Interface Engineering and Practical Applications of Amorphous Silicon Based Opto-electronic Devices for Large-Area Electronics

Abstract

In this dissertation, we extensively studied intrinsic amorphous silicon (a-Si) hybrid structure with inorganic / organic materials and explored the potential practical applications. Meanwhile, we still stepped into the various transparent electronic devices, including the transparent organic TFTs and memristors based on the developed transparent ultrathin metal film electrodes.

Top cathode contacts based on the conjugated organic materials were developed in our group, we further developed the air-stable cathode contacts based on the Polyethylenimine (PEI) and ZnO interlayers. We found the dipole induced in the PEI layers is highly related to the surface properties of the adjacent interlayers. The dipole between ZnO / PEI interlayers is supposed to be overcome by another one induced between PEI and cathode metal. Then we move to the investigation on the bottom anode contacts. We analyzed the band diagram and the carrier lateral transportation of the WOx / a-Si interfaces. We found the WOx / a-Si interface is able to to support centimeter-scale non-electrode area, without the additional conductive layers. From the developed anode and cathode contacts, we further characterized the electrical performance of the a-Si devices as photodetectors (PD), and re-designed device structure towards the applications of in-screen fingerprint scanners. The a-Si PDs with top cathode contacts based on ZnO NPs and PEI interlayers can achieve the LDR up to 190 dB, and at least 4 orders for the illuminations < 50 Lux. The current sequence in the short linear arrays exhibited good ratio up to 2 orders. The inverted a-Si PD with bottom cathode contacts were developed and characterized to fulfill the requirements of the architecture design for the sensing arrays. The ZnO and MoOx are employed as the ETL and HTL. The current leakage and LDR are analyzed for different a-Si thicknesses and device areas. Finally, the optimized a-Si PD arrays are fabricated on the LTPS TFT backplanes. In order to meet the development opportunities for the neuromorphic computation, innovative artificial synapses were demonstrated based on the interactions between the a-Si and PEI interlayers. The hysteresis and timing dependent plasticity of the artificial synapses can be excited by light illumination, coupled with electrical pulse stimuli. The device design is promising to serve as fundamental elements for neuromorphic functionalities towards the simultaneous visual information process in large-area electronics or IoT.

Transparent electronics is the hot research area to promote the information interactions. We also fabricated transparent devices based on the ultrathin transparent metal films and the corresponding optical management. Transparent resistive switching devices were demonstrated based on ultrathin doped Ag films and optically optimized dielectric / metal / dielectric structure. The overall transparency is higher than 80%, and the device conductance modulation is found to be analog and continuous. Transparent organic TFTs were achieved based on ultrathin Cu-based composite electrodes with Ni seeding and capping layers. The transparency is up to 71.4 % for the source/drain regions and even higher for the overall transparency.

Finally, based on the developed a-Si photodetectors, we proposed the potential development optimizations. The first one is the transparent a-Si optoelectronic devices based on all-inorganic interlayers. Then the optical design can be further conducted to modulate the absorption spectrum of a-Si active layers as well as the exterior apparent colors.