Calibration of the WCT-100 photoconductance instrument at low conductance

The WCT-100 photoconductance instrument is used throughout the photovoltaic industry providing a relatively simple and inexpensive means to assess recombination parameters. This paper presents a method to determine its output voltage V_WCT over the conductance range, S = 0002-20mS, a range not easily attained by the conventional calibration method. The relationship between V_WCTand S is found to transition from being non-linear to linear at S ∼ 1 mS, which equates to the conductance of a 300 μm thick 30 Ω-cm silicon wafer. For samples whose dark conductance is lower than the transition conductance, the non-linear relationship between V_WCT and S must be taken into account to prevent a gross underestimation of recombination rate. The method we describe can equally well be used to investigate the calibration of other photoconductance instruments. It involves the simultaneous measurement of a device's electrical conductance and the instrument's output voltage under a range of steady-state illumination intensities.