Projects

Polycrystalline Thin Films

University of Florida

Publications

Project Objective:  Identify critical issues associated with the design and manufacture of a monolithic two-junction tandem solar cell consisting of a CIS bottom cell and a CGS top cell connected via a heavily-doped tunnel junction.  University of Florida (UF) shall focus on the feasibility of fabricating a CGS top cell with an AM 1.5 conversion efficiency in the 10-13% range, a bottom CIS cell with a conversion efficiency in the 13-15% range, and a low resistance, high quality tunnel junction for the top- and bottom- cell interconnection.  Fabrication procedures for high-quality top and bottom cells and for a tunnel junction for the interconnection of the top- and bottom- cells of the CGS/CIS two-junction tandem cell will be developed.

Approach/Background:  Fabricating and testing the limits of performance and manufacturability of three separate devices representative the components of the tandem cell proposed, namely, (i) a top CGS solar cell, (ii) a tunneling diode, and (iii) a bottom CIGS cell.  The key processing parameters associated with the fabrication of each device will be investigated and optimized, and appropriate device models will be developed.

Status/Accomplishments:  Initial efforts have been carried out to model and optimize the performance of the top CIS cell using the one-dimensional simulator AMPS-1D.  Two parameters have been investigated fully, namely, the effect of the addition of sulfur (to increase the band gap above 1.68 eV) , the effect of the film thickness on the conversion efficiency.

We have initiated the retrofitting of our migration-enhanced epitaxial growth (MEE) reactor to include a new Ga source required for growing the absorber film for top bottom cell.  All the parts have been received, and a reactor-modification drawing has been completed.

The MEE reactor has been successfully used to grow CIS films on molybdenum-covered glass. 

Planned FY 2002 Activities:

• Produce epitaxial CGS on a suitable substrate with controlled p-type carrier concentration.
• Produce epitaxial CIS on a suitable substrate with controlled p-type carrier concentration
• Characterize electrical and optical properties of the  n⁺⁺ ZnCdS/p⁺⁺-Cd_1-xZn_xS.tunnel junction and determine the optimal thickness and doping level for low optical and low electrical losses.
• Develop a device model for each component of the tandem cells to facilitate simulation and prediction of single junction cell performance.
• Develop and evaluate a one-step electrodeposition method for growing CGS films on CdS-coated glass substrates.

University of Florida High-Performance PV Publications:
None at this time.