Muneyori Tachiyuki

Role of incorporated sulfur into the surface of Cu(InGa)Se2 thin-film absorber

Abstract High-performance Cu(InGa)Se 2 (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 H 2 Se gas. Additional sulfurization step to form a thin Cu(InGa)(SeS) 2 (CIGSS) surface layer on the absorber is necesarry to improve the device performance. In order to understand the role of S incorporated into CIGS absorber, approaches with S are discussed. One approach is carried out by changing the condition of our absorber formation process. It is verified to be possible to incorporate more S into the CIGS absorber, but difficult to improve the device performance with higher S contained CIGS absorbers because of decrease in FF. The incorporated S is concluded to be effective to improve the pn heterojunction quality due to the passivation of surface and grain boundary of CIGS absorber through the formation of a thin CIGSS surface layer.

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.

Yield issues on the fabrication of 30 cm×30 cm-sized Cu(In,Ga)Se2-based thin-film modules

The approaches to establish a more robust and reproducible baseline process for 30cm × 30 cm-sized CIGS-based thin-film circuits with a Zn(O,S,OH) x buffer layer are reported, which also lead to an achievement of 12.93% efficiency on an aperture area of 864cm 2 . Monitoring the transparency or transmittance (%T) of dip solution as a process control parameter in the chemical bath deposition (CBD)-buffer deposition step and setting the end point of dipping the CIGS-based absorbers in the solution as the %T of 60% remarkably contribute to make our CBD-buffer deposition process more reproducible. By considering carefully the growth process of metal-organic chemical vapor deposition (MOCVD)-ZnO:B window, a thin layer of high-resistivity, intrinsic ZnO is deposited on the Zn(O,S,OH) x buffer layer to simulate the film structure of MOCVD-ZnO:B window in the case of sputtered-5.7 GZO window. Achievement of the reproducibility of 85% for the CIGS-based thin-film circuits with a sputtered-5.7 GZO window confirms that the yield goal of 85% is surely attainable independent of window-layer deposition techniques, such as MOCVD and sputtering. In this study, it is emphasized how important to eliminate unknown factors in the fabrication process for CIGS-based thin-film modules to improve both reproducibility and efficiency.

The role of Cu(InGa)(SeS)/sub 2/ surface layer on a graded band-gap Cu(InGa)Se/sub 2/ thin-film solar cell prepared by two-stage method

The purpose of this study is to understand the current baseline process for the absorber formation by a two-stage method and make the process more reliable and reproducible through the investigation of the formation chemistry of the Cu(InGa)Se/sub 2/ (CIGS) thin-film absorbers with a graded band-gap structure and Cu(InGa)(SeS)/sub 2/ (CIGSS) surface layer. A 50-cm/sup 2/ aperture-area efficiency of 13% measured at NREL, which has been reported for the first time in a CIGS thin-film solar module with a Cd-free, sulfur-contained Zn-compound buffer layer, should be a good evidence of the role of CIGSS surface layer on the performance of CIGS thin-film solar cells.

Progress in large-area CIGS-based modules with sputtered-GZO window

An objective of this study is to reduce the efficiency gap of 30 cm/spl times/ 30 cm-sized CIGS-based thin-film modules with MOCVD-ZnO:B or sputtered-5.7GZO (ZnO:5.7wt%Ga) window. Based upon a growth model of MOCVD-ZnO:B window, a thin intrinsic-ZnO layer with the thickness of 50 nm is prepared on a CBD-Zn(O,S,OH)/sub x/ buffer by RIF sputtering in the case of sputtered-5.7GZO window. This approach contributes to maintain the reproducibility of our current baseline process and leads to the achievement of the module efficiency of 12.2 % with this window, which has been measured by NREL. It is emphasized that 1) this gap is reduced up to 1 % by making a module, and 2) low-cost packaging technologies without any J/sub sc/ loss and suitable to CIGS-based thin-film circuits should be developed promptly.

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.