UV Irradiation Effects on Charge Transport Characteristics in n-Doped POPy2

Abstract

Organic electronics, specifically OLEDs and organic solar cells, are widely studied because of their low-cost fabrication and flexible substrates. Doping is vital to the success of these devices and is used to improve charge carrier transport in a material and lower the charge injection barrier from metal contacts. However, n-doping of materials with low electron affinity still poses a challenge. Building on previous work, this thesis focuses on charge transport in thin films of phenyldi(pyren-2-yl) phosphine oxide (POPy2) n-doped with (pentamethylcyclopentadienyl) (1,3,4-trimethylbenzene) ruthenium dimer ([RuCp*Mes]2). Because POPy2 has a low electron affinity (EA = 2.2 eV), photoactivation by a UV LED was necessary to facilitate charge transfer from the dopant to the host. Using variable temperature current voltage (VTIV) analysis, the conductivity and activation energy of carrier hopping transport was measured in the dark and under UV irradiation. It was found that transport under UV illumination had an activation energy that was much lower relative to transport in the dark. There was also an observed temperature dependence of the measured activation energy under UV exposure, as the activation energy decreased significantly as temperature increased to 300 K. Further understanding of this phenomenon is necessary and could be explored by investigating the dependence of charge carrier mobilities on temperature and probing higher temperatures. Because this particular host:dopant system was found to be a viable candidate for high-energy emission OLEDs and other optoelectronic devices, it is important to further understand the charge transport properties and characteristics that are vital for the success of optoelectronic devices.