Takuya Satoh

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.

Cu(In,Ga)Se2 solar cells on stainless steel substrates covered with insulating layers

Abstract We have developed the flexible Cu(In,Ga)Se 2 (CIGS) solar cells on the stainless steel substrates with the insulating layer for the fabrication of the integrated module. The CIGS films have strong adhesion to the Mo films with insulating layers. An efficiency of 12.3% was achieved by the flexible CIGS solar cell with a structure of ITO/ZnO/CdS/CIGS/Mo/SiO 2 /stainless steel. The insertion of the SiO 2 insulating layer did not have an influence on the formation of the CIGS film and solar cell performances.

Production technology for CIGS thin film solar cells

Abstract Composition ratios such as Cu/(In+Ga) and Ga/In have a great influence not only on performances but on yields of Cu(In,Ga)Se 2 (CIGS) solar modules. We demonstrated on a composition monitoring of a Cu/(In+Ga) ratio and control of a Ga/In profile in the CIGS films using a Cu rich composition in Cu rich growth stage. A composition monitoring method based on emissivity of CIGS films is proposed and developed for the preparation of large area CIGS films. The substrate temperature cooling rate immediately after the deposition increases with increasing the Cu/(In+Ga) ratio in Cu-rich films, while Cu poor films have almost the same cooling rate. Measurement of the temperature cooling rate therefore enables the determination of whether the deposited film has Cu rich or poor composition. In and Ga depth profiles in completed CIGS films depend on the Cu/(In+Ga) ratios in Cu rich growth stage. Smooth gradient profile near the surface was formed in the CIGS film prepared from a heavily Cu rich film due to the fast diffusion rate of Ga into an excess Cu x Se layer. The bandgap profile, induced by the Ga/In depth profile, can be controlled by the Cu rich composition in Cu rich growth stage to improve performance of solar cells.

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se 2 (CIGS) thin film was proposed for production of CIGS solar cells. In-Ga-Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu-Se/In-Ga-Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF2/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V oc = 0.634 V, J sc = 36.4 mA/cm 2 , FF = 0.756).

Variable Light Soaking Effect of Cu(In,Ga)Se 2 Solar Cells with Conduction Band Offset Control of Window/Cu(In,Ga)Se 2 Layers

The impact of the conduction band offset (CBO) between window/Cu(In,Ga)Se 2 (CIGS) layers on the light soaking (LS) effect in CIGS solar cells has been studied with continuous CBO control using a (Zn,Mg)O (ZMO) window layer. Two types of CIGS solar cells with different window/buffer/absorber layers configurations were fabricated, i.e., ZMO/CIGS (without buffer layer) and ZMO/CdS/CIGS structures. The CBO values between ZMO and CIGS layers were controlled to -0.15~0.25 eV. Plus and minus signs of CBO indicate the conduction band minimums of ZMO above and below that of CIGS, respectively. Current-voltage ( J-V ) characteristics of the solar cells with different LS durations revealed that a positive CBO value higher than 0.16 eV induces J-V curve distortion, i.e., LS effect, and all the J-V characteristics stabilized in 30 min. The degrees of the LS effect were dominated by the CBO value between ZMO and CIGS layers in the both structure regardless of the existence of CdS buffer layers. These results indicate that the LS effect is dominated by the highest barrier for photo-generated electrons in the conduction band diagram, i.e., the CBO between ZMO and CIGS layers, and quantitatively the LS effect emerges the CBO value higher than 0.16 eV.

Cu(In,Ga)Se2 Solar Cells on Flexible Stainless-Steel Foils

We have developed flexible Cu(In,Ga)Se 2 (CIGS) solar cells on stainless-steel foils. The CIGS films deposited by an in-line evaporation system have str ong adhesion to Mo films. The phases of CIGS films on the stainless-steel foil was a lmost the same as that on the glass, but the grain size of CIGS films on the Mo/ stainless-steel foils are smaller than those of the films on the glass. We also examined insulating layers and resulted in t hat the insulating layers had no influence on the formation of CIGS and the cell performance. Furtherm or we fabricated CIGS solar cells with sodium compound layers to improve efficiency and achi eved an efficiency of 14.3% with the stainless-steel foils.