Junko Minemura

Process damage free thin-film GaAs solar cells by epitaxial liftoff with GaInP window layer

Abstract We report on process damage free thin-film GaAs cells detached from the GaAs substrates. GaAs cells grown by gas-source MBE were thinned by the epitaxial liftoff (ELO) technique. Photoluminescence spectroscopy showed a peak splitting in the band emission, indicating that a strain was induced in the thin-film cell fixed on the quartz glass substrate. The strain, however, was found not to affect the quality of the thin-film cells, based on the fact that the peak intensity was almost twice that before ELO. The thin-film GaAs cells showed no evidence of degradation in diode characteristics and spectral responses. The keys to avoiding damage on the active region of the solar cell during the thinning process are the introducing a GaInP window layer and improving the thin film process including metallization on thin film cells. These results demonstrates that the thinning and transfer processes dol-not affect the quality of the active region of the cells.

GaInP single junction and GaInP/GaAs two junction thin-film solar cell structures by epitaxial lift-off

Abstract The epitaxial lift-off (ELO) technique was used in forming a thin-film GaInP/GaAs two-junction monolithic tandem solar cell structure. First, the GaInP single junction solar cell to be used in the tandem cell structure as a top cell was thinned by the ELO process. Although the ELO process and the transfer to the quartz substrate caused a strain in the thin-film cell after separation from the GaAs substrate, the photoluminescence peak intensity was not decreased. This shows that defects, such as those causing carrier loss, were not introduced on the thin-film cell during the thinning process. The key issue for thin-film cell fabrication is to avoid damaging the AlInP window layer during the selective etching (HF etchant), by which the thin-film cell is released from the GaAs substrate. A GaInP/GaAs monolithic tandem structure was also thinned by the same process with a GaInP single junction cell. Characteristics of the single-junction GaInP cell and individual cells in the GaInP/GaAs tandem structure were examined. It was found that the spectral response remains almost the same as that for cells with a GaAs substrate, thus confirming the feasibility of using the ELO process to fabricate thin-film GaInP/GaAs cells.

Optimal growth procedure of GaInP/GaAs heterostructure for high-efficiency solar cells

Abstract We have developed an optimal growth procedure for gas-source MBE production of a GaInP/GaAs heterointerface. The interface quality is crucial to obtaining high-performance GaAs solar cells with a GaInP barrier layer because minority carrier lifetime depends strongly on the interface structure. In situ Reflective High-Energy Electron Diffraction (RHEED) observation during the growth across the GaInP/GaAs heterointerface revealed that the phosphorus atoms are replaced by arsenic atoms in the near-interface region of the GaInP layer, and a transient layer acting as a carrier trap is formed. Introduction of a GaP layer into the interface was found to be effective in suppressing carrier loss. From Composition Analysis by Thickness Fringe-Transmission Electron Microscopy (CAT-TEM) images, it was also found that the optimum thickness of inserted GaP to avoid the generation of misfit dislocations is 1 nm.

Carrier generation and transport in InGaAs/GaAs multiple quantum well solar cells

Carrier generation and transport properties of GaAs p-i-n structure solar cells incorporating an In/sub x/Ga/sub 1-x/As/GaAs multiple quantum well (MQW) are reported. In this type of solar cell, carrier recombination in the MQW should be suppressed to a level far below photogeneration, hence we studied the carrier transport by measuring the photocurrent of MQW solar cells with different potential depths, at various temperatures, under various applied voltages. Photocurrents for samples with 0/spl les/x/spl les/0.15 at room temperature showed that the ratio of the number of photogenerated carriers contributing to the photocurrent to the number of absorbed photons, expressed as quantum efficiency, was almost unity. The experimental results are well expressed with the theoretical efficiency of the carrier transport over the MQW in which recombination and escape of carriers are characterized by lifetimes depending on the potential depth of the well.

AlGaAs solar cells grown by MBE for high-efficiency tandem cells

Abstract AlGaAs solar cells for AlGaAs/GaAs tandem cell applications have been investigated. The performance of the AlGaAs cell grown by a molecular beam epitaxy (MBE) technique is believed to have strong dependence on growth temperature. The conversion efficiency, electrical properties, and the crystal quality of AlGaAs cells MBE grown at different temperatures have been characterized and correlations between each characteristic are discussed. The conversion efficiency is maximized at a growth temperature of 660°C. This agrees well with the results that the bulk recombination current is the lowest at this temperature. The structure of an AlGaAs cell in the AlGaAs/GaAs tandem configuration is optimized through numerical simulation using the parameters obtained from the AlGaAs and GaAs single junction cells. The efficiency for tandem cells is shown to be the highest when the thickness of the AlGaAs cell was chosen so that the incident light near the absorption edge might be shared with the bottom GaAs cell.

Photocurrent and photoluminescence in InGaAs/GaAs multiple quantum well solar cells

We have simultaneously measured photocurrent (PC) and photoluminescence (PL) in GaAs solar cells, including InGaAs/GaAs multiple quantum wells (MQWs) in order to clarify the optimum structure for MQW solar cells. MQW solar cells with shallow wells, for which the carrier escape probability η is calculated to be unity, exhibit a sharp rectifying PC profile. On the other hand, those with deep wells, in which η is nearly zero, exhibit degraded PC-voltage curves and strong PL from the MQWs. From these experimental results, it was found that the introduction of shallow wells with η=1 is essential to prevent degradation of cell performance and is expected to improve the conversion efficiency of MQW solar cells.

Vertical Transport Properties of Photogenerated Carrier in InGaAs/GaAs Strained Multiple Quantum Wells

We report on the carrier transport properties of a GaAs p-i-n structure including In x Ga 1−x As/GaAs (x = 0-0.15) strained multiple quantum wells (MQWs). Photocurrent measurements at room temperature showed that the spectral response extended to lower energies with increasing In content, and the quantum efficiency (ratio of the number of photogenerated carriers contributing photocurrent to the number of absorbed photons) was almost unity even when the well depth was several times greater than the thermal energy kT at room temperature. This is consistent with the calculation of the time for carriers to escape from the InGaAs/GaAs single quantum well. The calculation result also shows that applying a high electric field increases the photocurrent gain in MQW diodes

Epitaxial growth of GaAs on HF-treated Si substrates

Abstract Introduction of a Ga layer on an HF-treated Si substrate and its effect on the generation and behavior of dislocations in the over-grown GaAs layers are studied. There are no phases except for the crystal phase and many dislocations are observed in the lattice image at theGaAs/Si interface, although an amorphous Ga layer on the hydrogen-terminated Si is observed by RHEED before the growth of a GaAs layer. In spite of the high dislocation density at theGaAs/Si interface, the dislocation density at the GaAs surface is the same as that of conventional structures. This indicates that initial growth condition may control the formation of dislocations.