Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading


The power conversion efficiency of thin-film CuIn1−ξGaξSe2 (CIGS) solar cells has been previously reported to be about 22%. Generally, this efficiency compares well to the 26.7% efficiency of single-junction crystalline-silicon solar cells; however, the scarcity of indium remains the core obstacle for large-scale and low-cost production of thin-film CIGS solar cells. Were it possible to reduce the thickness of the CIGS layer without reducing the efficiency, the aforementioned shortfall would be overcome. Unfortunately, this is not the case as reduction of this thickness below a certain thickness limit lowers the efficiency significantly. As such, several techniques to offset this drawback have been proposed; they include: the use of light-trapping nanostructures, alternative back contacts, and back-surface passivation. In fact, recent publications have reported that optoelectronic simulations of thin-film Schottky-barrier solar cells with periodically nonhomogeneous absorbing layers of InGaAn and periodically corrugated backreflectors predict improvement in efficiency.

Although nonhomogeneity (i.e., bandgap grading) of the CIGS layer could increase efficiency by establishing drift fields, simple simulations as well as experiments such as the one mentioned above have shown that linear grading of the bandgap can significantly reduce the short circuit current density. Therefore, novel strategies are required for bandgap grading to maintain the current density and enhance the open circuit voltage. On this account, a team of researchers from the Pennsylvania State University: PhD student Faiz Ahmad and Professor Akhlesh Lakhtakia, in collaboration with Dr. Tom Anderson and Professor Peter Monk at the University of Delaware, engaged in a detailed optoelectronic optimization of ultrathin CIGS solar cells with a nonhomogeneous CIGS layer with back-surface passivation and backed by a periodically corrugated metallic backreflector. Their work is currently published in the research journal, Applied Optics.

In their approach, the power conversion efficiency of an ultrathin CIGS solar cell was maximized using a coupled optoelectronic model to determine the optimal bandgap grading of the nonhomogeneous CIGS layer in the thickness direction. The bandgap of the CIGS layer was either sinusoidally or linearly graded, and the solar cell was modeled to have a metallic backreflector corrugated periodically along a fixed direction in the plane.

The authors reported that their model was able to predict that specially tailored bandgap grading could significantly improve the efficiency, with much smaller improvements due to the periodic corrugations. To be specific, an efficiency of 27.7% with the conventional 2200-nm-thick CIGS layer was predicted with sinusoidal bandgap grading, in comparison to 22% efficiency obtained experimentally with homogeneous bandgap. Moreover, the researchers reported that the inclusion of sinusoidal grading increased the predicted efficiency to 22.89% with just a 600-nm-thick CIGS layer.

In summary, optoelectronic optimization was carried out for an ultrathin CIGS solar cell with a CIGS layer that was nonhomogeneous along the thickness direction and a metallic backreflector corrugated periodically along a fixed direction. Ideally, the bandgap in the CIGS layer was either sinusoidally or linearly graded. In a statement to Advances in Engineering, Professor Akhlesh Lakhtakia, the corresponding author and an internationally recognized pioneer in modeling solar cells highlighted that their reported high efficiencies arose due to a large electron–hole pair generation rate in the narrow-bandgap regions and the elevation of the open-circuit voltage due to a wider bandgap in the region toward the front surface of the CIGS layer. Overall, the bandgap nonhomogeneity, in conjunction with periodic corrugation of the backreflector, could be effective in realizing ultrathin CIGS solar cells that can help overcome the scarcity of indium.

Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading - Advances in Engineering

About the author

Akhlesh Lakhtakia joined the Pennsylvania State University in 1983, where he was elevated to the rank of Distinguished Professor of Engineering Science and Mechanics in January 2004. In 2006, he became the Charles Godfrey Binder (Endowed) Professor of Engineering Science and Mechanics. In 2018, he received the highest rank at Penn State: Evan Pugh University Professor.

Dr. Lakhtakia has published more than 880 journal articles; has contributed 33 chapters to research books and encyclopedias; has edited, co-edited, authored or co-authored 21 books and 26 conference proceedings; has authored or co-authored 369 conference papers; has reviewed for 175 journals; serves on the editorial boards of five electromagnetics journals; and became the first Editor-in-Chief of the online Journal of Nanophotonics published by SPIE from 2007. He serves as an international lecturer for the International Commission for Optics, SPIE, and the Optical Society of America; was twice a Visiting Professor of Physics at Universidad de Buenos Aires, a Visiting Professor of Physics at the University of Otago, a Visiting Professor of Physics at Imperial College London, a Visiting Fellow in Mathematics at the University of Glasgow, and Honorary International Professor National Taipei University of Technology; and headed the IEEE EMC Technical Committee on Nonsinusoidal Fields from 1992 to 1994.

Dr. Lakhtakia has been elected a Fellow of the Optical Society of America (1992), SPIE (1996), the UK Institute of Physics (1996), the American Association for the Advancement of Sciences (2009), the American Physical Society (2012), the Institute of Electrical and Electronics Engineers (2016), the Royal Society of Chemistry (2016), and the Royal Society of Arts (2017). He was named to the inaugural class of Outstanding Reviewers by the Optical Society of America in 2012. He also served as the 1995 Scottish Amicable Visiting Lecturer at the University of Glasgow. He received the PSES Outstanding Research Award in 1996, the PSES Premier Research Award in 2008, and the PSES Outstanding Advising Award in 2005. For his research on sculptured thin films and complex-medium electromagnetics, he received the Faculty Scholar Medal in Engineering in 2005 from the Pennsylvania State University, and was the sole recipient of the 2010 SPIE Technical Achievement Award. Nanotech Briefs recognized him in 2006 with a Nano 50 Award for Innovation. Sigma Xi bestowed on him the Walston Chubb Award for Innovation in 2016. The University of Utah made him a Distinguished Alumnus in 2007 and the Indian Institute of Technology (BHU) in 2014. IIT(BHU) conferred on him an “Alumnus of the Century” award in 2019.

His current research interests lie in the electromagnetics of complex materials, sculptured thin films, nanoengineered metamaterials, surface multiplasmonics, solar cells, engineered biomimicry, and forensic science. His research accomplishments have been discussed on CNN and in a NOVA movie. His recent research has been covered on several scientific media outlets.


Faiz Ahmad, Tom H. Anderson, Peter B. Monk, Akhlesh Lakhtakia. Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading. Applied Optics, Volume 58, No. 22 / 1 August 2019.

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