Abstract
The theoretical limit of solar energy conversion is over 85%, yet the maximum efficiency of any solar cell in the laboratory is less than half this value, and commercial solar cells are only one fifth. For solar cells to meet world’s future energy demands, the challenge is to develop solar cells that achieve efficiencies that approach the thermodynamic limit.
The Very High Efficiency Solar Cell program led by Drs. Christiana Honsberg and Allen Barnett of the University of Delaware is demonstrating new levels for high efficiency solar cells and modules. The program’s goal is to develop solar cell modules, photovoltaic (PV) modules, for portable applications that can operate at greater than 50 percent efficiency. The approach uses proven quantitative models for the solar cell design coupled with high optical efficiency and system integration. The resulting optical/solar cell allows efficiency improvements while retaining low costs, and expanding photovoltaic applications. The new cell design utilizes multiple junctions for the high and low energy photons, a new silicon cell for the mid-energy photons and novel cell architectures and optical elements. The DARPA team has produced multiple junction solar cells with a record-efficiency of 42.9%.
Multiple junction solar cells, called tandems cells, theoretically allow the approaching of the thermodynamic limit with an “infinite” stack of homojunctions. However, material-related issues have limited these devices to three-junction design. Recently, new physical mechanisms have been proposed which allow higher efficiency for a given number of materials and also offer other advantages such as reduced sensitivity to temperature or use of low-cost nano-materials.
These talks will present our approaches to producing ultra-high efficiency solar cells and discuss the experimental and theoretical challenges in making these cells. These routes include multi junction solar cells consisting of integrated pn junctions, multiple transition cells (quantum well or intermediate band quantum dot cells) and multiple exciton generation. In addition, physical insights into operation and design rules for advanced concept cells will be presented.