Daisuke Okumura

Fabrication of graded band-gap Cu(InGa)Se2 thin-film mini-modules with a Zn(O,S,OH)x buffer layer

Abstract High-performance Cu(InGa)Se2 (CIGS) thin-film absorbers with an intentionally graded band-gap structure have been fabricated by a simple two-stage method using In/CuGa/Mo stacked precursors and H2Se gas. Additional sulfurization step to form a thin Cu(InGa)(SeS)2 surface layer on the absorber is necessary to enhance the grain growth and improve the device performance. Improvement of the interface quality between the absorber and the Zn(O,S,OH)x buffer layer by applying a post-deposition light soaking has, for the first time, resulted in the efficiency of over 14% measured by JQA with a 50 cm2 aperture-area monolithic mini-module. The post-deposition light-soaking treatments would be utilized as an effective tool leading to the accelerated process development with high yield for the future commercial production.

Large area ZnO films optimized for graded band-gap Cu(InGa)Se2-based thin-film mini-modules

In this study, two deposition methods (i.e. MOCVD and sputtering methods) to prepare n-type ZnO window layers for CIGS-based thin-film solar cells are discussed. In order to make ZnO : Al transparent conductive oxide (TCO) films prepared by DC magnetron sputtering comparable to ZnO : B TCO prepared by MOCVD, a new ZnO sputtering process is proposed by introducing a multilayer structure. Using these films, CIGS thin-film solar cells with efficiencies of greater than 14% have been fabricated with an active area of 3.2 cm2. This structure was adapted to fabricate CIGS thin-film mini-modules with efficiencies around 11% having aperture area of 50 cm2.

Optimization of Al-doped ZnO Window Layers for Large-Area Cu(InGa)Se2-Based Modules by RF/DC/DC Multiple Magnetron Sputtering

In this report, a comparative study of physical properties of the multilayered ZnO:Al films prepared by a combination of RF and DC magnetron sputtering is presented. It has been found that a RF/DC/DC trilayered system consisting of a thin RF-sputtered ZnO:Al bottom layer with two identical DC-sputtered ZnO:Al layers deposited with a low DC current improved the physical properties when compared to those of the ZnO film of the baseline condition, [DC(2.0 A, thickness of about 6500 A) monolayer]. The sheet resistance and transmittance of the highest quality ZnO film deposited with the RF(600 A)/DC(1.2 A, thickness of 4200 A)/DC(1.2 A, thickness of 4200 A) sputtering condition were found to be 10 Ω/sq and 85% in the wavelength range of 350–1400 nm, respectively. With the newly improved transparent-conductive-oxide (TCO) window, Cu(InGa)Se2 (CIGS) modules (aperture area = 50 cm2) have been fabricated, and marked improvement in fill factor (FF) (+8%) and efficiency (+12%) have been obtained when compared to those of the ZnO:Al deposited under the baseline condition. The average efficiency of the above CIGS modules was found to be 11.1%.

Application of Stacked ZnO Films as a Window Layer to Cu(InGa)Se2-Based Thin-Film Modules

To overcome the disadvantages of DC sputtering while keeping its advantages, a stacked structure of ZnO films as a window layer has been developed for application to Cu(InGa)Se2 (CIGS)-based thin-film modules. Application of multilayered, 2 wt% Al2O3-doped ZnO (AZO) films prepared from a 2 wt% Al2O3-doped ZnO ceramic target and use of a combination of RF and DC sputtering techniques (i.e., sputtering conditions of RF 300 W/DC 1 A/DC 1 A) can lead to better crystallinity and electrical properties and, as a result, an improved device performance of CIGS-based thin-film modules. To improve the humidity resistance of the ZnO window layer, which is the top layer of CIGS-based thin-film modules, AZO is replaced with 5.7 wt% Ga2O3-doped ZnO (GZO) prepared from a 5.7 wt% Ga2O3-doped ZnO ceramic target. By adjusting the stacked structure and the layer thickness by DC sputtering, an improvement in device performance, especially the fill factor (FF), is achieved with the stacked structure of AZO (base)/AZO (middle)/0.6 GZO (top) as the window layer, and a total thickness of about 600 nm is realized, which means a thinner (about 60% of AZO top layer thickness) top layer.

Improved FF of CIGS thin-film mini-modules with Zn(O,S,OH)/sub x/ buffer by post-depostion light soaking

In order to explain the observations on the post-deposition light soaking and understand this unique and valuable effect, a model is proposed and confirmed to work well. Based on the model, released H/sub 2/O molecules through the dehydration of Zn(OH)/sub 2/ in the Zn(O,S,OH)/sub x/ buffer during the light soaking is considered as a major player to affect the form factor (FF). The most striking result in this study is the post-deposition light soaking effect can be controlled from reversible to irreversible by adjusting the light soaking conditions. Approach to reduce the Zn(OH)/sub 2/ concentration in the buffer contributes to make a better p-n heterojunction and improve the yield.

Formation of robust junction between Cu(InGa)Se/sub 2/-based absorber and Zn(O,S,OH)/sub x/ buffer prepared on a 30 cm/spl times/30 cm submodule

An important key to realize the strong requirement on the electrical yield (i.e. 85 % in the efficiency of over 10 % in 30 cm/spl times/30 cm-sized CIGS-based circuits with an aperture area of over 810 cm/sup 2/) has been indicated to make a robust junction between CIGS-based absorber and Zn(O,S,OH)/sub x/ buffer. In this study, the baseline process for CBD-Zn(O,S,OH)/sub x/ buffer deposition is investigated from the standpoint of reduction of the deviation of FF. By monitoring the transparency or transmittance (%T) of the CBD solution as a new control parameter, the Zn(O,S,OH)/sub x/ buffer deposition process is much stabilized especially on the thickness uniformity measured by LBIC technique. From this approach, it is confirmed that much narrower distribution of FF in the range of over 0.6 can be steadily achieved by improving the thickness uniformity of the buffer and, as a result, the achievement of the above goal is foreseeable.