Charge transport properties of organic semiconductors and their relevance in organic optoelectronic devices
Date Issued
2010
Date
2010
Author(s)
Liu, Shun-Wei
Abstract
Organic thin-films have been used as active materials in optoelectronic devices, such as organic light-emitting diodes (OLEDs) and organic solar cells. In this thesis, we study the charge transporting properties or carrier mobilities of pure or mixed organic thin films, polymer blended films, and their influences on the device performances.
The first part of this thesis presents the time-of-flight (TOF) photocurrent study of drift mobilities and transport properties of holes and electrons in the mixed organic semiconductors. Two kinds of mixed layer systems, i.e. planar vs. spherical (NPB:Alq3) and planar vs. tetrahedron (NPB:Bebq2) molecular packings, are studied by a Poole-Frenkel Model and the analysis of disorder parameters. This study is to obtain insightful information about the carrier transport mechanism in the mixed organic solids.
In the second part of this thesis, we study the thin film growth of a tetrahedron Bebq2 molecular material. Charge transporting properties and carrier mobilities of Bebq2 thin film are extracted and deciphered by TOF technique. Due to the tetrahedron molecular structure, we have demonstrated a simple fabrication control, the deposition rate of Bebq2, to increase the current efficiency and the operation lifetime of OLEDs, of which both light-emitting and electron-transport layers are Bebq2 thin films.
Organic solar cell based on the blends of P3HT and PCBM bulk heterojunction is the other chosen optoelectronic device in this study. The charge transporting properties and carrier mobilities of P3HT/PCBM blended films are studied in the third part of this thesis. From our experimental results, the initial increase of the blending concentration of PCBM induces P3HT aggregation in the formation of percolation paths, which leads to an enhancement of both hole and electron mobilities and the output photocurrent of organic solar cells. However, when the blending concentration of PCBM above certain level, the resulting PCBM clusters may dominate the charge transporting in hole and electron percolation paths, impair the efficiency of exciton dissociation (charge transfer) in such donor-acceptor bulk heterojunction. In addition to the charge carrier mobilities of the thin films determined by TOF technique, this is indirectly evidenced by the smooth surface morphology of the thin film (by AFM surface images) and the low photocurrent output from the solar cell devices.
Subjects
Organic light-emitting diode
organic solar cells
carrier mobility
charge transport properties
time-of-flight
mixed layer
Type
thesis
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