Graduation Year

2004

Document Type

Thesis

Degree

M.S.E.E.

Degree Granting Department

Electrical Engineering

Major Professor

Christos S. Ferekides

Keywords

optical properties, TCOs, front contact

Abstract

Transparent conductive oxides are an essential part of technologies that require both large-area electrical contact and optical access in the visible portion of the lightspectrum. SnO₂ doped with Fluorine (SnO₂: F) and In₂O₃ doped with tin (ITO) are the most popular choices of front contacts for CdTe solar cells. In this thesis, CdS/CdTe devices were fabricated with SnO₂: F (MOCVD) and ITO (sputtering) as front contacts without a high resistivity (resistivity relatively greater than front contact) buffer layer. The device characteristics of these devices were low but improved considerably after the inclusion of an intrinsic SnO₂ (SnO₂-i) deposited by MOCVD as buffer. Thus having emphasized and demonstrated the benefits of a buffer layer in these devices, the use of reactively sputtered SnO₂ (intrinsic), SnO₂ doped with Zinc (5% and 10% Zinc) and In₂O₃(intrinsic) as buffer layers in SnO₂:F/buffer/CdS/CdTe devices were explored.

Experiments were also carried out on the photovoltaic active layers of SnO₂:F/SnO₂-i/CdS/CdTe Solar cells. Namely, the effect of window layer thickness was studied by making a series of devices in which the CdS thickness was progressively reduced and the effect of substrate temperature (Tsub) during the deposition of the absorber layer was also studied by increasing Tsub > 600 degree C during CdTe CSS. In order to determine the effectiveness of In₂O₃ as a buffer layer, a series of ITO/In₂O₃/CdS/CdTe cells were fabricated with varying thickness of In₂O₃ (250 to 2000 Ǻ) and also the CdS thickness was reduced in steps (~800 Ǻ to~500 Ǻ) in these devices. ITO/ In₂O₃ device with efficiency greater than 14% (Voc: 820 mV, FF: 72% and Jsc: 24 mA/cm²) was fabricated for an In2O3 thickness of 250 Ǻ and CdS thickness of ~ 600 Ǻ. However the best efficiency of 14.7% (Voc: 830 mV, FF: 77%, Jsc: 23 mA/cm²) was achieved for SnO₂:F/SnO₂-i/CdS/CdTe device.

ITO films with resistivity as low as 1.9X10-4 ohm-cm, mobility 32 cm2V-1s-1 and average transmission ~ 90% in the visible region were obtained for carrier concentration in the order of 1.1XE21cm-3.

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