Abstract
GaInN-based photovoltaic (PV) devices are highly promising for application to optical wireless power transmission (OWPT) systems as well as solar cells. This paper reports the research results of Ga0.9In0.1N multiple-quantum-well (MQW) PV cells on sapphire, focusing primarily on the growth temperature managements in metalorganic chemical vapor deposition (MOCVD) processes. As a result of the MOCVD study, the epilayer qualities in the PV cell structures improved significantly through the adoption of an optimized growth temperature for the GaInN MQWs and the two-step growth for the top p-GaN layers. Furthermore, the improved epilayer qualities resulted in the decrease in carrier recombination currents and series resistance for the forward diode characteristics without a light illumination. Subsequently, a sample with the improved qualities exhibited a higher open-circuit voltage and a higher fill factor in the PV characteristics. Eventually, the highest power conversion efficiency (PCE) in this study was measured to be 1.6% at a 1-sun solar spectrum and 42.7% at a monochromatic light illumination with 389 nm in wavelength and 5 mW cm−2 in optical power density. The dependency of the PV performance on the optical power densities at a monochromatic light illumination predicted that a higher PCE value may be achievable at a higher optical-power-density illumination. This is a very promising prediction when considering the practical application to OWPT systems.
Highlights
GaInN-based alloys have been considered as potential materials for photovoltaic (PV) solar cells since they are nontoxic and have direct-transition energy bandgaps (Eg) varying with their alloy compositions from 3.4 eV for GaN to 0.64 eV for InN, which can encompass a wide region of the solar spectrum.1–24 In addition, GaInNbased PV cells are considered suitable for photoelectric transducers (PTs) utilized in the optical wireless power transmission (OWPT) systems,25–29 which have recently attracted interest as a promising technology applicable to the short-to-long distance wireless power transmission
We confirmed that the growth temperature management in the metalorganic chemical vapor deposition (MOCVD) process was effective in acquiring PV cell structures with good epilayer qualities and electrical characteristics
The evaluation of PV performance revealed that a PV cell with the high-quality structural and electrical properties exhibited highly improved VOC and FF values and thereby exhibited the highest power conversion efficiency (PCE) we have ever reported
Summary
GaInN-based alloys have been considered as potential materials for photovoltaic (PV) solar cells since they are nontoxic and have direct-transition energy bandgaps (Eg) varying with their alloy compositions from 3.4 eV for GaN to 0.64 eV for InN, which can encompass a wide region of the solar spectrum. In addition, GaInNbased PV cells are considered suitable for photoelectric transducers (PTs) utilized in the optical wireless power transmission (OWPT) systems, which have recently attracted interest as a promising technology applicable to the short-to-long distance wireless power transmission. For PT devices for OWPT systems, Eq (1) is rewritten as follows by assuming the operation under a monochromatic light illumination: JSC (A cm−2) × VOC(V) × FF Nλ (s−1 cm−2) × Eλ(eV). (JSC/q)/Nλ and qVOC/Eλ represent an EQE and a potential efficiency ηV at the specific λ, respectively, which, as well as the FF, are key factors for considering the performance of PT devices. Many researchers have reported GaInN-based PV cells utilizing multiple-quantum-well (MQW) structures, which include several thin GaInN well layers acting as light absorption layers.. Many researchers have reported GaInN-based PV cells utilizing multiple-quantum-well (MQW) structures, which include several thin GaInN well layers acting as light absorption layers.9–24 This is primarily because growing thick and high InN-mole-fraction GaInN films that are appliable to the PV solar cells is difficult due to their thermodynamic stability.. The fabricated PV cells were evaluated under artificial sunlight and monochromatic light illuminations for possible application to OWPT systems
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