pubs.acs.org/NanoLett
Broadband Absorbing Bulk Heterojunction Photovoltaics Using Low-Bandgap Solution-Processed Quantum Dots Kevin M. Noone,† Elisabeth Strein,† Nicholas C. Anderson,† Pei-Tzu Wu,‡ Samson A. Jenekhe,†,‡ and David S. Ginger*,† † ‡
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700 and Department of Chemical Engineering, University of Washington, Box 351750, Seattle, Washington 98195-1750 ABSTRACT We describe bulk heterojunction (BHJ) solar cells containing blends of colloidal PbS nanocrystal quantum dots with several new donor-acceptor conjugated polymers. Using photoinduced absorption spectroscopy we found that blends of PbS quantum dots with one polymer, poly(2,3-didecyl-quinoxaline-5,8-diyl-alt-N-octyldithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx), produce significantly more photoinduced charge than blends of PbS with the other donor-acceptor polymers or with traditionally studied polymers like [2-methoxy-5-(3′,7′-dimethyloctyloxy)-para-phenylene vinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT). Photovoltaic devices made with PDTPQx/PbS blends exhibit power conversion efficiencies 10-100 times larger than previously reported BHJ blends made with IR-absorbing quantum dots. KEYWORDS Quantum dot, nanocrystal, bulk heterojunction, photoinduced absorption, solar cell, photovoltaic, lead sulfide diode, PIA
B
ulk heterojunction (BHJ) photovoltaics are being studied as low-cost solar cells due to their compatibility with low-temperature roll-to-roll manufacturing.1 BHJ cells have been demonstrated with a number of solutionprocessable semiconductors ranging from all-organic polymer/fullerene blends to hybrid organic/inorganic systems.2-8 Performance has been increasing rapidly and champion power conversion efficiencies in solid-state BHJs now approach 8%.8,9 Nevertheless, there is room for improvement, particularly if cells can capture more of the solar spectrum without losing voltage, perhaps by utilizing a tandem cell configuration.10 Despite recent progress,11 synthesizing organic semiconductors with the broadband infrared light harvesting needed for such structures remains challenging, and BHJ devices made from low-bandgap quantum dots have not yet been able to emulate the success of analogous polymer/CdSe blends.4,12 While Schottky diodes made from PbS and PbSe nanoparticles exhibit encouraging performance,13-16 they can sacrifice many of the manufacturing advantages of BHJs. Here, we demonstrate a hybrid organic/ inorganic cell using low-bandgap PbS colloidal quantum dots blended with a new conjugated polymer that operates as a BHJ. The resulting hybrid devices exhibit quantum efficiencies 2 orders of magnitude larger than PbS quantum dot/ polymer blends made with traditional polymers, have a clear quantum-confined photocurrent response from the PbS dots, and produce open circuit voltages of up to 0.4 V.
Previously, we hypothesized that the poor performance exhibited by many BHJ blends of common conjugated polymer donors with PbSe quantum dots is a consequence of a lack of photoinduced charge transfer at the organic/ inorganic interface.17 In this study, we utilized solutionprocessable colloidal PbS quantum dots as our IR sensitizer and explored blends with several new conjugated donoracceptor copolymers as host materials (see Supporting Information Note 1 for PbS quantum dot synthetic protocol and other experimental methods). PbS has been identified as a material that is both abundant and low in cost, making it suitable for large scale production of photovoltaics,18 assuming efficiency can be tailored using quantum confinement. Organic semiconductor hosts are attractive because they provide many cost and manufacturing advantages by facilitating solution processability.6 Figure 1 shows the solution absorption spectra of the polymers, poly(2,3-didecyl-quinoxaline-5,8-diyl-alt-Noctyldithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx, red circles, Figure 1a), poly(2,6-bis(3-n-dodecylthiophen-2-yl)-alt-N-dodecyldithieno[3,2-b:2′,3′-d]pyrrole) (PDTPBT, green diamonds, Figure 1b), and poly(2,3-didecyl-pyrido[3,4-b]pyrazine-5,8-diyl-alt-N-dodecyldithieno[3,2-b:2′,3′-d]pyrrole) (PDTPPPz, blue squares, Figure 1c) in chloroform, along with the solution absorption of a sample of PbS quantum dots in tetrachloroethylene (black lines). The three copolymer semiconductors (PDTPQx, PDTPPPz, and PDTPBT) were prepared by Stille coupling polymerization of the distannyl derivative of N-alkyldithienopyrrole respectively with three different dibromides, 5,8-dibromo-2, 3-didecyl-quinoxaline, 5,8-dibromo-2,3-didecyl-pyrido-
* To whom correspondence should be addressed. E-mail: ginger@ chem.washington.edu. Received for review: 04/18/2010 Published on Web: 06/29/2010 © 2010 American Chemical Society
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FIGURE 2. X-Channel (in-phase) photoinduced absorption spectra of blends of ∼3 nm diameter PbS quantum dots with PDTPQx (red circles), PDTPPPz (green diamonds), and PDTPBT (blue squares).
PDTPQx were likely candidates for BHJ devices when blended into thin films with PbS quantum dots. Photoinduced absorption is a spectroscopic technique that measures the presence of long-lived species such as free charges (polymer polarons) following photoexcitation of polymers in a bulk heterojunction blend.17,22-24 Figure 2 shows the X-channel (in-phase) PIA spectra of BHJ blends of PbS quantum dots with PDTPQx (red circles), PDTPPPz (green diamonds), and PDTPBT (blue squares), respectively. The PDTPQx blend clearly exhibits new subbandgap absorptions and a bleach of the bandgap transition following photoexcitation. The PDTPBT blend exhibits some very weak PIA signal (at least 10× less than the PDTPQx), and the PDTPPPz blend shows no detectable PIA signals in the range from 0.8 to 2.2 eV. Since neat PDTPQx films exhibit no long-lived PIA signal (Supporting Information Figure 3), we attribute the broad feature exhibited by the blend between 0.9 and 1.6 eV, as well as the bleach of the ground state absorption above 1.7 eV, to the presence of long-lived polarons on the PDTPQx polymer chains following photoinduced electron transfer to the PbS quantum dots. Though the polaron absorption spectrum for PDTPQx has not previously been reported, the assignment of the broad subgap absorption to a polaron is consistent with the observed quenching of the polymer PL by the quantum dots (see Supporting Information Figure 4), solution-phase chemical oxidation experiments (see Supporting Information Figure 5), the well-established spectroscopy of model conjugated polymers such as polythiophenes23 and alkoxypolyphenylene-vinylenes (PPVs),24 and the device performance results described below. If the spectral features observed in the PIA spectrum of the PDTPQx/PbS blend in Figure 2 are indeed evidence of long-lived polarons produced by photoinduced charge transfer, then we would expect these PDTPQx/PbS blends to exhibit significantly better performance in BHJ photodiode structures than blends of PbS quantum dots with either PDTPPPz and PDTPBT, as well as with the more commonly
FIGURE 1. Chemical structures and solution absorbance spectra of (a) PDTPQx, red circles (b) PDTPPPz, green diamonds, and (c) PDTPBT, blue squares in chloroform. The black line on each spectrum is a solution absorption spectrum of PbS quantum dots in tetrachloroethylene.
[3,4-b]pyrazine, and 5,5′-dibromo-4,4′-didodecyl-2,2′-bithiophene, according to literature methods.19,20 These polymers were initially selected because their ionization potentials measured via cyclic voltammetry place their highest occupied molecular orbitals (HOMOs) within the reported bandgap of PbS,21 making them likely candidates to form type-II heterojunctions when blended with PbS quantum dots in BHJ cells (see Supporting Information Figure 2). Since both morphological and electronic effects in blend films make it difficult to predict solid-state device behavior solely from solution electrochemistry, we used quasi-steady state photoinduced absorption (PIA) spectroscopy to test which, if any, of the host polymers PDTPPPz, PDTPBT, and © 2010 American Chemical Society
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FIGURE 4. Light (blue inverted triangles) and dark (red triangles) J-V curves for a PDTPQx/PbS blend BHJ device. Inset Table: short circuit current density (Jsc, mA/cm2), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) under simulated AM 1.5.
derivative [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM, filled blue squares), and PDTPQx blended with PbS quantum dots (filled red circles). As expected, the pure PDTPQx film exhibits very little detectable photocurrent. The PDTPQx/ PCBM blend has a peak quantum efficiency of 2% near the polymer absorption maximum with no photoresponse past the polymer’s absorption limit of ∼800 nm. In contrast, the PDTPQx/PbS blend exhibits a photoresponse further into the IR with a clear feature at ∼1000 nm that coincides with the first quantum-confined excitonic transition of the PbS quantum dots. Furthermore, the PDTPQx/PbS blend produces significantly more photocurrent than the PDTPQx/ PCBM blend at all wavelengths. To date, BHJ devices incorporating near-IR quantum dots have been extremely difficult to realize. In our lab, the best blends of both PbS and PbSe with traditionally studied semiconducting polymers like alkoxy-PPVs and P3HT exhibit EQEs below 1% across the entire visible spectrum and even lower into the near-IR, consistent with literature reports.25-27 The new PDTPQx/PbS devices produce ∼100× more photocurrent than devices made from previously studied polymers (such as P3HT and alkoxy-PPV) that were made in our lab and tested under identical conditions. These photocurrents also compare favorably with literature reports of other polymer/IR quantum dot blends, for instance, by producing roughly 20× more photocurrent than the P3HT/ PbSe blend reported in ref 23. Figure 4 shows the current density/voltage (J-V) curves for the PDTPQx/PbS blend devices taken under simulated AM 1.5 illumination from a filtered Xe lamp. The device in Figure 4 has a Jsc of 4.2 mA/cm2, a Voc of 380 mV, and a FF of 34%, from which we estimate a PCE of ∼0.55%. While modest compared to the best all-organic (polymer/fullerene) BHJ cells, this PCE is significantly higher than BHJ devices made from previous polymer blends with low-bandgap quantum dots28 and in fact compares favorably to the early reports of BHJ blends incorporating spherical CdSe QDs,29-31 despite being produced by devices that are optically thin and
FIGURE 3. (a) EQE spectra of PbS/PDTPQx (open red circles), PbS/ PDTPPPz (open green diamonds), PbS/PDTPBT (open blue squares), and P3HT (open black triangles) blend devices (all EQE data shown are for devices with 9:1 blends of PbS-polymer w/w). (b) EQE spectra of PDTPQx/PbS (filled red circles), PDTPQx/PCBM (filled blue squares), and pristine PDTPQx (filled green diamonds).
studied poly(3-hexylthiophene) (P3HT). To test this, we fabricated a number of BHJ devices from each of these blends. Figure 3a compares the external quantum efficiency (EQE) spectra of devices made from 9:1 weight ratios (∼65% by volume) of colloidal PbS quantum dots with PDTPQx (open red circles), PDTPPPz (open green diamonds), PDTPBT (open blue squares), and P3HT (open black triangles). The EQE of the PDTPQx/PbS blend is approximately 2 orders of magnitude larger than blends with the other two polymers, consistent with the observation of polaron signal in the PDTPQx PIA spectrum but not any of the other polymers (Figure 2). It is possible that some previously reported polymer/Pb-chalcogenide-QD blends have shown photocurrents due to limited Schottky diode operation at high quantum dot loadings. Taken together, the observed polaron signal in the PIA spectra and the dramatically higher photocurrents of the PDTPQx/PbS blends provide strong evidence that this material combination is operating as a true BHJ. Figure 3b compares the EQE spectra of pure PDTPQx (filled green diamonds), PDTPQx blended with the fullerene © 2010 American Chemical Society
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through the use of low-bandgap quantum dots with controlled shapes.33-35 In summary, we have studied blends of colloidal PbS quantum dots and a new class of conjugated polymers for use in solution-processed, broadband-absorbing photodiodes and solar cells. BHJ blend devices made from blends of PbS and PDTPQx exhibit efficiencies 2 orders of magnitude greater than blends of PbS with conventional host polymers including P3HT, and alternatives such as PDTPPPz and PDTPBT. We used PIA spectroscopy both as an effective screening tool to predict device performance for these materials and to confirm that the devices are behaving as true BHJs rather than as low-volume fraction Schottky diodes. The external quantum efficiencies of the PDTPQx blend devices are significantly (∼100×) better than devices made with other conjugated polymers using identical PbS nanoparticles in our laboratories, and the open circuit voltages are good for such low bandgap materials. At around 0.55%, the overall power conversion efficiencies of these devices are still modest and are presently limited both by light absorption and by internal quantum efficiency. We believe that improvements in performance can be achieved both by increasing the molecular weight of the host polymer to facilitate the casting of thicker films and by controlling the shape of the colloidal particles.4,12,33-35 We anticipate that the viability of new organic host materials when blended with PbS should reinvigorate the study of solution-processable bulk-heterojunction excitonic solar cells made with a range of low-band gap nanoparticles and should facilitate their use in both hybrid photovoltaics and photodetectors36-38 with band gaps tailored via quantum confinement.
FIGURE 5. (a) Absorbance and (b) internal quantum efficiency spectra of a PDTPQx/PbS blend device estimated from a simple interference model.
use a low molecular weight polymer. Significantly, we note the PDTPQx/PbS devices exhibit a Voc of 380-410 mV. With a bandgap (Eg) of about 1 eV for these cells, this Voc is remarkably close to the “Eg - 0.6 V” upper limit observed in many BHJ cells.32 Figure 5 shows the absorbance (Figure 5a) and calculated IQE (Figure 5b, see Supporting Information Supplementary Note 6) spectra for a PDTPQx/PbS BHJ device with a 100 nm thick active layer. The absorbance spectrum in Figure 5a shows that these PDTPQx/PbS devices are optically thin and thus are limited in their performance by the amount of light they absorb. Thicker active layers and light-trapping strategies could thus result in significant improvement in device performance, particularly since we believe the relatively low fill factors may be the result of current shunts in these thin films. The IQE shown in Figure 5b is greater than 20% across most of the visible and near-IR and drops below 20% at wavelengths longer than 800 nm. Although notable for an IR quantum dot/polymer blend, these IQE’s are lower than optimized organic BHJ structures and suggest an obvious route to improve future device performance. Improving charge transport among the quantum dots through shape control has proven successful in polymer/CdSe blends4,12 and is likely to improve the performance of these devices © 2010 American Chemical Society
Acknowledgment. This report is based on work (polymer and hybrid organic/inorganic solar cells) supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science under Award No. DE-FG02-07ER46467. S.A.J. also acknowledges Seth Marder and Xuan Zhang, who developed methods for synthesis of distannyl derivatives of dithienopyrrole monomers and the NSF (DMR-0805259 and DMR-0120967) for the polymer synthesis. D.S.G. acknowledges the NSF (DMR-0120967 and DMR-0449422) for seeding the initial nanocrystal synthesis work, for funding the equipment used to make these measurements, and for supporting undergraduate participation in this project. K.M.N. acknowledges partial support from an IGERT Fellowship Award under NSF DGE-050457 at the Center for Nanotechnology at the UW. N.C.A. acknowledges support from the Mary Gates Scholarship and the Washington Research Foundation at the UW. The authors declare no competing financial interests. Supporting Information Available. Experimental methods, calculation of internal quantum efficiency, and additional figures and references. This material is available free of charge via the Internet at http://pubs.acs.org. 2638
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REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19 (7), 1924–1945. Snaith, H. J.; Moule, A. J.; Klein, C.; Meerholz, K.; Friend, R. H.; Gratzel, M. Nano Lett. 2007, 7 (11), 3372–3376. Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gratzel, M. Adv. Mater. 2005, 17 (7), 813–815. Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295 (5564), 2425–2427. Oosterhout, S. D.; Wienk, M. M.; van Bavel, S. S.; Thiedmann, R.; Jan Anton Koster, L.; Gilot, J.; Loos, J.; Schmidt, V.; Janssen, R. A. J. Nat. Mater. 2009, 8 (10), 818–824. Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21 (13), 1323–1338. Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42 (29), 3371–3375. Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135-E138. Solarmer Energy To Cap off a Magnificient Year, Solarmer Achieves 7.9% NREL Certified Plastic Solar Cell Efficiency. Press Release. Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317 (5835), 222–225. Hellstrom, S.; Zhang, F. L.; Inganas, O.; Andersson, M. R. Dalton Trans. 2009, (45), 10032–10039. Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.; Rumbles, G. Nano Lett. 2009, 10 (1), 239–242. Ma, W.; Luther, J. M.; Zheng, H.; Wu, Y.; Alivisatos, A. P. Nano Lett. 2009, 9 (4), 1699–1703. Koleilat, G. I.; Levina, L.; Shukla, H.; Myrskog, S. H.; Hinds, S.; Pattantyus-Abraham, A. G.; Sargent, E. H. ACS Nano 2008, 2 (5), 833–840. Johnston, K. W.; Pattantyus-Abraham, A. G.; Clifford, J. P.; Myrskog, S. H.; MacNeil, D. D.; Levina, L.; Sargent, E. H. Appl. Phys. Lett. 2008, 92 (15), 151115–3. Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2008, 8 (10), 3488–3492. Noone, K. M.; Anderson, N. C.; Horwitz, N. E.; Munro, A. M.; Kulkarni, A. P.; Ginger, D. S. ACS Nano 2009, 3 (6), 1345–1352. Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43 (6), 2072–2077. Zhang, X.; Steckler, T. T.; Dasari, R. R.; Ohira, S.; Potscavage, W. J.; Tiwari, S. P.; Coppee, S.; Ellinger, S.; Barlow, S.; Bredas,
© 2010 American Chemical Society
(20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38)
2639
J. L.; Kippelen, B.; Reynolds, J. R.; Marder, S. R. J. Mater. Chem. 2010, 20 (1), 123–134. Wu, P.-T.; Kim, F. S.; Champion, R. D.; Jenekhe, S. A. Macromolecules 2008, 41 (19), 7021–7028. Hyun, B.-R.; Zhong, Y.-W.; Bartnik, A. C.; Sun, L.; Abrun˜a, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F. ACS Nano 2008, 2 (11), 2206–2212. Ginger, D. S.; Greenham, N. C. Phys. Rev. B 1999, 59 (16), 10622. Osterbacka, R.; An, C. P.; Jiang, X. M.; Vardeny, Z. V. Science 2000, 287 (5454), 839–842. Wei, X.; Vardeny, Z. V.; Sariciftci, N. S.; Heeger, A. J. Phys. Rev. B 1996, 53 (5), 2187–2190. Jiang, X. M.; Schaller, R. D.; Lee, S. B.; Pietryga, J. M.; Klimov, V. I.; Zakhidov, A. A. J. Mater. Res. 2007, 22 (8), 2204–2210. Wang, Z.; Qu, S.; Zeng, X.; Zhang, C.; Shi, M.; Tan, F.; Wang, Z.; Liu, J.; Hou, Y.; Teng, F.; Feng, Z. Polymer 2008, 49 (21), 4647– 4651. Seo, J.; Kim, S. J.; Kim, W. J.; Singh, R.; Samoc, M.; Cartwright, A. N.; Prasad, P. N. Nanotechnology 2009, 20 (9), 6. Sargent, E. H. Nat. Photonics 2009, 3 (6), 325–331. Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. Rev. B 1996, 54 (24), 17628. Han, L.; Qin, D.; Jiang, X.; Liu, Y.; Wang, L.; Chen, J.; Cao, Y. Nanotechnology 2006, 17 (18), 4736–4742. Heinemann, M. D.; von Maydell, K.; Zutz, F.; Kolny-Olesiak, J.; Borchert, H.; Riedel, I.; Parisi, J. Adv. Funct. Mater. 2009, 19 (23), 3788–3795. Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Adv. Funct. Mater. 2009, 19 (12), 1939–1948. Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127 (19), 7140–7147. Hanrath, T.; Veldman, D.; Choi, J. J.; Christova, C. G.; Wienk, M. M.; Janssen, R. A. J. ACS Appl. Mater. Interfaces 2009, 1 (2), 244–250. Talapin, D. V.; Yu, H.; Shevchenko, E. V.; Lobo, A.; Murray, C. B. J. Phys. Chem. C 2007, 111 (38), 14049–14054. Rauch, T.; Boberl, M.; Tedde, S. F.; Furst, J.; Kovalenko, M. V.; Hesser, G.; Lemmer, U.; Heiss, W.; Hayden, O. Nat. Photonics 2009, 3 (6), 332–336. Arango, A. C.; Oertel, D. C.; Xu, Y. F.; Bawendi, M. G.; Bulovic, V. Nano Lett. 2009, 9 (2), 860–863. McDonald, S. A.; Konstantatos, G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4 (2), 138– 142.
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