Plasmonic Nanostructures for Enhanced ZnO/Si Heterojunction Optoelectronic Devices
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The objective of this work focuses on ZnO and Al doped ZnO (AZO) thin film deposition and characterization, and developing reliable ZnO/Si heterojunction thin film optoelectronic devices. Producing and integration of plasmonic nanostructures were also studied for improving device performance with plasmonic light trapping effects. Enhanced ZnO/Si heterojunction metal-semiconductor-metal (MSM) photodetectors with plasmonic Ag nanoparticles (NPs) were realized. Self-assembled Ag NPs with different sizes, densities and distributions were produced on the surface of ZnO/Si MSM photodetector devices. By tuning the characteristic of these NPs, a higher-performance MSM detector has been achieved with photocurrent enhancement up to 680%. The spectral enhancement was broadband from 350 nm to 850 nm. To investigate the nanoplasmonic effects for enhanced solar cell devices, a relatively simple device structure, Si Schottky solar cell with the metal-insulator-semiconductor (MIS) structure, was studied first. By introducing Ag NPs and SiO 2 spacer layers on top of Si Schottky solar cells, we demonstrated a positive and tunable light trapping effect introduced by metallic NPs. Enhanced light trapping effects at distinct resonance wavelengths were observed in the optical spectra of the plasmonic-enhanced devices. Electrical measurements confirmed the expected photocurrent improvement at these corresponding wavelengths. It was also revealed that the Ag NPs enhance the carrier generation rate inside of the Si active layer without sacrificing carrier collection efficiency of the device. The short-circuit current density (Jsc) of the best cell we obtained was improved from13.7 mA/cm 2 to 19.7 mA/cm 2 , with an enhancement factor of 43.7%. Periodic nanostructures formed with nanoimprint technique and annealing process were studies to utilize in the Al-ZnO/Si heterojunction solar cell devices. The size, inter-particle distance and shape of these nanostructures can be easily tuned by changing the Ag film thickness and deposition angle. These periodic nano-ellipsoid arrays provide an absorption enhancement by a factor of 83.8% over a wavelength range of 500-1000 nm. Theoretical simulations revealed that the electromagnetic field can be coupled between each two nanoparticles, which would contribute to the optical absorption enhancement. Thin film Al-ZnO/n-Si solar cells were fabricated. By introducing these well-designed Ag periodic nanostructures, device performance was enhanced in both visible and infrared (IR) wavelength regions. To achieve more reliable Al-ZnO/n-Si solar cell devices, we developed a novel and simple approach to realize high performance AZO films. Thin Al films were deposited on ZnO surfaces, followed by thermal diffusion processes, introducing the Al doping into ZnO thin films. During the thermal diffusion process, the chemical state of Al on the surfaces can be converted to a fully oxidized state, resulting in high electrical conductivity of 6.2 ohm/sq and excellent transparency (96.5% at 550 nm), which is superior compared with previously reported values for indium tin oxide (ITO) and most reported transparent conducting electrode (TCE) materials. These AZO films were used to fabricate AZO/n-Si solar cells and boost the device power conversion efficiency from <1% to 2.56%. Finally, the thermal diffusion process and co-sputtering process were combined for AZO deposition, giving a precise control of Al doping in buffer AZO layers and a high electrical conductivity for top AZO layers. Thus, the AZO/n-Si heterojunction solar cell performance was further improved to 4.1%.