Development of a Physical Model of Thermovoltaic Effects in the Thin Films of Zinc Oxide Doped with Transition Metals

Abstract

A model of the thermovoltaic effect emergence in ZnO/ZnO (Me = Cu, Fe), sandwich structures has been developed in the article. The samples were made by the sol-gel method. When they were uniformly heated in a laboratory furnace in the temperature range of 200–300 °C, there an electromotive force (EMF) of −7~10 mV, not associated with the Seebeck effect, emerged. The developed physical mechanisms of the effect emergence consist of the following well-known fact: iron and copper coexist in zinc oxide in two states, namely, Fe2+ and Fe3+ (donor), and Cu2+ and Cu+ (acceptor). During the heating of the ZnO/ZnO–Me system, the concentration of charge carriers in the layers will increase, while in the upper layer its value will be larger because of the presence of electrically active impurities. At room temperatures, Coulomb forces retain an electron that is located on the Fe2+ ion, as well as a hole on Cu2+ ion, and the main states undergo ionization. However, as the temperature increases, the carrier concentration can reach a critical level, when they can screen the ion charge (the Debye screening radius decreases to the Bohr radius of the impurity). In this case, an abrupt collective endothermic process of ionization of multivalent impurities takes place, accompanied by the appearance of a concentration gradient of free carriers in the sample, and accordingly, the emergence of an electromotive force. Quantitative calculations of the critical temperature, at which the onset of EMF generation is observed, performed within the framework of the developed models.