organic-based aphotovoltaic … · organic-based photovoltaics:toward low-cost power generation...

10 MRS BULLETIN • VOLUME 30 • JANUARY 2005

IntroductionHarvesting energy directly from sunlight

using photovoltaic (PV) technology* isbeing increasingly recognized as an essen-tial component of future global energyproduction. The finite supply of fossil fuelsources and the detrimental long-termeffects of CO2 and other emissions into ouratmosphere underscore the urgency of de-veloping renewable energy resources. PVtechnology is being increasingly recognizedas part of the solution to the growing en-ergy challenge and an essential componentof future global energy production. Pro-

vided that photovoltaics can be made trulyeconomically competitive with fossil fuels,large-scale manufacturing of these devicesoffers a pathway to a sustainable energysource that could supply a significant frac-tion of our energy needs. This comes withonly minimal impact on the environmentassociated with the energy needed to manu-facture the devices (this is embedded in thecost), disposal of small amounts of wastematerials from the manufacturing process,and the land use for the installation of large-scale PV systems. The impact of land usecan be minimized by locating large-scalearrays in desert regions of little ecologicalimportance and can be completely negatedby integrating PV systems into existingbuilding structures and rooftops.

A photovoltaic device, or solar cell, con-verts absorbed photons directly into elec-trical charges that are used to energize anexternal circuit (Figure 1). The photovoltaiceffect was discovered in 1839 by Frenchphysicist Edmond Becquerel. Attempts atcommercialization did not begin until acentury later, and the first crystalline sili-con PV device was developed in 1954 atBell Laboratories.

A typical conventional solar cell is fabri-cated from an inorganic semiconductor ma-terial, such as crystalline Si, that is dopedto form a p–n junction. The p side containsan excess of positive charges (holes), andthe n side contains an excess of negativecharges (electrons). In the region near thejunction, called the depletion region, anelectric field is formed. Electrons and holesgenerated through light absorption in thebulk of the Si diffuse to this junction, wherethey are accelerated by the electric field to-ward the proper electrode. If the Si is ofsufficient quality, the charges reach the elec-trodes and leave the device in order to drivethe external circuit. The power conversionefficiency of this process is defined as theratio of the electric power provided to theexternal circuit to the solar power incidenton the active area of the device. This is typi-cally measured under standard simulatedsolar illumination conditions, and efficien-cies of over 24% have been achieved in thelaboratory for devices such as the one justdescribed. As will be discussed later in thisarticle, and throughout this issue of MRSBulletin, the fundamental mechanism bywhich 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 electricalenergy were consumed worldwide.1 TheU.S. accounted for the largest percentageof 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 muchland area of PVs would be required to supply

Organic-BasedPhotovoltaics:Toward Low-CostPower Generation

Sean E. Shaheen, David S. Ginley, andGhassan E. Jabbour, Guest Editors

AbstractHarvesting energy directly from sunlight using photovoltaic technology is a way to

address growing global energy needs with a renewable resource while minimizingdetrimental effects on the environment by reducing atmospheric emissions.This issue ofMRS Bulletin on “Organic-Based Photovoltaics” looks at a new generation of solar cellsthat have the potential to be produced inexpensively. Recent advances in solar powerconversion efficiencies have propelled organic-based photovoltaics out of the realm ofstrictly fundamental research at the university level and into the industrial laboratorysetting. Fabricated from organic materials—polymers and molecules—these devices arepotentially easier to manufacture than current technologies based on silicon or other materials. In this introductory article, we describe the motivation for pursuing research inthis field and provide an overview of the various technical approaches that have beendeveloped to date. We conclude by discussing the challenges that need to be overcomein order for organic photovoltaics to realize their potential as an economically viable pathto 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 technologycan be found at Web site www.eere.energy.gov/solar/pv_basics.html.

Figure 1. Illustration of the operation ofa photovoltaic module.

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Organic-Based Photovoltaics:Toward Low-Cost Power Generation

MRS BULLETIN • VOLUME 30 • JANUARY 2005 11

the electricity needs of the U.S., the largestconsumer. In 2003, 39.6 quads of energy,largely from fossil fuels, were consumedto produce electricity in the U.S. After con-version losses, 13.1 quads of net electricalenergy were output by power plants for gen-eral consumption.2 This amount of electric-ity could be produced by a 100 km � 100 kmland area in a region of high solar insola-tion, such as in the deserts of the Nevadaor Arizona, covered with solar moduleswith a power conversion efficiency of15%. In order to meet the U.S. Departmentof Energy’s cost goal of $0.33/W,3 thesemodules would have to be manufacturedat a cost of $50/m2 or less. Similar calcula-tions can be done for desert regionsaround the globe.

Photovoltaic TechnologiesGiven the vast potential of photovoltaic

technology, worldwide production of ter-restrial solar cell modules has been growingrapidly over the last several years, withJapan recently taking the lead in total pro-duction volume (Figure 2). Current pro-duction is dominated by single-crystal andpolycrystalline silicon modules, which rep-resent 94% of the market. These devices,based on silicon wafers, have been termedthe “first generation” of PV technology.These are single-junction devices that arelimited by thermodynamic considerations,stemming largely from energy loss due tocarrier thermalization, to a maximum theo-retical power conversion efficiency of�31% when illuminated by the AM 1.5 solarspectrum† with an intensity of 1000 W/m2

(1 sun).4 Progress in efficiencies of research-scale crystalline PV devices over that lastseveral decades is shown in Figure 3.Clearly, significant improvements in deviceefficiencies have been steadily achieved.

One key to the development of anyphotovoltaic technology is the cost reduc-tion associated with achieving economiesof scale. This has been evident with the de-velopment of crystalline silicon PVs andwill presumably be true for other tech-nologies as their production volumes in-crease. Figure 4 shows the decrease in thecost of crystalline silicon PV modules as

the production rate has increased, as wellas predicted future costs for both c-Si onwafer and the emerging c-Si film on glasstechnologies. The current cost of �$4/Wis still too high to significantly influenceenergy production markets. Best estimatesare 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 hasbeen considerable effort in advancing thin-film, “second-generation” technologiesthat do not require the use of silicon wafersubstrates and can therefore be manufac-tured at significantly reduced cost.5 Steadyprogress 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 vapordeposition, and plasma-enhanced chemi-cal vapor deposition. Multijunction cellsbased on a-Si/a-SiGe have been the mostsuccessful second-generation technology todate because of their ability to be fabricatedat relatively low cost. Several companiesare manufacturing a-Si/a-SiGe modulesusing roll-to-roll processing on flexiblestainless steel and other substrates thatallow high-speed production as well aseasy integration into roofing materials.

Figure 2. World photovoltaic module production (megawatts), total consumer andcommercial use, per country.18

Figure 3. Progress of research-scale photovoltaic device efficiencies, under AM 1.5simulated 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 thepath through the earth’s atmosphere that thesolar radiation travels. The value is calculated as1/cos z, where z is the zenith angle between aline perpendicular to the earth’s surface and aline intersecting the sun. AM 1 describes the casein which the sun is directly overhead. The AM 1.5spectrum is commonly used for testing photo-voltaic devices meant for terrestrial use. AM 0 isthe spectrum of sunlight outside the earth’s at-mosphere and is used for testing PV devices in-tended for use in space.







These cells comprise 5.6% of the 6% of themarket not dominated by crystalline orpolycrystalline silicon.

The increasing manufacturing capacityin solar cells (as illustrated in Figure 4),coupled with the uncertainties of fossilfuel energy sources, is clearly a significantdriver for the current PV technologies. Ul-timately, however, there will be limitationson how much the costs of current tech-nologies can be reduced. Achieving trulycompetitive cost/efficiency ratios may re-quire breakthroughs even in mainstaytechnologies. This has engendered theconcept of third-generation PV technolo-gies, originally developed by Martin Greenof the University of New South Wales andillustrated in Figure 5.6 Here, the third-generation technologies (III) can be sepa-rated into two categories. In the first, IIIa,are novel approaches that strive to achievevery high efficiencies—by using conceptssuch as hot carriers, multiple electron–holepair creation, and thermophotonics—thathave theoretical maximum efficiencies wellin excess of the 31% target for a single-junction device. In this case, the allowedcost of the cell could be quite high. Re-search in this area is still in its infancy. Ex-amples of the fundamental mechanismshave been demonstrated, but working de-vices have yet to be achieved. In the sec-

ond type of third-generation device, IIIb,the goal is to achieve moderate efficiency(15–20%), but at a very low cost. To achievethis will require inexpensive materials forthe active components and the packaging,low-temperature atmospheric processing,and high fabrication throughput. It is inthis category that organic-based photo-

voltaics (OPVs) have the potential to makea significant impact. Achieving these goalswill require significant basic and appliedscience advancements over the next 10years. This issue of MRS Bulletin will focuson the strengths and weaknesses of thedifferent approaches that have been pro-posed for OPVs, along with the funda-mental understanding required to fulfilltheir potential.

Why Research the Development ofOrganic-Based Photovoltaics?

With a theoretical efficiency that is thesame as conventional semiconductor de-vices and a cost structure derived fromplastic processing, organic photovoltaicsoffer the long-term potential of achievingthe goal of a PV technology that is eco-nomically viable for large-scale powergeneration. The development of materialsfor OPVs can be leveraged by the increas-ing development of organics for transis-tors, actuators, packaging, waveguides,and displays.7,8 Some key advantages forOPVs are that organic small-molecule andpolymer materials are inherently inexpen-sive; they can have very high optical ab-sorption coefficients that permit the use offilms with thicknesses of only severalhundred nanometers; they are compatiblewith plastic substrates; and they can befabricated using high-throughput, low-temperature approaches that employ oneof a variety of well-established printingtechniques in a roll-to-roll process.9 Suchlow-temperature, and therefore low-energy-consuming, techniques require less capitalinvestment than fabrication techniques forSi-based devices, for instance. In addition,the possibility of using flexible plastic sub-strates in an easily scalable, high-speedprinting process can reduce the balance-of-system costs for OPVs to make themcompetitive with inorganic thin-film tech-nologies. Thus, if efficiencies are compara-ble or even slightly lower than existingtechnologies, there may be compellingcost arguments favoring OPVs.

A key issue faced by the organic elec-tronics community in general is the stabilityof the organic materials. It is encouragingthat, for instance, automotive paints con-tain chromophores that are similar to mole-cules commonly used in OPV devices, andthat organic light-emitting displays aredemonstrating acceptable lifetimes underhigh injection currents. Device degradationpathways stem largely from changes inmorphology, loss of interfacial adhesion,and interdiffusion of components, as op-posed to strictly chemical decomposition.Thus, careful design and materials engi-neering can substantially improve devicelifetimes.

Figure 4. Historical and projected costs for wafer c-Si and film c-Si photovoltaic modulesversus their cumulative production (in megawatts). “Distributed fossil fuel” refers to off-gridgeneration of power, using, for instance, gas-driven generators.19

Figure 5. Cost-efficiency analysis for first-, second-, and third-generationphotovoltaic technologies (labeled I, II,and III, respectively). Region IIIa depictsvery-high-efficiency devices that requirenovel mechanisms of device operation.Region IIIb—the region in which organicPV devices lie—depicts devices withmoderate efficiencies and very low costs.20







In addition to these factors, there are anumber of other key aspects of organicmaterials. First and foremost is their versa-tility. They possess an unprecedented flexi-bility in the synthesis of the basic molecules,allowing for alteration of a wide range ofproperties, including molecular weight,bandgap, molecular orbital energy levels,wetting properties, structural properties(rigidity, conjugation length, molecule-to-molecule interactions, etc.), and doping.This ability to design and synthesizemolecules and then integrate them into organic–organic and inorganic–organiccomposites provides a unique pathway in the design of materials for novel devices. Additionally, with the ability toalter the color of the device, fabricate devices on flexible substrates, and poten-tially print them in any pattern, OPVs may be integrated into existing buildingstructures and into new commercial prod-ucts in ways impossible for conventionaltechnologies.

The Nature of OrganicSemiconductors

Organic semiconductors (OSCs) can bebroadly classified as either small molecules,with molecular weights of less than a fewthousand atomic mass units, or polymers,with molecular weights greater than10,000 amu. Amore recent class of materials,called dendrimers, has also been devel-oped that span a molecular weight rangebetween small molecules and polymers.

The distinctions between these classes be-come most important in determining theprocessing that is required in making filmsand devices and for the subsequent mo-lecular arrangements (morphologies) thatare obtained. However, the fundamentalmechanisms that underlie the optical andelectronic properties of the different classesare identical. These processes are determinedessentially by the molecular orbitals thatare built up from the summation of indi-vidual atomic orbitals in the molecule. Thefrontier molecular orbitals, the highest occupied molecular orbital (HOMO) andthe lowest unoccupied molecular orbital(LUMO), largely determine a molecule’sproperties. Figure 6 depicts the HOMOsof several common OSCs.

Photoexcitations in OSCs are inherentlydifferent than those in conventional inor-ganic semiconductors.10 Whereas light ab-sorption in an inorganic semiconductortypically leads to the immediate productionof free carriers (electrons and holes thatare freely transported through the semi-conductor material), light absorption in anOSC results in the formation of excitons(electron–hole pairs that are bound to-gether by Coulomb attraction).

In order for free carriers to be generatedin this case, the excitons must be dissoci-ated. This can happen in the presence ofhigh electric fields, at a defect site in thematerial, or at the interface between twomaterials that have a sufficient mismatchin their energetic levels (band offset). Inthe case of dissociation at the interface be-tween two materials, an exciton created ineither material can diffuse to the interface,leading to either electron transfer from thedonor material to the acceptor material orhole transfer from the acceptor to thedonor.

Thus, one can fabricate a photo-voltaic device with the structure positiveelectrode/donor/acceptor/negative elec-trode. Tang, using a copper phthalocyaninelayer as the donor and a perylene deriva-tive as the acceptor, first accomplishedthis.11 This device had a power conversionefficiency of about 1% under simulatedsolar illumination. As a result of the exci-tonic nature of the photoexcitations inOSCs, the operational mechanisms of OPVsare different from those of conventionalphotovoltaics. These differences are de-scribed in detail in the article in this issueby Gregg.

Charge transport in OSCs is determinedby the intermolecular overlap of the fron-

tier orbitals of adjacent molecules.12 Thiscoupling is very weak compared with theinteratomic coupling of conventional semi-conductors. Additionally, the arrangementof molecules in a typical OSC is disorderedand more resembles an amorphous mixturethan a crystalline lattice. As a result, theconventional band transport descriptionof charge transport cannot be used forOSCs. Instead, descriptions of charge trans-port in OSCs are based on carrier hoppingbetween molecules. This is a temperature-activated process that results in an increasein carrier mobility with increasing tempera-ture, in contrast to the decreasing tempera-ture dependence of the carrier mobility inconventional semiconductors.

It has also been found that carrier mo-bilities in OSCs follow an exponential de-pendence on the square root of the electricfield.13 The temperature and field depend-ence for both small-molecule and polymerOSCs has been successfully modeled usingwhat is known as the disorder formalism,originally developed by Bässler.14 This modelassumes carrier hopping between molecu-lar sites with Gaussian distributions of en-ergies and geometric disorder parameters.A more recent version of the model hasbeen developed that takes into accountspatial correlations in the site energies and

Figure 6. Depiction of the highest occupied molecular orbitals of several common organicsemiconductors: (a) poly(3-hexylthiophene) (P3HT), (b) [6,6]-phenyl C61-butyric acid methylester (PCBM), and (c) N,N�-dimethyl-3,4,9,10-perylene bis(tetracarboxyl diimide) (MPP).Green and pink regions depict positive and negative orbitals, respectively.







results in a better fit to experimental dataover a wider range of electric fields.15 Typi-cal carrier mobilities in OSCs range from10�6 cm2 V�1 s�1 to 10�3 cm2 V�1 s�1. Whilethese values are several orders of magni-tude less than those found in conventionalsemiconductors, they are sufficiently highto allow for the efficient extraction ofcharges in OPVs before free carriers re-combine to the ground state. Higher car-rier mobilities have in fact been measuredin crystalline OSCs—in materials such asanthracene or pentacene. These materialshave been shown to be able to exhibit trueband transport. However, there has notbeen a large focus on using such systems inOPV devices, since the slow growth tech-niques required to produce the crystallinelattice offset some of the cost benefits oforganics and because additional problemsarise with the presence of grain bound-aries in these materials.

In trying to develop high-efficiency OPVdevices, it has become clear to researchersin the field that the details of the morphol-ogy of the molecules have a large impacton the charge carrier mobilities and, there-fore, device efficiencies. Learning how tocontrol exactly where along the scale fromamorphous to crystalline the structure ofthe material lies is important. Several tech-niques for doing this have been developed,including choice of solvent and castingconditions, thermal annealing, and inter-penetration of the organic semiconductorinto an inorganic lattice that influences themolecular alignment.15

Paradigms for OPV DevicesThe molecular nature of organic semi-

conductors places constraints on the waysin which OPV devices must be designed.As a further consequence of the weak inter-molecular coupling between molecules, theexciton diffusion length, which is the dis-tance an exciton will travel on averagebefore it decays to the ground state, is typi-cally on the order of 10 nm. This requiresthat planar devices, such as the Tang celldiscussed earlier, consist of very thin activelayers. Alternatively, the donor and accep-tor species can be blended on the nanoscalesuch that all excitons are produced withinapproximately 10 nm of an interface. Thelow values for charge carrier mobilities inOSCs dictate that the thickness of the ac-tive layer be kept thin, thus limiting theoptical density and the power conversionefficiency.

In this issue of MRS Bulletin, a variety ofschemes to fabricate efficient OPV devicesare discussed. These encompass differentapproaches to absorbing the sun’s emis-sion and accommodating the length scalesderived from exciton diffusion lengths and

carrier lifetimes in organic semiconductormaterials. These approaches include� Dye-sensitized nanostructured oxide cells.These cells use an organic dye to absorblight and rapidly inject electrons into ananostructured oxide such as anatase TiO2.The hole is scavenged by a redox couple,such as iodide/triiodide (I�/I3

�), in solutionor by a solid-state organic semiconductoror ionic medium. To date, the solid-stateapproaches have not been as effective, butprogress is being made in understandingthe nature of the redox behavior of the or-ganic hole transporter. These approachesare discussed in the article by Grätzel.� Multilayer devices in which small-molecule OSCs are deposited sequentiallyto form a stacked device. Recent progressin the reduction of the series resistance hasled to increased efficiencies. This technol-ogy has the added benefit that multijunc-tion tandem devices, which trade offcurrent for voltage and possess highertheoretical efficiencies, can be fabricated ina straightforward manner. These devicesare discussed in the article by Forrest.� Organic–organic composites in whichdonor and acceptor species, which functionas the hole and electron transporters, re-spectively, are intimately mixed to producea “bulk heterojunction.” Among the mostsuccessful bulk heterojunction devices todate are those employing a conjugatedpolymer such as poly(3-hexylthiophene)as the donor and a fullerene derivative asthe acceptor. These devices are describedin the article by Janssen et al.� Organic–inorganic composites that com-bine a light-absorbing conjugated polymeras the donor and hole transporter with ananostructured, large-bandgap inorganicsemiconductor such as TiO2 or ZnO as theacceptor and electron transporter. Thesework similarly to the bulk heterojunctions;however, the gross morphology of themixture is determined by that of the nano-structured oxide that is grown in a self-organizing manner on the electrode.Optimization of the organic–inorganicinterface is critical for these devices, whichare discussed in the article by Coakley et al.� Organic–inorganic composites consistingof nanocrystals of conventional semicon-ductors, such as CdSe, blended into a con-jugated polymer matrix. In this case, bothcomponents can efficiently absorb light, andthe bandgap of the nanocrystals can betuned by growing them to different sizes.As in the previous case, the energetics of theorganic–inorganic interface is crucial. Re-cent progress in this area is discussed in thearticle by Milliron et al.� Photoelectrochemical cells employing aconjugated polymer as the light absorberand electron donor in conjunction with a

liquid electrolyte as the redox mediatorand electron transporter. These devices,which are in very early stages of develop-ment, are discussed by Wallace et al.

Industrial InterestFueled by recent progress in the power

conversion efficiency of OPV devices andthe cost benefits of processing and fabricat-ing organics, many companies have begunresearch on developing OPV devices tobring them to the marketplace. A partiallist of companies performing research anddevelopment on OPV devices includesGeneral Electric, Konarka, Nanosys,Nanosolar, Solaronix, Shell, Sharp, Sony,CDT, and Toshiba.

The article by Brabec et al. discusses thepotential for high-throughput industrialmanufacture of OPV modules. Solution-based techniques such as inkjet, screen, orflexographic printing (a form of rotaryweb letterpress using flexible relief plates)are well suited for organic thin-film depo-sition and can be done under atmosphericconditions. The scalability and low cost ofsuch techniques could alter the paradigmfor solar cell manufacturing and couldopen new markets that make use of veryinexpensive photovoltaics requiring onlymoderate efficiencies and device lifetimes.

An important economic factor in the de-velopment of any PV technology is thebalance-of-system costs. As has beenbrought to light for thin-film technologies,the material costs of the active compo-nents will soon compose only a small partof the total module manufacturing costs.17

Glass substrates, transparent conductingelectrodes (TCOs), and packaging mate-rials instead will constitute a significantfraction of the final cost. In this area, reel-to-reel manufacture of OPV modules canhave an impact by using low-cost plasticsubstrates and conducting polymer alter-natives to TCOs. Packaging is likely to bea major concern for the development ofany practical organic semiconductor de-vice, due to sensitivity to oxygen and watervapor. Progress has been made in this areawith the development of flexible encapsu-lants for packaging organic light-emittingdiodes. Industrial efforts into the develop-ment of OPVs can build on the success ofsuch efforts to commercialize organic light-emitting diodes. Also encouraging is thatinitial efforts to fabricate OPV devicesusing high-throughput techniques showno decrease in performance over small-scale laboratory techniques.

Future Challenges for OrganicPhotovoltaics

Many formidable challenges must beovercome before OPV devices can be







Sean E. Shaheen, GuestEditor for this issue ofMRS Bulletin, has been a researcher for the National Center for Photovoltaics at the National Renewable Energy Laboratory since 2002, where heworks on the develop-ment of novel materials

for organic-based solarcells and simulation and modeling of thephysics of these devices. He received his PhD in physics from the University ofArizona (1999) and performed postdoctoralresearch at the Univer-sity of Linz, Austria,

as a Lise Meitner Post-doctoral Fellow. His research interests extend to modeling ofinformation processingin biological and othercomplex dynamical systems.

Shaheen can bereached by e-mail at [email protected].

David S. Ginley, GuestEditor for this issue ofMRS Bulletin, is a GroupManager in ProcessTechnology and Ad-vanced Concepts at theNational Renewable En-ergy Laboratory, leadingactivities in the applica-tions of nanotechnology,organic electronics,

transition-metal oxides(ferroelectric materials,rechargeable Li batter-ies, and transparent con-ductors) and inkjetprinting. He is also anadjunct professor at theUniversity of Coloradoat Boulder (physics) andthe Colorado School ofMines (materials sci-

considered a truly practical technology. Firstand foremost, higher power conversion effi-ciencies must be demonstrated. Severalgroups are now reporting values in the 5%regime. These are very encouraging results;however, efficiencies of laboratory-scale de-vices must be pushed higher before large-scale manufacturing can really be considered.Several fundamental issues must be ad-dressed in order for this to happen. Theseinclude� Bandgap. Current OPV devices have ex-hibited high (�70%) quantum efficiency,defined as the ratio of the number of pho-tons of a given wavelength incident on thedevice to the number of charges deliveredto the external circuit, for blue photons. How-ever, efficiently harvesting red photons,which contain a significant fraction of theenergy in the solar spectrum, has provenchallenging. Optical bandgaps of the light-absorbing components of OPVs must bereduced to approach the nominally optimalvalue of 1.4 eV while retaining good chargecarrier mobilities.� Interfaces. An improved understanding ofthe fundamental processes of charge trans-fer and carrier recombination across donor–acceptor interfaces is critical. Here, the bandoffset must be optimized to yield the highestpossible photovoltage from the device. Con-tact resistance between layers must be mini-mized in order to reduce the device seriesresistance, which is important in deter-mining the fill factor and thus the powerconversion efficiency. Additionally, physi-cal properties such as wetting between or-ganic and inorganic components andadhesion between layers are important forboth the device performance and lifetime.� Charge transport. Obtaining higher chargecarrier mobilities will allow for the use ofthicker active layers while minimizing carrier recombination and keeping the seriesresistance of the device low. The develop-ment of organic molecules and organic–inorganic composites that self-assemble intoordered phases with pathways for effi-cient charge transport is a promising routeto obtaining higher carrier mobilities.

Devising new ways to address theseissues, control the morphologies, and im-

prove the optoelectronic properties of thelight-absorbing and charge-transportingmaterials is the primary materials sciencethrust in all of the varieties of OPV devices.In addition to overcoming these fundamen-tal obstacles to higher efficiency, pathwaysof device degradation must be identifiedand mitigated before device lifetimes canbe sufficiently long for practical use. Manu-facturing issues such as printing and/orlamination of electrode materials, asopposed to vacuum deposition of metalelectrodes, must be researched, and opti-mization of device patterning and integra-tion of OPV devices into module assembliesmust also be done.

As a final note, we mention that most ofthe efficiency values reported or claimedfor OPV devices have not yet been certi-fied by an internationally recognized PVtest and measurement laboratory, such asthe National Renewable Energy Laboratoryin Golden, Colo. Photovoltaic device effi-ciencies are notoriously difficult to measureaccurately because, among other reasons,of the sensitivity of the device perform-ance to deviations between the emissionspectrum of the solar simulator used intesting and the true AM 1.5 spectrum. Untilrecently, certified measurements of OPVefficiencies have not been critical, since ef-ficiency values in the field have been rela-tively low. Recent developments to pushefficiencies into the 5% regime, which isapproaching that of amorphous-silicon-based devices, now make accurate measu-rements more important. These advancesare a testament to the rapid progress beingmade in the field of organic photovoltaicsand to the promise they hold for growinginto a viable technology as a low-cost re-newable energy resource.

AcknowledgmentsThe authors thank Dr. Howard Branz,

Dr. Brian Gregg, and Dr. Garry Rumbles atNREL for fruitful discussions and contin-ued collaborations. S.E. Shaheen and D.S.Ginley gratefully acknowledge the finan-cial support for research in organic photo-voltaics through the NREL DDRD program,DARPA, and the Xcel Energy Renewable

Development Fund. G.E. Jabbour wouldlike to thank NREL for financial support.

References1. International Electricity Information, U.S. Dept.of Energy, http://www.eia.doe.gov/emeu/international/electric.html#Consumption (ac-cessed December 2004).2. Annual Energy Review 2003, U.S. Dept. ofEnergy, http://www.eia.doe.gov/emeu/aer/contents.html (accessed December 2004).3. U.S. Photovoltaic Industry Roadmap, U.S.Dept. of Energy, http://www.nrel.gov/ncpv/pvmenu.cgi?site�ncpv&idx�3&body�infores.html (accessed December 2004).4. W. Shockley and H.J. Queisser, J. App. Phys.32 (3) (1961) p. 510.5. K. Zweibel, B. Von Roedern, and H. Ullal,“Finally: Thin-film PV!” Photon Int. (October2004) p. 48.6. M.A. Green, Physica E 14 (1–2) (2002) p. 65.7. G. Collins, Sci. Am. (August 2004) p. 74.8. D.M. de Leeuw, Phys. World 12 (3) (1999) p. 31.9. S.E. Shaheen, R. Radspinner, N. Peygham-barian, and G.E. Jabbour, Appl. Phys. Lett. 79(18) (2001) p. 2996. 10. B.A. Gregg and M.C. Hanna, J. Appl. Phys.93 (6) (2003) p. 3605.11. C.W. Tang, Appl. Phys. Lett. 48 (2) (1986) p. 183.12. J.L. Bredas, J.P. Calbert, D.A. da Silva Filho,and J. Cornil, Proc. Nat. Acad. Sci. 99 (9) (2002)p. 5804.13. P.W.M. Blom, M.J.M. de Jong, and M.G. vanMunster, Phys. Rev. B 55 (2) (1997) p. R656.14. H. Bässler, Phys. Status Solidi B 175 (1993)p. 15.15. S.V. Novikov, D.H. Dunlap, V.M. Kenkre,P.E. Parris, and A.V. Vannikov, Phys. Rev. Lett. 81(20) (1998) p. 4472.16. S.E. Shaheen and D.S. Ginley, “Photovoltaicsfor the Next Generation: Organic-Based SolarCells,” in Encyclopedia of Nanoscience and Nano-technology, edited by J.A. Schwarz, C. Contescu,and K. Putyera (Marcel Dekker, New York,2004), p. 2879.17. K. Zweibel, Sol. Energy Mater. Sol. Cells 63(4) (2000) p. 375.18. P. Maycock, PV News (February 2003).19. R.M. Margolis, “Photovoltaic TechnologyExperience Curves and Markets,” NCPV SolarProgram Review Meeting, Denver, CO (2003).20. M.A. Green, “Third generation photovoltaics:concepts for high efficiency at low cost,” Proc.Electrochem. Soc. 10 (2001) p. 3. ■■





Brian A. Gregg



ence). His work focuseson the development andbasic science of trans-parent conducting oxides,ferroelectric materials,organic materials, andnanomaterials and thedevelopment of next-generation process tech-nology for materials anddevice development(combinatorial methods,direct-write materials,composite materials, andnon-vacuum processing).

Ginley holds a BS de-gree in mineral engi-neering chemistry fromthe Colorado School ofMines and a PhD degreein inorganic chemistryfrom MIT. Ginley is afellow of the Electro-chemical Society. He haspublished more than 320 papers and holds 24 patents. He has beenhonored with a Depart-ment of Energy Awardfor Sustained Researchin Superconducting Ma-terials and R&D 100Awards for novel chemi-cal etches, nanoparticletechnology, ferroelectricfrequency agile electron-ics, and alumina-basednanofibers. He has alsoreceived two FLC tech-nology transfer awards.

Ginley can be reached by e-mail at [email protected].

Ghassan E. Jabbour,Guest Editor for thisissue of MRS Bulletin, isa professor of chemicaland materials engineer-ing and the technicalarea leader of optoelec-

tronic materials and de-vices at the Flexible Dis-play Center (FDC) atArizona State University.Jabbour is also leads theTechnical AdvisoryBoard on OptoelectronicMaterials, Devices, andEncapsulation at FDC.His research focuses onthe fabrication, charac-terization, and testing oforganic (polymeric andsmall-molecule) elec-tronics and photonics.His group was the firstto demonstrate the use of high-throughputprinting techniques in the fabrication of organic solar cells and diodes.

Jabbour holds a PhDdegree in materials sci-ence and engineeringfrom the University ofArizona. He is associateeditor of the Journal ofthe Society for InformationDisplay (JSID). He wasthe track chair of theNanotechnology Programfor the SPIE AnnualMeeting (2001–2004), andthe secretary general forthe Materials Secretariat

of the American Chemi-cal Society (2001). Jabbourhas been involved as achair, co-chair, or com-mittee member for morethan 40 conferences re-lated to photonic andelectronic properties oforganic materials andtheir applications in displays and lighting,transistors and solarcells, hybrid photo-sensitive materials, and hybrid integrationof semiconducting materials. He is an SPIEfellow and was namedone of the 100 NewLeaders of the USA in2001 by Asahi Shimbun(Japan).

Jabbour can bereached by e-mail at [email protected] and by telephone at 480-727-8930.

A. Paul Alivisatos is aprofessor of materialsscience at the Universityof California, Berkeley,and director of the Ma-terials Sciences Division.He is also the scientificdirector of the Molecu-

lar Foundry Program atthe Lawrence BerkeleyNational Laboratory(LBNL) and holds a sen-ior scientist appointmentat LBNL. In the privatesector, Alivisatos is thefounder and editor inchief of Nano Letters, and the scientific founderof Quantum Dot Corp.and Nanosys Inc.

Alivisatos’ researchseeks to understand thesynthesis and characteri-zation of semiconductornanocrystals. He holds aBA degree in chemistryfrom the University ofChicago and a PhD de-gree in chemical physicsfrom UC–Berkeley. Heheld a postdoctoral ap-pointment with LouisBrus at AT&T Bell Labo-ratories and returned toBerkeley in 1988, wherehe joined the faculty asan assistant professor ofchemistry. He wasnamed associate profes-sor in 1993, and becamea full professor of chem-istry in 1995. He was also given the honor of being named

Chancellor’s Professorof Chemistry from 1998to 2001. In 1999, he wasnamed professor of ma-terials science.

He has received manyawards for his contribu-tions to the field of nano-technology, the mostrecent being election intothe National Academyof Sciences and theAmerican Academy ofArts, and receiving the2005 ACS Award in Colloid and SurfaceChemistry.

Alivisatos can bereached by e-mail [email protected].

Christoph J. Brabec isdirector of the polymerphotovoltaics programat Konarka Technologies.Before he and his teamjoined Konarka, he wasproject leader at SiemensCorporate Technology,where he focused on thedevelopment of polymerelectronic devices (solarcells, displays, imaging).While working on hisPhD degree in 1995, he

Sean E. Shaheen David S. Ginley Ghassan E. Jabbour A. Paul Alivisatos Christoph J. Brabec

Ilan Gur Jens A. Hauch Jan C. (Kees) Hummelen







investigated the rheologyof polymer melts withrespect to molar masscorrelations. In 1996, hejoined the group of AlanHeeger at the Universityof Santa Barbara for asabbatical, and he con-tinued to work on theoptoelectronic propertiesof organic semiconduc-tors later on as an assis-tant professor at theUniversity of Linz. In1998, he worked as asenior scientist at theChristian Doppler Labo-ratory on organic solarcells, joining Siemens in2001. He is author or co-author of more than 100papers and 30 patents;he finished his habili-tation in physical chemistry in 2003.

Brabec can be reachedby e-mail at [email protected].

Kevin M. Coakley is arecent PhD graduatefrom the Department ofMaterials Science andEngineering at StanfordUniversity. His researchthere focused on fabri-

cating photovoltaic cellsfrom conjugated poly-mers infiltrated into or-dered nanoporous hostsand on the electrical andoptical properties ofconjugated polymers inconfined channels.

Coakley can bereached by e-mail [email protected].

Paul C. Dastoor is a senior lecturer in physics in the School of Mathe-matical and Physical Sciences at the University of Newcastle in Australia.He received his BA de-gree in natural sciencesfrom the University ofCambridge in 1990 andhis PhD degree in sur-face physics, also fromthe University of Cam-bridge, in 1995. Aftercompleting his doctorate,he joined the SurfaceChemistry Departmentat British Steel in 1994before taking up hispresent appointment atthe University of New-castle in 1995.

He was an EPSRCVisiting Research Fellow

at Fitzwilliam College,Cambridge, in 2002 and a CCLRC VisitingResearch Fellow at theDaresbury Laboratory,Cheshire, for 2004–2005.His research interestsencompass the growthand properties of thinfilms, surface coatings,and organic electronicdevices based on semi-conducting polymers.

Dastoor can bereached by e-mail [email protected].

Stephen R. Forrest isthe James S. McDonnellDistinguished UniversityProfessor of ElectricalEngineering at PrincetonUniversity. He holds aBA degree in physicsfrom the University ofCalifornia and MSc andPhD degrees in physicsfrom the University ofMichigan. From there,he investigated photo-detectors for opticalcommunications at BellLaboratories. In 1985,Forrest joined the Elec-trical Engineering and

Materials Science De-partments at the Univer-sity of SouthernCalifornia, where heworked on optoelec-tronic integrated circuitsand organic semi-conductors. He hasserved as director of the National Center for Integrated Photonic Technology as well asdirector of Princeton’sCenter for Photonicsand Optoelectronic Ma-terials (POEM). From1997 to 2001, he waschair of the ElectricalEngineering Departmentat Princeton. A fellow ofthe IEEE and OSA, anda member of the Na-tional Academy of Engi-neering, he received the IEEE/LEOS Distin-guished Lecturer Award for 1996–1997,and in 1998, he was co-recipient of the IPONational DistinguishedInventor Award as wellas the Thomas Alva Edison Award for inno-vations in organic LEDs.In 1999, Forrest receivedthe MRS Medal for

work on organic thinfilms. In 2001, he wasawarded theIEEE/LEOS WilliamStreifer ScientificAchievement Award foradvances made on photo-detectors for opticalcommunications systems.Forrest has authored ap-proximately 365 papersin refereed journals andholds 122 patents.

Forrest can be reachedby e-mail at [email protected].

Chiatzun Goh is a PhDcandidate in the Depart-ment of Materials Sci-ence and Engineering atStanford University. Hiscurrent research interestsinclude the investigationof charge transport inconjugated polymersand nanopatterning ofmetal oxide semicon-ductors for use in or-ganic photovoltaic cells.He holds a BA degree inchemistry from BeloitCollege and is currentlya Kodak fellow.

Goh can be reachedby e-mail at [email protected].

Michael Grätzel is aprofessor at the SwissFederal Institute of Technology in Lau-sanne, where he directsthe Laboratory of Pho-tonics and Interfaces,and he is a part-time distinguished visitingprofessor at the DelftUniversity of Technology.He has pioneered re-search on nanomaterials

Paul C. Dastoor Stephen R. Forrest Chiatzun Goh Michael Grätzel

Michael D. McGehee Delia J. Milliron

Kevin M. Coakley

René A.J. Janssen Yuxiang Liu







and display-related ap-plications, and he de-veloped a new type ofsolar cell based on dye-sensitized mesoscopicoxide particles.

Grätzel is the authorof more than 500 publi-cations, two books, andover 40 patents. He hasbeen an invited profes-sor at the University ofCalifornia at Berkeley,the University of Tokyo,and the Ecole NationalSupérieur de Paris. Hehas received numerousawards, including theMillennium 2000 Euro-pean innovation price,the 2001 Faraday Medalfrom the British RoyalSociety, the 2001 DutchHavinga Award, the 2003 Italgas Prize, andMcKinsey VentureAwards in 1998 and 2002.He holds a doctor degreefrom the TU Berlin andhonorary doctorates fromthe Universities of Uppsala and Turin.

Grätzel can bereached by e-mail [email protected].

Brian A. Gregg is a senior scientist at theNational Renewable Energy Laboratory. Hepioneered the use of liquid-crystalline organicsemiconductors in photo-voltaic cells, inventedthe self-powered “photo-electrochromic” smartwindow, and has madea number of contributionsto understanding the kinetics, thermodynamics,and recombination proc-esses in dye-sensitizedsolar cells and in solid-state organic PV cells.He has published morethan 60 papers on or-ganic photovoltaic cellsin the last 17 years.

Gregg can be reached by e-mail [email protected].

Ilan Gur is a NationalScience Foundationgraduate research fellow

and PhD candidate inmaterials science andengineering at the Uni-versity of California atBerkeley, where he hasalso earned BA and MSdegrees. His scientificresearch interests are inthe design of novel ma-terials systems and de-vice architectures toenable low-cost photo-voltaic energy conver-sion, with a focus onII–VI semiconductornanostructures and conjugated organic materials. He was the recipient of the National Defense Sci-ence and EngineeringGraduate fellowshipfrom the U.S. Depart-ment of Defense and the United Nations Industrial DevelopmentOrganization’s inter-national research fellowship.

Gur can be reachedby e-mail at [email protected].

Jens A. Hauch is headof polymer photovoltaicproduction developmentat Konarka Technologies.He holds a BS degree in physics from the University of Illinois at Urbana-Champaign,where he was a memberof the Center forComplex Systems Re-search. He holds a PhDdegree from the Centerfor Nonlinear Dynamicsat the University ofTexas at Austin. Beforejoining Konarka in 2004,he was active in the de-velopment of thin-filmmagnetic sensors, electro-chromic displays, andorganic photodetectorsat Siemens CorporateTechnology.

Hauch can be reached by e-mail [email protected].

Jan C. (Kees) Hummelenholds a master’s degreein chemistry and a doc-torate degree in science

from the University ofGroningen, the Nether-lands. After two years asa postdoctoral fellow inthe Institute for Polymersand Organic Solids atthe University of Cali-fornia, Santa Barbara,and six months at Eind-hoven Technical Univer-sity, he returned toGroningen, where he wasappointed UniversitairHoofdocent in 1998 and afull professor of chem-istry in 2000. Over thelast 10 years, his mainresearch activities havebeen in fundamental andapplied fullerene chem-istry, the developmentof plastic photovoltaictechnology, and molecu-lar and nanoelectronics.

Hummelen can bereached by e-mail [email protected].

René A.J. Janssen isprofessor of chemistryand physics at Eind-hoven University ofTechnology. His group’sresearch focuses onfunctional conjugatedmolecules, macromole-cules, nanostructures,and materials aiming at electro-optical appli-cations, using synthetic organic and polymerchemistry combinedwith advanced opticalspectroscopy, electro-chemistry, morphologicalcharacterization, and the preparation andanalysis of prototypedevices.

Janssen can bereached by e-mail [email protected].

Yuxiang Liu is a PhDcandidate in the Depart-ment of Chemistry atStanford University. His current research is focused on the synthesisof mesoporous metaloxides for photovoltaicapplications and thestudy of exciton diffu-sion in organic semicon-ductors. He holds a BAdegree in chemistryfrom Beijing University.

Liu can be reached by e-mail at [email protected].

Michael D. McGehee isan assistant professor inthe Materials Scienceand Engineering De-partment at StanfordUniversity. He holds anAB degree in physicsfrom Princeton Univer-sity, and a PhD degreein materials sciencefrom the University ofCalifornia at Santa Barbara, where he didresearch on polymerlasers in the laboratoryof Alan Heeger. Aftergraduation, he did post-doctoral research at UCSBand joined the Stanfordfaculty in 2000. His re-search interests have in-cluded organic–inorganic nanostruc-tures for applicationssuch as photovoltaics,light extraction frompolymer LEDs and thechemistry and physics

of organic semiconduc-tors. McGehee has won a National ScienceFoundation CAREERAward, the Henry andCamille Dreyfus NewFaculty Award and the Dupont Young Professor Award.

McGehee can bereached by e-mail [email protected].

Delia J. Milliron is pur-suing postdoctoral stud-ies at the IBM T.J. WatsonResearch Center. Sheholds an AB degree inchemistry with a certifi-cate in materials sciencefrom Princeton Univer-sity. She completed herPhD degree in chem-istry at the University of California at Berkeleyunder A. Paul Alivisatos.Her research has involvednext-generation electronicmaterials such as organicmolecular crystals, semi-conducting polymers,and inorganic nano-crystals. Her work hasthus far included devel-opment of new materi-als, their application in light-emitting diodes,photovoltaic cells, andfield-effect transistors,and advancing the understanding of dis-similar material interac-tions that take place inthese devices.

Milliron can bereached by e-mail [email protected].

David L. Officer is direc-tor of the Nanomaterials

Pavel SchilinskyDavid L. Officer Niyazi Serdar Sariciftci







Research Centre (NRC)and professor of chem-istry at the Institute ofFundamental Sciences,Massey University. He isa principal investigatorat the MacDiarmid Insti-tute for Advanced Mate-rials and Nanotechnology,a New Zealand Centreof Research Excellence,and he is a partner in-vestigator with the Aus-tralian Research CouncilCentre for Nanostruc-tured Electromaterials.He has collaborative re-search programs in thesynthesis and use ofporphyrins for photo-voltaic and moleculardevices and in the de-velopment and applica-tion of functionalizedpolythiophenes and car-bon nanotubes. Heholds a PhD degreefrom Victoria Universityof Wellington in NewZealand.

Officer can be reachedby e-mail at [email protected].

Niyazi Serdar Sariciftciis chair of the Institutefor Physical Chemistryat Johannes Kepler Uni-versity in Linz, Austria.He is also the foundingdirector of the Linzer Institut für OrganischeSolarzellen (LIOS). Heholds a BA degree fromSankt Georgs Kolleg inIstanbul, Turkey, andMA and PhD degrees inphysics from the Uni-versity of Vienna. Hedid his postdoctoral re-search at the Universityof Stuttgart’s Physics In-stitute. From 1992 to1996, he was a senior re-search associate at theInstitute for Polymersand Organic Solids atthe University of Cali-fornia, Santa Barbara,working in the areas ofphotoinduced optical,magnetic-resonance, andtransport phenomena inconducting polymers.

Sariciftci can bereached by e-mail at [email protected].

Pavel Schilinsky isfinalizing a PhD degreein physics from the Uni-versity of Oldenburg. InAugust 2004, he beganworking at KonarkaTechnologies in Germany.He also studied physicsat the University ofBremen and the Carlvon Ossietzky Univer-sity of Oldenburg andwrote his master’s thesison the topic of synthesisand characterization ofCdS and CuS nanoparti-cles for photovoltaicapplications.

Schilinsky can bereached by e-mail [email protected].

Chee On Too is the assistant director of the Intelligent Polymer Re-search Institute andchief operating officer ofthe Australian ResearchCouncil Centre forNanostructured Electro-materials at the Univer-sity of Wollongong,Australia. He has pub-lished 54 refereed pa-

pers on the synthesisand processing of con-ducting polymers aswell as on the applica-tions of conductingpolymers, includingsmart separation tech-nologies (chromatogra-phy and membranes),sensors, controlled-release and thermo-responsive conductingpolymer composites,metal-ion separationusing conductive elec-troactive polymer colloids, catalytic hydrogen generation,polymer batteries, andphotovoltaics.

Too can be reached by e-mail at [email protected].

Christoph Waldauf is aresearch and develop-ment scientist atKonarka Technologies,where he works on thedevelopment of organicphotovoltaic efficiencies,lifetimes, and processoptimization. He re-ceived his educationfrom the TechnicalSchool for Electronicsand Telecommunication,the Technical Universityof Graz, and the Univer-sity of Oldenburg. Hehas been a site acquisi-tion manager for ConnectAustria, a technician atthe department of Ad-vanced Technology atDatacon, and a scientistat Siemens CorporateTechnologies.

Waldauf can bereached by e-mail [email protected].

Gordon G. Wallace isresearch director of theAustralian ResearchCouncil Centre for Nano-structured Electromateri-als. In 1990, he wasappointed professor at theUniversity of Wollon-gong. He holds a DScdegree from DeakinUniversity. He wasawarded an AustralianResearch Council (ARC)QEII Fellowship in 1991and an ARC Senior Re-search Fellowship in 1995.He was appointed to anARC Professorial Fellow-ship in 2002 and receivedan ETS Walton Fellow-ship from the ScienceFoundation Ireland in2003. In the same year,he was elected fellow ofthe Australian Academyof Technological Sciencesand Engineering. He isalso a fellow of the RoyalAustralian Chemical In-stitute (RACI), and hereceived the inauguralPolymer Science andTechnology Award fromthe RACI in 1992 and theRACI Stokes Medal forresearch in electrochem-istry in 2004. Wallace haspublished more than300 refereed publicationsand a monograph on in-herently conductingpolymers for intelligentmaterials systems.

Wallace can bereached by e-mail [email protected]. ■■

Chee On Too Christoph Waldauf Gordon G. Wallace

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