Research On Cu(in,Ga)Se₂Thin-film Solar Cell

Solar power is one of the effective ways to provide clean and renewable energy. Cu(In,Ga)Se2(CIGSe) thin-film solar cell belongs to the second generation of solar cells. Not only because of the high light absorption coefficient of CIGSe, but also the high efficiency, the stable performance, good low-light performance and strong corrosion resistance of CIGSe solar cell, it is expected to become a new generation of widespread use of solar cell materials. At present, the world record efficiency of CIGSe thin film solar cell device has reached20.8%. However, due to the complex preparation process, high equipment cost and low raw material utilization rate, the large-scale industrialization is hindered. How to improve the efficiency of CIGSe solar cell and reduce the overall cost become two major purposes of the research in this field.This dissertation mainly composed of three parts. In the first part, we research on the prepara-tion of highly efficient CIGSe thin-film solar cell. By using "3-stage" method, high quality CIGSe film is prepared as the absorption layer. The preparation of other thin film layers which constitute the solar cell are studied and optimized. Then the physical properties of solar cell are measured and studied by variety of characterization methods. On the basis of the results, the preparation of solar cell is optimized. In the second part, we research on the thermal annealing effect of Mo film. The effect of annealed Mo film on CuInSe2(CISe) film and the solar cell are also studied. In the third part, we research on the low cost preparation of solar cell by non-vacuum method. The Cu(In,Ga)S2(CIGS) nano-particles are prepared by "hot-inject" method. Then a CIGS nano-ink is formed by dissolving and dispersing the nano-particles. After that a CIGS precursor film is prepared by "doctor-blade" method and selenized to form a Cu(In,Ga)(S,Se)2(CIGSSe) film. The physical properties of this CIGSSe film are measured. The CIGSSe solar cell is prepared by using the CIGSSe film as the absorption layer.The main contributions of this dissertation include:(1) Using the system of vacuum equipments which are designed and developed independently, CIGSe thin-film solar cell is fabricated with the efficiency of17.67%. The high quality CIGSe polycrystalline films are prepared by the "3-stage" co-evaporation process. X-ray diffractometer (XRD) patterns show the films are single phase of CIGSe. Com-bined with energy dispersive X-ray spectrometer (EDX) analysis, the components and phases of CIGSe polycrystalline films which are consistent with the experimental design. Microstructure and morphology of the films are chararterized by scanning electron microscope (SEM), showing that the grain size of CIGSe is greater than1?m. The bottom electrode of Mo films, buffer layer of CdS films and transparent conductive electrode of ZnO(Al2O3) films are prepared and optimized. The double-layer Mo flims have good adhesion and electrical conductivity, with the square resistance of0.2305?/sq. The thickness of CdS films are50nm. The ZnO(Al2O3) films have good trans-parency and electrical conductivity, with the resistivity of7.2x10-4?·cm. The CIGSe thin-film solar cells are prepared. The structure of devices is SLG/Mo/CdS/i-ZnO/ZnO(Al2O3)/Ni-Al-Ni.The highest efficiency of CIGSe thin-film solar cell small device (0.55cm2) is17.67%, with the open-circuit voltage (Voc) of697mV, the short-circuit current density (Jsc) of33.8mA/cm2and the fill factor (FF) of75%. The quality of this cell is among the first-class level in China.(2) The thermal annealing effect of Mo film is studied. The efficiency of thin-film solar cell is improved by using the annealed Mo film as the bottom electrode.The thermal annealing effect of double-layer Mo film is studied. The annealed Mo film has better adhesion with SLG and lower square resistance of0.1487Q/sq. To minimize the uncertainty in experiment, we start with CISe without adding Ga to eliminate alloy fluctuation. Not only CISe film but also the In2Se3(ISe) precursor are studied. The results show that both the crystal orien-tation of ISe and CISe film are changed when they are deposited on annealed Mo film. Compared with the (112)-orientation,(220/204)-orientation has been enhanced. By using the annealing Mo film as the bottom electrode, the efficiency of CISe thin-film solar cell is improved from10.54%to11.47%. The main reason of this improvement is the increase of FF from70.09%to75.81%. Then this method is introduced into the CIGSe thin-film solar cell. By using the annealing Mo film as the bottom electrode, the efficiency is also improved, from13.78%to14.29%. In the case of single-layer Mo film, the adhesion and electrical conductivity of Mo film are also improved, with the square resistance decreasing from0.470?/sq to0.147?/sq. By using the annealing single-layer Mo film as the bottom electrode, the efficiency of CISe thin-film solar cell is improved, from3.45%to9.06%.A series of experimental results show that the efficiency of thin-film solar cell is improved by using the annealed Mo film as the bottom electrode.(3) The non-vacuum process to prepare CIGSSe thin-film solar cells is studied. Photovoltaic effect is observed by the fabricated CIGSSe thin-film solar cell.The CIGS nano-particles are prepared by "hot-inject" method. XRD patterns show the nano-particles are single phase of CIGS. Combined with EDX analysis results, show that the components and phases are CIGS which are comply with the experimental design. The size of CIGS nano-particles are10-30nm characterized by transmission electron microscopy (TEM). With the UV-VIS measurement of CIGS nano-particles, the optical band gap of1.3eV is calculated. After dissolved and dispersed by hexanethiol, the CIGS nano-ink is prepared. Then the CIGS precursor film is formed by "doctor-blade" method. We studied the selenation process of CIGS precursor film. It shows that the optimized annealing time is10minutes, the temperature of Se source and sample are300?and520?, respectively. XRD and EDX results of selenized film show that single phase CIGSSe films are prepared. By using the CIGSSe film as absorption layer, a small solar cell is prepared, with the efficiency of0.92%. After redesigning and optimizing the preparation process of precursor films, that is using auxiliary mechanism of CIGSSe grain growth by Cu-rich phase, the more smooth CIGSSe films with larger grain size and better continuity are prepared. The efficiency of CIGSSe thin-film solar is also improved to1.3%.The Cu2ZnSnS4(CZTS) nano-particles are also prepared by "hot-inject" method. The phase and constituent are characterized by XRD and EDX. Combined with Raman spectrum (main peak at336cm-1) shows the product is CZTS nano-particles.Finally, this paper also briefly reports several other researches during my Ph. D. period. The system of "1111" iron-based superconductor is studied, including:(1) the suppression of supercon-ductivity by Zn-doping in LaFe0.925-yCo0.075ZnyAsO,(2) the magnetic properties of SmCoAsO. Moreover, the growth and magnetic properties of EuCuAs single crystal are also studied. Accord-ing to the data of magnetization and MR measurements, magnetic phase diagrams are established.