Visible Colloidal Nanocrystal Silicon Light-Emitting Diode - Nano

LETTER pubs.acs.org/NanoLett

Visible Colloidal Nanocrystal Silicon Light-Emitting Diode Daniel P. Puzzo,‡ Eric J. Henderson,† Michael G. Helander,‡ ZhiBin Wang,‡ Geoffrey A. Ozin,†,* and Zhenghong Lu‡,* † ‡

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4

bS Supporting Information ABSTRACT: We herein demonstrate visible electroluminescence from colloidal silicon in the form of a hybrid silicon quantum dot
organic light emitting diode. The silicon quantum dot emission arises from quantum confinement, and thus nanocrystal size tunable visible electroluminescence from our devices is highlighted. An external quantum efficiency of 0.7% was obtained at a drive voltage where device electroluminescence is dominated by silicon quantum dot emission. The characteristics of our devices depend strongly on the organic transport layers employed as well as on the choice of solvent from which the Si quantum dots are cast. KEYWORDS: Luminescent silicon, quantum dots, organic light-emitting diodes, organic semiconductors

L

uminescent semiconductor nanocrystals, or quantum dots (QDs),1,2 have rightfully received significant consideration for a wide variety of light-emitting applications3,4 as such materials are capable of displaying size-dependent photoluminescence with high quantum yield. Moreover, semiconductor QDs can be prepared as stable colloidal dispersions in common solvents enabling solution processability, a standard scalable process that has been widely used in the development of a range of optoelectronic devices, most notably for the present discussion being hybrid semiconductor QD
organic light-emitting diodes (OLEDs). To date, nanocrystals (NCs) of CdSe and PbS5 are most commonly employed in QD optoelectronics as the maturity of their chemistry has allowed for synthesis of colloidally stable systems exhibiting size-tunable photoluminescence6,7 throughout the visible and infrared spectral ranges with quantum yields greater than 50%. Indeed, the brilliant optical and electrical properties of such nanomaterials cannot be denied; however, a major drawback regarding the use of such materials, particularly when considering potential industrial scaling, is their toxicity.8
10 Research into less harmful alternatives bearing comparable properties is thus warranted, and potential candidates include the group IV semiconductor nanomaterials of carbon,11 germanium,12 and silicon. Examples of luminescent forms of each material have been demonstrated with a resounding emphasis being directed toward the development of light-emitting silicon-based nanostructures. The research effort into the development of luminescent forms of silicon that has now spanned two decades has been driven by both purely scientific interest as well as potential commercial and technological advancement. With regards to the former, bulk silicon is an indirect band gap semiconductor and, thus, bandedge luminescence from the material must be phonon assisted, the improbability of such an event renders Si a r 2011 American Chemical Society

poor emitter of light.13 In terms of the latter, since the majority of electronics today is manifest in Si, the preparation of a stable luminescent form of Si whose fabrication is amenable to CMOS conditions would allow for an integration of Si-based photonic structures into Si-based electronics ultimately leading to devices of enhanced capability.14 Toward this end, it is now wellestablished that Si crystallites of reduced dimensions, typically below 5 nm, emit light owing to the phenomenon of quantum confinement. Canham was the first to demonstrate photoluminescence from nanocrystallites of Si which were present in porous silicon (PSi) derived from anodic electrochemical etching of bulk Si.15 This report initiated a significant research effort into quantum-confined light-emitting silicon-based nanomaterials16,17 the majority of studies focusing on demonstration of electroluminescence from PSi.18,19 In addition to PSi, luminescent silicon nanocrystals or QDs can be prepared by bottom-up synthetic methods. Stable colloidal dispersions of Si QDs have been synthesized by annealing SiOx powders followed by etching with HF,20 plasma synthesis,21 or solution reduction of SiCl4.22 The resulting colloidal stability of such Si QD dispersions enables solution processability into thin films which in turn allows for their facile integration into a range of optoelectronic devices, such as a hybrid QD
OLED structure. We exploit this property here to demonstrate the development of hybrid Si QD
OLEDs operating at visible wavelengths. To date, the first Si QD electroluminescent devices were produced by incorporating Si QDs into nonconductive matrices such as oxides and nitrides.23,24 Very recently, Cheng Received: December 21, 2010 Revised: March 3, 2011 Published: March 28, 2011 1585

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Figure 1. (a, b) Schematics of the two Si QD-OLED devices considered. (c) Si valence band measured by XPS. (d) Cross-sectional SEM of the bilayer structure of composition Si QDs/PVK/Si wafer tilted in order to highlight the smoothness of the Si QD layer. (e, f) Energy dispersion of the refractive indices and extinction coefficients of thin films of each of the semiconductor materials used in this study.

et al. demonstrated infrared electroluminescence from a Si QD
OLED structure.25 Aside from the limitations of infrared electroluminescence however, the device also suffered from poor spectral purity owing to the use of a good polymer emitter (polyphenylene vinylene) as a charge transport layer. For this study, decyl-capped Si QDs were prepared by the method developed by Henderson et al.26 A brief description of the methodology as well as complete characterization of the Si QDs including high-resolution transmission electron microscopy, FTIR, and X-ray photoelectron spectroscopy (XPS) is provided in the Supporting Information (Figures S1
S3). The employed method yields highly transparent and stable colloidal dispersions in a wide range of solvents including tetrahydrofuran, hexane, toluene, and chloroform. As will be further highlighted below, it is a combination of such excellent colloidal stability as well as solvent selectivity that allowed for the spin coating of highquality Si QD films (i.e., continuous films with low surface roughness) from neat dispersions, an absolute necessity for fabricating high-quality hybrid LEDs.

We describe herein two different types Si QD-OLED devices (Figure 1, panels a and b). To prepare device 1, a 1 wt % solution of poly(vinylcarbazole) (PVK) in o-dichlorobenzene was first spin-cast onto patterned indium tin oxide (ITO) (1 mm wide lines) and then heated on a hot plate at 110 °C for 2 h in a nitrogen-filled glovebox. A 0.5 wt % solution of Si QDs was then spin-cast at 1000 rpm from either hexane or THF atop the PVK layer which was then again thermally treated at 110 °C for 2 h. As mentioned earlier, Si QD film quality as well as device performance (see below) was strongly dependent on the choice of solvent from which the Si QDs were cast with the best films being obtained from THF or hexane dispersions. The cross-sectional SEM (Figure 1d) of the bilayer structure composed of Si QDs/ PVK/Si wafer (the top face of which is tilted toward the viewer) highlights the impeccably low roughness of the top QD film cast from hexane. Hexane is a poor solvent for PVK, and thus, little interlayer mixing is expected when hexane is employed as the QD dispersion solvent. As the current
voltage characteristics provided below will attest, the aforementioned is not true for THF 1586

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Figure 2. (a, b) Luminance and current density vs voltage plots of devices 1 and 2. The insets are the proposed energy level diagrams of the devices at zero field. (c) Current density vs voltage curves for device 1 when the Si QDs are cast from hexane and THF as well as for device 2. (d) EL spectrum of device 1 at 10 V applied bias which note is dominated by QD emission.

QD dispersions. Following the coating of the Si QD film, in order to complete the device, the bilayer structure on patterned ITO was subsequently introduced into a Kurt J. Lesker LUMINOS cluster tool operating at a base pressure of ∼10
8 Torr where 50 nm of TPBi, the electron transport layer (ETL) material, was deposited followed by 100 nm of the Al cathode. To prepare device 2, which differs from device 1 solely in its hole transport layer (HTL) composition, the same procedure as outlined above for the fabrication of device 1 was followed except that PVK was substituted for polyethylenedioxythiophene (PEDOT). As detailed in the Supporting Information (Figure S4), spectroscopic ellipsometry (SE) was used to determine the thicknesses of each of the semiconductor layers in the multilayer LED devices (nk files of each material are provided in panels e and f of Figure 1).27 According to the energy level diagrams of both devices, only a small barrier for hole injection from PEDOT to the VB edge of the Si QDs is predicted. The valence band (VB) edge of the Si QDs (Figure 1c) was determined by referencing XPS measurements28 for the QDs against those for bulk silicon.29 The Si QD VB edge, taken as the intersection of the extrapolation of the low energy photoemission onset and the background, is shifted down by 0.40 eV relative to the VB edge of bulk silicon. Relative to vacuum then, the VB edge of the Si QDs lies at 5.5 eV since that for bulk Si lies at 5.1 eV. The valence band photoemission spectra were referenced to the Si 2p core level photoemission spectrum measured at the same photon energy and flux and, thus, the binding energy shift of the VB edge is attributed to quantum confinement. With the VB edge of the Si QDs determined, knowledge of the band gap thus affords the value of the QD conduction band (CB) edge. If bandedge luminescence is presumed, then Si QD
OLEDs with EL observed at

λ = 685 nm yields a band gap of 1.81 eV and thus nanocrystals with a CB edge lying at 3.7 eV relative to vacuum. The measured and calculated shifts in Si QD VB and CB edges are consistent with the findings of Wang et al.30 as well as van Buuren et al.29 It is relevant to note here that a significant drawback of CdSe based QD
OLEDs is efficient hole injection into the CdSe QDs owing to a large mismatch in energy between the valence band edge of CdSe (>6.6 eV) and the HOMO of typical organic HTLs (5.0
6.0 eV).31 In contrast, as validated by XPS measurements discussed above, the VB edge of the Si QDs lies approximately at 5.5 eV and thus hole injection should proceed at significantly lower field than in CdSe. The Si QD-OLED device characteristics depend strongly on device composition. An obvious difference between devices 1 and 2 is of course the high current densities (Figure 2c) that can be achieved at lower field which is attributed to the HTL thickness differences as well as the enhanced transport properties of PEDOT over PVK. Moreover, the JV characteristics of device 1 depend significantly on the Si QD dispersion solvent along with the final thickness of the Si QD layer. With regards to the former, for dispersions processed from THF, significant interlayer mixing is expected as THF is a good solvent for PVK. The thickness of the QD layer is thus expectedly reduced which accounts for the large current density that can be achieved at low field. The same is not true for devices where the QDs are cast from hexane as a truly multilayer structure is realized in such devices and we thus investigated how the JV characteristics of device 1 varied with the thickness which was easily controlled by manipulating the concentration of the Si QDs in the dispersion. A decrease in current density and increase in drive voltage are observed as the thickness of the Si QD layer is increased (Figure 2c), which can 1587

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Figure 3. (a) EL spectra of device 1 at 10, 15, and 20 V. Note the emergence of blue EL at biases above 15 V. (b) EQE vs voltage plot for device 1. An EQE of 0.7% is achieved under device conditions where the EL originates purely from QD emission. (c) Photograph of the Si QD
OLED at an applied bias of 15 V. (d) EL spectra from two Si QD LEDs (device 1 structure) with emitter layers consisting of Si QDs with different mean diameters. Because the QD luminescence arises from quantum confinement, EL centered at two different wavelengths is observed for the QDs of different mean diameter (i. e., 3.2 nm for 685 nm EL and 3.0 nm for 645 nm EL).

be attributed to the insulating nature of the QD layer. Indeed, device 1 with a 30 nm thick Si QD layer exhibits the highest drive voltage; however as will be highlighted below, the EQE, EL brightness (Figure 2, panels a and b) and color purity (i.e., device EL arising purely from QD emission) over a broader range of field strengths are superior to the other compositions shown in Figure 2c. We thus primarily focus for the remainder of this report on the characteristics of device 1 consisting of a 30 nm thick Si QD layer. The observed electroluminescence (EL) of device 1 with a 30 nm thick Si QD layer (Figure 2d) at an applied bias of 10 V is dominated by Si QD emission, indicating that a majority of the excitons generated in the device recombine on QD centers. The luminescence from the Si QDs is believed to originate from two processes: (1) direct injection of holes and electrons into the QDs; (2) exciton formation in the ETL or HTL organic layers followed by resonant energy transfer to the QDs.31 Energy transfer is indeed possible in both device structures since the emission of both PVK and TPBi overlap the excitation spectrum of the Si QDs (Figure S1, Supporting Information). A consideration of the significant difference in luminance between devices 1 and 2 however can potentially shed some light on the mechanism of EL generation. The luminance of the LED containing PVK as the HTL layer is significantly higher than that with PEDOT (Figure 2 panels a and b) which we attribute to a number of factors: (1) exciton energy transfer from PVK to the Si QD is the primary contribution

to the device EL; (2) efficient exciton energy transfer from the Si QDs to the PEDOT; (3) a lower energy barrier for hole injection from PVK over PEDOT to the VB edge of the Si QDs. In addition to the choice of HTL material, as was true for the JV characteristics of our devices, their respective luminances also varied depending on the QD solvent (i.e., the degree of mixing of QD emitter and HTL layers) and the thickness of the QD layer. Sufficiently bright luminance originating from purely QD emission was only achieved in LEDs of device 1 composition with minimal intermixing between layers which of course was ensured by simply processing the Si QDs from hexane. The EL of our Si QD-OLEDs is sufficiently bright and vivid to be easily visible by eye (Figure 3c). As mentioned above, at drive voltages below 15 V, the EL spectrum (Figure 3a) of device 1 consisting of a 30 nm QD layer is dominated by QD emission. However, at biases greater than 20 V, the Si QD layer begins to degrade slightly and blue EL from both PVK and TPBi (Figure 3a) is observed which interestingly offers the added possibility of color mixing in our QD
OLED. The QD
OLEDs of device 1 composition with 15 and 20 nm thick Si QD layers both behave in the same way as the device bearing a 30 nm Si QD emissive layer, the difference being that the devices turn on as well as mix with the blue EL of PVK at lower biases (Figure S5, Supporting Information). In general, the thicker the QD layer, the higher the luminance originating from solely QD emission and the larger bias at which blue begins to be observed. Most notably, the external 1588

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OLED (Figure 3b) of device 1 composition (30 nm thick QD layer) reaches a peak value of 0.7% under device conditions where the EL arises solely from QD emission alone. The aforementioned becomes evermore impressive when one recalls that the emitter layer is composed of an indirect band gap semiconductor as well as upon consideration that the highest efficiencies reported from from QD-OLEDs based on direct band gap semiconductors and PSi LEDs is 2.7%32 and ∼1%, respectively. An added feature of our system, owing again to the quantum confinement effect, is the ability to tune the luminescence of the Si QDs by simply varying their average size which of course can be controlled at the HF etching stage of the synthetic method. To demonstrate the size-tunable emission of such QDs, we fabricated two Si QD-OLEDs with device 1 structure, the difference between them being the size of the Si QDs present in their respective emitter layers. As expected, two spectra (Figure 3d) with separate EL maxima are observed, effectively demonstrating a mechanism of EL tunability in our system. We herein described an example of visible electroluminescence from colloidal silicon in the form of a Si QD
OLED. We acknowledge that a drawback of our system is indeed low brightness at operating voltages where device EL arises purely from QD emission achieved which we attribute to both the low photoluminescence quantum yield of Si QDs, the relative value of which was measured to be 3% (against Ru(bpy)32þ standard),33 along with the insulating nature of their respective thin films. We nevertheless hope to encourage with this report a greater research effort into alternative QD systems to the archetypal cadmium and lead based chalcogenides for optoelectronic applications. With regards to the system described herein, it should indeed be possible through more mature materials chemistry to increase the conductivity and mobility of the Si QD film by modifying the capping ligands on the QD surface which would lead to further enhancement in the performance of our Si QD
OLEDs. We are thus pursuing ways of introducing functional capping ligands onto the Si QD surface without compromising colloidal stability or their excellent film forming properties. In addition, enhancements and improvements are anticipated by moving toward all inorganic QD
LEDs.34 Experimental Details. Materials. o-Dichlorobenzene, hexane, THF, and poly(vinylcarbazole) were purchased from Sigma-Aldrich and stored as well as processed in an N2-filled glovebox. Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) used as hole injection layer for device 2 was provided by H. C. Starck. TPBi was purchased from Luminescence Technology Corp. and used as received. High-purity (99.99% trace metals basis) LiF was purchased from Sigma-Aldrich and thoroughly degassed in high vacuum prior to use. The synthesis of the Si QDs employed in this study is taken from the report of Henderson et al. Si QD/OLED Fabrication. Details are provided in the text. OLED Characterization. Current
voltage (IV) characteristics of the OLEDs were measured using an HP4140B pA meter in ambient air. Luminance measurements were taken using a Minolta LS-110 Luminance from the substrate side of the devices. Electroluminescence (EL) spectra were measured using an Ocean Optics USB2000 fiber spectrometer.

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bS

Supporting Information. Additional experimental detail and supporting figures. 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] and [email protected].

’ ACKNOWLEDGMENT G.A.O. and Z.L. are Government of Canada Research Chairs in Materials Chemistry and Organic Optoelectronics, respectively. D.P.P., E.J.H., and M.G.H. each hold scholarships from the Natural Sciences and Engineering Research Council (NSERC) of Canada and thus all authors thank the Council for strong and sustained support for research. ’ REFERENCES (1) Efros, A. L.; Efros, A. L. Sov. Phys. Semicond. 1982, 16, 772. (2) Brus, L. J. Phys. Chem. 1986, 90, 2555. (3) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (4) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314. (5) McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138. (6) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (7) Hines, M. A.; Scholes, G. D. Adv. Mater. 2003, 15, 1844. (8) Cho, S. J.; Maysinger, D.; Jain, M.; R€oder, B.; Hackbarth, S.; Winnik, F. M. Langmuir 2007, 23, 1974. (9) Kirchner, C.; Liedl, T.; Kudera, S.; Pellegrino, T.; Javier, A. M.; Gaub, H. E.; St€olzle, S.; Fertig, N.; Parak, W. J. Nano Lett. 2005, 5, 331. (10) Derfus, A. M.; Chan, W. C. W.; Bhatia, S. N. Nano Lett. 2004, 4, 11. (11) Baker, S. N.; Baker, G. A. Angew. Chem., Int. Ed. 2010, 49, 2. (12) Maeda, Y.; Tsukamoto, N.; Yazawa, Y.; Kanemitsu, Y.; Masumoto, Y. Appl. Phys. Lett. 1991, 59, 3168. (13) Kittel, C. Introduction to Solid State Physics, 8th ed.; Wiley: New York, 2005. (14) Shainline, J. M.; Xu, J. Laser Photonics Rev. 2007, 1, 334. (15) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (16) Lu, Z. H.; Lockwood, D. J.; Baribeau, J. M. Nature 1995, 378, 258. (17) Lockwood, D. J.; Pavesi, L. Silicon photonics; Springer-Verlag: Toronto, 2004. (18) Richter, A.; Steiner, P.; Kozlowski, F.; Lang, W. IEEE Electron Device Lett. 1991, 12, 691. (19) Langham, W.; Steiner, P.; Kozlowski, F. J. Lumin. 1993, 57, 341. (20) Liu, S. M.; Yang, Y.; Sato, S.; Kimura, K. Chem. Mater. 2006, 18, 637. (21) Jurberg, D.; Rogojina, E.; Mangolini, L.; Kortshagen, U., U. Appl. Phys. Lett. 2006, 88, 233116. (22) Zou, J.; Sanelle, P.; Pettigrew, K. A.; Kauzlarich, S. M. J. Clust. Sci. 2006, 17, 565. (23) Jambios, O.; Rinnert, H.; Devaux, X.; Vergnat, M. J. Appl. Phys. 2005, 98, 046105. (24) Lui, C.; Li, C.; Ji, A.; Ma, L.; Wang, Y.; Cao, Z. Appl. Phys. Lett. 2005, 86, 223111. (25) Cheng, K.-Y.; Anthony, R.; Kortshagen, U. W.; Holmes, R. J. Nano Lett. 2010, 10, 1154. (26) Henderson, E. J.; Kelly, J. A.; Veinot, J. G. C. Chem. Mater. 2009, 21, 5426. (27) Liu, Z. T.; Kwok, H. S.; Djurisic, A. B. J. Phys. D: Appl. Phys. 2004, 37, 678. (28) Kraut, E. A. Phys. Rev. Lett. 1980, 44, 1620. (29) van Buuren, T.; Dinh, L. N.; Chase, L. L.; Siekhaus, W. J.; Terminello, L. T. Phys. Rev. Lett. 1998, 80, 3803. 1589

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