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Risultati finali
Photo-Voltaic devices will be fabricated in the laboratory using the new discotic liquid crystalline semiconductor synthesized & characterized by non-industrial DISCEL partners.
The various aspects of semiconductor processability as well as device fabrication and performance will be assessed at the lab scale.
The most promising devices will be up-scaled to demonstration stage. A series of potential applications could benefit from this development, such as solar energy use in consumer electronics.
The fabrication of PVD prototypes on plastic substrates will be considered should the economics and attained performance level meet requirements for industrial production and commercialisation.
The identification and synthesis of new organic semiconductor materials is an important enabling factor for the development of (opto)-electronic devices. DISCEL contributes to this objective with new discotic molecules leading to significant potential advantages over existing systems.
Screening and identification of new liquid crystalline discotic hole, electron and exciton carriers is indeed achieved following a specific protocol. Successful candidates are synthesized and purified.
The resulting products are processed into thin films. Controlled orientation and mesophase stabilisation is achieved and the semi-conducting character is verified.
The selection protocol includes mechanical and electrical performance testing to assess their suitability for the envisioned applications.
The most promising molecules are then used by the DISCEL industrial partners to make electronic devices - LED, PVD & FET, respectively - and develop the corresponding fabrication processes for the most performing systems.
Liquid crystalline semiconductors are particularly difficult to purify by standard purification methods because of their high tendency to form aggregates in solution. Crystallization methods cannot be used either because compounds are liquid crystalline at room temperature. Therefore, metal scavengers bound on the surface of silica gel particles have been used. They allow substantial purification of metal and ionic contaminants.
Measurement of key physical parameters of liquid crystalline semiconductors:
- Charge carrier mobility
-- p-type materials: Charge carrier mobilities have been measured in four hexabenzocoronene (HBC) derivatives using pulse radiolysis time resolved microwave conductivity. Three are crystalline (K-phase) at room temperature and display high charge carrier mobilities (ca. 0.5 cm²/Vs), undergoing a transition to a columnar liquid crystalline phase (Dh) at ca 100°C above which the mobility drops by half. On cooling the D K transition is reversible with a hysteresis of ca 20°C. Neither side chain branching nor chirality significantly affected the measured mobilities, but purity of the sample was determined to be very important. The fourth compound maintained liquid crystallinity at all temperatures, and has a mobility of ca 0.2 cm²/Vs at room temperature, which increases gradually with temperature with no transition to a value close to the previous three compounds at the highest temperature.
-- n-type materials: Freshly prepared hexaazatrinaphthalene derivatives display a moderate charge mobility of ca 0.1cm²/Vs at room temperature, decreasing by a factor of ~2 at the transition to the Dh phase. On cooling back to room temperature the mobility remains low indicating that an irreversible phase transition has occurred.
Investigation of several perylene-diimide (PEDI) derivatives shows a large variation in mobility dependent on the alkyl chain substituents. For the derivative used in the HBC composites, a value of 0.01cm²/Vs at room temperature was found.
Conclusions: All HBC derivatives are capable of supporting rapid charge transport at RT. Chirality of the branched alkyl side chains does not have a pronounced positive influence on the mobility contrary to expectations. Purity is an important factor. The mobilities in the n-type components are in general substantially lower than for the HBCs.
- Photo-induced charge separation:
Photo-induced charge separation has been studied in thin, spin coated films of ca 50/50 mixtures of a perylenediimide derivative and two different HBCderivatives using flash-photolysis time resolved microwave conductivity. Charge separation in the blends is greatly enhanced compared with that for the separate components. This is attributed to electron transfer from HBC to PEDI with the efficiency being approximately equal for excitation of either component. Evidence is found for direct photo-induced charge separation at long wavelengths, which is attributed to absorption in regions of the composite in which HBC and PEDI are in intimate contact. The efficiency of charge separation decreases dramatically with increasing light intensity. This is attributed to exciton-exciton annihilation within the separate HBC and PEDI domains competing with exciton diffusion to the interfacial region where charge transfer takes place. Charge separation in the HBC derivatives can vary by a factor of about four. Room temperature liquid crystalline natures in HBC derivatives appears to have a positive influence on overall photocurrent generation which compensates for the higher mobility and charge separation efficiency found for crystalline derivatives.
Surface treatment allows orienting discotic liquid crystals either face-on or side-on. Alignment extends over micrometers. Face-on orientation is desirable for photovoltaic cells and light-emitting diodes whereas edge-on is required for field-effect transistors.
Field Effect Transistors will be fabricated in the laboratory using the new discotic liquid crystalline semiconductor synthesized & characterized by non-industrial DISCEL partners.
The various aspects of semiconductor processability as well as device fabrication and performance will be assessed at the lab scale.
The most promising devices will be up-scaled to demonstration stage. A series of potential applications could benefit from this development, such as flexible intelligent labels and identification tags.
The fabrication of finished device prototypes using flexible plastic substrates will be considered should the economics and attained performance level meet requirements for industrial production and commercialisation.
Organic Light Emitting Displays will be fabricated in the laboratory using the new discotic liquid crystalline semiconductor synthesized & characterized by non-industrial DISCEL partners.
The various aspects of semiconductor processability as well as device fabrication and performance will be assessed at the lab scale.
The most promising devices will be up-scaled to demonstration stage. A series of potential applications could benefit from this development, such as portable phones and video screens.
The fabrication of LED prototypes will be considered should the economics and attained performance level meet requirements for industrial production and commercialisation.
We have developed a theoretical approach based on quantum-chemical calculations to evaluate the parameters controlling at the molecular level the rate of charge hopping between two adjacent discotic molecules, and hence the charge mobility along discotic stacks. Since the calculated parameters are highly sensitive to the relative positions of adjacent discs in the stacks (in particular, the rotational angles), these quantum-chemical calculations are performed on the equilibrium structures of the stacks determined in a first stage by means of Molecular Mechanics and Dynamics calculations. Our approach proves very useful to screen prior to chemical synthesis molecules with potential interest as charge transporters and to help the synthetic chemists designing the most appropriate conjugated core with proper lateral substituents.
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