Nanoparticle Oxides Precursor Inks for Thin film Copper Indium Gallium Selenide (CIGS) Solar Cells
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Nanoparticle Oxides Precursor Inks for Thin film Copper Indium Gallium Selenide (CIGS) Solar Cells Vijay K. Kapur, Matthew Fisher and Robin Roe International Solar Electric Technology (ISET) 8635 Aviation Blvd., Inglewood, CA 90301 ABSTRACT The paper describes ISET’s patented non-vacuum process for low cost mass production of CIGS solar cells. In this process, the water based precursor inks of mixed oxides are deposited on various conducting substrates by a variety of non-vacuum coating techniques. The oxides are converted to CIGS by annealing and the device is completed by deposition of CdS by CBD followed by ZnO deposition by MOCVD. Small area solar cells with efficiency >13% have been fabricated by this process. The advantages of this non-vacuum process are: high compositional control of the absorber layer, high materials utilization and low cost. INTRODUCTION A number of labs working on thin film CIGS solar cells have clearly demonstrated their potential in achieving respectable conversion efficiency and their outdoor stability. Also, CIGS solar cells have been found to be robust against radiation damage in space and, therefore, are being favored as the candidate for fabricating light weight and flexible solar cells and modules for space power application. Most of the labs developing CIGS solar cells have used physical vapor deposition (PVD) techniques such as evaporation or sputtering for depositing the absorber layer of CIGS. The champion solar cell with conversion efficiency 18.8% (1) was first fabricated more then two years ago by the research team at the National Renewable Energy Laboratory (NREL), USA. Since then a number of other laboratories have demonstrated CIGS solar cells with conversion efficiencies in the 15–17% range. However, utilizing this knowledge and experience gained in labs around the world in defining a commercial process for manufacturing low cost CIGS modules remains a big challenge. It is well known that the electronic properties of chalcopyrite materials such as CIGS are affected by slight changes in their composition. Deposition of CIGS absorber layers with specific electronic properties and with uniform composition on large areas using PVD methods is very challenging. In PVD systems the composition of the deposited materials is affected by the relative geometry of the source and the substrate on which the material is being deposited. The geometry in a system changes as the deposition process progresses affecting the composition of the deposited material continuously. Variations in the composition of the CIGS absorber layers result in poor process yields. These limitations can be overcome by using a variety of feed back loops and controls in deposition systems, however at a substantially increased costs of the deposition equipment. Cost analysis of large-scale manufacturing reveals that the major costcontributing factor in manufacturing thin film PV modules is the cost of materials. Even though, in principle thin film solar cells use very little amount of materials, the cost of materi
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