• PDF / 353,998 Bytes
• 5 Pages / 585 x 783 pts Page_size
• 101 Downloads / 239 Views

DOWNLOAD

REPORT

---

Background World events and an increasing realization of the importance of clean, renewable sources of energy have fueled intense interest and activity in the development and implementation of a broad range of green energy technologies. Solar energy, including photovoltaic energy conversion, has enjoyed a prominent position within the green energy portfolio. In addition, sustained progress in mainstream solar energy technologies as well as revolutionary breakthroughs over the last two decades in the fabrication, understanding, and application of solid-state and soft-material nanostructures have reignited both commercial and research interest in photovoltaic devices. However, several challenges must be overcome for the tantalizing promise of photovoltaics to be fully realized. Among these, the fundamental tradeoff between light absorption and collection of photogenerated electrons and holes ranks as one of the most important. The engineering of materials and device structures to achieve new levels of control over photon propagation and light energy distribution in photovoltaic devices and systems— “photon management”—has emerged as a powerful approach for overcoming this tradeoff while also reducing reflectivity losses and enabling photovoltaic functionality to be achieved in new device and system-level form factors. The articles in this issue of MRS Bulletin address a number of directions and recent advances in this rapidly developing field. Traditional approaches to photon management in photovoltaics

have involved the use of antireflection coatings or surface texturing to maximize transmission of incident light into, or photon path length within, a solar cell; design of device structures in which absorption is maximized in regions for which the effects of minority carrier recombination are minimized; and integration with optical concentrators to direct sunlight incident over a large area onto a small, high-efficiency, high-value solar cell. More recently, a number of new directions for photon management have emerged in which subwavelength structures are employed to control, in a wavelength-dependent manner, photon propagation into and within photovoltaic devices. A defining feature of these approaches is that they exploit the wavenature of light and are therefore able to surpass classical limits of light confinement based on surface texturing. They allow for the use of thinner active layers or structures that are optimized for charge collection and potentially lower the cost of photovoltaic devices or increase their conversion efficiency.

Improving light collection The high refractive index n of inorganic semiconductor materials leads to high optical reflectivity at a direct, planar interface between such a semiconductor and a low-index medium (e.g., air or glass), as shown schematically in Figure 1a. Since reflected light is not available for conversion to electricity in a solar cell, approaches for reducing this reflectivity are an essential element of photon management in photovoltaic devices.

E.T. Yu, University of T