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A) Single Molecule Imaging and Spectroscopy of Organic Semiconductors for Electronics

The vision for organic electronics is to use a detailed knowledge of the structure of molecular aggregates to design new high-performance devices.  For example, the efficiency of organic solar may depend sensitively on the precise geometry and relative orientation of individual donor and acceptor molecules in an active layer.  The Dougherty research group seeks to discover new structure-function relationships by observing the spatial arrangement and electronic structure of organic semiconducting molecules.  The group operates a Scanning Tunneling Microscopy laboratory in ORaCEL that is capable of both visualizing single molecules and then immediately measuring their electronic properties using state of the art tunneling spectroscopy.

An example is shown in Figure 1a where a region showing both ordered and disordered domains of a-NPD (a common hole transporter in OLED’s) on a gold surface.   In this case, a statistical survey of the electronic tunneling spectra in different domains allows a molecule-by-molecule assessment of the electronic disorder in the film.  In the ordered domain a narrow distribution of highest occupied molecular orbital energies is obtained, while in the disordered domain a broader distribution is obtained.  This provides direct proof of the distribution fitted from macroscopic device models by other research groups.

Related experiments have been carried out to study the electronic impact of sliding defects in organic films.  In addition, the group has taken a single molecule spectroscopy approach to organic spintronics by characterizing the magnetic interfaces states created by organic semiconductors in contact with magnetic electrodes.

B) Transient Nano- and Femtosecond Spectroscopies

ORaCEL research also focus on carrier decay channels in organic and hybrid organic-inorganic perovskite (HOIPs) materials in ultrafast time scale. Gundogdu research group (Physics, NCSU) utilizes various spectroscopic pump-probe (see Figure) techniques such as time-resolved photoluminescence (TRPL), transient absorption spectroscopy (TAS), photoluminescence and photoluminescence excitation spectroscopy (PL and PLE), and time domain terahertz spectroscopy (TDTS) to probe charge dynamics in organic semiconductor and HOIP films.

TAS shows collective all carrier inclusive type of decay channels, while TRPL gives ultra-fast decay of radiative pathways of the carriers. TDTS gives photo-conductivity and free-carrier decay in ultra-fast scale. PL and PLE show how material emits and if the emission is charge transfer-like emission. Thus, these spectroscopic analyses complete one another and provide us the full understanding of carrier pathways in ultrafast time scales such us several picoseconds.