Date of Award

Spring 2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Sandra Zivanovic

Abstract

There is an important need for improvement in both cost and efficiency of photovoltaic cells. For improved efficiency, a better understanding of solar cell performance is required. An analytical model of thin-film silicon solar cell, which can provide an intuitive understanding of the effect of illumination on its charge carriers and electric current, is proposed. The separate cases of homogeneous and inhomogeneous charge carrier generation rates across the device are investigated. This model also provides for the study of the charge carrier transport within the quasi-neutral and depletion regions of the device, which is of an importance for thin-film solar cells. Two boundary conditions, one based on a fixed charge carrier surface recombination velocities at the electrodes and another based on intrinsic conditions for large size devices are explored. The device's short circuit current and open circuit voltage are found to increase with a decrease of surface recombination velocity at the electrodes. The power conversion efficiency of thin film solar cells is observed to depend strongly on impurity doping concentrations. The developed analytical model can be used to optimize the design and performance of thin-film solar cells without involving highly complicated numerical codes to solve the corresponding drift-diffusion equations.

The third generation polymer photovoltaic solar cells, the first generation includes monocrystalline silicon solar cells and second generation being thin-film solar cells, and photodetectors are researched widely in the last few years due to their low device processing cost, mechanical flexibility, and lightweight. Organic photovoltaic materials such as poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT: PCBM) blend are usually cheaper than inorganic materials, but have a limitation of lower power conversion efficiency (PCE) than their inorganic (for example, Si) counterparts. These organic devices need to be optimized to achieve the maximum possible PCE. One way to do this is to achieve the optimal thickness of the optically active layer of P3HT:PCBM while fabricating these organic photovoltaic devices. The influence of the active layer's thickness of P3HT:PCBM blend on performance of polymer solar cells and photodetectors are experimentally investigated. The fabricated device structure is glass/ITO/PEDOT:PSS/P3HT:PCBM/A1, where ITO is the indium tin oxide, and PEDOT:PSS stands for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) used as a buffer layer to collect holes effectively at the ITO anode. Aluminum is used as a cathode. Chlorobenzene is used as a solvent to prepare the polymer-fullerene blend. Spin coating technique was utilized to deposit the active layer and the concentration of P3HT, PCBM, and spin-coating speeds were varied to achieve a wide range of the active layer's thicknesses from 20 mn to 345 mn. The PCE of solar cell devices and the external quantum efficiency ( EQE) of the photodetectors are found to increase with the thickness of the active layer. The maximum PCE of 1.09% is obtained for the active layer's thickness of 345 mn.

The ongoing advanced space exploration requires the novel energy sources that can generate power for extreme duration without need of refill. The need for such extreme-duration lightweight power sources for space and terrestrial applications motivates the study and development of polymer-based betavoltaic devices. The betavoltaic devices based on the semiconductive polymer-fullerene blend of P3HT:ICBA, where ICBA is indene-C60 bisadduct, are demonstrated here for the first time. Both direct and indirect energy conversion methods were explored. For the indirect conversion method, a scintillator intermediate layer of cerium-doped yttrium aluminum garnet (Ce:YAG) was used. A high open circuit voltage of 0.56 V has been achieved in the betavoltaic device fabricated on polyethylene terephthalate (PET) substrate with the indirect energy conversion method at 30 keV electron kinetic energy. The directional and external interaction losses are significantly reduced using thin PET substrates. The maximum output electrical power of 62 nW was achieved at 30 keV input electron beam energy. The highest betavoltaic PCE of 0.78% was achieved at 10 keV of electron beam energy.

The performance of two different scintillators, Ce:YAG and Thallium doped Cesium Iodide (CsI:TI), were compared in the indirect conversion betavoltaic devices experimentally and the interaction of electron beam with Ce:YAG and CsI:TI was studied using Monte Carlo simulations. The catholuminescence profiles from simulation showed that CsI:TI is more-efficient to generate photons when hit by electron beam compared to Ce:YAG, which is further verified experimentally with 20% PCE enhancement using CsI:TI at 30 kV e-beam compared to betavoltaic devices with Ce:YAG. The directional loss in the indirect conversion devices is further reduced by applying thin reflecting aluminum film on top of the scintillator. The PCE increased by 26.7% with 30 nm thin aluminum film on top of Ce:YAG scintillator at 30 keV electron beam energy. The experimental results showed that the output electrical power from betavoltaic devices increased with the increase in incident electron beam energy.

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