Studies Of CIGS Solar Energy Materials And Solar Cells Deposited By Sputtering From A Single Quaternary Target

This paper is focused on the preparation and optical&electrical properties of CuInGaSe2(CIGS) solar cells. The quaternary CIGS target is used to prepare the absorbed layer by RF sputtering process from the ananlysis of photovoltaic industry overview, CIGS solar cells research stutation and status of industrial development. The structure of CIGS solar cells is glass/Mo/CIGS/CdS/i-ZnO/AZO, and the efficiency of solae cells is up to2.08%. In addition, the working principle of testing equipment and properties of each layers are discussed.Firstly the preparation of Mo electrode by magnetron sputtering is investigated. The Mo layer exhibits scaly structure, good adhesion, sheet resistance about0.26Ω/□, thickness of layer1080nm, and electrical resistivity about9.8×10-5Ω·cm by two-steps method, which contains the working pressure of2pa in the first step and0.5pa in the second step. Both of the power in two steps is130w.Secondly the effect of substrate tempreture, sputtering pressure and post-selenization process on the films’structure, optical properties and surface of the CIGS obsorbed layer fabricating by sputtering quaternary CIGS target was investigated. The XRD patterns shows that the intensity of (112) peak becomes stronger and the FWMA of (112) peak tend to be smanller with increasing the substrate tempreture, which indicate the crystallization quality become better. Besides the composition of films is closed to the composition of target, which shows temperature effect of poor Cu and poor Se is not obvious. The poor Cu state occurs as the increasing of working pressure, which is accordance with the report by Sigmund. The yield of Cu is lower than that of In and Ga. The patterns of AFM and SEM indicated that the grain size become smaller and average roughness decreases from11.36nm to5.12nm. The surface of small grain size exhibit smooth and dense, while the more grain boundaries in small grain-size films will enchance the carrier recombination, lower short-circuit current and drop the efficiency of cells. The optical bandgap of CIGS films shifts from1.0eV to1.32eV with the increasing working pressure. In general, the CIGS film prepared at1.0pa shows good properties and satisfy requirement of solar cell. The poor Se state in CIGS films prepared by one step method can be eliminated by post-selenization process. The results show that post-selenization films possess better crystalline, smoother suaface and pure phases. The peak located at176cm-1in Raman spectra is vertified as Al mode, while the broaded peak located at215-220cm-1is identified as B2mode of chalcopyrite structure. In conclusion, the CIGS films prepared by sputtering at0.9pa and post-selenization with10mg Se exhibit near stoichiometric ratio and smooth surface, which is suitable to fabricate the CIGS solae cell.The AZO windows layer prepared by magnetron sputter is studied. The effect of power, working pressure and substrate temperature on optical and electrical properties of AZO transparent conductive thin films has been studied. The result shows that the AZO thin films prepared at working pressure of0.8pa, sputtering power of225w, the substrate temperature of225℃possess lowest electrical resistivity about9.8×10-4Ω·2cm, and the average transmittance of AZO thin films in the visible region is up to85%with bandgap of3.32eV.Comparative analysis of CdS layer prepared by magnetron sputtering and chemical bath method. The test result of CdS thin films obtained by the chemical bath method in surface morphology, transmission spectrum and band gaps show that a smooth surface, a larger grain and an average transmittance reaches above80%in visible light range, which band gap is about2.3eV. However, the thin films deposited by magnetron sputtering indicate that a small grain, an average transmittance slightly less than80%and a similar band gap. Although the later films show certain feasibility, which can basically meets the requirement of the electrical properties in the CIGS solar cells. The previous method can make the absorption layers avoid the injury from sputtering method, clean the absorber layer surface, low cost, more stable performance and easily realize industrial production. Because of sputtering can bring pollution to vacuum chamber and cross reaction in the multilayer film preparation, the chemical bath method is preferred. The solar cells with the structure glass/Mo/CIGS/CdS/i-ZnO/AZO were obtained, the result indicate an efficiency of2.08%, an open circuit voltage of294mV, the short circuit current of21.6mA/cm2and the fill factor is32%on a solar cell unit area about0.07cm2, which were characterized by current-voltage measurement under AM1.5illumination at25℃. The reasons for the loss of solar cell efficiency and quantum efficiency have been analyzed though the combined with the SEM section and the quantum efficiency curve. There are four main reasons:(1) the smaller grain in the absorber layer, no obvious longitudinal grain boundary and the grain cannot clear observe form the section of thin films. Then, the thin film thickness is too thin to the higher efficiency, which only about750nm, the high efficiency of CIGS solar cells would need a1500nm thickness.(2) the CdS thin film is too thick for the cells. Buffer layer CIGS cell is generally high efficiency of the thickness is40-50nm, from the battery of the section, we prepared buffer layer thickness is about80nm, the buffer layer is too thick to affect the rate of cellvoltage.(3) The thickness of window layer also affects the conversion efficiency of the battery, the thickness of window layer was about350nm.(4) The study found photoelectric conversion efficiency of CIGS polycrystalline thin film solar cells is higher than that of CIS single crystal solar cell. The reason is:CIGS polycrystalline film crystal andcolumnar section, thin film is single grain, reducing carrier transmission boundary composite;and the formation of lean CIGS thin film on the surface of the ordered defect compounds layer (ODC), in each grain surface of polycrystalline CIGS also have ODClayer, internal the ODC and CIGS VB offset between grains by forming a hole barrier, prevent grain tograin boundary diffusion inside the hole, so that theinterface recombination is reduced; grain gettering effect on grain, the grain internal purity is higher thansingle crystal material.(5) Without antireflection layer MgF2.