Letter pubs.acs.org/JPCL
Toward Antimony Selenide Sensitized Solar Cells: Efficient Charge Photogeneration at spiro-OMeTAD/Sb2Se3/Metal Oxide Heterojunctions Néstor Guijarro,†,‡ Thierry Lutz,† Teresa Lana-Villarreal,‡ Flannan O’Mahony,† Roberto Gómez,‡ and Saif A. Haque*,† †
Department of Chemistry and Centre for Plastic Electronics, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, U.K. ‡ Institut Universitari d’Electroquímica i Departament de Química Física, Universitat d’Alacant, Ap. 99, E-03080, Alicante, Spain S Supporting Information *
ABSTRACT: Photovoltaic devices comprising metal chalcogenide nanocrystals as lightharvesting components are emerging as a promising power-generation technology. Here, we report a strategy to evenly deposit Sb2Se3 nanoparticles on mesoporous TiO2 as confirmed by Raman spectroscopy, energy-dispersive X-ray spectrometry, and transmission electron microscopy. Detailed study of the interfacial charge transfer dynamics by means of transient absorption spectroscopy provides evidence of electron injection across the Sb2Se3/TiO2 interface upon illumination, which can be improved 3-fold by annealing at low temperatures. Following addition of the spiro-OMeTAD hole transporting material, regeneration yields exceeding 80% are achieved, and the lifetime of the charge separated species is found to be on the millisecond time scale (τ50% ∼ 50 ms). These findings are discussed with respect to the design of solid-state Sb2Se3 sensitized solar cells. SECTION: Energy Conversion and Storage; Energy and Charge Transport
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porting material (HTM)) is required to further boost the conversion efficiency.14 Recently, Sb2S3 has emerged as one of the most promising sensitizers for the development of solid-state nanocrystal sensitized devices. The implementation of Sb2S3 in solid-state devices along with the optimization of the HTM for a better interaction with the sensitizer has led to a record efficiency of 6.2%.14 As a consequence of these encouraging results and in pursuit of a better performance, we consider the potential of Sb2Se3 nanocrystals in solid state nanocrystal-sensitized solar cells. The use of Sb2Se3 as the light harvesting component in solid-state nanocrystal-sensitized solar cells is particularly attractive as the metal selenide exhibits a red-shifted and wider spectral range response (from Eg ≈ 1−1.2 eV15) and a higher extinction coefficient than Sb2S3.16 Moreover, a recent computational investigation reported by Giustino et al. supports this idea.17 Surprisingly, up to now, this material has been scarcely exploited in solar cells. Bhattacharya et al. prepared photoelectrochemical cells using Sb2Se3 deposited by chemical bath deposition as a photoanode,18 whereas Messina et al. optimized the deposition method as well as the posttreatments, and implemented this material in Schottky-type cells.19 Herein, we report for the first time on a Sb2Se3sensitized TiO2 photoanode combined with spiro-OMeTAD
anocomposites of organic and inorganic materials are attracting considerable interest for the development of low-cost, scalable and robust power-generation technologies.1−3 A configuration of particular interest is based on the solid-state dye-sensitized solar cell (DSC). The typical architecture of such a solar cell is based upon a mesoporous titanium dioxide film coated with a light harvesting material. Solar to electrical power-conversion efficiencies exceeding 6% have been reported for devices employing molecular dyes as the light absorbing species and an organic semiconductor as the hole-transporting medium.4 In the past few years there has been significant attention paid to the use of inorganic nanocrystals (e.g., quantum dots (QDs)) as light harvesting materials for the development of nanocrystal-sensitized solar cells. Interest in the use of such materials has been primarily driven by the potential to further extend the light harvesting capability of DSC’s from the visible to near-infrared region of the solar spectrum, thereby improving the photocurrent response of the device. As such, a variety of inorganic nanocrystals have been investigated as light absorbers in such devices including metal sulfide, selenide, and phosphide nanoparticles.5 Moreover, significant efforts have been made to gain an insight into device function, including the influence of the materials interface properties on the charge transfer dynamics. For example, prudent selection of QD material,6 size,7,8 mode of attachment,9 and surface treatments10−13 have all proven to be essential for an efficient design, and both chemical and energetic coupling among the components (viz., metal oxide (MO), QD, and hole trans© 2012 American Chemical Society
Received: April 10, 2012 Accepted: May 4, 2012 Published: May 7, 2012 1351
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(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene) as the HTM, and focus on the charge transfer dynamics. Here, a successive ionic layer adsorption and reaction (SILAR) method for the direct growth of Sb2Se3 upon TiO2 is described, paying particular attention to the characterization of the as-grown material. We report a transient absorption spectroscopy study addressing the interfacial charge transfer processes (i.e., charge separation and recombination) at the organic hole-conductor/Sb2Se3/TiO2 heterojunction. In-situ growth of Sb2Se3 has been achieved by following a SILAR method with organic solvents, similar to that reported for CdSe.20 Briefly, the classical SILAR method entails the successive immersion of a substrate in ionic solutions of a metal and a chalcogen precursor. Selective adsorption of the cationic metal precursor to the substrate and subsequent surface reaction with the chalcogen source leads to generation of the chalcogenide nanocrystals. In the present work, mesoporous TiO2 or ZrO2 were successively immersed in a 10 mM SbCl3 acetone solution (antimony source) and a 30 mM selenide ethanol solution (selenium source). Each immersion was followed by a thorough rinse with ethanol and acetone to remove the excess of precursors not attached to the surface. The selenide solution was prepared by reduction of selenium powder with NaBH4 in ethanol and under a N2 atmosphere.21 The SILAR deposition of the metal selenide was also performed under N2. Figure 1A shows the absorption spectra of Sb2Se3/TiO2 thin films as a function of the number of SILAR deposition cycles. The absorption of the Sb2Se3 is both increased and red-shifted with repeating SILAR cycles. This implies that each cycle increases not only the coverage (i.e., amount of sensitizer) but also the mean nanocrystal size (decreasing the quantum confinement). This observation is in accordance with similar procedures for other metal chalcogenides reported elsewhere.22 Furthermore, the shape of the spectra, along with the apparent absence of an excitonic peak suggests a wide size distribution of the as-synthesized nanocrystals. In order to gain insight into the morphology of the deposited nanocrystals, transmission electron microscopy (TEM) measurements were undertaken. As can be seen in Figure 1A (inset), the TiO2 nanoparticles appear rougher after sensitization. This could be attributed to a homogeneous deposit of small and size-dispersed nanoparticles. It is pertinent to note that the degree of TiO2 modification increases as the SILAR progresses, i.e., higher coverage and bigger NCs and/or aggregates are observed for an increased number of cycles (see Figure S1 in the Supporting Information). Simultaneous elemental analysis by means of energy-dispersive X-ray (EDX) spectrometry reveals the presence of Sb and Se in approximately stoichiometric amounts, although an excess of antimony was apparent for a higher number of cycles (see Supporting Information). To explore in more detail the morphology of the asdeposited Sb2Se3 QDs, a rutile (110) single crystal was modified with eight SILAR cycles and analyzed by atomic force microscopy (AFM). Figure 1B shows a typical tappingmode AFM image of the single crystal after the Sb2Se3 deposition. It can be seen that after a significant number of cycles, the amount of deposited material remains rather low. The characteristic well-defined atomic steps of the monocrystal surface are clearly defined (see Figure S2 in the Supporting Information), and only slightly covered by isolated sizedistributed nanoparticles and aggregates. Cross-section analysis shows a nanoplate-type structure for the nanoparticles (smaller
Figure 1. (A) Absorption spectra of Sb2Se3/TiO2 films as a function of the number of cycles (1−5) and TEM images before (upper) and after (bottom) three SILAR cycles (inset, scale bar 5 nm). (B) AFM image of a rutile (110) single crystal after eight SILAR cycles. (C) Raman spectra of TiO2 before and after sensitization with three SILAR cycles; arrows indicate the features appearing after Sb2Se3 deposition at 120, 191, and 250 cm−1.
nanoparticles are of 60 nm diameter and 2.5 nm height; see Figure S3 in the Supporting Information). The poor deposition on the single crystal, together with the high coverage obtained in nanoporous structure using few cycles, suggests that the antimony precursor is only weakly bound to the TiO2 surface, and also physically trapped inside the nanochannels until its reaction with the selenide. This process resembles a precipitation inside nanovessels instead of a slower layer-bylayer atomic deposition typical for a SILAR process. It is likely that a number of SILAR methods using organic solvents reported elsewhere, which present a high degree of coloration in few cycles, progress in the same way. More detailed studies addressing the mechanism of metal selenide nanocrystal growth on TiO2 are currently underway and will be reported in due course. To obtain further evidence for the presence of Sb2Se3 on TiO2 film, Raman spectroscopy measurements were performed (Figure 1C). As a reference, the spectrum of the bare TiO2 film dominated by the anatase bands is also presented. After sensitization, additional features at 120, 191, and 250 cm−1 ascribed to Sb2Se3 are observed. The main bands at 191 cm−1 and 250 cm−1 have been attributed to the Sb−Se and Se−Se bond vibrations, respectively, in the literature.23−25 It is pertinent to note that the spectrum is in agreement with that reported by Farfán et al. for Sb2Se3 nanoparticles approximately 2−6 nm in size, including the bandwidth, which is much larger 1352
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than that of the bulk Sb2Se3 as a result of a broad size distribution of particles.26 We note that the presence of oxygenated species of antimony (such as Sb2O3) or unreacted SbCl3 is unlikely, since their characteristic peaks between 350 and 450 cm−1 were not observed.27,28 We further note that the spectra shown in Figure 1C appear to have a broad band overlapped between 150 and 200 cm−1. This signal can be attributed to amorphous Sb, as reported elsewhere.29 It is possible that during the SILAR process, an excess of NaBH4 in the selenide solution may react with Sb(III) species anchored on the substrate, resulting in metallic amorphous Sb detected by EDX (see Supporting Information). We consider next the interfacial charge transfer dynamics at the Sb2Se3/TiO2 heterojunction in the absence and presence of the HTM (spiro-OMeTAD). Transient absorption spectroscopy has arisen as a powerful tool to study the photocarrier population dynamics in QD-based systems. Depending on the time resolution of the experimental setup, both intrinsic photocarrier relaxation30 and charge transfer (fs−ps time scale)31 or recombination processes (μs−ms)32 have successfully been studied in the literature. Herein, transient absorption studies were conducted for both mesoporous TiO2 and ZrO2 films sensitized with Sb2Se3 on the submicrosecond to second time scale. For this study, we focus on samples sensitized with three SILAR cycles that showed particularly promising performance, as discussed in the Supporting Information. Upon photoexcitation (λ = 450 nm) of the Sb2Se3/TiO2 film, electron injection occurs from the metal selenide into the conduction band and density of acceptor states of the MO. In the absence of a hole-conductor, the injected electron will recombine with the hole in the metal selenide nanocrystal. In the present work, ZrO2 is employed as a control substrate, since the high energy of the conduction band prevents electron injection.33 Figure 2A shows the transient spectra for Sb2Se3/ TiO2 (blue curve) and Sb2Se3/ZrO2 (black curve) films recorded 10 μs after pulsed excitation at 450 nm. The spectrum for the Sb2Se3/TiO2 consists of a negative signal followed by a positive plateau. The negative signal is attributed to the ground state bleaching and, in agreement with the loss of quantum confinement observed in the absorption spectra, it is red-shifted with an increase in the number of SILAR cycles (see Figure S6 in the Supporting Information). Similar results have been recently reported for TiO2 modified with colloidal PbS QDs.34 The positive featureless broad band extending in the NIR from c.a. 900 nm onward (shown in Figure 2A) is most likely due to the presence of free and trapped electrons in TiO235,36 or localized holes in the Sb2Se3 following photoinduced electron injection. By contrast, the weaker positive absorption recorded for Sb2Se3/ZrO2 is indicative of poor electron injection from the photoexcited metal selenide into the ZrO2. The charge recombination reaction between the photoinjected electrons in the MO and the photooxidized Sb2Se3 nanocrystals is shown in Figure 2B. This data was obtained by monitoring the decay of the photoinduced transient band at 1200 nm. Typical data for the Sb2Se3/TiO2 and Sb2Se3/ZrO2 samples (blue and black curves, respectively) are shown. It is apparent from these traces that the transient absorption signal is significantly increased by attaching Sb2Se3 nanocrystals on TiO2 instead of ZrO2, as little or no charge injection is observed from Sb2Se3 to the ZrO2 semiconductor. Furthermore, no rise in the transient absorption decay is observed at early time scales, thus we conclude that the electron injection from the
Figure 2. Transient absorption spectra of Sb2Se3/TiO2 and Sb2Se3/ ZrO2 films, before and after heat treatment (annealed samples) measured 10 μs after excitation (A) and the corresponding transient kinetics monitored at 1200 nm (B). Energy levels diagram, depicting the recombination pathways followed during transient kinetics (solid arrows) and prior processes (dashed arrows) (C, D). Samples were excited at 450 nm (fluency 38 μJ·cm−2), all the measurements were carried out under N2 atmosphere. An estimate of the location of the conduction and valence bands of Sb2Se3 was obtained using optical and photoelectrochemical measurements (see Supporting Information).
photoexcited Sb2Se3 into the TiO2 is occurring on time scales faster than the instrument response of our transient absorption spectrometer (80% following the post fabrication thermal annealing step, while the lifetime of the separated charges remains strikingly long (>50 ms). The observation of an efficient charge separation (as reported here) can be accounted for by a favorable energy level alignment at the interface with an apparent 0.8 eV driving force for hole transfer from the photooxidized Sb2Se3 to the spiro-OMeTAD (see Figure 3B). On the other hand, the remarkably long recombination lifetime is consistent with the high Sb2Se3 coverage of the MO as observed by TEM, which could function as a blocking layer preventing direct contact between the TiO2 and the hole conductor, thereby retarding interfacial charge recombination. It is pertinent to note that our observation of the nanocrystal layer functioning as a blocking layer is in agreement with recent studies reporting that an increased QD coverage on TiO2 can result in an increase in the recombination resistance with the electrolyte.44 Transient absorption spectroscopy was used to interrogate the hole-transfer reaction at the spiro-OMeTAD/Sb2Se3/ZrO2 heterojunction. Although the electron injection efficiency appears to be very limited, a significant signal, attributed to the spiro-OMeTAD+ species, is recorded. The concentration of photogenenerated spiro-OMeTAD+ species is further increased upon thermal annealing at 80 °C (compare black and red curves, Figure 3A) presumably due to an improved interfacial contact as discussed above. However, it is apparent that the signal of the ZrO2 sensitized films decays faster than in the case of TiO2. Moreover, the presence of such a signal indicates that charge separation in this assembly can be initiated not only by electron transfer to the MO, but also by hole-transfer to the spiro-OMeTAD. This is in agreement with the observations of Plass et al. on spiro-OMeTAD/PbS/TiO2 assemblies, where hole transfer was determined to occur on the picosecond time scale, i.e., similar to that of electron injection.31 We note that these processes are likely to compete with electron and hole trapping, and hence charge transfer from trap states cannot be discarded. In the case where injection is limited, the decay in transient absorption with time is likely to represent recombination of holes in the spiro-OMeTAD and electrons localized within the QD layer, perhaps at QD/QD interfaces (Figure 3C). This could explain the increased rate of decay observed for ZrO2 systems relative to equivalent TiO2-based films (Figure 3A). In the former, recombining electrons and holes are expected to be in much closer proximity (within the sensitizer and the HTM, respectively) than in the latter, where electrons have been spatially separated by injection into the MO. An increase in spatial separation of donor and acceptor sites is expected to reduce the rate of electron transfer. Moreover, our observation of photoinduced hole-transfer from the Sb2Se3 nanocrystals to the organic HTM in the spiroOMeTAD/Sb2Se3/ZrO2 assembly, without appreciable electron injection into the oxide, may lead to a negatively charged QD, which is expected to experience an upward shift of the energy levels. This in turn may serve to facilitate electron
previous studies of CdSe-sensitized solar cells.39,40 We note that minimization of charge recombination losses in our Sb2Se3/ TiO2 architecture can be expected through the implementation of interface engineering strategies recently developed for hybrid solar cells.13,41,42 For example, our own preliminary studies demonstrate that the use of an Inx(OH)ySz passivation layer prior to Sb2Se3 deposition on the TiO2 electrode leads to retardation of interfacial charge recombination, as illustrated by the 10-fold increase in the time taken for the transient signal to decay to half of the value at 1 μs, while not affecting the yield of charge separation (see Figure S9 in the Supporting Information). This observation is in agreement with recent work by Itzhaik et al.43 It is interesting to note that annealing the Sb2Se3/TiO2 film at higher temperatures (>80 °C) resulted in a decrease in the yield of charge separation. It is reasonable to suppose that thermal annealing of the as-prepared Sb2Se3/ TiO2 film may lead to an increase in Sb2Se3 nanoparticle aggregation and surface oxidation (see Supporting Information), which in turn could influence charge separation yield and recombination lifetime. Our observation of a reduction in the yield and decrease in recombination lifetime with increasing annealing temperature is consistent with this notion. It is apparent that postfabrication temperature annealing resulted in little or no change in the yield of charge separation at the Sb2Se3/ZrO2 interface. The small signals observed could be ascribed to electron injection to MO surface states or interfacial QD/QD electron transfer (Figure 2D). Next we consider the charge transfer processes at a spiroOMeTAD/Sb2Se3/MO heterojunction. The data presented in Figure 3A was obtained by monitoring the spiro-OMeTAD+ species at 1600 nm following 450 nm pulsed excitation of the
Figure 3. (A) Transient kinetics for spiro-OMeTAD/Sb2Se3/TiO2 and spiro-OMeTAD/Sb2Se3/ZrO2 films recorded at 1600 nm. (B,C) The energy diagrams of the systems include the recombination pathways monitored by transient optical studies (solid arrows) and previous processes (dashed arrows). All the measurements were done in N2, exciting all the samples at 450 nm (fluency 6.9 μJ·cm−2). Energy level for spiro-OMeTAD was reported previously elsewhere.45 1354
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for Record Efficiency Solid-State Sensitized Heterojunction Solar Cells. Nano Lett. 2011, 11, 1452−1456. (5) Rühle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar Cells. ChemPhysChem 2010, 11, 2290−2304. (6) Vogel, R.; Hoyer, P.; Weller, H. Quantum-Sized PbS, CdS, Ag2S, Sb2S3 and Bi2S3 Particles as Sensitizers for various Nanoporous WideBandgap Semiconductors. J. Phys. Chem. 1994, 98, 3183−3188. (7) Robel, I.; Kuno, M.; Kamat, P. V. Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 4136−4137. (8) Cánovas, E.; Moll, P.; Jensen, S. A.; Gao, Y.; Houtepen, A. J.; Siebbeles, L. D. A.; Kinge, S.; Bonn, M. Size-Dependent Electron Transfer from PbSe Quantum Dots to SnO2 Monitored by Picosecond Terahertz Spectroscopy. Nano Lett. 2011, 11, 5234−5239. (9) Guijarro, N.; Shen., Q.; Giménez, S.; Mora-Seró, I.; Bisquert, J.; Lana-Villarreal, T.; Toyoda, T.; Gómez, R. Direct Correlation between Ultrafast Injection and Photoanode Performance in Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 22352−22360. (10) Lee, Y. L.; Lo, Y. S. Highly Efficient Quantum-Dot-Sensitized Solar Cell based on Co-sensitization of CdS/CdSe. Adv. Funct. Mater. 2009, 19, 604−609. (11) Shalom, M.; Rühle, S.; Hod, I.; Yahav, S.; Zaban, A. Energy Level Alignment in CdS Quantum Dot Sensitized Solar Cells Using Molecular Dipoles. J. Am. Chem. Soc. 2009, 131, 9876−9877. (12) Barea, E. M.; Shalom, M.; Giménez, S.; Hod, I.; Mora-Seró, I.; Zaban, A.; Bisquert, J. Design of Injection and Recombination in Quantum Dot Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 6834−6839. (13) Chi, C.-F.; Chen, P.; Lee, Y.-L.; Liu, I.-P.; Chou, S.-C.; Zhangm, X.-L.; Bach, U. Surface Modifications of CdS/CdSe Co-sensitized TiO2 Photoelectrodes for Solid-State Quantum-Dot-Sensitized Solar Cells. J. Mater. Chem. 2011, 21, 17534−17540. (14) Im, S. H.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Maiti, N.; Kim, H.-J.; Nazeeruddin, Md. K.; Grätzel, M.; Seok, S. I. Toward Interaction of Sensitizer and Functional Moieties in Hole-Transporting Materials for Efficient Semiconductor-Sensitized Solar Cells. Nano Lett. 2011, 11, 4789−4793. (15) Vadapoo, R.; Krishnan, S.; Yilmaz, H.; Marin, C. Electronic Structure of Antimony Selenide Sb2Se3 from GW Calculations. Phys. Status Solidi B 2010, 248, 700−705. (16) El-Sayad, E. A. Compositional Dependence of the Optical Properties of Amorphous Sb2Se(3−x)Sx Thin Films. J. Non-Cryst. Sol. 2008, 354, 3806−3811. (17) Patrick, C. E.; Giustino, F. Structural and Electronic Properties of Semiconductor-Sensitized Solar−Cell Interfaces. Adv. Funct. Mater. 2011, 21, 4663−4667. (18) Bhattacharya, R. N.; Pramanik, P. A Photo-electrochemical Cell Based on Chemically Deposited Sb2Se3 Thin-Film Electrode and Dependence of Deposition on Various Parameters. Sol. Energy Mater. 1982, 6, 317−322. (19) Messina, S.; Nair, M. T. S.; Nair, P. K. Antimony Selenide Absorber Thin Films in All-Chemically Deposited Solar Cells. J. Electrochem. Soc. 2009, 156, H327−H332. (20) Lee, H. J.; Wang, M.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Grätzel, M.; Nazeeruddin, Md. K. Efficient CdSe Quantum DotSensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Lett. 2009, 9, 4221−4227. (21) Klayman, D. L.; Griffin, T. S. Reaction of Selenium with Sodium-Borohydride in Protic Solvents. Facile Method for Introduction of Selenium into Organic Molecules. J. Am. Chem. Soc. 1973, 95, 197−199. (22) Guijarro, N.; Lana-Villarreal, T.; Shen, Q.; Toyoda, T.; Gómez, R. Sensitization of Titanium Dioxide Photoanodes with Cadmium Selenide Quantum Dots Prepared by SILAR: Photoelectrochemical and Carrier Dynamics Studies. J. Phys. Chem. C 2010, 114, 21928− 21937. (23) Zhang, Y.; Li, G.; Zhang, B.; Zhang, L. Synthesis and Characterization of Hollow Sb2Se3 Nanospheres. Mater. Lett. 2004, 58, 2279−2282.
injection from the QD into the TiO2. This phenomenon may be particularly attractive for systems that employ narrow band gap light-harvesting materials in which electron injection into TiO2 is a limiting factor. In this paper we have reported a strategy for the design of spiro-OMeTAD/Sb2Se3/TiO2 solar cell photoactive layers. More specifically, a new SILAR method for the deposition of Sb2Se3 has been described. The deposition and growth of the Sb2Se3 nanocrystals on mesoporous TiO2 films has been confirmed by Raman Spectroscopy, EDX analysis, and TEM. Transient absorption spectroscopy studies reveal an efficient charge separation yield at the spiro-OMeTAD/Sb2Se3/TiO2 heterojunction (yield of photons absorbed to charge separated species >80%) with an electron−hole recombination lifetime of 50 ms. We find that the yield and lifetime of the charge separated state at the spiro-OMeTAD/Sb2Se3/TiO2 heterojunction is strongly influenced by thermal treatment, with, for example, annealing at 80 °C leading to a 3-fold improvement in the overall yield. The present findings demonstrate that Sb2Se3 is a promising candidate as a light harvesting material in the design of low-cost, robust, and high-efficiency solid-state semiconductor sensitized solar cells.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed experimental section. Morphological and compositional analysis of the films. Determination of the Sb2Se3 energy levels. Discussion of the optimized number of cycles, annealing effect, and the role of Inx(OH)ySz layer, using transient absorption measurements. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Engineering and Physical Sciences Research Council − EPSRC (via the Excitonic Supergen (EP/G031088/ 1) and UK−India (EP/H040218/1) programmes) for financial support. S.A.H. thanks the Royal Society for an RS-URF. N.G. is grateful to the Spanish MEC for the award of an FPU grant. The Alicante group acknowledges support of the Spanish MICINN through project HOPE CSD2007-00007 (Consolider Ingenio 2010).
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REFERENCES
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dx.doi.org/10.1021/jz3004365 | J. Phys. Chem. Lett. 2012, 3, 1351−1356
