Chan-Wook Jeon

A band-gap-graded CZTSSe solar cell with 12.3% efficiency

Although Cu2ZnSn(S,Se)4 (CZTSSe) has attracted attention as an alternative to CuInGaSe2 (CIGS) as an absorber material in solar cells, its low efficiency is a serious shortcoming preventing its commercialization. To realize a high-efficiency CZTSSe solar cell, improved grain crystallinity, inhibited secondary-phase formation, controlled defect generation, adequate Na content, and band gap grading are required in the absorber layer. Few studies have focused specifically on band gap grading. In this study, a method of using SeS2, a new potential chalcogenization source material, to control the S and Se contents in a CZTSSe absorber and its effects were investigated. Using an appropriate SeS2/Se weight ratio, band gap grading was realized within the depletion region. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V was achieved. Furthermore, the possibility of enhancing JSC through the formation of a type-inverted n-type phase at the absorber surface in response to an appropriate alignment of the conduction-band minimum energy level and the Fermi energy pinning level is discussed. By introducing the chalcogenization source material SeS2 during the annealing process, CZTSSe solar cells with a maximum efficiency of 12.3% were obtained.

Improvement of Electro-Optic Properties in PDLC Device by Using New Cross-Linker for the Control of the Contrast Ratio, Response Time and Driving Voltage

We developed new cross-liker for the PDLC application that includes the bisphenol A and tris(hydroxyphenyl)ethane moieties in the structure with bis- or tris-acrylate at the end of molecules to induce photo-polymerization. Although the cross-linker was added as small as 2% ∼ 10% (more ratio) to compare with monomer, the electro-optic properties of PDLCs were found to exhibit significant change with material selection. We found that the more cross-linker added, the droplet size of LC became small which affected the properties of PDLC devices such as response time and driving voltage, directly. We found that tri-functionalized cross-linker synthesized from tris(hydroxyphenyl)ethane moiety could enhance the electro-optic property in PDLC. Also, in the PDLCs containing smaller size of liquid crystal droplets, although the response time and driving voltage were increased, the contrast ratio was enhanced, significantly.

Influence of Cr thin films on the properties of flexible CIGS solar cells on steel substrates

The material properties of molybdenum (Mo) and Mo/chromium (Cr) combined layers as back electrodes were compared with the electrical performance of copper indium gallium diselenide (CIGS)-based solar cells fabricated on steel foil. The addition of a Cr thin layer between the Mo films and stainless steel (STS) substrates increased the columnar grain size of Mo. These results could lead to enhancements in the solar cell efficiency for CIGS films evaporated on Mo/Cr, although the root mean square roughness increased from 187 to 251 nm compared with that of the Mo/STS sample.

Comparison of Aluminum Zinc Oxide and Indium Tin Oxide for Transparent Conductive Oxide layer in Cu(In,Ga)Se2Solar Cell

The photovoltaic performances of i-ZnO/CdS/Cu(In,Ga)Se2(CIGS) solar cell with different window architectures of SnO2:In2O3(ITO), Al2O3:ZnO (AZO) and ITO/AZO, were experimentally compared. The solar cell with ITO deposited directly on i-ZnO layer showed an abnormal current-voltage characteristic as having a shunt path. Both of AZO and ITO/AZO resulted in normal current-voltage behavior as far as AZO is contacting ZnO.

Control of Ga Distribution in Cu(In,Ga)Se2 Photovoltaic Absorber by Solid-State Selenizatoin of CuGa/In/Se/In/CuGa Stack

Among many key parameters required to obtain a record-efficiency Cu(In,Ga)Se2 (CIGS) cell, the band-gap of CIGS should have a double-graded profile in which the band-gap increases toward both of the back and front of the absorber. In an effort to obtain an increased Ga content near the junction area which will raise the band-gap energy of CIGS, a novel metal precursor layered with predetermined amount of Se was annealed in N2 ambient. By inserting the Se layer in between metallic precursor layers, it was found that the front band-gap was increased due to the high Ga content by changing the direction of selenization reaction from inside to outside of metallic precursor. The proposed method is expected to provide a simple process for high quality CIGS photovoltaic absorber layer. The conversion efficiency of 6.80% with Jsc = 37.65 mA/cm2, Voc = 0.51 V, and FF= 35.4% in an active area of 0.48 cm2 was achieved.

Electrical and Optical Properties of Cadmium Stannate Deposited by RF Magnetron Sputtering

CTO (Cadmium Stannate) thin films, one of TCOs (Transparent Conducting Oxide) having a potential application for photovoltaic or display product, were prepared by using rf magnetron sputtering system. The lowest resistivity of CTO thin film deposited at room temperature was 6.6 × 10−4 ohm · cm with carrier mobility of 9.1 cm2/Vs and carrier concentration of 10.4 × 1020 cm−3. The average transmittance of CTO thin film was found to be over 80% regardless of deposition condition. The transmittance of the annealed CTO thin film at 600°C in air atmosphere, was found to increase upto more than 90%, but the film resistivity degraded by two order of magnitude due to the decreased carrier concentration with minor increase of carrier mobility.

Effect of Alignment Layer on Electro-Optic Properties of Polymer-Dispersed Liquid Crystal Displays

The electro-optic properties of polymer-dispersed liquid crystal (PDLC) cell having parallel alignment layers were examined and compared to the control cell without any pre-treatment. Although the liquid crystal droplets of the treated PDLC was observed to be much smaller than that of the control cell, the contrast ratio of both cells are quite similar over a wide temperature range. The control cell performed better in terms of response time and operating voltage. The retardation characteristics of the cells showed sharp contrast, which may bring about inconsistent consequence in determining their electro-optic properties.

Dependence of Cu(In,Ga)Se2 Solar Cell Performance on Cd Solution Treatment Conditions

In the current study, chemical bath deposition (CBD) was used to grow CdS thin films on a Cu(In,Ga)Se2 (CIGS) absorption layer, in order to examine the effects of CdS deposition conditions on the properties of CIGS solar cell devices. The dip time leading up to the start of CdS synthesis is thought to be an important process variable determining the concentration of Cd ions diffused into the CIGS as well as the condition of the CIGS surface. Accordingly, the behavior of the CIGS solar cell efficiency variation was observed while different dip times were applied, at 4, 15 and 30 minutes, respectively. When the dip time was extended, the series resistance (Rs) of the device fell by a substantial margin, leading to improved photoelectric conversion efficiency and enhanced uniformity in device properties. This can be attributed to the effect of CIGS surface cleaning by the NH4OH contained in the reaction solution.

A Study on Cu(In,Ga)Se2 Thin-Film Characteristics During Three-Stage Process Using Real-Time Substrate Monitoring

In this study, to analyze the effect of the second stage (Cu–Se) deposition time on CIGS property, the experiment of controlling the time of the second stage was conducted. Especially, very low values were found for device properties when it was 3300s and 3500s in the second stage. It was observed that excess Cu2–xSe diffused excessively on the surface of the thin film, causing a lot of pores and voids on the surface. With the same evaporation ratio for each source of Cu, In, Ga, and Se, it is very important to apply the 2700s process time for the second stage in order to produce high-efficiency device. The efficiency property of device was checked through solar-simulator (AM1.5G, 100 mW/cm2 at 25°C). In case the process time of the second stage was 2700s, efficiency was as following: 10.1%, Voc: 0.53V, Jsc: 33.5 mA/cm3, fill-factor: 57.2%. The analysis results obtained in this study are expected to be useful for various domestic and overseas research which aim to make high-efficiency solar cells based on proper variation of the second-stage processing time.

XPS and Raman study of slope-polished Cu(In,Ga)Se2 thin films

The growth of quality Cu(In,Ga)Se2 photovoltaic absorber without secondary phases is very important for improving the solar cell efficiency. Although X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy can identify the secondary phases, they provide insufficient information because of their insufficient resolution and complexity in analysis. In general, normal Raman spectroscopy is better for the analysis of secondary phases. On the other hand, the Raman signal provides information for film depths of less than 300 nm, and the Raman information cannot represent the properties of the entire film. In this regard, the authors introduce a new way of identifying secondary phases in Cu(In,Ga)Se2 films using depth Raman analysis. The as-prepared film was polished using a dimple grinder, which expanded a 2 μm thick film to approximately 1 mm, which is more than sufficient to resolve the depth distribution. Raman analysis indicated that the Cu(In,Ga)Se2 film showed different secondary phases, such as CuIn3Se5, InSe and CuSe, present in different depths of the film, whereas XPS provided complex information about the phases. Overall, the present study emphasizes that the Raman depth profile is more efficient for the identification of secondary phases in Cu(In,Ga)Se2 thin films than XPS and XRD.