Scientific highlights

Molecular electrical doping of organic semiconductors
We review the broad range of phenomena observed upon molecularly doping organic semiconductors and identify two distinctly different scenarios: the pairwise formation of both organic semiconductor and dopant ions (IPA) on one hand and the emergence of ground state charge transfer complexes (CPXs) between organic semiconductor and dopant through supramolecular hybridization of their respective frontier molecular orbitals on the other hand. Evidence for the occurrence of these two scenarios is subsequently discussed on the basis of the characteristic and strikingly different signatures of the individual species involved in the respective doping processes in a variety of spectroscopic techniques. The critical importance of a statistical view of doping, rather than a bimolecular picture, is highlighted by employing numerical simulations, which reveal one of the main difference between inorganic and organic semiconductors to be their respective density of electronic states and the doping induced changes thereof. Engineering the density of states of doped organic semiconductors, the Fermi-Dirac occupation of which ultimately determines the doping efficiency, thus emerges as key challenge. As a first step, the formation of charge transfer complexes is identified as being detrimental to the doping efficiency, which suggests sterically shielding the functional core of dopant molecules as an additional design rule to complement the requirement of low ionization energies or high electron affinities in efficient n-type or p-type dopants, respectively. In an extended outlook, we finally argue that, to fully meet this challenge, an improved understanding is required of just how the admixture of dopant molecules to organic semiconductors does affect the density of states: compared to their inorganic counterparts, traps for charge carriers are omnipresent in organic semiconductors due to structural and chemical imperfections, and Coulomb attraction between ionized dopants and free charge carriers is typically stronger in organic semiconductors owing to their lower dielectric constant. 

Nevertheless, encouraging progress is being made towards developing a unifying picture that captures the entire range of doping induced phenomena, from ion-pair to complex formation, in both conjugated polymers and molecules. Once completed, such a picture will provide viable guidelines for synthetic and supramolecular chemistry that will enable further technological advances in organic and hybrid organic/inorganic devices.


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I. Salzmann et al., Acc. Chem. Res. 49 (2016) 370
DOI:10.1021/acs.accounts.5b00438

 

Doped organic semiconductors explored
Organic semiconductor materials are already being employed today in solar cells and organic LEDs (OLEDs) amongst others. Until now, however, little was known about how the doping molecules are integrated into the chemical structure of organic semiconductors. The Molecular Systems Joint Research Team of the Helmholtz-Zentrum Berlin and Humboldt-Universität zu Berlin at BESSY II have now analysed this with surprising results. The molecules are not necessarily uniformly dispersed in the host lattice, as it is usual with inorganic semiconductors, but instead form what are known as co-crystallites. The doped organic semiconductor consists of a matrix of undoped crystallites in which such “mixed crystallites” are embedded. It is this very species that takes over the role as the actually doping molecule. 

H. Méndez, et al., Nature Commun. 6 (2015) 8560
DOI: 10.1038/ncomms9560

 

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Efficient light emission from semiconductor hybrid structures by energy-level tuning
The unfavourable energy level offset in hybrid inorganic/organic structures (HIOS) is an inherent obstacle for efficiently exploiting such structures for light emitting applications. In this publication (see below), the CRC 951 shows how by introducing a [RuCp*mes]+ interlayer between ZnO and a triply spiroannulated ladder-type quarterphenyl (L4P-sp3) the energy levels of this hybrid structures can be optimized leading to an increment of the radiative emission yield by a factor of seven. Figure 1 exhibits the energy level diagrams of L4P-sp3 without interlayer (1a) and with [RuCp*mes]+ interlayer (1b) on Zn-terminated ZnO(0001). The energy values (in eV) were determined by UV-photoelectron spectroscopy and are referenced to the Fermi level. In Fig. 1a energy level alignment (ELA) is of type-II with a final offset between the respective filled/empty frontier levels of 1.1 eV. The type-II ELA rather facilitates charge transfer that quenches light emission.Introducing the [RuCp*mes]+ as interlayer (Fig. 1b), the organic semiconductor levels rigidly shift in energy with respect to those of the inorganic component with an offset as little as 0.1 eV. This type-I ELA is favorable for energy transfer and subsequent light emission. This is shown in Figure 2 which exhibits the PL spectra of a hybrid structure consisting of a ZnO/Zn0.9Mg0.1O QW structure overgrown with 3 nm L4P-sp3 (blue) and of an equivalent hybrid structure, but with a [RuCp*mes]+ interlayer between QW structure and L4P-sp3 (green). By introducing the interlayer the molecular emission of the L4P-sp3 increases by a factor of seven. This clearly shows the great potential of HIOS and is a major step towards HIOS based light-emitting applications. 

R. Schlesinger et al., Nature Commun. 6 (2015) 6754
DOI: 10.1038/ncomms7754

 

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Fig. 1: Energy level diagrams of a) L4P-sp3 without interlayer and b) with [RuCp*mes]+ interlayer on Zn-terminated ZnO(0001). Energy values are referenced to the Fermi level and in eV. The offset between the L4P-sp3 and ZnO energy levels is highlighted in red. The L4P-sp3 LUMO region is shaded with a gradient to represent uncertainties due to the unknown transport gap.

 

Fig. 2: PL spectra of hybrid a structure consisting of a ZnO/Zn0.9Mg0.1O QW structure overgrown with 3 nm L4P-sp3 (blue) and of an equivalent hybrid structure, but with a [RuCp*mes]interlayer between QW structure and L4P-sp3 (green).

Probing the energy levels in hole-doped molecular semiconductors
Understanding the nature of polarons – the fundamental charge carriers in molecular semiconductors – is indispensable for rational material design that targets superior (opto-) electronic device functionality. The traditionally conceived picture of the corresponding energy levels invokes singly occupied molecular states within the energy gap of the semiconductor (Fig. 1a). Here, by employing a combined theoretical and multi-technique experimental approach, we show that this picture needs to be revised. Upon introducing an excess electron or hole into the material, the respective frontier molecular level is split by strong on-site Coulomb repulsion (U) into an upper unoccupied and a lower occupied sub-level, only one of which is located within the semiconductor gap (Fig.1b). By including also inter-site Coulomb interaction between molecular ions and circumjacent neutral molecules, we provide a complete picture for the electronic structure of molecular semiconductors in the presence of excess charges. With this understanding, a critical re-examination of previous results is called for, and future investigations of the properties and dynamics of polarons in weakly interacting molecular systems are put on sound footing.

S. Winkler et al., Mat. Horiz. 2 (2015) 427
DOI: 10.1039/c5mh00023h

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Fig. 1a) From left to right: common perception on the single-particle energy levels for neutral molecules (green) surrounding a cation (red) with respect to a common vacuum level (phiel,infty), their experimental accessibility to (inverse) photoelectron spectroscopy depending on their occupancy and the resulting density of states (DOS) on a logarithmic scale, ln(DOS), that is to be challenged on the basis of this study.

Fig. 1b) Revision of these common perceptions: the on-site Coulomb repulsion U causes a splitting into two HOMO sub-levels of a molecular cation, while the inter-site Coulomb repulsion induces an energy-level shift to higher binding energies of the neutral molecules in close vicinity to the cation.

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