Solar energy has been recognized as an alternative energy source that can help address fuel depletion and climate change issues. As a renewable energy alternative to fossil fuels, it is an eco-friendly and unlimited energy source. Among solar cells, thin film Cu2ZnSn(S,Se)4 (CZTSSe) is currently being actively studied as an alternative to heavily commercialized Cu (In,Ga)Se2 (CIGS) thin film solar cells, which rely upon costly and scarce indium and gallium. Currently, the highest efficiency achieved by CZTSSe cells is 14.9 %, lower than the CIGS record of 23.35 %. When applied to devices, CZTSSe thin films perform poorly compared to other materials due to problems including lattice defects, conduction band offset, secondary phase information, and narrow stable phase regions, so improving their performance is essential. Research into ways of improving performance by doping with Germanium and Cadmium is underway. Specifically, Ge can be doped into CZTSSe, replacing Sn to reduce pinholes and bulk recombination. Additionally, partially replacing Zn with Cd can facilitate grain growth and suppress secondary phase formation. In this study, we analyzed the device’s performance after doping Ge into CZTSSe thin film using evaporation, and doping Cd using chemical bath deposition. The Ge doped thin film showed a larger bandgap than the undoped reference thin film, achieving the highest Voc of 494 mV in the device. The Cd doped thin film showed a smaller bandgap than the undoped reference thin film, with the highest Jsc of 36.9 mA/cm2. As a result, the thin film solar cells achieved a power conversion efficiency of 10.84 %, representing a 20 % improvement in power conversion efficiency compared to the undoped reference device.

Cu2ZnSn(S,Se)4 (CZTSSe) based thin-film solar cells have attracted growing attention because of their earthabundant and non-toxic elements. However, because of their large open-circuit voltage (Voc)-deficit, CZTSSe solar cells exhibit poor device performance compared to well-established Cu(In,Ga)(S,Se)2 (CIGS) and CdTe based solar cells. One of the main causes of this large Voc-deficit is poor absorber properties for example, high band tailing properties, defects, secondary phases, carrier recombination, etc. In particular, the fabrication of absorbers using physical methods results in poor surface morphology, such as pin-holes and voids. To overcome this problem and form large and homogeneous CZTSSe grains, CZTSSe based absorber layers are prepared by a sputtering technique with different RTA conditions. The temperature is varied from 510 oC to 540 oC during the rapid thermal annealing (RTA) process. Further, CZTSSe thin films are examined with X-ray diffraction, X-ray fluorescence, Raman spectroscopy, IPCE, Energy dispersive spectroscopy and Scanning electron microscopy techniques. The present work shows that Cu-based secondary phase formation can be suppressed in the CZTSSe absorber layer at an optimum RTA condition.

Recent advances in technology using ultra-thin noble metal film in oxide/metal/oxide structures have attracted attention because this material is a promising alternative to meet the needs of transparent conduction electrodes (TCE). AZO/ Ag/AZO multilayer films are prepared by magnetron sputtering for Cu2ZnSn(S,Se)4 (CZTSSe) of kesterite solar cells. It is shown that the electrical and optical properties of the AZO/Ag/AZO multilayer films can be improved by the very low resistivity and surface plasmon effects due to the deposition of different thicknesses of Ag layer between oxide layers fixed at AZO 30 nm. The AZO/Ag/AZO multilayer films of Ag 15 nm show high mobility of 26.4 cm2/Vs and low resistivity and sheet resistance of 3.58*10−5 Ωcm and 5.0 Ω/sq. Also, the AZO/Ag (15 nm)/AZO multilayer film shows relatively high transmittance of more than 65% in the visible region. Through this, we fabricated CZTSSe thin film solar cells with 7.51% efficiency by improving the short-circuit current density and fill factor to 27.7 mV/cm2 and 62 %, respectively.

Cu2ZnSn(S,Se)4(CZTSSe) thin film solar cells areone of the most promising candidates for photovoltaic devices due to their earth-abundant composition, high absorption coefficient and appropriate band gap. The sputtering process is the main challenge to achieving high efficiency of CZTSSe solar cells for industrialization. In this study, we fabricated CZTSSe absorbers on Mo coated soda lime glass using different pressures during the annealing process. As an environmental strategy, the annealing process is performed with S and Se powder, without any toxic H2Se and/or H2S gases. Because CZTSSe thin films have a very narrow stable phase region, it is important to control the condition of the annealing process to achieve high efficiency of the solar cell. To identify the effect of process pressure during the sulfo-selenization, we experiment with varying initial pressure from 600 Torr to 800 Torr. We fabricate a CZTSSe thin film solar cell with 8.24 % efficiency, with 435 mV for open circuit voltage(VOC) and 36.98 mA/cm2 for short circuit current density(JSC), under a highest process pressure of 800 Torr.

Cu2ZnSn(S,Se)4 (CZTSSe) films were prepared on Mo coated soda lime glass substrates by sulfo-selenization of sputtered stacked Zn-Sn-Cu(CZT) precursor films. The precursor was dried in a capped state with aqueous NaOH solution. The CZT precursor films were sulfo-selenized in the S + Se vapor atmosphere. Sodium was doped during the sulfo-selenization treatment. The effect of sodium doping on the structural and electrical properties of the CZTSSe thin films were studied using FE-SEM(field-emission scanning electron microscopy), XRD(X-ray diffraction), XRF(X-ray fluorescence spectroscopy), dark current, SIMS(secondary ion mass spectrometry), conversion efficiency. The XRD, XRF, FE-SEM, Dark current, SIMS and cell efficiency results indicated that the properties of sulfo-selenized CZTSSe thin films were strongly related to the sodium doping. Further detailed analysis and discussion for effect of sodium doping on the properties CZTSSe thin films will be discussed.

목 적: Ⅱ-Ⅲ2-Ⅵ4형 반도체 가운데 고용체 화합물에 대한 연구의 하나로 Ⅵ족인 S, Se를 상호 교환하여 성장시킨 ZnAl2Se3.6S0.4 고용체의 구조 및 광 발광 메카니즘을 규명하였고, 이로부터 광학적 energy band gap의 온도의존성과 기초적 열역학 함수를 추정고자 한다. 방 법: ZnAl2Se3.6S0.4 단결정은 수송매체로서 iodine을 이용한 화학수송법(CTR)으로 단결정을 성장시켰 다. 단결정을 성장시키기 위하여 시료 출발측을 950 ℃, 성장측을 850 ℃로 하여 7일간 성장시켰다. 기초 흡 수단 부근에서 에너지 띠 간격의 온도의존성을 구하기 위하여 저온장치가 부착된 UV-VIS-NIR spectrophotometer를 사용하여 광흡수 스펙트럼을 측정하였다. 광발광 특성은 광흡수 특성 측정에 사용하 였던 측정용 시편을 cryogenic system 의 cold finger 에 부착시키고, 여기 광원으로 325 ㎚의 He-Cd laser 를 사용하였으며, double-grating monochromator, data-mate control system, cryogenic system, PM tube 등으로 구성된 측정 system을 사용하여 측정하였다. 결과 및 고찰: ZnAl2Se3.6S0.4 단결정의 에너지 띠 간격과 온도의존성을 구하기 위하여 이들 단결정의 기 초 흡수단 영역인 320 ～ 420㎚ 파장영역과 13K ～ 289K 온도영역에서 광흡수 스펙트럼을 측정하였다. Varshni가 제안한 에너지 띠 간격의 온도의존성 특성에 대한 실험식에 잘 일치하였다. 측정된 energy gap(Eg)으로부터 열역학적 함수 물리량을 추정할 수 있었다. 또한 13K에서 측정한 광발광 스펙트럼에서 보 면 427㎚(2.904 )영역에서 비교적 넓고 세기가 강한 청색 발광 피크와 468㎚(2.648 )영역에서 세기가 약한 청색 발광 피크를 관측할 수 있었다. 결 론: ZnAl2Se3.6S0.4 단결정의 결정구조는 defect chacopyrite 구조였으며, 격자 상수는 a= 5.5563Å, c= 10.8324Å이었다. 또한 찌그러짐 인자값은 0.0504 이었다. 광학적 에너지 띠 간격의 온도 의존성을 규명하 였고, 이 때 Eg(0)=3.538(eV), α=2.02×10-3(eV/K), β=502.19(K)로 주어졌다. Energy band gap의 온도의 존성으로부터 entropy(SCV), heat capacity(CCV), enthalpy(HCV) 값을 추정 할 수 있었다. ZnAl2Se3.6S0.4 결 정의 광발광 특성은 비교적 넓고 세기가 강한 청색 발광 피크와 세기가 약한 청색 발광 피크를 관측할 수 있었 다. 이들 발광 전이기구는 두개의 주개 준위(SD1, DD1)와 한 개의 받개 준위(DA1)사이의 재결합에 의한 발광 메카니즘으로 설명된다.

A stoichiometric mixture of evaporating materials for ZnAl2Se4 single-crystal thin films was prepared in a horizontalelectric furnace. These ZnAl2Se4 polycrystals had a defect chalcopyrite structure, and its lattice constants were a0=5.5563Åand c0=10.8897Å.To obtain a single-crystal thin film, mixed ZnAl2Se4 crystal was deposited on the thoroughly etched semi-insulating GaAs(100) substrate by a hot wall epitaxy (HWE) system. The source and the substrate temperatures were 620oCand 400oC, respectively. The crystalline structure of the single-crystal thin film was investigated by using a double crystal X-ray rocking curve and X-ray diffraction ω-2θ scans. The carrier density and mobility of the ZnAl2Se4 single-crystal thin filmwere 8.23×1016cm−3 and 287m2/vs at 293K, respectively. To identify the band gap energy, the optical absorption spectra ofthe ZnAl2Se4 single-crystal thin film was investigated in the temperature region of 10-293K. The temperature dependence ofthe direct optical energy gap is well presented by Varshni's relation: Eg(T)=Eg(0)−(αT2/T+β). The constants of Varshni'sequation had the values of Eg(0)=3.5269eV, α=2.03×10−3eV/K and β=501.9K for the ZnAl2Se4 single-crystal thin film.The crystal field and the spin-orbit splitting energies for the valence band of the ZnAl2Se4 were estimated to be 109.5meVand 124.6meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicatethat splitting of the ∆so definitely exists in the Γ5 states of the valence band of the ZnAl2Se4/GaAs epilayer. The threephotocurrent peaks observed at 10K are ascribed to the A1-, B1-exciton for n=1 and C21-exciton peaks for n=21.

Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) thin films were prepared by sulfurization of evaporated precursor thin films. Precursor was prepared using evaporation method at room temperature. The sulfurization was carried out in a graphite box with S powder at different temperatures. The temperatures were varied in a four step process from 520˚C to 580˚C. The effects of the sulfurization temperature on the micro-structural, morphological, and compositional properties of the CZTSSe thin films were investigated using X-ray diffraction (XRD), Raman spectra, field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The XRD and Raman results showed that the sulfurized thin films had a single kesterite crystal CZTSSe. From the FE-SEM and TEM results, the Mo(Sx,Se1-x)2 (MoSSe) interfacial layers of the sulfurized CZTS thin films were observed and their thickness was seen to increase with increasing sulfurization temperature. The microstructures of the CZTSSe thin films were strongly related to the sulfurization temperatures. The voids in the CZTSSe thin films increased with the increasing sulfurization temperature.