Improved epilayer qualities and electrical characteristics for GaInN multiple-quantum-well photovoltaic cells and their operation under artificial sunlight and monochromatic light illuminations

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.

Over 43%-power-efficiency GaInN-based photoelectric transducer on free-standing GaN substrate for optical wireless power transmission system

“…On the other hand, the fabricated PTs exhibited the same V OC value as high as approximately 2.3 V regardless of the adoption of those surface processes. In addition, this V OC value was found to be fairly improved compared with approximately 2.1 V obtained for PTs on sapphire we have ever reported [8]. This difference corresponds to the improvement from approximately 66.5%-70.9% in the potential efficiency PE.…”

“…The two-terminal PT devices with an effective lightreceiving area of 1 mm 2 were fabricated using the conventional photolithographic method as follows [8]. First, the ntype contact region was etched by BCl 3 plasma reactive ion etching.…”

“…As we expected, the observation results confirmed that their surface quality was drastically improved with the use of FS-GaN as the growth substrate. More specifically, the sample on FS-GaN showed a clear step-like surface morphology with a small RMS roughness of approximately 0.8 nm, while the sample on sapphire showed a rough surface morphology [8]. The cross-sectional STEM images clarifies that the rough surface observed for the sample on sapphire was attributed to the surface V-shape pits caused by the threading dislocations in the epitaxial structure [8,23] and that those were markedly decreased with the use of FS-GaN.…”

“…Recently, we have reported that GaInN-based PTs, which were formed on a sapphire substrate to have 42-period Ga 0.9 In 0.1 N multiple-quantum-wells (MQWs) as a light absorption layer, exhibited a PCE value of 42.7% at a monochromatic light illumination with a wavelength of 389 nm and a power density of 5 mWcm −2 [8]. In more specific, the respective values of EQE, PE and FF were confirmed to be 89.6%, 66.5% and 71.7%, which indicates the particular importance of increasing PE and FF for achieving further performance improvement.…”

“…For that purpose, it seems effective to improve the crystal quality of epitaxial structures, which would allow the diode characteristics closer to be ideal ones with less carrier recombination and generation processes and thereby enlarge the shunt resistance R sh as well as V OC , resulting in high PE and FF [8,13,24]. In this study, therefore, we grew the GaInN MQW PT structure using a high-crystalquality free-standing (FS) GaN substrate as an underlying growth substrate instead of the sapphire substrate we have ever used [8,21,22]. In addition, to investigate their effects on the surface reflectance and its contribution to the device performance, we considered an adoption of a wet surface treatment with a tetramethyl ammonium hydroxide (TMAH) solution [25][26][27] and of the anti-reflection (AR) coating with an Al 2 O 3 film by the atomic layer deposition (ALD) [21,28].…”