Heterojunctions are inherent in and essential to all molecular electronic devices. In organic solar cells,
in particular, heterojunctions play a defining role in all of the major electrical processes and particularly
in stability. The p-type organic molecule diindenoperylene (DIP) is an interesting photoactive organic
molecule that forms inherently unstable interfaces within organic solar cell devices. Using scanning probe
microscopies, supported by x-ray scattering, we examined the stability of inorganic/organic interfaces at both
charge extraction electrodes. The DIP morphology can be stabilized by: 1. roughening the ITO surface,
which disrupts the molecular packing, 2. using an interlayer such as PEDOT:PSS, which modifies the surface
energy, or 3. depositing a dielectric layer, LiF, which pins the DIP grain boundaries. Any combination of
the approaches would lead to significant improvements in solar cell lifetime.
For diode device structures, the vertical transport of charge carriers across the device is expected to dominate its' performance. In planar organic molecules, where the π-stacking direction is normal to the main body of the molecule, charge transport is highly anisotropic and related to the orientation of the molecules relative to the substrate. On ITO, we have confirmed with AFM and X-ray scattering (reflectivity and grazing incidence X-ray diffraction ) that PTCDI-C8, an electron accepting photoactive molecule, forms large crystalline domains consisting of tilted, up-right standing molecules. Similarly to that observed on SiO2 substrates, the domain size can be tuned by adjusting the substrate temperature during growth. In such a configuration, the charge transport is dominated by carrier movement parallel to
the substrate, along the π-stacking direction, with vertical transport limited by hopping between layers. We observed that
in diodes using such films, the developed charge density increased with the lateral island size. In the vertical direction, charge transport is best described by a thermally activated hopping mechanism. This type of behaviour can have significant implications for nanostructured bulk heterojunctions of such films, when combined with electron donors such as pentacene or diindenoperylene, to be used in small molecule solar cells.
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