This project will exploit the ability to precisely control coupling between QDs and OSCs by virtue of the unique tunability of their relative energy levels. In contrast to bulk inorganic semiconductors, the energy levels in semiconductor QDs are not fixed, but depend on the diameter of the nanocrystal. Owing to the precision of state-of-the-art QD synthetic procedures, an almost continuous shift of the QD energy levels is achieved, which will be used for a targeted fine-tuning of coupling between the inorganic and organic moieties. The OSC component will not be merely mixed with the QDs, but is chosen such that it contains suitable functional groups to bind covalently to the QD surface. This will circumvent the problem of phase segregation and lead to robust COINs with a periodically alternating arrangement of the QD and OSC component. From an electronic perspective, the QDs provide the active centers in which charge-carrier generation, separation and recombination take place. The OSC acts as mediator and selective filter for carrier transport between adjacent QDs: Selective hole transport occurs if the OSC HOMO is in resonant alignment with the 1Sh state of the QD, whereas electron transport is achieved upon alignment of the 1Se state of the QD with the LUMO.
Our aim is to establish this new scheme of energy level tailoring as a versatile tool to study fundamental aspects of charge carrier transport and to provide a different approach to coupled QD-based optoelectronic devices. We will develop independently suitable QD/OSC combinations for hole-only or electron-only conductive layers and study the effects of carrier trapping or delocalization. Due to the wide interparticle spacings and structural rigidity imposed by the relatively large ligands utilized by us, the investigated COINs are expected to display a large degree of structural periodicity, long-range order and the formation of mesocrystalline domains. This important property, which is generally absent in QD films cross-linked with conventional small molecules, bears the potential for the beneficial formation of carrier minibands. Since the degree of order of this complex superstructure is expected to be crucial for the functional properties of the COINs, it will be studied by us with a variety of advanced scattering techniques.
A potential drawback in QD optoelectronics are incompletely passivated surfaces, unsaturated dangling bonds and the occurrence of large trap state densities, which limit the efficiency of carrier transport. These issues will be closely monitored by us for each QD-OSC combination by means of field-effect transistor measurements in the subthreshold regime following a previously reported procedure. Additionally, we will use transient electrical methods (displacement current measurements and impedance spectroscopy) as well as charge modulation spectroscopy to study charge carrier dynamics and energetics. The impact of varying interparticle spacings, QD packing types and directionality of QD assembly imposed by different OSCs onto carrier transport and trapping will be correlated to structural information obtained by X-ray scattering experiments. As a proof of principle, we will build a functional device with the developed COINs. By combining an electron-conducting with a hole-conducting COIN in a two-layer structure, we will fabricate a diode and investigate its electrical characteristics.
April 2016 - March 2019