Chris Elschner

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.

Interrelation between Crystal Packing and Small-Molecule Organic Solar Cell Performance†

X-ray investigations on single crystals of a series of terminally dicyanovinyl-substituted quaterthiophenes and co-evaporated blend layers with C(60) give insight into molecular packing behavior and morphology, which are crucial parameters in the field of organic electronics. Structural characteristics on various levels and length scales are correlated with the photovoltaic performance of bulk heterojunction small-molecule organic solar cells.

Impact of mesoscale order on open-circuit voltage in organic solar cells

Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor-donor-acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation.

Comparative Study of Microscopic Charge Dynamics in Crystalline Acceptor-Substituted Oligothiophenes

By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes, we find that the internal acceptor-donor-acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields, substantial energetic disorder, and pronounced Poole-Frenkel behavior, which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder, which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition, the presence of a well-defined π-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping.

Charge transport in amorphous and smectic mesophases of dicyanovinyl-substituted oligothiophenes

By analyzing electrostatic and polarization effects in amorphous dicyanovinyl-substituted oligothiophenes, we conclude that local molecular dipole moments result in a large, spatially correlated, energetic disorder. This disorder increases with the number of thiophene units in the oligomer and leads to an unexpected reduction of charge carrier mobility in a more ordered (smectic) mesophase, observed for the longest of the studied oligomers (hexamers). This reduction in mobilities contradicts the common belief that more ordered phases of organic semiconductors have a better charge carrier mobility. In this particular case, the amorphousness leads to a better-connected charge percolating network, helping to bypass deep energetic traps. By comparing mobilities of amorphous and crystalline mesophases we conclude that vacuum deposited thin organic films have well ordered polycrystalline morphologies.

Determining the C60 molecular arrangement in thin films by means of X-ray diffraction

The electrical and optical properties of molecular thin films are widely used, for instance in organic electronics, and depend strongly on the molecular arrangement of the organic layers. It is shown here how atomic structural information can be obtained from molecular films without further knowledge of the single-crystal structure. C60 fullerene was chosen as a representative test material. A 250 nm C60 film was investigated by grazing-incidence X-ray diffraction and the data compared with a Bragg-Brentano X-ray diffraction measurement of the corresponding C60 powder. The diffraction patterns of both powder and film were used to calculate the pair distribution function (PDF), which allowed an investigation of the short-range order of the structures. With the help of the PDF, a structure model for the C60 molecular arrangement was determined for both C60 powder and thin film. The results agree very well with a classical whole-pattern fitting approach for the C60 diffraction patterns.

Orientation of OLED Emitter Molecules Revealed by XRD

Thin films of the phosphorescent emitters Ir(ppy)3 and Ir(ppy)2(acac) are investigated by GIXRD and GIWAXS. Both molecules form crystalline grains and exhibit a preferred orientation that is pertained even when doped into a host.