Nanostructured Device Designs for Enhancing the Performance of Thin Film Cd'Te/CdS and CIS/CdS Solar Cell Devices

The increasing demand for energy (from 14 terawatts in year 2000 to 50 terawatts in year 2050) and its environmental impact requires a renewed effort and novel approaches to developing clean and efficient energy sources. Within this context nanoscience and nanotechnology offers exciting and requisite approaches to addressing these challenges. At the root of the opportunities provided by the nanotechnology is the fact that, all the elementary steps of energy conversion (such as charge transfer, molecular reairangement, chemical reactions etc) take place at the nanoscale. Thus the development of new materials, device structures as well as methods to characterize, manipulate and assemble them, creates an entirely new paradigm for developing new and revolutionary energy technologies. For the realization of the possibilities offered by nanoscale science and technology, development of novel techniques for fabricating large area, uniform, self-ordered films, is indispensable. Thus, there is a need for a process to economically fabricate large periodic arrays of semiconductor nanostructures that will allow (a) the size and composition to be varied, (b) encapsulation in a rugged host material, (c) flexibility to use a variety of substrate materials. Furthermore, to make practical devices, one must study and understand the electrical and optical properties of Nanostructured materials and their interfaces with other materials, so that new devices can be engineered. A clear understanding of charge transport in nanoscale hetero-junctions is essential for the development of a host of opto-electronic devices. The proposed research involves the fabrication, characterization and analysis of Nanoscale hetero- junctions inside an insulating Alumina (A1203) matrix and applying this understanding to increase the short circuit currents and efficiencies of solar cells based on above semiconductors. The potential applications of this research include energy conversion, display devices and sensors. The existing cadmium sulfide (CdS)/cadmium telluride (CdTe) thin film hetero junction solar cells have reached an efficiency level of 16.5%. The existing challenges for achieving still better performance are, (i) light absorption in the CdS window layer, (ii) interface states at the CdTe-CdS hetero junction and, (iii) less than optimal contact to CdTe. Our proposed solutions are, (i) use of nanowire design to reduce light absorption in the CdS window layer, and, (ii) carbon nanotubes (CNTs) based electrode to p-CdTe, instead of the traditional graphite paste electrode. These improvements are expected to yield not only stable electrode, but also efficiencies as high as 22%. Copper indium gallium diselenide (CIGS)/cadmium sulfide (CdS) solar cells have reached a power conversion efficiency of 19.6%. The path to higher efficiency is through higher open circuit voltage. In this proposal, a size dependent quantum confinement of the band gap energy is proposed as a route to achieve higher open circuit voltage in CIS/CdS solar cells. With our nanowire technology, the CIS energy band gap will be increased from 1.04 eV to 1.5 eV, and efficiency will increase from 19.6% to 25%. Objectives: (1) Fabricating controlled diameter nano-wire heterojunctions and, understanding the physics of electron transport in them, (2) Using nanowires as ideal absorbers in photovoltaic device structures and, performing a scientific study of the effect of size (wire diameter) on the electro-optical and material characteristics of films and junctions at nanoscale. (3) Applying this knowledge toward achieving higher efficiencies of power conversion in Nanostructured single junction CIS/CdS and CdTe/CdS solar cells.