Download Measuring CdS/CdTe solar cells back contact barrier height

Measuring CdS/CdTe solar cells back contact barrier height
Mohammed K. Al Turkestani1 and Ken Durose2
1
Department of Physics, Umm Al-Qura University, Saudi Arabia
2
Stephenson Institute for Renewable Energy/Department of Physics, University of Liverpool,
UK
The back side or dark contact to CdTe solar cells presents a fundamental limit to PV
performance since metallic contacts to p-CdTe inevitably have a non-Ohmic contact. In this
work a) methods to measure the back contact Schottky barrier height in thin film solar cells
are evaluated and b) the methods are used to evaluate the barrier heights of a number of
contacting procedures and materials are measured.
Two methods for measuring the back contact barrier height were compared. i) The first
relies on determining the voltage value at which the roll over effect (forward bias current
limitation) starts occurring in the J-V characteristics. However, the procedure for identifying
this voltage can lead to uncertainties, especially with weak roll over. ii) The second method
assumes that the solar cell series resistance, Rs, is predominantly due to the back contact.
Thus the back contact barrier height can be calculated from the dependence of Rs on
temperature, T. Simplifications, improvements and limitations of these methods are
presented. Both methods yielded results with a high level of agreement.
Barrier heights for a range of different contacts for CdTe solar cells were then measured.
Three contact materials were investigated, these being Au, Sb2Te3 and As2Te3. Different
etching procedures were also used as part of the contacting protocols, these being either
nitric-phosphoric acid (wet etching) or plasma (dry etching) or the control (no etching). Both
the values of the barrier height (generally in the range 0.2 – 0.45 eV) and the effect of the
back surface treatment on those values are presented and discussed.
Presentation: Poster