modularization and standardization in architecture and (ii) the role of color and reflectance. A standardized mounting system is proposed that will allow for modular attachment of solar panels, making it easy to adjust, repair, and replace individual panels. Biological inspiration can be used to improve the system further. The forced-air ventilation of the tunnels of prairie dogs shows how to enhance cooling. The non-iridescent wings of butterflies of the Morpho genus, proposes how a low-cost structurally colored film can be inserted into the solar panel during its assembly.