Tomah Sogabe

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.

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.

Experimental characterization and self-consistent modeling of luminescence coupling effect in III-V multijunction solar cells

In this paper, we focused on developing an accurate model to describe the luminescent coupling (L-C) effect in multijunction solar cells (MJSC) under light concentration. We present here a transcend current-voltage (I-V) formula combined with a self-consistent simulation algorithm to derive the coupling yield γ dependence on light intensity by including the electrical parameters such as shunt resistance (Rsh) and series resistance (Rs), which were ignored in previous simulation models. The effects of both Rsh and Rs on γ were revealed, and the dependence of γ on the external voltage bias Vbias was investigated. Meanwhile, we have performed experiments to determine coupling yield γ by measuring the I-V curves of individual subcell of InGaP/GaAs/Ge triple junction solar cell under varied light intensity. We found that the measured results are only in good agreement with the simulated data obtained from the model where the resistance parameters were included. Based on these results, we calculated the convers...

Theoretical analysis of GaAs/AlGaAs quantum dots in quantum wire array for intermediate band solar cell

A GaAs quantum dot (QD) array embedded in a AlGaAs host material was fabricated using a strain-free approach, through combination of neutral beam etching and atomic hydrogen-assisted molecular beam epitaxy regrowth. In this work, we performed theoretical simulations on a GaAs/AlGaAs quantum well, GaAs QD and QD array based intermediated band solar cell (IBSC) using a combined multiband k·p and drift-diffusion transportation method. The electronic structure, IB band dispersion, and optical transitions, including absorption and spontaneous emission among the valence band, intermediate band, and conduction band, were calculated. Based on these results, maximum conversion efficiency of GaAs/AlGaAs QD array based IBSC devices were calculated by a drift-diffusion model adapted to IBSC under the radiative recombination limit.

Enhancement of current collection in epitaxial lift-off InAs/GaAs quantum dot thin film solar cell and concentrated photovoltaic study

We report the fabrication of a thin film InAs/GaAs quantum dot solar cell (QD cell) by applying epitaxial lift-off (ELO) approach to the GaAs substrate. We confirmed significant current collection enhancement (∼0.91 mA/cm2) in the ELO-InAs QD cell within the wavelength range of 700 nm–900 nm when compared to the ELO-GaAs control cell. This is almost six times of the sub-GaAs bandgap current collection (∼0.16 mA/cm2) from the wavelength range of 900 nm and beyond, we also confirmed the ELO induced resonance cavity effect was able to increase the solar cell efficiency by increasing both the short circuit current and open voltage. The electric field intensity of the resonance cavity formed in the ELO film between the Au back reflector and the GaAs front contact layer was analyzed in detail by finite-differential time-domain (FDTD) simulation. We found that the calculated current collection enhancement within the wavelength range of 700 nm–900 nm was strongly influenced by the size and shape of InAs QD. In addition, we performed concentrated light photovoltaic study and analyzed the effect of intermediate states on the open voltage under varied concentrated light intensity for the ELO-InAs QD cell.

Self‐consistent electrical parameter extraction from bias dependent spectral response measurements of III‐V multi‐junction solar cells

Spectral response of multi-junction solar cell is complicated because of the interplay between external measurement conditions such as bias light intensity, monochromatic light intensity, bias voltage, and intrinsic electrical properties of series interconnected subcells. In this paper, we report an experimental study on the bias voltage-dependent spectral response (SR) for multi-junction solar cell. A self-consistent iteration loop was developed from a nonlinear least square Powell hybrid algorithm that was used for curve fitting the experimental SR versus bias voltage data of each subcell. We demonstrated for the first time that this approach enabled us to derive the electrical parameters such as dark saturation currents (J0), shunt resistance (Rsh), series resistance (Rs), and spectra response (Jphoto) for each subcell of a Ga0.99In0.01As/Ge dual junction solar cell with stable convergence. The accuracies of the fitting results were confirmed by the agreement between the J–V curves calculated on the basis of these parameters and the experimental J–V curve of multi-junction solar cell measured under AM1.5 and 1 sun condition. Copyright

Analysis of bias voltage dependent spectral response in Ga0.51In0.49P/Ga0.99In0.01As/Ge triple junction solar cell

Spectral response measurement plays great role in characterizing solar cell device because it directly reflects the efficiency by which the device converts the sunlight into an electrical current. Based on the spectral response results, the short circuit current of each subcell can be quantitatively determined. Although spectral response dependence on wavelength, i.e., the well-known external quantum efficiency (EQE), has been widely used in characterizing multijunction solar cell and has been well interpreted, detailed analysis of spectral response dependence on bias voltage (SR −Vbias) has not been reported so far. In this work, we have performed experimental and numerical studies on the SR −Vbias for Ga0.51In0.49P/Ga0.99In0.01As/Ge triple junction solar cell. Phenomenological description was given to clarify the mechanism of operation matching point variation in SR −Vbias measurements. The profile of SR−Vbias curve was explained in detail by solving the coupled two-diode current-voltage characteristic ...

Extraction of electrical parameters in multi-junction solar cells from voltage dependent spectral response without light bias

We have investigated a useful method of extracting the electrical parameters of each subcell in a multi-junction solar cell from bias-voltage-dependent spectral response data (SR-V) which are obtained without light bias. Colored bias lights are commonly employed for biasing the respective subcells except for the targeted subcell, which is to be characterized. The extracted parameters for each subcell are J0, dark saturation current; Rs, series resistance; Rsh, shunt resistance; and Jphoto, photo current. The program code for the analysis has been developed with a nonlinear least-square method, Powell Hybrid algorithm. It is shown that this algorithm results in a stable convergence and the experimental results on the dark current–voltage (I–V) characteristics and photo I–V curves can be reliably reproduced using the extracted electrical parameters using this method.

Growth of ErAs nanodots by molecular beam epitaxy for application to tunneling junctions in multijunction solar cells

ErAs nanodots (NDs) grown on GaAs(001) substrates by using molecular beam epitaxy (MBE) were investigated. Atomic force microscope images indicate that the size of ErAs NDs increases with deposition time and growth temperature. A calibration was performed to determine the deposition rate of ErAs in order that the size of NDs can be accurately controlled and hence optimized. Local current flow images and surface profiles around ErAs NDs were simultaneously measured to clarify the local conductivity distribution corresponding to a real space profile. Furthermore, we also fabricated and characterized an ErAs-ND-embedded GaAs tunnel junction (TJ), which resulted in a voltage drop of 30 mV for 15 A/cm2 operation current equivalent to 1000 suns concentration, which is less than one-third of that of a conventional heavily doped tunnel junction.

Impact of optically nonuniform luminescence coupling effect to the limiting cell conversion efficiency in InGaP/GaAs/Ge triple junction solar cell

Abstract. The impact of nonuniform spatial distribution of the luminescence coupling (LC) effect to the limiting cell conversion efficiency of multijunction solar cells (MJSCs) has been investigated. For this purpose, the laser beam induced current distribution maps of the limiting bottom cell have been acquired experimentally under varying middle-to-bottom cell LC efficiencies. The minimum and the maximum LC efficiencies demonstrated were 8.5% and 69%, respectively. To further analyze the measurement results, a quasi-two-dimensional simulation model considering the spatially nonuniform nature of the LC effect has been developed. A good agreement between the simulation and the measurement results suggests that the nonuniform LC current distribution is induced by optical phenomena such as photon escape and internal reflection. This nonuniformity then causes the absolute conversion efficiency of the limiting cell to be reduced by 1.35% at maximum LC efficiency. This reduction, when suppressed, can yield higher limiting cell conversion efficiency, which in turn may improve the overall MJSC conversion efficiency.