Spray CVD for Making Solar-Cell Absorber Layers

Spray CVD combines the advantages of metalorganic CVD and spray pyrolysis.

Spray chemical vapor deposition (spray CVD) processes of a special type have been investigated for use in making CuInS₂ absorber layers of thin-film solar photovoltaic cells from either of two subclasses of precursor compounds:
[(PBu₃) ₂Cu(SEt)₂In(SEt)₂] or [(PPh₃)₂Cu(SEt)2 In(SEt)₂] {where Bu, Et, and Ph signify butyl, ethyl, and phenyl groups, respectively}. CuInS₂ is a member of the class of chalcopyrite semiconductors described in the immediately preceding article. [(PBu₃)₂Cu(SEt)₂In(SEt)₂] and [(PPh₃)₂ Cu(SEt)₂In(SEt)₂] are members of the class of single-source precursors also described in the preceding article.

In a spray CVD process of this type, a room-temperature solution containing the precursor compound is first ultrasonically nebulized. The resulting aerosol is swept into a two-zone reactor by a flow of argon, which serves as a nonreactive carrier gas. In the reactor, the aerosol first encounters the evaporation zone, which is a warm zone wherein the solvent and precursor evaporate. The resulting mixture of gases then enters the deposition zone, which is a hot zone wherein the precursor decomposes and the semiconductor film grows on a substrate as in conventional CVD.

Spray CVD affords a combination of the most desirable features (but without the major difficulties) of metalorganic CVD and spray pyrolysis. These desirable features include growth of the film in an inert atmosphere, capability for deposition on a large area, laminar flow over the substrate, and storage and delivery of the precursor from a cool solution reservoir. The lastmentioned feature is especially advantageous in that it can prevent premature decomposition of a thermally labile precursor.

Two different spray CVD processes of this type have been tested in experiments thus far. In one process, a horizontal hot-wall reactor was used; in the other process, a vertical cold-wall reactor was used (see figure). In each process, the flow rate of argon was typically about 4 L/min. For the horizontal hot-wall reactor, the aerosol was generated by use of an ultrasonic plate nebulizer excited at a frequency of 2.5 MHz; for the vertical-cold-wall reactor, a syringe pump delivered the solution at a rate of 1.5 mL/min to the nebulizer, wherein the aerosol was generated by use of an atomizing ultrasonic nozzle excited at a frequency of 120 kHz. In the horizontal hot-wall reactor, the portion of the wall in the evaporation zone was heated to a temperature of 130 °C, while the portion of the wall in the deposition zone was heated to about 400 °C. In the vertical cold-wall reactor, as its name suggests, the wall was not heated; instead, the substrate was heated to 400 °C.

The CuInS₂ films produced in the experiments have been characterized by x-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and four-point-probe electrical tests. The results of these tests have provided some guidance for refinement of the spray CVD processes and for annealing and possibly other postprocess steps to improve the quality of the deposited CuInS2 films.

This work was done by Kulbinder K. Banger, Jerry Harris, Michael H. Jin, 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-17447-1.