The origin of changes in electrical properties of organic films fabricated at various vacuum-deposition rates
Introduction
Organic optoelectronic technology has drawn a huge potential, during last decades, in both science and industry by taking advantage of unlimited organic molecular design, low-cost processing, flexibility, and lightweight [[1], [2], [3]]. A comprehensive understanding of the electrical properties of organic thin films can be helpful for establishing fundamental science and leads to fabricate high performance devices [4]. The electrical properties of organic thin films can be improved by controlling their molecular orientation and aggregation structures through optimization of deposition conditions such as substrate temperature and deposition rate during film fabrication. The effects of deposition rates on electrical and optical characteristics of organic thin films were investigated by many researchers [1,[5], [6], [7], [8], [9]]. For example, it has been demonstrated that hole and electron carrier mobilities of organic thin films are changed by deposition rates [6,[10], [11], [12]]. However, the physical origins of these effects are still not fully understood.
In the present paper, the physical origins of electrical properties changed by vacuum deposition rates for films of N,N՛-diphenyl-N,N՛-bis(1-naphthyl)-1,1՛-biphenyl-4,4՛-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminium (Alq3) were investigated. Currents of hole-only devices (HODs) were clearly enhanced by increasing deposition rates of α-NPD. In sharp contrast to HODs, currents of electron-only devices (EODs) decreased when an Alq3 layer was fabricated at a high deposition rate. To make this opposite behavior clear, we studied charge carrier trap densities and depths, surface morphologies, molecular orientations, and densities of α-NPD and Alq3 films fabricated at different deposition rates. We found that the increased and decreased electrical properties are probably because of a horizontal orientation of α-NPD molecules relative to a substrate plane and increased surface roughness of Alq3 films, respectively. Finally, we demonstrated an enhancement of the operational stability when HODs were fabricated at a high deposition rate.
Section snippets
Experimental details
Glass substrates coated with an indium tin oxide (ITO) layer (100 nm) and silicon substrates were cleaned using ultrasonic bath in pure water, acetone, and isopropanol, respectively. To complete the cleaning process, ultraviolet-ozone treatment was carried out on the substrates.
For measuring the electrical properties and stability, HODs containing an α-NPD layer and EODs containing an Alq3 layer were fabricated. The structures of the HOD and EOD samples were glass substrate/ITO anode (100 nm)/
Results and discussion
The J-V characteristics of the α-NPD HODs and Alq3 EODs are shown in Fig. 1(a) and b. No electroluminescence was observed from these devices during the J-V measurements, indicating completely unipolar current flow. When the deposition rates were increased from 0.01 to 1, the current densities of HODs increased while the current densities of EODs decreased, which is in agreement with the results reported in Refs. [6,23]. In addition, in order to examine the reproducibility of our
Conclusion
We found that the electrical properties of α-NPD and Alq3 films are influenced by their molecular orientation and surface roughness. By measuring the electrical, physical and structural properties of films deposited at different vacuum-deposition rates, the electrical properties of α-NPD HODs were enhanced at higher deposition rates due to the horizontal orientation of α-NPD molecules which can improve the overlap of π orbitals between neighbouring molecules. In the case of Alq3, high surface
Acknowledgement
Authors would like to thank Ministry of Science, Research and Technology of IR of Iran and Shahid Behehsti University for providing facilities associated with this research. Also supports from Kyushu University of Japan and OPERA members is appreciated.
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