nexafs “microscopy” of organic devices and related...

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NEXAFS “microscopy” oforganic devices and related systems

- Scientific opportunities- Contrast control/compositional “mapping” in real and reciprocal space

NSRRC user meeting and workshopTaipei, Taiwan, Oct. 21, 2010

H. AdeDepartment of Physics

North Carolina State Universityhttp://www.physics.ncsu.edu/stxm/

Thanks to the organizers (My first visit to NSRRC)

Financial support: DOE Office of Science, Basic Energy Science, Division of Materials Science and Engineering, National Science Foundation, Division of Materials Research


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Context: Generic Characterization NeedsSpatial resolution

Microscopy and scatteringContrast

Inherent contrast or artificial contrast (metal stains, deuteration, fluorescent labels)Cross section matched to thickness of interestCompositional mapping

Appropriate sample environmentMight require wet/solvated stateElectric fields/currentsMagnetic fields

Time resolutionStatic structure is often only half the information ♦ Magnetism in STXM and PEEM♦ XPCS with enhanced compositional contrast

Soft x-rays have a unique combination of attributes

Decision on that to build might be driven by applications and the user community


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Soft X-rays: Unique interaction

with organic materials

Scattering factorsand optical constants of

C,N, and O

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δ

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β

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Assumed density of 1 g/cm3

Assumed density of 1 g/cm3

“Natural” scattering contrast:242 |)()(|)()( EiEEEFEI βδ +∝∝

Quantitative absorption microscopy:• Beer’s Law: I=I0e-µρt

20-200 nm thick samples

Complex index of refraction: n=1-δ+iβ


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e-

hν

C 1s edge ~ 290 eVN 1s edge ~ 405 eVO 1s edge ~ 540 eV

CH2 CH2

n[ ]

[ ]C C

n

CH2C

CH3

C

O

O

CH3

n[ ]

N

H

CC

C

C

CC N C

H

C

O

C

C

C

CC C

O][n

Unoccupied Molecular Orbital

Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy“Resonant effects” are more than just elemental

Unsaturation C=C

Unsaturation C=O

Photon energy


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1895

1993

2003

1 µm

1992

1993 1996

NEXAFS microscopy XMCD microscopy

XLD microscopy XMLD microscopy Dynamics

Science 258, 972 (1992)

Science 262, 1427 (1993)

X-ray Microscopy: A short historic context!(Examples “biased” towards polymers/magnetic materials. Most applications are in these fields.)


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X-Linear Dichroism Microscopy

Pattern rotates with rotation of polarization due to radial orientation of phenyl groupsPhenyl and carbonyl groups point (on average) radiallyoutwardπ* and σ* resonances show complementary dichroism

Kevlar 149 fibers, 200 nm thick section, imaged at 285.5 eV

H. Ade and B. Hsiao, Science 262, 1427 (1993)

Photon Energy (eV)

280 285 290 295 300 305 310 315 320O

ptic

al D

ensi

ty0

1

2

3

4

5

6

7

Kevlar® 149

Kevlar® 49

N N C C

OO

HH][

n

EE

Spectra with horizontal polarization in locations indicated

A. P. Smith and H. Ade. Appl. Phys. Lett. 69, 3833 (1996)

-30°

38°

8°

Determine degree of orientational order


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Resonant Scattering/ReflectivityR-SoXS/R-SoXR

(contrast is almost as good as selective deuteration)

280 290 3000.000

0.001

0.002

0.003

0.004

0.005

β

E (eV)

PS PMMA P2VP

270 280 290 300 310

-0.003

-0.002

-0.001

0.000

0.001

0.002

δ

E (eV)

PS PMMA P2VP

Scattering factors f’ and f” (optical const. δ and β, respectively) show strong energy dependence

“Bond specific” scattering!Substantial potential as complementary tool!

NP2VPPS PMMA

R or I ∝ (Δδ2+Δβ2)

Absorption (NEXAFS)

Dispersion

270 280 290 300 310 320

1x10-6

2x10-6

3x10-6

4x10-6

5x10-6

Energy (eV)

PS-PMMA Interface PS-Vacuum Interface

(c)

Ref

lect

ivity

βδ in +−=1 222

2211

221121212 sinsin

sinsin βδθθθθ

∆+∆∝+−

==nnnnrR


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Structure/morphology Determination in

Real Space: Absorption X-ray

Microscopy

Best for NEXAFSRelatively slowLow damage

Structure/morphology Determination in

Reciprocal Space: Resonant X-ray

Scattering

Small Angle Scattering/RSoXSCoherence length larger than domains,but smaller than illuminated area

log (intensity)40

-20

-40

0

20

-40 -20 0 20 40-40

-20

0

20

40

scattering vector q (µm-1)

scat

terin

g ve

ctor

q(µ

m-1

)

informationabout

domainstatistics

Figure courtesy of/after J. Stohr


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Lets look at opportunities from perspective of scientific applications and needs

not just from the “tool” perspective


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Organic Electronics: An interesting area of applications and characterization needs

Context: Energy Security/Independence, Global Warming

organic photovoltaics (OPV)

(from Nicole Cappello, Gatech)

Flexible organic light emitting diodes (OLED)

(from Sony)

organic thin film transistors,

(from www. livescience.com.)


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Organic Photovoltaic Devices:Interfaces, morphology, domain purity, energy levels are

critically important

“Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies”, Wiley-VCH (August 25, 2008)

Bulk heterojunction device Light

What makes fullerene-based devices to successful?What are the primary shortcomings of polymer-polymer devices and how can they be overcome?

What role can soft x-ray methods play?

PCBMP3HT


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Ideal morphology is easy to depictbut hard to create

glas

sIT

OPE

DOT:

PSS

Dono

r

Acce

ptor

Cath

ode

Ideal

Model bilayer

+‐+‐+ ‐

glas

sIT

OPE

DOT:

PSS

Dono

r

Acce

ptor

Cath

ode

Ideal

Model bilayer

+‐+‐+‐+‐+ ‐+ ‐

What do we have in reality?

Size? (recall, exciton diff length ~10 nm)Percolation/interpenetration?Sharp or rough/diffuse interfaces?Wetting layers?Domain purity?


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Are donor and/or acceptor domains pure?


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Thermodynamics of blends used in organic solar cellsP3HT:PCBM 1:1 w/w

• Quantitative mapping• Diffusion constant ~ 2.5 × 10−14 m2/s. • The PCBM concentration at the crystal boundary was found to be ~19% (v/v)

Watts, B., Belcher, W. J., Thomsen, L. et al., Macromol. 42,. 8392 (2009)

PCBM

P3HT


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Miscibility in P3HT:PCBM from NEXAFS microscopy1:1 blends annealed 48 hrs, large PCBM crystals next to

“equilibrium” matrix

30

25

20

15

10

5

0

PCBM

Con

cent

ratio

n [%

]

200180160140120100Temperature [ºC]

P3HT Grade Random Middle High

300295290285Energy [ev]

1.0

0.8

0.6

0.4

0.2

Abs

orpt

ion

[OD

]

400350

UnannealedHigh MW Mixture

Data; FitPCBM: 59.7(3)%

300295290285Energy [ev]

0.6

0.5

0.4

0.3

0.2

0.1

Abs

orpt

ion

[OD

]

400350

Annealed at 180ºCHigh MW Mixture

Data FitPCBM: 13.1(3)%

10 µm

PCBM

P3HT

All grades of P3HT are partially miscible

B. Collins et al. J. Phys Chem Lett 1, 3160 (2010)

Absolute accuracy <1%


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regio-random P#HT:PCBM blends(a) 284.4 eV (PCBM π* absorption peak) (Ib) 285.7 eV (P3HT π* absorption peak) Scale bars are 100 µm.

(c) Mid RR-P3HT blend(d) High RR-P3HT blend

MDMO-PPV:PCBM blends of initial wt. ratios of (e) 1:1 (f) 1:4

9% miscibility

B. Collins et al. J. Phys Chem Lett 1, 3160 (2010)

Domains are never going to be pure implications for device physics?


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Fullerene-based devices very popular, but polymer:polymer devices are still of interest because:

Voc in P3HT:PCBM is only a small fraction, i.e. ~0.6 eV / 1.9 eV = 32%, of absorbed photon energy.

If even just 50% of this loss could be avoided, efficiency could be doubled to ~12%.This can be partially fixed by bandgap and bandoffset engineeringPrecise loss factors not completely understood

In PFB/F8BT bilayers (for example) this fraction is 1.6 eV / 2.0 eV = 80%. However, this well studied, all-polymer system has poor efficiency. ♦ WHY?♦ How can the high Voc advantage be harnessed?


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1:1 PFB:F8BT blend all-polymer solar cell model system cast from chloroform to 150 nm smallest domains to date

140 °C yields max in efficiency, but peak EQE is ~25%, IPCE still <2%

C. R. McNeill et al. Nanotechnology 19, 424015 (2008)

Effective resolution < film thicknessNeed 3D resolution, i.e. tomographyor scattering

Lateral composition maps from x-ray microscopy

280 285 290 295 300 305 310 315 3200.0

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105

1.2x105

F8BT PFB

Mas

s A

bsor

ptio

n C

o-ef

ficie

nt (

cm2/g

)

Energy (eV)

283 284 285 286 287

F8BT

NS

N

N N n

PFB

n

Donor

Acceptor

NEAFS contrast


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Resonant Soft X-ray Scattering (R-SoXS) High enough contrast to characterize PFB:F8BT blend thin films in transmission

283 284 285 286 287 2880.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7284.7

Scat

terin

g in

tens

ity (a

rb. u

nits

)

Photon energy [eV]

Scattering at 1º fixed detector angle150 nm thick film

Optimum energy shifted to below absorption peaks

contrast mostly from phase scattering

S. Swaraj,C Wang, H Yan, B Watts,J. Lüning, C. R. McNeill, and H. Ade, Nano Letters ASAP, 2010


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R-SoXS shows domains are too large or too impure1:1 PFB:F8BT blends cast from chloroform

1E-3 0.01 0.1 11E-3

0.01

0.1

1

10

100

1000

As spun 140oC annealed 160oC annealed 180oC annealed 200oC annealed

Inte

nsity

q(nm-1)Good S/R and Information content to 1 nm-1

R-SoXS284.7 eV

Small domains get more pure

Small domains disappear at 200 ºC

~7 nm feature size

Average domain much larger than exciton diffusion length poor efficiency partially explained

Pair distance distribution function P(r)

S. Swaraj,C. Wang, H Yan, B Watts,J. Lüning, C. R. McNeill, and H. Ade, Nano Letters ASAP, 2010

~250 nm

~100 nm

~85 nm~80 nm~80 nm

STXM

~260 nm

~110 nm

~89 nm~71 nm~77 nm

RSoXSDomain size

200 oC180 oC160 oC140 oCAs spun

Sample

.)sin()(2

)(0

2 ∫∞

= dqqrqqIrrPπ


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What is the role of the interfaces?

Improved exciton dissociation

Decreased charge transport/separation in mixed layer

We know that molecular mixed systems don’t work well


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Role of heterojunction interface:PFB/F8BT model bilayer

Devices made by lamination, then annealingVoc=1.6 eV (Blends have Voc of only 1.3 V)

F8BT

NS

N

N N n

PFB

n

H. Yan, S. Swaraj, C. Wang, I. Hwang, N C. Greenham, C. Goves, H. Ade. C.R. McNeill. Adv Funct Mater.ASAP

F8BT

ITO/PEDOT:PSS

PFB

Al cathode

Photocurrent with dark-current subtracted


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R-SoXR of 70nm/70nm PFB/F8BT bilayer devices

Annealing increases interface and surface widths substantially!

MC confirms that sharp interfaces are best!

Need to use maximally incompatible polymers or new processing methods!?

Bulk transport of PFB and F8BT also change

MC simulations

H. Yan, S. Swaraj, C. Wang, I. Hwang, N C. Greenham, C. Goves, H. Ade. C.R. McNeill. Adv Funct Mater.ASAP


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Summary of polymer:polymer systems:(Possibly profound implications)

Domain size in present polymer:polymer blends is still way too large

Blockcopolymers?♦ How to make sharp interfaces though?

Present manufacturing paradigm is casting from same solventBut, we need maximally incompatible polymers for sharp interfaces

Totally new processing methods♦ Orthogonal solvents?♦ Multilayers?


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Dichroism in STXM and Scattering

probing domain size and domain correlation in TFT applications


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Specific molecular orbitals are probed via x-ray photons at resonant energiesAbsorption/Scattering enhanced if photon polarization is parallel to orbital dipole moment

Contrast Mechanism in STXM and scattering

Max Contrast@ 284.2 & 285.7 eV

Min Contrast@ 293 eV

A. Salleo et. al., Advanced Materials, 22, 3812 (2010).


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Pentacene Domains

Crystalline domains with oriented molecules/orbitals withinNearby domains exhibit correlated orientation

Linearly polarized x-ray beam reveals orientation of domains if at resonance

500nm 500nm

285.7eV 293eV

Scanning Transmission X‐ray Microscopy

Zone PlateDiffractiveFocusing

OrderSorting

Aperture

Sample LinearDetector

xyz‐stage

Method

Sample courtesy of Rainer Fink

Individual domains ~100 nm, correlated domains ~1000 nm


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We are not the first to use STXM for PentaceneCharacterization

B. Brauer et. al., Chem. Mater. 22, 3693 (2010)C. Hub et al. J. Mater. Chem. 20, 4884 (2010)


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Scattering to get complete and easy domain statisticsFirst results from dichroic scattering of pentacene are promising

Ratio of linear/circular polarization scattering

2 week old data: background subtraction and photon energy dependence needs to be sorted out.


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PBTTT single and PBTTT/PMMA bilayer comparisonCollins, Gann, Yan, Ade (NCSU), Chabinyc, Cochran (UCSB)

Color scale the same for equal contrastPostannealed bilayer has the most contrast and largest domainsPost annealing the bilayer clearly affects the morphology more than annealing the PBTTT first

1μm

Preannealed pBTTTNo Annealing

1μm

Postannealed Bilayer

1μm

Pre and Post Annealed

1μm

PBTTT is popular polymeric material for TFT


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Recent developments


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Simultaneous Surface and Bulk Imaging of Polymer Blends with X-ray

Spectromicroscopy

PS:PMMA blend(PMMA matrix, PS dispersions)

C. Hub, S. Wenzel, J. Raabe, H. Ade, and R. H. Fink, Rev Sci Instrum 81, 033704 (2010)

E-yield form ~5 nm layer surface sensitive


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Simultaneous Surface and Bulk Imaging of Polymer Blends with X-ray

Spectromicroscopy

B. Watts, C R. McNeill , Macromol. Rapid Commun. 2010 ; DOI: 10.1002/marc.201000269

PFB capping layer on top of the F8BTrich phase which appeared to be pinned to the underlying PFB-rich droplets. Bottom surface showed that the F8BT-rich droplets in the PFB phase penetrate through the PFB wetting layer connecting the top and bottom surfaces. Similarly, PFB-rich droplets in the F8BTrich phase were observed to connect to both the PFB capping and wetting layers.

285 eV, Strong PFB absorptionContinuous phase is F8BTDispersion is PFB


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The challenges with organic devices

OPVs have 3D – structure5-10 nm lateral, 150 nm thickRequire compositional mapping (not just morphology) with ~ 5nm resolutionPlenty other materials systems need to be investigated ♦ P3HT:F8TBT, P3HT:N2200, PCPDTBT:PCBM, etc.♦ Some of these have very small implied structures

Need TomographyIncreased doseDamage?


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What about zone plates?X-ray optics: best resolution

Figure courtesy of C. Jacobsen

5 nm target

New data points in 2010T. Tyliszczak’s talk


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Radiation DamageTypical critical dose ~ 1000 eV/nm3

Rose criterion for detection: S/N=5(10 nm)3 PS feature has S/N of 5 for 1400 incident photons, and 253 absorbed photons in pi* peak at 285 eV

Dose= ~72 eV/nm3

Quantitation/spectra/tomography typically require >100x dose

~ critical at 10 nm for PSDose roughly ∝ 1/(∆α)

Cryo does not help!C. Jacobsen has done a lot of work in this area.

Scattering as complement

T. Coffey et al. J. Electron Spectroscopy 122 (2002) 65 –78


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The Great DilemmaSolutions?????

Organic PV are great research problem of (inter-)national importance!!There are lots of other interesting scientific problems

How do we do have maximum impact?Need higher spatial resolution and 3D information♦ Quantitative phase mapping? (less damage)♦ Combine STXM with scattering?♦ Ptychography?

The backbone of the semi-conducting polymers are much more radiation resistant than ordinary polymers, but not the side-chains

Scattering as complement!


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Outlook for Soft X-rays

Lots of great science possible (It’s also fun!)Great to see so many young scientists here

Lots of interesting new method developmentsSoftware development are required to capture information from all energies (particularly true for scattering/reflectivity)

Instruments to be developed at TPS might very well depend on the applications, not the intrinsic features of a technique.


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Thank you for your attention

Thanks to members of my group:B. Collins, B. Watts (now SLS), S. Swaraj (now Soleil),

T. Araki (now Toyota), C. Wang (now ALS), H. Yan, Z. Gu, J. Seok

and Bill Slotter, Jan Luning (while at Stanford). C. McNeill (Cambridge), A.

Hexemer, D. Kilcoyne and T. Tyliszczak, (ALS), A. Garcia, T.-Q. Nguyen, G. C. Bazan, K.E. Sohn, and E.J. Kramer (UCSB),

and others as indicated previously


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41Photon Energy / eV285 290

C OO

73O

C

O

27

vectra

CH3

CH3

O CO

O n

PC

PAR

CH3

CH3

O CO C O

O

n

CCO O

NH

NH

n

Kevlar™

CCO

OO

O (CH2)4 n

PBT

PNIC

O

O

CH2 CH3

CH3

CH3

CH2 *CO

O*

n

CCO

OO

O (CH2)2 n

PET

Photon Energy / eV285 290

PS

PBrS

SAN

n

78

Br

22

n

*

C N

m

CH2 NH CO

NHNH ** n

MDI polyurea

CH2 NH CO

ONHCO

OCH2 4* *n

MDI polyurethane

CH3

N

HN

H

C

O

n

TDI polyurea

CH3

NH

NH

CO

OC

O*

O

(CH2)4 *n

TDI polyurethane

Photon Energy / eV285 290

CH2 CC

CH3

OO

CH3(CH2)3

n PBMA

CH2 CC

CH3

OO

CH3

n PMMA

* NH

C *

O

n

On PEO

Nylon-6

EPRCH3

* n

PP *CH3

n

PE * n

Fingerpt.ppt 8 May 1998

Spectroscopy = selective contrast Dhez, Ade, and UrquhartJ. Electron Spectrosc. 128, 85 (2003)

Data: Stony Brook STXM at NSLS

NEXAFS microscopy/Resonant scattering is really only game in town for quantitative compositional analysis of soft matter at high spatial resolution

nexafs “microscopy” of organic devices and related...

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