Improving Photovoltaics with High Luminescence Efficiency Quantum Dot Layers

A solar cell relies on its ability to turn photons into current. Because short wavelength photons are typically absorbed near the top surface of a cell, the generated charge carriers recombine before being collected. But when a layer of quantum dots (nanoscale semiconductor particles) is placed on top of the cell, it absorbs short wavelength photons and emits them into the cell at longer wavelengths, which enables more efficient carrier collection. However, the resulting power conversion efficiency of the system depends critically on the quantum dot luminescence efficiency – the nature of this relationship was previously unknown. Our calculations suggest that a quantum dot layer must have high luminescence efficiency (at least 80%) to improve the current output of existing photovoltaic (PV) cells; otherwise, it may worsen the cell’s efficiency. Our quantum dot layer (using quantum dots with over 85% quantum yield) slightly reduced the efficiency of our PV cells. We observed a decrease in short circuit current of a commercial-grade cell from 0.1977 A to 0.1826 A, a 7.6% drop, suggesting that improved optical coupling from the quantum dot emission into the solar cell is needed. With better optical coupling, we predict current enhancements between ~6% and ~8% for a solar cell that already has an antireflection coating. Such improvements could have important commercial impacts if the coating could be deployed in a scalable fashion.

Engineering thin films of magnetic alloys and semiconductor oxides at the nanoscale

The thesis aims to exploit properties of thin films for applications such as spintronics, UV detection and gas sensing. Nanoscale thin films devices have myriad advantages and compatibility with Si-based integrated circuits processes. Two distinct classes of material systems are investigated, namely ferromagnetic thin films and semiconductor oxides. To aid the designing of devices, the surface properties of the thin films were investigated by using electron and photon characterization techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), and energy-dispersive X-ray spectroscopy (EDS). These are complemented by nanometer resolved local proximal probes such as atomic force microscopy (AFM), magnetic force microscopy (MFM), electric force microscopy (EFM), and scanning tunneling microscopy to elucidate the interplay between stoichiometry, morphology, chemical states, crystallization, magnetism, optical transparency, and electronic properties. Specifically, I studied the effect of annealing on the surface stoichiometry of the CoFeB/Cu system by in-situ AES and discovered that magnetic nanoparticles with controllable areal density can be produced. This is a good alternative for producing nanoparticles using a maskless process. Additionally, I studied the behavior of magnetic domain walls of the low coercivity alloy CoFeB patterned nanowires. MFM measurement with the in-plane magnetic field showed that, compared to their permalloy counterparts, CoFeB nanowires require a much smaller magnetization switching field , making them promising for low-power-consumption domain wall motion based devices. With oxides, I studied CuO nanoparticles on SnO2 based UV photodetectors (PDs), and discovered that they promote the responsivity by facilitating charge transfer with the formed nanoheterojunctions. I also demonstrated UV PDs with spectrally tunable photoresponse with the bandgap engineered ZnMgO. The bandgap of the alloyed ZnMgO thin films was tailored by varying the Mg contents and AES was demonstrated as a surface scientific approach to assess the alloying of ZnMgO. With gas sensors, I discovered the rf-sputtered anatase-TiO2 thin films for a selective and sensitive NO2 detection at room temperature, under UV illumination. The implementation of UV enhances the responsivity, response and recovery rate of the TiO2 sensor towards NO2 significantly. Evident from the high resolution XPS and AFM studies, the surface contamination and morphology of the thin films degrade the gas sensing response. I also demonstrated that surface additive metal nanoparticles on thin films can improve the response and the selectivity of oxide based sensors. I employed nanometer-scale scanning probe microscopy to study a novel gas senor scheme consisting of gallium nitride (GaN) nanowires with functionalizing oxides layer. The results suggested that AFM together with EFM is capable of discriminating low-conductive materials at the nanoscale, providing a nondestructive method to quantitatively relate sensing response to the surface morphology.