Dae-Hyung Cho

Electronic effect of Na on Cu(In,Ga)Se2 solar cells

We report the effect of Na on the electronic properties of Cu(In,Ga)Se2 (CIGS) thin-film solar cells with a structure of grid/ITO/i-ZnO/CdS/CIGS/Mo/SiOx/soda-lime glass (SLG). The diffusion of Na from the SLG into the CIGS layer was systematically controlled by varying the thickness of SiOx. As the Na content increased, the hole concentration of CIGS was enhanced, while the band-gap was nearly constant, which led to a lower Fermi level in the CIGS towards its valence-band edge. The Na-induced increment in the built-in potential (Vbi) across the n-(ITO/i-ZnO/CdS)/p-CIGS junction yielded an increment of open-circuit voltage that well agreed with the calculated Vbi.

Incorporation of Cu in Cu(In,Ga)Se2-based Thin-film Solar Cells

of 11.1%. The other prepared with a 3-step process had nearly the same In-Ga and Cu concentrations and showed a conversion eciency of 15.5%. We discuss the incorporation of Cu in the two types of thin-film solar cells.

Photovoltaic Performance and Interface Behaviors of Cu(In,Ga)Se2 Solar Cells with a Sputtered-Zn(O,S) Buffer Layer by High-Temperature Annealing

We selected a sputtered-Zn(O,S) film as a buffer material and fabricated a Cu(In,Ga)Se2 (CIGS) solar cell for use in monolithic tandem solar cells. A thermally stable buffer layer was required because it should withstand heat treatment during processing of top cell. Postannealing treatment was performed on a CIGS solar cell in vacuum at temperatures from 300-500 °C to examine its thermal stability. Serious device degradation particularly in VOC was observed, which was due to the diffusion of thermally activated constituent elements. The elements In and Ga tend to out-diffuse to the top surface of the CIGS, while Zn diffuses into the interface of Zn(O,S)/CIGS. Such rearrangement of atomic fractions modifies the local energy band gap and band alignment at the interface. The notch-shape induced at the interface after postannealing could function as an electrical trap during electron transport, which would result in the reduction of solar cell efficiency.

Quantitative analysis of Cu(In,Ga)Se2 thin films by secondary ion mass spectrometry using a total number counting method

The relative atomic fraction of Cu(In,Ga)Se2 (CIGS) films is one of the most important measurements for the fabrication of CIGS thin film solar cells. However, the quantitative analysis of multi-element alloy films is difficult by surface analysis methods due to the severe matrix effect. In this study, the quantitative analysis of CIGS films was investigated by secondary ion mass spectrometry (SIMS). The atomic fractions of Cu, In, Ga and Se in the CIGS films were measured by alloy reference relative sensitivity factors derived from the certified atomic fractions of a reference CIGS film. The total ion intensities of the constituent elements were obtained by a total number counting method. The atomic fractions measured by SIMS were linearly proportional to those certified by inductively coupled plasma mass spectrometry using an isotope dilution method. The uncertainties were determined from the standard uncertainties in the measurements and those of a CIGS thin film certified reference material.

Photoreflectance characteristics of chemical-bath-deposited-CdS layer in Cu(In,Ga)Se2 thin-film solar cells

The authors have characterized the CdS layer in Cu(In,Ga)Se2 thin-film solar cells using photoreflectance (PR) spectroscopy and investigated its influence on the photovoltaic performance. The CdS layer was fabricated by chemical bath deposition with various concentrations of ammonia (1.0–3.0 M), thiourea (0.025–0.1 M), and Cd-salt (0.0004–0.003 M) as well as various thicknesses (30–90 nm). The PR transition energy in CdS increased from 2.282 to 2.366 eV as the thiourea concentration increased from 1.0 to 3.0 M, whereas it decreased as the thickness of CdS increased. The conversion efficiency depended on neither the ammonia and the Cd-salt concentrations nor the thickness of CdS, whereas it changed from 14.72% to 15.81% as the thiourea concentration decreases from 3.0 to 1.0 M.

Light-soaking effects and capacitance profiling in Cu(In,Ga)Se2 thin-film solar cells with chemical-bath-deposited ZnS buffer layers

We fabricated Cu(In,Ga)Se2 (CIGS) solar cells with chemical-bath deposited (CBD) ZnS buffer layers with different deposition times. The conversion efficiency and the fill factor of the CIGS solar cells reveal a strong dependence on the deposition time of CBD-ZnS films. In order to understand the detailed relationship between the heterojunction structure and the electronic properties of CIGS solar cells with different deposition times of CBD-ZnS films, capacitance-voltage (C-V) profiling measurements with additional laser illumination were performed. The light-soaking effects on CIGS solar cells with a CBD-ZnS buffer layer were investigated in detail using current density-voltage (J-V) and C-V measurements with several different lasers with different emission wavelengths. After light-soaking, the conversion efficiency changed significantly and the double diode feature in J-V curves disappeared. We explain that the major reason for the improvement of efficiency by light-soaking is due to the fact that negatively charged and highly defective vacancies in the CIGS absorber near the interface of CBD-ZnS/CIGS were formed and became neutral due to carriers generated by ultra-violet absorption in the buffer layer.

ZnS buffer layer prepared by sulfurization of sputtered Zn film for Cu(In, Ga)Se 2 solar cells

We report on a novel method of a fabrication of a ZnS buffer layer for an application of Cu(In, Ga)Se2 (CIGS) solar cells. The ZnS thin film was prepared by a sulfurization of a sputtered Zn film using a sulfur cracker. The structural and optical properties of the ZnS films and the photovoltaic performance of the ZnS-employed CIGS solar cells were investigated. The thin ZnS film played a sufficient role as a buffer layer achieving the power conversion efficiency of 10.5%.