Yoshitaka Okada

Strain-compensated InAs/GaNAs quantum dots for use in high-efficiency solar cells

We have investigated GaAs-based p-i-n quantum dot solar cells (QDSCs) with 10 up to 20 stacked layers of self-assembled InAs quantum dots (QDs) grown by atomic hydrogen-assisted molecular beam epitaxy. The net average lattice strain was minimized by using the strain-compensation technique, in which GaNAs dilute nitrides were used as spacer layers. The filtered short-circuit current density beyond GaAs bandedge was 2.47 mA/cm2 for strain-compensated QDSC with 20 stacks of InAs QD layers, which was four times higher than that for strained QDSC with identical cell structure.

Increase in photocurrent by optical transitions via intermediate quantum states in direct-doped InAs/GaNAs strain-compensated quantum dot solar cell

We have developed a technique to fabricate quantum dot (QD) solar cells with direct doping of Si into InAs QDs in GaNAs strain-compensating matrix in order to control the quasi-Fermi level of intermediate QD states. The Si atoms were evenly incorporated into QDs during the assembling stage of growth such that a uniform array of partially filled QDs has been obtained. Nonradiative recombination losses were also reduced by Si doping and a photocurrent increase due to two-step photon absorption was clearly measured at room temperature detected under filtered air-mass 1.5 solar spectrum.

Characteristics of InAs/GaNAs strain-compensated quantum dot solar cell

We have fabricated and compared the performance of GaAs-based p-i-n quantum dot solar cells with ten multilayer stacked structures of self-assembled InAs quantum dots embedded with GaNxAs1−x strain-compensating spacer layers. Reducing the thickness of the spacer layer, and hence increasing the nitrogen composition in GaNxAs1−x, from 40 nm (x=0.5%) to 15 nm (x=1.5%) thereby fulfilling the net strain-balanced condition, resulted in a steady increase in the short-circuit density, while a decreasing trend for the open-circuit voltage was observed. The observed results can be interpreted in terms of the difference in the quantum confinement structure.

Intermediate band solar cells: Recent progress and future directions

Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

Does a magnetic barrier or a magnetic-electric barrier structure possess any spin polarization and spin filtering under zero bias?

In this letter, we have clarified that there is no spin polarization and spin filtering in a magnetic barrier structure as well as in a magnetic–electric barrier structure using our explicit expressions for electron transmission probability. Our results are found to be contradictory to those of A. Majumdar [Phys. Rev. B 54, 11911 (1996)] and G. Papp and F. M. Peeters [Appl. Phys. Lett. 78, 2184 (2001)]. We have shown the significant spin polarization and spin filtering observed by these authors were caused by a mistake in their transmission probability.

Two-photon excitation in an intermediate band solar cell structure

We present evidence for the production of photocurrent due to two-photon excitation in an intermediate band solar cell structure. The structure consists of an n-GaNAs intermediate band layer sandwiched between a p-AlGaAs emitter and an n-AlGaAs barrier layer with suitable doping level to block electron escaping from the intermediate band to the bottom n-GaAs substrate. Multi-band transitions observed in two-photon excitation experiments are explained using photo-modulated reflectance spectrum, and further support for intermediate band solar cell operation of this structure is given by current-voltage measurements.

Highly packed InGaAs quantum dots on GaAs(311)B

We have fabricated highly packed and ordered In0.4Ga0.6As quantum dots (QDs) array on GaAs(311)B substrate without coalescence of QDs. Reflection high-energy electron diffraction and Auger spectra suggest the inhomogeneous distribution of In and Ga in QD. In concentration near the surface of QD is larger than that of the inside, and the inhomogeneous distribution of In and Ga in QDs prevents QDs from merging.

Intermediate-band dynamics of quantum dots solar cell in concentrator photovoltaic modules

We report for the first time a successful fabrication and operation of an InAs/GaAs quantum dot based intermediate band solar cell concentrator photovoltaic (QD-IBSC-CPV) module to the IEC62108 standard with recorded power conversion efficiency of 15.3%. Combining the measured experimental results at Underwriters Laboratory (UL®) licensed testing laboratory with theoretical simulations, we confirmed that the operational characteristics of the QD-IBSC-CPV module are a consequence of the carrier dynamics via the intermediate-band at room temperature.

Growth modes in atomic hydrogen‐assisted molecular beam epitaxy of GaAs

It has been shown that irradiation with atomic hydrogen during the growth of GaAs in molecular beam epitaxy (MBE) promotes an ideal layer‐by‐layer two‐dimensional nucleation and step‐flow growth mode on GaAs(001) substrates, thereby resulting in atomically flat surfaces. Fundamentally important observations related to elementary processes have been presented based on the reflection high‐energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements. A growth model for the atomic hydrogen‐assisted GaAs MBE has been postulated.

Bipolar transistor carrier-injected optical modulator/switch: Proposal and analysis

Bipolar transistor structures can be used, instead of conventional p-n (p-i-n) junctions, to realize high-speed optical modulators and switches, which are operated by free-carrier injection. Some basic results obtained by the theoretical analysis of the structure applied to a DH X crossing are presented, with emphasis on the switching speed. It is shown that switching times can be as fast as 60 ps for the required ON-OFF optical switching, which is considerably faster than those expected for a diode structure.