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8th World Conference on Photovoltaic Energy Conversion; 119-122, Ge solar cells were prepared using three types of p-Ge wafers with carrier concentrations ranging from 1.0 × 1016 / cm3 (0.28 Ωcm) to 3.0 × 1018 / cm3 (0.0063 Ωcm). The surface treatment of Ge wafers and the passivation of the n-a-Si:H (n-µc-Si:H) / p-Ge interface have a great impact on the conversion efficiency. As for the substrate pretreatment, the HF treatment and the method of adding the HCl treatment to the HF treatment were also attempted. At the n-a-Si:H (n-µc-Si:H) / p-Ge emitter interface, i-a-Si:H with a thickness of 4 to 14 nm was inserted as a passivation layer. A BSF structure of p-Ge /i-a-Si:H /p-µc-Si was deposited on the back surface side. Voc was greatly improved by lowering the resistivity of the substrate. In a cell in which an i-a-Si:H layer with a thickness of 14 nm was introduced at the emitter interface on a 0.016 Ω cm substrate, a conversion efficiency 8.6 % (Voc = 291 mV, Jsc = 45.0 mA/cm2, FF = 0.656, AM1.5, 100mW/cm2, 25 ° C) was obtained. The interface was observed with a transmission electron microscope (TEM). As a result, It was found that the island-shaped heteroepitaxial growth suppressed the misfit dislocations that would otherwise occur due to lattice mismatch, and exhibited good interfacial characteristics.

37th European Photovoltaic Solar Energy Conference and Exhibition; 295-301, Bifacial a-SiOx:H/a-SiOx:H/a-Si:H/a-SiOx:H/a-SiOx:H quintuple-junction and a-SiOx:H/a-SiOx:H/aSi:H/a-Si:H/a-SiOx:H/a-SiOx:H sextuple-junction solar cells were prepared by the PE-CVD. The solar cell has a structure in which both sides are sandwiched between ITO layers, and the structure allows light irradiation from both sides. A total of 49 cells having an area of 0.25 cm2 , 0.5 cm2 , and 1 cm2 was fabricated on a 10cm x 10cm substrate. The distribution of open-circuit voltage, Voc, under low illumination in all 49 solar cells was investigated. As one example of applying bifacial multi-junction a-Si based solar cells as an independent power source for the Internet of Things (IoT) devices, a flexible bifacial quintuple-junction solar cell was also prepared. In the sextuple-junction solar cell, an open-circuit voltage of 4.1 and 3.5 V was obtained even at a low illuminance of 3,000 and 100 lux, respectively. In the quintuple-junction solar cell formed on the polyimide film substrate, an open-circuit voltage of 3.2 and 2.7 V was obtained at 3,000 and 100 lux, respectively. The decreasing amount of the open circuit voltage when the irradiation intensity became 1/10, Voc (1/10), was 62-64 mV/cell.

36th European Photovoltaic Solar Energy Conference and Exhibition; 140-146, Hydrogenated amorphous Si(a-Si:H) Quintuple-junction solar cells which consist of a-SiOx:H/aSiOx:H/a-Si:H/a-SiOx:H/a-SiOx:H were fabricated by plasma CVD method. The total thickness was 0.6-0.8 m. Irradiation intensity (Pin) dependence of the open circuit voltage (Voc) of quintuple-junction solar cells was measured. The decreasing amount Voc (1/10) of the open-circuit voltage when the irradiation intensity became 1/10 was 62mV/cell. Voc drops rapidly from around the irradiation intensity of 1mW/cm2 (approximately 1,000 lux). This large Voc reduction is due to leakage current. Then, we discussed the origin of the leakage current, and finally by improving the leakage current, a very high open-circuit voltage Voc of 3.5 V was demonstrated under LED light illumination. Furthermore, we theoretically analyzed Voc as a function of the irradiation intensity, including effects of the leakage current and the film quality of i-a-Si(O):H. It was found from the simulation results that it is necessary to increase the shunt resistance Rsh and to lower the defect density of i-a-Si(O):H in order to obtain a sufficient Voc-Pin characteristics for IoT devices application under low illuminance.

36th European Photovoltaic Solar Energy Conference and Exhibition; 522-526, The passivation properties of the hydrogenated amorphous silicon oxide (a-SiOx:H) film with the thin thickness applied to the Si-based heterojunction solar cells were investigate. It is confirmed that the ~2nm thickness of the a-SiOx:H film is enough to obtain the satisfactory effective lifetime of excess minority carrier using the structure passivated by the a-SiOx:H/n-type a-Si:H bilayer, although the at least 3nm overlayer of a-Si:H is necessary in the emitter side to probably protect the a-SiOx:H passivation film against the influence of p-layer deposition. The Cody gap estimated from the spectroscopic ellipsometry measurement showed that the optical bandgap of this ~2nm-thick a-SiOx:H film is narrower than that of the bulk state, which was suggested probably to be resulted from the non-steep changes of hydrogen content and from the existence of the partially epitaxial growth and/or nanocrystallized structure film at the a-SiOx:H/c-Si interface by the measurements of high resolution Rutherford backscattering spectrometry and high resolution elastic recoil detection analysis. These high passivation quality and comparably narrow bandgap of the a-SiOx:H film at the thin thickness should be beneficial to obtain high Voc and FF.

35th European Photovoltaic Solar Energy Conference and Exhibition; 262-265, Heterojunction solar cells with a Rib structure are prepared aiming at ultra-thin Si solar cells. In the Rib structure, the Rib region of about 10% in area ratio mechanically supports the thinned region. In this paper, evaluation was carried out with a focus on the in-plane characteristic distribution of Rib solar cells. Both electroluminescence (EL) and dark lock-in thermography (DLIT) imaging were carried out to evaluate the local electronic properties of Rib solar cells. It was found that EL intensity from Rib-region is 2.2 times larger than EL from thin-region. The difference in EL intensity corresponds to the difference of Voc, about 23 mV. DLIT imaging also showed that the current density in the Rig-region is higher than the thin-region. In addition, evaluation by PL imaging is also carried out. And the relationship between the PL intensity distribution and the Rib wafer thickness distribution is discussed based on the thickness dependence of the effective carrier lifetime.

31st European Photovoltaic Solar Energy Conference and Exhibition; 176-180, A crystalline Si based tandem solar cell composed of a nano-wall top cell and a conventional p/n junction bottom cell has been studied. The best bandgap of the top cell is 1.7eV for the conventional Si bottom cell. To increase the bandgap by the quantum size effect, the width of nano-walls should be decreased to 2 nm or narrower. In this work, we studied how to make the Si tandem cell with the nano-wall top cell. The cell structure employed was a 4-terminal cell consisting of the Si nano-wall top cell and an interdigitated back-contact bottom cell. A top-down process was adopted to fabricate the nano-wall, and we succeeded in obtaining an array of nano-walls with a width of about 2.0 nm. We have not fabricated the nano-wall solar cells with this width yet, but preliminary experiments for cell fabrication have been carried out by using samples with wider nano-walls. We will describe the structure and the I-V characteristics of the fabricated cells in addition to the nano-wall fabrication process for the top cell.

31st European Photovoltaic Solar Energy Conference and Exhibition; 205-209, The influence of Aluminum Oxide (Al1-xOx) films grown on two types of silicon micro wire (Si-MW) substrates by thermal atomic layer deposition (ALD) technique as a passivation layer was investigated, compared to the one grown on conventional flat wafer substrate. When the growth rate of the Al1-xOx film became closed to the ideal growth rate (0.11 nm/cycle), the Al1-xOx film has then shown good-passivation effect and were found to be uniformly deposited. Compared to the samples grown on flat substrate and the Si-MW substrate fabricated using ICP-dry etching technique, the Al1-xOx film deposited on the Si-MW substrate using Deep-RIE technique, blessed us with implied Voc of 0.646 V after annealing. We investigated implied Voc of Al1-xOx films and reflectance properties for Si-MW substrates with various wire-array sizes ((1)D=2 um, P=4 um, (2)D=3 um, P=5 um, (3)D=5 um, P=8 um, (4)D=7 um, P=10 um, (5)D=10 um, P=13 um). We found that the relation between implied Voc and reflectance properties for each of wire array size of Si-MW substrates, is in a trade-off.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2550-2553, To investigate the effects of using a-SiOx:H/a-Si:H tandem solar cells, numerical simulations of low concentration spectrum-splitting thin-film solar cells have been done. In addition, the effects of the width of the splitting filter’s cutting profile have been investigated. These calculations showed that using tandem solar cells whose i-layer thickness of bottom cell was 200 nm improved concentration characteristics and increased total conversion efficiency under 10 suns. Furthermore, it was shown that the total efficiency decreased parabolically as the width of the cutting wavelength increased.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2184-2186, In this work, the effects of CO2 plasma treatment, introduced before the deposition of a bottom p-μc- SiOx:H layer, on the n-μc-SiOx:H / p-μc-SiOx:H interface were invetigated. It was found that CO2 plasma treatment before p-μc-SiO:H deposition can help the crystallization of post-deposited p-μc-SiO:H and thus improve the n-μc- SiOx:H / p-μc-SiOx:H interface properties. Also, the a-Si:H/μc-Si:H tandem solar cells with the novel structure of pa- SiO:H/ i-a-Si:H/ n-μc-SiO:H /CO2 plasma / p-μc-SiO:H/ i-μc-Si:H / n-μc-SiO:H/ Ag/ Al were then fabricated on the textured Soda-lime glasses coated with ZnO:B to compare with the one without CO2 plasma. From the results, it shows that by using the CO2 plasma treatment optimized so far, the better cell interface property was achieved, the Voc and FF were improved and thus an efficiency of 12.1 % could be achieved.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2573-2575, To increase the performance of thin-film solar cells, solar spectrum splitting technique has been considered and studied. It was found from the simulation that the total efficiency of nearly 25% can be obtained at the splitting wavelength of 600 nm using top cell with higher band gap material. The experiment has been carried out to verify the simulation results. Up to now the total efficiency of about 17.9 % has been obtained using a-Si:H and Cu(In1- x,Gax)Se2 as the top and bottom cells, respectively, at the splitting wavelength of 614 nm. Then after optimization we obtained an advanced conversion efficiency of up to 21.8% which is close to the simulation results. This result shows that our developed splitting technique is a promising technique for high-performance thin-film solar cell.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1616-1618, We have focused on intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) for application to passivation layers of heterojunction solar cells. i-a-SiO:H can be deposited at high temperatures without epitaxial growth and, consequently, can be a solution for epitaxial growth that occurs when conventional i-a-Si:H is used. In this work we report on the application of i-a-SiO:H as the passivation layer for flat and textured substrates and improvement of effective lifetimes through annealing treatment. The effective lifetimes for annealed samples were 1.7-2.2 times higher than the as-deposited ones. These values correspond to surface recombination velocities of 4-5 cm/s and 6-7 cm/s for flat and textured substrates, respectively. By introducing the annealing process to solar cell fabrication, VOC increased from 659 to 703 mV for the textured wafer case due to improvement of passivation. Thanks to this increase, the conversion efficiency improved from 15.8% to 17.3%. For the flat substrate, a VOC of 717 mV and efficiency of 18.9% were achieved. After optimization of passivation layer thickness, an efficiency of 20.1% was achieved. These results indicate that application of i-a-SiO:H as passivation material and proper annealing conditions are promising for improving the passivation quality of heterojunction solar cells.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2120-2122, We have developed zinc oxide (ZnO) transparent conductive oxide (TCO) films fabricated on Eagle- XG type glasses with a high-haze value using Metal Organic Chemical Vapor Deposition (MOCVD) technique and glass substrate surface treatment . Effects of glass substrate treatment time on surface morphology changes for glass and ZnO were investigated. As a result, glass surface morphology was changed to a deep canyon-like texture and with increasing time of glass substrate etching the canyon size was increased. It was found that ZnO film surface morphology was similar to W-textured one, but its valley part is very precipitous. We have also fabricated p-i-n type a-Si:H solar cells on newly developed ZnO substrates. By optimizing Ar treatment condition and solar cell fabrication condition, up to now we could obtain an initial efficiency of 11.0% showing that high-haze ZnO fabricated by MOCVD technique on RIE-etched EAGLE-XG type glass is a promising TCO for thin film silicon solar cells.

27th European Photovoltaic Solar Energy Conference and Exhibition; 3926-3928, There are a lot of unused water surfaces such as of the hydroelectric dams and balancing reservoirs. These surfaces of the water can be used for the PV system deployment. Floating PV systems are developed and installed on the water surface of the “Aichi ike” balancing reservoir in Aichi, Japan in 2007. PV module cooling system with intermittent water spray is installed in a part of the systems and performance is compared with the other system which does not have cooling. As a result, water spray cooling is effective in lowering the module temperature, but periodical cleaning is required. Detergent with citric acid is effective to clean the water stain on the PV module surface. Floating PV systems have almost no damage after the five years deployment.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2620-2623, As reported, TiO2 /ZnO layers were inserted at TCO/Si interfaces in order to suppress optical reflection loss. However, such TiO2 /ZnO layers have to be coated by sputtering which is different from PECVD used for thin-film silicon-based solar cell fabrication. Recently, n-SiOx fabricated by PECVD, has widely been studied as a window layer in n-i-p solar cells or a front anti-reflection layer (FAL) inserted into glass/TCO interface. Here in this work, we have studied how to apply this n-μc-SiOx as a new refractive-index matching layer inserted into ZnO/ p-layer interface (FALp). In this study, we have theoretically and experimentally investigated the optical effect of a FALp on the solar cell performance. Optical calculation of FALp was firstly performed in order to estimate the optimal thickness and refractive index (n) of FALp. Next, the optimization of n-μc-SiOx films was experimentally carried out by investigating the effect of CO2 flow rate on the properties of films. Finally, the performance of silicon-based thin film solar cells, i.e., both of a-Si:H and μc-Si:H solar cells, with FALp optimized up to now was analyzed. It was found that by employing the FALp the spectral response, especially in range 500 to 900 nm, was improved even though the spectral response at short wavelength region was decreased due to absorption loss resulted from un-optimized thickness. With this FALp layer the Jsc for the μc-Si:H solar cell improved by ~0.4 mA/cm2. The results show that this novel n-μc-SiOx:H film can be used as a front anti-reflection layer inserted at ZnO/p interface as well instead of TiO2 /ZnO.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2559-2561, In this work, we have tried to improve the interface property of sputtered ZnO film and p-layer of thinfilm silicon solar cell by introducing low-temperature deposited p-μc-Si as a interfacial layer (IL). It was found that higher cell performance with better fill factor was obtained with low-temperature deposited IL and up to now an efficiency of 10.5% has been achieved showing that this approach is the promising method to achieve high efficiency on high-haze sputtered ZnO substrates.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2301-2306, The present status and future prospects of amorphous Si (a-Si) thin-film solar cells, which are expected to enter the stage of full-scale practical application, are first described. For a-Si single-junction solar cells, the conversion efficiency of their large-area modules has now reached 6-8%, and their practical application to megawatt solar systems has started. Meanwhile, the focus of R&D has been shifting to a-Si and microcrystalline Si (μc-Si) tandem solar cells. Thus far, a-Si/μc-Si tandem solar cell modules with stabilized conversion efficiency exceeding 10% have been reported. Triple-junction solar cells, whose target year for practical application is 2015 or later, are also introduced, as well as innovative thin-film full-spectrum solar cells, whose target year of realization is 2030 or later. In addition, an overview of the recent status of Cu(InGa)Se2 (CIGS), CdTe, dye-sensitized and organic semiconductor solar cells is presented.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2633-2636, Zinc oxide (ZnO) thin films with high haze value were successfully developed using soda-lime glasssubstrate etching before ZnO film deposition. Effects of soda-lime glass treatment conditions on the properties of ZnO films were systematically investigated. It was found that the larger crater surface and high rms roughness of soda-lime glass could be obtained when the glass treatment gas pressure was 13 Pa. It was also found that the surface morphology of ZnO films can be modified by controlling the surface morphology of soda-lime glass substrate without deterioration of their good electrical properties. By employing developed high-haze ZnO film deposited on etched soda-lime glass substrate as front TCO films in μc-Si:H single-junction solar cells, we obtained an efficiency of 8.15% with improved quantum efficiency and largely enhanced Jsc. Thus, the obtained ZnO films deposited on etched soda-lime glass have a lot of potentials to be used as the front TCO layers in silicon-based thin film solar cells as well as a new promising low-cost substrate.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2392-2394, We have developed novel structure a-Si1-xOx:H/a-Si:H/μc-Si:H triple-junction solar cells with the Eff target of around 14%. Firstly, we optimized the fabrication condition of a-Si1-xOx:H single-junction solar cells for an application to the top cell. Our best result showed a high Voc of 1.059 V ( Jsc = 9.85 mA/cm2, FF = 0.69 and Eff = 7.2% ) when the a-Si1-xOx:H material was used in p, buffer and i layers. The achievement of high Voc for the top cells indicated that the effective built-in potential (Vb) was improved due to the wided optical bandgap (Eopt) of ilayer as well as the high potential barrier at the p/i interface from wider Eopt of p-layer. Secondly, we fabricated a-Si1-xOx:H/a-Si:H tandem solar cells and the high Voc of 1.94 V (Jsc = 7.92 mA/cm2, FF = 0.71 and Eff = 10.9%) was achieved. Finally, we fabricated a-Si1-xOx:H/a-Si:H/μc-Si:H triple-junction solar cells. Up to now, we have achieved the high Voc of 2.41 V ( Jsc = 6.91 mA/cm2, FF = 0.70 and Eff = 11.6% ). However, in order to reach the target, it is necessary to optimize Eopt of the middle cell as well as the reflectance of each intermediate layers.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2177-2179, We investigated the effects of light scattering by metal nanoparticles (NPs) formed on the rear surface of Si solar cells on photocurrent enhancements of the cells. The two-dimensional finite-difference time domain (FDTD) method was used to analyze the optical behavior of light scattered by the NP arrays. The arrays reflect the incident light back to the active layer with a wide range of angles, thereby enhancing the optical absorption near bandgap wavelengths of Si without extinction loss by the NPs. Based on the simulation of the model structure, quasiperiodic Ag NP arrays were formed on the rear surface of a planar heterojunction Si solar cell. Spectral response measurements revealed that the Ag NPs improved photocurrent at wavelengths longer than 1000 nm, resulting in a 2 % increase in the short circuit current density of the cell.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2915-2918, Optimization of p-type hydrogenated microcrystalline silicon oxide thin films (p-µc-Si1-xOx:H) by RFPECVD 13.56 MHz method for use as a p-layer of hetero-junction µc-Si:H solar cells was performed. The properties of p-µc-Si1-xOx:H films were characterized by conductivity, activation energy, Raman scattering spectroscopy, and spectroscopic ellipsometry. The wide-gap p-µc-Si1-xOx:H were optimized by CO2/SiH4 ratio and H2/SiH4 dilution effect. Besides, we also investigated effects of wide-gap p-µc-Si1-xOx:H layer on the performance of hetero-junction µc-Si:H solar cells with various optical band gap of p-layer. We observed that a wide-gap p-µc-Si1-xOx:H is effective to improve the open circuit voltage of the solar cells. These results indicate that wide-gap p-µc-Si1-xOx:H is promising for use as window layer in hetero-junction µc-Si:H solar cells.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 591-594, SiOx/ SiO2 superlattices have been developed by RFPECVD technique by varying the deposition conditions and annealing temperatures. Presence of nanocrystals has been confirmed from Raman study, high resolution transmission electron micrographs and X-ray diffraction studies. Size of the nanocrystals as observed from XRD varies from 8 nm to 12 nm for the superlattices deposited under different conditions and crystallized at different temperatures. Raman studies show that the crystalline volume fraction varies from 17 % to 62.4 % for the SL’s formed under different conditions. Photoluminesence study show two peaks, one at ~ 465 nm in blue region and the other at 525 nm in the green region. These two peak intensities increase with decreasing SiOx layer thickness. Peak intensities are more for the superlattice annealed at 800oC than 900oC. Incorporation of nitrogen in the network alters the structure as it replaces electronegative oxygen atoms in the Si-(Si4-n-On) tetrahedral structures.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1282-1285, This contribution reports the recent work performed on a development of p-SHJ solar cells with a- SiOx:H(i) buffer layer between a-Si:H(n) and c-Si(p) (front side), instead of a-Si:H(i) buffer layer. The aim of this work is to improve the open circuit voltage (VOC) by decreasing Dit with a high optical band gap, using a-SiOx:H(i) layer. Furthermore, the experimentally on the effect on CO2/SiH4 ratio in a-SiOx:H(i) buffer layer were investigated. The p-SHJ solar cell performance clearly improved with increasing the CO2/SiH4 ratio in a-SiOx:H(i) buffer layers from 0 to 0.33. This is due to the decrease in Dit at the a-SiOx:H(i)/c-Si(p) front interface, probably by avoiding the formation of the defective interfacial layer. With this buffer layer concept, a high open circuit voltage of 659 mV with an active area efficiency as high as 18.5% for a-Si:H(n+)/a-SiOx:H(i)/c-Si(p)/μc-SiOx:H(p+) solar cell structure has been achieved.