Britt A. Minch

Two pathways for photon upconversion in model organic compound systems

We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. The bimolecular upconversion process in these systems is based on the presence of a metastable triplet excited state of the macrocycles giving rise to dramatically different photophysical characteristics relative to the other known methods for photon upconversion such as two-photon absorption, parametric processes, second harmonic generation, and sequential multiphoton absorption. The chosen blue emitter molecules have suitably positioned triplet levels: in the case of an oligofluorine—essentially higher and in the case of diphenylanthracene—lower than the sensitizer porphyrin platinum triplet level and thus two excitation pathways for photon upconversion were observed and investigated.

Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities

The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir–Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10 -6 cm 2 ·V -1 ·s -1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10 -4 cm 2 ·V -1 ·s -1 . Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad.

Organic photovoltaic cells containing discotic liquid crystalline phthalocyanines

We report on the photovoltaic properties of solar cells containing a new discotic liquid crystalline material (DL-CuPc) based on copper phthalocyanine. In addition to being soluble, these materials can self-organize into highly ordered structures which can lead to good transport properties that can potentially be superior to those of amorphous materials. Increase in short-circuit current density and fill factor was obtained by thermal annealing of spin-coated DL-CuPc layer in bi-layer solar cells based on junction between DL-CuPc and C60. These improvements are explained by change in structure and morphology upon thermal annealing.

Response to "Comment on 'Two pathways for photon upconversion in model organic compound systems' [J. Appl. Phys. 101, 023101 (2007)]"

We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. Aggregation between the active molecules (metallated porphyrins and anthracene derivatives) indeed plays a very important role in the triplet-triplet annihilation supported photon upconversion process. Contrary to the suggestions of Steer [J. Appl. Phys. 102, 076102 (2007)] aggregation (followed from phase separation) is a highly undesired effect: external quantum yields of the level of 1%–3% are observed only in systems with a low degree of aggregation (solutions), whereas the solid state films exhibit at least order of magnitude lower upconversion quantum yield than solutions. Additionally, the “only excitons in aggregates model” cannot explain the increase of the linear absorption of the sensitizer at the presence of the emitter. Furthermore, this hypothesis cannot explain why the upconversion quantum yield increases whe...