Transport Properties Of Charge Carriers In Mixed Organic Semiconductors

The interest in photoelectric devices based on disordered organic semiconductors is rapidly increasing, which is excited by the extremely promising applications of organic light-emitting diodes, organic filed-effect transistors, and organic photovoltaic devices. Unfortunately, as one of the most promising breakthroughs optoelectronic devices based on mixed organic semiconductors have not got due attention from the scientists. Therefore, our research work on the transportation properties of charge carriers in mixed materials has great significance for the application of the mixed organic semiconductors to optoelectronic devices.The charge-carrier mobility of mixed films of Alq3/MEH-PPV, Alq3/PVK and Alq3/CdSeS quantum dots with various weight fractions was studied by the TOF technique. A sensitive dependence of the charge carrier mobility on the electric field and dopants'weight fraction was observed. The observed field dependence of mobility is attributed to the increased positional disorder and the changed LUMO and HOMO levels, both of which result form the mixing.On the one hand, Monte Carlo simulations were performed thereby explaining the influence of mixing on the hopping transport mechanisms in disordered materials by assuming the mixed materials as a whole; on the other hand, Monte Carlo simulations were performed as well so that we can understand how the electron transports in Alq3/CdSeS films by treating the two components differently. It is found that by adjusting the simulation parameters the simulation results can give solid supports to our experimental data not only qualitatively but also quantitatively.By comparing the simulation results with the experimental data, we concluded the transport mechanisms of charge carriers in disordered materials: on condition that the charge carrier transports in pure organic materials it obeys the hopping transport law and the energetical disorder and the positional disorder play the leading role in the transport mechanisms. However, in terms of the mixed materials, the electron transfer from QDs to organic molecule is an indispensable complementarity to the hopping transport between the two components. As the consequence of the big energy gap between the QDs and the organic materials, the role of the disorder becomes negligible. Under the circumstances, two parameters,ΓandΧ(throughout the simulation we assumed thatΓhas the same value asΧ), which measure the electronic wave function overlap and the dependence degree of the electron transfer on the distance between the acceptor and the donor respectively, determine the electron's transport processes. Therefore, we can attribute the sensitive dependence of the electron mobility on the QDs'weight fraction to the changedΓandΧresulting from mixing the QDs to organic materials.