Shannon Fields

Bandgap gradients in (Ag,Cu)(In,Ga)Se2 thin film solar cells deposited by three-stage co-evaporation

(Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells are optimized at bandgaps greater than 1.2 eV by varying composition profile of the absorber layer using a three-stage evaporation process. Numerical modeling and cumulative process data provides insight into the process. Silver alloying CIGS changes the optimized bandgap profile by reducing carrier concentration, and reducing bandgap gradients. The minimum bandgap position is controlled by the point when the film reaches I/III stoichiometry during the second stage of the three-stage process. We achieved a 19.9% efficient solar cell with VOC = 732 mV at a bandgap of 1.2 eV based on quantum efficiency.

Critical issues in vapor deposition of Cu(InGa)Se/sub 2/ on polymer web: source spitting and back contact cracking

This paper presents solutions to two critical problems encountered during the physical vapor deposition of Cu(InGa)Se/sub 2/ films on Mo coated polymer web in a roll-to-roll reactor. Spitting out of the Cu source, resulting in Cu-rich particles imbedded in the growing films and causing shorts in the devices, has been eliminated by changing the shape of the effusion nozzles. Sources having conical nozzles eliminated the spitting by reducing the temperature gradient along the nozzle. Heavy cracking of the Mo back contact film during the deposition of the Cu(InGa)Se/sub 2/ films has been found to be related to the reaction of Mo with Se in the reactor. Introducing an appropriate amount of oxygen into the Mo film eliminated the cracking by reducing the reactivity of the film to Se.

Cu(InGa)Se2 photovoltaics on insulated Stainless Steel Web substrate

This paper presents data on the use of Stainless Steel (SS) foils, coated with 3 to 5µm silicone based resin (SBR) developed by Dow Corning Corporation (DCC) for Cu(InGa)Se2 based PV devices. The substrate validation was performed by the device performances on R2R deposited Cu(InGa)Se2 via elemental evaporation and, to a limited extent, on Cu(InGa)Se2 films from metal precursor selenization. Initially, it is shown that a 430SS base is preferred to others due to a better thermal expansion match to Cu(InGa)Se2. The SBR-coated SS was found to be an excellent substrate in terms of (i) the adhesion of Mo and Cu(InGa)Se2 films, (ii) its surface smoothness, and (iii) its operational stability up to 600 °C. Na was introduced into the R2R deposition process by co-evaporating NaF during deposition. Presence of Na in the films was determined by SIMS depth profiling. The highest small area device performance from R2R deposition was 15.2%. In addition, 9ft and 29ft long 8″ wide Cu(InGa)Se2 webs fabricated in the reactor showed machine direction center line, and efficiencies that averaged more than 10%. Precursor selenization process gave the highest efficiency of 12.3% with 10nm NaF deposited on the SBR or on the precursors. Work on developing monolithically integrated 3″×3″ mini-modules has been initiated. Encouraging results were obtained in laser scribing the Mo back contact.

Commercial-scale sources for the evaporation of elemental Cu, Ga, and In: Modeling, design, and validation

A methodology for the design of commercial-scale sources for the evaporation of Cu, Ga, and In is described. Semi-empirical flow equations are utilized in predicting the internal pressure profile, and compared against temperature profiles generated by finite element modeling. This approach allows for the prediction of condensation within the source, and subsequent droplet ejection towards the substrate. Using this methodology, a commercial-scale prototype source was designed, fabricated, and validated for Cu, Ga, and In evaporation. Deposition profiles were determined, and rates sufficient for commercial-scale manufacturing were achieved.

Design and testing of pilot-scale Cu and mixed-vapor Ga-In evaporation sources

There are a number of requirements in the design of evaporation sources for the inline deposition of Cu(InGa)Se2 absorber layers. These include evaporation rates ≫10g/hr without anomalous behavior such as spitting, spatial composition uniformity across the area of thousands or tens of thousands of consecutive substrates, and low capital and maintenance costs. To meet these requirements, both quantitative and qualitative design considerations must be made, including thermal and flow modeling, fabrication methods, and material properties such as thermal and chemical compatibilities.

Design Strategy for Scale-Up of Physical Vapor Deposition of Cu(InGa) Se2 on Flexible Substrates

It has been observed that scale-up of the Cu(InGa)Se2 physical vapor deposition (PVD) onto flexible substrates to wide webs and long runtimes will lead to unacceptable film qualities due to issues associated with the thermal characteristics of the source-boats and the effect of melt level reduction with time. These issues are the subject of the present paper. The difficulties encountered with the design of the sources presently in operation are addressed, and new designs are proposed that resolve the problem of thickness and composition uniformity over wider substrates (~12 inches). Simulation results are presented comparing the old source design with the proposed new one for a 12-inch wide substrate. It is shown that improved film thickness uniformity and hence composition uniformity is achieved with the proposed designs, and material utilization can be improved by an appropriate choice of system design parameters such as nozzle-to-substrate distance