Investigation of ALD thin films to improve the reliability of organic electronic devices

Abstract

Organic electronic devices are attractive for their potential to produce lightweight, scalable, and flexible electronics as opposed to traditional inorganic electronics. In spite of these advantageous properties, the implementation of organic electronics in many applications is still challenging because of the potential for rapid degradation upon environmental exposure to oxygen, humidity, and mechanical stress. To enhance the stability of the devices, a reliable barrier layer to prevent the ingress of moisture and oxygen is required as well as more stable functional layers inside the device. Both of these goals can be partially achieved using defect-free conformal atomic layer deposition (ALD) films integrated into the devices. However, the practical electrical performance as well as the chemical stability of ultrathin ALD films has not received much attention from researchers. In some cases, characterization methods for the ultrathin ALD films have not been established. Therefore, this dissertation work proposes to investigate the functional properties of ultrathin (<10 nm) ALD films to create an encapsulation barrier film as well as to create environmentally robust coatings for electron selective contacts in organic electronics to improve their reliability. First, the chemical stability of ALD in aqueous environments was evaluated. Based on the results, select ALD films were applied as either electron selective functional layers to modify ITO contacts in an organic solar cell or as a robust encapsulation barrier layers for organic solar cells. In addition, the gas barrier performance of ultrathin ALD films was investigated using an improved calcium corrosion test, which can discriminate between the intrinsic film permeation and the defect-assisted permeation. Also, the mechanical failure of the barrier films in the presence of particle defects observed in the fabrication process of ultrathin thin films are discussed, and a method to circumvent such issues is suggested. Finally, this research developed two methods to improve the characterization of ultrathin barrier films, first in improving the measurement of the effective water vapor transmission rate and then a method to measure the intrinsic and defect assisted permeation rate through barrier films. These methods together have established chemical, electrical, mechanical, and permeation characterization methods to evaluate the potential of ultrathin ALD film for the enhancement of the stability of organic photovoltaic devices.