10 MRS BULLETIN ��� VOLUME 30 ��� JANUARY 2005 Introduction Harvesting energy directly from sunlight using photovoltaic (PV) technology* is being increasingly recognized as an essen- tial component of future global energy production. The finite supply of fossil fuel sources and the detrimental long-term effects of CO2 and other emissions into our atmosphere underscore the urgency of de- veloping renewable energy resources. PV technology is being increasingly recognized as part of the solution to the growing en- ergy challenge and an essential component of future global energy production. Pro- vided that photovoltaics can be made truly economically competitive with fossil fuels, large-scale manufacturing of these devices offers a pathway to a sustainable energy source that could supply a significant frac- tion of our energy needs. This comes with only minimal impact on the environment associated with the energy needed to manu- facture the devices (this is embedded in the cost), disposal of small amounts of waste materials from the manufacturing process, and the land use for the installation of large- scale PV systems. The impact of land use can be minimized by locating large-scale arrays in desert regions of little ecological importance and can be completely negated by integrating PV systems into existing building structures and rooftops. A photovoltaic device, or solar cell, con- verts absorbed photons directly into elec- trical charges that are used to energize an external circuit (Figure 1). The photovoltaic effect was discovered in 1839 by French physicist Edmond Becquerel. Attempts at commercialization did not begin until a century later, and the first crystalline sili- con PV device was developed in 1954 at Bell Laboratories. A typical conventional solar cell is fabri- cated from an inorganic semiconductor ma- terial, such as crystalline Si, that is doped to form a p���n junction. The p side contains an excess of positive charges (holes), and the n side contains an excess of negative charges (electrons). In the region near the junction, called the depletion region, an electric field is formed. Electrons and holes generated through light absorption in the bulk of the Si diffuse to this junction, where they are accelerated by the electric field to- ward the proper electrode. If the Si is of sufficient quality, the charges reach the elec- trodes and leave the device in order to drive the external circuit. The power conversion efficiency of this process is defined as the ratio of the electric power provided to the external circuit to the solar power incident on the active area of the device. This is typi- cally measured under standard simulated solar illumination conditions, and efficien- cies of over 24% have been achieved in the laboratory for devices such as the one just described. As will be discussed later in this article, and throughout this issue of MRS Bulletin, the fundamental mechanism by which organic PV devices work is substan- tially different than that of Si and other in- organic semiconductor devices. In 2002, 49.3 quads (1 quad 1 quad- rillion Btu 2.9 1011 kWh) of electrical energy were consumed worldwide.1 The U.S. accounted for the largest percentage of consumption (25.6%), followed by West- ern Europe (19.2%), China (10.2%), Japan (6.8%), and Russia (5.5%). As a case study, let���s examine how much land area of PVs would be required to supply O rganic-Based Photovoltaics: T oward Low-Cost Power Generation Sean E. Shaheen, David S. Ginley, and Ghassan E. Jabbour, Guest Editors Abstract 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. Keywords: electron acceptors, energy production, excitons, metal oxide semiconductors, nanostructures, organic semiconductors, photovoltaics, polymers, power generation, quantum dots, solar cells. *An introduction to photovoltaic technology can be found at Web site www.eere.energy.gov/ solar/pv_basics.html. Figure 1. Illustration of the operation of a photovoltaic module. www.mrs.org/publications/bulletin

Organic-Based Photovoltaics:Toward Low-Cost Power Generation MRS BULLETIN ��� VOLUME 30 ��� JANUARY 2005 11 the electricity needs of the U.S., the largest consumer. In 2003, 39.6 quads of energy, largely from fossil fuels, were consumed to produce electricity in the U.S. After con- version losses, 13.1 quads of net electrical energy were output by power plants for gen- eral consumption.2 This amount of electric- ity could be produced by a 100 km 100 km land area in a region of high solar insola- tion, such as in the deserts of the Nevada or Arizona, covered with solar modules with a power conversion efficiency of 15%. In order to meet the U.S. Department of Energy���s cost goal of $0.33/W,3 these modules would have to be manufactured at a cost of $50/m2 or less. Similar calcula- tions can be done for desert regions around the globe. Photovoltaic Technologies Given the vast potential of photovoltaic technology, worldwide production of ter- restrial solar cell modules has been growing rapidly over the last several years, with Japan recently taking the lead in total pro- duction volume (Figure 2). Current pro- duction is dominated by single-crystal and polycrystalline silicon modules, which rep- resent 94% of the market. These devices, based on silicon wafers, have been termed the ���first generation��� of PV technology. These are single-junction devices that are limited by thermodynamic considerations, stemming largely from energy loss due to carrier thermalization, to a maximum theo- retical power conversion efficiency of 31% when illuminated by the AM 1.5 solar spectrum��� with an intensity of 1000 W/m2 (1 sun).4 Progress in efficiencies of research- scale crystalline PV devices over that last several decades is shown in Figure 3. Clearly, significant improvements in device efficiencies have been steadily achieved. One key to the development of any photovoltaic technology is the cost reduc- tion associated with achieving economies of scale. This has been evident with the de- velopment of crystalline silicon PVs and will presumably be true for other tech- nologies as their production volumes in- crease. Figure 4 shows the decrease in the cost of crystalline silicon PV modules as the production rate has increased, as well as predicted future costs for both c-Si on wafer and the emerging c-Si film on glass technologies. The current cost of $4/W is still too high to significantly influence energy production markets. Best estimates are that costs will level off in the region of $1/W���$1.50/W in the next 10 years, sub- stantially higher than the $0.33/W target. Thus, over the last decade, there has been considerable effort in advancing thin- film, ���second-generation��� technologies that do not require the use of silicon wafer substrates and can therefore be manufac- tured at significantly reduced cost.5 Steady progress has been made in laboratory effi- ciencies, as can be seen in Figure 3 for de- vices based on CdS/CdTe, Cu(In,Ga)Se2 (CIGS), and multijunction a-Si/a-SiGe. These devices are fabricated using tech- niques such as sputtering, physical vapor deposition, and plasma-enhanced chemi- cal vapor deposition. Multijunction cells based on a-Si/a-SiGe have been the most successful second-generation technology to date because of their ability to be fabricated at relatively low cost. Several companies are manufacturing a-Si/a-SiGe modules using roll-to-roll processing on flexible stainless steel and other substrates that allow high-speed production as well as easy integration into roofing materials. Figure 2. World photovoltaic module production (megawatts), total consumer and commercial use, per country.18 Figure 3. Progress of research-scale photovoltaic device efficiencies, under AM 1.5 simulated solar illumination, for a variety of technologies. ���Solar spectra are defined by an air mass (AM) value, which is a measure of the length of the path through the earth���s atmosphere that the solar radiation travels. The value is calculated as 1/cos z, where z is the zenith angle between a line perpendicular to the earth���s surface and a line intersecting the sun. AM 1 describes the case in which the sun is directly overhead. The AM 1.5 spectrum is commonly used for testing photo- voltaic devices meant for terrestrial use. AM 0 is the spectrum of sunlight outside the earth���s at- mosphere and is used for testing PV devices in- tended for use in space.