RSC Advances
Total Page：16
File Type：pdf, Size：1020Kb
RSC Advances REVIEW The case for organic photovoltaics Seth B. Darling*ab and Fengqi Youc Cite this: RSC Advances, 2013, 3, 17633 Increasing demand for energy worldwide, driven largely by the developing world, coupled with the tremendous hidden costs associated with traditional energy sources necessitates an unprecedented fraction of the future global energy mix come from sustainable, renewable sources. The potential solar energy resource dwarfs that of all other renewable sources combined, yet only two photovoltaic technologies are known to have the potential to be scaled up to make dramatic impact on the overall Received 14th June 2013, energy mix: silicon and organic photovoltaics. In this paper, we present the long-term sustainability Accepted 4th July 2013 advantages of organics when compared to silicon and other photovoltaic technologies in terms of energy DOI: 10.1039/c3ra42989j payback time and global warming potential while also discussing the outlook for transitional applications www.rsc.org/advances of organic solar cells. Introduction In any discussion of energy, units play a central (and often confusing) role. Here we will adopt the convention of focusing The need for sustainable energy on the average rate of energy use, measured in Watts, rather Global demand for energy has been growing essentially than energy itself, which is measured in any of a panoply of unabated since the dawn of the Industrial Revolution, and different units (e.g. Joules, Watt-hours, Btus, quads, tons of oil nearly all projections provide a picture of continued expansion equivalent, therms, etc.). Fig. 1 depicts historical and projected of energy use for the foreseeable future. Vast supplies of fuels global energy demand in terawatts (TW) from 1990–2035 as 1 have served as the engine for economic development and reported by the U.S. Energy Information Administration (EIA). increased affluence for enormous numbers of people. While These data are broken down by OECD and non-OECD metrics for economic development can include per capita countries to highlight the relatively larger contribution to the consumption of materials, waste product emissions, or future growth from the latter. From this chart, one can see that expenditure on goods and services, energy use is an essential today we consume approximately 18 TW worldwide, a number and easily quantified element of economic activity. Energy use that is projected to rise to over 25 TW by 2035 and widely reflects the intensity of industrial, agricultural, and transport activity, and is often a reliable indicator of GDP. Therefore, it is not surprising that the majority of the projected growth in energy demand over the coming decades will originate in the developing world, e.g. China, India, Brazil, and eventually African countries, which can be loosely collected under the heading of countries outside of the Organization for Economic Cooperation and Development (OECD). In other words, if these countries are to continue their development, growth in demand is inevitable. Energy efficiency and conservation can temper a portion of the global demand, and indeed represent the most cost-effective approaches available to address the energy challenge, but no matter how pervasive, efficiency and conservation alone far fall short of matching the anticipated need. aCenter for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois, 60439, USA. E-mail: [email protected]; Fax: 630-252-4646; Tel: 630-252-4580 bInstitute for Molecular Engineering, The University of Chicago, Chicago, Illinois, USA cDepartment of Chemical and Biological Engineering, Northwestern University, Fig. 1 Global energy demand based on data through 2010 and EIA projections Evanston, Illinois, USA from 2015–2035.1 This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 17633–17648 | 17633 Review RSC Advances central to society, taking an immense toll on human life and quality of life. Though global reserves of coal, gas, and especially oil will eventually be depleted, current reserves and resources are sufficient—particularly in light of developments in hydraulic fracturing technology to extract shale gas and oil—to provide the vast majority of our energy needs on one hand and to inflict unprecedented damage on the other. Emissions from fossil fuel power plants are associated with acid rain that deteriorates infrastructure; particulates and ozone that aggravate asthma, bronchitis, and other pulmonary conditions; heavy metals that lead to acute poisoning, cancer, and developmental disorders; air and water pollution that decrease crop yields and produce smog; and thermal pollution that kills aquatic ecosystems. Coal is the worst offender among the fossil sources, and though natural gas is the cleanest of the fossil fuels and is steadily displacing coal in the U.S. system, this means the emissions are less hazardous, not that they are innocuous. Moreover, coal use is expanding dramatically in the developing world. These impacts have a direct and substantial cost in our health insurance, food prices, tourism dollars, and taxes, among others yet do not show up on our electric bills. Dwarfing these hidden costs, though, are the ever increasing effects of another emission: greenhouse gases. Fig. 2 Global energy consumption breakdown by energy source in 2013 derived from the EIA reference case.1 Climate disruption caused by carbon dioxide, methane, nitrous oxide, and other greenhouse gases contributes to more frequent and more substantial droughts, floods, wild- fires, and storms as well as biodiversity loss, sea level rise, and believed to reach 30 TW by the year 2050. In other words, the numerous other deleterious effects. When all of these hidden projected demand in 2050 is about twice what we use today. costs are folded into the overall economic decision making, This represents an enormous expansion of energy use, raising fossil fuels no longer represent the most attractive option. serious societal questions about (1) where we can obtain this Nuclear energy is one of the major secondary sources in use amount of energy and (2) what are the consequences today, and one whose use could in principle be expanded far associated with the utilization of those energy sources at beyond the current capacity. Though nuclear fission carries those scales. These questions are among the most important few of the same hidden costs associated with fossil fuels, it has issues facing humanity, and how we answer them will have several serious hidden costs of its own: (1) nuclear weapons tremendous impact on the lives of future generations. proliferation, or so-called dual-use capabilities, and nuclear Before addressing our future energy mix, it is informative to materials use for terrorism represent ongoing challenges that survey the makeup of our current supply. Fossil fuels are difficult to connect to a specific economic cost. (2) There dominate today’s energy mix, collectively representing over has been little progress on long-term management of high- 80% of our total energy supply (Fig. 2). In contrast, renewable level radioactive waste, though in principle deep underground sources represent about 10%, the vast majority of which is repositories could eventually provide a solution. (3) Nuclear contributed by hydropower. Despite their considerable pre- accidents such as those at Three Mile Island in 1979, sence in media and common conversation, sources such as Chernobyl in 1986, and most recently at Fukushima Daiichi wind, solar, and geothermal are currently tiny fractions of the in 2011 have both terrible short-term local effects on overall blend (Fig. 2). The dominance of fossil fuels is a individuals and ecosystems and very long-lasting regional consequence of their abundance and a long history of aftermaths. Estimates are that cleanup in Fukushima will end government subsidies. U.S. land grants for coal in the 19th up costing over $250 billion,3 with far larger impact on the century and tax expenditures for oil and gas in the early 20th overall Japanese economy. In all likelihood nuclear fission will century, as well as various incentives continuing to the present continue to play a key role in our energy mix for decades to day, have helped drive economic growth for more than 200 come, but the scale of that role must be tempered by these years. With the expense of many decades of infrastructure hidden costs. already invested, fossil fuel energy has a very low apparent Beyond fossil fuels and nuclear, the only other major energy cost, making it generally the most attractive in the energy sources are renewable energy sources. These sources are marketplace. associated with vastly lower hidden costs (though still The challenge going forward is not that fossil fuels are a important to consider and minimize, such as with negative limited resource, though this is indeed the case, but rather impacts on local wildlife); their relatively small current role in that their continued use on a massive scale—and a scale that our energy mix is based primarily on their apparently higher continues to grow with time—is associated with tremendous cost. Here it is important to note that despite the common hidden costs2 that threaten to eventually overwhelm structures perspective that government subsidies for renewable energy 17634 | RSC Adv., 2013, 3, 17633–17648 This journal is ß The Royal Society of Chemistry 2013 RSC Advances Review are unfairly propping up inferior technologies, subsidies have used directly from sunlight—a