ZnO is attracting significant interest as a candidate for hybrid photovoltaic and light-emitting devices. We studied electronic coupling at interfaces of ZnO with conjugated organic molecules like ladder-type oligo(phenylenes) (LOP) and NTCDA whose fundamental optical excitations are resonant to the ZnO band gap as well as with polymers employing a combination of time-resolved techniques as well as in situ differential reflectance and photoemission spectroscopy.
Our studies provide evidence for the formation of hybrid charge transfer excitations (HCTE) across (Zn,Mg)O/organic interfaces. We show that by interfacial design the properties of these HCTE can be tuned and by that the charge separation process. The impact of the HCTE on photovoltaic parameters like the open circuit voltage and short circuit current is exemplarily demonstrated in (Zn,Mg)O/P3HT diodes.
Furthermore, we show that by proper alignment of the frontier molecular orbitals with the semiconductor valence and conduction band edges, exciton dissociation at the interface can be switched off while exciton transfer efficiencies of up to 80 % are maintained. Thus, efficient conversion of ZnO excitons into highly emissive excitons of the organic (LOP) layer is achieved which is essential for the realization of hybrid light-emitting diodes.
Innovative hybrid inorganic/organic structures (HIOS) should implement exciton creation by electrical injection in inorganic semiconductors followed by resonant energy transfer and light emission from the organic semiconductor. An inherent obstacle of such designs is the typically unfavorable energy level alignment at HIOS interfaces, which assists in exciton separation thus quenching light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels: ZnO and a tailored ladder-type oligophenylene. Using an organometallic donor interlayer the ZnO work function is substantially lowered eliminating the ZnO - L4P-sp3 interfacial energy level offsets enhancing the hybrid structure's radiative emission yield sevenfold.
Molecular-beam epitaxial growth far from thermal equilibrium allows us to overcome the standard solubility limit
and to alloy ZnO with CdO in strict wurtzite phase up to mole fractions of several 10%. In this way, a band-gap
range extending from 3.3 eV down to 2.3 eV can be covered. Strong improvement of the crystalline quality
indicated by a rocking curve width of only 45 arc sec is achieved when growing the ternary on ZnO substrates.
Despite very low growth temperatures (~150 °C), layer-by-layer growth indicated and controlled by RHEED
oscillations is accomplished. This enables us the fabrication of atomically smooth heterointerfaces and well-defined quantum well structures exhibiting prominent band-gap related light emission in the whole composition
range. Post-growth annealing increases the radiative efficiency up to two orders of magnitude and demonstrates
thermal stability of the structures with respect to phase separation even up to temperatures of about 500°C.
Low-energy shifts of the photoluminescence features reaching the order of 1 eV as well as a dramatic increase of
the lifetime from the sub-ns to the 100-μs time-scale uncover the presence of huge polarization-induced electric
fields of some 108 V/m in ZnCdO/ZnO single quantum well structures. Carrier injection by moderate optical
excitation in the 10 kW/cm2 screens these fields and recovers practically the bare quantum-confined energy
transitions. On appropriately designed structures, laser action from the UV down to the green wavelength
range is observed under optical pumping. The threshold at low temperature is only 60 kW/cm2 and increases
only moderately up to room temperatures. All these findings make ZnO-based heterostructures a promising
alternative to group-III-nitrides for opto-electronic applications in the short-wavelength range.