Small-molecule Organic Materials Research Articles

Informatic Calibration of a Materials Properties Database for Predictive Assessment of Mechanically Activated Disordering Potential for Small Molecule Organic Solids

The potential for small molecule organic crystalline materials to become disordered as a result of high shear mechanical processing was investigated. A data-driven model was generated from a database of critical materials properties, which were expected to correlate with the potential of a small molecule organic crystalline solid to become fully disordered by the application of mechanical energy. The model was compared with a previously published disordering model based on fundamental thermodynamic relationships. Samples of 23 crystalline solids were subjected to extensive comminution under controlled temperature conditions; powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC) were used to confirm disordering. Logistic regression was used to investigate the significance of each materials property with respect to the prediction of disordering potential. Seven materials properties (glass transition temperature, melting temperature, heat of fusion, crystallographic density, Young's modulus, molar volume and attachment energy) were identified as having a significant correlation with the potential for material disordering. Stepwise multivariate logistic regression was used to further assess the correlation between disordering potential and each of the seven properties. A linear probability model based on two materials properties (glass transition temperature and molar volume) was developed for the prediction of disordering potential.

Energy transfer dynamics between Alq 3 and CdTeS/ZnS core shell nanocrystals

The photoluminescence (PL) properties of the guest–host films, using CdTeS/ZnS core shell quantum dots (QDs) as the guest and organic small-molecule material Alq 3 as the host, are studied by steady-state and time-resolved PL spectroscopy. Both the relative intensity and the PL lifetime are intensively dependent on the weight ratio of Alq 3 and CdTeS/ZnS QDs. The detailed analysis provides clear evidence for a Förster energy transfer from Alq 3 host to QDs guest, based on the nonradiative resonant transfer mechanism. The results are relevant to the application of hybrid organic/inorganic systems to OLEDs.

Nanoimprinted large area heterojunction pentacene-C60 photovoltaic device

A pentacene-C60 bilayer heterojunction organic photovoltaic device with interpenetrating donor-acceptor interface was fabricated by nanoimprinting the pentacene layer prior to C60 deposition. An amorphous silicon substrate nanostructured using an excimer laser was imprinted onto the pentacene layer at high temperature and pressure, using a nanoimprinting lithography system to form a textured pentacene surface. A fivefold improvement in power conversion efficiency was observed due to increased exciton dissociation at the large area heterojunction. Using the proposed technique, highly efficient bilayer large area heterojunction photovoltaics based on small molecule organic materials can be envisaged.

Colloquium: Electronic transport in single-crystal organic transistors

Small-molecule organic semiconductors, together with polymers, form the basis for the emerging field of organic electronics. Despite the rapid technological progress in this area, our understanding of fundamental electronic properties of these materials remains limited. Recently developed organic field-effect transistors (OFETs) based on single crystals of small-molecule organic materials are characterized by an unprecedented quality and reproducibility. These devices provide a unique tool to study the fundamentals of polaronic transport on organic surfaces and to explore the limits of OFET performance. This Colloquium focuses on the intrinsic, not limited by static disorder, charge transport in single-crystal OFETs and on the nature of defects on surfaces of organic crystals. In the conclusion, an outline of the outstanding problems that are now becoming within experimental reach owing to the development of single-crystal OFETs is presented.

Trap concentration dependence of percolation in doped small molecule organic materials

The thermally stimulated current (TSC) technique is used to investigate the effect of doping of organic glassy thin films of the hole transport material N,N′-di(1-naphthyl)N,N′-diphenylbenzidine (α-NPD) with various concentrations of 4,4′,4″-tris(N-(1-naphthyl)-N-phenylamino)triphenylamine (1-NaphDATA). The mobility is estimated from current-voltage characteristics. At small dopant concentrations a TSC peak appears at about 200K. Increasing the dopant concentration to about 4vol% leads to a peak shift towards higher temperatures, related to decreasing mobility. When increasing the dopant concentration further, the peak shifts again to lower temperatures towards the peak position for pure 1-NaphDATA. The energy distribution of the trap structure is obtained utilizing the fractional TSC technique. In accordance to a higher-lying highest occupied molecular-orbital level of 1-NaphDATA as compared to the α-NPD matrix, the activation energy of a deep trap level of about 0.5eV was reported previously for low doping concentrations up to a few percent. At higher dopant concentrations the deep traps vanish from the trap structure. The behavior can be interpreted as a change from a trap-controlled transport for small doping concentrations to a percolating transport on the dopant molecules themselves. It is shown that the onset of percolation at rather low concentrations can be explained by hopping including not only nearest neighbors.

Trap concentration dependence of thermally stimulated currents in small molecule organic materials

The effect of doping of thin organic glassy films of the hole transport material $N$,${N}^{\ensuremath{'}}$-di(1-naphthyl)$N$,${N}^{\ensuremath{'}}$-diphenylbenzidine ($\ensuremath{\alpha}$-NPD) with various concentrations of 4,${4}^{\ensuremath{'}}$, ${4}^{\ensuremath{''}}$-tris($N$-(1-naphthyl)-$N$-phenylamino)-triphenylamine (1-NaphDATA) is investigated with respect to the resulting charge carrier mobility and the effect on thermally stimulated currents (TSC). The energetic trap structure is obtained utilizing the fractional TSC technique. At low doping concentrations of a few percent the activation energy of an about 0.5 eV deep trap level is measured, which is related to the higher lying highest occupied molecular orbital level of 1-NaphDATA compared to the $\ensuremath{\alpha}$-NPD matrix. A strong shift of the TSC peak towards higher temperatures with increasing dopant concentration is observed. It can be attributed to re-trapping despite the low film thickness of 200 nm. Thereby the effect of re-trapping is theoretically investigated and found to be composed of a mobility change and a change of the trap depth, as it is measured in fractional TSC measurements. The latter is related to enhanced re-trapping into deeper traps. The maximal peak shift occurs at about 4 vol % of dopant concentration, corresponding to about 2.7% molecular concentration. For higher concentrations of 1-NaphDATA the mobility increases again, and the TSC peak gradually changes to that of pure 1-NaphDATA, explainable by a progressive change from a deep-trap-controlled transport to a percolating transport on the 1-NaphDATA molecules.

Nondispersive hole transport in amorphous films of methoxy-spirofluorene-arylamine organic compound

Hole transport in the organic small molecule material 2,2′,7,7′-tetrakis-(N,N-di-4- methoxyphenylamino)-9,9′-spirobifluorene (methoxy-spiro) is studied by three different experimental techniques: Time of flight (TOF), dark-injection space-charge-limited current transients and steady-state current–voltage characteristics, and over a wide range of electric fields, sample thicknesses and temperatures. It is shown that room-temperature mobilities measured by all three methods agree well over a range of studied thicknesses from 4 μm down to 135 nm. Temperature-dependent TOF measurements are analyzed within the Gaussian disorder model and the parameters of charge transport in this material are extracted. Comparison with the data reported for other spirolinked compounds indicate a significant increase of the mobility prefactor in the methoxy-spiro, which is attributed to the increased spread of the highest occupied molecular orbital wave function in this material.