Gestión de Contratos

1.4.1 Material Purification

High purity of an organic semiconductor is essential for achieving optimal optical and electrical device performance, and a reproducible, comprehensible physical char- acterization of the organic material system. The intrinsic properties of the material system can only be accurately studied without extrinsic behaviors introduced from foreign species. Chemical impurities have invariably different HOMO and LUMO levels than the major constituents, and may form traps that result in a decrease of charge mobility and exciton diffusion length.[18,19] _{The impurities may also diminish}

Figure 1.11: The apparatus for thermal gradient sublimation purification. luminescence efficiency due to exciton quenching or introduce parasitic luminescence at an undesired wavelength.

A variety of solution-based purification techniques compatible with both polymer and small molecule organics is available including fractional crystallization,[20] _chro-

matography,[21] _{centrifugation,}[22] _{etc. Effective separation of the organic compound}

from impurities relies on their difference in solubility, molecular size, and weight. These methods are usually the first purification steps following synthesis, and yet the purity level is still unacceptable although labeled as > 98% by the manufacturer.

Thermal gradient sublimation is the widely adopted method for small molecule organics to refine the purity level.[23] _{As illustrated in Fig. 1.11, a horizontal three-}

zone furnace is used to maintain a thermal gradient along a quartz tube, where the raw organic material is loaded in a quartz boat at the hot end. The tube is evacuated to 10−6 Torr vacuum using a combination of dry mechanical and turbo pumps. The temperature is gradually ramped up until slow sublimation of the raw material is observed. Organic compounds with different volatility diffuse along the thermal gradient and condense at different catching sleeves in the tube, allowing for spatial separation and collecting the purified material from impurities. A permeable stopper, e.g. quartz wool, is placed near the tube open end to stop the organic

vapor from contaminating the pump system. Each cycle of purification process lasts approximately one week. Several iterations are sometimes needed to reach acceptable purity levels.

1.4.2 Organic Thin Film Deposition

The intermolecular van der Waals bond of organic materials enables a wide range of deposition methods, free from the requirement of lattice matching characteristic of inorganic semiconductors. An even larger variety of material combination, thin film morphologies, and device geometries on various substrates can therefore be achieved. This section provides a brief introduction of widely used organic deposition tech- niques, in particular, those used in the subsequent chapters.

The most common organic deposition method for both research and large scale manufacturing purposes is vacuum thermal evaporation (VTE).[23,24] The configura- tion of VTE is illustrated in Fig. 1.12. The source material is placed in a baffled metal boat made of tungsten or molybdenum that is resistively heated in vacuum by passing current through it. The evaporation is generally operated at 10−6 to 10−9 Torr vacuum, resulting in a ballistic and highly directional organic vapor stream to- wards the substrate. The shadow mask attached to the substrates as shown in Fig. 1.12 is used to pattern growth features and define the device area. The deposition rate is controlled in real time by a quartz crystal monitor and a feedback loop to the boat heating controller. Multiple organic sources can be simultaneously deposited, with the deposition rate and thus concentration of each source controlled by individ- ual crystal monitors. Substrate rotation enables the uniformity of film thickness and composition.

A promising alternative to VTE is organic vapor phase deposition (OVPD),[26–28]

whereby organic materials are thermally evaporated at a low pressure (0.1-10 Torr) into a heated carrier gas stream and transported through a hot wall reactor to a

Figure 1.12: Schematic of the vacuum thermal evaporation (VTE) system. Image adapted from ref.[25]

cooled substrate. In OVPD, the process of evaporation and transport are essentially decoupled. Molecules evaporated from the sources undergo collisions with the gas molecules, resulting in a loss of kinetic energy. The molecules remain near the source region until being carried away by the gas stream. Control over the evaporation temperature and gas flow rate results in a shift between equilibrium and kinetically limited growth regime,[29] and therefore vastly different morphology of the film. One subclass of OVPD is organic vapor jet printing (OVJP),[30] [31]where the vapor stream is passed through a heated nozzle towards the substrate with < 1 mm substrate-to- nozzle distance. Instead of using a shadow mask for film patterning, OVJP is designed to directly pattern the features with < 10µm resolution.[31]_{Also note that OVJP offers}

a superior material utilization efficiency compared to VTE and OVPD.

Solution-based depositions are another major class of organic deposition tech- niques. They are applicable for processing large molecular weight or low decomposi- tion temperature organic molecules. Most notable of these techniques include spin-, dip-, and blade-coating, inkjet printing[32]_{and screen-printing,}[33]_{etc. Organic mate-}

rials are dissolved or suspended in the solvent, and subsequently contact the substrate as the solvent evaporates. Post annealing by heat or solvent vapor can be used to

Figure 1.13: Three types of organic heterojunctions (HJs).

further alter film morphology. Solution processing has the advantage of very-high- speed deposition over large substrate areas, but has difficulty in growing complicated multi-layer structures and device patterning.

1.5

Fundamentals of Organic Donor-Acceptor Heterojunc-