Björn Kobin

Efficient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structures energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structures radiative emission yield sevenfold, validating the relevance of our approach.

Sterically Crowding the Bridge of Dithienylcyclopentenes for Enhanced Photoswitching Performance

Better switching: The introduction of bulky substituents into the bridge moiety of dithienylethenes led to derivatives exhibiting high photocyclization quantum yields. This novel and versatile form of substitution facilitated tuning of the switching performance without compromising on the optical and redox properties of the ring-open and ring-closed forms (see scheme).

Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers.

We used aromatic phosphonates with substituted phenyl rings with different molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface in order to engineer the energy-level alignment at hybrid inorganic/organic semiconductor interfaces, with an oligophenylene as organic component. The work function of ZnO was tuned over a wide range of more than 1.7 eV by different SAMs. The difference in the morphology and polarity of the SAM-modified ZnO surfaces led to different oligophenylene orientation, which resulted in an orientation-dependent ionization energy that varied by 0.7 eV. The interplay of SAM-induced work function modification and oligophenylene orientation changes allowed tuning of the offsets between the molecular frontier energy levels and the semiconductor band edges over a wide range. Our results demonstrate the versatile use of appropriate SAMs to tune the energy levels of ZnO-based hybrid semiconductor heterojunctions, which is important to optimize its function, e.g., targeting either interfacial energy- or charge-transfer.

Cascade energy transfer versus charge separation in ladder-type oligo(p-phenylene)/ZnO hybrid structures for light-emitting applications

Usability of inorganic/organic semiconductor hybrid structures for light-emitting applications can be intrinsically limited by an unfavorable interfacial energy level alignment causing charge separation and nonradiative deactivation. Introducing cascaded energy transfer funneling away the excitation energy from the interface by transfer to a secondary acceptor molecule enables us to overcome this issue. We demonstrate a substantial recovery of the light output along with high inorganic-to-organic exciton conversion rates up to room temperature.

Making Nonsymmetrical Bricks: Synthesis of Insoluble Dipolar Sexiphenyls

A versatile synthesis of nonsymmetrical, terminally substituted p-sexiphenyl (6P) derivatives has been developed. The synthesis makes use of a nonsymmetrical starting material as well as modular functionalization using Suzuki cross-coupling to yield a soluble precursor, which finally is converted to the insoluble target 6P derivatives. These derivatives display similar electronic and optical properties to the parent 6P, yet the permanent dipole along their molecular axis allows for tuning of their self-assembly on various substrate surfaces.

Photochemical Degradation of Various Bridge-Substituted Fluorene-Based Materials

Photochemical degradation is an important issue to be overcome in advancing the lifetime of fluorene-containing conjugated polymers. In order to optimize the inertness of the materials, a quantitative measure for the efficiency of degradation is needed. Here, we introduce a method to measure a relative quantum yield of the photochemical degradation by monitoring the kinetics of the process by means of UV/vis spectroscopy and liquid chromatography (LC) techniques. This method is employed to a set of differently substituted 2,7-diphenylfluorenes, serving as model compounds for polyfluorene materials. Our measurements show that the quantum yield changes by orders of magnitude upon varying the bridge substituents and that altered kinetics indicate changing degradation mechanisms.

Hybrid polaritons in a resonant inorganic/organic semiconductor microcavity

We demonstrated the strong coupling regime in a hybrid inorganic-organic microcavity consisting of (Zn,Mg)O quantum wells and ladder-type oligo(p-phenylene) molecules embedded in a polymer matrix. A Fabry-Perot cavity is formed by an epitaxially grown lower ZnMgO Bragg reflector and a dielectric mirror deposited atop of the organic layer. A clear anticrossing behavior of the polariton branches related to the Wannier-Mott and Frenkel excitons, and the cavity photon mode with a Rabi-splitting reaching 50 meV, is clearly identified by angular-dependent reflectivity measurements at low temperature. By tailoring the structural design, an equal mixing with weights of about 0.3 for all three resonances is achieved for the middle polariton branch at an incidence angle of about 35°.

Strong Coupling and Laser Action of Ladder-Type Oligo(p-phenylene)s in a Microcavity

We investigate the coupling of ladder-type quarterphenyl to the photon modes of a dielectric ZrOx /SiOx microcavity at ultraviolet wavelengths. For a relatively long cavity (≈10 μm) with high-reflectivity mirrors (0.998), optically pumped laser action is demonstrated in the weak-coupling regime. We observe single-mode operation with a threshold of 0.4 mJ cm(-2) . Strong coupling is achieved by using a short λ/2 cavity. We find pronounced anti-crossing features of the molecular (0,0) and (0,1) vibronic transitions and the cavity mode in angle-dependent reflectivity measurements providing Rabi splittings of (90±10) meV. All these features occur spectrally resonant to the exciton transition of ZnO demonstrating the potential of ladder-type oligo(p-phenylene)s for the construction of inorganic/organic hybrid microcavities.

ZnO/organic interfaces: formation of hybrid excitations and relation to charge separation(Conference Presentation)

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.

Efficient light emission from hybrid inorganic/organic semiconductor structures by energy level optimization

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 structures 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 structures radiative emission yield sevenfold.