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Title:Zirconium nitride contacts for cadmium telluride solar cells
Author(s):Yang, Zhengfeng
Advisor(s):Rockett, Angus A.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):cadmium telluride (CdTe)
back contact
Schottky barrier
transition metal nitrides
Abstract:CdTe/CdS solar cells were produced with ZrN as a back contact by magnetron sputtering. ZrN films have been produced with crystalline quality and uniformity on CdTe. With good crystalline quality and uniformity on substrate materials, ZrN films have a low resistivity of 12.3 to 22.0 μΩ-cm, comparable to that of good conductive metals, which would permit thin back contacts. Annealing at temperature up to 270°C for 30 minutes did not affect the structure or properties of the ZrN. The effect of the thickness of the ZrN films on their back contact performance was studied. As the thickness of the ZrN films becomes larger, the improved microstructure of ZrN films contributes to the better cell performance. However, thicker ZrN leads to peeling off of films. Thicknesses beyond 300 nm do not result in better contacts. Cu is known to improve the performance of CdTe/CdS solar cells. However, a dual effect of Cu on the performance of CdTe/CdS solar cells indicates that the ones with more Cu doping perform worse and even drop to zero. SIMS depth profiles exhibit that Cu is a fast diffuser and higher temperatures stimulate more Cu to diffuse into ZrN films and bulk of CdTe. Compared to the conventional back contact material Au, ZrN films have lower optical reflectance and higher transmission. This advantage would permit thinner absorber layers for the resulting solar cells. The work function of ZrN was measured to be 4.1 eV by ultraviolet photoelectron spectroscopy (UPS). Transmission electron microscopy (TEM) indicated that the ZrN grew as columnar crystals perpendicular to the surface of CdTe with a rock-salt structure and (111) texture. A 2-3 nm thick amorphous layer of ZrN was observed at the CdTe/ZrN interface. Energy dispersive spectroscopy (EDS) showed that Cd and Te were present in the amorphous layer and the Zr:N ratio was not 1, off stoichiometric ZrN. The low work function combined with the amorphous layer at the interface contributed to the resulting observed Schottky barrier contact to the CdTe.
Issue Date:2014-09-16
Rights Information:Copyright 2014 Zhengfeng Yang
Date Available in IDEALS:2014-09-16
Date Deposited:2014-08

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