Improved Single-Source Precursors for Solar-Cell Absorbers

John H. Glenn Research Center, Cleveland, Ohio

Improved single-source precursor compounds have been invented for use in spray chemical vapor deposition (spray CVD) of chalcopyrite semiconductor absorber layers of thin-film solar photovoltaic cells. The semiconductors in question are denoted by the general formula CuIn_xGa1_–xS_ySe_2–y, where x≤1 and y≤2. These semiconductors have been investigated intensively for use in solar cells because they exhibit longterm stability and a high degree of tolerance of radiation, and their bandgaps correlate well with the maximum photon power density in the solar spectrum. In addition, through selection of the proportions of Ga versus In and S versus Se, the bandgap of CuIn_xGa_1–xS_ySe_2–y can be tailored to a value between 1.0 and 2.4 eV, thus making it possible to fabricate cells containing high and/or graded bandgaps.

These Sequences of Chemical Reactions are representative of the synthesis of a single- source precursor, in this case, [(LR3)2Cu(ER′)2M(ER′)2] followed by formation of the chalcopyrite semiconductor (in this case, CuInE2) from the precursor.

A "single-source precursor compound" is a single molecular compound that contains all the required elements, which when used under the spray CVD conditions, thermally decomposes to form CuIn_xGa1_–xS_ySe_2–y. Relative to the use of multiple precursor reagents, the use of single-source precursors offers the advantage of better regulation of the chemical composition of the deposit, less susceptibility to contamination, and, most importantly, a simplified fabrication process.

The improved single-source precursor compounds of the invention are denoted by the general formula [(LR₃)₂Cu(ER′)₂M(ER′)₂], where L signifies P, As, or Sb; R or R′ signifies an alkyl or aryl group; and E signifies S or Se. The general formula and molecular structure afford flexibility for tailoring the precursor to suit a specific CVD spray process and to tailor the chemical composition (more specifically, the proportions x and y) of CuIn_xGa1_–xS_ySe_2–y formed in the process. In addition, by choosing L, R, R′, and E according to their steric and electronic properties, one can tailor decomposition temperatures and the phases in the deposit.

A single-source precursor of the invention can by synthesized by the reaction of a stabilized Cu(I) cation with an In(III) or Ga(III) chalcogenide anion prepared in situ by reaction of the conjugate acid of the thiol or senenol with NaOEt (where Et signifies an ethyl group) in CH₃OH, as in the example of the figure. Alternatively, one could synthesize a single-source precursor by use of commercially available reagents in a new facile one-pot process: For example one could react NaSCH₃ with InCl₃ and add CuCl and PR₃.

This work was done by Kulbinder K. Banger, Jerry Harris, and Aloysius Hepp of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Materials category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17445-1.