Copper Indium Diselenide and Related Alloys

Cu(InGa)Se2 grown on a Mo/glass substrate

Cu(InGa)Se₂ based solar cells have often been touted as being among the most promising of solar cell technologies for cost effective power generation. This is partly due to the advantages of thin films for low cost, high rate semiconductor deposition over large areas using layers only a few microns thick and for fabrication of monolithically interconnected modules.  Perhaps more importantly, very high efficiencies have been demonstrated with Cu(InGa)Se₂ at both the cell and module levels. Currently the highest solar cell efficiency is 19.9% for small area solar cells and 13.5% for large area commercial modules and modules have shown excellent long term stability in outdoor testing.  In addition to its potential advantages for large area terrestrial applications, Cu(InGa)Se₂ solar cells have shown high radiation resistance, compared to crystalline silicon solar cells and can be made very lightweight with flexible substrates so they are also promising for space applications.

Research at the Institute of Energy Conversion and other laboratories worldwide have shown that polycrystalline thin film CuInSe2-based alloys are remarkable materials with wide compositional tolerance, inherently passive grain boundaries, and the ability to control the bandgap and other material properties by alloying with Ga to make Cu(InGa)Se2, Al to make Cu(InAl)Se2, S to make Cu(InGa)(SeS)2, and more.  Various different approaches to the deposition of CuInSe2-based films have been explored. There has been tremendous progress in Cu(InGa)Se2 solar cells as evidenced by high module and cell efficiencies, the range of deposition and device options that have been developed, and the growing base of science and engineering knowledge of these materials and processes.  Still, there is a lack of understanding of many of the critical problems associated with the semiconductor processing and a need for further research at both the laboratory scale, to address fundamental issues, and on the pilot line, to address equipment and scale-up problems and to validate processes.

At IEC, current research and development focus includes several different aspects, three of which are described below.  A baseline process for complete cell fabrication is maintained and cells with efficiencies as high as 18% have been produced. In addition, IEC maintains active collaborations and cooperation with several companies and other university researchers.

I.  Wide Bandgap Alloys

CuInSe2-based alloys with wider bandgap (E_g) are important due to potential advantages in the performance of cells and modules.  Reduced module losses should result from the tradeoff between higher voltage and lower current at maximum power. Wider bandgap alloys will also yield devices with a lower coefficient of temperature for output power which will improve performance at the elevated temperatures experienced in most real terrestrial applications.  Finally, high efficiency wide bandgap devices are necessary for the development of tandem or multijunction cell structure which are envisioned as a pathway to higher overall efficiency.  However, in all results to date, the efficiency of devices with CuInSe₂-alloy absorber layers decreases with increasing bandgap for E_g > 1.3 eV.

Current-voltage (left) and quantum efficiency (right) measurements show comparable device perfromance in Cu(InGa)Se₂ and Cu(InAl)Se₂ solar cells with 17 % efficiency.

At IEC we have investigated deposition, materials properties, and device characteristics with high-Ga Cu(InGa)Se₂, Cu(InAl)Se₂, and Cu(InGa)(SeS)₂ alloys.  A new program under the Future Generation program of the Department of Energy’s Solar America Initiative is continuing this focus.  Specifically, we are comparing alloys with Ag to those alloyed with Ga and S and developing novel approaches to post-deposition materials processing.

At IEC Cu(InGa)(SeS)₂ films are deposited by five source elemetal evaporation (right) and by the reaction of Cu-Ga-In films in H₂Se and H₂S

II.  In-line Evaporation of Cu(InGa)Se₂

Elemental in-line evaporation has been shown to be a viable process for the large-area manufacture of Cu(InGa)Se₂ photovoltaics.  IEC has developed an in-line evaporation process in which Cu(InGa)Se₂ is deposited on glass substrates with linear translation through the system or with flexible (high temperature polyimide or metal foil) substrates in a roll-to-roll configuration.  Lightweight and flexible cells using Cu(InGa)Se₂ are attractive in space power, military and other high value applications. In addition, flexible substrates have an advantage in manufacturability as the roll-to-roll configuration allows continuous, high yield, and low-cost production.

IEC's in-line evaporation system for the deposition of Cu(InGa)Se2 on a moving substrate can be used with flexible web (plastic or metal foil) in a roll-to-roll configuration with glass substrates.

Research and development of in-line evaporation at IEC has focused on source design, process control including the development and incorporation of in-line diagnostic tools, and material and process characterization.  Using evaporation sources that provide a high degree of uniformity and reproducibility, solar cells with efficiencies of 12.1 % on polyimide and 15.8 % on glass have been demonstrated. 

Cu(InGa)Se2 deposited using continuous in-line evaporation at IEC on a 100 foot long by 13 inch wide polyimide web.

Recently IEC has added the capability to do laser and mechanical scribing on square foot size samples.  This is being used to investigate monolithic integration for fabrication of Cu(InGa)Se₂ modules on polyimide substrates like that shown and on glass substrates. A new program under the University Photovoltaic Process and Product Development program of the Department of Energy’s Solar America Initiative will, in cooperation with Dow Corning develop a low-cost insulated foil substrate for Cu(InGa)Se2 manufacturing and demonstrate its application with roll-to-roll evaporation and monolithic integration for module production.

Cu(InGa)Se2 deposited on a polyimide web by roll-to-roll evaporation at IEC can be used to fabricate ultra-lightweight modules.

III.  Reaction of Cu-Ga-In Precursors

Manufacturing of Cu(InGa)Se₂ is also being developed using a process in which the metals, Cu, Ga and In, are deposited first and then reacted to form Cu(InGa)Se₂ or Cu(InGa)(SeS)₂.  This process has the potential advantage of enabling low-cost, commercially available tools for the metal deposition to be implemented.  At IEC, Cu(InGa)(SeS)₂ thin films are grown using sputtered Cu0.8Ga0.2 and In layers which are reacted in an atmospheric pressure, tubular quartz reactor with H₂Se, H₂S, and O₂ process gases.