: interfaces, heterojunctions and semiconductor device engineering"

Richard Friend Cavendish Laboratory, Cambridge

Ras Al Khaimah February 22, 2009 PPV: the H H Delocalised π- prototypical H H H electrons provide semiconducting H H H H both conduction : H H H and valence bands H H H H H H

Solutions of a range of semiconducting : Polymer Light-Emitting Diodes

poly(p-phenylenevinylene) aluminium, magnesium or calcium indium/tin n oxide External Circuit

glass substrate

Burroughes et al. Nature, 347, 539 (1990), US patent 5,247,190

1992 - foundation of Cambridge Display Technology, CDT How to pattern the red, green and blue pixels: direct printing

Inkjet Deposition Process: • Polymer deposition by ink-jet printing

Direct patterning deposition Printed Polymer in Bank Holes Non-contact printing Minimum material P-OLED Display Prototypes

Full color prototype displays from 0.28” to 40” demonstrated 0.28” Micro-displays on Si Larger displays on a-Si or LTPS active matrix backplanes 13”

40” Molecules or Polymers?

Molecular semiconductors: Polymers

Single crystals – fragile! Solution processing – excellent film-forming properties Vacuum-sublimed thin films – non- crystalline structures can give Disorder inherent – limits uniform and stable structures semiconductor mobilities Stacked structures and Multilayer structures are hard to demonstration of clean make (orthogonal solvents or heterojunctions: breakthrough by cross-linking chemistry needed) Ching Tang, Kodak (1987) Novel architectures – distributed- heterojunctions good for solar cells Sony launched an ultra-thin, flat, OLED-based TV in December XEL-1 Technical specifications 2007. Called the XEL-1, the 11- Pixel resolution QHD (960H x 540V) inch OLED TV has a thickness of just 3mm. Contrast ratio 1,000,000:1

Panel size (effective 251mm x 141 mm (287 mm diagonal) picture)

Power consumption 45W (0.84W) (stand-by)

Weight 2.0Kg

Lifetime (viewing 30,000 hours (equivalent to 10 years hours) viewing at 8 hours per day) 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 TiO2 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 , Advanced Materials (August 2008) 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 Photovoltaic Charge separation at a ‘heterojunction’ diodes: between different polymer semiconductors

step 1 photon absorbed in electron Energy polymer creates electron and hole on same polymer chain

OR step 2 electron drops down to lower energy site on the other n OR OR CN polymer chain RO MEH-PPV

n Outcome of exciton at CN OR OR heterojunction = light in CN-PPV charge transfer when: ∆ Ip criterion for charge hole transfer: Halls, Cornil, Silbey et al. Phys. Eexciton < ∆Ip, ∆EA Rev. B60 5721, (1999) ‘Dispersed Interface’ Photovoltaics

‘mixed’ polymers generally Halls et al. Nature 376, 498 (1995), phase-separate due to low Yu et al. Science 270, 1789 (1995) entropy of mixing – spinodal decomposition ITO Polymer blend Al Electron acceptor

h Exciton e n h N e F8BT C H C8H17 N 8 17 S

N N

n

PFB Hole acceptor

[similar approach: Dye dispersed in TiO2 nanoparticles (Grätzel) ] 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) Charge separation at the heterojunction - what limits it? (i) photoexcitation Energy (ii) photoinduced charge transfer (ii) - fast and efficient - electron and hole are on (i) (iii) 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….. Bound charge-transfer states: ‘exciplexes’ or ‘charge-transfer’ excitons

F8BT – PFB heterojunctions: These systems can have sufficient π- electron wavefunction Energy overlap across the heterojunction to see

∆EA luminescence direct Exciton to the ground state Charge-transfer character gives n rise to: C H N N F8BT C8H17 8 17 S •red-shift PFB •longer ∆Ip radiative decay time Arne Morteani, Carlos Silva, Exciplexes in PFB:F8BT Adv. Mater. 15 1708 (2003)

Photoluminescence: Time-resolved: F8BT and PFB < 100ps 50:50 PFB:F8BT Exciplex ~47ns

F8BT PFB

intensity (a.u.)

-

Blends from chloroform solution – little de-mixing PL Wavelength 0 Quantum chemical models: (ns) (nm) Time - exciplexes, about 100 meV lower in energy, 50 with radiative lifetime about 100 nsecs Ya-Shih Huang, David Beljonne (Mons) - ‘polaron pairs’ also about 100 meV lower in energy, with radiative lifetimes > 1 µsec Nature Materials 7, 484 (2008) Time-resolved transient absorption spectra of PFB/F8BT

Sebastian Westenhoff, Justin Hodgkiss, Ian Howard, Neil Greenham JACS 130 13653 (2008)

a) 0.01 (a) transient transmission spectra 900 ps of F8BT 0.00

(b) transient transmission for 50 % 350 fs PFB : 50 % F8BT. T/T -0.01 100% F8BT

∆ 0.004 b) [The delay times of the spectra were integrated over ±150 fs and 0.000 ±100 ps for 350 fs and 900 ps 900 ps spectra, respectively. Excitation was at 490 nm with a fluence of ~3 -0.004 350 fs 13 2 50% F8BT : 50% PFB × 10 photons/cm .]

550 600 650 700 750 800 Wavelength /nm All films prepared from chloroform solution Extending the time range:

PFB:F8BT: Long-Lived Excitations

0.0010 Mechanical delay 500 ps 532 nm 0.0005 Electronic delay Q-switched Nd 0.0000 laser at 1 kHz, electronic delay -0.0005 between this

T / T -0.0010 and 200fs ∆ pulses derived -0.0015 from 1 kHz Ti- -0.0020 sapphire laser system -0.0025 1E-12 1E-9 1E-6 1E-3 t / s Evidence for Triplet generation in F8BT:PFB polymer blends

N O a) 0.0 Ir O (a): time-resolved 7 -0.5 Blend: 5 ns transient absorption Blend: 75 ns R spectra of a PFB:F8BT

-1.0 R T/T (arb.u.)

∆ R=C H 8 17 (50 %/50 %) blend and Ir-F8BT: 75 ns N -1.5 S Ir-F8BT: 5 ns N Ir-F8BT films at 2 600 800 indicated delay times. Wavelength /nm Ir-F8BT b) (b) photoluminescence decay of the Exciplex decay exciplex as measured by time-correlated

single photon counting at 650 nm (symbols) PL intensityPL together with a monoexponential fit (line). 0.003 c) 0.002 470 nm 0.001 (c) transient absorption kinetics with magic 0.000 0 angle polarization between pump and probe

-0.001 [Triplet] ∆Τ/Τ 2200 nm x3 at wavelengths as indicated in the figure. -0.002 Excitation was at 355 nm with fluences of -0.003 625 nm 13 2 -0.004 ~5 × 10 photons/cm (at 650 nm, 775 nm, 13 2 -0.005 780 nm x1.66 and 2200 nm), and ~2.5 × 10 photons/cm -0.006 (at 475 nm). The dashed black line is the 1 10 100 Time /ns triplet density reconstructed from the global fit. F8BT photophysical model:

2.8 Singlet exciton

kS• CP 2.6 (~ 1011 s–1) Separated charges eV 2.4 Interfacial kCP• SSP Charge Pair (4 × 106 s–1) kCP•T 7 –1 2.2 (2.6 × 10 s )

kCP 6 –1 Energy / Triplet exciton (6 × 10 s ) 2.0 kT (2.9 × 106 s–1)

Ground State 0

Charge separation coordinate

See also: Ford, T. A.; Avilov, I.; Beljonne, D.; Greenham, N. C. Phys. Rev. B 2005, 71, 125212. Ohkita, H.; Cook, S.; Astuti, Y. et. al., Chem. Commun. 2006, 3939-3941. Offermans, T.; van Hal, P. A.; Meskers, S. C. J. et. al., Phys. Rev. B 2005, 72, 045213. Inkjet-Printed All- Polymer Transistors

Structure of Device: O O n Source, drain and gate Inkjet Printing S n Gate PEDOT:PSS SO3H

Insulator Semiconductor S S Spin coating Source Drain F8T2

n Glass substrate Insulator Spin coating

PVP OH

Sirringhaus, Kawase et al. Science 290, 2123 (2000) The First Inkjet Printed TFT

-1 10-6 V =-60V g -8 10-7 -6 10-7 [A] -48V d I -4 10-7 -2 10-7 0 100 250µm 0 -10 -20 -30 -40 -50 -60 V [V] ds Gate Source & Drain L=200µm, W=2mm 10 µm channel length TFTs: performance and stability:

10-5 30 I (V = -40V, V = -40V) -40V on g ds W = 10 mm 10-6 25 L = 10 µm

-7 20 10

A] W = 200 µm µ 15 -30V -8

[ 10 s I L = 10 µm 10 10-9 Current (A) 5 -20V 10-10 -10V I (V = +20V, V = -40V) 0 off g ds 0 -5 -10 -15 -20 -25 -30 -35 -40 0V -11 10 V [V] d 10-12 – Field-effect mobility µ = 0.04 0 5 106 1 107 1.5 107 2 107 2.5 107 3 107 3.5 107 4 107 cm2/Vs Number of switches – ON-OFF current ratio between – Continuous switching at 50 Hz 0V and -40V : 5 · 105 in air and ambient light – sufficient for driving A5 100PPI – No encapsulation electronic paper display – No degradation seen in device performance after 107 switch cycles – Shelf life excellent E-ink electrophoretic display with active-matrix drive 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) 2 7 Dresden Display Factory • First plastic electronics factory in the world • Best and largest plastic electronics

• Capacity 700,000 units per year expandable to 1.2M units per year • Display module cost comparable to LCD • 1st Customer Shippable Displays late 2009 28 “Our Solution: the ultimate digital reading experience – form factor & user interface”

• Developing the thinnest, lightest, large-screen device

• A simple, elegant, and intuitive user interface (UI)…finger gestures & soft- buttons

• Establish THE platform mobile content, documents & attachments

28 Field-Effect Transistor: n-type operation?

Vd

Source Drain

current flow

Semiconductor +- +- +- +- +- +- +- +- +- Vg Insulator Gate +- +- +- +- +- +- +- +- +-

p-type (Vg negative)

n-type (Vg positive) n-type operation – traditionally very hard to obtain. With BCB as gate dielectric, we can now make n-type FETs from most polymers Si Si O 10-7 Vds 30 V n-type FET from OC1C10-PPV : -8 10 O A)

µ n O ( d I 10-9 d = 300 nm Chua et al., Nature L = 25 µm 434 194 (2005) w = 2.5 mm

–5 2 10-10 electron mobility, 6x10 cm /Vs 0 10 20 30 40 50 60 hole mobility, 3x10–5 cm2/ Vs Vgs (V) Light-emitting F8BT Transistors

Camera

F8BT: annealed above TM Channel length: 20 µm Dielectric: PMMA (450 nm)

10-5

Current (A)

10-6

0 20 40 60 80 100 Gate Voltage (V) 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