Hironori Komaki

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.

Potential-induced degradation of Cu(In,Ga)Se2 photovoltaic modules

Potential-induced degradation (PID) of Cu(In,Ga)Se2 (CIGS) photovoltaic (PV) modules fabricated from integrated submodules is investigated. PID tests were performed by applying a voltage of −1000 V to connected submodule interconnector ribbons at 85 °C. The normalized energy conversion efficiency of a standard module decreases to 0.2 after the PID test for 14 days. This reveals that CIGS modules suffer PID under this experimental condition. In contrast, a module with non-alkali glass shows no degradation, which implies that the degradation occurs owing to alkali metal ions, e.g., Na+, migrating from the cover glass. The results of dynamic secondary ion mass spectrometry show Na accumulation in the n-ZnO transparent conductive oxide layer of the degraded module. A CIGS PV module with an ionomer (IO) encapsulant instead of a copolymer of ethylene and vinyl acetate shows no degradation. This reveals that the IO encapsulant can prevent PID of CIGS modules. A degraded module can recover from its performance losses by applying +1000 V to connected submodule interconnector ribbons from an Al plate placed on the test module.

Highly Efficient Cu(In,Ga)Se2 Thin-Film Submodule Fabricated Using a Three-Stage Process

Using a three-stage process, a highly efficient, integrated chalcopyrite Cu(In,Ga)Se2 (CIGS) submodule was fabricated with a certified efficiency of 18.34% and an open circuit voltage of 2.963 V, a short circuit current of 29.05 mA, a fill factor of 0.762, and a designated area of 3.576 cm2. The diode properties and parasitic resistances of the submodule and a reference single cell containing a CIGS absorber layer identical to that in the submodule were determined using a distributed diode model. In addition, the fundamental loss mechanisms for the submodule were investigated.

Buried p-n junction formation in CuGaSe2 thin-film solar cells

CuGaSe2/CdS interfaces and the mechanism behind the hetero p-n junction formation were investigated using solar cell devices which demonstrated about a 10% energy conversion efficiency. It was found that the CuGaSe2/CdS interface could be described as a CuGaSe2/Cu-deficient Cu-Ga-Se layer (CDL)/CdS structure and the p-n junction was located at the CuGaSe2/CDL interface that was present 50–100 nm from the CDL/CdS interface. While the difficulty of the realization of equilibrium n-type CuGaSe2 has been generally recognized, this result suggests that CDL consisting of ordered vacancy compound phases such as CuGa3Se5 can play the role of an n-type material.

Piezoelectric Photothermal and Photoreflectance Spectra of InxGa1-xN Grown by Radio-Frequency Molecular Beam Epitaxy

Piezoelectric photothermal spectroscopy (PPTS) measurements were carried out on InxGa1-xN (x=0.01?0.32) thin films grown by radio-frequency molecular beam epitaxy. We found that the band energy shifts to the lower energy side of the spectrum (red shift) with an increase in the indium composition from 0.01 to 0.32. For samples with a lower indium composition, we were able to observe the exciton contribution, and the binding energy was estimated to be 27 meV (x=0.01). Since conventional photoreflectance (PR) spectroscopy was unable to observe signals for the samples with a higher indium content (x=0.13, 0.2, and 0.32), the usefulness of this PPTS method for samples with phase fluctuation is demonstrated.

Surface morphology of evaporated CuInS2 thin films grown by single source thermal evaporation technique

Single source thermal evaporation technique was carried out for CuInS2 thin films on the glass substrates. The films were annealed from 100 °C to 500 °C in air. The crystal structures of the films were examined by x-ray diffraction and surface morphology investigated by optical microscope. The CuInS2 thin films were grown by annealing at 200 °C. The sample indicated high resistivity and n-types conductivity examined by a four-point probe method and thermoprobe analysis, respectively.

Structural and optical characterization of CuInS2 thin films grown by vacuum evaporation method

Structural and optical properties of CuInS2 thin films grown by the single-source thermal evaporation method have been studied. The films were annealed from 100 to 500 °C after an evaporation in air. The surface morphology was investigated by scanning electron microscopy. The maximum grain size of the samples after annealing at 400 °C was over 500 nm. The EPM analysis concluded that the polycrystalline CuInS2 thin films after annealing below 100 °C were Cu-rich, and those annealed above 200 °C were In-rich. The bandgap energy of the CuInS2 films after annealing above 300 °C was about 1.48 eV.

Highly efficient and reliable mechanically stacked multi-junction solar cells using advanced bonding method with conductive nanoparticle alignments

This paper shows a high-efficiency mechanically stacked multi-junction solar cells using conductive nanoparticle alignments at the bonding interfaces. We fabricated a GaInP/GaAs/InGaAsP/InGaAs four-junction solar cell with the total efficiency of 30.4% under 1 sun AM1.5G. In addition, we discuss the reliability of our solar cells showing the results of accelerated aging test and thermal cycling test. It was confirmed that our solar cell has high long-term reliability under sever conditions (high and low temperature). Thus, our bonding method is promising to achieve highly efficient mechanically stacked multi-junction solar cells for practical use.

Fabrication of integrated CIGS modules using the in-line three-stage process

We have attempted an in-line three-stage process for the deposition of CIGS (CuInGaSe2) absorber layers. The allowed growth area of CIGS deposition apparatus is 30 × 30 cm2, and the compositional uniformity and distribution of cell performance have been investigated. Large-size grains and band-grading typical for the three-stage process have been observed. The composition ratios and the performance of small area cells (0.5 cm2) have shown good uniformity over the whole deposition area, with an average efficiency of 15.8 % without anti-reflection coating. By improving CIGS deposition process, efficiency of small cells enhanced up to 17.3 %. Minimodules with aperture area of 76.5 cm2 were also fabricated on 10 × 10 cm2 SLG substrates. Preliminary results showed efficiency as high as η = 14.2 % with anti-reflection coating.