Effects of Na on the electrical and structural properties of CuInSe{sub 2}
- National Renewable Energy Laboratory, Golden, Colorado 80401 (United States)
We found theoretically that Na has three effects on CuInSe{sub 2}: (1) If available in stoichiometric quantities, Na will replace Cu, forming a more stable NaInSe{sub 2} compound having a larger band gap (higher open-circuit voltage) and a (112){sub tetra} morphology. The ensuing alloy Na{sub x}Cu{sub 1{minus}x}InSe{sub 2} has, however, a positive mixing enthalpy, so NaInSe{sub 2} will phase separate, forming precipitates. (2) When available in small quantities, Na will form defect on Cu site and In site. Na on Cu site does not create electric levels in the band gap, while Na on In site creates acceptor levels that are shallower than Cu{sub In}. The formation energy of Na{sub (In{sub Cu})} is very exothermic, therefore, the major effect of Na is the elimination of the In{sub Cu} defects and the resulting increase of the effective hole densities. The quenching of In{sub Cu} as well as V{sub Cu} by Na reduces the stability of the (2V{sub Cu}{sup {minus}}+In{sub Cu}{sup 2+}), thus suppressing the formation of the {open_quotes}Ordered Defect Compounds.{close_quotes} (3) Na on the surface of CuInSe{sub 2} is known to catalyze the dissociation of O{sub 2} into atomic oxygen that substitutes Se vacancy (shallow donors), converting them into O{sub Se}. We find, however, that O{sub Se} is an (isovalent) {ital deep} rather than shallow acceptor. We also find that having removed the donors, O atoms in CuInSe{sub 2} form Cu{sub 2}O and In{sub 2}O{sub 3} compounds, and phase separate, forming precipitates at the surfaces and grain boundaries. Our results are compared with previous models and provide new insights into the defect physics of Na in CIS. {copyright} {ital 1999 American Institute of Physics.}
- OSTI ID:
- 337501
- Journal Information:
- Journal of Applied Physics, Vol. 85, Issue 10; Other Information: PBD: May 1999
- Country of Publication:
- United States
- Language:
- English
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