Kangho Kim

Narrow band gap (1 eV) InGaAsSbN solar cells grown by metalorganic vapor phase epitaxy

Heterojunction solar cell structures employing InGaAsSbN (Eg ∼ 1 eV) base regions are grown lattice-matched to GaAs substrates using metalorganic vapor phase epitaxy. Room temperature (RT) photoluminescence (PL) measurements indicate a peak spectral emission at 1.04 eV and carrier lifetimes of 471–576 ps are measured at RT from these structures using time-resolved PL techniques. Fabricated devices without anti-reflection coating demonstrate a peak efficiency of 4.58% under AM1.5 direct illumination. Solar cells with a 250 nm-thick InGaAsSbN base layer exhibit a 17% improvement in open circuit voltage (Voc), 14% improvement in fill factor, and 12% improvement in efficiency over the cells with a thicker (500 nm-thick) base layer.

Highly efficient single-junction GaAs thin-film solar cell on flexible substrate

There has been much interest in developing a thin-film solar cell because it is lightweight and flexible. The GaAs thin-film solar cell is a top contender in the thin-film solar cell market in that it has a high power conversion efficiency (PCE) compared to that of other thin-film solar cells. There are two common structures for the GaAs solar cell: n (emitter)-on-p (base) and p-on-n. The former performs better due to its high collection efficiency because the electron diffusion length of the p-type base region is much longer than the hole diffusion length of the n-type base region. However, it has been limited to fabricate highly efficient n-on-p single-junction GaAs thin film solar cell on a flexible substrate due to technical obstacles. We investigated a simple and fast epitaxial lift-off (ELO) method that uses a stress originating from a Cr/Au bilayer on a 125-μm-thick flexible substrate. A metal combination of AuBe/Pt/Au is employed as a new p-type ohmic contact with which an n-on-p single-junction GaAs thin-film solar cell on flexible substrate was successfully fabricated. The PCE of the fabricated single-junction GaAs thin-film solar cells reached 22.08% under air mass 1.5 global illumination.

Impact of thermal annealing on bulk InGaAsSbN materials grown by metalorganic vapor phase epitaxy

Two different thermal annealing techniques (rapid thermal annealing (RTA) and in-situ post-growth annealing in the metalorganic vapor phase epitaxy (MOVPE) chamber) were employed to investigate their impact on the optical characteristics of double-heterostructures (DH) of InGaAsSbN/GaAs and on the performance of single-junction solar cell structures, all grown by MOVPE. We find that an optimized RTA procedure leads to a similar improvement in the photoluminescence (PL) intensity compared with material employing a multi-step optimized anneal within the MOVPE reactor. Time-resolved photoluminescence techniques at low temperature (LT) and room temperature (RT) were performed to characterize the carrier dynamics in bulk InGaAsSbN layers. Room temperature carrier lifetimes were found to be similar for both annealing methods, although the LT-PL (16 K) measurements of the MOVPE-annealed sample found longer lifetimes than the RTA-annealed sample (680 ps vs. 260 ps) for the PL measurement energy of 1.24 eV. InGaAsSbN-based single junction solar cells processed with the optimized RTA procedure exhibited an enhancement of the electrical performance, such as improvements in open circuit voltage, short circuit current, fill factor, and efficiency over solar cells subjected to the in-situ MOVPE annealing technique.

Influences of InGaP conical frustum nanostructures on the characteristics of GaAs solar cells

Conical frustums with quasihexagonal nanostructures are fabricated on an InGaP window layer of single junction GaAs solar cells using a polystyrene nanosphere lithography technique followed by anisotropic etching processes. The optical and photovoltaic characteristics of the conical frustum nanostructured solar cells are investigated. Reflectance of the conical frustum nanostructured solar cells is significantly reduced in a wide range of wavelengths compared to that of the planar sample. The measured reflectance reduction is attributed to the gradual change in the refractive index of the InGaP conical frustum window layer. An increase of 15.2% in the power conversion efficiency has been achieved in the fabricated cell with an optimized conical frustum nanostructure compared to that of the planar cell.

Characteristics of bulk InGaAsN and InGaAsSbN materials grown by metal organic vapor phase epitaxy (MOVPE) for solar cell application

Bulk, lattice-matched InGaAsSbN material has been grown by metal organic vapor phase epitaxy (MOVPE) for applications in concentrated multi-junction solar cells. By optimizing the growth conditions for high Sb and As partial pressures, we achieved background hole concentrations as low as 2 x 1018 cm-3. After thermal annealing, the background hole concentration increased from 2x1018 to 2 x 1019 cm-3, although PL intensity increased by a factor of 7. We recently grew single junction (1eV) solar cells incorporating dilute-nitride materials and devices were fabricated and characterized for solar cell application. Performance characteristics of these cells without anti-reflection coating included the efficiency of 4.25% under the AM1.5 (air mass) direct illumination, Voc of 0.7 V, and a spectral response extended to longer wavelength compared with GaAs cells.

1.25-eV GaAsSbN/Ge Double-Junction Solar Cell Grown by Metalorganic Vapor Phase Epitaxy for High Efficiency Multijunction Solar Cell Application

Dilute-nitride-antimonide materials grown by metalorganic vapor phase epitaxy (MOVPE) with bandgap energies of 1.25 eV have been integrated into solar cell structures employing a Ge bottom cell on Ge substrate. Single homo- and heterojunction solar cells employing narrow bandgap GaAsSbN (E g ~ 1.25 eV) are grown normally lattice-matched on a GaAs substrate, using MOVPE. Homojunction solar cell structures were realized by employing GaAsSbN material with low carbon background concentration and Si doping to form a p/n junction. External quantum efficiency measurements in the range (870 nm-1000 nm) reveal that the efficiency of the homojunction solar cell is significantly improved over that of the heterojunction structure. The GaAsSbN homojunction cell was integrated with a Ge single-junction bottom cell on Ge substrate. Under AM1.5 direct illumination, the fabricated GaAsSbN (1.24 eV)/Ge double-junction solar cell with a 600-nm-thick GaAsSbN base layer exhibits Jsc, Voc, FF, and efficiency values of 11.59 mA/cm 2, 0.83 V, 72.58%, and 7% with anti-reflection coating (ARC), respectively.

Ge nanopillar solar cells epitaxially grown by metalorganic chemical vapor deposition

Radial junction solar cells with vertically aligned wire arrays have been widely studied to improve the power conversion efficiency. In this work, we report the first Ge nanopillar solar cell. Nanopillar arrays are selectively patterned on p-type Ge (100) substrates using nanosphere lithography and deep reactive ion etching processes. Nanoscale radial and planar junctions are realized by an n-type Ge emitter layer which is epitaxially grown by MOCVD using isobutylgermane. In situ epitaxial surface passivation is employed using an InGaP layer to avoid high surface recombination rates and Fermi level pinning. High quality n-ohmic contact is realized by protecting the top contact area during the nanopillar patterning. The short circuit current density and the power conversion efficiency of the Ge nanopillar solar cell are demonstrated to be improved up to 18 and 30%, respectively, compared to those of the Ge solar cell with a planar surface.

Internal stress-assisted epitaxial lift-off process for flexible thin film (In)GaAs solar cells on metal foil

Flexible thin film (In)GaAs solar cells are grown by metalorganic chemical vapor deposition on GaAs substrates and transferred to 30 μm thick Au foil by internal stress-assisted epitaxial lift-off processes. The internal stress is induced by replacing the solar cell epi-layers from GaAs to In0.015Ga0.985As, which has a slightly larger lattice constant. The compressive strained layer thickness was varied from 0 to 4.5 μm to investigate the influence of the internal stress on the epitaxial lift-off time. The etching time in the epitaxial lift-off process was reduced from 36 to 4 h by employing a GaAs/In0.015Ga0.985As heterojunction structure that has a compressive film stress of −59.0 MPa. We found that the partially strained epi-structure contributed to the much faster lateral etching rate with spontaneous bending. Although an efficiency degradation problem occurred in the strained solar cell, it was solved by optimizing the epitaxial growth conditions.

Carrier dynamics in QW and bulk bismide and epitaxial lift off GaAs-In(Al)GaP double heterostructures grown by MOVPE for multi-junction solar cells

III-V multi-junction solar cells are based on a triple-junction design that consists of an InGaP top junction, a GaAs middle junction, and a bottom junction that employs either a 1eV material grown on the GaAs substrate or InGaAs grown on the Ge substrate. The most promising 1 eV materials under extensive investigation are the bulk dilute nitride such as InGaAsN(Sb) lattice-matched to GaAs substrate and the dilute-bismide quantum well materials, such as GaAsBi, strain-compensated with GaAsP barriers. Both approaches have the potential to achieve high performance triple-junction solar cells. In addition, space satellite applications utilizing III-V triple-junction solar cells can have significantly reduced weight and high efficiency. An attractive approach to achieve these goals is to employ full-wafer epitaxial lift off (ELO) technology, which can eliminate the substrate weight and also enable multiple substrate re-usages. For the present study, we employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in MOVPE-grown bulk dilute bismide double heterostructures (DH). Carrier lifetime measurements are crucial to optimizing MOVPE materials growth. We have studied carrier dynamics in GaAsBi QW structures with GaAsP barriers. Carrier lifetimes were measured from GaAsBi DH samples at different stages of post-growth thermal annealing steps. Post-growth annealing yielded significant improvements in carrier lifetimes. Based on this study, single junction solar cells (SJSC) were grown and annealed under a variety of conditions and characterized. The SJSC annealed at 600 – 650 °C exhibited improved response in EQE spectra. In addition, we studied carrier dynamics in MOVPE-grown GaAs-In(Al)GaP DH samples grown on GaAs substrates. The structures were grown on top of a thin AlAs release layer, which allowed epitaxial layers grown on top of the AlAs layer to be removed from the substrate. The GaAs active layers had various doping densities and thicknesses. Our TR-PL results from both pre- and post-ELO processed GaAs-In(Al)GaP DH samples are reported.

Single junction solar cell employing strain compensated GaAs0.965Bi0.035/GaAs0.75P0.25 multiple quantum wells grown by metal organic vapor phase epitaxy

Single junction solar cells employing 30-period and 50-period GaAs0.965Bi0.035/GaAs0.75P0.25 (Eg ∼ 1.2 eV) multiple quantum wells (MQWs) as base regions were grown by metal organic vapor phase epitaxy. Room temperature photoluminescence measurements indicated a peak spectral emission at 1.18 eV, and the spectral dependence of the external quantum efficiency measured from the fabricated devices shows the extended absorption edge relative to that of GaAs. The fabricated devices with anti-reflection coating employing a 50-period MQW structure exhibit 23% improvement in the conversion efficiency, 4% in the open-circuit voltage, 9% in the short-circuit current density, and 9% in the fill factor, compared to those from the devices employing a 30-period MQW structure in the base region, under AM1.5 direct illumination.Single junction solar cells employing 30-period and 50-period GaAs0.965Bi0.035/GaAs0.75P0.25 (Eg ∼ 1.2 eV) multiple quantum wells (MQWs) as base regions were grown by metal organic vapor phase epitaxy. Room temperature photoluminescence measurements indicated a peak spectral emission at 1.18 eV, and the spectral dependence of the external quantum efficiency measured from the fabricated devices shows the extended absorption edge relative to that of GaAs. The fabricated devices with anti-reflection coating employing a 50-period MQW structure exhibit 23% improvement in the conversion efficiency, 4% in the open-circuit voltage, 9% in the short-circuit current density, and 9% in the fill factor, compared to those from the devices employing a 30-period MQW structure in the base region, under AM1.5 direct illumination.