Date of Graduation
7-2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Physics (PhD)
Degree Level
Graduate
Department
Physics
Advisor/Mentor
Ware, Morgan
Committee Member
Salamo, Gregory
Second Committee Member
Churchhill, Hugh
Keywords
Indium gallium nitride (InGaN), solar cells, tunable bandgap, Shockley limit, electric fields
Abstract
Indium gallium nitride (InGaN) materials have shown significant potential for photovoltaic and optoelectronic applications due to their excellent optical and electrical properties. The unique direct bandgap, strong optical absorption, thermal stability, and high radiation resistance make InGaN alloys particularly promising for high performance solar cells. One of the most important advantages of InGaN is its tunable bandgap, which ranges from 0.7 eV for InN to 3.4 eV for GaN. This wide bandgap range enables absorption across a broad portion of the solar spectrum, extending from infrared to ultraviolet. Despite these advantages, the power conversion efficiency of conventional single p-n junction solar cells remains restricted by the Shockley limit. To overcome this limitation, alternative device structures have been proposed, including the p1-n1-p2-n2 homojunction structure, which introduces multiple junctions and internal electric fields that enhance carrier separation and reduces recombination losses, thereby enabling efficiency improvements beyond the Shockley limit. This work investigated the potential of a InGaN homojunction solar cell based on the p1-n1-p2-n2 structure as a pathway toward achieving power conversion efficiency beyond those attainable with conventional single p-n junction devices. Numerical simulations were performed using nextnano++ to systematically analyze the electrical and optical behavior of the proposed structure. The study examined the effects of varying doping concentrations, from uniform doping to modifying the second or third layers, adjusting the thickness of the second or third layers, applying light doping, modifying the top layer thickness, and observing the effects of incident light intensity on the p1-n1-p2-n2 structure. The impact of these parameters on open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and overall efficiency (η) were evaluated through current-voltage characteristics and energy band diagram analysis. The results show that, under uniform doping of 6.5E+16 cm-3, the p1-n1-p2-n2 homojunction structure achieves an efficiency of 32.91%, representing an improvement of approximately 7% compared with p-n junction’s 25.31% efficiency [1]. The doping concentration of 6.5E+16 cm-3 was selected as the standard for experiment. A slight increase in efficiency to 33.67% was observed after varying the thickness of the second layer to 750 nm. Further enhancements in efficiency are achieved through structural optimization, where variations in the top layer thickness yield efficiency approaching 40% with a thickness of 30 nm. Most notably, as we increase the third layer thickness to 180 nm, it results in a sharp turn on behavior in the I-V characteristics, near unity fill factor and a maximum demonstrated efficiency of 57%, indicating a transition from conventional photovoltaic operation to a bistable, thyristor-like switching device. Under varying illumination levels, the optimum efficiency of 33.85% is achieved at an intensity of 6 suns, with standard uniform doping of 6.5E+16 cm-3 and fixed layer thicknesses. These results show the p1-n1-p2-n2 structure is a promising pathway for improving efficiency beyond that of a single p-n junction solar cell. They also provide a better insight into how internal electric fields across three junctions enhance device performance.
Citation
ALHEJJI, M. (2026). Investigating the Efficiency of InGaN p‑n‑p‑n Homojunction Solar Cells. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/6312
