(Invited) InGaN/GaN Multiple Quantum Well Solar Cells for Energy and Hydrogen Generation
Authors: Jiang Hongxing , Lin Jingyu 
Published: February 2020

III-nitride wide bandgap semiconductors have been at the center stage of the semiconductor R & D effort over the last decade. Photonic/electronic devices based on III-nitrides, including UV/blue/green/white LEDs, violet/blue LDs, UV detectors, and high power/temperature transistors, have reached high performance level.
This talk will provide a brief overview on recent advances made by our group and others in the area of III-nitrides for energy and hydrogen generation. The presentation will concentrate on topics of material and device challenges of III-nitride solar cells and photoelectrochemical cells (PECs). The bandgap of InGaN expands from about 0.65 to 3.4 eV, which covers the entire solar spectrum. In principle, a multi-junction solar cell based on multiple layers of InGaN with different In-contents is capable of capturing the entire spectrum of the sunlight and can thus be highly efficient. To count for the relatively low materials quality for high In content InGaN alloys, InGaN/GaN multiple quantum wells (MQWs) have been adopted for the construction of nitride solar cells. We have grown, fabricated, and characterized InGaN based solar cells utilizing the InGaN/GaN MQWs p-i-n structures.1-3The fabricated solar cells exhibited open circuit voltage of about 2 V, a fill factor of about 60% and overall efficiency of about 3% under AM 1.5 irradiation. The performance of these nitride solar cells under concentrated sunlight has also been investigated.
In additional to its bandgap match to the solar spectrum, the bandgap of InGaN alloys can also be engineered to optimally match with conditions necessary for direct hydrogen generation by water splitting using sunlight.4-6 The results on hydrogen gas generation via solar water splitting accomplished using n- and p-type InGaN epilayer as working electrodes will be discussed. Direct solar water splitting and hydrogen gas generation under zero bias has been realized using an InGaN/GaN multiple quantum well solar cell as a photoelectrochemical cell. Under the irradiation by a simulated sunlight (AM1.5G solar simulator), a 1.5% solar-to-fuel conversion efficiency has been achieved under zero bias, setting a new benchmark of employing III-nitrides as photoreactive electrodes for artificial photosynthesis. Time dependent hydrogen gas production photocurrent measured over a prolonged period revealed an excellent chemical stability of InGaN in aqueous solution. Our results indicate that InGaN is an excellent artificial photosynthesis material which is capable to provide usable clean fuel (hydrogen gas) with the sunlight being the only energy input.
InGaN materials growth effort was supported by NSF (DMR-1206652) and the energy device effort was by DOE (FG02-09ER46552). Jiang & Lin are grateful to AT&T Foundation for the support of Ed & Linda Whitacre endowed chairs.

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