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Dissertation Proposal Defense – Ankit K. Singh
MSE Grad Presentation
Friday, May 26, 2017 - 2:30pm
Prof. Samuel Graham, Advisor, ME/MSE
Prof. Preet Singh, MSE
Prof. Mark Losego, MSE
Prof. Elsa Reichmanis, CHBE
Prof. Maysam Ghovanloo, ECE
"Study of ALD Films for Improving the Stability and Reliability of Electronic Devices in Harsh Environments"
Over the past decade, a wide range of electronic devices have been developed that require barrier film technologies to improve lifetime. For example, organic electronic devices have the ability to produce light weight, scalable and flexible electronics as opposed to conventional electronic devices. Despite having several advantages over peers, commercial application of such devices is still challenging as they are prone to rapid degradation on exposure to atmospheric species like oxygen and water vapor. The stability of such devices can be increased by using a barrier layer that prevents the ingress of oxygen and water vapor to the active layer in the devices. More recently, perovskite cells have attracted significant attention of photovoltaics community through their steep advancement in power conversion efficiency which has gone above 22% in less than a decade. However, their poor environmental stability is a major challenge that needs to be addressed before any practical application. Just like organic electronic devices, perovskite cells are also prone to degradation by atmospheric species.
In the quest to make barrier films for these applications, several studies have been conducted using variety of vacuum deposition techniques, out of which, ALD has shown the greatest potential for making ultra-thin barrier films for their ability to form conformal and pinhole free films. Low temperature ALD is desirable as organic materials cannot withstand high temperatures. For this, plasma-enhanced ALD (PEALD) has been used. Recent studies have got water vapor transmission rate of PEALD films up to the order of 10-6 g/m2/day which is what is required for commercial application. However, existence of defects, in the barrier films, like particle defects and cracks significantly deteriorate the quality of an otherwise excellent barrier. Various architectures using nanolaminates and hybrid structures have been tested to minimize defects in the barrier, but none have been able to achieve the desired level. Thus, there is a need to develop a robust structure of barrier film which has sufficient tolerance against these defects while maintaining high quality. Direct use of ALD on perovskite cells also has numerous issues. Several studies have shown that plasma and ozone can lead to degradation of the devices. High temperature deposition is not possible due to their thermal instability. This negates the use of ALD for direct deposition of encapsulation barrier. This calls for devising an alternate strategy for perovskite cell encapsulation to enhance their stability and lifetime. Moreover, very little has been done to understand the corrosion resistance of ALD barrier films which may exist in solar cell encapsulation or in newer applications such as biomedical implants.
In this dissertation, I propose to study the performance of ALD barrier films used in harsh environments defined as mechanical and chemically corrosive environments. Firstly, I propose to investigate different barrier architectures and material combinations to fabricate a mechanically robust barrier film that is free from particle defects and crack formation. Preliminary results have shown that use of PECVD SiNx along with ALD significantly improves performance of the barrier film. Further studies will be conducted to understand the mechanism behind improved defect tolerance like filling up of the pores and closing of the defects in ALD layer. Secondly, chemical stability of different materials like Al2O3, HfO2, TiO2 and ZrO2 will be investigated. Initial experiments have been conducted with these materials in 3.5% NaCl solution using electrochemical impedance spectroscopy (EIS), which has shown that TiO2 and ZrO2 are the most stable ALD material. This study will be further conducted in different ionic (Sea Water, HCl and H2SO4) and simulated biological solutions (Sweat, Saliva and DMEM) to understand the chemical stability and corrosion resistance of ALD materials, and their mechanism of degradation. Once a chemically stable high quality barrier has been fabricated, it will be used for encapsulating perovskite cells through indirect encapsulation. In this method, instead of direct deposition of ALD on the devices, barrier will be fabricated separately and then sandwiched onto the device using an adhesive in between. A buffer layer will be used to protect the devices from coming in direct contact of the adhesive as it can also degrade the devices. Also, to improve the stability of existing active material, low temperature thermal ALD films will be deposited on the perovskite material to understand the chemistry at the interface. This thin ALD layer is expected to act like a mild barrier layer as well. This research will contribute to different aspects of barrier film encapsulation like quality, mechanical reliability and chemical stability. It will also demonstrate the application of barriers on active devices.