Printing efficient solar cells

As worldwide demand for fossil fuels depletes reserves, scientists are increasingly focused on generating alternative energy— especially if it can be produced cleanly and inexpensively. Hydroelectric, solar, wind, nuclear, and biomass technologies are replacing coal, oil, and natural gas. Many of these new technologies have similar costs. Yet the availability of materials or existing infrastructure can affect how widely they are adopted. With about 125,000TW of solar power hitting the planet, photovoltaic (PV) technologies could satisfy a significant portion of the world’s energy demand. In addition, PV cells are economical, renewable, and well suited for large-scale deployment. First-generation solar panels use silicon technology. Unfortunately, their manufacturing expense prevents them from being cost competitive. Second-generation PV devices, like those using thin films of amorphous silicon, cadmium telluride, or copperindium-gallium diselenide, are cheaper than silicon. However, they have not been adopted widely because the device materials elicit environmental and manufacturing concerns. Another option, third-generation organic-photovoltaic (OPV) cells based on inherently conductive polymers, may lead to much cheaper solar power. The polymers can be printed just like any other commercially available ink and applied to a substrate using conventional, high-volume, low-cost printing. In addition, the raw materials are ubiquitous and their use has no real impact on the environment. To further this technology’s reach, we have developed an organic photovoltaic-ink system. It may address several issues faced by traditional PV technologies, enabling increased use of printed solar power. Our ink systems are already available for research-scale printed solar-cell development. Figure 1 shows the architecture of a typical p–n-junction OPVdevice stack. The anode, a transparent electrode such as indiumtin oxide (ITO) coated onto plastic or glass, is usually covered with up to 100nm of hole-transport-layer (HTL) ink. The HTL planarizes the ITO surface and facilitates collection of positivecharge carriers (holes) from the light-harvesting layer. The phoFigure 1. An organic-photovoltaic (OPV)-device stack. The holetransport and photoactive layers form the photovoltaic-ink system. ITO: indium-tin oxide, Ca: calcium, Al: aluminum.