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.

Excitonic solar cells (XSCs) function by a mechanism that is different than that of conventional solar cells.They have different limitations on their open circuit photovoltages, and their behavior cannot be interpreted as if they were conventional p–n heterojunctions. Exciton dissociation at the heterojunction produces electrons on one side of the interface already separated from the holes produced on the other side of the interface. This creates a powerful photoinduced interfacial chemical potential energy gradient that drives the photovoltaic effect, even in the absence of a built-in electrical potential. The maximum thermodynamic efficiency achievable in an XSC is shown to be identical to that of a conventional solar cell, with the substitution of the optical bandgap in the XSC for the electronic bandgap in the conventional cell. This article briefly reviews the photovoltaic mechanism of XSCs, the limitations on their photovoltage, and their maximum achievable efficiency.

The dye-sensitized solar cell (DSSC) provides a technically and economically viable alternative concept to present-day p–n junction photovoltaic devices. In contrast to conventional silicon systems, where the semiconductor assumes both the task of light absorption and charge carrier transport, these two functions are separated in DSSCs. The use of sensitizers having a broad absorption band in conjunction with wide-bandgap semiconductor films of mesoporous or nanocrystalline morphology permits harvesting a large fraction of sunlight. There are good prospects that these devices can attain the conversion efficiency of liquid-electrolyte-based dye-sensitized solar cells, which currently stands at 11%. In this article, we present the current state of the field, the realm of our review being restricted to the discussion of organic molecular hole conductors, which have demonstrated the best performance to date.

We consider the fundamental limits to organic solar cell efficiency, and the schemes that have been used to overcome many of these limitations. In particular, the use of double and bulk heterojunctions, as well as tandem cells employing materials with high exciton diffusion lengths, is discussed.We show that in the last few years, a combination of strategies has led to a power conversion efficiency of ηp = 5.7% (under AM 1.5 G simulated solar radiation at 1 sun intensity) for tandem cells based on small-molecularweight materials, suggesting that even higher efficiencies are possible.We conclude by considering the ultimate power conversion efficiency that is expected from organic thinfilm solar cells.

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.

Fabrication of bulk heterojunctions with well-ordered arrays of organic and inorganic semiconductors is a promising route to increasing the efficiency of polymer photovoltaic cells. In such structures, almost all excitons formed are close enough to the organic–inorganic interface to be dissociated by electron transfer, all charge carriers have an uninterrupted pathway to the electrodes, and polymer chains are aligned to increase their charge carrier mobility. Furthermore, ordered structures are interesting because they are relatively easy to model. Studies of ordered cells are likely to lead to better design rules for making efficient photovoltaic cells.

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.

This review of photoelectrochemical cells (PECs) based on inherently conducting polymers (ICPs) deals with the mechanisms of operation and the various factors that influence the overall efficiency of PECs. The factors addressed include ICP composition and oxidation state, the use of nanostructured surfaces and interfaces, and the PEC electrolyte and redox mediator.

The essential cost-driving factor for the production of classical photovoltaic devices is the expensive investment in costly semiconductor processing technologies. This unfavorable cost structure has so far prohibited the technology from having a significant impact on global energy production. Nevertheless, the continued high interest in photovoltaics originates from the fact that they represent the only truly portable renewable-energy conversion technology available today. Therefore, the potential of fabricating organic photovoltaic elements on low-cost, thin plastic substrates by standard printing and coating techniques and packaged by lamination is not only intriguing, but highly attractive from a cost standpoint. In this article, we discuss the economic and technical production aspects for organic photovoltaics.