Effects of Transition Metal Oxide Doping on Delocalized Charge Density and Charge Energetics at Organic Donor/acceptor Interfaces

Several recent reports in Science, Nature and its sister journals showed that carrierdissociation in organic solar cell is mainly due to delocalized charge-transfer (CT) stateswhich in turn depends strongly on the materials’ dielectric constants. It is thus of interest toexplore possible ways for tuning dielectric properties of organic donor/acceptor (D/A)interfaces. The principal investigator has recently reported infrared active solar cells andphotodetectors obtained by doping wide bandgap transition metal oxides (TMO) into smallmolecule organic materials to form CT complexes (CTCs). These CTCs not only provide anew strategy for fabricating infrared devices using wide bandgap materials, they arecharacterized by notable dipole moments that can also substantially modify the dielectricproperties of the small molecule semiconductors. It thus provides a possible approach fortuning charge delocalization at their D/A interfaces.This proposal aims to control the delocalized charge population at D/A junctions ofsmall molecule semiconductors by TMO doping. Charge energetics and dynamics at theseD/A junctions will mainly be characterized with charge-modulated electroabsorptionspectroscopy (CMEAS) and ultraviolet/ x-ray photoemission spectroscopies (UPS/XPS).CMEAS is a recently developed technique, modified from the conventional electroabsorptionspectroscopy, which can study CT states at the D/A interfaces under excitation.Complementary studies in the ground state would be carried out using conventionalUPS/XPS. Dielectric properties of selected small molecule semiconductors (e.g. C60,pentacene, rubrene) doped with TMOs (e.g. MoO3, ReO3, V2O5) would be characterized byboth standard impedance spectroscopy (IS) and controlled intensity modulated photocurrentspectroscopy (CIMPS), which could monitor the impedance/ dielectric change uponirradiation with different wavelengths and/or intensities. Selected TMO:organic CTCs wouldalso be examined with other techniques including SEM, AFM, PL, I-V measurements etc forstructural, optical properties and devices characterization.The uniqueness of this proposal is the combined use of the conventional UPS/XPS andthe newly developed CMEAS techniques. These techniques are complementary to each otherand would provide charge energetics of CT states at organic D/A interfaces in both theexcited and the ground states. Such combined information at both the ground and the excitedstates is seldom available from a single laboratory. The proposed works are expected to givenew insights on approaches for controlling the interface energetics, charge delocalization andthus carrier extraction efficiency at organic D/A interfaces.