Organic semiconductors (OSCs) have recently become viable for a wide range of electronic devices, some of which have already been commercialized. With the mechanical flexibility of organic materials and promising performance of organic field effect transistors (OFETs) and organic bulk heterojunction devices, OSCs have been demonstrated in applications such as radio frequency identification tags, flexible displays, and photovoltaic cells.
Transient phenomena play decisive roles in the performance of electronic devices and OFETs in particular. The dynamics of the establishment and depletion of the conducting channel in OFETs are investigated theoretically. The device structures explored resemble typical organic thin-film transistors with one of the channel contacts removed. By calculating the displacement current associated with charging and discharging of the channel in these capacitors, transient effects on the carrier transport in OSCs may be studied. In terms of the relevant models it is shown that the non-linearity of the process plays a key role. The non-linearity arises in the simplest case from the fact that channel resistance varies during the charging and discharging phases. Traps can be introduced into the models and their effects examined in some detail. When carriers are injected into the device, a conducting channel is established with traps that are initially empty. Gradual filling of the traps then modifies the transport characteristics of the injected charge carriers. In contrast, dc measurements as they are typically performed to characterize the transport properties of organic semiconductor channels investigate a steady state with traps partially filled. Numerical and approximate analytical models of the formation of the conducting channel and the resulting displacement currents are presented.
For the process of transient carrier extraction, it is shown that if the channel capacitance is partially or completely discharged through the channel resistance, qualitatively different time dependences of the displacement current may be obtained. Depending on the final state of the capacitor, either fully discharged or remaining partially charged, the displacement current in the long time limit shows power law or exponential dependence on time. The salient effect of including fast traps in this model is to change the apparent exponent in the resulting approximate power law for the case of full depletion. The exponent of the power law decreases in magnitude as the ratio of the emission to capture rates decreases. In contrast, the effects of slow traps are quite different. If significant, slow traps can give rise to a slowly decaying displacement current.
In organic bulk heterojunction photovoltaic cells, large interfacial areas that impact the efficiency of photo-generation of charge carriers are of critical importance. Carrier recombination at the interface is studied on the basis of two bilayer structures with interfaces perpendicular and parallel to the current direction, which idealize the island-like morphology of mixed materials. Overall recombination currents at the interface are found to depend strongly on the interface orientation and on parameters that control the time scales of interfacial recombination and carrier transport.
University of Minnesota Ph.D. dissertation. May 2012. Major: Electrical Engineering. Advisor: Dr. P. Paul Ruden. 1 computer file (PDF); ix, 121 pages, appendix A.
Modeling of electronic properties in organic semiconductor device structures..
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