Date of Award
Doctor of Philosophy
Electrical and Computer Engineering
Electrical Engineering (Microelectronics and Photonics)
Solar energy is the first useful, abundant, environmentally friendly and efficient source of renewable energy in the world. The conversion of solar energy into electricity using photovoltaic (PV) technology is clearly an important way, with significant potential to mitigate global warming by lowering the emission of greenhouse gases. The costs of the PV system are determined by the overall cost of the technology, such as structural costs, field wiring and the costs of chemical encapsulation materials. In order to remarkably reduce the total costs of solar PV technology, the conversion efficiency needs to be increased from the current ~15-20% to above 30%, because most of the systems costs reduce proportionately with the reduction in material consumption and the increase in conversion efficiency. The best way to achieve such higher efficiencies is to use a tandem junction solar cell, comprising two materials, one with a high bandgap as top cell and another with a lower bandgap as bottom cell. In this project, we will investigate the fundamental optical and electronic properties of an inorganic material, CdSe, then make proof-of-concept heterojunction solar cells in this material.
Among the properties to be studied will be doping concentration, mobility of electrons and holes, deep defect densities and recombination phenomena, minority carrier diffusion lengths, electron affinities and energies of band edges. Structural, electronic and optical measurement techniques will be used to measure the relevant properties. CdSe films will be deposited using multi-source evaporation techniques.
CdSe is an II-VI group semiconductor chalcogenide which can be considered as one of the most successful materials for photovoltaic and optoelectronic applications. CdSe is more appropriate for photovoltaic application compare to other inorganic compound semiconductor. Especially easy to deposit using mass-production vapor deposition techniques. It is binary compound-by definition, stoichiometry easier to achieve than in ternary of quaternary. CdSe is not water soluble and does not thermally decompose, also it has reliable optical and electrical properties, like direct bandgap and high absorption coefficient in the visible range which makes it a good candidate as an absorber layer for solar cells.
In the first part of this work, we have shown the deposition of CdSe thin films using thermal evaporation method at high growth temperature of 400°C. The transmittance and reflection were analyzed with Cary 5000 UV-Vis-NIR spectrophotometer. The optical measurement shows CdSe is a direct band gap material with a perfect optical band gap of 1.72eV and high absorption coefficient about α ≈ 9×10^4 cm^-1 for higher energy photons range which is in the range needed to make tandem cells with crystalline silicon (c-Si).
In the next part, we study the critical processes like Cadmium Chloride (CdCl2) treatment and Post-deposition selenization to improve the electrical properties of CdSe thin films. CdCl2 activation process is a vital step which reduces the density of mid-gap states inside the bandgap and helps to obtain recrystallization, grain-growth and passivation of grain boundaries and also prevents from recombination. CdSe thin films were deposited on fluorine doped thin oxide (FTO) glass substrate at different thicknesses and their grain size and mobility were systematically analyzed before and after CdCl2 activation using scan electron microscopy (SEM) and space charge limited current (SCLC) techniques respectively. CdCl2 activated CdSe films showed larger grain size (from 0.9 to 1.9µm) and higher electron mobility (from 0.09 to 3.52cm^2/V.s) by increasing film thicknesses (from 0.5 to 3µm) respectively. Post-deposition selenization of CdSe thin films shows much larger grain size (from 0.3 to 5.7µm), higher photoconductivity value (from 2.94×10^-4 to 1.21×10^-3 Ω-1.cm-1) and higher mobility lifetime product for electron as majority carrier (from 1.37×10^-6 to 5.62×10^-6 cm2/V) by increasing the selenization time (from 0 to 120min) respectively.
In the last part of this thesis, we have studied, designed and fabricated the efficient heterojunction CdSe solar cell. We developed new NP superstrate and substrate device structure with different n-layer and p-layer such as n-CdS and p-PTAA, and p-PEDOT:PSS. In addition, cadmium chloride and post-deposition selenization were developed to passivate the recombination centers. 2µm grains were achieved under CdCl2 treatment (at 500C for 10h). The highest Voc (world record) ever achieved in CdSe solar cells (0.8V) and current density 8mA/cm2. Moreover, we found dopant density, optical bandgap, urbach energy, shallow and deep traps equal to 2.5×10^15 cm-3, 1.7eV, 15.6meV, 0.24eV and 0.53eV respectively. Attempt-to-escape frequency and relative dilectric constant (ϵr) were measured 2×10^9 Hz and 10.4 respectively.
Bagheri, Behrang, "Research project to study cadmium selenide (CdSe) solar cells" (2020). Graduate Theses and Dissertations. 17865.