Minoru Kaneiwa

Evaluation of InGaP/InGaAs/Ge triple-junction solar cell under concentrated light by Simulation Program with integrated Circuit Emphasis

The characteristics of a multi-junction solar cell under concentrated light were evaluated by Simulation Program with Integrated Circuit Emphasis (SPICE). We developed the multi-unit model and analyzed the affects of the chromatic aberration and intensity distribution for the multi-junction cells. In the multi-unit model, the same numbers of units as grid numbers are installed for every electrode, and the units were connected to each other via lateral resistances. In order to obtain the generation current from each diode, we measured the intensity of concentrated light through the pinhole using single-junction solar cells consisting of InGaP, GaAs and Ge as detectors. By using the multi-unit model, we could successfully calculate the electrical cell performances taking the chromatic aberration and intensity distribution into account, and the calculated value agreed well with the experimental value. The multi-unit model will be very useful for cell designs and performance analysis of the concentrator cells.

Future development of InGaP/(In)GaAs based multijunction solar cells

Although technologies for the InGaP/InGaAs/Ge cell have been matured, there is still room for improvement of the InGaP/(In)GaAs/Ge cell in practical level. Band gap of the top cell should be increased a little to get higher V/sub OC/. Thinning the Ge substrate is thought to be effective to increase a power per weight even for rigid panel. For concentrator application, grid pitch, cell size and current matching design should be optimized with taking account of the spectrum of concentrated light. The InGaP/(In)GaAs based solar cells shall be cornerstone in high efficiency multijunction solar cells in future. High efficiency cell consisted of 1 eV lattice-match material such as InGaAsN is strongly desired for high efficiency 4-junction or 6-junction cell. Wafer bonding and layer transfer techniques might be sophisticated to make solar cells. Paper-like InGaP/GaAs solar cells with efficiency of 29.4% on flexible metal film developed by SHARP Corp. are newly reported. Material cost of the cell is basically very low, because it has only very thin layers of III-V compounds and cheap metal film. Thin film technology shall be a hint for future cells.

Paper-Thin InGaP/ GaAs Solar Cells

A paper-thin, lightweight InGaP/GaAs solar cell with high efficiency and flexibility has been developed. A high-efficiency thin-film cell can be obtained for cell fabrication both before and after removing the substrate. Introducing a tunnel junction as the contact layer between the cell and metal film improves cell characteristics (Fill Factor (FF) and open-circuit voltage (Voc)). A highly doped n-type layer is necessary for good ohmic contact at the metal film interface. High radiation resistance of a thin-film cell was confirmed for a GaAs cell with a one-micron base layer. The thin-film cell was laminated for better handling. The laminated cell efficiency was about 22%. Anti-reflective coating is necessary on the laminate film to improve cell efficiency. A prototype unit panel using the laminated cells was developed for space application. An output power per weight of over 1W/g is possible for the unit panel. However, development of a bypass diode with thin-film structure is currently a problem, and reliability tests need to be performed for the unit panel

Development of a new 550/spl times/ concentrator module with 3J cells - performance and reliability

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. The peak uncorrected efficiency for a 7,056 cm/sup 2/ 400/spl times/ module with 36 solar cells connected in series was 26.6 % was measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1,176 cm/sup 2/ area measured by Fraunhofer ISE and NREL were 27.4 % and 24.8 % respectively. The peak uncorrected efficiency for a 550/spl times/ and 5,445 cm/sup 2/ module with 20 solar cells connected in series was 28.9 %. The temperature corrected efficiency under the best sunshine condition in Japan for the 550/spl times/ module was 31.5/spl plusmn/2%. For reliability, some new degradation modes inherent to high concentration III-V solar cell system are discussed and a 20 year lifetime under concentrated flux exposure proven. The fail-safe issues of the concentrated sunlight are also discussed. For cost, the overall scenario for the reduction of material cost is discussed.

Influence of the dopant species on radiation-induced defects in Si single crystals

Observations on deep levels introduced in silicon by 1 MeV electron irradiation are reported using boron- or gallium-doped Czochralski (CZ) grown Si space solar cells with different doping concentrations, deep level transient spectroscopy analysis has been carried out to detect the radiation-induced deep levels. Present results provide evidence for new defect states in addition to those previously reported in gallium- and boron-doped Si. The combined boron and gallium data provide enough information to gain valuable insight into the role of the dopants on radiation induced defects in Si. The dominant donor-like electron level at EC−0.18 eV in boron-doped Si has not been observed in gallium-doped CZ-grown Si. A noticeable suppressing generation of the radiation-induced defects in gallium-doped Si is also observed, especially hole level EV+0.36 eV, which is thought to acts as a recombination center.

Radiation response analysis of wide-gap p-AlInGaP for superhigh-efficiency space photovoltaics

We present here the direct observation of the majority and minority carrier defects generation from wide-band-gap (2.04eV) and thick (2μm) p-AlInGaP diodes and solar cells structures before and after 1MeV electron irradiation by deep level transient spectroscopy (DLTS). One dominant hole-emitting trap H1 (EV+0.37±0.05eV) and two electron-emitting traps, E1 (EC−0.22±0.04eV) and E3 (EC−0.78±0.05eV) have been observed in the temperature range, which we could scan by DLTS. Detailed analysis of the minority carrier injection annealing experiment reveals that the H1 center has shown the same annealing characteristics, which has been previously observed in all phosphide-based materials such as InP, InGaP, and InGaAsP. The annealing property of the radiation-induced defects in p-AlInGaP reveals that multijunction solar cells and other optoelectronic devices such as light-emitting diodes based on this material could be considerably better to Si and GaAs in a radiation environment.

23.5% efficient silicon solar cell with rear micro contacts of c-Si//spl mu/c-Si:H heterostructure

To obtain high efficiency silicon solar cells, we have investigated the rear heterostructure comprising a p-type single crystalline silicon (c-Si) substrate and a highly boron doped (p/sup +/) hydrogenated microcrystalline silicon (/spl mu/c-Si:H) film. This heterostructure was formed by rear micro contacts where a SiO/sub 2/ film was opened on the rear surface of the substrate. Voc was improved by an effective BSF using this heterostructure. With optimal design of finger electrode patterns, a conversion efficiency of 23.5% (AM1.5, 25/spl deg/C, 100 mW/cm/sup 2/) was obtained for a single crystalline silicon solar cell in 5/spl times/5 cm/sup 2/ area.

Progress and future view of silicon space solar cells in Japan

The progress and the future view of silicon (Si) space solar cells in Japan are reviewed. In 1991, two types of the high-efficiency silicon (HES) cell were developed: (1) NRS/LBSF cell; (2) NRS/BSF cell. The former shows a conversion efficiency of 18.0% (AM0, 28/spl deg/C) at beginning-of-life (BOL), the latter shows superior radiation tolerance. In 1998, the radiation tolerance of the NRS/BSF cell was improved; moreover, the advanced high-efficiency silicon-1 (AHES-1) cell was accomplished. It shows 13.1% at end-of-life (EOL). The development has progressed to make the 13.7% EOL cell: AHES-2 cell. In 1994, the integrated bypass function (IBF), which prevents failures due to reverse biasing of cells, was proposed. The NRS/BSF cells with IBF have been used already on several satellites. The structures, performance, and radiation tolerances of these cells are introduced. New approaches for further improvement are proposed.

Carrier removal in lattice-mismatched InGaP solar cells under 1-MeV-electron irradiation

Radiation-induced majority carrier removal is investigated from n+∕p− lattice-mismatched In0.56Ga0.44P solar cells under 1-MeV-electron irradiation. The change in carrier concentration in the 1×1017cm−3p− base layer is determined using standard capacitance–voltage techniques and found to proceed at a rate Rc=1.3cm−1, in agreement with that observed in lattice-matched InGaP. However, the observation of an increased short-circuit current and short-wavelength quantum efficiency over the unirradiated values at electron fluence levels in excess of 3×1015cm−2, allows the carrier concentration from the n+ emitter layer to be measured. By modeling the quantum efficiency of these solar cells, it is shown that the main photoresponse from these lattice-mismatched solar cells is due to drift transport, making the spectral response highly sensitive to changes in the width of the depletion region. Using this technique, the carrier concentration in the 2×1018cm−3 n+ emitter layer is found to be reduced to 1×1018cm−3 aft...

Deep-level defects introduced by 1 MeV electron radiation in AlInGaP for multijunction space solar cells

Presented in this paper are 1 MeV electron irradiation effects on wide-band-gap (1.97 eV) (Al0.08Ga0.92)0.52In0.48P diodes and solar cells. The carrier removal rate estimated in p-AlInGaP with electron fluence is about 1cm−1, which is lower than that in InP and GaAs. From high-temperature deep-level transient spectroscopy measurements, a deep-level defect center such as majority-carrier (hole) trap H2 (Eν+0.90±0.05eV) was observed. The changes in carrier concentrations (Δp) and trap densities as a function of electron fluence were compared, and as a result the total introduction rate, 0.39cm−1, of majority-carrier trap centers (H1 and H2) is different from the carrier removal rate, 1cm−1, in p-AlInGaP. From the minority-carrier injection annealing (100mA∕cm2), the annealing activation energy of H2 defect is ΔE=0.60eV, which is likely to be associated with a vacancy-phosphorus Frenkel pair (Vp‐Pi). The recovery of defect concentration and carrier concentration in the irradiated p-AlInGaP by injection relat...