Junggyu Nam

Influence of surface properties on the performance of Cu(In,Ga)(Se,S)2 thin-film solar cells using Kelvin probe force microscopy

We have investigated the sulfurization process in a Cu(In,Ga)(Se,S)2 (CIGSS) absorber layer fabricated by a two-step sputter and selenization/sulfurization method in order to make an ideal double-graded band-gap profile and increase the open circuit voltage (Voc). The sulfurization process was controlled by temperature from 570 °C to 590 °C without changing H2S gas concentration and reaction time. Although the energy band-gap of the CIGSS absorber layer was increased with increasing sulfurization temperature, the Voc of the completed CIGSS device fabricated at 590 °C sulfurization temperature did not increase. In order to investigate this abnormal Voc behavior, the CIGSS absorber layer was measured by local electrical characterization utilizing Kelvin probe force microscopy, especially in terms of grain boundary potential and surface work function. Consequently, the abnormal Voc behavior was attributed to the degradation of grain boundary passivation by the strong sulfurization process. The optimum sulfurization temperature plays an important role in enhancement of grain boundary passivation. It was also verified that the Voc degradation in the CIGSS solar cell fabricated by the two-step method is more influenced by the grain boundary passivation quality in comparison with the slight non-uniformity of material composition among grains.

Direct evidence of void passivation in Cu(InGa)(SSe)2 absorber layers

We have investigated the charge collection condition around voids in copper indium gallium sulfur selenide (CIGSSe) solar cells fabricated by sputter and a sequential process of selenization/sulfurization. In this study, we found direct evidence of void passivation by using the junction electron beam induced current method, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The high sulfur concentration at the void surface plays an important role in the performance enhancement of the device. The recombination around voids is effectively suppressed by field-assisted void passivation. Hence, the generated carriers are easily collected by the electrodes. Therefore, when the S/(S + Se) ratio at the void surface is over 8% at room temperature, the device performance degradation caused by the recombination at the voids is negligible at the CIGSSe layer.

Defect visualization of Cu(InGa)(SeS)2 thin films using DLTS measurement

Defect depth profiles of Cu (In1−x,Gax)(Se1−ySy)2 (CIGSS) were measured as functions of pulse width and voltage via deep-level transient spectroscopy (DLTS). Four defects were observed, i.e., electron traps of ~0.2 eV at 140 K (E1 trap) and 0.47 eV at 300 K (E2 trap) and hole traps of ~0.1 eV at 100 K (H1 trap) and ~0.4 eV at 250 K (H2 trap). The open circuit voltage (VOC) deteriorated when the trap densities of E2 were increased. The energy band diagrams of CIGSS were also obtained using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and DLTS data. These results showed that the valence band was lowered at higher S content. In addition, it was found that the E2 defect influenced the VOC and could be interpreted as an extended defect. Defect depth profile images provided clear insight into the identification of defect state and density as a function of depth around the space charge region.

Effect of various encapsulants for frameless glass to glass Cu(In,Ga)(Se,S)2 photovoltaic module

Cu(In,Ga)(Se,S)2 (CIGSS) modules with a frameless glass to glass (G2G) structure were successfully fabricated by replacing the currently used ethylene vinyl acetate (EVA) with new encapsulants such as polyolefins (POE), ionomer (IO) and liquid silicon (LSI), and their characteristics including optical, mechanical, and reliability properties were investigated. Most of the solid films for encapsulation except POE exhibit similar light transmittance in the visible-infrared wavelength range, while LSI shows the highest light transmittance with a difference of more than 2% in the overall wavelength range. The G2G structure encapsulated with IO exhibits superior values for the adhesion strength at which interface failure occurs (about 7.5 MPa) compared to other encapsulants. Water vapor transmittance ratio (WVTR) values of the encapsulants were evaluated to determine their suitability as water-resistant materials in terms of the CIGSS G2G structure. The IO encapsulant had the lowest WVTR value of about 1.75 g per m2 per day. Changes of cell-to-module (CTM) conversion ratio in the CIGSS G2G structure using various encapsulants were investigated using I–V measurements. The CIGSS G2G structure encapsulated by LSI shows the best CTM conversion ratio, since the module process occurs at the lowest temperature as compared with other encapsulants, therefore the module has a smaller thermal transient effect. Finally, the long-term reliability of the CIGSS modules based on various encapsulants was also evaluated by damp heat testing.

Enhancement of the photo conversion efficiencies in Cu(In,Ga)(Se,S)2 solar cells fabricated by two-step sulfurization process

Cu(In,Ga)(Se,S)2 (CIGSS) absorber layers were fabricated by using a modified two-stage sputter and a sequential selenization/sulfurization method, and the sulfurization process is changed from one-step to two-step. The two-step sulfurization was controlled with two different H2S gas concentrations during the sulfurization treatment. This two-step process yielded remarkable improvements in the efficiency (+0.7%), open circuit voltage (+14 mV), short circuit current (+0.23 mA/cm2), and fill factor (+0.21%) of a CIGSS device with 30 × 30 cm2 in size, owing to the good passivation at the grain boundary surface, uniform material composition among the grain boundaries, and modified depth profile of Ga and S. The deterioration of the P/N junction quality was prevented by the optimized S content in the CIGSS absorber layer. The effects of the passivation quality at the grain boundary surface, the material uniformity, the compositional depth profiles, the microstructure, and the electrical characteristics were exa...

Direct band gap measurement of Cu(In,Ga)(Se,S)2 thin films using high-resolution reflection electron energy loss spectroscopy

To investigate the band gap profile of Cu(In1−x,Gax)(Se1−ySy)2 of various compositions, we measured the band gap profile directly as a function of in-depth using high-resolution reflection energy loss spectroscopy (HR-REELS), which was compared with the band gap profile calculated based on the auger depth profile. The band gap profile is a double-graded band gap as a function of in-depth. The calculated band gap obtained from the auger depth profile seems to be larger than that by HR-REELS. Calculated band gaps are to measure the average band gap of the spatially different varying compositions with respect to considering its void fraction. But, the results obtained using HR-REELS are to be affected by the low band gap (i.e., out of void) rather than large one (i.e., near void). Our findings suggest an analytical method to directly determine the band gap profile as function of in-depth.

Correction to: Optimization of Controllable Factors in the Aluminum Silicon Eutectic Paste and Rear Silicon Nitride Mono-Passivation Layer of PERC Solar Cells

The original version of this article unfortunately contained a mistake. The acknowledgements section was incomplete. The correct information is given below.

Effects of Laser Doping on Selective Emitter Si Solar Cells

Laser-doped selective emitter process requires dopant source deposition, spin-on-glass, and is able to form selective emitter through SiNx layer by laser irradiation on desired locations. However, after laser doping process, the remaining dopant layer needs to be washed out. Laser-induced melting of pre-deposited impurity doping is a precise selective doping method minimizing addition of process steps. In this study, we introduce a novel scheme for fabricating highly efficient selective emitter solar cell by laser doping. During this process, laser induced damage induces front contact destabilization due to the hindrance of silver nucleation even though laser doping has a potential of commercialization with simple process concept. When the laser induced damage is effectively removed using solution etch back process, the disadvantage of laser doping was effectively removed. The devices fabricated using laser doping scheme power conversion efficiency was significantly improved about 1% abs. after removal the laser damages.