Abstract: In this work, we present the growth of full composition graded ZnCdSSe thin film structures on a single substrate and development of solar cells with enhanced solar conversion efficiency. This is realized by a temperature dependent composition deposition method in CVD reactor. In addition to thin film growth, we also grew nanowires on top of various compositional thin films within the ZnCdSSe material system. After the successful growth of those materials we proposed a method to build thin film/nanowire hybrid structures to form 3D junctions for enhanced solar conversion efficiencies in solar cells. Introduction: The increasing demand of energy and the limited reserves of fossil fuels require the development of renewable energy sources such as solar, wind, geothermal, and etc. Solar among the others is the most promising one as it provides 1400 times more energy than what the whole world currently consume in a year. The best way to capture the solar energy is the photovoltaic cells and those are composed of various semiconductors with different properties. It has been a great research area to grow semiconductors with proper bandgap energy values making them suitable for solar cell applications. II-VI materials ZnS and CdSe can be used to form quaternary alloys and their full compositions can cover a wide range of the spectrum and can be utilized for solar cell application. Aim: Our aim is to grow hybrid structures having bandgap energy suitable for solar cell applications and then build 3D junction based solar cell for enhanced solar conversion efficiencies. Constrains: Producing planar junctions with conventional methods for bulk materials are easy; however creating 3D junctions require a dozens of steps taken in clean room environment. By utilizing bottom-up approach we can easily form 3D junctions and problems related to the limited diffusion length can be solved through our proposed method with minimum number of steps. Method: Single crystal wide composition graded ZnxCd1-xSySe1-y quaternary alloy thin films on a single substrate were grown via vapor-solid (VS) growth mechanism using a single zone chemical vapor deposition (CVD) reactor. Growth was carried out utilizing the temperature dependent composition deposition and CdSe and ZnS powders were used as source materials. By placing the precursors and substrate at the right position full composition graded quaternary films can be grown on the substrate as various compositions prefer distinctive growth temperatures. Results: By using various growth parameters we demonstrated the growth of full composition graded ZnCdSSe thin films on a single substrate and all the characterizations results revealed that those materials can be grown in high crystal quality and exhibit promising optical features. In addition to the thin film growth we have also grown hybrid structures with thin film at the bottom and nanowires on top. In order to grow those structures we coated the pre grown thin film layer with catalyst material and then the growth was taken place at the right position to grow CdSe-rich nanowire structures. As a result of that pin brush-like materials were obtained. By filling the gaps of the nanowires with p-type materials we can easily create 3D pn junctions. If we design a solar cell based on 3D pn junctions we can eliminate the short diffusion length problem of the carrier because photo generated carriers can become majority carrier by moving in the perpendicular direction to the substrate. By this way external quantum efficiency of the cells can be greatly enhanced. Conclusions: We demonstrated wide composition graded quaternary alloy thin films grown by a facile method based on the temperature dependent composition deposition. The emission wavelength coverage of the materials is between 400nm-700nm and thus making the structures very important for various optoelectronic applications. We also grew pin brush-like thin film/nanowire hybrid structures for the first time with a single growth steps to potentially create 3D pn junctions. Our results pave the way for high efficiency full spectrum solar cells as it reduces the carrier recombination in the device and collects more photo-generated carriers. Along with device application, the growth method can be also adopted for other important material systems to develop novel optoelectronic applications.
Anahtar Kelimeler: Thin Films, Nanowires, 3D pn Junctions, Solar Cells, Bandgap Engineering
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