Molecular control of pentacene/ZnO photoinduced charge transfer Josef W. Spalenka, Peerasak Paoprasert, Ryan Franking, Robert J. Hamers, Padma Gopalan et al. Citation: Appl. Phys. Lett. 98, 103303 (2011); doi: 10.1063/1.3560481 View online: http://dx.doi.org/10.1063/1.3560481 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v98/i10 Published by the AIP Publishing LLC.
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APPLIED PHYSICS LETTERS 98, 103303 共2011兲
Molecular control of pentacene/ZnO photoinduced charge transfer Josef W. Spalenka,1 Peerasak Paoprasert,2 Ryan Franking,1 Robert J. Hamers,1,2 Padma Gopalan,1,2,3 and Paul G. Evans1,3,a兲 1
Materials Science Program, University of Wisconsin, Madison, Wisconsin 53706, USA Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA 3 Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706, USA 2
共Received 18 November 2010; accepted 10 February 2011; published online 9 March 2011兲 Photoinduced charge transfer modifies the device properties of illuminated pentacene field effect transistors 共FETs兲 incorporating ZnO quantum dots at the gate insulator/pentacene interface. The transferred charge is trapped on electronic states associated with the ZnO quantum dots, with a steady state population approximately proportional to the rate of organic-inorganic charge transfer. Trapped charge shifts the threshold voltage of the FETs, providing the means to evaluate the rate of organic/inorganic charge transfer and the effects of interface modification. Monolayers of the wide-gap alkane stearic acid and the conjugated oligomer terthiophene attached to the ZnO suppress or permit charge transfer, respectively. © 2011 American Institute of Physics. 关doi:10.1063/1.3560481兴 Understanding and controlling electronic transport through isolated molecules and single-molecule-thick layers has been a defining challenge both in molecular electronics and in organic electronics. These phenomena have a crucial role in the properties of devices including transistors and photovoltaics.1–3 Interface effects are particularly important in composites or blends incorporating both organic and inorganic semiconductors because the short exciton diffusion length in organic materials makes phenomena within a few nanometers of interfaces particularly important.4 The problem of short exciton diffusion length is typically solved in devices by using blends and nanostructured interfaces which have large surface area, maximizing transport of photoexcited carriers to interfaces. In this geometry, however, it is difficult to distinguish interface charge transfer from other effects.5,6 Charge transfer through these interfaces is not accurately probed by device-scale quantities, such as photovoltaic external quantum efficiency because charge collection in photocells depends on electron transport through additional interfaces at contacts and hole or electron blocking layers, or through bulk-composite blends with complex morphology.7 The effects of the surface chemistry of nanocrystalline ZnO, for example, on the rates of ZnO/organic charge transfer are difficult to separate from other transport phenomena in bulk blends and nanorod composites.8 In particular, QDs are often capped with organic ligands, such as alkanethiols,9 that act as stabilizers in solution and prevent aggregation during longterm storage. Such organic stabilizers have large effects on the field-effect mobility of electrons in ZnO quantum dot 共QD兲 transistors cast from ligand-stabilized solutions,10 but the microscopic origin of this effect has remained unexplored. In this Letter, we demonstrate molecular-layer-scale characterization and control of photoinduced charge transfer using an interfacial organic monolayer on ZnO QDs. By probing charge transfer using pentacene field effect transistors 共FETs兲, we apply a strategy we have previously devela兲
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oped to probe charge transfer to C60 acceptor molecules,11 adapting it to the problem of isolating the effects of an intervening molecular layer between pentacene and ZnO, separate from other bulk transport effects. We probe charge transfer using FETs incorporating a sparse layer of 4 nm diameter ZnO QDs at the gate oxide of a bottom-contact pentacene FET, as shown in Fig. 1共a兲. The coverage of QDs was deliberately low in order to avoid percolative conduction through ZnO. We evaluated FETs with bare QDs at the interface and devices in which ZnO was functionalized with molecular layers prior to pentacene deposition. The bottom-contact FETs were fabricated on highly doped silicon 共resistivity 0.02 ⍀ cm兲 with a 200 nm SiO2 gate dielectric. Source and drain contacts, formed from 60 nm of Au on a 10 nm Cr adhesion layer, were patterned by photolithography. ZnO QDs were prepared via sol-gel synthesis by injection of 0.85 ml of tetramethylammonium hydroxide into a solution of 20 mmol zinc acetate dihydrate 共Zn关Ac兴2兲 in absolute ethanol in a sealed container.12 Growth for 10 min. at room temperature 共RT兲 resulted in QDs with diameter of 3–4 nm, as measured using ultraviolet/visible absorption spectroscopy.13,14 After the growth stage, the QD solution was diluted by a factor of 10 with absolute ethanol and immediately spin-coated onto the FET structures. The electrode/QD structure was then annealed in air at 300 ° C for 10 min. Storing QDs was impractical due to the absence of capping molecules. A Zn coverage of 2.5⫻ 1015 Zn atoms/ cm2 was observed via x-ray photoelectron spectroscopy 共XPS兲 measurements of the relative intensities of emission from Zn in QDs and Si in the SiO2 gate dielectric. The corresponding area density of QDs with 4 nm diameter is 1.8⫻ 1012 cm−2, less than the 7.2⫻ 1012 cm−2 that would be expected in a closepacked monolayer of QDs. An incomplete layer of QDs is also observed in scanning electron microscope images, as shown in Fig. 1共b兲. Forming the QD layer did not create a ZnO QD transistor, nor, after the deposition of pentacene, an ambipolar ZnO-pentacene transistor. The stearic acid 共C18H36O2兲 monolayers were formed on the QDs by immersing FET electrodes structures, on which
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FIG. 1. 共Color online兲 共a兲 Schematic of a pentacene FET incorporating a layer of ZnO QDs at the pentacene/SiO2 interface. 共b兲 Scanning electron micrograph of a submonolayer of ZnO QDs spin-coated onto the SiO2 gate insulator. 共c兲 Energy band alignments for pentacene and nanocrystalline ZnO and the relevant rate constants for charge transfer.
QDs had already been deposited, in 1 mM stearic acid in tetrahydrofuran 共THF兲 at RT for three days.15 Alternatively, conjugated terthiophene 共2 , 2⬘ : 5⬘ , 2⬙-terthiophene-5carboxylic acid, 3TCOOH兲 monolayers were deposited by immersing the QD/electrode structure in 1 mM 3TCOOH in THF for three days at RT.14 Samples were rinsed with THF and dried with nitrogen. A S 2p peak appeared in XPS spectra after forming the 3TCOOH monolayer. The choice of solvent was crucial. Depositing 3TCOOH from a 1 mM solution in N,N-dimethylformamide, acetonitrile, dimethyl sulfoxide, or ethanol, completely dissolved the ZnO QDs. From the S 2p and Zn 2p XPS lines, we find that the terthiophene-functionalized sample has one terthiophene molecule per 16 Å2 of QD surface area, a 3TCOOH coverage of 0.5 to 1 ML on ZnO. The pentacene layer was deposited under vacuum in a deposition system that allowed in situ electrical measurements without exposing the sample to air.16 The gate voltage was scanned from +60 to ⫺60 V, with a drain-source voltage of ⫺30 V. The mobility of holes in pentacene FETs fabricated in this way was reduced from 8 ⫻ 10−4 cm2 / V s without QDs to 3 ⫻ 10−4 cm2 / V s with ZnO QDs. Illumination for photoinduced charge transfer measurements was provided at a wavelength of 656 nm by a laser diode at an intensity of 39 mW/ cm2. The relative energies of the valence band of ZnO with respect to the lowest unoccupied molecular orbital level of pentacene, as in Fig. 1共c兲, indicate that ZnO QDs act as electron acceptors.17 Excitons generated by photon absorption in the pentacene dissociate at the ZnO-pentacene interface, and electrons are subsequently trapped on ZnO. Light absorption directly in the ZnO QDs can be neglected because we have found that these QDs have negligible absorption at wavelengths longer than 340 nm. The role of the rate of charge transfer in determining the threshold voltage can be illustrated using a system of rate equations. The volume concentration of excitons in the volume of pentacene adjacent to the QDs, nep, and the area concentration of trapped electrons in ZnO QDs, nZnO, depend on the rate at which excitons are generated per unit volume ⌫, and the rate constants for exciton recombination, krec, charge transfer from pentacene to ZnO, ktransfer, and escape
of trapped electrons from ZnO, kescape. Two coupled rate equations give nep and nZnO: dnep = ⌫ − krecnep − ktransfernep , dt
共1兲
dnZnO = ktransfernep − kescapenZnO . dt
共2兲
The recombination rate constant for excitons in pentacene, krec, which is on the order of 1.2⫻ 109 s−1,18 is much larger in this case than ktransfer and the solution to the system of rate equations gives nZnO = ktransfer⌫ / kreckescape. With the assumption that krec and kescape are not significantly altered by molecules at the interface, the steady-state population of charges on ZnO is proportional to ktransfer, and nZnO provides a probe of ktransfer. Charge transfer to unfunctionalized ZnO QDs is evident in electrical measurements during illumination, Fig. 2共a兲. The FET threshold voltage VT shifts by ⌬VT to more positive values, indicating that the trapped species are electrons. The area density of charge transferred to QDs can be calculated using nZnO = 共Ci / q兲⌬VT. Here, Ci is the capacitance per unit area of the gate insulator, and q is the electron charge. The +33 V threshold voltage shift in Fig. 2共a兲 corresponds to 3.4⫻ 1012 cm−2 trapped electrons on ZnO QDs, or 1.9 electrons per ZnO QD. The nonzero threshold voltage in the
FIG. 2. 共Color online兲 共a兲 Transfer characteristics of a pentacene FET incorporating ZnO QDs at the pentacene/gate insulator interface, acquired in the dark 共solid line兲 and under illumination 共dashed line兲. 共b兲 Transfer characteristics of pentacene on an SiO2 gate dielectric without ZnO in the dark 共solid line兲 and under illumination 共dashed line兲.
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FIG. 3. 共Color online兲 Transfer characteristics in the dark 共solid line兲 and under illumination 共dashed line兲 for 共a兲 pentacene on ZnO QDs functionalized with stearic acid and 共b兲 pentacene on ZnO QDs functionalized with 3TCOOH.
dark, VT = 20 V in Fig. 2共a兲, arises from charge trapped on ZnO QDs or on the gate insulator, as surface charge has not been removed by a surface treatment. VT recovers by more than 50% within 2 min after illumination ceases, and returns to within 1 V of the original value in the dark after several hours. In a control experiment without ZnO, Fig. 2共b兲, the threshold voltage shift upon illumination is +4.5 V, equivalent to a concentration of 4.6⫻ 1011 cm−2 trapped electrons. This is only 14% of the charge transferred with ZnO, and corresponds to charge transfer to traps in the pentacene or at the pentacene/gate insulator interface. Capping ZnO QDs with organic monolayers leads to dramatic changes in photoinduced charge transfer across the pentacene/ZnO interface. Figure 3共a兲 shows the transfer characteristics of an FET in which the surfaces of the ZnO QDs are capped with stearic acid. With the stearic acid layer, the VT shift under illumination is +5.4 V, or 5.4 ⫻ 1011 charges/ cm2, approximately equal to the trap densities found in the pentacene on the control samples without QDs. We thus conclude that the stearic acid layer blocks charge transfer between the pentacene and ZnO. A conjugated terthiophene monolayer, however, inset in Fig. 3共b兲, allows charge transfer to occur at a rate nearly as large as with unfunctionalized ZnO. Transfer curves for FET devices with 3TCOOH-functionalized ZnO are shown in Fig. 3共b兲. Illumination causes a threshold voltage shift of +24 V, indicating a trapped charge density of 2.5⫻ 1012 cm−2. The rate of charge transfer via tunneling through an interfacial layer depends on the chemical structure of the molecular layer and on the linkage groups attaching it to the interface.19–21 The current through the pentacene/monolayer/ ZnO junction decreases exponentially with increasing molecular length, with a decay constant that depends on the energy barrier introduced by the intervening layer. Long alkanes, such as stearic acid, have a large energy gap of ⬃7 eV, creating a large barrier to electron tunneling and decay constants on the order of 1 Å−1, an order of magnitude larger than -conjugated molecules.19 The decay constant for conjugated oligothiophene, such as terthiophene, is much lower, with calculated values of 0.2 Å−1.22 Stearic acid forms monolayer films with a thickness of 20 Å and the length of the 3TCOOH molecule is ⬃12 Å.23,24 We thus expect the tunneling through the stearic acid layer to be eight orders of magnitude smaller than 3TCOOH. We observe at
least a factor of 5 difference in the rate of charge transfer through these molecules, and the actual difference in rate is likely much larger. The discrepancy arises from the difficulty in isolating photoinduced charge transfer to ZnO from charge transfer to defects inherent in the pentacene thin film. These results suggest that the molecular materials chosen to form inorganic/organic interfaces in transistors,10 or to stabilize QD suspensions, can be designed to optimize the electronic properties of subsequent devices. FET-based devices that rely on tuning the threshold voltage using surface molecules may also find applications in new organic gas sensors25 and memories.26 This work was supported by the University of Wisconsin Materials Research Science and Engineering Center, under NSF Grant No. DMR-0520527. R. M. Metzger, J. Mater. Chem. 18, 4364 共2008兲. S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa, and Y. Iwasa, Nature Mater. 3, 317 共2004兲. 3 H. L. Yip, S. K. Hau, N. S. Baek, H. Ma, and A. K.-Y. Jen, Adv. Mater. 20, 2376 共2008兲. 4 N. Koch, ChemPhysChem 8, 1438 共2007兲. 5 W. J. E. Beek, M. M. Wienk, and R. A. J. Janssen, Adv. Mater. 16, 1009 共2004兲. 6 S. D. Oosterhout, M. M. Wienk, S. S. van Bavel, R. Thiedmann, L. J. A. Koster, J. Gilot, J. Loos, V. Schmidt, and R. A. J. Janssen, Nature Mater. 8, 818 共2009兲. 7 M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S. Ginley, Appl. Phys. Lett. 89, 143517 共2006兲. 8 D. C. Olson, Y. J. Lee, M. S. White, N. Kopidakis, S. E. Shaeen, D. S. Ginley, J. A. Voigt, and J. W. P. Hsu, J. Phys. Chem. C 112, 9544 共2008兲. 9 N. S. Pesika, Z. Hu, K. J. Stebe, and P. C. Searson, J. Phys. Chem. B 106, 6985 共2002兲. 10 S. Bubel, D. Nikolova, N. Mechau, and H. Hahn, J. Appl. Phys. 105, 064514 共2009兲. 11 B. Park, P. Paoprasert, I. In, J. Zwickey, P. E. Colavita, R. J. Hamers, P. Gopalan, and P. G. Evans, Adv. Mater. 19, 4353 共2007兲. 12 A. Wood, M. Giersig, M. Hilgendorff, A. Vilas-Campos, L. M. LizMarzan, and P. Mulvaney, Aust. J. Chem. 56, 1051 共2003兲. 13 E. A. Meulenkamp, J. Phys. Chem. B 102, 5566 共1998兲. 14 J. Kagan, S. K. Arora, and A. Üstünol, J. Org. Chem. 48, 4076 共1983兲. 15 A. Lenz, L. Selegård, F. Söderlind, A. Larsson, P. O. Holtz, K. Uvdal, L. Ojamäe, and P.-O. Käll, J. Phys. Chem. C 113, 17332 共2009兲. 16 B.-N. Park, S. Seo, and P. G. Evans, J. Phys. D: Appl. Phys. 40, 3506 共2007兲. 17 B. N. Pal, P. Trottman, J. Sun, and H. E. Katz, Adv. Funct. Mater. 18, 1832 共2008兲. 18 H. Marciniak, M. Fiebig, M. Huth, S. Schiefer, B. Nickel, F. Selmaier, and S. Lochbrunner, Phys. Rev. Lett. 99, 176402 共2007兲. 19 D. Segal, A. Nitzan, W. B. Davis, M. R. Wasielewski, and M. A. Ratner, J. Phys. Chem. B 104, 3817 共2000兲. 20 W. B. Davis, W. A. Svec, M. A. Ratner, and M. R. Wasielewski, Nature 共London兲 396, 60 共1998兲. 21 G. Peng, M. Strange, K. S. Thygesen, and M. Mavrikakis, J. Phys. Chem. C 113, 20967 共2009兲. 22 A. Salomon, D. Cahen, S. Lindsay, J. Tomfohr, V. B. Engelkes, and C. D. Frisbie, Adv. Mater. 15, 1881 共2003兲. 23 S.-L. Ren, S.-R. Yang, J.-Q. Wang, W.-M. Liu, and Y.-P. Zhao, Chem. Mater. 16, 428 共2004兲. 24 S. Hotta and K. Waragai, J. Mater. Chem. 1, 835 共1991兲. 25 A.-M. Andringa, M.-J. Spijkman, E. C. P. Smits, S. G. J. Mathijssen, P. A. van Hal, S. Setayesh, N. P. Willard, O. V. Borshchev, S. A. Ponomarenko, P. W. M. Blom, and D. M. de Leeuw, Org. Electron. 11, 895 共2010兲. 26 K.-J. Baeg, Y.-Y. Noh, H. Sirringhaus, and D.-Y. Kim, Adv. Funct. Mater. 20, 224 共2010兲. 1 2
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