On the role of Na and modifications to Cu(In,Ga)Se/sub 2/ absorber materials using thin-MF (M=Na, K, Cs) precursor layers [solar cells]

Abstract

The growth and characterization of Cu(ln,Oa)Se, polycrystalline thin films under the presence of thin-MF (M=Na, K, Cs) precursor layers is presented. Some electrical, structural, and electronic absorber properties due to the presence of such Group la impurities are quantified along with their influence in device performance. We present a growth model for the role of Na in Cu(ln,Ga)Se, that attributes the enhancemeiits in electrical conductivity and photovoltaic device performance to the extinction of a finite number of donor states (i.e., I n , ) at the bulk and grain-boundary regions.

Development of thin‐film Cu(In,Ga)Se2 and CdTe solar cells

“…The methods include the co-evaporation or the deposition of a thin precursor of a Na compound such as NaF, Na 2 Se or Na 2 S. The effects of other alkali metal salts (e.g., KF, CsF, LiF) have been investigated. 97,98 The observed effects of the KF and LiF precursors were similar to those of NaF, but less pronounced, while the CsF precursor had only a minor influence on the CIGS properties. Generally, Na in CIGS improves the cell efficiency by increasing the V OC and fill factor.…”

“…Generally, Na in CIGS improves the cell efficiency by increasing the V OC and fill factor. 97,99,100 The optimum Na dose is often considered to be equal to the amount diffusing from a soda-lime glass substrate, resulting in a typical Na concentration of approximately 0Á1 at.%.…”

“…104,114,115 Various models have been proposed to explain the effects of Na on the electronic and structural properties of layers and influence on solar cells. 97,110,[116][117][118] CdTe layers CdTe thin films have been succesfully grown by a variety of vacuum and non-vacuum deposition methods. Generally, CdTe growth methods such as CSS or close-spaced vapor transport with deposition temperature above 500 C are classified as high-temperature processes, while methods such as electrodeposition, HVE and sputtering with deposition temperature below 450 C are classified as low-temperature processes.…”

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

“…[5][6][7][8] Na incorporation into the CIGSe absorber results in a significant improvement of solar cell efficiency, [9][10][11][12] and so for devices based on alternative (i.e., Na-free) substrates, Na must be added deliberately as part of the solar cell manufacturing process. Na can be incorporated prior to, 13,14 during, 12,15 or after 2,16 CIGSe growth. (Note that the latter approach requires a subsequent annealing process.)…”

Atomic structure of the (001) surface of CuGaSe2

“…This results in the quenching of the In Cu and V Cu defects, which results in a decrease of the concentration of the (2V −1 Cu +In +2 Cu ) complex defect. The presence of small quantities of Na then contributes to the synthesis of Cu-poor chalcopyrites, avoiding the formation of the so called ordered defective compounds [45], increasing the p-type layer conductivity [50], and reducing the dependence on the compositional ratio of Cu/In [51].…”