Improving CdSe Quantum Dot/Polymer Solar Cell Efficiency Throughthe Covalent Functionalization of Quantum Dots: Implications in theDevice Recombination KineticsJosep Albero,† Paola Riente,† John N. Clifford,† Miquel A. Pericas,*,†,‡ and Emilio Palomares*,†,§

†Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans 16, E-43007 Tarragona, Spain‡Department de Química Organica, Universitat de Barcelona, c/Martı I Franques 1-11, 08080 Barcelona, Spain§Catalan Institution for Research and Advanced Studies (ICREA), Avda. Lluis Companys 23, E-08010 Barcelona, Spain

*S Supporting Information

ABSTRACT: Novel quantum dot capping ligands based onfullerene derivatives were attached through click-chemistry tothe surface of semiconductor CdSe nanocrystals (C70−CdSe).Steady-state and time-correlated luminescence studies insolution show efficient quenching of the quantum dot (QD)emission in C70−CdSe. When this material was blended withthe polymer poly-3-hexyl thiophene (P3HT) to fabricate bulk-heterojunction solar cells, P3HT/C70−CdSe devices doubledthe light-to-energy conversion efficiency when compared toP3HT/Py−CdSe reference devices prepared using pyridine asthe capping agent. This is due to an increase in bothphotocurrent and fill factor showing the beneficial efficienteffect of fullerene to improve light harvesting and chargetransport in these devices. However, C70 also appears to increase recombination in these devices as evidenced by both transientabsorption spectroscopy and transient photovoltage measurements. This work also discusses the effects on the CdSefunctionalization with C70 over the device charge recombination kinetics that limit the efficiency in CdSe QDs/polymer solarcells.

■ INTRODUCTIONSemiconductor nanocrystals with quantum properties (orquantum dots (QDs)) such as CdSe, CdS, PbS, and PbSehave been extensively used in biology as biomarkers,1−3 inphysics for infrared (IR) photodetectors,4−7 and in otheradvanced technological applications such as light emittingdiodes. Recently, their use in so-called molecular photovoltaicdevices has attracted much attention,8,9 and the mainarchitectures for QD-based molecular solar cells are summar-ized in Scheme 1. Quantum dot sensitized solar cells havereached power conversion efficiencies for liquid and solid state

in the range of 5−6%.10,11 Organic/quantum dot bilayer singlejunction solar cells show light-to-energy conversion efficienciesof above 5%.12 Schottky type solar cells made of PbS QDs havealso been reported to achieve efficiencies of over 7% when awide band gap metal oxide such as nanocrystalline TiO2 is usedas a selective contact for electrons.13 In addition, powerconversion efficiency up to 6.6% has been reported in p−nhomojunction architectures.14

However, efficiencies of bulk-heterojunction QD/polymersolar cells (CdSe, CdS, or PbS; polymers, P3HT (chemicalname, poly-3-hexyl thiophene) or PCPDTBT (chemical name,poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis-(2-ehtylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]])) remain com-paratively low.15

One of the main factors limiting the performance of quantumdot polymer bulk heterojunction solar cells is the poor chargetransport in quantum dots, due to the low carrier mobilitywhen compared with C60 or C70, or the fullerene derivativePCBM (chemical name [6,6]-phenyl-C61−butyric acid methylester), and this is directly related to the experimentally

Received: April 10, 2013Revised: May 29, 2013Published: June 7, 2013

Scheme 1. Different Types of Quantum Dot BasedMolecular Solar Cells: (a) QD/Polymer Bulk-Heterojunction Solar Cell, (b) QD/Metal Oxide BilayerType Solar Cell, and (c) QD Single Type Solar Cell

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observed faster carrier recombination dynamics that thesematerials show.16,17 The low carrier mobility in quantum dotfilms is often ascribed to the inefficient charge transferprocesses between the nanocrystals themselves due to thepresence of organic capping agents, which are used during theirsynthesis in order to avoid nanocrystal aggregation and the lossof quantum properties.18 These organic ligands are usually longalkyl chains terminated by an anchoring group such as acids(i.e., oleic acid), amines (i.e., hexadecylamine), phosphines (i.e.,trioctylphosphine), and phosphonic acids (i.e., tetradecylphos-phonic acid). The problem is, therefore, that most organiccapping ligands used in the synthesis of QD nanocrystals inorder to obtain nanocrystals with a narrow size distribution,good solubility in organic solvents, and well-controlled shapeinhibit the performance of complete photovoltaic devices byforming insulating barriers between neighboring QDs. Thus,ligand exchange of the original capping agent shell by smallermolecules or ions has been widely explored to make thenanoparticles soluble in different solvents, to introduce newfunctional groups or decrease the thickness of the capping-agent shell to increase the charge transfer rate. However,although different treatments, using halide anions,19,20

amines,21,22 or thiols23 have been investigated, to improve thecarrier transport in semiconductor quantum dots/polymerblends, pyridine is still nowadays the most widely employedligand for capping ligand exchange.24 Unfortunately, it has alsobeen demonstrated that the pyridine ligand exchange does notreplace entirely the capping ligand shell in CdSe quantumdots.25 Among the approaches in the literature to replacepyridine and improve the device efficiency, we highlight thefollowing. Olson et al. reported P3HT/CdSe bulk-hetero-junction solar cells exhibiting efficiencies up to 1.77% whenbutylamine was used as a shorter capping ligand instead ofpyridine.22 Friend and co-workers developed a method to growdirectly polymer chains on the surface of CdSe nanoparticles,26

while Haque and collaborators investigated the in situ synthesisof CdS nanocrystals inside a semiconductor polymer. In both ofthese cases the central idea was to avoid the use of cappingligands entirely.27 Finally, Kruger et al. developed a differentstrategy of postsynthetic treatment where the quantum dotswere treated with an hexanoic acid-assisted washing procedure,exhibiting efficiencies up to 2.1%.28 However, in all these cases,the effect of these changes over the interfacial chargerecombination kinetics that limit light conversion efficiency infunctional devices was not explored.Our own group has studied recently in detail the charge

recombination kinetics in PCPDTBT/CdSe QD solar cells anddemonstrated the direct relationship between chemical

capacitance, Cμ (charge density), and the carrier lifetime (τ)for the first time in quantum dot polymer bulk-heterojunctionsolar cells.16 Following on from the work cited and to furtherexplore the nature of device efficiency-limiting processes inbulk-heterojunction QD/polymer solar cells, we carried out thesynthesis of hybrid electron acceptor materials composed ofCdSe nanocrystals and fullerenes, which are anchored to thequantum dots through simple and practical click chemistry.Kamat and co-workers have previously demonstrated efficientelectron transfer kinetics in C60−CdSe diads linked covalentlyin nonpolar organic solvents.29 Fullerene was covalentlyattached to CdSe using the chemical approach reported byBrittain and Lander,30 (Scheme 2) exploiting the reaction ofC70 with azides,31 and thus, functionalizing the quantum dotsurface with a C70 monolayer, in a similar way as it was reportedbefore.32 The C70−CdSe cluster was mixed with the semi-conductor polymer P3HT, and complete bulk-heterojunctiondevices were fabricated showing a remarkable increase inefficiency when compared with pyridine capped P3HT/CdSereference devices. The charge transfer recombination kineticswas also studied in complete devices under operatingconditions by using advanced time-resolved techniques.

■ EXPERIMENTAL SECTION

Semiconductor Nanocrystal Synthesis. CdSe quantumdots were synthesized using a wet chemical synthetic method.In brief, 384 mg of cadmium oxide and 6 mL of oleic acid (OA)were mixed together and put under vacuum. After addition of60 mL of ODE, the round-bottom flask was heated up to 120°C. After 15 min, argon atmosphere was provided, and thetemperature was increased to 250 °C. In the meantime, 590 mgof selenium was solved in 5 mL of tri-n-octylphosphine (TOP)under nitrogen (stock solution). As soon as the solutionreached 250 °C, 1.02 mL of the selenium stock solution wasadded as fast as possible. The temperature was held at 250 °Cfor 90 s. Afterward, the heating was removed, and the solutionwas left to cool down under continuous stirring. The CdSequantum dots were precipitated with copious amounts ofmethanol and collected by centrifugation and decantation. Theprecipitated nanocrystals were recovered by adding a smallamount of chloroform and reprecipitated with methanol. Thispurification process was repeated three times. Finally, part ofthe OA coated CdSe quantum dots was dissolved in pyridineand refluxed at 90 °C overnight under dark conditions.Pyridine-coated CdSe was precipitated with hexane andcollected by centrifugation and decantation. The precipitatewas dissolved in a mixture of pyridine and chlorobenzene (1:9,

Scheme 2. Ligand Exchange Procedure for C70-CdSe; Notice That the Picture Is Not Scaled in Size but Was Done to Illustratethe Anchoring Reaction Sequence

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v/v), and saved as stock solution at a concentration of 30 mg/mL.Azide functionalized CdSe quantum dots were synthesized

using ligand exchange methodology.33 The material wasprepared mixing 128 mg of CdSe in 6 mL of degassed toluene,(3-azidopropyl)trimethoxysilane (0.5 mmol, 102.4 mg), aceticacid (0.2 mmol, 11.5 μL) and ultrapure water (0.7 mmol, 13μL) under argon atmosphere. The reaction was stirred at refluxfor 24 h. The functionalized CdSe quantum dots were collectedby centrifugation and dried under vacuum at 40 °C overnight.The covalent attachment of fullerene to the azide functionalizedCdSe quantum dot was done following the methodologydescribed by Wudl and co-workers.31 The azide-functionalizedCdSe (0.032 mmol, 11.7 mg) was suspended in 5 mL ofchlorobenzene, and C70 (SES Research, 99.0%) was added(0.032 mmol, 27.3 mg). The reaction was stirred at 132 °Covernight. The C70 anchored onto CdSe quantum dots waswashed with MeOH and collected by centrifugation threetimes. Finally, the functionalizated CdSe quantum dots weredried under vacuum at 40 °C overnight. The precipitate wasdissolved in chlorobenzene and saved as stock solution in aconcentration of 30 mg/mL under nitrogen environment.Bulk-Heterojunction Device Preparation. For the device

preparation, ITO (indium tin oxide, 5Ω/cm2 resistance)substrates were cleaned and covered with an aqueousPEDOT:PSS dispersion (Baytron AL4083, from H.C. Starck)by spin coating (4500 rpm/90 s). The films were annealed at120 °C for 30 min under N2. Afterward, the QD:polymersolution was spin-coated onto the surface of PEDOT:PSS toform photoactive films. Aluminum (Al) cathodes (80 nm) werethermally deposited onto the QD:polymer blend, obtaining anactive area of 9 mm2. Device thermal annealing was done on ahot plate under nitrogen atmosphere (150 °C for 15 min). Thesemiconductor polymer employed in this work was poly(3-hexylthiophene-2,5-dyil) (P3HT), and it was purchased fromRieke Metals, Inc. and used without further purification. Thepolymer molecular weight was 50−70 K in average, and it has a91−94% regioregularity.Spectroscopic Characterization. UV−visible absorption

spectra was recorded with a Shimadzu 1600 spectrophotom-eter. Steady-state luminescence emission spectra and theexcited-state lifetimes were measured employing a LifeSpec-psapparatus from Edinburgh Instruments.FT-IR spectra were recorded on a Thermo Nicolet 5700

FTIR spectrometer, using KBr pellets. Potassium bromide usedin the preparation of the pellets was kept in an oven at 50 °C.Elemental analyses (C; H; N) were performed in LECOCHNSmodel 932 by C.A.I. microanalysis elemental, Uni-versidad Complutense de Madrid, Madrid, Spain.The I−V characteristic measurements were carried out with

an ABET 150 W xenon light source equipped with the correctset of filters to achieve the solar spectrum 1.5 a.m. G. The lightintensity was adjusted to 100 mW/cm2 using a calibrated Siphotodiode. The applied potential and cell current weremeasured with Keithley model 2600 digital source meter. Thecurrent to voltage (I−V curve) was measured automaticallywith a home-built Labview software. The measurement ofincident photon-to-current conversion efficiency (IPCE) wasplotted as a function of the excitation wavelength by using theincident light from a 300 W xenon lamp (ILC Technology,USA), which was focused through a Gemini-180 doublemonochromatic (Jobin Yvon Ltd.)

For transient photovoltage (TPV) measurements, the deviceswere connected to the 1 MOhm input terminal of anoscilloscope and illuminated with white light to set the lightbias. A small optical perturbation was applied using a nitrogenpumped PTI GL-301 dye laser as the excitation source at awavelength of 470 nm (frequency 1.5 Hz, pulse duration <1ns), which resulted in voltage transient amplitudes of 4 mV.The intensity of the laser pulse was attenuated as necessaryusing a circular neutral density filter. The photoinduced chargeextraction (CE) technique was performed employing a ring ofwhite LEDs as the illumination source. The devices wereilluminated under different light intensities (from darkness to 1sun intensity) at open circuit conditions, which allow the deviceto reach an equilibrium in open circuit voltage, depending onthe applied light intensity. This applied light bias was turned offin <1 μs, while simultaneously, the cell is switched from opento short circuit and the voltage decay recorded by anoscilloscope.Laser transient absorption spectroscopy (L-TAS) measure-

ments were performed by excitation of the blend films at 470nm with pulses from a nitrogen-pumped PTI GL-301 dye laser(<1 ns pulse duration, 1.5 Hz, intensity 0.09 mJ/cm2). Theresulting photoinduced changes in the optical density weremonitored at different wavelengths by employing a 150 Wtungsten lamp, with 1 nm bandwidth monochromators locatedbefore and after the sample, a home-built photodiode baseddetection system, and a TDS-200 Tecktronik oscilloscope.

■ RESULTS AND DISCUSSIONThe CdSe quantum dots were synthesized with an averagediameter of 3 nm, calculated from the empirical formulaproposed by Peng et al.34 and confirmed with transmissionelectron microscopy image (see Figure SI1 in SupportingInformation). The UV−visible and emission spectra of thecolloidal quantum dots are also shown in Figure SI2 of theSupporting Information. The as-prepared CdSe quantum dotscapped with the OA ligand were ligand-exchanged with,pyridine (Py) and C70 molecules. The C70 exchange procedureis depicted in Scheme 2.Ligand exchange was confirmed by FT-IR spectroscopy

(azide signal at 2092 cm−1) (see Figure 1). The loading of the

Figure 1. FT-IR: (red) C70 supported onto CdSe quantum dots;(black) azide-funtionalized CdSe quantum dots; and (blue) C70.

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azide group was determined by elemental analysis of nitrogenas 2.78 mmol·g−1. In the FT-IR spectrum, the appearance ofcharacteristic bands at 526, 566, and 1428 cm−1, correspondingto the stretching vibration C70,

35 confirms their incorporationonto the azide functionalized CdSe quantum dots. The loadingof the fullerene group was determined by elemental analysis ofnitrogen as 1.61 mmol·g−1, indicating a fullerene load of 1.17mmol·g−1. The UV−vis spectra of the C70 coated CdSe (C70−CdSe) quantum dots and a mix of quantum dots and thederivative fullerene PC70BM (CdSe + PC70BM) were measured(see Figure SI3 in Supporting Information). The absorbancespectrum of the C70−CdSe presented different features locatedat 433, 498, and 698 nm when compared with the mix solution,indicating that the linking process produces singular vibrationalchanges in the UV−visible spectra. Moreover, using Lambert−Beer law for dilute solutions we have estimated that themolecular extinction coefficient for the fullerene derivative is21 565 L mol−1 cm−1 and for the CdSe is 97 396 L mol−1 cm−1.By tacking those numbers as a reference and the UV−visiblefrom the Supporting Information (Figure SI3), we haveestimated a fullerene/CdSe quantum dot ratio of 4:1.The yield of the CdSe fluorescence quenching by C70 in

C70−CdSe was studied by steady-state fluorescence spectros-copy, and the estimation of electron transfer kinetics fromCdSe to C70 was studied using and time-correlated singlephoton counting (TCSPC). The inset in Figure 2 is the

emission spectra for chlorobenzene solutions of C70−CdSe,CdSe, and PC70BM that were iso-absorbing at the excitationwavelength (470 nm). Negligible emission from the PC70BMwas observed under these conditions. Dramatic quenching ofthe CdSe emission band is observed in C70−CdSe, and byintegrating the area under the curve and comparison with theCdSe reference emission spectrum, quenching is estimated tobe ∼99%. We did not observe photoluminescence from the C70in our measurement range indicating energy transfer has notoccurred, and therefore, electron transfer is the origin of theemission quenching process.The main panel in Figure 2 shows the emission decay of

PC70BM, CdSe quantum dots and the quenched emissiondecay in C70−CdSe upon excitation with a diode laser (λex =470 nm) for iso-absorbing chlorobenzene solutions. Theemission decays were measured for a fixed acquisition time

period of 60 s. The first point to note is that emission lifetime isshorter in C70−CdSe than in CdSe (this is more evident whenemission lifetimes were recorded for a fixed number of emissioncounts for each sample as shown in Figure SI4 (SupportingInformation). By integrating the area under the curve of C70−CdSe and comparing this to the CdSe reference, CdSe emissionquenching due to electron injection from quantum dot excitedstate to the fullerene lowest unoccupied molecular orbital(LUMO) is estimated to be higher than 95%. The electrontransfer in C70−CdSe is calculated to be of 215 ps following themathematical adjustment of our TCSPC data as reported byKoops et al.36 This value is in good agreement with the above-mentioned electron transfer kinetics of C60 thiol derivate CdSequantum dots as reported previously by Kamat and co-workers.29 In order to investigate the role of the linkingbetween the quantum dot and the fullerene, we added PC70BMfullerene to the CdSe solution with identical relation to that ofthe reported elemental analysis, and we measured the emissionlifetime. As can be seen in Figure SI4 (SupportingInformation), the simple addition of fullerenes in the quantumdot solution did not produce a significant quenching of theemission kinetics, indicating that the linkage between thenanocrystals and the fullerenes is key for an efficient chargetransfer from the CdSe to the C70.The interfacial charge transfer recombination kinetics

between the free carriers in the polymer/modified nanocrystalheterojunction was studied using L-TAS in thin films of P3HT/Py-CdSe and P3HT/C70-CdSe. These films were prepared byspin coating and were roughly 100 nm thick, as measured withan Ambios XP-1 perfilometer. For comparison purposes, filmsof P3HT/PC70BM/Py-CdSe were also measured. Figure 3shows the decays corresponding to the P3HT positive polaronsin the microsecond−millisecond time scale in the differentfilms.

The positive signal in the L-TAS measurements at 980 nm isindicative of long-lived free carriers in the films, implyingcharge separation between the P3HT polymer and the modifiedquantum dots. The 980 nm was chosen as the wavelength tomonitor the kinetics in these films as this is near the maximumof the P3HT polaron spectrum.37,38 The transient decays werefitted to both stretched exponential (eq 1) or power law (eq 2)functions and, in both cases, can be ascribed to non-geminaterecombination processes due to the time-window monitored(microsecond-to-seconds), which discounts more rapid gemi-

Figure 2. CdSe quantum dots (red), C70 derivative fullerene (blue),and C70−CdSe (orange) emission kinetics in chlorobenzene plottedlinear−linear. The same acquisition time (60 s) was used for allsamples. λex = 470 nm, λprobe = 554 nm for CdSe, C70−CdSe, andPC70BM. The inset corresponds to the steady-state emission spectrafor these samples.

Figure 3. Transient absorption decay of P3HT/Py−CdSe (red),P3HT/C70−CdSe (orange), P3HT/PC70BM/Py−CdSe (green), andP3HT (black) thin films monitored at 980 nm after laser excitation at470 nm.

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nate recombination and the lack of metal contacts in thesample, which could induce carrier recombination between theorganic layer and the metal interface.

τ= · −

α⎜ ⎟⎛⎝

⎞⎠f t A

t( ) exp

(1)

= τf t At( ) (2)

The P3HT/PC70BM and P3HT/C70−CdSe samples werefitted to the power law function as fitting with a stretchedexponential was found to be unsatisfactory as shown in theSupporting Information (Figure SI5). In these cases, the half-lifetime of the P3HT polaron was found to be 2.8 and 4.4 μs,respectively. The P3HT/Py−CdSe transient decay, on thecontrary, was fitted to a stretch exponential function,37 and ahalf-lifetime of 11.7 μs was extracted. Faster recombination inP3HT/PCBM devices showing power law behavior38 hasgenerally been ascribed to the fast electron mobility of thefullerene, whereas our own studies16 and others17,27 indicatethat in polymer/QDs devices, the slower recombinationlifetime and stretched exponential behavior is controlled bypoor electron mobility in the QD. For comparison purposes,we also prepared thin films consisting in the combination ofP3HT/C70/CdSe without any linkage between the quantumdots and the fullerene. A positive signal was also found,indicating P3HT polaron generation and showing similardynamics to the P3HT/C70−CdSe but a half-lifetime of 8.2 μs.From the L-TAS data, it can be observed that the yield of

polarons, which is proportional to the decay ΔOD, formedupon excitation with the laser pulse is similar in themicroseconds time scale for the hybrid P3HT/C70−CdSe andthe P3HT/Py−CdSe films. Hence, the modification of thecapping ligand with C70 moieties does not influencesignificantly the formation of free carriers in the organic film.The films nanomorphology was studied by atomic force

microscopy. In Figure SI6 (Supporting Information) we canobserve (a) P3HT, (b) P3HT/PC70BM, (c) P3HT/C70−CdSe,(d) P3HT/Py−CdSe, and (e) P3HT/PC70BM/Py−CdSe filmstopography. The images were acquired in tapping mode. As canbe seen, the Py−CdSe films (d) show a rougher nano-morphology when compared to C70−CdSe (c) films, which ismuch similar to the P3HT/PC70BM films (b). The mix of thethree components (e) separately showed a very rough surfacewith the presence of big agglomerates.Bulk heterojunction solar cell devices comprising P3HT/Py−

CdSe and P3HT/C70−CdSe were fabricated to investigate theirpower conversion efficiencies.The J−V characteristics are shown in Figure 4. The P3HT/

Py−CdSe devices showed a short circuit current density of 2.65mA/cm2, an open-circuit voltage of 724 mV, 52.8% fill factor,and an overall light-to-energy conversion efficiency of 1.01% at1 sun. However, the P3HT/C70−CdSe showed a currentdensity of 5.61 mA/cm2, 548 mV of open-circuit potential,65.7% fill factor, and an overall efficiency of 2.02% at 1 sun. Wewould like to notice that the measured fill factor forP3HT7C70−CdSe devices is among the highest values reportedfor QD/polymer solar cells. We have also prepared P3HT/PC70BM/Py−CdSe devices, where the fullerenes and thequantum dots were just mixed but not linked, maintainingthe fullerene/quantum dot ratio of the linked fullerene/quantum dot, derived from the elemental analysis above-mentioned. The short circuit current density for P3HT/PCBM/Py−CdSe was 0.73 mA/cm2. The open-circuit voltage

was 579 mV and the fill factor 26.9%. The overall efficiency forthose devices was 0.11%. The incident photon-to-currentefficiency (IPCE) of these devices is shown in Figure 5.

The P3HT/C70−CdSe device clearly shows higher IPCE incomparison with P3HT/Py−CdSe device, which is in goodagreement with the obtained photocurrent from the J−Vcurves. The P3HT/C70−CdSe spectrum is red-shifted from 600to 650 nm when compared with the P3HT/Py−CdSe one. TheP3HT/C70−CdSe devices also display an enhancement in theUV region (peaks at 380 and 460 nm) of the IPCE spectrumcorresponding to the quantum dot contribution as observed inthe Py−CdSe/P3HT IPCE spectrum. Therefore, the use of C70modified CdSe quantum dots in these devices produces adouble beneficial effect to photocurrent, in one hand due to thecontributions of photogenerated charges from both thefullerene and the quantum dot materials; on the other hand,the enhanced photocurrent point out to an improved chargetransport due to the higher electron mobility in fullerenes.The accumulated charge and charge recombination dynamics

under operating conditions at 1 sun illumination in P3HT/C70−CdSe and P3HT/Py−CdSe devices were studied usingcharge extraction and transient photovoltage techniques. InFigure 6, the correlation of capacitance in these devices atdifferent open circuit voltages corresponding to different lightintensities (so-called light bias) is shown, measured with thecharge extraction technique as described previously.39

As can be see in Figure 6, all types of devices showedchemical capacitance in the same range under light bias. Thedata has been fitted in all cases to an exponential function (eq3), with initial capacitance of C0 = 9 × 10−10 F/cm2 and C0 = 2

Figure 4. J−V curves of P3HT/Py−CdSe (red), P3HT/PC70BM/Py−CdSe (green), and P3HT/C70−CdSe (orange) devices measured at 1sun light illumination and dark conditions.

Figure 5. Incident photon to current efficiency (IPCE) of P3HT/Py−CdSe, P3HT/C70−CdSe, and P3HT/PCBM/Py−CdSe devices.

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× 10−8 F/cm2; and slope of γ = 10.9 V−1 and γ = 4.8 V−1 forP3HT/C70−CdSe and P3HT/Py−CdSe, respectively. Thesevalues have been obtained from the average of the dataextracted from several devices.

γ= ·C C Vexp( )0 OC (3)

Transient photovoltage measurements were employed toinvestigate recombination lifetimes in P3HT/C70−CdSe andP3HT/Py−CdSe devices and are plotted vs capacitance inFigure 7a. The relationship between the capacitance withrecombination lifetime follow a relationship (eq 4) with λ = 1.7and λ = 4.1 for P3HT/C70−CdSe and P3HT/Py−CdSe,respectively, where λ is the parameter related to the chargerecombination order.40

τ τ=λ

⎜ ⎟⎛⎝

⎞⎠

CC0

0

(4)

Therefore, P3HT/Py−CdSe devices show a strongerdependence of carrier lifetime with charges than P3HT/C70−CdSe or other organic devices.39 The stronger dependence ofthe recombination dynamics of the polymer/QD devicescompared with the fullerene/polymer ones has been reportedbefore and is related also to the lower fill factor in the devices.16

Moreover, the P3HT/C70−CdSe devices presented recombi-nation kinetics in the time scale (see Figure 7b), showingcarrier lifetimes at open circuit voltage of 2 and 18 μs for

P3HT/C70−CdSe and P3HT/Py−CdSe, respectively. Thus,the introduction of C70 does not have a strong influence in themean carrier lifetime under working conditions. However, themodification of the polymer/quantum dot interface with thesemolecules produced an important effect on the recombinationdynamics by reducing its dependence with the charge density.As a consequence, the kinetic competition between chargetransport and recombination is biased toward the fastest chargecollection when the C70 moieties are used instead of pyridine,increasing the current photogenerated in the device and the fillfactor.

■ CONCLUSIONS

The use of fullerene covalently linked to CdSe quantum dotshas improved notably the power conversion efficiency inpolymer/quantum dot solar cell devices due to an increase inthe solar cell photoresponse and the increase in the fill factor.The increase in device photocurrent is due to the efficient CdSeemission quenching and ultrafast electron transfer to thefullerene that was shown to occur on picosecond time scales.Moreover, non-geminate charge recombination at the polymer/nanocrystal interface results in microsecond to millisecondkinetics. However, in CdSe/P3HT solar cells, the strongerdependence of the recombination kinetics versus theaccumulated charge at the device under operation (λ) isresponsible for the overall low efficiency in the CdSe/P3HTsolar cells mainly due to the poor fill factor measured.Therefore, decreasing the recombination order (λ) through(a) careful film morphology control and/or (b) engineering thenanocrystal/polymer interface with organic molecules, asshown in this work, results in a great enhancement of thelight-to-power conversion efficiency.

■ ASSOCIATED CONTENT

*S Supporting InformationUV−vis and emission spectra as well as a TEM image of thecolloidal quantum dots in solution; emission kinetics of thecolloidal quantum dots and the hybrid C70-CdSe; L-TAS datafitted to different functions. UV-vis spectra of the differentcoated CdSe quantum dots in solution; AFM images of thefilms topography. This material is available free of charge viathe Internet at http://pubs.acs.org.

Figure 6. Capacitance vs light bias induced open circuit voltage ofcomplete P3HT/C70−CdSe and P3HT/Py−CdSe devices measuredunder standard operating conditions. In all cases, geometricalcapacitance has been subtracted, which has been found to be 14nF/cm2 for P3HT/C70−CdSe and 63 nF/cm2 for P3HT/Py−CdSe.

Figure 7. Carrier lifetime as a function of the capacitance (a) and the applied light bias (b) in P3HT/C70−CdSe and P3HT/Py−CdSe devicesrecorded using transient photovoltage (TPV) measurements.

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■ AUTHOR INFORMATION

Corresponding Author*E-mail: epalomares@iciq.es (E.P.); mpericas@iciq.es(M.A.P.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

E.P., J.N.C., and J.A. would like to acknowledge the EuropeanResearch Council starting Grant ERCstg-POLYDOT and thenational projects CONSOLIDER HOPE 0007-2007 andMICINN CTQ-2007-60746-BQU as well as Catalonianregional government for the project 2009 SGR 207. P.R.thanks MINECO for a Torres Quevedo postdoctoral grant.

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