Organic electronics: interfaces, heterojunctions and semiconductor device engineering" Richard Friend Cavendish Laboratory, Cambridge

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1 Organic electronics: interfaces, heterojunctions and semiconductor device engineering" Richard Friend Cavendish Laboratory, Cambridge Ras Al Khaimah February 22, 2009

2 PPV: the prototypical semiconducting polymer: H H H H H H H H H H H H H H H H H H Delocalised π- electrons provide both conduction and valence bands Solutions of a range of semiconducting polymers:

3 Polymer Light-Emitting Diodes indium/tin oxide poly(p-phenylenevinylene) n aluminium, magnesium or calcium External Circuit glass substrate Burroughes et al. Nature, 347, 539 (1990), US patent 5,247, foundation of Cambridge Display Technology, CDT

4 How to pattern the red, green and blue pixels: direct printing Inkjet Deposition Process: Polymer deposition by ink-jet printing Direct patterning deposition Non-contact printing Minimum material Printed Polymer in Bank Holes

5 P-OLED Display Prototypes Full color prototype displays from 0.28 to 40 demonstrated Micro-displays on Si Larger displays on a-si or LTPS active matrix backplanes 40

6 Molecules or Polymers? Molecular semiconductors: Single crystals fragile! Vacuum-sublimed thin films noncrystalline structures can give uniform and stable structures Stacked structures and demonstration of clean heterojunctions: breakthrough by Ching Tang, Kodak (1987) Polymers Solution processing excellent film-forming properties Disorder inherent limits semiconductor mobilities Multilayer structures are hard to make (orthogonal solvents or cross-linking chemistry needed) Novel architectures distributedheterojunctions good for solar cells

7 Sony launched an ultra-thin, flat, OLED-based TV in December Called the XEL-1, the 11- inch OLED TV has a thickness of just 3mm. XEL-1 Technical specifications Pixel resolution QHD (960H x 540V) Contrast ratio 1,000,000:1 Panel size (effective picture) 251mm x 141 mm (287 mm diagonal) Power consumption (stand-by) 45W (0.84W) Weight 2.0Kg Lifetime (viewing hours) 30,000 hours (equivalent to 10 years viewing at 8 hours per day)

8 Recent work in Cambridge: Metal oxide charge transport layers in polymer LEDs Thin-film and nanostructured metal oxides much studied for e.g. photovoltaic diodes. ZnO and TiO 2 provide wide band gap and high refractive index Deposition via sol-gel, spray coating etc. + thermolysis LEDs with efficiencies above 2 cd/a Much reduced need for encapsulation Dinesh Kabra, Myoung Hoon Song, Bernard Wenger, Henry Snaith and Richard Friend, Advanced Materials (August 2008)

9 Device Engineering: the polymer-polymer heterojunction device architectures excitons bound at heterojunction Molecular semiconductors and excitons: Dielectric constants are low (typically about 3) so the Coulomb interaction between electron and hole is poorly screened. Excitons are strongly localized and the exciton binding energy of order 0.5 ev

10 Photovoltaic diodes: electron Energy Charge separation at a heterojunction between different polymer semiconductors step 1 photon absorbed in polymer creates electron and hole on same polymer chain RO OR n MEH-PPV OR OR CN step 2 electron drops down to lower energy site on the other polymer chain light in OR CN OR CN-PPV n Outcome of exciton at heterojunction = charge transfer when: hole Halls, Cornil, Silbey et al. Phys. Rev. B , (1999) I p criterion for charge transfer: E exciton < I p, E A

11 Dispersed Interface Photovoltaics mixed polymers generally phase-separate due to low entropy of mixing spinodal decomposition Halls et al. Nature 376, 498 (1995), Yu et al. Science 270, 1789 (1995) ITO Polymer blend Al Electron acceptor h Exciton e e F8BT C 8 H 17 C 8 H 17 N S N n h N N n PFB Hole acceptor [similar approach: Dye dispersed in TiO 2 nanoparticles (Grätzel) ]

12 Best organic solar cells: Poly(3-hexyl thiophene) hole acceptor Fullerene electron acceptor Solar energy conversion efficiencies above 5% cf: silicon cells >20% Santa Barbara, Konarka Problem: band edge offsets are very large (1 ev), so open circuit voltage is low (less than 1V) This limits efficiency severely.. (but may be needed to avoid triplet exciton formation)

13 Charge separation at the heterojunction - what limits it? (i) Energy (ii) (iii) (i) photoexcitation (ii) photoinduced charge transfer - fast and efficient - electron and hole are on adjacent chains and are still bound by Coulomb interaction - geminate recombination is likely decay route to ground state (iii) long-range charge separation - necessary for photovoltaic operation - hard to achieve..

14 Bound charge-transfer states: exciplexes or charge-transfer excitons F8BT PFB heterojunctions: Energy E A Exciton These systems can have sufficient π- electron wavefunction overlap across the heterojunction to see luminescence direct to the ground state PFB F8BT I p C 8 H 17 C 8 H 17 N S N n Charge-transfer character gives rise to: red-shift longer radiative decay time

15 Exciplexes in PFB:F8BT Arne Morteani, Carlos Silva, Adv. Mater (2003) Photoluminescence: 50:50 PFB:F8BT PFB F8BT Blends from chloroform solution little de-mixing Time-resolved: PL-intensity (a.u.) Exciplex ~47ns F8BT and PFB < 100ps Quantum chemical models: - exciplexes, about 100 mev lower in energy, with radiative lifetime about 100 nsecs - polaron pairs also about 100 mev lower in energy, with radiative lifetimes > 1 µsec Wavelength (nm) 50 Ya-Shih Huang, David Beljonne (Mons) Nature Materials 7, 484 (2008) Time (ns) 0

16 Time-resolved transient absorption spectra of PFB/F8BT Sebastian Westenhoff, Justin Hodgkiss, Ian Howard, Neil Greenham JACS (2008) a) T/T % F8BT 900 ps 350 fs (a) transient transmission spectra of F8BT (b) transient transmission for 50 % PFB : 50 % F8BT b) % F8BT : 50% PFB 900 ps 350 fs [The delay times of the spectra were integrated over ±150 fs and ±100 ps for 350 fs and 900 ps spectra, respectively. Excitation was at 490 nm with a fluence of ~ photons/cm 2.] Wavelength /nm All films prepared from chloroform solution

17 Extending the time range: PFB:F8BT: Long-Lived Excitations T / T E-12 1E-9 1E-6 1E-3 t / s Mechanical delay Electronic delay 500 ps 532 nm Q-switched Nd laser at 1 khz, electronic delay between this and 200fs pulses derived from 1 khz Tisapphire laser system

18 Evidence for Triplet generation in F8BT:PFB polymer blends a) b) c) Τ/Τ T/T (arb.u.) PL intensity Blend: 5 ns Blend: 75 ns Ir-F8BT: 5 ns Ir-F8BT: 75 ns 600 Wavelength /nm 800 Exciplex decay 470 nm 2200 nm x3 625 nm 780 nm x Time /ns 100 [Triplet] 0 N S N Ir-F8BT 7 N R R R=C H O Ir O (a): time-resolved transient absorption spectra of a PFB:F8BT (50 %/50 %) blend and Ir-F8BT films at indicated delay times. (b) photoluminescence decay of the exciplex as measured by time-correlated single photon counting at 650 nm (symbols) together with a monoexponential fit (line). (c) transient absorption kinetics with magic angle polarization between pump and probe at wavelengths as indicated in the figure. Excitation was at 355 nm with fluences of ~ photons/cm 2 (at 650 nm, 775 nm, and 2200 nm), and ~ photons/cm 2 (at 475 nm). The dashed black line is the triplet density reconstructed from the global fit.

19 F8BT photophysical model: Energy /ev Singlet exciton k CP T ( s 1 ) k T ( s 1 ) Triplet exciton Interfacial Charge Pair k S CP (~ s 1 ) k CP ( s 1 ) Ground State k CP SSP ( s 1 ) Separated charges Charge separation coordinate See also: Ford, T. A.; Avilov, I.; Beljonne, D.; Greenham, N. C. Phys. Rev. B 2005, 71, Ohkita, H.; Cook, S.; Astuti, Y. et. al., Chem. Commun. 2006, Offermans, T.; van Hal, P. A.; Meskers, S. C. J. et. al., Phys. Rev. B 2005, 72,

20 Inkjet-Printed All- Polymer Transistors Structure of Device: Gate Source, drain and gate Inkjet Printing PEDOT:PSS O S O n SO 3 H n Insulator Source Drain Semiconductor Spin coating F8T2 S S Glass substrate Insulator Spin coating PVP OH n Sirringhaus, Kawase et al. Science 290, 2123 (2000)

21 The First Inkjet Printed TFT I d [A] V g =-60V -48V Gate 250µm Source & Drain V [V] ds L=200µm, W=2mm

22 10 µm channel length TFTs: performance and stability: W = 10 mm L = 10 µm -40V I on (V g = -40V, V ds = -40V) I s [µa] V d [V] Field-effect mobility µ = 0.04 cm 2 /Vs -40 ON-OFF current ratio between 0V and -40V : sufficient for driving A5 100PPI electronic paper display -30V -20V -10V 0V Current (A) W = 200 µm L = 10 µm Number of switches I off (V g = +20V, V ds = -40V) Continuous switching at 50 Hz in air and ambient light No encapsulation No degradation seen in device performance after 10 7 switch cycles Shelf life excellent

23 E-ink electrophoretic display with active-matrix drive

24 Active-matrix backplane for e-ink electrophoretic display Paradigm shift: Display with 600 x 800 pixels (100 dpi) or 900 x 1200 pixels (150 dpi) on flex using E Ink display media: multi-level patterning without mask alignment (needed for photolithography) active, real-time distortion correction for shape changes to substrate (PET film)

25 Dresden Display Factory First plastic electronics factory in the world Best and largest plastic electronics 2 7 Capacity 700,000 units per year expandable to 1.2M units per year Display module cost comparable to LCD 1 st Customer Shippable Displays late 2009

26 Our Solution: the ultimate digital reading experience form factor & user interface 28 Developing the thinnest, lightest, large-screen device A simple, elegant, and intuitive user interface (UI) finger gestures & softbuttons Establish THE platform mobile content, documents & attachments 28

27 Field-Effect Transistor: n-type operation? V d Source Drain Semiconductor Insulator Gate current flow Vg p-type n-type (V g negative) (V g positive)

28 n-type operation traditionally very hard to obtain. With BCB as gate dielectric, we can now make n-type FETs from most polymers V ds 30 V Si O Si n-type FET from OC 1 C 10 -PPV : O I d (µa) 10-9 d = 300 nm L = 25 µm w = 2.5 mm O n Chua et al., Nature (2005) V gs (V) electron mobility, 6x10 5 cm 2 /Vs hole mobility, 3x10 5 cm 2 / Vs

29 Light-emitting F8BT Transistors Camera F8BT: annealed above TM Channel length: 20 µm Dielectric: PMMA (450 nm) Current (A) Gate Voltage (V)

30 Acknowledgements: Photovoltaics: Chris McNeil, Neil Greenham, Jonathan Halls, Jeremy Burroughes, Richard Wilson CDT Excitons and exciplexes: Astrid Gonzales-Rabade, Justin Hodgkiss, Ian Howard, Ya-Shih Huang, Arne Morteani, Carlos Silva, Sebastian Westenhoff, Transistors: Lay Lay Chua, Peter Ho, Jana Zaumseil, Henning Sirringhaus