Nanoscale Morphology Revealed at the Interface Between Colloidal

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Nanoscale Morphology Revealed at the Interface Between Colloidal Quantum Dots and Organic Semiconductor Films Matthew J. Panzer,*,† Katherine E. Aidala,‡ Polina O. Anikeeva,§ Jonathan E. Halpert,| Moungi G. Bawendi,| and Vladimir Bulovic´§ †

Department of Chemical and Biological Engineering, Tufts University, Medford, Massachusetts 02155, Department of Physics, Mount Holyoke College, South Hadley, Massachusetts 01075, and § Department of Electrical Engineering and | Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ‡

ABSTRACT The degree of interpenetration at the interface between colloidal quantum dots (QDs) and organic semiconductor molecules commonly employed in hybrid light-emitting devices (QD-LEDs) has been examined using tapping-mode atomic force microscopy. Both phase separation-driven and Contact Printing-enabled QD/semiconductor heterojunction fabrication methodologies lead to significant QD embedment in the underlying organic film with the greatest degree of QD penetration observed for QD monolayers that have been contact printed. The relative performance of QD-LEDs fabricated via three different methods using the same materials set has also been investigated. KEYWORDS Colloidal quantum dots, organic semiconductors, morphology, QD-LEDs, phase separation, contact printing

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eveloping strategies for the rational control of nanoscale morphology, especially at the interface between two dissimilar materials, is of critical importance for applications ranging from catalysis1 to solar energy conversion.2 The realization of well-defined structures at the nanometer length scale is particularly important in the development of organic thin-film optoelectronic devices, including light-emitting devices (LEDs)3 and photovoltaics/ photodetectors,4 where each constituent layer in these multilayered, highly integrated structures is typically only 10-100 nm in thickness. Recently, several groups have reported on the development of hybrid organic/colloidal quantum dot (QD) LEDs (QD-LEDs) with a device structure comprising a close-packed monolayer of QDs sandwiched between two organic semiconductor (OSC) charge transport layers and charge carrier-injecting electrodes, at least one of which is transparent.5-10 Using this type of device architecture (Figure 1), one can obtain narrowband emission across the entire visible wavelength region and an external quantum efficiency (EQE) as high as 2.7% for orangeemitting devices (peak wavelength λ ) 600 nm).10 The relative contributions of both direct charge carrier injection and Fo¨rster energy transfer to the operation of these hybrid QD-LEDs have also been examined.6,9-12 The goal of the present work is to determine whether or not nanoscale morphology at the QD/OSC interface in QD-LEDs is

influenced by the selection of a particular fabrication method. This investigation is motivated by recent experimental evidence9 and computational modeling results11 that reveal the importance of the exact positioning of the QD monolayer within a QD-LED stack on the external quantum efficiency. A specific question under investigation is to determine to what extent the cartoon commonly used to illustrate the p-OSC/QD/ n-OSC double heterojunction (Figure 1) represents the physical reality of an actual QD-LED. In this report, we demonstrate that such a simplistic picture is not, in fact, an accurate representation of the true morphology observed in QD-LED structures, and the potential impact of actual QD/OSC interfacial morphology on QD-LED performance is discussed. In the fabrication of QD-LEDs, two different methodologies are commonly utilized to incorporate a close-packed QD monolayer at the interface between the organic hole- and electron-transporting (p-OSC and n-OSC, respectively) semiconductor films. The first approach (Self-Assembly) exploits the natural tendency of QDs to phase-separate from a variety

FIGURE 1. Schematic illustration of an archetypical QD-LED (not to scale). The close-up view shows the highly idealized situation of a close-packed monolayer of quantum dots located at the interface between two organic semiconductor (OSC) charge transport layers with precisely half the volume of each QD embedded into both the hole-transporting (p-OSC) and electrontransporting (n-OSC) films.

* To whom correspondence should be addressed. E-mail: matthew.panzer@ tufts.edu. Received for review: 02/17/2010 Published on Web: 06/14/2010 © 2010 American Chemical Society

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of p-OSC molecules during the drying of a spin-coated film containing both materials.6,13 Using this technique, one can vary the QD and OSC concentrations in the spin-coating solution to obtain a close-packed QD monolayer that selfassembles at the top surface of the organic film. The second fabrication strategy (Contact Printing) employs the direct transfer of a QD monolayer from a parylene-coated polydimethylsiloxane (PDMS) stamp to the top of a predeposited OSC film.14-16 This alternative approach decouples the formation of the QD layer from that of the underlying organic layer and allows more flexibility in the selection of the semiconductor material; namely, solubility in a common solvent with the QDs is no longer required. Since Contact Printing facilitates the deposition of a dry QD monolayer, potential damage to the underlying organic film by solvent is also minimized using this fabrication technique. In this work, nanoscale morphology at the QD/p-OSC interface is probed using tapping-mode atomic force microscopy (AFM) because it is a nondestructive technique. In addition, AFM can be used to resolve subnanometer variations in topography (height) and can provide phase contrast between materials with different mechanical properties.17,18 To determine the degree to which QDs are embedded within the surrounding organic films in a QD-LED, samples consisting of a submonolayer array of close-packed QDs integrated on top of a p-OSC film are prepared on an indium tin oxide (ITO)-coated glass substrate. An incomplete monolayer is intentionally formed to measure the height of the QDs with respect to the surrounding organic film using tapping-mode AFM. Samples are prepared using both the Self-Assembly and Contact Printing deposition techniques, which are illustrated in Figure 2. The Contact Printing samples can be further classified into two subgroups, those for which the organic film is formed via spin-coating from solution (spinContact Printing), and those for which the organic film is deposited in vacuum via thermal evaporation (evap-Contact Printing). QD/p-OSC interfaces have been fabricated via SelfAssembly, spin-Contact Printing, and evap-Contact Printing using a single batch of in-house synthesized, alloyed ZnCdSe QDs10,19 in conjunction with the following p-OSC materials: N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD), N,N′-bis(3-methylphenyl)-N,N′-bis-(phenyl)-9,9-spiro-bifluorene (sp-TPD), and N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2-dimethylbenzidine (NPD). The diameter of the highly monodisperse, approximately spherical QDs used in this study (which includes the surrounding oleic acid capping ligands) is approximately 9.1 ( 0.2 nm, as determined by AFM (see Figure 3b). QD size has also been confirmed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), which show a QD diameter (without ligands) of approximately 9 ( 1 nm (Supporting Information). All p-OSC films were prepared to obtain a final organic layer thickness of 40 ( 5 nm, regardless of deposition method. Further details regarding sample preparation can be found in the Supporting Information. © 2010 American Chemical Society

FIGURE 2. (a) Spin-coating an appropriate concentration of QDs and organic semiconductor molecules from the same solution onto a substrate yields a monolayer of QDs that naturally phase separates from the underlying OSC film upon drying (Self-Assembly). (b) Photograph of a parylene-coated PDMS stamp. (c) By spin-coating QD solution onto a stamp, a monolayer QD film can first be formed on the stamp. The dry QD film can then be transferred to a predeposited OSC film on a substrate via Contact Printing. The OSC film can either be deposited by spin-coating from solution or by thermal evaporation under high vacuum.

The major result of this study is plotted in Figure 3, which shows (in panels e-h) that QDs located on top of p-OSC films commonly used in QD-LEDs are mostly embedded/depressed into the underlying organic film. In contrast, a reference sample (Figure 3, panels a-d) consisting of QDs deposited by Contact Printing on top of a nanometer-scalesmooth, template-stripped gold surface20 shows QDs sitting on top of the gold substrate in a close-packed submonolayer arrangement that is sharply defined in the AFM topography image. Assuming that QDs printed on top of the gold surface do not penetrate into the underlying gold film to a significant degree, a cross-sectional AFM linescan of this reference sample indicates a total QD diameter (including ligands) of approximately 9.1 nm (Figure 3b). This is comparable to the center-to-center distance between adjacent QDs within the islands observed in the AFM scan of Figure 3a. Comparing the submonolayer array of QDs on gold to the QDs contact printed on top of an evaporated TPD film (Figure 3, panels e-h) reveals the large extent to which QDs are embedded in the underlying organic layer. Cross-sectional AFM linescans of QD submonolayers on top of OSC films show apparent QD height protrusions of only 1 to 2 nm above the surrounding organic film. To quantify the degree to which QDs protrude above an underlying film, the following AFM image analysis method2422

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scanned clearly identifies the locations of the QDs with a flat phase background. This image is then used to create a mask that is copied to the height image. Separate histograms are created with this mask (and its inverse) and the average value of each of the height distributions is calculated, yielding the average height difference (∆h) for that image. Further details can be found in the Supporting Information. While this method, even in an ideal case, will slightly underestimate the true distance from the underlying film to the very tops of the quantum dots, the systematic offset introduced by using an average height differential will not alter the trends we observe. Figure 4 illustrates how QD/p-OSC interfacial morphology varies across the different heterojunction fabrication methods employed in this study for sp-TPD. Analogous figures for TPD and NPD are included in the Supporting Information. In general, ∆h values were observed to follow the trend: SelfAssembly > spin-Contact Printing ≈ evap-Contact Printing (except for sp-TPD, spin-Contact Printing > evap-Contact Printing). Table 1 summarizes the ∆h values obtained for the three QD/p-OSC combinations fabricated using the different methods described above. The largest ∆h value (2.5 nm, for the QD/sp-TPD interface formed via Self-Assembly) is still smaller than the QD radius (∼4.5 nm), indicating that QDs are significantly depressed into the underlying p-OSC layer in QD-LEDs. As an aside, it should be noted that an exception to this behavior has been observed with the use of a spin-coated polymeric OSC layer (poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine), poly-TPD). For QDs contact-printed on top of a poly-TPD film (spin-Contact Printing), a value of ∆h ) 6.2 nm has been observed (Supporting Information, Figure S4). The reduced degree of QD penetration into poly-TPD as compared with the oligomeric p-OSC materials TPD, sp-TPD, and NPD can most likely be attributed to the denser, entangled nature of the polymer film. The ∆h values shown in Table 1 reveal two important observations: (i) QDs are significantly embedded in an underlying p-OSC layer, which forces one to revise the idealized heterojunction cartoon shown in Figure 1; (ii) Contact Printing results in a larger degree of QD penetration compared with the Self-Assembly fabrication method. Although making quantitative height measurements using tapping-mode AFM on different samples can be a challenge,21 an effort was made to use comparable tip drive conditions and to verify that relative height differences were in agreement across multiple samples and at multiple locations within the same sample. In addition, as a result of varying the feedback loop set point between 1.3 and 0.9 V (a substantial change in the tip-sample tapping force), a QD height difference of only 0.02 nm was observed. For small molecule p-OSCs such as TPD, sp-TPD, and NPD, the amount of QD/OSC interpenetration may be partially dependent on the strength of the intermolecular interactions within the p-OSC film. One measure of the

FIGURE 3. (a) AFM topography (height) image of a close-packed, submonolayer QD film contact printed on top of a template-stripped gold substrate. (b) Cross-sectional linescan showing the substrate/ QD step edge height. (c) Three-dimensional representation of the topography data. (d) Histogram of relative pixel heights for the goldQD sample (details regarding the formation of this plot are discussed in the main text). (e-h) Corresponding figures for a submonolayer QD/evaporated TPD sample fabricated using the evap-Contact Printing method. QDs are significantly embedded into the underlying TPD layer.

ology was developed. For QDs stamped on gold (Figure 3a) a histogram can be plotted of the relative height of every pixel recorded during the AFM scan (Figure 3d). Two peaks are visible with virtually no overlap, and the average height of the gold and the average height of the QDs can easily be obtained. We define the difference between these two average heights as ∆h, a measure of the extent to which QDs protrude from the underlying film. Negative relative pixel height values in a histogram are an artifact of the AFM analysis software and do not affect the value of ∆h (which is a difference between two average pixel heights). Because the height variations within the exposed OSC films are observed to be as large as (or even larger than) those found at the QD/OSC step edges, a histogram of the full height image often does not reveal two well-defined distributions (Supporting Information, Figure S1) and a more rigorous method is required. In all cases, the phase image of the area © 2010 American Chemical Society

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FIGURE 4. AFM topography images and relative pixel height histograms for submonolayer QD/sp-TPD interfaces formed using (a) Self-Assembly, (b) spin-Contact Printing, and (c) evap-Contact Printing.

via Auger recombination),23,24 is reduced, minimizing one oftheprimaryfactorsthatlimitsQD-LEDquantumefficiency.9,10 If the embedded QD monolayer still remains relatively close to the p-OSC/n-OSC junction (only a few nanometers away), Fo¨rster energy transfer of excitons formed near the OSC heterojunction to the QDs can lead to efficient excitation of the QD film, reflected in an increased external quantum efficiency for the QD-LED.6,10 The results of the present study indicate that both of the QD/p-OSC heterojunction fabrication routes employed (Self-Assembly and Contact Printing) can facilitate the realization of an embedded QD nanoscale morphology, as desired. On the basis of the previous discussion, it follows that the further the QD layer is depressed into the p-OSC film (smaller ∆h), the greater the QD-LED efficiency one might expect. Therefore, QD-LEDs fabricated using Contact Printing should exhibit higher external quantum efficiency (EQE) as compared to QD-LEDs made via Self-Assembly, since Contact Printing results in smaller ∆h values (Table 1). Indeed, this agrees with what is observed experimentally, as shown in the plots of Figure 5 for devices made with spTPD25 and a 2,2′,2′′-(l,3,5-benzenetriyl)-tris(1-phenyl-l-Hbenzimidazole) (TPBi) n-OSC layer. However, it may be an oversimplification to ascribe the improved QD-LED performance solely due to a variation in ∆h. An additional consideration is that the mobility of charge carriers within the p-OSC layer almost certainly depends on the manner in which that film is deposited (i.e., with or without solvent present). Lower carrier mobilities in spin-coated films may result due to an unfavorable nanoscale morphology within the p-OSC layer or a higher carrier trap density, both of which can be caused by residual solvent molecules.26,27 Reduced hole mobilities in solution processed sp-TPD films may explain the higher turn-on voltages and lower current

TABLE 1. Average Height Differences (∆h) for QD/Organic Semiconductor Interfaces (nm)a semiconductor

Self-Assembly

spin-Contact Printing

evap-Contact Printing

TPD sp-TPD NPD

1.2 2.5 2.3

0.84 1.7 0.86

0.64 0.80 0.91

a

All values (0.2 nm.

cohesive forces within an OSC film is the temperature required for the onset of weight loss as measured by thermogravimetric analysis (TGA). This temperature is comparable (310-340 °C) for all three p-OSC materials studied here,22 which may partially explain the variation between ∆h values in Table 1 for a given fabrication method. Variation in p-OSC film hardness between samples may also affect ∆h values obtained via AFM. Chemical interactions between the organic ligands that coat the outside surface of the QDs (oleic acid, in this study) and the underlying p-OSC molecules may also play a role in determining the degree of QD/p-OSC penetration. While the present study is focused primarily on comparing different p-OSC materials paired with the same QDs, possible effects on morphology due to changing the QD ligands should be separately investigated. The present analysis reveals that the degree to which QDs are embedded in the underlying p-OSC film is significantly larger than 50%, which could be expected to influence the operation of QD-LEDs containing the same QD/p-OSC heterointerfaces. Recent QD-LED studies indicate that in the highest efficiency QD/OSC devices, the QD monolayer is embedded in the p-OSC approximately 5 to 10 nm away from the interface between the p-OSC and n-OSC layers.9 By placing the QD monolayer within the p-OSC layer, QD charging, which can lead to nonradiative carrier losses (e.g., © 2010 American Chemical Society

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layer is deposited in a solvent-free manner and QDs are embedded to a large extent in the underlying film. These results, together with a present understanding of the benefits of locating QDs slightly away from the p-OSC/nOSC junction in QD-LEDs, can explain why Contact Printing has become the preferred QD-LED fabrication method to realize the highest degree of performance in these devices. The phenomenon of significant QD embedment in an underlying OSC layer may also have important implications for understanding and developing other hybrid optoelectronics, including solar cells, photodetectors, and memory storage devices. Acknowledgment. This work is supported by the Institute for Soldier Nanotechnologies (DAAD-19-02-0002), a Presidential Early Career Award for Scientists and Engineers (V.B.), and the Clare Boothe Luce Foundation (K.E.A.). This work made use of MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under Award DMR-02-13282. The authors thank Darcy Wanger for her assistance with the TEM imaging. Supporting Information Available. Details regarding sample preparation, relative pixel height histogram generation using AFM data, additional AFM images and histograms, and a description of QD-LED fabrication and characterization. This material is available free of charge via the Internet at http://pubs.acs.org. FIGURE 5. (a) Current density-voltage characteristics of QD-LEDs consisting of the same device structure (ITO/PEDOT-PSS (100 nm)/ sp-TPD (40 nm)/QD monolayer/TPBi (40 nm)/Mg-Ag (100 nm)/Ag (20 nm)) fabricated using three different sp-TPD/QD heterojunction formation techniques: Self-Assembly (SA), spin-Contact Printing (sCP), and evap-Contact Printing (e-CP). Inset shows the QD-LED architecture in cross-section (not to scale). (b) External quantum efficiency (EQE) of these QD-LEDs as a function of current density. The largest EQE observed is 0.7% for the device fabricated using evap-Contact Printing.

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densities observed in the QD-LEDs fabricated via SelfAssembly and spin-Contact Printing. Thus, the best QD-LED performance is obtained using the evap-Contact Printing method, which features a small ∆h value and a solvent-free, evaporated p-OSC layer. For the ZnCdSe QDs employed in this work paired with sp-TPD as the p-OSC material, this corresponds to a peak EQE of 0.70% at a bias voltage of 3.75 V (peak wavelength λ ) 649 nm), which is comparable to the previously reported performance of QD-LEDs containing similar QDs.10 In summary, three distinct methodologies for QD-LED fabrication have been compared using various organic semiconductors and a single batch of QDs. Nanoscale morphology at the QD/p-OSC interface has been examined using tapping-mode AFM, revealing a difference in QD penetration depth that depends on the fabrication method selected, although all methods result in QD films that are significantly embedded in the underlying p-OSC layer. The highest QD-LED performance is achieved using the evap-Contact Printing method in which the organic © 2010 American Chemical Society

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