Unsteady-state lock-in thermography - Application to shunts in solar cells

This paper presents the fast implementation of Lock-in Thermography (LIT) before reaching steady-state conditions. The errors caused by the initial temperature drift are compensated by a simple correction formula based on the measurement of the temperature drift image. The effect of this correction has been studied on the in-phase, quadrature, amplitude and the phase signals of lock-in thermography. Simulations and experiments have been performed for this study while considering a specific application of LIT for the investigation of solar cells. This study shows that in our case the error caused by the temperature drift is basically a baseline shift in the in-phase and the quadrature signal, and does not significantly affect the relative shape and spatial resolution of these signals. However, for the amplitude and phase signal this correction is more significant and affects also the relative shape of the signals. Experimental results validate the correction method and its effectiveness in eliminating errors from LIT data measured before thermal equilibrium has been established.