Process Study Of Cu（In,Ga）Se₂and Cu₂ZnSnSe₄Thin Film Solar Cells Fabricated By Nanoparticle Ink Technology

In Twenty-first Century, energy shortage is one of the serious problems. Looking for a new green, environmentally friendly, low-cost and sustainable energy becomes critical. Solar photovoltaic power generation is one of main methods to solve this problem. Cu(In,Ga)Se2(CIGS) and Cu2ZnSnSe4(CZTSe) thin films have become the research hotspot in the field of photovoltaic cell due to their stable performance, high conversion efficiency, high optical absorption coefficient, good performance in low-light, long-term radiation stabilities. Since the CIGS solar cell preparation technology becomes more mature, the photoelectric conversion efficiency of the CIGS solar cells has reached20.3%. However, the scarcity of indium (In) and gallium (Ga) is an important issue in mass production of such solar cells. CZTSe has attracted lots of attention as a potential alternative to CIGS in thin film solar cells due to its abundance of elements zinc (Zn) and tin (Sn). Nowadays, the mainstream technology of the CIGS thin film solar cell is vacuum co-evaporation and sputter deposition method, which is expensive, complicated operation, low material utilization and difficult to scale-up for commercial production. Therefore, it is highly desirable to develop a robust, low-cost and high throughput method for the fabrication of high quality CIGS and CZTSe thin films.In this dissertation, the CIGS and CZTSe thin films were successfully synthesized using a simple, green and inexpensive nanoparticle ink based method, involving deposition of CIGS and CZTSe nano-inks onto SLG and SLG/Mo substrates followed by post-anneal treatment at Ar and/or Ar/selenium atmosphere. Then, we study the synthesis methods and properties of the Mo thin film, CdS buffer layer, i-ZnO thin film and ZnO:Al transparent conduction oxide layers. Lastly, the photovoltaic cell devices with structure of Soda-lime glass (SLG)/Mo/Cu(In,Ga)Se2/CdS/i-ZnO/ZnO:Al/Al and SLG/Mo/Cu2ZnSnSe4/CdS/i-ZnO/Zn0:Al/Al were trial-produced using selenized CIGS thin films and CZTSe thin films as absorber layers and their photovoltaic properties were trial-tested. This thesis is divided into three parts, as follows:First, CIGS thin films were successfully fabricated by a facile and low-cost non-vacuum synthesis method. At first, the CIGS nanoparticles with appropriate size distributions and preferred (112) orientation structure were synthesized by a non-vacuum mechanochemical method using commercially available Cu2Se3,In2Se3Ga2Se3and Selenium powders as original materials. Secondly, the fluffy CIGS precursor films were deposited via spin-coating from nano-inks consisting of CIGS nanoparticles and two-step preheating treatments in air. Thirdly, the chalcopyrite CIGS thin films were obtained by a rapid thermal annealing (RTA) process rather than conventional furnace as thermal treatment equipment at Ar atmosphere and/or Ar/selenium atmosphere. The selenization was done with elemental Se vapor thus avoiding the use of highly toxic H2Se The effects of various thermal treatment parameters including substrate temperatures and selenization time on the structural and morphological properties of the CIGS thin films were inverstigated. Lastly, the dense and compact CIGS thin films with chalcopyrite structure were synthesized by a solution-filling treatment in which a solution containing Cu-In organic ligands. The CIGS thin films are p-type conductivity confirmed by a hot point probe method. The optical bandgap of the CIGS thin films are1.18eV with a high absorption coefficient exceeding104cm-1.Secondly, high quality CZTSe thin films were successfully synthesized by a novel, green and simple solution-based deposition process for the first time, involving deposition of non-toxic solvent-based CZTSe nano-inks onto SLG and SLG/Mo substrates followed by post-selenization with elemental Se vapor at different temperatures. The CZTSe nano-inks were prepared by a non-vacuum mechanochemical method using non-toxic and cheap ethanol as solvent and commercially available Cu2Se, elemental Zn, Sn, selenium powders as original materials for the first time. Then, the CZTSe precursor films were deposited by spin coating the nano-inks onto the SLG and SLG/Mo substrates. Lastly, the high quality CZTSe thin films with tetragonal phase stannite-type structure were synthesized by selenization process using a rapid thermal annealing (RTA) furnace rather than conventional furnace as thermal treatment equipment. Within this work, we evaluate the influence of different selenization temperatures, time and flow rate of carrier gas conditions on the structural and morphological properties of the CZTSe thin films. The CZTSe thin films are p-type conductivity confirmed by a hot point probe method. Photoluminescence spectrum shows slightly asymmetric narrow bands with a maximum of intensity at0.92eV. An absorption coefficient exceeding104cm-1and the band gap energy about0.91eV of the studied films are determined by an UV-VIS-IR absorption spectroscopy. Thirdly, preparation and characterization of CdS buffer layers, Mo thin films, i-ZnO thin films and ZnO:Al transparent oxide layers were studied by chemical bath deposition (CBD) method and sputter depositon method, respectively. And the photovoltaic cell devices were trial-produced using CIGS thin films and CZTSe thin films as absorber layers and their photovoltaic properties were studied. At first, the flat and dense CdS thin films with cubic structure were synthesized by CBD method. The structural, morphological, and optical properties were studied. Secondly, the dense, flat and smooth Mo thin films, i-ZnO thin films and ZnO:Al thin films were fabricated by RF magnetron sputtering. The effects of the sputter power, gas work pressure and target-substrate distance on the structural, morphological, optical and electrical properties of the Mo thin films, i-ZnO thin films and ZnO:Al thin films were systematically studied in this article. Lastly, the photovoltaic cell devices with structure of SLG/Mo/CIGS/CdS/i-ZnO/ZnO:Al/Al and SLG/Mo/CZTSe/CdS/i-ZnO/ZnO:Al/Al were trial-produced with active area is about0.5cm-2. Optical-electrical tests indicate that the CIGS and the CZTSe photovoltaic cell devices have obvious photoelectric conversion effect. The CIGS photovoltaic device shows a photoelectric conversion efficiency (PCE) of2.53%(Voc=0.369V, JSC=21.99mA/cm2, FF=31.2%). The CZTSe photovoltaic device shows a photoelectric conversion efficiency (PCE) of0.18%(Voc=0.204V, JSC=3.5mA/cm2, FF=25.3%).