Investigation of work function and chemical composition of thin films of borides and nitrides

In the last decades, an increasing interest of the scientific community has been addressed to the renewable energy field [1]. In particular, the solar radiation is considered as an abundant, cheap, clean and sustainable energy source. In this context, promising solid state devices for the conversion of primary energy into electricity like thermionicphotovoltaic (TIPV) converters have been developed. They consist of three elements: electron emitter, collector and photovoltaic (PV) cell [2]. The main characteristics of the emitter are the enhanced capability to emit electrons and the selective thermal emittance, which should be able to satisfy the specific requirements of the PV cell in terms of radiation absorption. Basically, an ideal cathode must be characterized by a low work function, but, for a thermionic energy converter, it has to be higher than that of the anode. In the present work, thin films of different borides and nitrides deposited by pulsed laser deposition (PLD) are proposed in order to improve the performance of TIPV devices. The emitting layers of lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) have been well established, therefore they are considered good candidates in terms of low work function and high melting point. As it regards nitrides, amorphous carbon nitride (CNx) and hydrogenated aluminium nitride (AlN:H) have been individuated as valid alternatives to borides, because they are characterized by an even lower work function. The films of these materials with a thickness up to 200 nm have been grown on different substrates by PLD. Because the properties of borides and nitrides are strongly dependent on the content of boron or nitrogen, their chemical composition and work function were investigated by XPS and UPS techniques. These analyses were carried out by using an Escalab 250 Xi, equipped with a monochromatized Al Kα and ultraviolet He sources. The work function was determined from the cutoff in the He I spectra. Finally, the homogeneity of the chemical composition along the thickness of the samples has been investigated by XPS depth profiling carried out by means of Ar+ ion sputtering. [1] Fatih Bayrak, Nidal AbuHamdeh, Khaled A. Alnefaie, Hakan F. Öztop, Renewable and Sustainable Energy Reviews, 74 (2017) 755. [2] A. Datas, Appl. Phys. Letters, 108 (2016) 143503.