SUPPORTING INFORMATION
SUPPORTING INFORMATION Highly Efficient Quantum-Dot Light-Emitting Diodes with DNA-CTMA as A Combined Hole-Transporting and Electron-Blocking Layer Qingjiang Sun, Guru Subramanyam and Liming Dai* School of Engineering, University of Dayton, Dayton, Ohio 45469, USA. Email: [email protected] Michael Check, Angela Campbell, Rajesh Naik and James Grote Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RX, WrightPatterson AFB, Ohio 45433, USA. Yongqiang Wang Ocean NanoTech, LLC, 700 Research Center Blvd., Fayetteville, AR 72701
1. Surface Morphology of DAN-CTMA Layers Deposited by Solution Spinning and MAPLE As can be seen from the atomic force microscopy (AFM) images of DNA-CTMA shown in Figure S1a, uniform and smooth thin films can be formed by both solution-spinning and MAPLE techniques. The root-mean-square (rms) roughness is 0.76 nm for the spin-on DNA-CTMA, and is 0.86 nm for the MAPLE DNA-CTMA (Figure S1a). Both are comparable to that of a polymer film including poly-TPD. It is well known that a hydrophobic substrate surface is essential for spin-coating QDs from non-polar organic solvent (such as toluene). The nanoscale morphologies of QDs on the top of poly-TPD and DNA-CTMA layers were examined by AFM, in terms of the different polarities of the two polymers: poly-TPD is soluble in polar organic solvents and insoluble in alcohols whereas the solubility of DNA-CTMA is reversed. As can be seen from the contact-angle images (the insets of Figure S1b), both the surfaces of poly-TPD and DNA-CTMA after film-forming are hydrophobic: the contact angle for toluene is ~12o on poly-TPD, and is ~3o on DNA-CTMA. 1
As a result, relatively uniform and densely-packed thin QD layer (~2 monolayers) can be spincoated on top of both poly-TPD and DNA-CTMA layers (Figure S1b), which is very important for demonstrating high-performance QD-LEDs. A thick QD film often lead to a high operating voltage and low carrier injection efficiency due to the insufficient dot-to-dot transport.S1-S3 The rms roughness of QDs on top of both poly-TPD and DNA-CTMA is also similar (~1.6 nm, Figure S1b).
Figure S1. Tapping-mode atomic force microscopy (AFM) images and line analysis of (a) spinon DNA-CTMA (left) and MAPLE DNA-CTMA (right) thin films on ITO/PEDOT substrates, and (b) CdSe/CdS/ZnS core/shell/shell QDs on top of ITO/PEDOT/poly-TPD (left) and ITO/PEDOT/DNA-CTMA (MAPLE) (right). The insets of (b) show the contact angles of toluene on the surfaces of ITO/PEDOT/poly-TPD and ITO/PEDOT/DNA-CTMA. The scale bars are 1µm. 2
2. Thermal Annealing of the QD-LEDs The possibility for performing the high-temperature thermal annealing of QDs in lightemitting devices strictly relies on the thermal stabilities of the underlying HIL/HTLs. We thus examined the thermal stabilities of the HIL/HTLs used in QD-LEDs investigated in this study. It was well demonstratedS4 that PEDOT HIL is thermally stable up to ~250 oC. Figure S2 shows differential scanning calorimetry (DSC) of poly-TPD and PVK. As can be seen, the glasstransition temperature is ~100 oC for PVK, and is ~230 oC for poly-TPD. Since no glasstransition was observed for DNA-CTMA, the decomposition temperature of DNA-CTMA was determined by thermo-gravitational analysis (TGA) to be ~220 oC (Inset of Figure S2). From these results, we understood that poly-TPD and DNA-CTMA are thermally stable for performing the thermal annealing of QDs (up to ~200 oC) on top of them whereas PVK should be excluded due to its relatively poor thermal stability. Therefore, Device C was chosen for the thermal annealing study. Upon annealing at different temperatures, the changes of the nanoscale morphology of the QD layer on top of the ITO/PEDOT/poly-TPD/DNA-CTMA multilayer were monitored by AFM imaging. Figure S3 shows the surface features for the QD layer after having been annealed at 100 oC, 120 oC, 140 oC, 160 oC, 180 oC, and 200 oC, along the rms roughness derived from the corresponding images. As can be seen, the surface of the QD layer was initially smoothened upon annealing with the rms roughness being reduced from ~1.6 nm for the QD layer annealed at 80-100 oC to 1.2 nm annealed at 120-180 oC. However, the rms roughness of the QD layer annealed at 200 oC was found to increase up to ~1.4 nm. The observed increase in the roughness of the QD layer annealed at 200 oC is probably due to the re-aggregation of QDs induced by the excess loss (~1.58%) of surface bound ligands (Figures 4b & S3). 3
Figure S2. Differential scanning calorimetry of poly-TPD and PVK. The inset shows TGA of DNA-CTMA.
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Figure S3. Tapping-mode AFM images and line analysis of the QDs on top of the ITO/PEDOT/poly-TPD/DNA-CTMA multilayer annealed at different temperatures for 10 mins with the rms roughness derived from the corresponding images (Scale bars: 1µm), and thermogravimetric analysis of CdSe/CdS/ZnS core/shell/shell QDs on the ITO/PEDOT/polyTPD/DNA-CTMA multilayer. 5
References (S1) Hikmet, R. A. M.; Talapin D. V.; Weller, H. Study of Conduction Mechanism and Electroluminescence in CdSe/ZnS Quantum Dot Composites. J. Appl. Phys. 2003, 93, 35093514. (S2) Niu, Y.; Munro, A. M.; Cheng, Y.; Tian, Y.; Liu, M. S.; Zhao, J. L.; Bardecker, J. A.; Plante, I. J.; Ginger, D. S.; Jen, A. K. Improved Performance from Multilayer Quantum Dot Light-Emitting Diodes via Thermal Annealing of The Quantum Dot Layer. Adv. Mater. 2007, 19, 3371-3376. (S3) Sun, Q. J.; Wang, Y. A.; Li, L.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y. F. Bright, Multicoloured Light-Emitting Diodes Based on Quantum Dots. Nature Photonics 2007, 1, 717-722. S4. Carter, J. C.; Grizzi, I.; Heeks, S. K.; Lacey, D. J.; Latham, S. G.; May, P. G.; Delospanos, O. R.; Pichler, K.; Towns, C. R.; Wittmann, H. F. Operating Stability of Light-Emitting Polymer Diodes Based on Poly(p-phenylene vinylene). Appl. Phys. Lett. 1997, 71, 34-36.
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1. Surface Morphology of DAN-CTMA Layers Deposited by Solution Spinning and MAPLE As can be seen from the atomic force microscopy (AFM) images of DNA-CTMA shown in Figure S1a, uniform and smooth thin films can be formed by both solution-spinning and MAPLE techniques. The root-mean-square (rms) roughness is 0.76 nm for the spin-on DNA-CTMA, and is 0.86 nm for the MAPLE DNA-CTMA (Figure S1a). Both are comparable to that of a polymer film including poly-TPD. It is well known that a hydrophobic substrate surface is essential for spin-coating QDs from non-polar organic solvent (such as toluene). The nanoscale morphologies of QDs on the top of poly-TPD and DNA-CTMA layers were examined by AFM, in terms of the different polarities of the two polymers: poly-TPD is soluble in polar organic solvents and insoluble in alcohols whereas the solubility of DNA-CTMA is reversed. As can be seen from the contact-angle images (the insets of Figure S1b), both the surfaces of poly-TPD and DNA-CTMA after film-forming are hydrophobic: the contact angle for toluene is ~12o on poly-TPD, and is ~3o on DNA-CTMA. 1
As a result, relatively uniform and densely-packed thin QD layer (~2 monolayers) can be spincoated on top of both poly-TPD and DNA-CTMA layers (Figure S1b), which is very important for demonstrating high-performance QD-LEDs. A thick QD film often lead to a high operating voltage and low carrier injection efficiency due to the insufficient dot-to-dot transport.S1-S3 The rms roughness of QDs on top of both poly-TPD and DNA-CTMA is also similar (~1.6 nm, Figure S1b).
Figure S1. Tapping-mode atomic force microscopy (AFM) images and line analysis of (a) spinon DNA-CTMA (left) and MAPLE DNA-CTMA (right) thin films on ITO/PEDOT substrates, and (b) CdSe/CdS/ZnS core/shell/shell QDs on top of ITO/PEDOT/poly-TPD (left) and ITO/PEDOT/DNA-CTMA (MAPLE) (right). The insets of (b) show the contact angles of toluene on the surfaces of ITO/PEDOT/poly-TPD and ITO/PEDOT/DNA-CTMA. The scale bars are 1µm. 2
2. Thermal Annealing of the QD-LEDs The possibility for performing the high-temperature thermal annealing of QDs in lightemitting devices strictly relies on the thermal stabilities of the underlying HIL/HTLs. We thus examined the thermal stabilities of the HIL/HTLs used in QD-LEDs investigated in this study. It was well demonstratedS4 that PEDOT HIL is thermally stable up to ~250 oC. Figure S2 shows differential scanning calorimetry (DSC) of poly-TPD and PVK. As can be seen, the glasstransition temperature is ~100 oC for PVK, and is ~230 oC for poly-TPD. Since no glasstransition was observed for DNA-CTMA, the decomposition temperature of DNA-CTMA was determined by thermo-gravitational analysis (TGA) to be ~220 oC (Inset of Figure S2). From these results, we understood that poly-TPD and DNA-CTMA are thermally stable for performing the thermal annealing of QDs (up to ~200 oC) on top of them whereas PVK should be excluded due to its relatively poor thermal stability. Therefore, Device C was chosen for the thermal annealing study. Upon annealing at different temperatures, the changes of the nanoscale morphology of the QD layer on top of the ITO/PEDOT/poly-TPD/DNA-CTMA multilayer were monitored by AFM imaging. Figure S3 shows the surface features for the QD layer after having been annealed at 100 oC, 120 oC, 140 oC, 160 oC, 180 oC, and 200 oC, along the rms roughness derived from the corresponding images. As can be seen, the surface of the QD layer was initially smoothened upon annealing with the rms roughness being reduced from ~1.6 nm for the QD layer annealed at 80-100 oC to 1.2 nm annealed at 120-180 oC. However, the rms roughness of the QD layer annealed at 200 oC was found to increase up to ~1.4 nm. The observed increase in the roughness of the QD layer annealed at 200 oC is probably due to the re-aggregation of QDs induced by the excess loss (~1.58%) of surface bound ligands (Figures 4b & S3). 3
Figure S2. Differential scanning calorimetry of poly-TPD and PVK. The inset shows TGA of DNA-CTMA.
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Figure S3. Tapping-mode AFM images and line analysis of the QDs on top of the ITO/PEDOT/poly-TPD/DNA-CTMA multilayer annealed at different temperatures for 10 mins with the rms roughness derived from the corresponding images (Scale bars: 1µm), and thermogravimetric analysis of CdSe/CdS/ZnS core/shell/shell QDs on the ITO/PEDOT/polyTPD/DNA-CTMA multilayer. 5
References (S1) Hikmet, R. A. M.; Talapin D. V.; Weller, H. Study of Conduction Mechanism and Electroluminescence in CdSe/ZnS Quantum Dot Composites. J. Appl. Phys. 2003, 93, 35093514. (S2) Niu, Y.; Munro, A. M.; Cheng, Y.; Tian, Y.; Liu, M. S.; Zhao, J. L.; Bardecker, J. A.; Plante, I. J.; Ginger, D. S.; Jen, A. K. Improved Performance from Multilayer Quantum Dot Light-Emitting Diodes via Thermal Annealing of The Quantum Dot Layer. Adv. Mater. 2007, 19, 3371-3376. (S3) Sun, Q. J.; Wang, Y. A.; Li, L.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y. F. Bright, Multicoloured Light-Emitting Diodes Based on Quantum Dots. Nature Photonics 2007, 1, 717-722. S4. Carter, J. C.; Grizzi, I.; Heeks, S. K.; Lacey, D. J.; Latham, S. G.; May, P. G.; Delospanos, O. R.; Pichler, K.; Towns, C. R.; Wittmann, H. F. Operating Stability of Light-Emitting Polymer Diodes Based on Poly(p-phenylene vinylene). Appl. Phys. Lett. 1997, 71, 34-36.
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