Polymer-Fullerene+Bulk-Heterojunction+Solar+Cells[1]

Polymer-Fullerene Bulk-Heterojunction Solar Cells

By Gilles Dennler,Markus C.Scharber,and Christoph J.Brabec*

1.Introduction

Solution-processed bulk-heterojunction photovoltaic cells were ?rst reported in 1995.[1]It took another 3–4years until the scienti?c community realized the huge potential of this technology,and suddenly,in 1999,the number of publications in that ?eld started to rise exponentially.Since then,the number of publications on organic semiconductor photovoltaics has increased by about 65%per year.While the best ef?ciency reported eight years ago barely reached values higher than 1%,ef?ciencies beyond 5%are achieved today.[2–6]

This article reviews the recent developments that have guided the community and the whole ?eld to the current performance of organic photovoltaic devices (OPVs).We start with reviewing the performance of the currently most prominent material system in OPVs,namely the mixture of poly(3-hexylthiophene):1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C 61(P3HT:PCBM).In the second part of this article,we discuss new and promising active materials that have already shown promising performances in actual devices,and have the potential to go to signi?cantly higher ef?ciencies than those achieved by P3HT-based solar cells.The third part is devoted to the recent development of a tandem technology for the organic ?eld.The last two sections go beyond pure advanced material science,and discuss necessary require-ments to ensure that OPVs will become a sustainable technology in the market.The ?rst part analyzes the impact of the fundamental,OPV-speci?c losses on the maximum theoretical

ef?ciency,in a simpli?ed Shockley-Queisser approach.The second part tries to answer the question of what are the minimum ef?ciency and lifetime a low-cost PV technology needs to demonstrate in order to become competitive for grid-connected energy supply.

Despite the great progress of several different organic/hybrid approaches,like solution-processed or evaporated small mole-cules,polymer–polymer blends,or organic–inorganic blends,this review will focus exclusively on bulk-heterojunction compo-sites from polymer–fullerene blends.

2.The P3HT:PCBM Blend

2.1.Estimation of the Maximum Expectable Ef?ciency

For more than 5years,the P3HT:PCBM blend has been dominating the organic-solar-cell research.Although the material blend is well known and investigated,there are still discussions on the practical ef?ciency one may expect from that system.Although the device physics of polymer:fullerene bulk hetero-junctions has been the object of many recent review articles [7]and book chapters,[8]it is still important to set the ef?ciency expectations for that material system.

Consider a material,say P3HT,that absorbs photons with wavelengths smaller than 675nm (a band-gap energy E g %1.85eV).Assuming that in a P3HT:PCBM blend the polymer de?nes the optical gap of the composite,one can calculate the absorbed photon density as well as the power density by combining the absorption spectrum with the sun ’s spectrum.The typical spectrum of the light impinging on the surface of the Earth is given by the ASTM Standard G159,[9]and named Air Mass 1.5(AM1.5).The so-called AM1.5G,the overall reference for solar-cell characterization,[10]cumulates an integrated power density of 1000W m à2(100mWcm à2),and an integrated photon ?ux of 4.31?1021s à1m à2,distributed over a large range of wavelengths (280–4000nm).Under these assumptions,a P3HT:PCBM layer can absorb,at best,27%of the available photons and 44.3%of the available power,while the ultimate ef?ciency ,as de?ned by Shockley and Queisser,[11]predicts a value of 34.6%for a semiconductor with a band gap of 1.85eV.This difference arises from the fact that each photon having an energy E n larger than E g produces only one electronic charge q ,extracted at a maximum potential E g .

The external quantum ef?ciency (EQE)of a device is de?ned by the ratio of the collected electrons to the incident photons.The

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[*]Dr.C.J.Brabec,Dr.G.Dennler,Dr.M.C.Scharber

Konarka Austria GmbH Altenbergerstrasse 694040Linz,Austria

E-mail:cbrabec@https://www.wendangku.net/doc/2d11620944.html,

DOI:10.1002/adma.200801283

Solution-processed bulk-heterojunction solar cells have gained serious

attention during the last few years and are becoming established as one of the future photovoltaic technologies for low-cost power production.This article reviews the highlights of the last few years,and summarizes today’s sta-te-of-the-art performance.An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic tech-nology towards the magic 10%regime.A cost model supplements the technical discussions,with practical aspects any photovoltaic technology needs to ful?l,and answers to the question as to whether low module costs can compensate lower lifetimes and performances.

R E V I E W A R T I C L

E

hc q

Z l 2

l 1

P AM1:5G el TáEQE el Tád l

l

(1)

where h is Plank’s constant,c is the speed of light in vacuum,and l 1and l 2are the limits of the active spectrum of the device.In the case of the P3HT:PCBM blend,and for an EQE of 100%,the maximum possible J sc is about 18.7mA cm à2.If the average EQE is only 50%,J sc would then be only about 9.35mA cm à2.More information about expected ef?ciencies and accuracy of mea-surement can be found in the literature.[10,12]

In a real device,the absorption in the photoactive blend cannot be 100%,because the active layer (AL)is embedded within a stack of several layers,which have different complex refractive indexes.Thus,absorption can occur in some layer located between the incident medium and the AL,and re?ection can happen at any interface located before the bulk of the active layer.In order to precisely quantify the amount of light absorbed within the active layer,one needs ?rst to calculate the 1D distribution of the optical electromagnetic ?eld E (x )across the device in any of the layers involved.This is usually solved by the so-called transfer-matrix formalism (TMF),which incorporates both the absorption and the re?ection events in each subsequent layer.[13–15]

Figure 1summarizes the number of photons (N ph )absorbed in the P3HT:PCBM layer versus the thickness of this layer for an organic solar cell having the following structure:glass (1mm)/indium tin oxide (ITO,140nm)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS,50nm)/P3HT:PCBM (x nm)/Al (100nm).The refractive indexes used for this calculation can be found elsewhere.[16]It appears,in this ?gure,that N ph generally increases with increasing thickness,but not monotonically.If the thickness of the layers is smaller than the coherence of the light,interference occurs,because the light is re?ected by the opaque electrode.About 9.5?1016photons s à1cm à2are absorbed in an AL of 5m m.Assuming an average internal quantum ef?ciency (IQE)of 100%,this represents a J sc value of 15.2mA cm à2,or approximately 20%less than in the theoretical consideration.In the case of an AL that has a more realistic thickness of 400nm,the maximum J sc (IQE ?100%)is 12.8mA cm à2.If the average IQE is lower than 100%,J sc is further reduced.At 80%average IQE,J sc should be around 10.2mA cm à2.Thus,despite the fact that the theoretical short-current density of

a

Gilles Dennler received his Engineering and Masters Degrees at the National Institute for Applied

Sciences,Lyon,France,in 1999.He obtained a ?rst Ph.D.in plasma physics at the University of Toulouse,France,and a second in Experimental Physics at Ecole Polytechnique of

Montre

′al,Canada.In 2003,he moved to the Linz Institute for Organic Solar Cells (Austria),where he was appointed Assistant Professor.He joined Konarka in September 2006,where he is currently Director of European

Research.

Markus Scharber received an Applied Physics B.Sc.degree from Napier University Edinburgh,Scotland,a Masters Degree from the Johannes Kepler University Linz,Austria,and a Ph.D.at the Linz Institute for Organic Solar Cells.He joined the company Quantum Solar Energy Linz (QSEL)in 2002,which was acquired by

Konarka Technologies Inc.in 2003.Over the last 5years,

his main research activities have been new materials for ef?cient plastic solar cells and their ef?ciency

limitations.

Christoph J.Brabec is the CTO of Konarka technologies Inc.He received his PhD in physical chemistry in 1995from Linz university,joined the group of Prof Alan Heeger at UCSB for a

sabbatical,and continued to work on organic

semiconductors as assistant professor at Linz university with Prof.Serdar Sariciftci.He joined the SIEMENS research labs as project lea-der for organic optoelectro-nic devices in 2001and ?nally joined Konarka in

2004.

Figure 1.Number of photons (N ph )absorbed in the active layer (AL)under AM1.5G calculated by TMF,for a device having the following structure:glass (1mm)/ITO (140nm)/PEDOT:PSS (50nm)/P3HT:PCBM (x nm)/Al (100nm).The right axis represents the corresponding short-circuit current density J sc at various IQE,indicated in the graph.

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P3HT:PCBM blend could be close to 19mA cm à2,the practically achievable J sc of real devices will be in the range of 10–12mA cm à2.

2.2.Review of Experimental Results

The ?rst years of OPVs were dominated by poly[2-methoxy,5-(20-ethyl-hexyloxy)-p -phenylene vinylene)(MEH-PPV)/C 60com-posites,which were later on substituted by the better-processable combination of poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylene vinylene](MDMO-PPV)/1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C 61(PCBM).[1,17–21]Because of the rather large gap and low mobility of the PPV-type polymers,ef?ciencies remained at 3%at best,[22,23]and the general interest in this class of material faded.

During the last ?ve years,research efforts have focused on poly(alkyl-thiophenes),and in particular on P3HT.In 2002,the ?rst encouraging results for P3HT:PCBM solar cells with a weight ratio of 1:3were published.[24]At that time,the short-circuit current density was the largest ever observed in an organic solar cell (8.7mA cm à2),and resulted from an EQE that showed a maximum of 76%at 550nm.This paper appeared to be a starting point for a rapid development for the P3HT:PCBM blend,followed by the ?rst explicit reports on ef?ciency enhancement in P3HT/PCBM cells as a result of thermal annealing.[25]The main development over the last years has consisted in understanding and optimizing the processing of the active layer and,especially,the device annealing conditions,which,until recently,appeared to be mandatory to achieve high ef?ciencies.Table 1gives a nonexhaustive survey of reports that deal with ef?cient photovoltaic cells based on a P3HT:PCBM blend.[2,26–35]

Controlling the morphology of the bulk heterojunction in order to ensure maximum exciton dissociation at the interface between the donor and the acceptor,in parallel to an ef?cient charge-carrier extraction,was found to be the key for high performance.The optimum P3HT:PCBM weight ratio for that is about 1:1,and the two best-suited solvents for this blend are chlorobenzene (CB)and ortho-dichlorobenzene (oDCB).Upon annealing,the open-circuit voltage (V oc )was usually found to

decrease slightly,while both the J sc and the ?ll factor (FF)increased signi?cantly.Figure 2illustrates a typical enhancement of the EQE upon thermal annealing,as reported by Yang et al.[27]This phenomenon is attributed mainly to an enhancement of the charge-carrier transport,by a larger hole mobility,[36,37]a reduced dispersivity,[38]and a reduced recombination kinetics.[39,40]X-Ray investigations allowed the development of a microscopic picture of the annealing process,[41]as depicted in Figure 3.Several detailed morphological studies revealed that the organization of the P3HT:PCBM is modi?ed upon annealing,[27,32,36]with ?brillar-like P3HT crystals embedded in a matrix believed to comprise mostly PCBM nanocrystals and amorphous P3HT.[27]

The in?uence of the molecular weight (M w )on the performance of P3HT:PCBM was quickly addressed once the annealing process was understood.[42]Too-short molecular-weight fractions were shown to have inferior mobility,most likely because of main-chain defects and low mobility.[43]Furthermore,the role of smaller M w fractions was found to initiate or facilitate the growth of crystalline ?brils during the annealing step,leading to a large

Table 1.Nonexhaustive survey of reports focusing on photovoltaic devices based on P3HT:PCBM blends.

Year

P3HT Provider M w [g mol à1]Ratio to PCBM (weight)

Layer thickness [nm]

Solvent

Annealing time [min]Annealing Temp.[8C]Max EQE [%]V oc [V]FF

J sc

[mA cm à2]Eff [%]Light intensity [mW cm à2]

Ref.

2002––1:3350–––760.580.558.7 2.8100[24]2003–––110DCB 475700.550.68.5 3.580[25]2004Rieke –1:2350CB 475650.540.3715.2 3.1100[26]2005Rieke 1000001:170DCB 60120580.6150.617.2 2.7100[27]2005Merck 116001:1–CB 15140580.610.539.4 3.0100[28]2005––1:163DCB 10110–0.610.6210.6 4.0100[29]2005Rieke –1:1220DCB 10110630.610.6710.6 4.4100[30]2005Aldrich 870001:0.8–CB 5155–0.650.5411.1 4.980[31]2005Rieke –1:0.8–CB 30150–0.630.689.5 5.080[32]2006Merck 211001:1175CB 120140700.60.5212 4.485[33]2006––1:0.8–CB 10150880.610.6611.1 5.090[2]2006Rieke –1:1320DCB 10110820.560.4811.2 3.0100[34]2008Rieke

–

1:1

220

DCB 10

120

87

0.64

0.6911.3

5.0

100

[118]

Figure 2.EQE of different P3HT:PCBM devices reported in the literature.Adapted from [27,2].

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Figure 3.a,b)Schematic pictures showing the microscopic process during annealing.c)Grazing incidence X-ray spectrum on a blend before and after annealing,showing the evolution of the a -axis oriented P3HT crystals.Reproduced with permission from [41].

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solar cells.In both cases,ef?ciencies of >5%are reported for bulk-heterojunction solar cells prepared from a blend of a conjugated polymer and a fullerene.The devices were character-ized by the NREL (National Renewable Energy Laboratory,Boulder Colorado)calibration laboratory.The report lists the ef?ciency numbers,and includes the open-circuit voltage,the electrical ?ll factor,and the short-circuit current,but does not disclose a detailed description of the applied materials.However,

analyzing the device parameters reveals that both solar cells are not composed of a blend of regioregular P3HT and PCBM,both deliver signi?cantly higher open-circuit voltages (V oc >850mV)compared with the best P3HT-PCBM solar cells (see Table 1),and either alternative donor or acceptor materials were applied to achieve these record ef?ciencies.

The ef?ciency limitations of organic solar cells have been described earlier,[59,60]discussing the importance of the band gap,that is,the highest occupied molecular orbital (HOMO)and lowest unoccupied molecular orbital (LUMO)levels of the donor and the acceptor molecules.Figure 6shows a schematic drawing of the energy levels in an organic solar cell.The maximum short-circuit current is determined by the smaller optical band gap of the two materials,and V oc is proportional to the difference between the HOMO level of the donor material and the LUMO level of the acceptor compound.For an ef?cient charge generation in the donor–acceptor blend,a certain offset of the HOMO and LUMO levels (D E HOMO ,D E LUMO )is required,[61]which is believed to be a few hundred milli-electron Volts.

This offset,which is often referred to as the exciton binding energy,[62]determines the ultimate device ef?ciency of bulk-heterojunction solar cells.[59,60]For a minimum energy offset of 0.3eV between the donor and acceptor,power conversion ef?ciencies of >10%are pratical,[60]for a semiconductor with an ideal optical band gap of $1.4eV (Fig.7),at an EQE of 65%,and a FF of 65%.The maximum ef?ciency does not depend on the absolute position of the HOMO and LUMO levels,but is solely a function of the smaller band gap and the donor–acceptor level offsets.

For donor band gaps smaller than $3eV,Figure 7describes the ef?ciency of bulk-heterojunction solar cells that comprise a donor with a variable band gap in conjunction with an acceptor with a variable LUMO.For highest ef?ciencies,the difference between the LUMO levels needs to be 0.3eV,and a band gap in the range of 1.2–1.7eV,which would correspond to donor HOMO levels of –5.2to –5.7eV if the acceptor is PCBM (whose LUMO is assumed to be à4.3ev).The material-design rules described above suggest that optimising the LUMO-level difference is the most promising strategy to develop high-ef?ciency bulk-heterojunction solar

cells.

Figure 5.a)Scanning electron microscopy and b)atomic force micro-scopy images obtained for a 0.05wt%P3HT solution in cyclohexanone.b)Absorption spectra of a 1wt%P3HT solution in p -xylene,with different proportions of nano?bers and well-solubilized P3HT:a)97%,b)75%,c)50%,d)39%,and e)0%nano?bers.Reproduced with permission from

[56].

Figure 4.UV-vis spectra of 3:2P3HT:PCBM as-cast PV devices with 0%(solid line),0.33%(dashed line),0.67%(dotted line),1.6%(dashed–dotted line),3.2%(short dashed line),and 6.3%(solid line)nitrobenzene added into the chlorobenzene solvent.Offset from the other spectra is the as-cast PV device from the o -xylene dispersion (triangles).Reproduced with permission from [56].

R E V I E W A R T I C L E

Figure 7.Calculated ef?ciency under AM1.5G illumination for single-junction devices based on composites that consist of a donor with a variable band gap and LUMO level and an acceptor with a variable LUMO level.

Figure 8.Promising polymers for OPV devices:1)poly[9,9-didecane?uorene-alt-(bis-thienylene)

benzothiadiazole][65],2)APFO-Green 5[66],3)poly[N -9

′-heptadecanyl-2,7-carbazole-alt -5,5-(40,70di-2-thienyl-20,10,30

-benzothiadiazole)][67],4)poly[2,6-(4,4-bis-(2-ethylhexyl)-4H -cyclopenta[2,1-b;3,4-b2]-dithiophene)-alt -4,7-(2,1,3-benzothiadiazole)][68],5)poly{5,7-di-2-thienyl-2,3-bis(3,5-di(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine}[69],and 6)platinum(II )polyyne polymer [70].7)and 8)are PCBM and P3HT,repectively.

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can compensate the rather low short-circuit current (7.7mA cm à2)and ?ll factor (54%).A high open-circuit voltage is a typical feature of ?uorene-based polymer devices,as the polymers often have a low-lying HOMO level.An interesting variation of polymer 1in Figure 8is obtained by replacing the ?uorene unit by dibenzosilole.[74,75]Replacing the bridging C atom of the ?uorene by a Si atom is motivated by the expectation of a positive impact on the charge-transport properties.This idea is supported by the work of Wang et al.,[75]who reported an uncerti?ed power-conversion ef?ciency of 5.4%for an alternating copolymer of 2,7-sila?uorene and 4,7-di(20-thienyl)-2,1,3-benzothiadiazole PCBM mixture.3.1.2.Carbazole-based Copolymers

A few recent reports [67,76]have described the use of carbazole copolymers in solar cells.This material class appears to have identical electrical and optical properties to the poly?uorene class.Moulin et al.tested polymer 3from Figure 8in bulk-heterojunction solar cells with PCBM.The best device performance was in the range of 3.6%(measured at 90mWcm à2,2,AM1.5,not certi?ed or veri?ed by EQE measurement),with a high V oc of 890mV and a high FF (63%).Overall,this speci?c polymer performed very similarly to the poly?uorene or polysila?uorene pendants (structure 1in Fig.8).Further work from the Leclerc group demonstrated the similarity between these material classes,and,by that,the high potential of 2,7-carbazole copolymers for solar-cell applications.[76]3.1.3.Cyclopentadithiophene-based Copolymers

Cyclopentadithiophene-based polymers have attracted a lot of attention in the last two years,[3,68,77–79]with poly[2,6-(4,4-bis-(2-ethylhexyl)-4H -cyclopenta[2,1-b ;3,4-b 2]-dithiophene)-alt -4,7-(2,1,3-benzothiadiazole)][PCPDTBT,structure 4,Fig.8]as the most prominent candidate of this novel class of copolymers.This polymer is a true low-band-gap material (E g $1.45eV),as well as an excellent charge transporter,[80]with high hole mobility,thereby ful?lling all the requirements for highly ef?cient solar cells.When PCBM is blended into PCPDTBT,an unfavourably intimate nanomorphology is formed,and the composites typically suffer from short carrier lifetimes and considerable recombina-tion.[68]It takes the use of additives like alkanedithiols to form a more course nanomorphology.Heeger and coworkers [3]inves-tigated the use and function of these additives in great detail,and reported solar cells with uncerti?ed ef?ciencies beyond 5%for PCPDTBT/PCBM composites.

Konarka has explored the cyclopentadithiophene class in great detail,and,as one of the outcomes,Figure 9shows an ef?ciency certi?cate for a device submitted to NREL.The solar cell delivers a short-circuit current of $15mA cm à2and a V oc of 575mV,which results,together with an FF of 61%,in an ef?ciency of $5.2%.The EQE of the certi?ed device reaches $63%at $780nm,with an estimated IQE of 85%at the same wavelength.

The only drawback of PCPDTBT is the rather high-lying HOMO level ($–5.2eV),which does not allow open-circuit voltages higher than 600–700mV when mixed with PCBM.The current research is,therefore,focused on two strategies to overcome this limitation.On the one hand,synthetic efforts are strengthened to design novel bridged bithiophene copolymers

with lower-lying HOMO levels;on the other hand,novel acceptors with higher-lying LUMO levels are investigated.[81]3.1.4.Metallated Conjugated Polymers

Metallated conjugated polymers have attracted a lot of attention as emitter materials in polymer light-emitting diodes (PLED).[82–85]The metal atom integrated into the polymer backbone can increase the mixing of the ?rst excited singlet and triplet states,leading to higher electroluminescence quantum ef?ciencies of PLEDs.In contrast,metallated conjugated polymers have rarely been tested as donor materials in bulk-heterojunction solar cells.[86,87]In early reports,power-conversion ef?ciencies sig-ni?cantly below 1%were published.Recently,Wong et al.[70]demonstrated highly ef?cient bulk-heterojunction solar cells using polymer 6(Fig.8)as a donor and PCBM as an acceptor material.The authors report $5%power-conversion ef?ciency,with EQEs as high as 87%at 570nm.Several groups raised serious doubts that the reported ef?ciencies were signi?cantly overestimated,[88,89]and a veri?cation by an independent institution is still missing today.Nevertheless,the concept to design polymers involving triplet states and long-lived triplet excitons in charge generation could become interesting for a next-generation organic PV

material.

Figure 9.NREL certi?cate of the device LS1submitted by Konarka.

R E V I E W A R T I C L E

Figure 10.Simpli?ed band diagram of tandem cells composed of two subcells connected in series by a recombination layer.

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technology,[105]based on a bottom cell processed from solution (P3HT:PCBM)and a top cell evaporated (ZnP C :C 60),both subdevices being separated by 1nm of Au.Further reports followed up this hybrid solution,with other material combina-tions.[106]

In parallel,the ?rst tandem cells that comprise two solution-processed subcells,based on a wide-band-gap poly?uor-ene-type polymer and a low-band-gap poly(terthiophene)-type polymer,were reported.[107]Dissolution of the ?rst layer was prevented by using a composite middle electrode of 15nm of evaporated metal,which is still semitransparent.The most signi?cant innovation in the tandem technology reported the use of a solution-processed recombination layer,which,for the ?rst time,allowed complete solution-processing of tandem cells.[108]This recombination layer was realized by spin-coating a ZnO nanoparticle [109]n-type layer as an electron selective electrode on the semiconductor,followed by an again spin-coated pH-neutralized PEDOT ?lm as a hole-selective electrode for the top cell.The combination of a p-type and an n-type semiconductor layer created a barrier for Ohmic transport,enforcing recombination of electrons and holes at the interface with equal rates.

The highest-ef?ciency tandem devices reported to date are entirely solution processed.These devices had a 38%perfor-mance increase versus the best single device,[4]and an uncerti?ed ef?ciency of 6.5%was reported (see Fig.11).The intermediate layer comprised a TiO x sol–gel layer and a PEDOT:PSS layer;the bottom cell was made of a blend of PCPDTBT and PCBM,and the top cell was based on a P3HT:PC 70BM mixture.Noticeably,the selective usage of PC60BM or PC70BM allowed maximization of the number of photons absorbed in each subcell,because of a reduction of the overlap between the respective absorption spectrum of the active blends.[110]

Although all the devices reviewed above are based on a two-terminal concept comprising cells connected in series,several groups followed other approaches.The optimization of semi-transparent top electrodes allows the superposition of two independent devices,and connects them either in series or in parallel.[111]Monolithic four-terminal devices [112]were reported using a transparent and insulating polymer (polytri?uoroethy-lene)to separate the two stacked cells.[113]The most innovative device architecture,which is also accounted for under tandem

cells,is probably the so-called folded re?ective tandem device,[114]as depicted in Figure 12.

This geometry has three major advantages.First of all,the re?ected light of one cell is directed toward the second device,which ideally has a complementary absorption spectrum.Second,the tilting of each cell enlarges the light path within the active layer.[115]Finally,using an angle between the cells smaller than 908can cause a light-trapping effect to occur,signi?cantly enhancing the absorption,and hence the photogeneration,of charge carriers.In the case of solar cells with thin active layers (50–60nm)and rather low EQEs,an almost two-fold enhance-ment of the performance was reported for an angle of 408between the cells.In the case of highly ef?cient single-junction cells,the V-shape geometry is only bene?cial if semiconductors with two different band gaps are operated.

Table 2gives an overview on the literature reports for organic tandem devices,and includes reports on small-molecular cells.Finally,and in analogy to the performance prediction for single-junction cells,Figure 13predicts the ef?ciency for tandem cells in relation to the band gap of the single-junction materials.The prediction was calculated for the case of optimal aligned

LUMO levels with only a 0.3eV difference to the PCBM LUMO.The 2D contour lines show that the ef?ciency can reach values as high as 14%.[103]

5.Fundamental Losses and

Theoretical Ef?ciency of Organic Solar Cells

The fundamental question for any new solar technology is the determination of the ultimate ef?ciency.The analysis of the last two chapters predicted a technical feasible ef?ciency of over 10%for organic single-junction solar cells,and close to 15%for the tandem junction cells.Clearly,one could argue that the

assumptions

Figure 11.Structure and current-voltage characteristics of the tandem cells realized by Kim et al.Reproduced with permission from [4].Copyright 2007American Association for the Advance-ment of

Science.

Figure 12.Sketch of the folded tandem cell realized by Tvingstedt et al.[114].

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E

Table 2.Nonexhaustive survey of reports dealing with stacked or tandem organic solar cells.

Year

Intermediate

layer

Bottom cell

Top cell Tandem cell Ref.

Active materials

V oc [V]

FF

J sc [mA cm à2

,(mW cm à2)]Eff [%]Active materials

V oc [V]

FF

J sc [mA cm à2,(mW cm à2)]Eff [%]

V oc [V]

FF

J sc [mA cm à2

,(mW cm à2)]Eff [%]19902nm Au H2Pc/Me-PTC 0.44– 2.7(78)–as bottom as bottom as bottom as bottom as bottom 0.78–0.9(78)–

[97]20020.5nm Ag CuPc/PTCBI 0.45–– 1.0as bottom as bottom as bottom as bottom as bottom 0.9

0.43

6.5(100) 2.6[98]20040.5nm Ag CuPc:C60–0.64– 4.6as bottom as bottom as bottom as bottom as bottom 1.030.599.7(100) 5.7[99]20050.8nm Au ZnPc:C600.5

0.37

15.2(130) 2.1as bottom as bottom as bottom as bottom as bottom 0.990.4710.8(130) 3.8[101]200620nm

ITO t

PEDOT:PSS MDMO-PPV:PCBM

0.840.58

4.6(100)

2.3

as bottom as bottom as bottom as bottom

as bottom 1.340.56

4.1(130)

3.1[104]

20061nm Au P3HT:PCBM 0.550.558.5(100) 2.6ZnPc:C600.470.59.3(100) 2.2 1.020.45 4.8(100) 2.3[105]20060.5nm LiF t

0.5nm Al t15nm Au t60nm

PEDOT:PSS PFDTBT:PCBM 0.90.5 1.0(100)0.4PTBEHT:PCBM

0.5

0.64

0.9(100)0.23

1.40.550.9(100)0.6[107]

200730nm ZnO t

PEDOT MDMO-PPV:PCBM 0.820.55 4.1(100) 1.9P3HT:PCBM 0.750.48 3.5(100) 1.3 1.530.42 3.0(100) 1.9[108]20078nm

TiO x t25nm PEDOT:PSS

PCPDTBT:PCBM

0.66

0.5

9.2(100)

3.0

P3HT:PCBM

0.63

0.69

10.8(100)

4.7

1.240.67

7.8(100)

6.5

[4]

Figure 13.Ef?ciency of an OPV tandem device versus the band gap of both donors.We assumed that the difference between the LUMO of the donor and the acceptor is 0.3eV,that the maximum EQE of the subdevices is 0.65,and that the IQE of the bottom device is 85%[103].

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early PV-cell ef?ciency calculations were based on a black-body spectrum,the AM1.5G spectrum has gained acceptance as the best representation of the sun ’s spectrum at the earth ’s surface. Possible dark current effects are neglected.

The FF will be taken as a ?xed value,instead of as a function of the band gap.

As introduced by Shockley,we will consider only a thermalized carrier.Excess energy,that is,the photon energy larger than the semicondcutor band gap,will be dissipated as heat.

Thermal radiation from the environment as well as thermal radiation from the solar cell itself will be neglected.

The balance of i)the photoinduced charge-transfer loss and ii)the polaron loss are material-related phenomena.Assuming i)thermalized-carrier loss equals the difference in the LUMO energies of the donor and the acceptor.Following Marcus theory,the rate of electron transfer in polarizable media is related to the driving force D G 0(energy difference between the initial and the transferred state),which is related to the difference in the HOMO and LUMO energies.Although functional bulk-heterojunction composites with LUMO differences as low as 0.1eV were reported,[117]we will use a value of 0.25eV in the simulation.For the ii)polaron loss we will again assume a value of 0.25eV.[116]It is important to note that these loss values are ‘educated’assumptions,which allow one to discuss the impact of these fundamental loss mechanisms on performance.These losses will vary for the individual composites,and ii)speci?cally loss may deviate from the assumption for individual material systems.Figure 14shows ef?ciency versus band gap calculations for the various loss mechanisms.The local minima and maxima in the curves re?ect the spectroscopic shape of the AM1.5G spectrum.The maximum ef?ciency of $50%at $1100nm for a single-junction photovoltaic converter is reduced to approx.40%at <1000nm (extrapolated),in the case of the charge-transfer loss (i),and to approximately 30%at 900nm,in the case

of combining the charge-transfer (i)with the polaron (ii)losses.Introduction of the two loss mechanisms shifts the optimum band gap to larger values,from 1100nm (1.12eV)down to 900nm (1.37eV).

To obtain more realistic benchmark values,we repeat the calculations for reasonable though challenging EQE and FF values.The highest EQE value reported for an organic solar cell is 87%.[118]FFs of >70%have already been reported a few times.Note that the assumption of a ?xed FF is a major difference compared with the Shockley model,[10]where the maximum power point is calculated as a function of the band gap,and reaches values between 0.8and 0.9.As a compromise,an EQE of 90%and a FF of 70%were used to calculate the highest possible ef?ciency (Fig.15).The reduction in FF and EQE do not change the spectroscopic shape of the ef?ciency versus band gap correlation.For each loss mechanism,the optimum band gap remains at the same position,but the absolute ef?ciency numbers are reduced.Even in the most unfavorable case,including all V oc ,EQE,and FF losses,an ef?ciency of a little less than 20%is realistic.

Inorganic and organic solar cells follow similar recombination mechanisms.Both their short-circuit current and FF are determined by the spectroscopic absorption,mobility,carrier lifetime,and defect distribution.The relation between radiative and nonradiative recombination may shift depending on the practical values,with the theoretical maximum remaining the same.As such,the Shockley–Queisser model would predict identical performance for inorganic and organic solar cells.The main difference,which is not accounted for in the Shockley–Queisser model,is the speci?c energetic loss (i),which leads to a reduction of the maximum-possible open-circuit voltage.The question arises as to whether we need to include the radiative recombination losses,which Shockley balanced under the dark current,for the organic-solar-cell prediction as well.The answer to this question is yes.Practically,we have accounted for

these

Figure 14.Plot of the theoretical maximum ef?ciency versus band gap for organic bulk-heterojunction solar cells:loss free case (full squares),including loss (i)(full circles)and including losses (i)and (ii)(full triangles).Both FF and EQE were set to 100%for this

calculation.Figure 15.Plot of the practical maximum ef?ciency versus band gap for organic bulk-heterojunction solar cells:loss free case (full squares),including loss (i)(full circles)and including loss (i)and (ii)(full triangles).A FF of 70%and an EQE of 90%were used for this calculation.

R E V I E W A R T I C L E

Figure 16.Plot of the V oc for a few inorganic and organic semiconductors.The values for the inorganic semiconductors were taken from [119].V oc values for the organic semiconductors were taken from the following publications:PCDTQX and PCDTBX [76],APFO Green-1[117],and PSiF-DBT [75].The arrow indicates the potential for higher V oc in case of a better-matched acceptor.

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The BOM costs are varied between 10to 100s m à2costs. The module ef?ciency is varied between 3and 10%.

Discounting future investments:all future replacements and investment costs necessary to replace the shorter-lifetime PV modules are discounted by 7%to come with the net present value.

The last point is a major assumption on how to ?nance roof-top PVs.Debit ?nancing,where a customer needs to take credit,is one way to ?nance an installation.In that case,the credit interest rates need to be added to the total costs for the installation.Alternatively,the installation can be ?nanced from an upfront investment.Less upfront investment is required for a module technology with shorter lifetimes,which allows discount of the future investments.We have chosen to work with the discount model,since it is clearly the more attractive model for a solar technology with a shorter lifetime.Nevertheless,it is important to note that the cost difference between these two models can be up to a factor of two.

Table 3shows the ?rst results from the calculations.The s per W p costs of a 1kW p plant with a BOM of 50s m à2and a BOS of 70s m à2are summarized.Taking 3s per W p as a benchmark,a

low-cost technology like OPV can become competitive at an ef?ciency of around 7%and a lifetime of 7years.

It is by far more interesting and relevant to answer the question as to whether a low-cost and lower-performance technology,such as OPV,can be a sustainable solution for the supply of future energy.For this question,one has to calculate the costs of electricity in s cents per kilowatt hour.In our calculation,we assumed 1000h of sun a year,a value typical for regions such as middle Europe (e.g.,Germany).Figure 17shows the costs in the case of 70s m à2BOS,whereas the performance parameters were varied to meet energy production costs of 50,25,10,and 5cents kWh à1,respectively.Additional calculations [120]for other BOS assumptions showed how great the BOS impacts the energy costs for a low-cost technology.Another clear relation is seen by the trend lines that connect the ef?ciency error bars.An increase in lifetime ?attens out the dependence between costs and ef?ciency.Modules with a longer lifetime are much less susceptible to cost reduction upon ef?ciency increase (in absolute numbers).The calculations further show that it really pays off to have low module costs.The lower the module costs,the less important lifetime and ef?ciency become.At a BOM of 70s m à2,energy costs of 10cents kWh à1can be generated with module ef?ciencies between 8and 16%for life times between 5and 10years.A module with 30s m à2costs can do the same with ef?ciencies between 5and 8%.Better solar insulation is always favourable,independent of the costs.Running the calculation for 2000h of sun per year would predict the same cost and lifetime values at only half of the ef?ciency.

The outcome of the cost calculation strongly supports the idea of low-cost and lower-performance PV technologies such as OPV.A shorter lifetime and lower ef?ciency can be compensated for by lower module costs.Low cost modules (i.e.,a BOM of 30–50s m à2)with a lifetime between 5and 10years and an ef?ciency between 10and 5%can produce electricity at 10s cents kWh à1in middle Europe,even at a BOS of 70s m à2.

7.Summary

Although P3HT is still dominating organic photovoltaic publica-tion records,there are already several very promising alternative polymers available,which have led to certi?ed ef?ciencies beyond the best values reported for P3HT.Polymers with various band gaps produced certi?ed ef?ciencies of >5%.Novel material classes that are optimised for photovoltaic requirements

will

Figure 17.Energy-cost calculations in s cents kWh à1for the presented model of 1kWp grid-connected roof-top plant under the following set of assumptions:BOS 70s m à2;BOM:varied from 10–100s m à2;lifetime:varied from 3–10years,ef?ciency:varied from 3–10%.The full symbols indicates the value at 5years of lifetime.The error bars and the guided lines around the symbols show the parameter variation in the case of a 3year and 10year product,

respectively.

Table 3.Cost calculations in s per W p for the presented model of 1kW p grid-connected roof-top plant under the following assumption:BOM ?50s m à2and BOS 70s m à2.

s per W p 3years 4years 5years 6years 7years 8years 9years 10years 3%12.29.27.3 6.1 5.2 4.6 4.1 3.7

R E V I E W A R T I C L E

rapidly lead to ef?ciencies beyond 7%,and their combination in multi-junction devices will lead to even higher ef?ciencies.Further improvement in the power-conversion ef?ciency of organic solar cells will come from donor–acceptor pairs with an optimised LUMO-level offset,as was demonstrated,for instance,by using multi-adduct fullerenes instead of single substituted fullerenes.Overall,10%ef?cient organic solar cells appear to be within reach in the next few years.

The energy offset between the donor and acceptor LUMO levels required for an ef?cient electron transfer is a unique loss mechanism among photovoltaic technologies.This loss mechan-ism reduces the maximum achievable ef?ciency for the organic and hybrid bulk heterojunction technologies,and practical ‘maximum ’ef?ciencies between 20–25%appear reasonable.Cost ef?cient power generation is achievable with low-cost solar-cell technologies,which show ef?ciencies at least between 5and 10%and lifetimes between 5and 10years.These values depend on the precise module and installation costs.Cost-model calculations prove that the lower ef?ciency and lower lifetime of organic solar cells as compared with inorganic technologies can be compensated by their low-cost structure.

Received:May 10,2008Revised:July 23,2008

Published online:

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