Hybrid Nanorod-Polymer solar cells

Hybrid Nanorod-Polymer solar cells

(Parte 1 de 2)

DOI: 10.1126/science.1069156 , 2425 (2002); 295Science et al.Wendy U. Huynh, Hybrid Nanorod-Polymer Solar Cells

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Hybrid Nanorod-Polymer

Solar Cells Wendy U. Huynh, Janke J. Dittmer, A. Paul Alivisatos*

We demonstrate that semiconductor nanorods can be used to fabricate readily processed and efÞcient hybrid solar cells together with polymers. By controlling nanorod length, we can change the distance on which electrons are transported directly through the thin Þlm device. Tuning the band gap by altering the nanorod radius enabled us to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum. A photovoltaic device consistingof7-nanometerby60-nanometerCdSenanorodsandtheconjugated polymer poly-3(hexylthiophene) was assembled from solution with an external quantum efÞciency of over 54% and a monochromatic power conversion efÞciency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers. Under Air Mass (A.M.) 1.5 Global solar conditions, we obtained a power conversion efÞciency of 1.7%.

The widespread expansion in the use of inorganic solar cells remains limited due to the high costs imposed by fabrication procedures involving elevated temperature (400° to 1400°C), high vacuum, and numerous lithographic steps. Organic solar cells that use polymers which can be processed from solution have been investigated as a low-cost alternative with solar power efficiencies of up to 2.5% (1). Nonetheless, conventional inorganic solar cells routinely exhibit solar power conversion efficiencies of 10%, and the most advanced, but also the most expensive models, can reach up to 30% efficiency (2). The main factor for the superior efficiency of inorganic over organic devices lies in the high intrinsic carrier mobilities that exist in inorganic semiconductors. Higher carrier mobilities mean that charges are transported to the electrodes more quickly, which reduces current losses via recombination. For many conjugated polymers, electron mobilities are extremely low, typically below 1024 cm2V21s21, due to the presence of ubiquitous electron traps such as oxygen (3). Therefore, polymer photovoltaic (PV) devices rely on the introduction of another material for electron transport. The presence of a second material also provides an interface for charge transfer. Compounds such as small conjugated molecules, therefore, have been blended with polymers at a concentration that enables the formation of percolation pathways for electron transport (1, 4–6 ). The efficiency of these devices is limited by inefficient hopping charge transport (7), and electron transport is further hindered by the presence of structural traps in the form of incomplete pathways in the percolation network (6).

Onewaytoovercomethesechargetransport limitations is to combine polymers with inor- ganic semiconductors. Charge transfer is favored between high electron affinity inorganic semiconductors and relatively low ionization potential organic molecules and polymers (8, 9). Chargetransferratescan be remarkablyfast in the case of organics that are chemically bound to nanocrystalline and bulk inorganic semiconductors,which have a high density of electronicstates(10). Becauseof the nanoscale natureof lightabsorptionandphotocurrentgenerationin solarenergyconversion,theadventof methods for controllinginorganicmaterialson the nanometer scale opens new opportunities for the developmentof future generationsolar cells. Therefore,we have used colloidal semiconductingnanorodsas the inorganicphase in theconstructionof thesesolarcellsviasolutionphase nano-assembly.By varyingthe radiusof the rods,the quantumsize effectcan be used to control the band gap; furthermore, quantum confinement leads to an enhancement of the absorptioncoefficientcomparedwith the bulk, such that devices can be made thinner (1). One-dimensional(1D) nanorodsare preferable to quantum dots or sintered nanocrystals in solarenergyconversion,becausethey naturally provide a directed path for electricaltransport. The length of the nanorods can be adjusted to the device thickness required for optimal absorptionof incidentlight.

We have establishedrecently that both diameter and length of colloidal semiconductor nanorodsof CdSe can be systematicallyvaried to aspect ratios above 20 and lengths greater than100 nm usinga solutionphasesynthesisat temperaturesbelow300°C(12, 13).Here,CdSe nanorods were combined with the conjugated polymerpoly (3-hexylthiophene)(P3HT) (Fig. 1A) to create charge transfer junctions with high interfacialarea. From the schematicenergy level diagram for CdSe nanocrystals and P3HT, it can be seen that CdSe is electronaccepting and P3HT is hole-accepting (Fig. 1B). CdSe is used as the electron transport material, whereas P3HT is an effective hole transportmaterialin its regioregularform,demonstrating the highest field effect hole mobilitiesobservedin polymersso far [reachingup to 0.1 cm2V21s21 (14)].

The mechanical properties of P3HT allow for the room-temperature solution-casting of uniform,flexiblethin films. However,dispersingtheinorganicnanorodsat highdensitywith-

Department of Chemistry, University of California, Berkeley and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

*To whom correspondence should be addressed. E- mail: alivis@uclink4.berkeley.edu

Fig. 1. (A) The structure of regioregular P3HT. (B) The schematic energy level diagram for CdSe nanorods and P3HT showing the charge transfer of electrons to CdSe and holes to P3HT. (C) The device structure consists of a Þlm ;200 nm in thickness sandwiched between an aluminum electrode and a transparent conducting electrode of PEDOT:PSS (Bayer AG, Pittsburgh, PA), which was deposited on an indium tin oxide glass substrate. The active area of the device is 1.5 m by 2.0 m. This Þlm was spin-cast from a solution of 90% wt % CdSe nanorods in P3HT in a pyridinechloroform solvent mixture.

Fig. 2. Cadmium selenidenanocrystalswith aspect ratios ranging from1 to 10. The samples,shownbytransmission electron micrographs (TEMs) at the samescale,havedimensions(A)7n mby7n m, (B)7n mby30nm,and (C)7n mb y6 0n m.

w.sciencemag.org SCIENCE VOL 295 29 MARCH 2002 2425 on May 28, 2009 w.sciencemag.org Downloaded from in P3HT to facilitatecharge transfer is a challengebecausethecomplexsurfacechemistryof the nanocrystals can limit its solubility in the polymer.In Fig. 2, nanocrystalsof 7 nm diameter and various lengths of up to 60 nm are shown.As nanocrystalsincreasein aspectratio from sphericalto rod-like,they move from the molecular regime closer to the realm of a 1D wire and they become less readily soluble.Although we can control microphase separation andaggregationforlow–aspectratioCdSenanocrystals dispersed in polymers (Fig. 3A), the performanceof the resultingPV device is limited by inefficient hopping electron transport

(15). In order to accommodatehigh–aspectratio nanorods, we developed a solvent mixture consisting of a good solvent and ligand for CdSe nanorods and a good solvent for the polymer. Nanorods were co-dissolved with P3HT in a mixtureof pyridineand chloroform andspin-castto createa uniformfilmconsisting of dispersed nanorods in polymer (Fig. 3B) (16). We show that by controlling the aspect ratio of CdSe nanorodsdispersedin P3HT, we can tailorthe lengthscaleand directionof electron transportthrough a thin film PV device.

We fabricatedan efficientinorganic-organic hybrid solar cell by spin-castinga solution of 90% by weight (90 wt %) CdSe nanorods in P3HT onto an indium tin oxide glass substrate coated with poly(ethylene dioxythiophene) dopedwith polystyrenesulfonicacid (PEDOT: PSS, a conductingpolymer)with aluminumas the top contact(Fig. 1C). With CdSe nanorods 7 nm by 60 nm, a power conversionefficiency of 6.9% was obtainedunder 0.1 mW/cm2 illuminationat 515 nm inside an inert atmosphere of flowing argon. For plastic PV devices, this monochromaticpower conversionefficiencyis one of the highestreported(1). At thisintensity, the open-circuitvoltageis 0.5 V, the photovoltage at the maximumpower point is 0.4 V, and the fill factor (F) is 0.6 (Fig. 4B). When our devicesweretestedin simulatedsunlight[under A.M. 1.5 Global solar conditionsand flowing argon (18)], a power conversion efficiency of 1.7%, an open-circuitvoltage of 0.7 V, a photovoltageat maximum power point of 0.45 V, and a F of 0.4 were obtained (Fig. 4C) (17). The maximum open-circuit voltage is determinedbythedifferencebetweentheworkfunctions of the electrodes,PEDOT:PSS,and aluminum, as well as the difference between the lowest unoccupied energy level in the CdSe nanorodand the highestoccupiedenergy level in P3HT. This difference is close to what we obtainedat solar intensity.

BecauseCdSeandP3HThavecomplementary absorptionspectra in the visible spectrum, these nanorod-polymer blend devices have a very broad photocurrent spectrum extending from 300 to 720 nm (Fig. 4A). Unlike other electronacceptorssuch as C60 in organicblend devicesand sinteredTiO2 in dye-sensitizedsolar cells, CdSe nanorodsabsorb a large part of the solar spectrum (1, 19). Furthermore, the absorptionspectrumof the hybrid devicespresented here can be tuned by alteringthe diameter of the nanorods in order to optimize the overlap with the solar emissionspectrum(Fig. 4D). The onset of absorption for 60-nm-long nanorodswith3 nm diameteris at a wavelength of 650 nm, whereas the onset for those with 7 nm diamteris at 720nm.BecauseP3HTshows no absorptionbeyond650 nm, we can tune the onset of the photocurrentactionspectrumfrom 650to 720nm by increasingthediameterof the nanorods while maintaininga constant length. Thecharacteristicsof thesetwodevices,suchas

Fig. 3. Thin Þlms of 20 wt%(A)7n mby7n m and(B)7n mb y6 0n m CdSe nanocrystals in P3HT were studied via transmission electron microscopy. In (C), a TEM of a cross section of a Þlm with 60 wt % 10 nm by 10 nm CdSe nanocrystals in P3HT reveals the distribution and organization of nanoparticles across the 110-nm-thickÞlm. A solution of CdSe in P3HT was spin-cast onto a Polybed epoxy substrate (Ted Pella, Redding, CA) and we used an ultramicrotometo obtainultrathin cross sections. In (D), a TEM of a cross sectionof a 100-nm-thickÞlm consistingof 40 wt%7n mb y6 0n m CdSe nanorodsin P3HT reveals that most nanorodsare partiallyalignedperpendicularto the substrateplane. The solutionswere cast onto an Epon/Araditeepoxy substrate (Ted Pella), embedded into epoxy, cured, and sliced using an ultramicrotome.This extra supportis requiredto slice the nanorodswith the microtome,becausethe blend Þlm has a tendencyto tear as the rods get longer.

Fig. 4. (A) EQEs of 7-nm-diameternanorodswith lengths7, 30, and 60 nm. The intensityis at 0.084 mW/cm2 at 515 nm. (B) The current-voltagecharacteristicsof the 7 nm by 60 nm nanorod device exhibit rectiÞcation ratios of 105 in the dark and a short-circuit current of 0.019 mA/cm2 under illuminationof 0.084 mW/cm2 and at 515 nm. (C) Solar cell characteristicsof this 7 nm by 60 nm nanorod device illuminatedwith simulatedAM 1.5 Global light, include a short-circuitcurrent of 5.7 mA/cm2.( D) Photocurrentspectrafortwodeviceswith60-nm-longnanorodswithdiameters7 and3 nm.

29 MARCH 2002 VOL 295 SCIENCE w.sciencemag.org2426 on May 28, 2009 w.sciencemag.org Downloaded from open-circuitvoltage and fill factor, are comparable.This furthersuggeststhat nanorodlength not diameter determines device performance. Moreover,bandgaptuningin nanorodsenables the realizationof high-efficiencydevice architectures,suchas tandemsolarcellsin whichthe different band gaps can be obtained by modifying only one chemicalcompound.

We can understand the origin of the high efficiency of nanorod-polymer devices by studyingthe dependenceof chargetransporton nanorodlength.The externalquantumefficiency (EQE), which is the percentageof electrons collected per incident photon (with no correction for reflection losses), can be used as a measure of the efficiency of charge transport given that the followingquantitiesare comparable for a set of devices: (i) incident light intensity; (i) fraction of light absorbed; (ii) charge collection efficiency at the electrodes, which is mainly given by the choice of electrodes; and (iv) the charge transfer efficiency, as determined from photoluminescence quenching. These four conditions are met for thedevicesforwhichEQEdataarepresentedin Fig.4A. Therefore,we can concludethatas the aspectratioof the nanorodsincreasesfrom 1 to 10, the chargetransportmust improvesubstantiallyto yieldan EQE enhancementby a factor of approximately3. In networks consisting of shorternanoparticles,electrontransportis dominated by hopping and, in those consisting of longer particles, band conduction is prevalent. In a cross section of the blend film (Fig. 3D), most nanorods were oriented partly in the direction of the electric field and, thus, in the direction of electron transport. Because the thicknessof the nanorod-polymerfilm is ;200 nm, 60-nm-long nanorods can penetrate through a large portion of the device whereas 30-nmand 7-nm-longparticlesare progressively less effective (Fig. 3C). The best device, which contained 7 nm by 60 nm nanorods, performedwithamaximumEQEof55%under 0.1 mW/cm2 illuminationat 485 nm (Fig.4A), and this value has been remarkably reproducible (20).

(Parte 1 de 2)