Deposition thin films of Organic Materials by Thermal Evaporation for Electro-Optical Applications

Electronic components based on thin-film organic semiconductors have many applications because of their flexibility and large-scale fabrication at low cost. These semiconductors allow the creation of electronic devices such as OFET (Organic-Field Effect Transistor), OLED (Organic-Light Emitting Diode) and OPV (Organic Photovoltaic Cell) as a new generation of electronics.

The electronic properties of the organic molecules depend on the bonds between the atoms. The location and properties of these interatomic bonds determine how light is absorbed and the charge transferred by molecules. The most important of these bonds are the conjugated bonds and how they are joined inside the molecules, where the shared electrons are able to move part of the molecule.

Figure 1: Molecular structure of polyethylene and polyacetylene

In non-conductive molecules such as polyethylene (shown in Figure 1), each carbon atom is bonded to the other 4 atoms and all of its electrons are in bonds and the conduction band in this structure is empty of electrons and there is no electron that can move. On the opposite side are unsaturated bonds that exist in conductive materials. For example, as shown in Figure 1, the structure of the polyacetylene contains an additional electron in each carbon atom, which contributes to the electrical conductivity of the molecule. In large molecules such as polymers these electrons expand along the molecule and form π junctions. The conductive or semiconducting property of the molecule depends on whether the bonds are full or semi-full.

Bonds in organic semiconductors are fundamentally different from bonds in their inorganic counterparts, which have a direct impact on their properties. Reduction of hardness, low melting point and reduction of electric charge transitions are the characteristics of these materials. The high absorption coefficient of these materials makes it possible to absorb high light in the thin layers of these materials (less than 100 nm) in a way that compensates for their low mobility.

Poor electronic displacement of organic semiconductors provides two characteristics for these materials. The existence of single and triple spin modes seen in non-conductive materials and the second is formation of excitons, which are the light-dependent composition of a pair of electrons and holes. Excitons arise from the collision of light with a photovoltaic molecule. The subtracted photon stimulates the valence bond electron and forms a pair of electron and hole. The energy of the incoming photon must be equal to or greater than the band gap energy of the desired material.

In order to separate this electron-hole pair in order to separate positive and negative electric charges to generate electric current, in non-organic photovoltaic cells, energy of about 0.5 to 1 eV is required which must be supplied by an external electric field. However, this energy in organic photovoltaic cells is about a few millivolts, which is easily obtained at room temperature.

Formation and separation of excitons is a major process in organic solar cells, which is why they are also called excitonic solar cells.

Thermal evaporation deposition is a low-cost and common method used for deposition of the organic materials. Thermal evaporation is used as an efficient and low-cost method in the manufacture of large-scale OLEDs as well as in the manufacture of organic solar cells.

One of the advantages of the thermal evaporation method is the deposition of complex structures in a single deposition step. The deposition process of organic semiconductors by thermal evaporation has many points in common with the process of deposition by thermal evaporation of non-organic materials but requires more careful control. Interatomic bonds in organic materials are weaker than inorganic materials. Therefore, the application of thermal energy breaks more bonds in organic materials. The evaporation temperature of organic materials is often below 500 ° C. The deposition of the organic materials by thermal evaporation in order to prevent the decomposition of organic matter requires careful control of the process temperature.

Due to the different forms of the molecules, the control of the morphology of the thin film deposited from organic materials is much more complicated than the control of non-organic materials. The type of substrate, the substrate temperature, and the deposition rate in the thermal evaporation process of organic materials must be carefully controlled. Many organic materials are degraded in the presence of oxygen or water. As a result, the deposition environment of these materials must be completely water and oxygen free. Suitable chamber pressure for the thermal evaporation of organic materials shall not exceed 5*10-6 millibars. Vapor pressure is exponentially dependent on temperature and temperature rise rate. Figure 2 shows the evaporative behavior of Alq3 material used in the manufacture of OLEDs.

Figure 2: Thermal evaporation rate with increasing temperature

In addition to the evaporative behavior of the materials used, the vapor flux distribution and the thickness of the layers are also important. Creating homogeneous and directional layers is only possible if the mentioned characteristics are fully known. The shape of the sources of evaporation (e.g. boats or baskets) is also important in the proper evaporation of organic materials and in determining the vapor flux distribution. Usually these sources do not form to accommodate a thin layer of evaporative material and are deep crucibles. The walls of crucibles should be designed to prevent heat reflection so that the temperature inside the crucible is more homogenous. The crucible material must be of high heat conductivity material to maintain uniform temperature inside the plant and to prevent decomposition of organic matter due to the temperature gradient inside the crucible. The materials used to make these crucibles are usually aluminum oxide or boron nitride. 

It is essential to use thermal sensors to accurately control the temperature of the crucible and substrate in the organic material deposition. Although direct temperature control in the deposition of inorganic materials by thermal evaporation is not required, this is one of the requirements of deposition organic materials.

Deposition of organic materials by thermal evaporation is that by initially attaining the appropriate pressure, the temperature of the evaporated material from ambient temperature to standby temperature which is lower than the evaporation threshold temperature. This increase in temperature should be as low rate as possible. Increasing the temperature with a steep slope causes temperature uniformity inside the crucible which may cause decomposition of organic matter. After a short period of time when the plant is kept at standby temperature, the temperature rises to evaporation temperature in moderate rate and immediately after the thermal evaporation process is completed, the temperature returns to standby temperature again. Figure 3 shows the temperature rise of Alq3 material.

Figure 3: Temperature rise rate in Alq3 layer process by thermal evaporation

See the following resources for more information in field of organic material evaporation process.

References:

  1. Deposition of Functional Organic Thin Layers by Means of Vacuum Evaporation, Jens Drechsel, Hartmut Fröb, 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  2. VACUUM DEPOSITION OF ORGANIC MOLECULES FOR PHOTOVOLTAIC APPLICATIONS, Peter Kovacik, A thesis submitted for the degree of DPhil in Materials University of Oxford
  3. Comparison of organic solar cells and inorganic solar cells, Askari Mohammad Bagher, Department of Physics, Payame Noor University, PO Box 19395-3697 Tehran, Iran
  4. https://www.researchgate.net/post/Is_it_possible_to_deposit_organic_films_using_an_ordinary_thermal_evaporator

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