Katsumi Kushiya

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.

Application of Zn-Compound Buffer Layer for Polycrystalline CuInSe2-Based Thin-Film Solar Cells

In order to meet the requirement for the development of a more environment-friendly device structure for polycrystalline CuInSe2-based thin-film solar cells, Zn compound buffer layers for use as an alternative to CdS buffer layers have been fabricated by a chemical-bath deposition (CBD) method which is believed to be cost-effective and applicable to large-area deposition. CIS-based thin-film solar cells with a Zn(O, OH)x buffer layer showed a relatively low efficiency of about 10% because of the difficulty in the reduction of the amount of OH- ions in the buffer layer. By adding a sulfur source to the CBD solution as a new approach to reduce the amount of OH- ions, a dramatic decrease in the content of hydroxide in the buffer layer and the formation of a better heterointerface between the CIS-based thin-film absorber and the ZnO window layer were achieved simultaneously for the first time. The application of Zn(O, S, OH)x as a buffer layer to CIS-based thin-film solar cells led to the higher efficiency of 12.8% on the active area of 3.2 cm2. Zn(O, S, OH)x fabricated by a CBD method was demonstrated to be a promising alternative to CdS and a more environment-friendly buffer layer, although further investigation is required to understand the mechanisms of the irradiation effect.

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.

Stabilization of PN Heterojunction between Cu(InGa)Se2 Thin-Film Absorber and ZnO Window with Zn(O, S, OH)x Buffer

Dramatic improvement of current–voltage (I–V) performance, especially the fill factor (FF) observed in the Cu(InGa)Se2-based thin-film circuits with Zn(O, S, OH)x buffer after postdeposition light soaking is discussed in this study. Considering the composition of Zn(O, S, OH)x buffer and the reversible behavior with respect to postdeposition light soaking, a model is proposed, in which H2O molecules released by the dehydration of Zn(OH)2 in the Zn(O, S, OH)x buffer are considered to play a dominant role in this behavior. Based upon this model, attempts to stabilize the pn heterojunction by making the reversible behavior irreversible are, for the first time, successfully achieved by adjusting the postdeposition light-soaking conditions. The reduction of the Zn(OH)2 concentration in the Zn(O, S, OH)x buffer through the combination of heating at 130°C for at least 40 min and irradiation using a constant-light solar simulator contributes to stabilizing the pn heterojunction and improving the I–V performance as well as the electrical yield.

Development of High-Efficiency CuInxGa1-xSe2 Thin-Film Solar Cells by Selenization with Elemental Se Vapor in Vacuum.

Achievements in the development of CuInSe2 (CIS) and CuInx Ga1-x Se2 (CIGS). thin-film absorbers with efficiency over 12% by the two-stage method are reported as follows: 12.3% efficiency with CIS, 13.3% efficiency with CIGS (Ga/III ratio=0.25) and 10.1% efficiency with CIGS (Ga/III ratio=0.40, graded band-gap structure). These results clearly indicate that the simple two-stage method developed in this study has high capability to fabricate high-quality CIS and CIGS thin-film absorbers. The Se content in the precursor layer and the Se beam flux intensity during the selenization stage are key factors that must be controlled precisely in order to achieve a high efficiency of over 15%. It is reported, for the first time, that the spontaneous formation of a graded band-gap structure is possible by changing Se and Ga contents in the precursor layer, which should be applicable to the preparation of a high-quality CIS or CIGS thin-film absorber. The formation chemistry models of CIS and CIGS thin films related to this study are proposed.

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%.

Annealing Enhancement Effect by Light Illumination on Proton Irradiated Cu(In,Ga)Se2 Thin-Film Solar Cells

In this paper, we investigated the high radiation tolerance of copper indium gallium di-selenide (CIGS) thin-film solar cells by conducting in situ measurements of short circuit current and open circuit voltage of CIGS thin-film solar cells during and after proton irradiation under short circuit condition. We found that the annealing rate of proton-induced defects in CIGS thin-film solar cells under light illumination with an AM0 solar simulator is higher than that under dark conditions. The activation energy of proton-induced defects in the CIGS thin-film solar cells with (without) light illumination is 0.80 eV (0.92 eV), which implies on enhanced defect annealing rate in CIGS thin-film solar cells due to minority-carrier injection.

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.