Non-technical: Solar photovoltaic (PV) energy conversion is an important technology for generating low-cost electricity to replace coal-generated power. It is also very important for providing electricity to half of the people in the world who currently lack grid-connected power. For economical generation of PV power, efficiency of solar cells is a very important consideration, since area-related costs such as encapsulation, wiring and structure decrease proportionately as the efficiency of the cell increases. A very recent development in this field has been the discovery that a new thin film material, hybrid organic-inorganic metal halide based perovskites, can be used to generate PV power with a conversion efficiency of ~20%. This new material has a larger bandgap than commonly used thin film materials such as copper-indium-gallium selenide (CIGS), and therefore, can be used as a higher bandgap first cell in a monolithic tandem solar cell arrangement with CIGS. Calculations predict that such a series-connected tandem cell structures can increase the efficiency of thin film CIGS solar cells to 30% from the current value of 20%, a 50% increase. This proposal is aimed towards fundamental material and device research to achieve such high efficiency tandem cell structures. It is hoped that such a structure will lead to further developments which result in a significant decrease in the cost of solar-electric power. A graduate student will participate in this project and new teaching materials will be incorporated into courses dealing with solar energy conversion.
The proposal is focused on designing and fabricating a monolithic tandem cell structure with a high gap perovskite cell deposited on top of a high efficiency CIGS cell. The two cells are connected using novel tunnel junctions. Several new materials, processes and device structures need to be developed for the concept to work. Among these are: -A perovskite material with a bandgap in the 1.7-1.8 eV range as opposed to the current 1.57 eV material. The higher bandgap is needed to make efficient tandem cells with a CIGS bottom cell. We will develop efficient Pb(I-Br) perovskites to increase the bandgap. -A perovskite material which is physically stable at higher temperatures so that transparent contacts can be deposited on the device to allow light to be incident form the top of the cell. We will use novel organic precursors such as formamidinium iodide and urea hydroiodide to achieve thermally stable perovskites. -A new vacuum process for depositing perovskites at higher temperatures which avoids the instability of the solution growth process. -The use of inorganic heterojunction layers for electron and hole extraction , thus avoiding unstable organic heterojunction layers. -ITO/ZnO tunnel junctions to connect the two cells. -Innovative CIGS cells in1.1 to1.15 eV range with high efficiency achieved by using bandgap grading strategies along with larger grain growth and deliberate Na or K doping. The project includes comprehensive material and device analysis tasks so as to understand the physics of the device.