Dong Hyeop Shin

Study of Band Structure at the Zn(S,O,OH)/Cu(In,Ga)Se2 Interface via Rapid Thermal Annealing and Their Effect on the Photovoltaic Properties

This study focused on understanding the mechanisms of the photovoltaic property changes in Zn(S,O,OH)/Cu(In,Ga)Se2 solar cells, which were fabricated via annealing, using reflection electron energy loss spectroscopy (REELS), ultraviolet photoelectron spectroscopy (UPS), low temperature photoluminescence (LTPL), and secondary ion mass spectroscopy (SIMS). A pinhole-free Zn(S,O,OH) buffer layer was grown on a CIGS absorber layer using the chemical bath deposition (CBD). When the Zn(S,O,OH) film was annealed until 200 °C, the Zn-OH bonds in the film decreased. The band gap value of the annealed film decreased and the valence band offset (VBO) value at the Zn(S,O,OH)/CIGS interface with the annealed film increased. Both results contribute to the conduction band offset (CBO) value at the Zn(S,O,OH)/CIGS interface and, in turn, yield a reduction in the energy barrier at the interface. As a result of the annealing, the short circuit current (JSC) and quantum efficiency (QE) values (400-600 nm) of the cell increased due to the improvement in the electron injection efficiency. However, when the Zn(S,O,OH) film was annealed at 300 °C, the cell efficiency declined sharply due to the QE loss in the long wavelength region (800-1100 nm). The SIMS analysis demonstrated that the Cu content in the CIGS bulk decreased and the Cu element also diffused into CIGS/Mo interface. Through LTPL analysis, it was seen that the considerable drop of the Cu content in the CIGS bulk induced a 1.15 eV PL peak, which was associated with the transition from a deep donor defect to degrade the quality of the CIGS bulk. Accordingly, the series resistance (RS) and efficiency of the cell increased.

Growth of Ultrathin Zn Compound Buffer Layer by a Chemical Bath Deposition for Cu ( In , Ga ) Se2 Solar Cells

A Zn compound buffer layer for Cu(In,Ga)Se 2 (CIGS) solar cells was grown from an alkaline aqueous solution using chemical bath deposition (CBD). To improve the film quality and exclude the cracks in the film, processing parameters such as reagent concentration, deposition time, and temperature profile were varied. Under the optimized CBD process, a uniform and crack-free film was grown on a CIGS substrate with thicknesses ranging from 10 to 60 nm. The controllable thickness of the film was as low as 10 nm. X-ray diffraction and Auger analysis showed that the Zn compound film was in an amorphous state with the ZnS x (OH) y O z composition. A 26% increase in the optical transmittance in the spectral range of 380―600 nm, as compared to a standard CdS buffer layer, was achieved. Finally, by optimization of the CBD process, we formed buffer layers, which enabled the transmission of the short wavelength of the solar spectrum for CIGS absorption.

Development of High-Efficiency Cd-Free Cu(In,Ga)Se2 Solar Cells Using Chemically Deposited ZnS Film

A ZnS film covered completely the CIGS surface without pinholes. The thickness of the ZnS film could be controlled according to the cycle number of the CBD process. As the thickness of the ZnS film increased, the JSC value of a CIGS solar cell with the ZnS film decreased sharply due to the increase in the series resistance of the cell. The QE spectrum of the CIGS solar cell with a 50 nm-thick ZnS film consequently showed very small values through the whole wavelength range. On the other hand, the VOC and FF values of the CIGS solar cell with a 27 nm-thick ZnS film were significantly influenced by the sputtering power for deposition of the ZnO:Al film. By decreasing the sputtering power during deposition of the ZnO:Al film, the VOC and FF values of the CIGS solar cells with a 27 nm-thick ZnS film were sharply improved due to a reduction of sputtering damage at the ZnS surface and ZnS/CIGS interface. Additionally, the JSC value of the CIGS solar cell with a 27 nm-thick ZnS film was improved by annealing the ZnS film. As a result of an annealing process, the QE value in a wavelength range of 400–700 nm increased. Finally, we achieved a conversion efficiency of 14.21 % for a CIGS solar cell with a ZnS film by optimizing the fabrication conditions.

Sn(O, S) 2 thin films by chemical bath deposition for Cd-free CIGS thin film solar cells

The buffer layers of tin compound semiconductor were deposited on the Cu(In, Ga)Se2 (CIGS) substrates by chemical bath deposition. Sn(O, S)2 films were grown in an alkaline ammonia solution by reaction of tin(IV) chloride with thiourea at the bath temperature of 70oC. The smooth and conformal coverage films were achieved on the CIGS substrates with thickness of 20nm. The morphological and chemical properties were evaluated using scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). On the other hand, layer thickness of 90nm could be grown in an acidic solution with sodium sulfide (Na2S) within 60min. When we applied the Sn compound thin films as the alternative buffers in thin film CIGS solar cells, the efficiency of 9.4% small area cells were achieved. This result provided the potential for the Sn(O, S)2 films to be used as buffer or window layers in solar cell devices.