Thin-film solar cells are cheap and easy to manufacture but require improvements as their efficiencies are low compared to that of the commercially dominant crystalline-silicon solar cells. An optoelectronic model is formulated and implemented along with the differential evolution algorithm to assess the efficacy of grading the bandgap of the CIGS, CZTSSe, and AlGaAs photon-absorbing layer for optimizing the power-conversion efficiency of thin-film CIGS, CZTSSe, and AlGaAs solar cells, respectively, in the two-terminal single-junction format. Each thin-film solar cell is modeled as a photonic device as well as an electronic device. Solar cells with two (or more) photon-absorbing layers can also be handled using the optolelectronic model, whose results will stimulate experimental techniques for bandgap grading to enable ubiquitous small-scale harnessing of solar energy.

This book will guide Photovoltaics researchers in a new way of thinking about harvesting light energy from all wavelengths of the solar spectrum. It closes the gap between general solar cells books and photovoltaics journal articles, by focusing on the latest developments in our understanding of solid-state device physics. The material presented is experimental and based on II-VI thin-film materials, mainly CdTe-based solar cells. The authors describe the use of new device design, based on multilayer graded bandgap configuration, using CdTe-based solar cells. The authors also explain how the photo-generated currents can be enhanced using multi-step charge carrier production. The possibility of fabricating these devices using low-cost and scalable electroplating is demonstrated. The value of electroplating for large area electronic devices such as PV solar panels, display devices and nano-technology devices are also demonstrated. By enabling new understanding of the engineering of electroplated semiconductor materials and providing an overview of the semiconductor physics and technology, this practical book is ideal to guide researchers, engineers, and manufacturers on future solar cell device designs and fabrications. Discusses in detail the processes of growths, treatments, solar cell device fabrication and solid state physics, improving readers’ understanding of fundamental solid state physics; Enables future improvements in CdTe-based device efficiency; Explains the significance of defects in deposited semiconductor materials and interfaces that affect the material properties and resulting device performance.

Crystalline-silicon (c-Si) photovoltaic solar cells are increasingly taking over the energy production sector nowadays. Even in comparison to coal-fired and nuclear plants for generation of electricity, the cost of harnessing solar energy by photovoltaic means has gone down considerably during the last decade. However, microwatt-scale generators of electricity are needed for human progress to become effectively unconstrained by economics. Large-scale adoption of thin-film solar cells is necessary for that to happen. However, Earth-abundant materials with low toxicity and high power-conversion efficiency must be used for thin-film solar cells. A series of theoretical investigations were performed to tackle the problem of materials scarcity as well as to explore potential enhancements of power-conversion efficiency in thin-film solar cells by thinning the absorber layer, grading the bandgap in the absorber layer, and modifying the back end. Three different types of thin-film solar cells were considered: CIGS, CZTSSe, and AlGaAs. The bandgap of the absorber layer was varied either sinusoidally or linearly. The thickness of the absorber layer was varied from 100 nm to 2200 nm. Back-end modifications incorporating a periodically corrugated backreflector and a back-surface passivation layer were considered as well. A coupled optoelectronic model was used along with the differential evolution algorithm to maximize the efficiency in relation to geometric and bandgap-grading parameters. Furthermore, as colored solar cells can promote large-scale adoption of rooftop solar cells, efficiency loss due to color-rejection filters was estimated. The coupled optoelectronic optimization predicted that tailored bandgap grading could significantly improve efficiency for all three considered thin-film solar cells. For CIGS solar cells with a 2200-nm-thick absorber layer, an efficiency of 27.7% was predicted with a sinusoidally graded bandgap absorber layer along with back-end modifications in comparison to 22% efficiency achieved experimentally with a homogeneous CIGS absorber layer. An efficiency of 21.7% was predicted with sinusoidal grading of a 870-nm-thick absorber CZTSSe layer in comparison to 12.6% efficiency achieved experimentally with a 2200-nm-thick homogeneous CZTSSe layer. Similarly, an efficiency of 34.5% was predicted through optoelectronic optimization of AlGaAs solar cells with a sinusoidally graded bandgap absorber layer along with back-end modifications in comparison to 27.6% efficiency achieved experimentally with a homogeneous AlGaAs absorber layer. For colored thin-film solar cells, predictions of the efficiency loss varied from 10% to 20%, depending upon the percentage of rejection of incoming solar photons. Thus, optoelectronic optimization by bandgap grading and back-end modifications is more than enough to swallow efficiency reduction by the rejection of a certain percentage of incoming solar photons. Thus, the proposed design strategies provide a way to realize more efficient thin-film solar cells for the ubiquitous harnessing of solar energy at low-wattage levels, thereby promoting widespread adoption of thin-film solar cells as local energy sources. Also, cheap, small-scale off-grid generation of electricity will provide access to energy for populations living without electricity far from central grids in less-developed and developing regions of our planet, thus equalizing opportunity and decreasing income and gender gaps.

This book concentrates on the latest developments in our understanding of solid-state device physics. The material presented is mainly experimental and based on CdTe thin-film solar cells. It extends these new findings to CIGS thin-film solar cells and presents a new device design based on graded bandgap multilayer solar cells. This design has been