Improved low-resistance, semitransparent back contacts, and a method of fabricating them, have been developed for solar photovoltaic cells that are made from thin films of I-III-VI2 semiconductor materials on flexible, high-temperature-resistant polyimide substrates or superstrates. [The term 'I-III-VI2' is an abbreviated indication that the semiconductor materials are compounds of elements in periods IB, IIIA, and VIA of the periodic table in the stoichiometric ratio of 1:1:2. More specifically, these are compounds of general empirical formula Cu(In, Ga, or Al)(Se or S)2.] The innovative aspect of the present development lies in the extension, to polyimide substrates or superstrates, of a similar prior development of improved low-resistance, semitransparent back contacts for I-III-VI2 solar cells on glass substrates or superstrates. A cell incorporating this innovation can be used either as a standalone photovoltaic device or as part of a monolithic stack containing another photovoltaic device that utilizes light of longer wavelengths.
The figure depicts a generic device incorporating these innovations in the substrate configuration. The semitrans-parent back contact that is the main focus of this article consists of two layers: The first layer deposited on the substrate is a transparent, electrically conductive oxide (for example, ZnO, InSnO2, or SnO2). This layer acts mainly as a current collector. The second layer performs as contact interface layer capable of making good electrical contact with the solar-absorber material; this layer is deposited over the conducive oxide to a thickness of
A solar-absorber layer — a p-doped I-III-VI2 semiconductor layer, possibly having an n-doped surface sublayer — is grown over the thin metal layer by co-evaporation or another suitable thin-film deposition technique. Next, a layer of CdS that serves as a window and/or a heterojunction partner with the I-III-VI2 semiconductor is deposited on the semiconductor surface by a chemical-bath or other suitable technique that does not damage the semiconductor surface. Finally, another transparent, electrically conductive oxide layer (typically of InSnO2) that is mostly transparent to the solar spectrum is deposited over the CdS.
The semitransparency of the back contact enables the cell to function whether illuminated from the front or the back surface. Also relative to the opaque back contacts of prior such cells, the semitransparent back contact enables this cell to operate at a lower temperature, and, consequently, with greater energy-conversion efficiency. During the course of development, it was discovered that the innovative semitransparent back contact increases the adhesion between the polyimide and the solar-absorber (I-IIIVI2 semiconductor) layer — an important advantage, inasmuch as adhesion between polyimide substrates and traditional opaque molybdenum back contacts had been found to be problematic.
This work was done by Lawrence M. Woods and Rosine M. Ribelin of ITN Energy Systems, Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Semiconductors & ICs category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Refer to LEW-17376.