Iype, Mercy, et al. “Temperature Dependence of the Activation Energy of Thin Films of Zinc Sulphide, Copper Phthalocyanine and Multi- Layer Zinc Sulphide-Copper Phthalocyanine ”. Asian Journal of Chemistry, vol. 14, no. 3, Mar. 2010, pp. 1667-72, https://asianpubs.org/index.php/ajchem/article/view/22458.