Polymer Solar Cell Efficiency Through

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Improving CdSe Quantum Dot/Polymer Solar Cells Efficiency Through the Covalent Functionalization of Quantum Dots: Implications in the Devices Recombination Kinetics. Josep Albero, Paola Riente, John N Clifford, Miquel A. Pericàs, and Emilio J Palomares J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403523j • Publication Date (Web): 07 Jun 2013 Downloaded from http://pubs.acs.org on June 15, 2013

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The Journal of Physical Chemistry

Improving CdSe Quantum Dot/Polymer Solar Cells Efficiency Through the Covalent Functionalization of Quantum Dots: Implications in the Devices Recombination Kinetics. Josep 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 Orgánica, Universitat de Barcelona, c/Martı´ I Franqués 1-11, 08080, Barcelona, Spain

‡Catalan Institution for Research and Advanced Studies (ICREA), Avda. Lluis Companys 23, E08010 Barcelona, Spain.

ACS Paragon Plus Environment

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ABSTRACT

Novel quantum dot capping ligands based on fullerene derivatives were attached through “clickchemistry” to the surface of semiconductor CdSe nanocrystals ( C70-CdSe). Steady state and time-correlated luminescence studies in solution show efficient quenching of the QD emission in C70-CdSe. When this material was blended with the polymer P3HT to fabricate bulkheterojunction solar cells, P3HT/C70-CdSe devices doubled the light-to-energy conversion efficiency when compared to P3HT/Py-CdSe reference devices prepared using pyridine as the capping agent. This is due to an increase in both photocurrent and fill factor showing the beneficial efficient effect of fullerene to improve light harvesting and charge transport in these devices. However, C70 also appears to increase recombination in these devices as evidenced by both Transient Absorption Spectroscopy and Transient Photovoltage measurements. This work also discusses the effects on the CdSe functionalization with C70 over the device charge recombination kinetics that limit the efficiency in CdSe QDs/polymer solar cells.


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KEYWORDS: Quantum dots, click chemistry, fullerene, charge recombination.


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INTRODUCTION. Semiconductor nanocrystals with quantum properties (or quantum dots (QDs)) such as CdSe, CdS, PbS and PbSe have been extensively used in biology as biomarkers,1-3 in physics for infrared (IR) photodetectors4-7 and in other advanced technological applications such as light emitting diodes. Recently, their use in so-called molecular photovoltaic devices has attracted much attention8, 9 and the main architectures for QD-based molecular solar cells are summarized in Scheme 1. Quantum dot sensitized solar cells have reached power conversion efficiencies for liquid and solid state in the range of 5-6%.10, 11Organic/quantum dot bilayer single junction solar cells show light-to-energy conversion efficiencies of above 5%.12 Schottky type solar cells made of PbS QDs have also been reported to achieve efficiencies of over 7% when a wide band-gap metal oxide such as nanocrystalline TiO2 is used as a selective contact for electrons.13 In addition, power conversion efficiency up to 6.6% has been reported in p-n homojunction architectures. 14

Scheme 1. Different types of quantum dot based molecular solar cells. (a) QD/polymer bulkheterojunction solar cell, (b) QD/Metal oxide bilayer type solar cell and (c) QD single type solar cell.


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On the other hand, efficiencies of bulk-heterojunction QD/polymer solar cells (QD: CdSe, CdS, PbS and polymers: P3HT (chemical name: 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 comparatively low15. One of the main factors limiting the performance of quantum dot polymer bulk heterojunction solar cells is the poor charge transport in quantum dots, due to the low carrier mobility when compared with C60 or C70, or the fullerene derivative PCBM (chemical name:[6,6]-phenylC61–butyric acid methyl ester), and this is directly related to the experimentally observed faster carrier recombination dynamics that these materials show16,

17

. The low carrier mobility in

quantum dot films is often ascribed to the inefficient charge transfer processes between the nanocrystals themselves due to the presence of organic capping agents, which are used during their synthesis in order to avoid nanocrystal aggregation and the loss of quantum properties18. These organic ligands are usually long alkyl chains terminated by an anchoring group such as acids (i.e. oleic acid), amines (i.e. hexadecylamine), phosphines (i.e. tri-octylphosphine) and phosphonic acids (i.e. tetradecylphosphonic acid). The problem is, therefore, that most organic capping ligands used in the synthesis of QD nanocrystals in order to obtain nanocrystals with a narrow size distribution, good solubility in organic solvents and well-controlled shape, inhibit the performance of complete photovoltaic devices by forming insulating barriers between neighboring QDs. Thus, ligand exchange of the original capping agent shell by smaller molecules or ions has been widely explored to make the nanoparticles soluble in different solvents, to introduce new functional groups or decrease the thickness of the capping-agent shell to increase the charge transfer rate. However, although different treatments, using halide anions19, 20, amines21, 22 or thiols23 have been investigated , to improve the carrier transport in


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semiconductor quantum dots/polymer blends, pyridine is still nowadays the most widely employed ligand for capping ligand exchange.24 Unfortunately, it has also been demonstrated that the pyridine ligand exchange does not replace entirely the capping ligand shell in CdSe quantum dots.25 Among the approaches in the literature to replace pyridine and improve the device efficiency we highlight the following. Olson et al. reported P3HT/CdSe bulkheterojunction solar cells exhibiting efficiencies up to 1.77% when butylamine was used as a shorter capping ligand instead of pyridine.22 Friend and co-workers, developed a method to grow directly polymer chains on the surface of CdSe nanoparticles26 while Haque and collaborators investigated the “in situ” synthesis of CdS nanocrystals inside a semiconductor polymer . In both of these cases the central idea was to avoid the use of capping ligands entirely.27 Finally, Krüger et al. developed a different strategy of post-synthetic treatment where the quantum dots were treated with an hexanoic acid-assisted washing procedure, exhibiting efficiencies up to 2.1%.28 However, in all this cases, the effect of these changes over the interfacial charge recombination kinetics that limit light conversion efficiency in functional devices was not explored. Our own group has studied recently in detail the charge recombination kinetics in PCPDTBT /CdSe QDs solar cells and demonstrated the direct relationship between chemical capacitance, Cµ (charge density) and the carrier lifetime (τ) for the first time in quantum dot polymer bulkheterojunction solar cells.16 Following on from the work cited and to further explore the nature of device efficiency-limiting processes in bulk-heterojunction QD/polymer solar cells, we carried out the synthesis of hybrid electron acceptor materials composed of CdSe nanocrystals and fullerenes which are anchored to the quantum dots through simple and practical “click” chemistry. Kamat and co-workers have previously demonstrated efficient electron transfer


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kinetics in C60-CdSe diads linked covalently in non-polar organic solvents.29 Fullerene was covalently attached to CdSe using the chemical approach reported by Brittain and Lander,30 (Scheme 2) exploiting the reaction of C70 with azides,31 and thus, functionalizing the quantum dot surface with a C70 monolayer, in a similar way as it was reported before.32 The C70-CdSe cluster was mixed with the semiconductor polymer P3HT, and complete bulk-heterojunction devices were fabricated showing a remarkable increase in efficiency when compared with pyridine capped P3HT/CdSe reference devices. The charge transfer recombination kinetics was also studied in complete devices under operating conditions by using advanced time resolved techniques. EXPERIMENTAL SECTION Semiconductor nanocrystal synthesis. CdSe quantum dots were synthesized using a wet chemical synthetic method. In brief, 384 mg cadmium oxide and 6 ml oleic acid (OA) are mixed together and put under vacuum. After addition of 60 ml ODE, the round bottom flask is heated up to 120 °C. After 15 minutes, argon atmosphere is provided and the temperature is increased to 250 °C. In the meantime, 590 mg of selenium is solved in 5 ml tri-n-octylphosphine (TOP) under nitrogen (stock solution). As soon as the solution reach 250 °C, 1.02 ml of the selenium stock solution is added as fast as possible. The temperature is held at 250 °C for 90 seconds. Afterwards, the heating is removed and the solution is left to cool down under continuous stirring. The CdSe quantum dots were precipitated with copious amounts of methanol and collected by centrifugation and decantation. The precipitated nanocrystals were recovered by adding a small amount of chloroform and reprecipitated with methanol. This purification process was repeated three times. Finally, part of


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the OA coated CdSe quantum dots were dissolved in pyridine and refluxed at 90ºC overnight under dark conditions. Pyridine-coated CdSe were precipitated with hexane and collected by centrifugation and decantation. The precipitate was dissolved in a mixture of pyridine and chlorobenzene (1:9, 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 was prepared mixing 128 mg of CdSe in 6 mL of degassed toluene, (3-azidopropyl)trimethoxysilane (0.5 mmol, 102.4 mg), acetic acid (0.2 mmol, 11.5 uL) and ultrapure water (0.7 mmol, 13 uL) under argon atmosphere. The reaction was stirred at reflux for 24 hours. The functionalized CdSe quantum dots were collected by centrifugation and dried under vacuum at 40 °C overnight. The covalent attachment of fullerene to the azide functionalized CdSe quantum dot was done following the methodology described by Wudl and co-workers.31 The azide-functionalized CdSe (0.032 mmol, 11.7 mg) were suspended in 5 ml of chlorobenzene and C70 (SES Research, 99.0 %) was added (0.032 mmol, 27.3 mg). The reaction was stirred at 132 °C overnight. The C70 anchored onto CdSe quantum dots were washed with MeOH and collected by centrifugation three times. Finally, the functionalizated CdSe quantum dots were dried under vacuum at 40 °C overnight. The precipitate was dissolved in chlorobenzene and saved as stock solution in a concentration 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 aqueous PEDOT:PSS dispersion (Baytron AL4083, from H.C. Starck) by spin coating (4500rpm/90 seconds). The films were annealed at 120ºC for 30 minutes under N2.


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Afterwards, the QD:Polymer solution was spin-coated onto the surface of PEDOT:PSS to form photoactive films. Aluminum (Al) cathodes (80nm) were thermally deposited onto the QD:Polymer blend, obtaining an active area of 9 mm2. Device thermal annealing was done on a hot plate under nitrogen atmosphere (150 ºC for 15 minutes). The semiconductor polymer employed in this work was Poly(3-hexylthiophene-2,5-dyil) (P3HT) and it was purchased from Rieke Metals, Inc. and used without further purification. The polymer molecular weight was 5070 K in average and it has a 91-94% of regioregularity.

Spectroscopic characterization. UV-Visible absorption spectra was recorded with a Shimadzu© 1600 spectrophotometer. Steady state luminescence emission spectra and the excited-state lifetimes were measured employing a LifeSpec-ps apparatus from Edinburgh Instruments©. FT-IR spectra were recorded on a Thermo Nicolet 5700 FTIR spectrometer, using KBr pellets. Potassium bromide used in the preparation of the pellets was kept in an oven at 50 1C. Elemental analyses (C; H; N) were performed in LECO CHNSmodel 932 by C.A.I. microanalysis elemental, Universidad Complutense de Madrid, Madrid, Spain. The I-V characteristics measurements were carried out with an ABET 150 W Xenon light source equipped with the correct set of filters to achieve the solar spectrum 1.5 AM G. The light intensity was adjusted to 100 mW/cm2 using a calibrated Si photodiode. The applied potential and cell current were measured with Keithley model 2600 digital source meter. The current to voltage (I-V curve) was measured automatically with a home-built Labview© software. The measurement of incident photon-to-current conversion efficiency (IPCE) was plotted as a


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function of the excitation wavelength by using the incident light from a 300 W xenon lamp (ILC Technology, USA), which was focused through a Gemini-180 double monochromatic (Jobin Yvon Ltd.) For transient photovoltage (TPV) measurements, the devices were connected to the 1 MOhm input terminal of an oscilloscope, and illuminated with white light to set the light bias. A small optical perturbation was applied using a nitrogen pumped PTI GL-301 dye laser as the excitation source at a wavelength of 470 nm (frequency 1.5 Hz, pulse duration < 1 ns), which resulted in voltage transient amplitudes of 4 mV. The intensity of the laser pulse was attenuated as necessary using a circular neutral density filter. The photo-induced Charge Extraction (CE) technique was performed employing a ring of white LEDs as the illumination source. The devices were illuminated under different light intensities (from darkness to 1 sun intensity) at open circuit conditions, which allow the device to reach an equilibrium in open circuit voltage, depending on the applied light intensity. This applied light bias was turned off in

Polymer Solar Cell Efficiency Through

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