Projects

Multijunction Concentrators

University of Illinois

Publications

Project Objective:  Demonstrate the potential for use of CuInSe₂ and related materials as the 1.00 eV energy-gap material in multijunction, extremely high efficiency, solar cells.  Intermediate objectives include demonstration of solar cells based on p-CIS/n⁺-GaAs and p-CIS/n-Ge heterojunctions as components of multijunction, high-efficiency solar cell devices. 

Approach/Background:  Photovoltaic devices based on Cu(In,Ga)Se₂ (CIGS) have the highest performance of any thin film technology.  However, the possibilities for even higher performances are significant.  Multijunction devices involving CIGS either in conjunction with III-V compound semiconductors (GaAs and related materials) or various Cu chalcopyrite compounds (CuGaSe₂, CuInS₂, or others) remain to be exploited. This program develops CIGS as a narrow-gap component of multijunction solar cells both in single crystal epitaxial and polycrystalline structures.  In previous efforts, we have developed a well-characterized and reproducible method for deposition of single-crystal epitaxial layers of Cu(In,Ga)Se₂ alloys on GaAs substrates of each of the three major surface orientations.  The technique consists of sputtering Cu or Cu-Ga and in targets in Ar gas and simultaneously evaporating molecular Se (and/or S) from an effusion cell or cells.  The present work will begin with a detailed study of the electrical properties of CIGS-GaAs heterojunctions.  This is critical to application of CIGS in high efficiency cells for two reasons.  First, because the only way to produce a two-contact multijunction solar cell involving CIGS is to use one of the surrounding semiconductors as the heterojunction partner. Therefore, it is necessary to establish the performance of junctions of candidate materials with the CIGS.  Second, because the CIGS epitaxial layers are high-quality single crystals, growth of multilayer structures will be possible.  Such growth is required in current designs of non-mechanically-stacked high efficiency devices where the 1.0 eV gap device is surrounded both above and below by additional devices.  Our preliminary studies will concentrate on demonstration of solar cells based on p-CIS/n⁺-GaAs and p-CIS/n-Ge heterojunctions.

Status/Accomplishments:  Hiring and Training of students has begun for the described effort.  Laboratory space is being prepared for solar cell device production and analysis.  Several samples of single crystal epitaxial layers have been supplied to NREL already to assist in calibration of their instruments and to improve their analysis capabilities. 

Planned FY 2002 Activities:  Preliminary efforts will focus on acquiring and installing equipment to activate a high-performance CuInSe₂ deposition system and fitting both the existing system and this new system with facilities for ionized physical vapor deposition for low-temperature deposition.  Substrates are being purchased and depositions will begin immediately for production of test heterojunctions.  During FY2002, we anticipate deposition of heterojunctions on both GaAs and Ge substrates and testing of these junctions.  We anticipate formation of photovoltaically-active junctions on both of these substrates in the next year.  This will provide a basis for proof-of-concept of the proposed program.

University of Illinois High-Performance PV Publications:
"Research Toward High Performance Epitaxial and Low-temperature Cu(In,Ga)Se₂ Solar Cells," (PDF 35 KB) National Center for Photovoltaics Program Review Meeting, Oct. 14-17, 2001 (Lakewood, CO).

A. Rockett, D.X. Liao, and C.M. Mueller