Research on Organic Electronic Materials - Rubrene Crystals

Role of synthesis for oxygen defect incorporation in crystalline rubrene

Understanding of the impact of molecular structure on charge transport in organic molecular crystals and thin films is the basis for control and manipulation of their electronic properties. It is critical however to recognize that morphology and presence of defects, either structural or chemical, affect the charge transport in organic solids as much as the molecular structure.

In crystalline rubrene, the material that exhibits one of the highest charge carrier mobilities among acenes, incorporation of the oxygen-related defect can significantly increase electrical conductivity. In this work, a photoluminescence signature of the defect is used to investigate the defect formation mechanisms. Our experimental results show that impurity incorporation can occur during the growth, if residual oxygen is present in the furnace, as well as after the crystal growth in a normal environment.

The post growth oxidation appears to be driven by the presence of structural disorder in rubrene crystals in a form of vacancies or grain boundaries. This correlation is likely caused by the structure of the impurity, a rubrene peroxide molecule. To accommodate a rubrene peroxide molecule within the crystal lattice of rubrene a vacancy is required at a neighbouring molecule site. The lattice therefore suppresses the postgrowth rubrene-oxygen reaction, making high-quality rubrene crystals less susceptible to the oxidation process. These findings are central in understanding of the material quality impact on electronic and optical properties of rubrene crystals.

Applied Physics Letters 91, 212106 (2007) PDF

Packing of rubrene molecules (Carbon atoms in the tertacene backbone are coloured yellow for clarity).

Oxygen-Related Bandgap State in Single Crystal Rubrene

Using photoluminescence spectroscopy with single- and two-photon excitation and charge transport analysis this work shows that large variations of the carrier density in rubrene are caused by an oxygen-related impurity, which acts as an acceptor state.

Among several photoluminescence bands corresponding to different electronic and vibronic transitions in rubrene crystals, we identified a band emitted due to the presence of an oxygen-related impurity. Our experiments show direct correlations of this O-band, sometimes mistakenly regarded as the photoluminescence signature of pure crystalline rubrene, and the defect.

This impurity however plays an important role in charge transport in rubrene crystals. A large increase in electrical conductivity of oxidized crystals and the well-defined spectroscopic signature of the defect indicate that the impurity forms a bandgap state near the valence band. Control of the impurity presence therefore allows modifying the material conductivity.

Physical Review Letters 97, 166601 (2006) PDF

PL characteristics of rubrene crystals in the direction normal to ab-surface: band III is the dominant feature for pure crystals, while band O around 645 nm (1.92 eV) indicates crystal oxidation

Research was done in collaboration with
D.V. Lang, C. Kloc, M.J. Wikberg, T. Siegrist, W.-Y. So, M.A. Sergent, and A.P. Ramirez (Bell Labs/Columbia U.)