Nanostructured phase-separated blends, or bulk heterojunctions, of conjugated polymers and fullerene derivatives form a very attractive approach to large-area, solid-state organic solar cells. The key feature of these cells is that they combine easy processing from solution on a variety of substrates with good performance. Efficiencies of up to 5% in solar light have been achieved, and lifetimes are increasing to thousands of hours. Further improvements can be expected and some of the promising strategies towards that goal are presented in this article.

Harvesting energy directly from sunlight using photovoltaic technology is a way to address growing global energy needs with a renewable resource while minimizing detrimental effects on the environment by reducing atmospheric emissions. This issue of MRS Bulletin on “Organic-Based Photovoltaics” looks at a new generation of solar cells that have the potential to be produced inexpensively. Recent advances in solar power conversion efficiencies have propelled organic-based photovoltaics out of the realm of strictly fundamental research at the university level and into the industrial laboratory setting. Fabricated from organic materials—polymers and molecules—these devices are potentially easier to manufacture than current technologies based on silicon or other materials. In this introductory article, we describe the motivation for pursuing research in this field and provide an overview of the various technical approaches that have been developed to date.We conclude by discussing the challenges that need to be overcome in order for organic photovoltaics to realize their potential as an economically viable path to harvesting energy from sunlight.

Recent results have demonstrated that hybrid photovoltaic cells based on a blend of inorganic nanocrystals and polymers possess significant potential for low-cost, scalable solar power conversion. Colloidal semiconductor nanocrystals, like polymers, are solution processable and chemically synthesized, but possess the advantageous properties of inorganic semiconductors such as a broad spectral absorption range and high carrier mobilities. Significant advances in hybrid solar cells have followed the development of elongated nanocrystal rods and branched nanocrystals, which enable more effective charge transport. The incorporation of these larger nanostructures into polymers has required optimization of blend morphology using solvent mixtures. Future advances will rely on new nanocrystals, such as cadmium telluride tetrapods, that have the potential to enhance light absorption and further improve charge transport. Gains can also be made by incorporating application-specific organic components, including electroactive surfactants which control the physical and electronic interactions between nanocrystals and polymer.