P. G. Evans, R. J. Hamers, T. R. Kuech, P. Gopalan (DMR #0520527)
Evans, Hamers, Kuech, and Gopalan have found that the transport of electrons between ZnO and organic electronic materials can be observed directly using device-like structures that electronically sense the charge transferred to the inorganic semiconductor. The team has developed a new technique in which thin film transistors are used to analyze scientifically and technologically relevant organic/inorganic semiconductor interfaces. This represents an important improvement over studies of charge transfer in photovoltaic and light emitting diode structures because the impact of charge transfer in those structures is convoluted with the transport through a number of layers in series, for example at potentially highly resistive contacts far from the charge-transfer interface.
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They have already demonstrated that FETs can be used to study the effects of functional self-assembled monolayers on the charge transfer characteristics of these semiconductor interfaces.[i] Evans, Hamers, and Gopalan completed in 2008 a study of photoinduced charge transfer to a functionalized fullerene monolayer, which provides electron-accepting traps at the gate interface of this modified pentacene transistor.[ii]
The carrier accumulation layer of FETs resides within a few nanometers of the gate dielectric and is uniquely sensitive to changes in morphology, the presence of dipolar molecules, and the effects of local charge trapping. The FET in the image above incorporates ZnO quantum dots into organic FETs to create a probe for charge transfer dynamics between pentacene and ZnO. ZnO nanoparticles at the pentacene/gate dielectric interface produce large changes in transistor device characteristics, including a large shift in the threshold voltage of the device. By fabricating and analyzing such devices we gain insight into the mechanisms of charge transfer at the interface between pentacene and ZnO. The FET structure is inherently versatile, and the team is in the process of using this structure to understand how the functional SAMs developed in the University of Wisconsin MRSEC affect interfacial charge transfer kinetics.
Electronically and optically reconfigurable organic/inorganic interfaces: Reconfigurable electronic interfaces have the potential to exploit the interfacial control of charge transport in sensors, actuators, and other smart devices. The team has produced reversibly reconfigurable electronic interfaces by designing self-assembled monolayers that exhibit a large change in dipole moment when they are incorporated at the gate insulator/organic semiconductor interface. Gopalan, Hamers, and Evans have shown that molecular reconfiguration can be detected through changes in electronic transport characteristics of FETs. Developing the charge-separated state leads to changes in current by a factor of more than 105 at low gate bias voltages.[iii] Incorporating reconfigurable molecular layers at electronic interfaces, as we have shown here, has the potential to be the basis for sensors and active layers that rely on subtle changes in a small number of molecules.
Dynamics of charge transfer at interfaces: IRG2 has shown this year that the FET-based characterization strategy can be used to experimentally probe the time constants for interfacial charge transfer (Evans, Gopalan, Kuech). The time constants determined in this way are exactly analogous to those determined in capacitance-voltage spectroscopy to probe bulk electronic defects in organic semiconductors.[iv] The interface specificity of this approach, and the potential to combine it with the ZnO transistor structures shown in Figure 11, promise to allow the long-time-constant processes associated with back-transfer of excitations in solar applications to be probed as a function of functionalization of the ZnO.
References
[i] B. Park, I. In, P. Gopalan, P. G. Evans, S. King, and P. F. Lyman, Appl. Phys. Lett. 92, 133302 (2008).
[ii] B. Park, P. Paoprasert, I. In, J. Zwickey, P. E. Colavita, R. J. Hamers, P. Gopalan, P. G. Evans, Adv. Mater. 19, 4353 (2007).
[iii] P. Paoprasert, B. Park, H. Kim, P. Colavita, R. J. Hamers, P. G. Evans, and P. Gopalan, Adv. Mater. 20, 4180 (2008).
[iv] B. Park, P. Paoprasert, P. Gopalan, T. F. Kuech, and P. G. Evans, Appl. Phys. Lett., Appl. Phys. Lett. 94, 073302 (2009).
