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We have presented the results of the deep level transient spectroscopy (DLTS) study of the electron-radiation-induced defects in n-type InAsxP1-x/InP MQWs solar cell structures. Irradiation of CBE grown n-type InAsxP1-x/InP by 1 MeV electron with a dose of 3times1016 cm-2 results in the appearance of a dominant electron emitting level E1, in the upper half of the band gap. The level E1 shows striking behavior during room temperature storage. It was found to completely anneal out with storage at room temperature with no change in other majority carrier levels. Detailed characteristics of these levels including thermal emission rates, carrier activation energies, and capture cross-sections have been determined. No any other defect has been observed in electron-irradiated n-type InAsxP1-x/InP MQWs solar cell structures, which contrasts to conventional n-InP solar cell counterparts (structures without QWs). This reveals that introduction of QWs in the intrinsic region of p-i-n structures enhances significantly the device radiation tolerance well beyond what has been thus far reported for conventional Si or III-V semiconductor-based solar cells

Submicron surface textures were investigated to reduce the optical loss of Si solar cells. Numerical simulations based on the rigorous coupled-wave approach were performed to obtain the spectral reflectivity of pyramid-type Si textures with submicron period. The results showed that a low mean reflectivity below 3% can be obtained in a wide range of period, and that a suitable combination of period and aspect ratio exists to reduce the reflection loss effectively. Based on the calculation results, tapered subwavelength structures with the period of about 0.1 mum were fabricated on multi-crystalline Si as well as single crystalline Si wafers by fast atom beam etching with a porous alumina mask. A low reflectivity below 2% was successfully obtained from 0.35 to 1.0 mum without AR coatings. Test cells with the SWS showed a gain on the short circuit current density due to the suppression of the optical loss

The recovery of defects introduced in n+/p AlInGaP solar cells under the 1 MeV electron/30keV proton irradiation by the minority-carrier injection-enhanced annealing is presented here. As a forward bias injection (100 mA/cm2) time increases, DLTS signal of H1 and H2 defects, which are introduced by the 1 MeV electron irradiation, decreases, indicating that they are annealed out due to nonradiative electron-hole recombination enhanced process. The activation energy DeltaE for the recovery of defect centers (H1 and H2) by the injection was 0.50 eV and 0.60 eV, respectively. Under the 30 keV proton irradiation, HP1 and HP2 defect centers were produced, and the annealing activation energy of AlInGaP solar cells obtained from the injection (1 A/cm2) was 0.42 eV

The CuPc crystal structure and molecular orientation on the C60 was investigated by AFM and XRD as preliminary step to clarify the relationship the interface structure and electric property for organic solar cells. It is figure out that the CuPc grows grainy on the C60 by the AFM measurement. The area density of the CuPc grain on the C60/mica became larger, and the grain diameter became smaller with increasing the substrate temperature. By the XRD, it became clear that the CuPc forms alpha-type mainly. The molecular column of deposited CuPc on C60 would be parallel to the substrate surface

SiO2 films prepared by atmospheric pressure chemical vapor deposition (APCVD) were investigated for surface passivation of crystalline Si solar cells. The passivation effects of APCVD SiO2 for p-type Si substrates were evaluated by measuring excess minority carrier lifetimes. Effective surface recombination velocities S eff were decreased by successive annealing in N2 atmosphere at 300-700 degC without the degradation of the bulk lifetime. A substantial reduction of Seff was observed especially after annealing over 500 degC. To investigate the origin of this improvement, interface state density Dit and fixed charge density Q were extracted from capacitance-voltage curves of metal-oxide-semiconductor structures. The results revealed that the decrease of the Seff was caused from the decrease of Dit and the increase of Q/Dit ratio induced by the annealing. The field-effect passivation attributed to a large Q/Dit was discussed using the extended Shockley-Read-Hall model

The radiation resistance of individual InGaP and GaAs sub-cells is assessed as a function of base doping under low-energy proton irradiation. It is found that InGaP sub-cells with low base doping are the most radiation tolerant. However, in GaAs, carrier removal leads to a rapid degradation in sub-cells with low base doping and the most radiation hard sub-cell is achieved using a linearly graded base doping profile.

The film quality improvement of amorphous carbon (a-C) layers for C60/a-C superlattice solar cells has been intended. a-C:N films were deposited by C60-cage breaking, and the layer-by-layer hydrogen treatment method was applied to the a-C:N films for the improvement of the film quality, and the structural change by the hydrogen treatment was investigated. Hydrogen radicals reconstruct carbon bonding structure of a-C:N film surface by etching the weak carbon bonds. Such structural change causes reducing the optical gap and changing the peak intensity ratio of Raman scattering spectra, which is concerned with sp2 cluster size. The spin defect density much depends on the hydrogen treatment time. It is clear that the spin defect density has minimum at the particular time of the hydrogen treatment, so that hydrogen radicals also affect to cut the carbon bonds and induce defects. It is necessary to optimize hydrogen treatment time to improve film quality.

GaAsN epitaxial thin films were grown on GaAs (001) substrate by chemical beam epitaxy with dimethylhydrazine ((CH3)2 N2H2). In the dependence of growth temperature on the N concentration, there were three distinct regions in which the dependence was different. At the growth temperature of 420degC, the N concentration increased with increasing surface step density that corresponds to misorientation angle of GaAs (001) substrate. When the samples grown on 2 and 10deg off substrates were compared, the root-mean-squares of surface roughness decreased from 1.1 nm to 0.3 nm, despite of the increase of N concentration. In addition, the reduction of the impurity concentrations (hydrogen and carbon) in the sample grown on the 10deg off substrate was also observed. These results showed the surface morphology of the GaAsN thin films is affected by the step density, and the possibility to obtain (In)GaAsN thin film that has high N and low impurity concentration with keeping crystal quality

Carbon materials can be expected as one of the promising materials for next generation low-cost solar cells. We have proposed C60/a-C superlattice structures as absorption layers. Superlattice structure has advantages to control the optical and electrical properties easily. In this paper, we will report Mg-doping in C60 films to control the conductivity of C60 layers for C60/a-C superlattice solar cells. Mg doping in C 60 films was done by the co-evaporation method of C60 and Mg sources. The resistivity of Mg-doped C60 films reduces in 6 orders of magnitude at the ratio of Mg/C60 of 0.57. It is thought that Mg atoms act as dopant effectively

Deep level transient spectroscopy (DLTS) is the best technique for monitoring and characterizing deep levels introduced intentionally or occurring naturally in semiconductor materials and complete devices. DLTS has the advantage over all the techniques used to-date in that it fulfils almost all the requirements for a complete characterization of a deep centre and their correlation with the device properties. In particular the method can determine the activation energy of a deep level, its capture cross-section and concentration and can distinguish between traps and recombination centers. In this invited paper we provide an overview of the extensive R & D work that has been carrier out by the authors on the identification of the recombination and compensator centers in Si and III-V compound materials for space solar cells. In addition, we present an overview of key problems that remain in the understanding of the role of the point defects and their correlation with the solar cell parameters.

Back contact type cells have large area contacts at the back, and have no front grid contact preventing light absorption. The cells are able to simultaneously offer reduced resistance loss for high current densities and high light absorption. However, the structure requires low surface recombination loss to obtain high efficiency. We studied various structures to reduce the loss for Si and Ge back contact type cells. A floating emitter at the surface of the Si cell is fabricated to reduce recombination loss with a SiO2 passivation layer. The Ge cell for thermophotovoltaics has a high refractive index SiNx passivation layer at the front surface. The layer also serves as a bottom layer of the anti reflection coating. In a GaInP/GaAs/Ge triple-junction cell with a Ge back contact type bottom cell, the energy band at the front of the Ge cell is bent due to the diffusion layer, and is effective in reducing recombination loss.

Crystal quality of GaAsN films can be improved by using chemical beam epitaxy method for low-temperature growth. However, low-temperature growth increases carbon (C) incorporation in the films, which degrades their electrical properties. To reduce the C concentration in the films, C incorporation process was investigated in view of the surface reaction of nitrogen (N) sources on a substrate surface, and monomethylhydrazine (MMHy) and 1,1-dimethylhydrazine (DMHy) were compared. When MMHy was used as an N source, the C concentration in GaAsN drastically increases below 380degC than that in GaAs due to insufficient CHX desorption. In the case of DMHy, N(CH3)2 is desorbed more readily than CHX. Therefore, the C concentration can then be reduced by using DMHy

This paper discusses on possibilities of HCPV, including (1) cost (2) reliability (3) performance and (4) applications. The reduction of the cost is achieved by the reduction of the material itself. The material is not special. HCPV does not covered by special materials including pure Si, TCO, Ru-based dye, or Pt electrode or other materials using CVD or other vacuum process. HCPV modules are made without special tools. Local assembly is also possible. The weight of the 300 W HCPV module is 22 kg corresponding 0.07 kg/W. It is about 70 % of the MC-Si flatplate modules and less than half of the high efficiency tandem thin film modules. The recent 1-year field test done in Japan showed that the HCPV generated 1.6 times energy per area than that of the crystalline Si module (fixed installation). This implies that the HCPV has an advantage not only desert area but wet and cloudy area. Intensive reliability tests were done to the HCPV module. One of the keys are lifetime of the receiver against 20 years of accumulated CONCENTRATED UV flux in wet environment and other accelerated environment. Finally, the new applications of HCPV including roof-top installation and see-through design are discussed.

In order to produce high-quality multicrystalline(mc-) silicon ingots, mc-Si ingots were grown by the directional solidification technique with various heater transfer rate, VH=15, 30, 60mm/hr . The grown ingots were sliced to a thickness of 0.5mm. The effective lifetime, substitutional carbon concentration, and etch-pit density (EPD) in the substrates were measured to characterize the grown ingots. The EPD was decreased with decreasing VH. However, the lifetime of the ingot with VH=15mm/hr was almost the same as that with VH=30mm/hr. The reason is that the degree of iron contamination from a crucible and coating materials is increased with increasing the growth time. The dopant striation was observed by the thermal pulse technique, and it was confirmed that the crystal/melt interface is almost flat. In addition, the striation showed discontinuity at the grain boundaries. From these results, we focused on the constitutional supercooling, and developed new growth method, named as the successive relaxation of supercooling (SRS) method. The effectiveness of the SRS method was demonstrated

GaAsN thin films were grown by chemical beam epitaxy using monomethylhydrazine ((CH3)N2H3) as the N sources, and the role of the nitrogen (N) and impurities (hydrogen (H), carbon (C)) on free carrier concentration was investigated. The N, H, and C concentrations increased with decreasing growth temperature. GaAsN thin film grown at 470degC was n-type conduction, and the donor is thought to be the same in GaAs thin film. On the other hand, the films grown at 390~460degC were p-type conduction. In the range of 420~460degC, the hole concentration increased with decreasing growth temperature, and it decreased below 420degC, respectively. In order to clarify the origin of acceptor in p-GaAsN thin films, the dependence of growth temperature on the hole concentration and N, H, and C concentration were investigated, and the relationship between them was discussed

Higher conversion efficiency has been predicted theoretically for InGaP/In0.16Ga0.84As/Ge lattice-mismatched triple junction solar cells owing to widening of the effective range of the solar spectrum than the conventional lattice-matched solar cells. The most serious problem in lattice-mismatched system is the large lattice mismatch (~1%) between In0.16Ga0.84As and Ge cells. The effect of a thermal cycle annealing (TCA) process which is expected to reduce the defect density in this system has been discussed from electrical and structural viewpoints. The minority carrier lifetime in In0.16Ga0.84As emitter layers were improved after TCA treatment from TR-PL measurement. EBIC measurements showed a reduction of the structural defect such as misfit dislocations due to the TCA process in In0.16Ga0.84As base layers. The misfit components observed in the base layers may have some influence through the emitter layers.