OPTOELECTRONIC MODELING AND OPTIMIZATION OF GRADED-BANDGAP THIN-FILM SOLAR CELLS
Open Access
- Author:
- Ahmad, Faiz
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 15, 2020
- Committee Members:
- Akhlesh Lakhtakia, Dissertation Advisor/Co-Advisor
Akhlesh Lakhtakia, Committee Chair/Co-Chair
Michael T Lanagan, Committee Member
Slava V Rotkin, Committee Member
James Kenneth Breakall, Outside Member
Judith Todd Copley, Program Head/Chair - Keywords:
- Thin-film solar cells
Graded-bandgap absorbers
Optoelectronic optimization
Earth-abundant materials - Abstract:
- 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.