As energy costs rise and petroleum supplies dwindle, science is searching for clean, renewable sources to meet global energy needs. Hydrogen, fuel cells, and solar cells are among sources being explored. Since the 1950s, solar cells have been developed to convert solar radiation into electrical energy. Most of today's commercial solar cells are based on silicon and have efficiencies of about 15%. More advanced solar cell designs based on silicon and other semiconductors have been developed in laboratory environments and have been demonstrated to have efficiencies of about 30%. Unfortunately, the cost to produce these more advanced solar cells is too high for commercial applications.

Photovoltaic materials must meet several basic requirements. They need to efficiently absorb light in the region of solar radiation, create electrical charges (electron hole pairs), prevent premature recombination of the charges, and allow efficient collection of the charges.

Existing types of solar cells have one fundamental problem: limited absorption range and resulting limited overlap with the solar radiation spectrum. Most solar radiation is concentrated within the wavelength range from 300 nm to 2,500 nm. Commercial silicon solar cells can absorb only about 45% of this spectral range, limiting overall efficiency to about 15%. While new materials are being investigated, none has yet been shown to absorb light over a broad enough wavelength range, covering the visible and infrared regions.

ASL has many solar cell research focuses, including:

• Enhancing the absorption range of solar cells. A potential solution to the limited absorption range is the use of nanostructured metals, which have been shown to absorb light over a broad range of wavelengths. To increase sensitivity of photovoltaic devices, nanostructured metals are combined with electroluminescent materials for upconversion in silicon solar cells.
• Creating broadband-absorbing solar cells based on polymer-metal nanocomposites. This may be accomplished by fabricating an organic photovoltaic device that absorbs solar radiation between 400 nm and 2500 nm.
• Enhancing cell efficiency through upconversion of long wavelength. ASL is fabricating a coating, consisting of an electroluminescent phosphor and metallic nanoparticles, which can be applied to existing silicon solar cells. This coating will absorb infrared solar radiation that is not directly absorbed by the silicon solar cell and upconvert this radiation into visible light. The visible light is back-reflected into the silicon solar cell, thus increasing the overall conversion efficiency.
• Evaluating replacement of dye molecules with metallic nanoparticles in order to increase the absorption range of Graetzel cells. In a typical Graetzel cell, light is being absorbed by dye molecules adsorbed at the surfaces of TiO² nanoparticles. The photoexcited dye molecules transfer electrons to the TiO² nanoparticles which efficiently transfer the electrons to an electrode. After moving through an external circuit and performing work, the electrons re-enter the cell through an electrolyte which is in direct contact with the dye molecules, replacing the charge and completing the circuit.
• The absorption range of the Graetzel cell is determined by the absorption of the dye molecules. Typically, the absorption covers a few hundred nanometers in the visible and near-infrared range. Conversion efficiencies of about 11% have been accomplished in the lab with this particular design.
• Creation of high-efficiency organic LEDs (OLEDs) for lighting applications. Organic Light Emitting Diodes (OLEDs) lack the required efficiency of about 100 lumens/Watt and the required lifetime of about 50,000 hours needed for general lighting applications. State-of-the-art devices are delivering about 20 lumens/Watt and about 500 hours of lifetime. These numbers illustrate the need for significant improvements. This project will evaluate the potential of metal-enhanced luminescence for increasing the luminescence efficiency and lifetime of OLEDs.