Optical and electrical analysis in various organic-inorganic stacked structures

Abstract

Understanding heterojunctions in layered organic semiconductor devices is crucial in achieving efficient optoelectronic and advanced organic electronic devices. In this paper, we investigated the formation of charge transfer complex (CTC) and the generation of free carriers at the interface of the transition metal oxide (TMO)/organic semiconductor (OS) bi-layer structures. First, the relationship between the CTC and conductivity is analyzed for bi-layers composed of molybdenum oxide (MoO 3 ) and OSs with different highest occupied molecular orbital (HOMO) levels, respectively. Same analysis is done for bi-layers involving N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and various TMOs with different work-functions, respectively. It is investigated that CTC is formed at the interface of the TMO/organic bi-layer structure and the conductivity in the TMO is increased. The absorbance intensities of the CTCs at the heterojunctions are proposed to depend on the density of the OS radicals formed from the charge transfer process. It is demonstrated that conductivity is low at bi-layer interfaces with high density of CTC. This is because the CTC aligned at the interface acts as a hindrance to electron mobility. Consequently, it is demonstrated that MoO 3 /2TNATA and MoO 3 /NPB exhibited low conductivity despite the formation of more CTCs than MoO 3 /CBP. We propose that the CTCs at the OS/TMO heterojunction with low charge separation efficiency, causes negative charges to align at the interface. This negatively charged electric field at interface act as resistance to the mobility of electrons inside the TMO layer. The investigation is key in achieving efficient organic electronic devices and stacked organic semiconductor functional structures. • Heterojunctions with organic hosts and transition metal oxides are investigated. • The formation of CTC and free carriers at the heterojunctions are analyzed. • Conductivity is low at the heterojunctions with high CTCs. • CTCs with low charge separation efficiency cause resistance to electron flow.