The first Focus Issue on Solar Concentrators, published in the inaugural issue of Energy Express last April, illustrated how power conversion efficiency can be increased by fabricating multi-junction solar cells. Cells with efficiencies higher than 40% have been demonstrated with highly crystalline films of the semiconductors GaInP and GaInAs grown on Ge to be compared with efficiencies of 13 to 19% for silicon modules. However, to reach high performance nearly perfect materials must be grown, which considerably increases their manufacturing costs. That is why these multi-junction cells are combined with solar concentrators that concentrate the light by a factor of 500–1000, reducing the use of semiconductor materials.
Newcomers in the commercial photovoltaic market such as organic photovoltaics (OPV) and dye-sensitized solar cells (DSSC) still lag behind their inorganic counterparts, but progress in these technologies is steady. OPV technologies have recently reached efficiencies of 8.1% (Solarmer Energy, Inc.) in laboratory polymer bulk heterojunctions and 7.7% (Heliatek) in molecular multilayer devices with an active area of 1 cm2. An example of advances in DSSC technologies is provided in the paper by Beomjin Yoo and colleagues from KIST Korea. The paper by Andrew J. Medford and co-workers from RISO in Denmark illustrates how OPV modules can be manufactured using roll-to-roll manufacturing, providing solar technologies with flexible form factors. Despite their great potential, OPV technologies must overcome significant barriers, both in efficiency and lifetime, before they can compete with existing PV technologies. The controlled growth of organic materials can be quite complex. The importance of controlling their morphology in processes that are easily scalable is illustrated in the paper by Brian E. Lassiter and co-workers from the University of Michigan. Alternatives to organic materials are inks or dispersions of inorganic nanocrystals that lend themselves to manufacturing using printing and moderate processing conditions that can lead to devices with the performance and stability of those based on inorganic materials. The paper by Vahid A. Akhaven and colleagues from the University of Texas at Austin describes such an approach using CuInSe2 nanocrystals that are environmentally friendlier than semiconductor nanocrystals used to obtain the highest efficiencies. The paper by D. Aaron R. Barkhouse and co-workers from the University of Toronto illustrates how detailed optical characterization and modeling techniques can serve to better understand limitations to the performance of these hybrid technologies based on nanocrystals. A paper by Seungkeun Choi and colleagues from the Georgia Institute of Technology shows that substitutes to indium tin oxide (ITO) such as conducting polymers can be combined with thick metal grid electrodes in large-area organic solar cells.
In my introduction to the inaugural issue of Energy Express, I highlighted the power of optical solutions to complex and challenging problems such as renewable and clean energy production. The paper by Brendan O’Connor and his colleagues at the University of Michigan provides a good example of creative and completely new approaches based on fiber-based architectures that can be used to harvest the sunlight.
I hope that the readers of Optics Express and its supplement Energy Express will enjoy reading the latest advances described in this Focus Issue and that it will motivate them to publish their latest discoveries in the area of optics for energy in Energy Express. I also want to express my sincere gratitude to the contributors who accepted my invitation to contribute an article and who worked around stringent publication deadlines. This second Focus Issue, Thin-Film Photovoltaic Materials and Devices, would not have been possible without the efforts of C. Martijn de Sterke (University of Sydney), Editor-in-Chief of Optics Express, and the work of the Associate Editors, reviewers, and the staff coordinating OSA’s publications. I want to express my gratitude to all of them.
Atlanta, September 13, 2010