KEYWORDS: Copper indium gallium selenide, Thin films, Solar cells, Doping, Simulations, Thin film solar cells, Quantum experiments, Optoelectronics, Solar energy, Film thickness
Detailed optoelectronic simulations of thin-film photovoltaic solar cells (PVSCs) with a homogeneous photon-absorber layer made of with CIGS or CZTSSe were carried out to determine the effects of defect density, minority carrier lifetime, doping density, composition (i.e., bandgap energy), and absorber-layer thickness on solar-cell performance. The transfer-matrix method was used to calculate the electron-hole-pair (EHP) generation rate, and a one-dimensional drift-diffusion model was used to determine the EHP recombination rate, open-circuit voltage, short-circuit current density, power-conversion efficiency, and fill factor. Through a comparison of limited experimental data and simulation results, we formulated expressions for the defect density in terms of the composition parameter of either CIGS or CZTSSe. All performance parameters of the thin-films PVSCs were thereby shown to be obtainable from the bulk material-response parameters of the semiconductor, with the influence of surface defects being small enough to be ignored. Furthermore, unrealistic values of the defect density (equivalently, minority carrier lifetime) will deliver unreliable predictions of the solar-cell performance. The derived expressions should guide fellow researchers in simulating the graded-bandgap and quantum-well-based PVSCs.
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 thinfilm CIGS solar cells. According to optics, this coating can be effective only for a narrow spectral 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 antireflection 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.
We model the effect of concentrated sunlight on CIGS thin-film 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 field 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 efficiency of the solar cell can improve with sunlight concentration. This is illustrated via a numerical study.
Rooftop solar cells may become more acceptable if they are colored, e.g., red or bluish green, which requires that a certain part of the incoming solar spectrum be reflected. We implemented and optimized an optoelectronic model for Cu2ZnSn(SξSe1-ξ)4 (CZTSSe) solar cells containing (i) a conventional 2200-nm-thick CZTSSe layer with homogeneous bandgap, or (ii) an ultrathin CZTSSe layer with optoelectronically optimized sinusoidally nonhomogeneous bandgap, or (iii) a CZTSSe layer with optoelectronically optimized linearly nonhomogeneous bandgap. Either complete or partial rejection of either red or bluish green photons was incorporated in the model. Calculations show that on average, the efficiency of a typical solar cell will be reduced by about 9% if 50% red photons are reflected or by about 13% if 50% blue-green photons are reflected. The efficiency reduction increases to about 18% if all red photons are reflected or about 26% if all blue-green photons are reflected.
Silicon photovoltaic solar cells generally have a black or blue appearance that makes them aesthetically very different from traditional red roofs that either comprise burned-clay tiles or composite-material shingles. Rooftop solar cells may become more acceptable if they are similar in appearance to traditional roofs. This objective requires that the red part (620–700 nm wavelength) of the incoming solar spectrum be reflected so that it becomes unavailable for photovoltaic generation of electricity. Complete reflection of red photons would result in the reduction of useful solar photons (300– 1200 nm wavelength) by 12.5%. Calculations show that the optical short-circuit density will then decline by: 17% for 100-μm-thick crystalline-silicon solar cells, 20–22% for triple-junction tandem thin-film solar cells of amorphous silicon, 15-16% for 2.2-μm-thick CIGS solar cells, and 16–20% for ultrathin CIGS solar cells. On average, the efficiency of a typical solar cell will have to be multiplied by a factor of 0.8 if all red photons were reflected. This reduction in efficiency can be offset by wider adoption of rooftop solar cells. Red-rejection filters can be made of particulate composite materials containing, say, silica nanospheres. Typically, the solar cells will be iridescent then, which may not be aesthetically pleasing to many. Non-iridescent red-rejection filters can be fabricated by upscaling the linear dimensions of biomimetic filters nano-imprinted to reproduce the Morpho blue, this possibility being guaranteed by the scale invariance of the Maxwell equations and the weak dispersion of the refractive indexes of numerous polymers in the visible spectral regime. Non-uniformly red rooftop solar cells would also become feasible.
We studied the optimization of an ultrathin CuIn1-ξGaξSe2 (CIGS) solar cell with a nonhomogeneous CIGS absorber layer and backed by a 1D metallic periodically corrugated back-reflector (PCBR) with a rectangular profile. Nonhomogeneity in the CIGS absorber layer was modeled through either a sinusoidal or a linear bandgap variation along the thickness direction. The maximum power density for the AM1.5G spectrum was determined from the spectrum of the useful solar absorptance computed using the rigorous coupled-wave approach. Ultrathin solar cells with optimized PCBR and homogenous bandgap depending on the thickness of the CIGS layer were found to deliver the best photonic absorption characteristics. The open-circuit voltage, efficiency, and fill factor were calculated for the optimal designs using values of the reverse-saturation current density, ideality factor, and the series resistance density obtained from experimental results. The overall trend is that the effect of the PCBR becomes less prominent as the thickness of the CIGS absorber layer increases. Higher efficiency and fill factor can be achieved with a solar cell containing as 400-nm-thick CIGS layer compared to the conventional solar cell with a 2200-nm-thick CIGS layer.
KEYWORDS: Solar cells, Absorption, Optimization (mathematics), 3D modeling, Thin film solar cells, Tandem solar cells, Semiconductors, Thin films, Optical engineering, Photons
The rigorous coupled-wave approach (RCWA) and the differential evolution algorithm (DEA) were coupled in a practicable approach to maximize absorption in optical structures with three-dimensional morphology. As a model problem, optimal values of four geometric parameters and the bandgaps of three i-layers were found for an amorphous-silicon, multiterminal, thin-film tandem solar cell comprising three p – i – n junctions with a metallic hexagonally corrugated backreflector. When the optical short-circuit current density was chosen as the figure of merit to be maximized, only the bandgap of the topmost i-layer was significant and the remaining six parameters played minor roles. While this configuration would absorb light very well, it would have poor electrical performance. This issue arises because the optimization problem allows for the thicknesses and bandgaps of the semiconductor layers to change. We therefore devised another figure of merit that takes into account bandgap changes by estimating the open-circuit voltage. The resulting configuration was found to be optimal with respect to all seven variable parameters. The RCWA + DEA optimization approach is applicable to other types of photovoltaic solar cells as well as optical absorbers, with the choice of the figure of merit being vital to a successful outcome.
KEYWORDS: Thin film solar cells, Solar cells, Amorphous silicon, Semiconductors, Tandem solar cells, Polarization, Absorption, Metals, Waveguide modes, Waveguides
The rigorous coupled-wave approach (RCWA) was implemented to investigate optical absorption in a triple-p-i-n-junction amorphous-silicon solar cell with a two-dimensional (2-D) metallic periodically corrugated backreflector (PCBR). Both total and useful absorptances were computed against the free-space wavelength λ0 for both s- and p-polarized polarization states. The useful absorptance in each of the three p-i-n junctions was also computed for normal as well as oblique incidence. Furthermore, two canonical boundary-value problems were solved for the prediction of guided-wave modes (GWMs): surface-plasmon-polariton waves and waveguide modes. Use of the doubly periodic PCBR enhanced both useful and total absorptances in comparison with a planar backreflector. The predicted GWMs were correlated with the peaks of the total and useful absorptances. The excitation of GWMs was mostly confined to λ0 < 700 nm and enhanced absorption. As excitation of certain GWMs could be correlated with the total absorptance but not with the useful absorptance, the useful absorptance should be studied while devising light-trapping strategies.
The rigorous coupled-wave approach (RCWA) was used to calculate the optical absorption in a dielectric material deposited over a two-dimensional (2D) metallic surface-relief grating. The dielectric material was taken to be nonhomogeneous in the direction normal to the mean plane of the grating. The grating was chosen to comprise hillocks on a square grid. On illumination by a monochromatic plane waves, the chosen structure should support the excitation of two types of guided-wave modes: surface-plasmon-polariton (SPP) waves and waveguide modes (WGMs). Two cases were considered: (i) a 1D photonic crystal made from layers of silicon oxynitrides of differing composition, and (ii) a tandem solar cell comprising three amorphous-silicon p-i-n junctions. Optical absorption was studied in relation to the direction of propagation, polarization state, and the free-space wavelength of the incident plane wave. Several but not all absorptance peaks were correlated with the excitations of SPP-wave modes and WGMs predicted by the solutions of the underlying canonical boundary-value problems for guided-wave propagation. Some peaks of useful absorptances in the solar cell were also predicted by solutions of the canonical problems.
KEYWORDS: Amorphous silicon, Tandem solar cells, Thin film solar cells, Thin films, Coating, Waveguides, Spiral phase plates, Solar cells, Reflection, Reflectors
The rigorous coupled-wave approach was implemented in a three-dimensional setting to calculate the chargecarrier-generation rate in a thin-film solar cell with multiple amorphous-silicon p-i-n junctions. The solar cell comprised a front antireflection window; three electrically isolated p-i-n junctions in tandem; and a periodically corrugated silver back-reflector with hillock-shaped corrugations arranged on a hexagonal lattice. The differential evolution algorithm (DEA) was used to maximize the charge-carrier-generation rate over a set of selected optical and electrical parameters. This optimization exercise minimized the bandgap of the topmost i–layer but all other parameters turned out to be uninfluential. More importantly, the exercise led to a configuration that would very likely render the solar cell inefficient. Therefore, another optimization exercise was conducted to maximize power density. The resulting configuration was optimal over all parameters.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.