CHEMPHYSCHEM COMMUNICATIONS DOI: 10.1002/cphc.201300825
PbS Colloidal Quantum-Dot-Sensitized Inorganic–Organic Hybrid Solar Cells with Radial-Directional Charge Transport Sungwoo Kim,[a, b] Jin Hyuck Heo,[a, c] Jun Hong Noh,[a] Sang-Wook Kim,*[b] Sang Hyuk Im,*[c] and Sang Il Seok*[a, d] Colloidal quantum dots (CQDs) have been intensively studied owing to their unique optical and physical properties such as convenient electronic bandgap control by the quantum confinement effect, strong absorption over broad wavelength regions, an intrinsically large dipole moment, and multiple exciton generation by impact ionization.[1] In particular, it is expected that the unique properties of CQDs will greatly improve solar cells because their solution processability at relatively lower processing temperatures can reduce the fabrication cost and yield flexible thin-film solar cells. Accordingly, intensive studies have been performed to develop efficient CQD solar cells with inorganic metal chalcogenides such as CdS(e),[2] PbS(e),[3] HgTe,[4] and CuInTe(Se).[5] Among them, near-infrared (NIR)-responsive PbS CQDs have been of great interest because PbS has a low bulk energy bandgap of ~ 0.4 eV and a large Bohr radius of 18 nm. Since Grtzel et al.[6] first reported dye-sensitized solar cells, these sensitized solar cells have been intensively studied over the past two decades in an effort to develop cost-effective solar cells. Sensitized solar cells are composed of an electron conductor, a sensitizer, and a hole conductor. This setup allows the generated electron–hole pairs to quickly separate into elec[a] S. Kim,+ J. H. Heo,+ Dr. J. H. Noh, Prof. S. I. Seok Solar Energy Materials Research Group Division of Advanced Materials Korea Research Institute of Chemical Technology 141 Gajeong-ro, Yuseong-gu Daejeon 305-600 (Korea) E-mail: [email protected] [b] S. Kim,+ Prof. S.-W. Kim Department of Molecular Science and Technology Ajou University Suwon, 443-749 (Korea) E-mail: [email protected] [c] J. H. Heo,+ Prof. S. H. Im Department of Chemical Engineering College of Engineering Kyung Hee University 1 Seochon-dong, Giheung-gu Yongin-si, Gyeonggi-do 446-701 (Republic of Korea) E-mail: [email protected] [d] Prof. S. I. Seok Department of Energy Science Sungkyunkwan University Suwon 440-746 (Korea) E-mail: [email protected] [+] Equal contributions. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201300825.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
tron conductors and hole conductors. The probability of recombination is thus greatly reduced, even when relatively impure materials are used. However, the conventional Ru dyes and liquid electrolytes used in dye-sensitized solar cells might limit the ability to fabricate flexible optoelectronics because the weak absorption coefficient of Ru dyes requires a mesoporous TiO2 electrode over 10 mm thick to fully absorb the light. Further, the liquid electrolyte could potentially leak when subjected to bending. Therefore, it is desirable to develop an allsolid-state sensitized solar cell with a relatively thin photoelectrode. As part of an effort to develop all-solid-state inorganic CQDsensitized solar cells (SSCs), we previously demonstrated PbS CQD-SSCs with a device architecture of mesoporous TiO2/PbS CQDs and poly-3-hexylthiophene (P3HT) and spiro-MeOTAD [2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene], which serve as organic hole-transporting materials (HTMs).[3b,c] From our previous studies, we found that the PbS CQDs can be more densely packed in the top section of a mesoporous TiO2 electrode, even though some PbS CQDs successfully infiltrate into the bottom section of the TiO2 electrode and the effectiveness of the organic HTMs is reduced by the infiltration of the HTMs into the narrow pores of the PbS CQDs deposited on the mesoporous TiO2. Therefore, we considered that a one-dimensional (1D) TiO2 electrode would provide sufficient pore space, enabling a good penetration of HTM into the surface of the PbS CQD/TiO2 electrode. Using a device architecture of 1D TiO2 nanorod electrode/PbS CQD/P3HT HTM, the generated charge carriers in the PbS CQDs could be transported in a radial direction, which is the shortest pathway to deliver the charge carriers into the electrode. Therefore, we could significantly improve the fill factor as compared to conventional mesoscopic TiO2 nanoparticle-based PbS CQD-SSCs. Scheme 1 shows an illustration of the PbS CQD-SSCs with radial-directional charge transport. We thought that the shortest pathway to extract the charge carriers generated in multistacked PbS CQDs was to use 1D TiO2 and separate the charge carriers in the radial direction. Holes can be efficiently transported to the Au counter electrode through the P3HT HTM and PEDOT:PSS hole-conducting layer and at the same time the electrons can be done as well. Upon illumination with solar light, the PbS CQDs generate electron–hole pairs and the generated electrons (holes) are injected into the TiO2 photoelectrode (P3HT HTM). When an n–p heterojunction is made by n-type TiO2 and p-type PbS CQDs stacked into multiple layers, the charge carriers have many opportunities to recomChemPhysChem 2014, 15, 1024 – 1027
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Scheme 1. 1D TiO2 nanorod-based PbS CQD-SSCs with radial-directional charge transport.
bine with surface traps before reaching the electrode because the generated charge carriers travel through the relatively thick PbS CQDs. Therefore, it is difficult for n–p heterojunction cells to attain a high fill factor due to the inherent potential for the charge carriers to recombine. However, it is expected that the newly designed PbS CQD-SSCs with radial-directional charge transport, shown in Scheme 1, will efficiently deliver the charge carriers without significant recombination owing to the significantly reduced pathway. These newly designed PbS CQD-SSCs consequently exhibit a higher fill factor (FF) than the mesoscopic PbS CQD-SSCs. In our previous report,[7] we found that PbS CQDs co-capped with oleic acid and 1-dodecanethiol exhibit better air stability and a higher open-circuit voltage than PbS CQDs capped with only oleic acid, owing to the strong passivation and the increased conduction band of TiO2/PbS CQDs. In addition, PbS CQDs co-capped with oleic acid and 1-dodecanethiol showed similar recombination characteristics as PbS CQDs capped with oleic acid, even though it was expected that the co-capped PbS CQDs would have poor electronic transportation owing to the passivation by 1-dodecanethiol. Hence, we used PbS CQDs co-capped with oleic acid and 1-dodecanethiol in the present study. Figure 1 shows the morphologies of the 1D TiO2 nanorod electrode and the multilayered PbS CQD/TiO2 electrode. The scanning electron micrograph (SEM) cross-sectional image in Figure 1 a indicates the formation of 1D TiO2 nanorods that are approximately 1 mm long and 150 nm in diameter. There are sufficient macropores in the 1D TiO2 for HTM infiltration. The SEM cross-sectional image of the PbS CQDs/TiO2 electrode in Figure 1 b shows that the PbS CQDs are deposited in multiple layers on the surface of the 1D TiO2 electrode and that the pores in the electrode are not completely filled up by the PbS CQDs. The SEM top view of the PbS CQDs/TiO2 electrode in Figure S1 confirms that sufficient pores between the PbS CQDs/1D TiO2 electrodes remain for HTM infiltration. The transmission electron microscopy (TEM) image of the PbS CQDs/1D TiO2 electrode in Figure 1 c indicates that the diameter increased to approximately 200 nm by the deposition of multiple layers of PbS CQDs on the surface of the 1D TiO2 electrode. The magnified TEM image of the PbS CQDs/1D TiO2 electrode in Figure 1 d confirms that PbS CQDs less than 5 nm in size are densely stacked on the surface of the 1D TiO2 electrode. The small, black crystalline dots in Figure 1 d are likely to be PbS CQDs because the crystalline domain size of the 1D TiO2 electrode was approximately 10 nm in size, as shown in Figure S2. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Morphologies of 1D TiO2 nanorod electrode and multilayered PbS CQDs/TiO2 electrode. The SEM cross-sectional image of the a) 1D TiO2 nanorod electrode and b) multilayered PbS CQDs on 1D TiO2 nanorod. c) the TEM image of multilayered PbS CQDs on 1D TiO2 and d) its magnified TEM image.
It is also noted that the 1D TiO2 electrode provided a flatter surface for the deposition of PbS CQDs as compared to the mesoscopic TiO2 nanoparticle electrode, thus enabling the PbS CQDs to be more densely packed on its surface and to form intimate interfacial contact between the TiO2 electrode and PbS CQDs. Figure 2 a shows the visible–near-infrared absorption and external quantum efficiency (EQE) spectrum of the 1D TiO2 nanorod electrode-based PbS CQD-SSCs. The absorption spectrum confirms that the size of the PbS CQDs is less than 5 nm because the maximum absorption occurs at 1200 nm (0.97 eV) and the bandgap (Eg) of a PbS CQD with a diameter d can be determined using Equation (1):[8] E g ¼ 0:41 þ 1=ð0:0252d 2 þ 0:283dÞ
ð1Þ
The TEM image in the inset of Figure 2 a shows that the average size of the PbS CQDs is ~ 4.7 nm, which is consistent with the value found with the absorption spectrum. The EQE spectrum shows that the PbS CQD-SSCs have a photoresponse to a wavelength of up of 1400 nm. The slight red-shift of the EQE spectrum as compared with the absorption spectrum of the PbS CQD solution could be attributed to the multiple packing of PbS CQDs through EDT linker molecules. Figure 2 b shows the photocurrent density-voltage (J–V) curves of the PbS CQD-SSCs and the device performance is summarized in Table 1. The 1D TiO2 nanorod (NR)-based PbS CQD-SSCs exhibited a short-circuit current density (Jsc) of 12.1 mA cm2, an open-circuit voltage (Voc) of 0.53 V, and a fill factor (FF) 60.3 % under an illumination of 1 sun (100 mW cm2), resulting in an overall power conversion efficiency (h) of 3.9 %. The mesoscopic TiO2 ChemPhysChem 2014, 15, 1024 – 1027
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Figure 2. a) Visible–near-infrared (NIR) absorption spectrum of PbS CQD solution and EQE spectrum of the 1D TiO2 nanorod electrode-based PbS CQDSSCs; and b) J–V curves of the 1D TiO2 nanorod electrode-based PbS CQDSSCs and the conventional mesoscopic TiO2 nanoparticle electrode-based PbS CQD-SSCs. NR = 1D TiO2 nanorods, NP = TiO2 nanoparticles, NIR = measured by placing a neutral density filter with a cut-off below 715 nm wavelength on the solar cell under an illumination of 1 sun.
Table 1. Summary of the device performance.
NR-1 sun NR-NIR NP-1 sun
Jsc [mA cm2]
Voc [V]
FF [%]
h [%]
Rs [W]
Rsh [kW]
12.1 5.3 10.7
0.53 0.43 0.62
60.3 60.3 46.7
3.9 1.4 3.1
87.4 145.7 120.9
23.7 31.1 2.7
nanoparticle (NP)-based PbS CQD-SSCs showed a Jsc of 10.7 mA cm2, a Voc of 0.62 V, an FF of 46.7 %, and a h of 3.1 %. It is thus apparent that the major contribution to the improvement of device efficiency in the 1D TiO2 NR-based PbS CQDSSCs can be attributed to the enhanced FF. The FF is related to series resistance (Rs) and shunt resistance (Rsh); to attain high FF, the solar cells should have low Rs and high Rsh. From the J– V curves under 1 sun conditions, the 1D TiO2 NR-based PbS CQD-SSCs had an Rs of 87.4 W and an Rsh of 23.7 kW, whereas the mesoscopic TiO2 NP-based PbS CQD-SSCs had an Rs of 120.9 W and an Rsh of 2.7 kW. Therefore, the 1D TiO2 NR-based PbS CQD-SSCs exhibited better device performance than the mesoscopic TiO2 NP-based PbS CQD-SSCs. This implies that the 1D TiO2 NR-based PbS CQD-SSCs with radial-directional charge transport have lower backward recombination of charge carriers because a lower Rs implies better charge transport in the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
forward direction and a higher Rsh implies lower charge transport in the backward direction. The relatively lower Voc in the 1D TiO2 NR-based PbS CQD-SSCs than in the mesoscopic TiO2 NP-based PbS CQD-SSCs could be attributed to the fact that the conduction band of 1D TiO2 NRs could be significantly shifted by deposition of a semiconductor such as Sb2S3 on the surface of the NRs, whereas the conduction band of mesoscopic TiO NPs was not significantly changed[9a] . One way to enhance the efficiency of solar cells is to fabricate a tandem device that can enhance the Voc once two solar cells having similar Jsc values are properly integrated. Because there are many efficient solar cells in the visible region, including organic solar cells and inorganic semiconductors or QDs-SSCs, we checked the J-V curve in the near-infrared (NIR) region (NRNIR) by placing a neutral density (ND) filter cut-off below the 715 nm wavelength. The overall power conversion efficiency (h) of the NR-NIR cell was 1.4 % of which the contribution of the NIR region corresponds to 36 % of the overall efficiency. In summary, we fabricated newly designed, all-solid-state PbS CQD-SSCs with radial-directional charge transport by depositing PbS CQDs on the surface of a 1D TiO2 nanorod electrode in multiple layers. The 1D TiO2 nanorod (NR) electrode provided a smoother surface than did the mesoscopic TiO2 nanoparticle (NP) electrode, such that the PbS CQDs could be more densely packed on the surface of the 1D TiO2 NR electrode. The 1D TiO2 NR electrode exhibited sufficient porosity, even after the deposition of multi-layered PbS CQDs, to allow P3HT HTM to effectively infiltrate the 1D TiO2 NR pores. Therefore, the 1D TiO2 NR-based PbS CQD-SSCs exhibited a significantly improved FF as compared with the mesoscopic TiO2 NPbased PbS CQD-SSCs owing to the improved charge transport. The newly designed PbS CQD-SSCs could efficiently extract the generated charge carriers in the PbS CQDs in the radial direction, which was the shortest pathway to deliver the charge carriers to the electrode. As a result, we attained an overall power conversion efficiency of 3.9 % at 1 sun conditions. However, further work is required to fully understand the radial-directional charge transport mechanism that is responsible for the enhanced solar cell performance.
Experimental Section Preparation of the TiO2 Nanorod Photoanode: The 1D TiO2 electrode was prepared by the method of previously reported literature.[9] In a typical procedure, 50 nm-thick of TiO2 blocking layer (bl-TiO2) was deposited on a cleaned fluorine-doped tin oxide (FTO, Pilkington, TEC8) electrode by spray pyrolysis deposition method with 20 mm of titanium diisopropoxide bis(acetylacetonate) (Aldrich) solution at 450 8C. To grow the sacrificial ZnO nanorods, the ZnO seed layer was formed on the bl-TiO2/FTO film by dip-coating in 0.1 m of zinc acetate dihydrate (ACS reagent) ethanolic solution at 90 8C and subsequent heat treatment at 450 8C for 30 min. The sacrificial ZnO nanorods were then grown to ~ 2 mm in thickness by immersing the ZnO seeds/bl-TiO2/FTO film in the aqueous solution mixture of 0.015 m zinc hexahydrate (ACS reagent) and 0.015 m hexaethylenetetramine (ACS reagent) at 95 8C for 12 h. The 1D TiO2 electrodes were then formed by dipping the sacrificial ZnO nanorod film in the aqueous solution mixture of ChemPhysChem 2014, 15, 1024 – 1027
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CHEMPHYSCHEM COMMUNICATIONS 0.075 m ammonium hexafluorotitanate (Aldrich) and 0.2 m boric acid (Yakuri Pure Chemicals) at 30 8C for 1.5 h. Finally, the 1D TiO2 electrode films were dipped in 40 mm TiCl4 aqueous solution at 60 8C for 1 h and were annealed at 450 8C for 30 min to improve the interfacial contact. Synthesis of PbS CQDs: The PbS CQDs were synthesized according to the method previously reported in the literature.[7] We mixed 0.25 mmol of lead(II) acetate trihydrate (94.375 mg, Aldrich), 0.5 mmol of oleic acid (141 mg, TCI), and 0.5 mmol of 1-dodecanethiol (101 mg, Aldrich) in a round-bottomed flask by magnetic stirring. The mixed solution was then degassed by stirring it under vacuum at 100 8C for 2 h. After cooling down to 60 8C, we added 4 mL of diphenyl ether (Sigma–Aldrich) and degassed the resulting solution at 65 8C for 30 min. We then increased the reaction temperature to 150 8C under N2 atmosphere and immediately injected 0.05 mmol of tri-n-octylphosphine (TOP, 97 %; Stream) and 0.05 mmol of bis(trimethylsilyl)sulfide (TMS, 95 %; Acros) into the reaction solution. We kept the reaction temperature at 120 8C for 30 min and then cooled the solution down to room temperature. The synthesized PbS CQDs were purified by precipitating them three times using ethanol-isopropanol and redispersion in hexane/ 1,2 dichlorobenzene (10/1 vol/vol). Finally, we adjusted the concentration of the PbS CQD solution to 15 mg mL1. Device Fabrication: To fabricate 1D TiO2 electrode-based PbS CQDSSCs, we first spin-coated 100 mL of 3-mercaptopropionic acid (MPA, Aldrich) on a 1D TiO2 electrode at 2500 rpm for 60 s. We then washed the film by dropping ethanol and chloroform onto the spinning film at 2500 rpm for 90 s in order to sufficiently remove the residual MPA in the 1D TiO2 electrode because residual MPA causes aggregation of the PbS CQDs in the electrode film. Applying an MPA pretreatment on the surface of the 1D TiO2 electrode helps bind the PbS CQDs to the electrode surface owing to the chemical bonding between MPA and PbS CQD. PbS CQDs in a hexane/1,2 dichlorobenzene (10/1 vol/vol) solution (100 mL, 15 mg mL1) was spin-coated on the 1D TiO2 electrode at 2500 rpm for 60 s. In order to stack the PbS CQDs on the surface of the 1D TiO2 electrode, 250 mL of 1 wt % 1,2-ethanedithiol (EDT) in ethanol was spin-coated at 2500 rpm for 60 s, and the PbS CQD solution was again spin-coated at 2500 rpm for 60 s. We thus fabricated multistacked PbS CQDs on the surface of the 1D TiO2 electrode by spin-coating the EDT and PbS CQDs a total of nine times. The P3HT (poly-3-hexylthiophene) solution (> 98 % regioregular, 15 mg mL1 in 1,2-dichlorobenzene; Rieke Metals) was then spincoated on the PbS CQD-deposited film at 2500 rpm for 60 s. Poly(3-4-ethylenedioxythiophene) doped with poly(4-stylenesulfonate) [PEDOT:PSS/methanol (1/2 vol/vol); Baytron AI 4083] was spin-coated at 2000 rpm (33 s1) for 30 s, and a 60 nm-thick Au counter electrode was then deposited with a thermal evaporator. To improve the interfacial contact between the PbS CQDs, the PbS CQD-SSCs were immersed in a 10 wt % EDT ethanolic solution for 10 h. The EDT post-treated devices were then rinsed with ethanol and dried at 60 8C in an oven for 5 min. The mesoscopic TiO2 electrode-based PbS CQD-SSCs were fabricated by following the procedure previously reported in the literature.[7] Characterization of Device Performance: The current-voltage (I–V) characteristics of the PV cell were measured by a solar simulator (Class A, 91195A; Newport) with a source meter (Keithley 2420)
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org and a calibrated Si-reference cell (certified by NREL) under 1 sun (100 mW cm2) AM 1.5G illumination. For the measurement of the J–V characteristics in the near-infrared region, we measured the J– V curve by placing a neutral density (ND) filter below the 715 nm wavelength on the solar cell under illumination of 1 sun. The external quantum efficiency (EQE) spectra were measured by a completely computerized home-designed system comprising a light source (1000 W xenon lamp, 69935, Newport) with a monochromator (Cornerstone 260, Newport) and a multimeter (Keithley 2002). The active area of the PbS CQD-SSCs for the J–V curve was fixed at 0.16 cm2 and the J–V curves were measured by covering the active area with a 0.096 cm2 metal mask.
Acknowledgements This study was supported by the Global Research Laboratory (G.R.L.) Program and the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea, and by a grant from the KRICT 2020 Program for Future Technology of the Korea Research Institute of Chemical Technology (KRICT), Republic of Korea. Keywords: charge transport · colloids · nanostructures · photovoltaics · quantum dots
[1] a) A. J. Nozik, Chem. Phys. Lett. 2008, 457, 3 – 11; b) R. D. Schaller, V. I. Klimov, Phys. Rev. lett. 2004, 92, 186601. [2] a) Y. L. Lee, Y. S. Lo, Adv. Func. Mater. 2009, 19, 604 – 609; b) S. H. Im, Y. H. Lee, S. I. Seok, Electrochim. Acta 2010, 55, 5665 – 5669; c) S. H. Im, Y. H. Lee, S. I. Seok, S. W. Kim, S. W. Kim, Langmuir 2010, 26, 18576 – 18580; d) T. Zeng, H. Tao, X. Sui, X. Zhou, X. Zhao, Chem. Phys. Lett. 2011, 508, 130 – 133; e) Y. H. Lee, S. H. Im, J. A. Chang, J. H. Lee, S. I. Seok, Org. Electron. 2012, 13, 975 – 979. [3] a) H. J. Lee, P. Chen, S. J. Moon, F. Sauvage, K. Sivula, T. Bessho, D. R. Gamelin, P. Comte, S. M. Zakeeruddin, S. I. Seok, Langmuir 2009, 25, 7602 – 7608; b) S. H. Im, J. A. Chang, S. W. Kim, S. W. Kim, S. I. Seok, Org. Electron. 2010, 11, 696 – 699; c) S. H. Im, H. j. Kim, S. W. Kim, S. W. Kim, S. I. Seok, Energy Environ. Sci. 2011, 4, 4181 – 4186. [4] a) S. Kim, T. Kim, S. H. Im, S. I. Seok, K. W. Kim, S. Kim, S. W. Kim, J. Mater. Chem. 2011, 21, 15232 – 15236; b) S. H. Im, H. j. Kim, S. W. Kim, S. W. Kim, S. I. Seok, Nanoscale 2012, 4, 1581 – 1584. [5] S. Kim, M. Kang, S. Kim, J. H. Heo, J. H. Noh, S. H. Im, S. I. Seok, S. W. Kim, ACS Nano 2013, 7, 4756 – 4763. [6] B. O’Regan, M. Gratzel, Nature 1991, 353, 737 – 740. [7] S. Kim, S. H. Im, M. Kang, J. H. Heo, S. I. Seok, S. W. Kim, I. Mora-Sero, J. Bisquert, Phys. Chem. Chem. Phys. 2012, 14, 14999 – 15002. [8] B. R. Hyun, Y. W. Zhong, A. C. Bartnik, L. Sun, H. D. AbruÇa, F. W. Wise, J. D. Goodreau, J. R. Matthews, T. M. Leslie, N. F. Borrelli, ACS Nano 2008, 2, 2206 – 2212. [9] a) J. H. Heo, S. H. Im, H. j. Kim, P. P. Boix, S. J. Lee, S. I. Seok, I. Mora-Ser, J. Bisquert, J. Phys. Chem. C 2012, 116, 20717 – 20721; b) Y. H. Lee, J. H. Heo, S. H. Im, H. j. Kim, C. S. Lim, T. K. Ahn, S. I. Seok, Chem. Phys. Lett. 2013, 573, 63 – 69. Received: September 6, 2013 Published online on January 20, 2014
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PbS Colloidal Quantum-Dot-Sensitized Inorganic–Organic Hybrid Solar Cells with Radial-Directional Charge Transport Sungwoo Kim,[a, b] Jin Hyuck Heo,[a, c] Jun Hong Noh,[a] Sang-Wook Kim,*[b] Sang Hyuk Im,*[c] and Sang Il Seok*[a, d] Colloidal quantum dots (CQDs) have been intensively studied owing to their unique optical and physical properties such as convenient electronic bandgap control by the quantum confinement effect, strong absorption over broad wavelength regions, an intrinsically large dipole moment, and multiple exciton generation by impact ionization.[1] In particular, it is expected that the unique properties of CQDs will greatly improve solar cells because their solution processability at relatively lower processing temperatures can reduce the fabrication cost and yield flexible thin-film solar cells. Accordingly, intensive studies have been performed to develop efficient CQD solar cells with inorganic metal chalcogenides such as CdS(e),[2] PbS(e),[3] HgTe,[4] and CuInTe(Se).[5] Among them, near-infrared (NIR)-responsive PbS CQDs have been of great interest because PbS has a low bulk energy bandgap of ~ 0.4 eV and a large Bohr radius of 18 nm. Since Grtzel et al.[6] first reported dye-sensitized solar cells, these sensitized solar cells have been intensively studied over the past two decades in an effort to develop cost-effective solar cells. Sensitized solar cells are composed of an electron conductor, a sensitizer, and a hole conductor. This setup allows the generated electron–hole pairs to quickly separate into elec[a] S. Kim,+ J. H. Heo,+ Dr. J. H. Noh, Prof. S. I. Seok Solar Energy Materials Research Group Division of Advanced Materials Korea Research Institute of Chemical Technology 141 Gajeong-ro, Yuseong-gu Daejeon 305-600 (Korea) E-mail: [email protected] [b] S. Kim,+ Prof. S.-W. Kim Department of Molecular Science and Technology Ajou University Suwon, 443-749 (Korea) E-mail: [email protected] [c] J. H. Heo,+ Prof. S. H. Im Department of Chemical Engineering College of Engineering Kyung Hee University 1 Seochon-dong, Giheung-gu Yongin-si, Gyeonggi-do 446-701 (Republic of Korea) E-mail: [email protected] [d] Prof. S. I. Seok Department of Energy Science Sungkyunkwan University Suwon 440-746 (Korea) E-mail: [email protected] [+] Equal contributions. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201300825.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
tron conductors and hole conductors. The probability of recombination is thus greatly reduced, even when relatively impure materials are used. However, the conventional Ru dyes and liquid electrolytes used in dye-sensitized solar cells might limit the ability to fabricate flexible optoelectronics because the weak absorption coefficient of Ru dyes requires a mesoporous TiO2 electrode over 10 mm thick to fully absorb the light. Further, the liquid electrolyte could potentially leak when subjected to bending. Therefore, it is desirable to develop an allsolid-state sensitized solar cell with a relatively thin photoelectrode. As part of an effort to develop all-solid-state inorganic CQDsensitized solar cells (SSCs), we previously demonstrated PbS CQD-SSCs with a device architecture of mesoporous TiO2/PbS CQDs and poly-3-hexylthiophene (P3HT) and spiro-MeOTAD [2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene], which serve as organic hole-transporting materials (HTMs).[3b,c] From our previous studies, we found that the PbS CQDs can be more densely packed in the top section of a mesoporous TiO2 electrode, even though some PbS CQDs successfully infiltrate into the bottom section of the TiO2 electrode and the effectiveness of the organic HTMs is reduced by the infiltration of the HTMs into the narrow pores of the PbS CQDs deposited on the mesoporous TiO2. Therefore, we considered that a one-dimensional (1D) TiO2 electrode would provide sufficient pore space, enabling a good penetration of HTM into the surface of the PbS CQD/TiO2 electrode. Using a device architecture of 1D TiO2 nanorod electrode/PbS CQD/P3HT HTM, the generated charge carriers in the PbS CQDs could be transported in a radial direction, which is the shortest pathway to deliver the charge carriers into the electrode. Therefore, we could significantly improve the fill factor as compared to conventional mesoscopic TiO2 nanoparticle-based PbS CQD-SSCs. Scheme 1 shows an illustration of the PbS CQD-SSCs with radial-directional charge transport. We thought that the shortest pathway to extract the charge carriers generated in multistacked PbS CQDs was to use 1D TiO2 and separate the charge carriers in the radial direction. Holes can be efficiently transported to the Au counter electrode through the P3HT HTM and PEDOT:PSS hole-conducting layer and at the same time the electrons can be done as well. Upon illumination with solar light, the PbS CQDs generate electron–hole pairs and the generated electrons (holes) are injected into the TiO2 photoelectrode (P3HT HTM). When an n–p heterojunction is made by n-type TiO2 and p-type PbS CQDs stacked into multiple layers, the charge carriers have many opportunities to recomChemPhysChem 2014, 15, 1024 – 1027
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Scheme 1. 1D TiO2 nanorod-based PbS CQD-SSCs with radial-directional charge transport.
bine with surface traps before reaching the electrode because the generated charge carriers travel through the relatively thick PbS CQDs. Therefore, it is difficult for n–p heterojunction cells to attain a high fill factor due to the inherent potential for the charge carriers to recombine. However, it is expected that the newly designed PbS CQD-SSCs with radial-directional charge transport, shown in Scheme 1, will efficiently deliver the charge carriers without significant recombination owing to the significantly reduced pathway. These newly designed PbS CQD-SSCs consequently exhibit a higher fill factor (FF) than the mesoscopic PbS CQD-SSCs. In our previous report,[7] we found that PbS CQDs co-capped with oleic acid and 1-dodecanethiol exhibit better air stability and a higher open-circuit voltage than PbS CQDs capped with only oleic acid, owing to the strong passivation and the increased conduction band of TiO2/PbS CQDs. In addition, PbS CQDs co-capped with oleic acid and 1-dodecanethiol showed similar recombination characteristics as PbS CQDs capped with oleic acid, even though it was expected that the co-capped PbS CQDs would have poor electronic transportation owing to the passivation by 1-dodecanethiol. Hence, we used PbS CQDs co-capped with oleic acid and 1-dodecanethiol in the present study. Figure 1 shows the morphologies of the 1D TiO2 nanorod electrode and the multilayered PbS CQD/TiO2 electrode. The scanning electron micrograph (SEM) cross-sectional image in Figure 1 a indicates the formation of 1D TiO2 nanorods that are approximately 1 mm long and 150 nm in diameter. There are sufficient macropores in the 1D TiO2 for HTM infiltration. The SEM cross-sectional image of the PbS CQDs/TiO2 electrode in Figure 1 b shows that the PbS CQDs are deposited in multiple layers on the surface of the 1D TiO2 electrode and that the pores in the electrode are not completely filled up by the PbS CQDs. The SEM top view of the PbS CQDs/TiO2 electrode in Figure S1 confirms that sufficient pores between the PbS CQDs/1D TiO2 electrodes remain for HTM infiltration. The transmission electron microscopy (TEM) image of the PbS CQDs/1D TiO2 electrode in Figure 1 c indicates that the diameter increased to approximately 200 nm by the deposition of multiple layers of PbS CQDs on the surface of the 1D TiO2 electrode. The magnified TEM image of the PbS CQDs/1D TiO2 electrode in Figure 1 d confirms that PbS CQDs less than 5 nm in size are densely stacked on the surface of the 1D TiO2 electrode. The small, black crystalline dots in Figure 1 d are likely to be PbS CQDs because the crystalline domain size of the 1D TiO2 electrode was approximately 10 nm in size, as shown in Figure S2. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Morphologies of 1D TiO2 nanorod electrode and multilayered PbS CQDs/TiO2 electrode. The SEM cross-sectional image of the a) 1D TiO2 nanorod electrode and b) multilayered PbS CQDs on 1D TiO2 nanorod. c) the TEM image of multilayered PbS CQDs on 1D TiO2 and d) its magnified TEM image.
It is also noted that the 1D TiO2 electrode provided a flatter surface for the deposition of PbS CQDs as compared to the mesoscopic TiO2 nanoparticle electrode, thus enabling the PbS CQDs to be more densely packed on its surface and to form intimate interfacial contact between the TiO2 electrode and PbS CQDs. Figure 2 a shows the visible–near-infrared absorption and external quantum efficiency (EQE) spectrum of the 1D TiO2 nanorod electrode-based PbS CQD-SSCs. The absorption spectrum confirms that the size of the PbS CQDs is less than 5 nm because the maximum absorption occurs at 1200 nm (0.97 eV) and the bandgap (Eg) of a PbS CQD with a diameter d can be determined using Equation (1):[8] E g ¼ 0:41 þ 1=ð0:0252d 2 þ 0:283dÞ
ð1Þ
The TEM image in the inset of Figure 2 a shows that the average size of the PbS CQDs is ~ 4.7 nm, which is consistent with the value found with the absorption spectrum. The EQE spectrum shows that the PbS CQD-SSCs have a photoresponse to a wavelength of up of 1400 nm. The slight red-shift of the EQE spectrum as compared with the absorption spectrum of the PbS CQD solution could be attributed to the multiple packing of PbS CQDs through EDT linker molecules. Figure 2 b shows the photocurrent density-voltage (J–V) curves of the PbS CQD-SSCs and the device performance is summarized in Table 1. The 1D TiO2 nanorod (NR)-based PbS CQD-SSCs exhibited a short-circuit current density (Jsc) of 12.1 mA cm2, an open-circuit voltage (Voc) of 0.53 V, and a fill factor (FF) 60.3 % under an illumination of 1 sun (100 mW cm2), resulting in an overall power conversion efficiency (h) of 3.9 %. The mesoscopic TiO2 ChemPhysChem 2014, 15, 1024 – 1027
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Figure 2. a) Visible–near-infrared (NIR) absorption spectrum of PbS CQD solution and EQE spectrum of the 1D TiO2 nanorod electrode-based PbS CQDSSCs; and b) J–V curves of the 1D TiO2 nanorod electrode-based PbS CQDSSCs and the conventional mesoscopic TiO2 nanoparticle electrode-based PbS CQD-SSCs. NR = 1D TiO2 nanorods, NP = TiO2 nanoparticles, NIR = measured by placing a neutral density filter with a cut-off below 715 nm wavelength on the solar cell under an illumination of 1 sun.
Table 1. Summary of the device performance.
NR-1 sun NR-NIR NP-1 sun
Jsc [mA cm2]
Voc [V]
FF [%]
h [%]
Rs [W]
Rsh [kW]
12.1 5.3 10.7
0.53 0.43 0.62
60.3 60.3 46.7
3.9 1.4 3.1
87.4 145.7 120.9
23.7 31.1 2.7
nanoparticle (NP)-based PbS CQD-SSCs showed a Jsc of 10.7 mA cm2, a Voc of 0.62 V, an FF of 46.7 %, and a h of 3.1 %. It is thus apparent that the major contribution to the improvement of device efficiency in the 1D TiO2 NR-based PbS CQDSSCs can be attributed to the enhanced FF. The FF is related to series resistance (Rs) and shunt resistance (Rsh); to attain high FF, the solar cells should have low Rs and high Rsh. From the J– V curves under 1 sun conditions, the 1D TiO2 NR-based PbS CQD-SSCs had an Rs of 87.4 W and an Rsh of 23.7 kW, whereas the mesoscopic TiO2 NP-based PbS CQD-SSCs had an Rs of 120.9 W and an Rsh of 2.7 kW. Therefore, the 1D TiO2 NR-based PbS CQD-SSCs exhibited better device performance than the mesoscopic TiO2 NP-based PbS CQD-SSCs. This implies that the 1D TiO2 NR-based PbS CQD-SSCs with radial-directional charge transport have lower backward recombination of charge carriers because a lower Rs implies better charge transport in the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
forward direction and a higher Rsh implies lower charge transport in the backward direction. The relatively lower Voc in the 1D TiO2 NR-based PbS CQD-SSCs than in the mesoscopic TiO2 NP-based PbS CQD-SSCs could be attributed to the fact that the conduction band of 1D TiO2 NRs could be significantly shifted by deposition of a semiconductor such as Sb2S3 on the surface of the NRs, whereas the conduction band of mesoscopic TiO NPs was not significantly changed[9a] . One way to enhance the efficiency of solar cells is to fabricate a tandem device that can enhance the Voc once two solar cells having similar Jsc values are properly integrated. Because there are many efficient solar cells in the visible region, including organic solar cells and inorganic semiconductors or QDs-SSCs, we checked the J-V curve in the near-infrared (NIR) region (NRNIR) by placing a neutral density (ND) filter cut-off below the 715 nm wavelength. The overall power conversion efficiency (h) of the NR-NIR cell was 1.4 % of which the contribution of the NIR region corresponds to 36 % of the overall efficiency. In summary, we fabricated newly designed, all-solid-state PbS CQD-SSCs with radial-directional charge transport by depositing PbS CQDs on the surface of a 1D TiO2 nanorod electrode in multiple layers. The 1D TiO2 nanorod (NR) electrode provided a smoother surface than did the mesoscopic TiO2 nanoparticle (NP) electrode, such that the PbS CQDs could be more densely packed on the surface of the 1D TiO2 NR electrode. The 1D TiO2 NR electrode exhibited sufficient porosity, even after the deposition of multi-layered PbS CQDs, to allow P3HT HTM to effectively infiltrate the 1D TiO2 NR pores. Therefore, the 1D TiO2 NR-based PbS CQD-SSCs exhibited a significantly improved FF as compared with the mesoscopic TiO2 NPbased PbS CQD-SSCs owing to the improved charge transport. The newly designed PbS CQD-SSCs could efficiently extract the generated charge carriers in the PbS CQDs in the radial direction, which was the shortest pathway to deliver the charge carriers to the electrode. As a result, we attained an overall power conversion efficiency of 3.9 % at 1 sun conditions. However, further work is required to fully understand the radial-directional charge transport mechanism that is responsible for the enhanced solar cell performance.
Experimental Section Preparation of the TiO2 Nanorod Photoanode: The 1D TiO2 electrode was prepared by the method of previously reported literature.[9] In a typical procedure, 50 nm-thick of TiO2 blocking layer (bl-TiO2) was deposited on a cleaned fluorine-doped tin oxide (FTO, Pilkington, TEC8) electrode by spray pyrolysis deposition method with 20 mm of titanium diisopropoxide bis(acetylacetonate) (Aldrich) solution at 450 8C. To grow the sacrificial ZnO nanorods, the ZnO seed layer was formed on the bl-TiO2/FTO film by dip-coating in 0.1 m of zinc acetate dihydrate (ACS reagent) ethanolic solution at 90 8C and subsequent heat treatment at 450 8C for 30 min. The sacrificial ZnO nanorods were then grown to ~ 2 mm in thickness by immersing the ZnO seeds/bl-TiO2/FTO film in the aqueous solution mixture of 0.015 m zinc hexahydrate (ACS reagent) and 0.015 m hexaethylenetetramine (ACS reagent) at 95 8C for 12 h. The 1D TiO2 electrodes were then formed by dipping the sacrificial ZnO nanorod film in the aqueous solution mixture of ChemPhysChem 2014, 15, 1024 – 1027
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CHEMPHYSCHEM COMMUNICATIONS 0.075 m ammonium hexafluorotitanate (Aldrich) and 0.2 m boric acid (Yakuri Pure Chemicals) at 30 8C for 1.5 h. Finally, the 1D TiO2 electrode films were dipped in 40 mm TiCl4 aqueous solution at 60 8C for 1 h and were annealed at 450 8C for 30 min to improve the interfacial contact. Synthesis of PbS CQDs: The PbS CQDs were synthesized according to the method previously reported in the literature.[7] We mixed 0.25 mmol of lead(II) acetate trihydrate (94.375 mg, Aldrich), 0.5 mmol of oleic acid (141 mg, TCI), and 0.5 mmol of 1-dodecanethiol (101 mg, Aldrich) in a round-bottomed flask by magnetic stirring. The mixed solution was then degassed by stirring it under vacuum at 100 8C for 2 h. After cooling down to 60 8C, we added 4 mL of diphenyl ether (Sigma–Aldrich) and degassed the resulting solution at 65 8C for 30 min. We then increased the reaction temperature to 150 8C under N2 atmosphere and immediately injected 0.05 mmol of tri-n-octylphosphine (TOP, 97 %; Stream) and 0.05 mmol of bis(trimethylsilyl)sulfide (TMS, 95 %; Acros) into the reaction solution. We kept the reaction temperature at 120 8C for 30 min and then cooled the solution down to room temperature. The synthesized PbS CQDs were purified by precipitating them three times using ethanol-isopropanol and redispersion in hexane/ 1,2 dichlorobenzene (10/1 vol/vol). Finally, we adjusted the concentration of the PbS CQD solution to 15 mg mL1. Device Fabrication: To fabricate 1D TiO2 electrode-based PbS CQDSSCs, we first spin-coated 100 mL of 3-mercaptopropionic acid (MPA, Aldrich) on a 1D TiO2 electrode at 2500 rpm for 60 s. We then washed the film by dropping ethanol and chloroform onto the spinning film at 2500 rpm for 90 s in order to sufficiently remove the residual MPA in the 1D TiO2 electrode because residual MPA causes aggregation of the PbS CQDs in the electrode film. Applying an MPA pretreatment on the surface of the 1D TiO2 electrode helps bind the PbS CQDs to the electrode surface owing to the chemical bonding between MPA and PbS CQD. PbS CQDs in a hexane/1,2 dichlorobenzene (10/1 vol/vol) solution (100 mL, 15 mg mL1) was spin-coated on the 1D TiO2 electrode at 2500 rpm for 60 s. In order to stack the PbS CQDs on the surface of the 1D TiO2 electrode, 250 mL of 1 wt % 1,2-ethanedithiol (EDT) in ethanol was spin-coated at 2500 rpm for 60 s, and the PbS CQD solution was again spin-coated at 2500 rpm for 60 s. We thus fabricated multistacked PbS CQDs on the surface of the 1D TiO2 electrode by spin-coating the EDT and PbS CQDs a total of nine times. The P3HT (poly-3-hexylthiophene) solution (> 98 % regioregular, 15 mg mL1 in 1,2-dichlorobenzene; Rieke Metals) was then spincoated on the PbS CQD-deposited film at 2500 rpm for 60 s. Poly(3-4-ethylenedioxythiophene) doped with poly(4-stylenesulfonate) [PEDOT:PSS/methanol (1/2 vol/vol); Baytron AI 4083] was spin-coated at 2000 rpm (33 s1) for 30 s, and a 60 nm-thick Au counter electrode was then deposited with a thermal evaporator. To improve the interfacial contact between the PbS CQDs, the PbS CQD-SSCs were immersed in a 10 wt % EDT ethanolic solution for 10 h. The EDT post-treated devices were then rinsed with ethanol and dried at 60 8C in an oven for 5 min. The mesoscopic TiO2 electrode-based PbS CQD-SSCs were fabricated by following the procedure previously reported in the literature.[7] Characterization of Device Performance: The current-voltage (I–V) characteristics of the PV cell were measured by a solar simulator (Class A, 91195A; Newport) with a source meter (Keithley 2420)
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org and a calibrated Si-reference cell (certified by NREL) under 1 sun (100 mW cm2) AM 1.5G illumination. For the measurement of the J–V characteristics in the near-infrared region, we measured the J– V curve by placing a neutral density (ND) filter below the 715 nm wavelength on the solar cell under illumination of 1 sun. The external quantum efficiency (EQE) spectra were measured by a completely computerized home-designed system comprising a light source (1000 W xenon lamp, 69935, Newport) with a monochromator (Cornerstone 260, Newport) and a multimeter (Keithley 2002). The active area of the PbS CQD-SSCs for the J–V curve was fixed at 0.16 cm2 and the J–V curves were measured by covering the active area with a 0.096 cm2 metal mask.
Acknowledgements This study was supported by the Global Research Laboratory (G.R.L.) Program and the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea, and by a grant from the KRICT 2020 Program for Future Technology of the Korea Research Institute of Chemical Technology (KRICT), Republic of Korea. Keywords: charge transport · colloids · nanostructures · photovoltaics · quantum dots
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