Sodium-doping of ZnTe film by close-spaced sublimation for back contact of CdTe solar cell

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Abstract

Low resistivity p-type ZnTe film was formed by close-spaced sublimation utilizing sodium compounds as sodium source for doping. Sodium was incorporated during film deposition with ZnTe sources containing various amount of Na2Te, Na3PO4, or NaCl. Film resistivity of less than 1 Ω cm with hole concentration of 6 × 1017 cm−3 was achieved by doping with Na2Te. However, doping with Na3PO4 or NaCl was less effective in reducing the resistivity of ZnTe film due to low carrier concentration. Hole mobility was correlated with the microstructure of the ZnTe film, which strongly depends on the kind and content of sodium compound in the source.

Introduction

CdTe thin film solar cells have long been of great interest due to their low cost manufacturing process and good device performance. The back contact to the CdTe layer is one of the most critical aspects in optimizing efficiency and stability. This contact is typically prepared by deposition of copper or Cu2Te to form a heavily doped p+ region adjacent to the metal contact in order to allow tunneling transport across the interface [1], [2]. Copper, however, is a fast migrating impurity in CdTe and may lead to a degradation of cell properties [3], [4].

ZnTe has also been used as a back contact for CdTe solar cells. The insertion of a ZnTe layer produces a back surface field to reduce recombination loss of minority carriers at the back contact interface, which may enhance the device performance [5], [6]. Extensive research into ZnTe doped with copper has been done and hole densities have been reported in the range of 1019–1020 cm−3 for films with 2–6 at% copper [7]. Group V elements (N, P, As, Sb) have also been tried for doping of p-type ZnTe [8], [9], [10], [11]. But, doping of thin film ZnTe with sodium has not been explored and the influence of sodium on device properties is not well known, although considerable amounts of sodium are present in the CdTe solar cells prepared on the soda-lime substrates used by the industry. However, deliberate incorporation of sodium is not expected to be so harmful because the ionization energies of acceptor levels introduced by sodium-doping are very low (59 meV in CdTe [12] and 169 meV in CdS [13]). In the case of CIGS solar cells, a significant amount of sodium is incorporated in CIGS absorber diffused from soda-lime glass substrates, which is considered to improve the cell performance due to the enhancement of p-type doping and grain growth of CIGS.

In this work, we studied sodium-doping of ZnTe thin films deposited by a close-spaced sublimation process using ZnTe sources mixed with various sodium compounds.

Section snippets

Experimental

ZnTe films were deposited on soda-lime glass substrates by close-spaced sublimation (CSS). The source materials were prepared by mixing 99.99% purity ZnTe powder with 1, 3, and 5 wt% of Na2Te, Na3PO4, or NaCl, respectively. During deposition, the temperatures of the source and the substrate were maintained at 700 and 400 °C, respectively. Deposition time was fixed at 5 min for all specimens. To compare the effect of sodium diffused from the soda-lime glass substrates, some films without sodium

Results and discussion

Fig. 1 shows the morphology of the ZnTe film deposited by CSS method without any additives in the ZnTe source. The microstructure is very dense with large grains; no voids or pinholes are visible. The resistivity of this undoped film was over 106 Ω cm and did not decrease with the additional heat treatment up to 700 °C, meaning that the doping of the sodium or other impurities diffused from the soda-lime substrate below 700 °C is not effective. Note that, for the application of a back contact

Summary

Low resistivity p-type ZnTe film was formed by close-spaced sublimation utilizing Na2Te as a sodium source for doping. Film resistivity of less than 1 Ω cm with hole concentration of 6 × 1017 cm−3 was achieved by doping with Na2Te. Hole mobility increases with Na2Te content due to an increase in grain size of ZnTe. By using Na3PO4 as a source, a dense microstructure can be obtained, but hole density is less than 1017 cm−3. NaCl has a limited doping effect because of the compensation effect of

Acknowledgments

This work was supported by the High Risk High Return Project (HRHRP) funded by the Korea Advanced Institute of Science and Technology (KAIST) and by the Engineering Research Center Program (2008-0062289) funded by the Korean Ministry of Education, Science, and Technology (MEST).

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