Shigenori Furue

Ultra-high stacks of InGaAs/GaAs quantum dots for high efficiency solar cells

We report ultra-high stacked InGaAs/GaAs quantum dot (QD) solar cells fabricated by the intermittent deposition of In0.4Ga0.6As under an As2 source using molecular beam epitaxy. We obtain a 400-stack In0.4Ga0.6As QD structure without using a strain balancing technique, in which the total number of QDs reaches 2 × 1013 cm−2. Photoluminescence and cross-sectional scanning transmission electron microscope measurements indicate that the In0.4Ga0.6As QD structure exhibits no degradation in crystal quality, no dislocations and no crystal defects even after the stacking of 400 QD layers. The external quantum efficiency and the short-circuit current density of multi-stacked In0.4Ga0.6As QD solar cells increase as the number of stacked layers is increased to 150. Such ultra-high stacks and good cell performance have not been reported for QD solar cells using other material systems. The performance of the ultra-high stacked QD solar cells indicates that InGaAs QDs are suitable for use in high efficiency solar cells requiring thick QD layers for sufficient light absorption.

Highly stacked and well-aligned In0.4Ga0.6As quantum dot solar cells with In0.2Ga0.8As cap layer

We report In0.4Ga0.6As quantum dot (QD) solar cells with In0.2Ga0.8As cap layers, which extends the photoabsorption spectra toward a wavelength longer than those of In0.4Ga0.6As QD solar cells without cap layers. Well-aligned 50-stack In0.4Ga0.6As QD structures with In0.2Ga0.8As cap layers can be grown without using a strain balancing technique. The photoluminescence wavelength of ten-stack In0.4Ga0.6As QDs with an In0.2Ga0.8As cap layer becomes longer, as a result of the reduced strain in the QDs achieved by using the cap layer. The cell characteristics of multistacked In0.4Ga0.6As QD solar cells are improved by employing In0.2Ga0.8As cap layers.

Gain-Enhanced InGaAs–InP Heterojunction Phototransistor With Zn-Doped Mesa Sidewall

A gain-enhanced InGaAs-InP heterojunction phototransistor (GE-HPT) with a Zn-doped mesa sidewall designed to reduce both base-emitter recombination currents and base-collector dark currents is realized. The developed GE-HPT has an optical conversion efficiency as large as 24 kA/W at an incident optical power of 29 nW and a wavelength of 1.55 mum, which is the highest gain yet reported in HPTs. The measured dark current in the base-collector junction is comparable to that of planar-type PIN photodiodes. The high gain and low dark current characteristics of the phototransistors make them promising for use in a weak light detection system.

Effects of Zn Doped Mesa Sidewall on Gain Enhanced InGaAs/InP Heterobipolar Phototransistor

The excellent detectability of the gain enhanced InGaAs/InP heterobipolar phototransistor (GE-HPT) is demonstrated and attributed to a reduction in the reverse leakage current at the base-collector junction and the enhancement of current gain at the emitter-base junction achieved by using a current blocking structure with a Zn doped mesa sidewall. The common emitter grounded current gain agrees well with the photo-conversion efficiency of several tens of thousands of A/W at incident optical powers in the hundred nanowatt to sub-picowatt range over several orders of magnitude. The deep mesa structure in the GE-HPT is also effective in ensuring superior isolation of better than 25 dB between adjacent arrays.

Characteristics of highly stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs/GaAs QD structure of over 100 layers without using a strain compensation technique. The external quantum efficiency of multistacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the efficiency increases as the number of stacking layers increases. The short-circuit current density of the solar cells increases as the number of InGaAs QD layers increases. Moreover, InGaAs QD solar cells have high open circuit voltage and good cell characteristics even though an interdot spacing is reduced to 3.5 nm. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers with a minband for sufficient light absorption.

Ultra-high stacks of InGaAs quantum dots for high efficiency solar cells

We report ultra-high stacks of quantum dots (QDs) for high efficiency solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a 400-stack InGaAs/GaAs QD structure without using a strain balancing technique, in which the total number of QDs reaches 2 × 1013 cm−2. Photoluminescence and cross-sectional scanning transmission electron microscope measurements indicate that the In0.4Ga0.6As QD structure exhibits no degradation in crystal quality, no dislocations and no crystal defects even after the stacking of 400 QD layers. The external quantum efficiency and the short-circuit current density of multistacked In0.4Ga0.6As QD solar cells increase as the number of stacked layers is increased to 150. Such ultra-high stacks and good cell performance have not been reported for QD solar cells using other material systems. The performance of the ultra-high stacked QD solar cells indicates that InGaAs QDs are suitable for use in high efficiency solar cells requiring thick QD layers for sufficient light absorption.

Correlated photon emission in a thick barrier coupled quantum dot

Correlated photon emission from a thick barrier coupled quantum dot (QD) has been observed by using selective two-color excitation spectroscopy and second-order photon correlation spectroscopy. Surprisingly, the carrier creation in both QDs induced an anomalous increase in the luminescence intensity, and furthermore the cross photon correlation spectrum between two QDs exhibited photon antibunching with a long recovery time. These significant findings can be interpreted in terms of the electromagnetic interaction between QDs with a thick barrier layer.

Growth and properties of Cu(In, Ga)(S, Se) 2 films

Cu(In, Ga)(S, Se)2 (CIGSSe) films were grown on Mo/soda-lime glass substrates in a three-stage process using a molecular beam epitaxy apparatus equipped with an rf-cracked S-radical beam source. CIGSSe films have been studied for their application in the development of solar cells with wide-bandgap semiconductors. The S/(S+Se) composition ratios in the fabricated films were determined from electron probe microscopy analysis (EPMA), and scanning electron microscopy (SEM) images of the films were obtained. The results showed that the grain sizes in the films decreased with increasing S concentrations. The surfaces of the films with higher S/(S+Se) composition ratios had greater roughness. To determine the compositional depth profile of the films, the acceleration voltage dependencies of EPMA on the S/(S+Se) ratios, and the group-VI atoms (S and Se) were obtained. The fabricated CIGSSe solar cell achieved an efficiency of 15% and had a fill factor of 0.72. However, since the open circuit voltage was lower than the expected value improvement in the band profile is necessary.

Critical issues for high-efficiency low-cost CIGS solar cells and modules

High-efficiency CIGS solar cells and submodules have been developed at AIST. By developing the water-vapor assisted deposition technique, widegap CIGS solar cells with conversion efficiencies of over 18% with Voc=0.744V have been demonstrated. The conversion efficiencies of integrated submodules on 10x10cm2 sodalime glass substrates have been improved up to η=16.2% by using coevaporation process. These results indicate that the CIGS technologies are competitive with the current Si and CdTe technologies in terms of both cost and performance.