Yoshihiro Hamakawa

Theoretical analysis of the effect of conduction band offset of window/CIS layers on performance of CIS solar cells using device simulation

Abstract One of the most important factors of CdS leading to high performance in Cu(In,Ga)Se 2 (CIGS) solar cells is appropriation of the conduction band offset of CdS/CIGS layers. However, it is not clearly explained. In this study, device modeling and simulation were conducted to explain the effect of conduction band offset of window/CIGS layers on performance of CIGS solar cells. As a result of calculation, excellent performance can be obtained when the conduction band of window layer positions higher by 0–0.4xa0eV than that of CIGS.

Preparation of Zn1-xMgxO films by radio frequency magnetron sputtering

Abstract II–VI widegap semiconductors such as ZnO are widely utilized in various electronic and optical devices. To widen the band gap of ZnO, we conducted a systematic investigation of solid solution thin films of Zn 1− x Mg x O, a group of ternary compounds of the Znue5f8Mgue5f8O system. We prepared the thin films by radio frequency (RF) magnetron co-sputtering on fused silica substrates at room temperature. The thin films were a single phase of Zn 1− x Mg x O having the basic structure of ZnO at x ≤0.46 and the basic structure of MgO at x ≥0.62, with segregation of the ZnO and MgO phases at x =0.58. The band gap of Zn 1− x Mg x O having the basic structure of ZnO increased from 3.24 eV at x =0 (ZnO) to 4.20 eV at x =0.46. Transmittances of Zn 1− x Mg x O thin films were nearly equivalent to those of ZnO. Zn 1− x Mg x O has a wider band gap than ZnO and can be expected to provide a useful window layer of solar cells that improves the overall efficiency by decreasing the absorption loss.

Cu(In,Ga)Se2 solar cells with controlled conduction band offset of window/Cu(In,Ga)Se2 layers

Our group studied the effects of conduction band offset of window/Cu(In,Ga)Se2 (CIGS) layers on CIGS-based solar cell performance. To control the conduction band offset, we considered the use of a window layer of Zn1−xMgxO thin film with a controllable band gap as an alternative to the conventional window layer using CdS film. From the measurement of valence band offset between Zn1−xMgxO/CIGS layers and the band gap of each layer, we confirmed that the conduction band offset of Zn1−xMgxO/CIGS layers could be controlled by changing the Mg content of the Zn1−xMgxO film. The CIGS-based solar cells prepared for this study consisted of an ITO/Zn1−xMgxO/CIGS/Mo/soda-lime glass structure. When the conduction band minimum of Zn1−xMgxO was higher than that of CIGS, the performance of CIGS-based solar cells with a Zn1−xMgxO window layer was equivalent to that of CIGS-based solar cells with CdS window layers. We confirmed that the control of the conduction band offset of the window/CIGS layers decreases the majority...

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se 2 (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn 1 - x Mg x O film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn 1 - x Mg x O/CdS/CIGS/Mo/glass structure with various CIGS band gaps (Eg 0.97-1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.

Microstructural dependence of electron and hole transport in low-temperature-grown polycrystalline-silicon thin-film solar cells

Carrier transport properties of undoped polycrystalline silicon (poly-Si) thin films prepared by SiH4–H2 plasma at low temperature have been investigated. The ac-conductivity measurement technique has been applied to poly-Si i layers with an n-i-n junction structure in order to characterize the electron conductivity along the growth direction. Furthermore, the hole conductivity has been measured with p-i-p junction structures. The temperature dependence of ac conductivity reveals that poly-Si films with relatively low crystalline volume fraction (Xc∼50%) exhibit intrinsic character, while the poly-Si films with high Xc (>50%) exhibit n-type character with activation energies less than 0.15 eV. Based on these results, the relationship among microstructure, carrier transport, and photovoltaic performance of poly-Si solar cells is discussed.

Evaluation of electric energy performance by democratic module PV system field test

A systematic investigation has been made on annual accumulated generated PV power from different solar arrays consisting of three kinds of silicon-based solar cells. To clarify seasonal output power variations with temperature in c-Si and a-Si cells might be an important issue for the operations of PV system. It has been shown from the results that electric output power from a-Si array in summer is 20% larger than that from c-Si. On the other hand, in winter, this scene should be reverted. However, output power from c-Si array is only 5% larger than that from a-Si. The analyzed data also shows that annual accumulated electric power generated from a-Si array corresponds to 90% of its nominal efficiency in the year. While in case of c-Si array, this ratio is about 84%.

Influence of texture feature size on spherical silicon solar cells

Abstract The effects of surface texturing on spherical silicon solar cells were investigated. Surface texturing for spherical Si solar cells was prepared by immersing p-type spherical Si crystals in KOH solution with stirring. Two kinds of texture feature sizes (1 and 5 μm pyramids) were prepared by changing stirring speed. After fabrication through our baseline processes, these cells were evaluated by solar cell performance and external quantum efficiency. The cell with 1 and 5 μm pyramids shows the short circuit current density (Jsc) value of 31.9 and 33.2 mA·cm−2, which is 9% and 13% relative increase compared to the cell without texturing. Furthermore, the cell with 5 μm pyramids has a higher open-circuit voltage (0.589 V) than the cell with 1 μm pyramids (0.577 V). As a result, the conversion efficiency was improved from 11.4% for the cell without texturing to 12.1% for the cell with 5 μm pyramids.

Tilt angle dependence of output power in an 80 kWp hybrid PV system installed at Shiga in Japan

One-year field experience of an 80 kW PV system on a rooftop of the ROHM Memorial VLSI Research Center at the Ritsumeikan University is reported. All kinds of live technology available materials, c-Si, poly Si and a-Si solar cells are installed on the three tilt angles of 26.5° south, horizontal and north 26.5°. Systematic PV performances have been measured from the beginning of June 2000 to the end of May 2001. Measurements were made mainly on DC output power from four kinds of PV arrays; c-Si south side, a-Si of horizontal and poly Si, a-Si north side. It has been shown from analyses of monthly data on each material that almost 70% of with that in the south side in the annual average. In summer a-Si module yields the maximum output power normalized to 1 kWp. On the contrary c-Si module shows larger output in winter. Some other unique results are demonstrated and discussed.

2D-numerical analysis and optimum design of thin film silicon solar cells

Abstract Device modeling for p–i–n junction μc-Si basis thin film polycrystalline Si solar cells has been examined with a simple model of columnar grain structure and its boundary condition utilizing two-dimensional device simulator. As the simulation results of solar cell characteristics show, open-circuit voltage ( V oc ) and curve fill factor (FF) considerably depend on those structural parameters, while short-circuit current density ( J sc ) is comparatively stable by courtesy of homogeneous built-in electric field in the i layer. It has also been found that conversion efficiency over 12% could be expected with 1xa0μm grain size and well-passivated condition with 3xa0μm thick i-layer.

Recent advances in amorphous and microcrystalline silicon basis devices for optoelectronic applications

The current state of the art in recent advances hydrogenated amorphous and microcrystalline silicon (a-Si and μc-Si) technologies and their applications to optoelectronic devices are reviewed. With the recent progress in material preparation and characterization technologies, we now have an age that considerably high quality thin films having valency electron controllability can be produced. In this paper, recent progress in thin film solar cell fabrication with a-Si and μc-Si technologies for active materials for optoelectronic devices are reviewed first, and their significance are pointed out, then some typical newly developed devices such as integrated amorphous solar cells, flexible solar cells etc., are demonstrated. Secondly, new kinds of thin film light-emitting devices, including solid-state flat panel displays are introduced. In the final part of this paper, the remarkable industrial progress in the field of optoelectronics and the prospects of market expansion toward the 21st century are briefly discussed.