An analytic expression for the tempoerature coefficient of the open-circuit voltage has been derived in general, which may be applied to different kinds of p-n junction solar cells. Based on the dominance of the saturation dark current density, the simplified expression have also been developed and justified by experimental measurements. It has been shown that the negative temperature coefficient of the open-circuit voltage is decreased with the increasing light level and the interaction between minority carriers and the high-low junction. Hence, the temperature coefficient of the open circuit voltage is good measure for providing a guide to test the effectivness of a high-low junction in a back-surface-field (BSF) solar cell. Moreover, based on the measured temperature coefficient of the open-circuit voltage from the BSF solar cells fabricated on thin epitaxial substrate, a new method for measuring the effective energy-gap narrowing in the highly doped emitter has been proposed and studied. It has shown that the effective energy-gap narrowing measured is in good agreement with the eempirical expression proposed by Slotboom.