during the elevated temperatures of crystal synthesis and annealing), our model predicts a bandgap of 1.50 eV at the experimental annealing temperature of 573 K used during the fabrication of thin film devices (Figure ), compared to the experimentally measured optical bandgap of 1.41 eV. Comparison to the optical bandgap of 1.5 eV measured by Starosta et al. using photoconductivity measurements is more challenging due to their chosen synthesis method. Oriented ingots were obtained using a modified Bridgman method in which an ampoule, heated to 773.15 K, was pulled across a 400 K/cm temperature gradient to form ingots of length 50-60 mm. Our model at 570 K agrees best with the optical bandgap measurement of 1.5 eV, deviating between -0.05 eV and +0.2 eV across the temperature gradient.