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162 NATURE PHOTONICS| VOL 6 | MARCH 2012 | www.nature.com/naturephotonics

Dye-sensitized solar cells (DSCs) are attractive because theyare made rom cheap materials that do not need to be highlypurified and can be printed at low cost 1. DSCs are unique

compared with almost all other kinds o solar cells in that electron

transport, light absorption and hole transport are each handledby different materials in the cell2,3. Te sensitizing dye in a DSC isanchored to a wide-bandgap semiconductor such as iO2, SnO2orZnO. When the dye absorbs light, the photoexcited electron rapidlytransers to the conduction band o the semiconductor, which car-ries the electron to one o the electrodes4. A redox couple, usuallycomprised o iodide/triiodide (I/I3

), then reduces the oxidized dyeback to its neutral state and transports the positive charge to theplatinized counter-electrode5.

In 1991, ORegan and Grtzel demonstrated that a film o tita-nia (iO2) nanoparticles deposited on a DSC would act as a mes-oporous n-type photoanode and thereby increase the availablesurace area or dye attachment by a actor o more than a thou-sand1. Tis approach dramatically improved light absorption and

brought power-conversion efficiencies into a range that allowedthe DSC to be viewed as a serious competitor to other solar celltechnologies6. A schematic and energy level diagram showing theoperation o a typical DSC is shown in Fig. 1. During the 1990sand the early 2000s, researchers ound that organometallic com-plexes based on ruthenium provided the highest power-conversionefficiencies7,8. Iodide/triiodide was ound to be the most effectiveredox couple913. Te record power-conversion efficiency rapidlyclimbed to 10% in the late 1990s and then slowly settled to 11.5%1417. Figure 2 shows a currentvoltage curve under 1 Sun illumina-tion, together with a plot o the external quantum efficiency as aunction o photon wavelength.

Te iodide/triiodide system has been particularly successul inDSCs because o the slow recombination kinetics between elec-

trons in the titania with the oxidized dye and the triiodide in theelectrolyte, which leads to long-lived electron lietimes (between1 ms and 1 s)1820. Iodide reduces the oxidized dye to orm an inter-mediate ionic species (such as I2

) that then disproportionates toorm triiodide and diffuses to the counter-electrode, providing twoelectrons per molecule, as shown in Fig. 1b4,19. Te slow recombi-nation and relatively ast dye regeneration rates o the I/I3

redoxcouple have resulted in near-unity internal quantum efficienciesor a large number o dyes, providing the high external quantumefficiencies shown in Fig. 2a. Te small size o the I/I3

redox

The renaissance of dye-sensitized solar cellsBrian E. Hardin1, Henry J. Snaith2and Michael D. McGehee 3*

Several recent major advances in the design of dyes and electrolytes for dye-sensitized solar cells have led to record power-

conversion efficiencies. Donorpiacceptor dyes absorb much more strongly than commonly employed ruthenium-based dyes,thereby allowing most of the visible spectrum to be absorbed in thinner films. Light-trapping strategies are also improvingphoton absorption in thin films. New cobalt-based redox couples are making it possible to obtain higher open-circuit voltages,leading to a new record power-conversion efficiency of 12.3%. Solid-state hole conductor materials also have the potentialto increase open-circuit voltages and are making dye-sensitized solar cells more manufacturable. Engineering the interfacebetween the titania and the hole transport material is being used to reduce recombination and thus attain higher photocurrentsand open-circuit voltages. The combination of these strategies promises to provide much more efficient and stable solar cells,paving the way for large-scale commercialization.

components allows or relatively ast diffusion within the mes-opores, and the two-electron system allows or a greater currentto be passed or a given electrolyte concentration. Unortunately,the I/I3

system is corrosive and dissolves many o the commonly

used sealants and metal interconnects (such as silver, copper, alu-minium and gold).

Obtaining maximum DSC power-conversion efficienciesTe ShockleyQueisser limit o ~32% is the maximum theoreti-cal power-conversion efficiency o a single-junction solar celldevice21. For highly efficient inorganic solar cells, the main devia-tion rom this ideal limit is through the loss-in-potential, whichcan be roughly defined as the difference between the optical band-gap o the photoactive semiconductor divided by the charge o anelectron, and the open-circuit voltage (VOC). As an example, therecord 25%-efficient silicon photovoltaic cell has a loss-in-poten-tial o around 400 mV, whereas the record 28.1%-efficient GaAsdevice has a loss-in-potential o 300 mV (re. 6). Unlike traditional

inorganic solar cells, DSCs require relatively large over-potentialsto drive electron injection to titania and regenerate the oxidizeddye, as shown in Fig. 1b. Tis requirement results in significantlylarge loss-in-potentials o more than 700 mV and defines the mini-mum bandgap o the sensitizing dye (and thereore the onset olight absorption). A plot o the maximum obtainable efficiency

versus loss-in-potential and absorption onset is shown in Fig. 322.ypically, a potential difference between the lowest unoccupiedmolecular orbital (LUMO) level o the dye and the conductionband o titania is required or ast electron injection3. Te mag-nitude o the required offset has not been precisely determinedbut is likely to be around 100150 mV (re. 23), which is muchlower than the over-potential usually required to regenerate thedye. Regeneration o a ruthenium metal complex dye with the I/I3

redox couple has a loss o around 600 mV, with over 300 mV othat being directly related to the reaction within the iodide elec-trolyte4,19. We estimate that the lowest loss-in-potential or theruthenium complex/iodide system is 750 mV, which limits themaximum obtainable conversion efficiency to 13.8%, as shownin Fig. 322.

Tere are two main ways in which the efficiency o a DSC can beimproved: extend the light-harvesting region into the near-inra-red (NIR), and lowering the redox potential o the electrolyte toincrease VOC. Using a dye that absorbs urther into the NIR, say to

1The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 2Department of Physics, Clarendon Laboratory,

University of Oxford, Oxford OX1 3PU, UK. 3Department of Material Science and Engineering, Stanford University, Stanford, California 94305-4045, USA.

*e-mail: [email protected]

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NATURE PHOTONICS| VOL 6 | MARCH 2012 | www.nature.com/naturephotonics 163

around 940 nm, while still managing to generate and collect thecharge carriers efficiently, could increase the current by over 40%,as shown in Fig. 322,24. Further increasing the power-conversionefficiency beyond 14% will require improved dyes and electrolyteswith smaller over-potentials to efficiently transer charge. Single-electron redox mediators based on cobalt and errocene complexeshave two advantages over iodide. First, they do not require an

intermediary step during regeneration and can thereore reducethe loss-in-potential. Second, unlike iodide, which does not havean ideal redox potential (0.350.40 eV over the normal hydrogenelectrode), alternative electrolyte couples can be tuned closer to thehighest occupied molecular orbital (HOMO) level o the sensitiz-ing dye to obtain a higher VOC.

Although efforts to solve these problems were stymied or manyyears25, new approaches have recently emerged and the world-record efficiency is climbing again26. Over the past 15 years, therehas been a great deal o research and improved understanding othe associated chemistry and device physics o DSCs, which iscomprehensively described elsewhere25,24,2731. Tis Review ocuseson several recent promising innovations in the field that we believewill lead to power-conversion efficiencies o more than 15% in thenear uture.

Strongly absorbing donorpiacceptor dyesTe sensitizing dye in a DSC is anchored to the n-type metal oxidesurace7. Light absorption is determined by the molar extinctioncoefficient o the sensitizing dye, the surace coverage o the dyeand the total surace area o the oxide film29. Sensitizing dyes gener-ally pack tightly on the titania surace, with a density o 0.51 dyemolecules per square nanometre29. Dyes typically contain a light-

harvesting portion, acidic ligands (or example, carboxylic or phos-phonic acid) to attach to the semiconductor surace, and ligands toincrease the solubility in solution and reduce aggregation betweendyes7. Aggregation occurs when the dye molecules are packed sotightly that their waveunction overlap is large enough to changetheir electronic character, which ofen causes the dyes to quench inthe excited state beore electron transer can occur.

Sensitizing dyes have traditionally been made rom ruthenium-based complexes such as N3, N719, C106 and CYC B1114,16,32,which have airly broad absorption spectra ( 350 nm) but lowmolar extinction coefficients (10,00020,000 M1 cm1)15,33. Tesecomplexes also have extremely weak absorption at the band-edge(around 780 nm), which restricts NIR light harvesting22. Althoughruthenium-based complexes work well and have been the mostwidely used dyes over the past two decades, it seems that increased

Electrolyte

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Figure 1 | Dye-sensitized solar cell device schematic and operation.a, Liquid-based DSCs are comprised of a transparent conducting oxide (such asfluorine-doped tin oxide, FTO) on glass, a nanoparticle photoanode (such as titania) covered in a monolayer of sensitizing dye, a hole-conducting

electrolyte and a platinum-coated, FTO-coated glass back-contact70. b, Energy level and device operation of DSCs; the sensitizing dye absorbs a photon

(energy h), the electron is injected into the conduction band of the metal oxide (titania) and travels to the front electrode (not shown). The oxidized dye

is reduced by the electrolyte, which is regenerated at the counter-electrode (not shown) to complete the circuit. VOCis determined by the Fermi level (EF)

of titania and the redox potential (I3/I) of the electrolyte.

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bulky cobalt redox couple, recombination in the system is reducedby at least one order o magnitude without affecting the electrontranser rate. Grtzel and co-workers recently took this approachto the next level by applying the insulating ligand technique to adonorpiacceptor dye YD2-o-C8 (Fig. 2c), which has a broadabsorption spectrum. In doing so, they achieved similarly lowrecombination rates and demonstrated DSCs with a laboratory-measured world record efficiency o 12.3% under 1 Sun illumina-

tion26(able B1). Te improved perormance was linked to a 16%increase in VOCover cells containing an iodide-based redox couple,which demonstrates the importance o tuning the redox level toincrease VOC. Te dye had an absorption onset at 725 nm and thecell had a total loss-in-potential o around 775 mV. In the shortterm, moving the dye absorption out to 830 nm could increasethis efficiency to 13.6% without any urther undamental advancesin technology. In recent work, an over-potential o only 390 mVwas sufficient to regenerate the oxidized dye and achieve an exter-nal quantum efficiency o more than 80%49. Given a total loss-in-potential o 500 mV, and assuming a required over-potential o100 mV on the electron-transer side, it may be possible to increasethe efficiency o the cobalt system to 19% by extending the absorp-tion out to 920 nm (Fig. 3).

One o the shortcomings o cobalt-based complexes is that theirbulky groups significantly decrease the speed at which the ions candiffuse through the electrolyte up to an order o magnitude lessthan conventional iodide ions50. Grtzel and co-workers ound thatreducing the illumination intensity increased the power-conver-sion efficiency to 13.1%, as it is less important or the ions to diffuseto the electrode quickly when the carrier density is lower26. Onecould imagine obtaining this efficiency under 1 Sun illuminationby using even thinner films to reduce the required diffusion dis-tance. Later in this Review we will describe potential techniquesor slowing recombination and attaining adequate light absorptionin thin films.

Long-term stability studies have not yet been perormed oncobalt complexes in dye-sensitized solar cells. It will be important

to make sure that cobalt complexes do not undergo irreversiblechanges at the counter-electrode42while providing stabilities simi-lar to (or better than) iodide-based electrolytes.

Solid-state dye-sensitized solar cellsSolid-state DSCs (ss-DSCs), which use solid hole conductorsinstead o a liquid electrolyte, are also capable o delivering high

voltages51. Te hole conductor is typically made rom either wide-bandgap small molecules (such as spiro-OMeAD) or semicon-ducting polymers (such as PEDO or P3H). Tese DSCs are inprinciple more industrially compatible than standard DSCs becausethey do not contain a corrosive liquid electrolyte, which requirescareul packaging. Te highest values o VOC (>1 V) achieved soar have been demonstrated in devices that exploit a small-mol-

ecule hole conductor52

. In ss-DSCs, hole transer occurs directlyrom the oxidized dye to the HOMO level o the hole conductor,which then transports the charge to the (typically silver) counter-electrode53,54. Dye regeneration occurs over a period o tens tohundreds o picoseconds several orders o magnitude asterthan regeneration with the I/I3

redox couple53. We believe that anover-potential o only 200 mV may be sufficient or hole regen-eration, thus allowing or power-conversion efficiencies o morethan 20%, as shown in Fig. 3 (again assuming a loss o 100 mV onthe electron-transer side). Although the first ss-DSCs made withsolution-processable small molecules achieved power-conversionefficiencies o less than 1%, researchers have recently increased this

value to 7.1%51,55. Significant recombination rates, together with thedifficulty in achieving high levels o pore-filling in thicker films,means that ss-DSCs currently work best at thicknesses o only a ew

micrometres56. Te greatest issues acing ss-DSCs are their incom-plete light harvesting and lower internal quantum efficiency, whichtogether result in current densities that are lower than liquid-basedDSCs, as shown in Fig. 2 (Y123, a spiro-OMeAD system)55.

Tere are many actors that affect the recombination betweenconduction-band electrons in the titania layer and holes in the holeconductor layer. Under solar operating conditions, the hole densityin ss-DSCs is highest near the dye-sensitized interace (the point o

generation) and charge screening is not as effective as it is in cellscontaining liquid electrolytes, which typically leads to recombina-tion rates that are an order o magnitude larger than those in thebest I/I3

systems. Recombination in ss-DSCs can be significantlyinhibited by interacial engineering (discussed below) and havingthe correct mix o ionic additives in the hole transporter phase54,57.Chemical p-dopants are ofen added to the hole transporter toincrease the conductivity, resulting in increased values o VOCandthe fill actor55.

Solid hole conductors are almost exclusively abricated throughsolution-deposition techniques. However, pore-filling can never becomplete through such procedures because space is lef when thesolvent evaporates58,59. Te pore-filling raction, which is definedas the raction o porous volume taken by the hole conductor,

can be as high as 6080% with small-molecule hole conductors,and the pores are generally uniormly filled throughout the entirefilm thickness. Uniormly covering the dye/metal oxide surace isextremely important to ensure good charge separation and col-lection; as a rule o thumb, around 50% pore-filling is required ina mesoporous network to ensure monolayer surace coverage59.Improving the pore-filling raction is an important strategy orreducing recombination and might be achieved by infiltrating holeconductors rom the melt60.

Light-harvesting in ss-DSCs has benefited tremendously romdonorpiacceptor dyes, which provide significantly enhancedlight absorption in 2-m-thick films. Careul control over thep-dopant, in combination with the use o a strongly absorbingdonorpiacceptor dye, has recently led to efficiencies o over 7%in ss-DSCs that exploit small-molecule hole conductors55. Another

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Figure 3 | Maximum obtainable power-conversion efficiencies versus

absorption onset for various loss-in-potentials.The model assumes

an external quantum efficiency of 90% with the rise from the onset of

absorption occurring over a range of 50 nm and a constant fill factor of

0.75 (ref. 22).


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way o improving light absorption in ss-DSCs is to use light-absorbing polymers as the hole conductor61. Polymer hole con-ductors, typically used in organic photovoltaic cells, have recentlyachieved power-conversion efficiencies o more than 5%62. Polymerhole conductors are also solution-processed, although the pore-fill-

ing ractions are much lower (

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solar cell73,74. Light excites the plasmon resonances o these parti-cles and so significantly enhances the electric field (and thereoreabsorption) in the regions surrounding them.

Although light-trapping techniques are certainly helpul, theextent to which they can be used to solve absorption problems islimited because light-trapping also enhances parasitic absorptionby the transparent electrode and hole conductor.

Co-sensitization and energy relay dyesOne o the greatest opportunities or improving the efficiency o alltypes o DSC is to reduce the energy gap o the dyes so that morelight in the spectral range o 650940 nm can be absorbed (Fig. 2a).However, finding one dye that absorbs strongly all the way rom350940 nm is extremely difficult. ypically, the peak absorptioncoefficient and spectral width o a dye are inversely related to eachother. Te most promising strategy or harvesting the whole spec-trum is to use a combination o visible- and NIR-absorbing dyes.

In the past, the co-sensitization o ruthenium metal complex dyeswas considered to be problematic because their low molar extinc-tion coefficient required ull monolayer coverage on the titania orelatively thick films to absorb all the incident red photons. However,organic dyes have significantly higher molar extinction coefficients

than ruthenium metal complex dyes and thus require smaller suraceareas, making it possible to co-sensitize thinner DSC films withoutsignificantly reducing light-harvesting in any portion o the spec-trum76. odays record-efficiency DSC employs a co-sensitizationstrategy to boost absorption at a wavelength o 550 nm (re. 26).Although co-sensitization or this device results in an overall increasein the power-conversion efficiency due to an increased short-circuitcurrent density, VOCis reduced slightly because the co-sensitized dyeused to absorb light at 550 nm is not as good at blocking recombina-tion as the YD2-o-c8 dye with which the device is co-sensitized26.

Only a ew NIR dyes (that is, peak absorption at >700 nm) haveso ar demonstrated good charge injection efficiencies in DSCs,although no NIR dye has yet independently achieved a VOCgreaterthan 460 mV in an electrolyte-based cell7779. NIR-sensitizing dyes

that do not require large over-potentials to regenerate and do nothave high recombination rates will be required to push efficienciestowards 15%. Te most significant challenge o co-sensitizationusing NIR dyes is maintaining a large VOC, which requires that eachdye adequately prevents recombination. Te problem o VOCreduc-tion is likely to be even more pronounced with NIR dyes becausethey have small bandgaps. Te resulting energy and hole transerrom neighbouring visible sensitizing dyes80 can increase recombi-nation and lower VOC(re. 81).

Energy relay dyes (ERDs) decouple the light-harvesting andcharge-transer processes, and thereore have a range o potentialadvantages over co-sensitization techniques. In DSCs, ERDs absorbsunlight and then transer energy non-radiatively to sensitizingdyes, which are responsible or charge separation (Fig. 4)82,83. ERDs

have been placed inside the electrolyte82,84

and the semiconductor85

,co-sensitized81,8688on the semiconductor surace, and tethered tosensitizing dyes83. Te use o ERDs has several important advan-tages over co-sensitization. Because ERDs do not participate in thecharge-transer process, they do not require precise energy levelsor charge transer, which allows or a wide range o dyes to beimplemented in DSC systems82. ERDs can be used to fill absorp-tion gaps in the sensitizing dye or a liquid-based device, and alsoto increase the overall light-harvesting efficiency o solid-state sys-tems89. ERDs do not need to attach to the semiconductor surace inorder to contribute to light-harvesting, and thus their addition canboth widen and strengthen the overall absorption spectrum or thesame film thickness.

ERDs typically transer energy via Frster resonant energy trans-er, which involves dipoledipole coupling between the ERD and the

sensitizing dye90. Te distance over which energy transer can occurefficiently is determined primarily by the molar extinction coefficiento the sensitizing dye and the overlap between the emission spectrumo the ERD and the absorption spectrum o the sensitizing dye91.When designing ERDs, it is important to use dyes with relativelyshort photoluminescent lietimes (25 nm) or ERDs that have a strong emission overlapwith the absorption spectrum o tightly packed organic dyes onthe semiconductor surace93. Tis allows or high excitation trans-er efficiencies o >90% or ERDs placed inside liquid-electrolytesystems94and >60% or ERDs placed in the hole conductor in ss-DSCs95. It has not yet been possible to dissolve enough ERDs intothe electrolyte to absorb all o the light, although this should be pos-sible to achieve by increasing the solubility and molar absorptioncoefficients o the ERDs94. It is still possible or systems with weakerdipoledipole coupling to efficiently transer energy over short dis-tances, although this requires the ERDs to be within 13 nm o thesensitizing dye requiring co-sensitization87or tethering83.

The path to commercializationTe ultimate goal o any emerging solar cell technology is toachieve an installed cost-per-watt level that reaches grid parity ver-sus conventional ossil uel technologies and competes avourablyagainst incumbent photovoltaic technologies. Silicon photovoltaicmodule costs have continued to reduce rom US$4 W1in 2008 to

just US$1.25 W1 in 2011, with module efficiencies ranging rom15% to 20% and lietimes guaranteed to 25 years. It is realistic toexpect that silicon photovoltaic modules could continue to reducein manuacturing costs to around US$0.70 W1, with module effi-ciencies rising to 1822%. Great strides have also been made inthe commercialization o thin-film technologies, where Cde hasachieved module efficiencies o 1012.5% at costs o US$0.70 W1

and current roadmaps expect to achieve module efficiencies o 14%at costs o US$0.50 W1. Copper indium gallium selenide modulesare now commercially available, with efficiencies o 1215% andmodule costs expected to be less than US$0.50 W1.

How DSCs will compete in the uture photovoltaic marketdepends not only on our ability to increase power-conversion effi-ciencies and develop ultralow-cost architectures that are stable over20 years, but also on market actors such as the overall photovoltaicdemand and the scarcity o rare elements. DCSs can be constructedrom abundant non-toxic materials, which is a significant benefitover current thin-film technologies1.

Commercializing 10%-efficient modules may require ultralow-cost architectures that reduce inherent costs by removing at leastone glass substrate, thereby pushing costs down to US$20 m2. It is

important to note there is an increased non-module balance-o-systems cost associated with using less-efficient solar modules; orexample, installing 10%-efficient modules costs US$0.30 W1morethan 15%-efficient modules96. 10%-efficient DSC modules willthereore probably need to be priced at US$0.200.30 W1and thusmanuactured at US$2030 m2to compete or utility-scale powergeneration. Substrates represent the largest module costs. At thegigawatt scale, glass covered with fluorine-doped tin oxide costsUS$812 m2, whereas uncoated glass costs US$5 m2. Te glassglass laminate or DSCs would thereore cost at least US$13 m2,leaving only US$717 m2 or the remainder o manuacturing,which is possible but challenging.

Ultralow-cost DSCs could be built rom cheap metal oils (suchas stainless steel and aluminium) and plastic sheets to reduce glasscosts. Although iodide is known to dissolve aluminium and stainless


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steel, there is significant opportunity to create pinhole-ree protec-tive coatings on oils and develop electrolytes that are less corrosivethan iodide. Additional stability issues emerge when using plas-tics sheets instead o glass, which have significantly higher water

vapour transport rates and thus allow moisture to ingress into theDSC. Researchers have yet to produce a plastic sheet that is cheaperthan glass while also having an adequate water vapour transportrate. Developing water-tolerant DSCs is an interesting pathway that

is unique to this technology97. Furthermore, sputtered transparentconducting oxides on plastics are more expensive, less transpar-ent and more resistive than when deposited on glass, which pro-

vides lower perormance levels. Cheaper transparent conductingelectrodes or DSCs must thereore be developed to match the effi-ciency o glass-based designs98,99.

Increasing the module efficiencies o DSCs to more than 14%would relax the ultralow-cost constraints, thus providing substan-tial incentive to create laboratory-scale devices with efficienciesgreater than 15%. Te relatively slow increase in record values orDSCs over the past ten years has lef the impression o a peror-mance ceiling, which is partially justified given that conventionaliodide- and ruthenium-based DSCs have a realistic maximum pos-sible efficiency o little more than 13%22. Te loss-in-potential can

realistically be reduced to 500 mV by better matching the energylevels at the heterojunction, using more strongly absorbing dyes inthinner films and urther inhibiting recombination losses, pushingefficiencies to 19% with a dye capable o absorbing out to 920 nm.Finally, although there have been a number o initial studies intothe development o DSC modules, a thorough understanding o theoverall lietimes and degradation mechanisms o new DSC cell andmodule designs requires a great deal o urther investigation5,100.

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REVIEW ARTICLES| FOCUS NATURE PHOTONICS

DOI: 10.1038/NPHOTON.2012.22
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