Analysis of Effect of High Voltage Power Supply on Film Quality in Organic Electronic Device Fabrication
Organic electronic devices including organic light emitting diodes, organic photovoltaics, and organic field effect transistors have attracted intense research and commercial interest. These devices use thin films of organic semiconductors deposited by various techniques. The deposition process often involves high voltage for processes such as organic molecular beam deposition or electrostatic spray deposition. The characteristics of the high voltage power supply affect the film quality and thus the device performance.
Organic thin films can be deposited by thermal evaporation, solution processing, or vapor phase deposition. Thermal evaporation heats the organic material in vacuum, and the evaporated molecules condense on a substrate. Solution processing deposits the organic material from a solution by spin coating, inkjet printing, or spray coating. Electrostatic spray deposition uses high voltage to charge and direct droplets containing the organic material.
Electrostatic spray deposition, also called electrospray, applies a high voltage to a nozzle containing a solution of the organic material. The electric field at the nozzle tip creates a Taylor cone, and a fine jet emerges and breaks into charged droplets. The droplets travel to the grounded substrate, where the solvent evaporates and the organic material deposits. The film morphology depends on the droplet characteristics and the deposition conditions.
The high voltage power supply determines the electric field at the nozzle and the charge on the droplets. The voltage level affects the cone jet mode and the droplet size. Higher voltages generally produce smaller droplets but may cause instabilities in the jet. The optimal voltage depends on the solution properties, the nozzle geometry, and the desired film characteristics.
Voltage stability affects the consistency of the deposition. Variations in voltage cause variations in the droplet size and charge, leading to non uniform film deposition. The power supply must provide stable output for consistent film quality. The stability requirement depends on the sensitivity of the deposition process to voltage variations.
Current flow in electrospray depends on the solution conductivity and the voltage. The current is related to the charge on the droplets and the flow rate. Monitoring the current provides information about the deposition process. Constant current operation can maintain consistent charge per droplet despite variations in solution properties.
Film morphology affects the device performance. For organic light emitting diodes, the film morphology affects the charge transport and recombination efficiency. Rough films may have current crowding and non uniform emission. Smooth, uniform films provide consistent device characteristics across the device area.
Film thickness uniformity depends on the uniformity of the deposition across the substrate. The electric field distribution affects the droplet trajectories and the deposition pattern. Non uniform fields cause some areas to receive more deposition than others. The electrode geometry and the voltage distribution must be designed for uniform deposition.
Organic molecular beam deposition can also involve high voltage for substrate biasing or ion bombardment. A biased substrate can attract charged species or influence the molecular orientation. Ion bombardment can densify the film or modify the surface. The bias voltage affects the energy of the incident species and the resulting film structure.
Molecular orientation in organic films affects the device performance. Many organic semiconductors have anisotropic properties, with different electrical characteristics along different molecular axes. The orientation depends on the deposition conditions, including the energy of the incident molecules. High voltage processes that control the incident energy can influence the orientation.
Contamination from the power supply or the deposition environment can degrade organic films. Organic materials are sensitive to impurities that can act as traps or recombination centers. The power supply must not introduce contaminants into the deposition environment. This is particularly important for vacuum deposition where even small amounts of contamination can be significant.
Process monitoring enables real time control of the film quality. Thickness monitors measure the deposition rate and accumulated thickness. Current monitoring in electrospray indicates the charge and flow characteristics. Optical monitoring can detect film formation in real time. This feedback enables adjustment of the power supply parameters to maintain optimal deposition conditions.
