Aldo BrillanteAlma Mater Studiorum, University of Bologna,

Department of Industrial Chemistry "Toso Montanari"

Polymorphism and phase mixing in organic semiconductors probed by Lattice Phonon Raman Microscopy

aldo.brillante@unibo.it http://www.unibo.it/docenti/aldo.brillante

OUTLINE

1) Polymorphism and OSC: problems and solutions2) Raman Mapping in bulk crystals3) Extension to thin films down to sub-ML4) Applications to actual devices

MATERIALS FOR ORGANIC ELECTRONICSMATERIALS FOR ORGANIC ELECTRONICS

What are we dealing with…………

Classical building blocks

S

S

S

S

Polymorphism is intrinsic to molecular materialsselected as best bet for organic electronics

Large molecules, with extended π conjugation kept together by weak forces.

A variety of different molecular geometry into the lattice may easily lead toconformational or packing polymorphism

This situation is ideal for the existence of many alternative crystal structures, withsmall differences in energy.

How polymorphism can be probed

- Not only XRD, GIXRD- Multidisciplinary approach

where Raman gets in

Lattice phonons probe the inter-molecular interactions and thus are very sensitive to differences in molecular packing

WHY LATTICE PHONONS?

Different crystalline polymorphs can be identified by their Ramanspectra in the region of lattice phonons (10-150 cm–1)

CONFOCAL RAMAN MICROSCOPY AND CRYSTAL STRUCTURECONFOCAL RAMAN MICROSCOPY AND CRYSTAL STRUCTURE

Lattice phonon Raman

spectroscopy

X-ray diffraction

Lattice dynamics

Lattice geometry

Lattice phonon Raman

spectroscopy

X-ray diffraction

Lattice dynamics

Lattice geometry

ExperimentalExperimental procedureprocedure

………some typical examples for organic materials:

In molecular crystals lattice phonons are the fingerprints of the crystal structure.

XX

XX

H

C

X-RAY DIFFRACTION LATTICE PHONON SPECTRA

Lattice phonon Raman spectra of the two polymorphic forms of pentacene, tetracene, α-sexithiophene and α-quaterthiophene and their corresponding crystal structures.

POLYMORPHISM in

ORGANIC ELECTRONICSis

AN ISSUE

1) Organic molecular materials commonly show polymorphism2) Carrier mobility in organic electronics depends on crystal structure3) Polymorphism may occur as phase mixing on a micro-scale4) Phase in-homogeneity acts as an intrinsic source of disorder with

detrimental effects on the charge transport properties of organic semiconductors.

Lattice phonon Raman spectra show that polymorphism may occur as phase mixing on a microscale

PENTACENE TETRACENE SEXITHIOPHENE

LT

HT

LT+HT

HT+LT

LT

HT

LT+HT

S

S

S

S

?

Phase mixing in DB-TTF

20 µm

20

40

60

80

100

120

1400 50 100 150

CrystEngComm. 10, 1899 (2008)

Phase mixing in DT-TTF(more subtle…)

Phase homogeneity cannot be taken for granted, even for well formed large crystal domains.

PHASE MIXING AND ORGANIC ELECTRONICSPHASE MIXING AND ORGANIC ELECTRONICS

HOW TO COPE WITH IT?

1) fast2) versatile3) “in situ”4) non-destructive5) spatially resolved (below 1m) 6) depth resolved7) easy mapping

1)1) Helps in discriminating crystal structures of domains of ~1Helps in discriminating crystal structures of domains of ~1m (m (and belowand below))2)2) Can be used as control technique of phase homogeneityCan be used as control technique of phase homogeneity

CONFOCAL RAMAN MAPPING

Sketch of a confocal Raman mapping experiment.

with a false color palette different phases

can be discriminated

HOW

1. Assign spectra to structures2. Scan selected crystal regions3. Identify phase mixing4. Map of structural homogeneity

WHAT

Get a visual displayof phase purity

CONFOCAL RAMAN MAPPINGCONFOCAL RAMAN MAPPING

CrystEngComm. 10, 937 (2008)

Optical Images and Raman Maps of PolymorphsOptical Images and Raman Maps of Polymorphs RESULTS

No relationship between morphology and the actual crystal phase:phase homogeneity can be efficiently monitored only from the lattice phonon confocal Raman maps.

Adv. Mater. 17, 2549 (2005), Adv. Funct. Mater. 17, 3119 (2007)

GETTING INSIDE THE BULK

getting into the bulk

450 m

25 m

7.5 m

theoreticalfield depth

LT Phase

HT Phase

LT+HT

The effect of penetration depthThe effect of penetration depth:Use confocal properties of micro-Raman to

investigate phase coexistence inside the bulk

The less stable form grows preferentially on surface/edges of the otherwise “perfect” crystal.Related examples are T6 etc.

0.15

0.10

0.05

0.00

%HT

LT Phase

HT Phase

LT+HT

getting into the bulk

450 m

25 m

7.5 m

theoreticalfield depth

1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (Raman mapping)

Fast, visual and efficient test: - quality “test” for deposition- quality test for the operating device

fromBULK CRYSTALS

toTHIN FILMS

andTEST PATTERNS FOR ELECTRONIC DEVICES

1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (confocal Raman)

Fast, visual and efficient test (quality “test” for deposition)

fromBULK CRYSTALS

toTHIN FILMS

andTEST PATTERNS FOR ELECTRONIC DEVICES

BULK

thin film(60 nm)

HT

LT

LT

LT PHASE IS CLEARLY IDENTIFIED

The presence of a lattice phonon spectrum is a proofof crystalline order.Polarized spectra can reveal orientation, if any.

furtherhints

PHASE RECOGNITION OF T6 DEPOSITED ON MICA

PHASE RECOGNITION OF T6 DEPOSITED ON Si/SiOx

aa

A crystal structure (LT phase) starts between 8 and 16 ML(20-40 nm).The initial deposition may bedisordered.

J.-F. Moulin, F. Dinelli, M. Massi, C. Albonetti, R. Kshirsagar, F. Biscarini,Nucl. Instrum. Methods Phys. Res. Sect. B 2006, 246, 122.

> 6 ML (15 nm)

“……..Upon further growth of the films, the bulk structure appearsaround 6 ML…………”

THE EXTRAORDINARY CASE OF PENTACENE

1ML (15Å) ofPentaceneon Si/SiOx

Ideal case to investigate

growth dynamics

ML growth is along c

High rate growthΦ1= 0.36 ± 0.02 Å/s

2D regime

PHASE RECOGNITION OF PENTACENE DEPOSITED ON Si/SiOx

The presence of low wavenumberphonons, indicates that we do have a lattice structure startingfrom 1ML, in agreement withGIXRD experiments.

The structure formed is rate-dependent

Relationship between morphology and lattice structure

3D 2Dintra-layerinter-layer

surface depositiongrowth

direction

Topography StructureΦ1= 0.36 Å/s Φ2= 0.033 Å/s

Phys. Rev. B 85 195308 (2012)

The bulk crystal structureinitially formed is the lessstable HT phase!

High rate growth: Φ1= 0.36 ± 0.02 Å/sLow rate growth Φ2= 0.033 ± 0.0016 Å/s

2D

3D

The structure formed is rate-dependentLowering growth rate shifts

from 2D to 3D regime

Φ1 Φ2

HT-phase

LT-phase

Calculated lattice modes dispersion relation shows:

1) There is a loss of periodicity along the direction normal to the substrate (ab plane), in a 2D-like structure

2) The more a mode depends on k (i.e. is dispersed), the higher its dependence on order and therefore dimensionality

3) The high frequency lattice phonons undergo no dispersion: they show at 1ML and in a 2D-like thin film structure.

4) The low frequency lattice phonons have high dispersion and will be washed out when correlation along c* is lost.

5) At high growth rate the spatial correlation along c* is lost

Thin film structure (TF) vs bulk structure

“Surface selected crystal structure”

“Surface induced crystal structure”

Two examples

Non-eq

eq

….a new grammar….

Pentacenequinone

six allowed lattice phonons

Z = 2 monoclinic

three allowed lattice phonons

Z = 1 surface selected structure

Perfluoropentacene

“Surface selected crystal structure”

ACS Nano 12, 10874 (2012)

1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (confocal Raman)

Fast, visual and efficient test (quality “test” for deposition)

fromBULK CRYSTALS

toTHIN FILMS

andTEST PATTERNS FOR ELECTRONIC DEVICES

x2

Raman signal is detected down to 2 monolayers

(nominal thickness ~5 nm)

T6

Topography of layer by layer growth

Unlikepentacene, no

lattice structure in the

first few ML

T6 on Si/SiOx

aa

T6 on gold

Phonon spectra vs coverageDifferent spectralevolution for differentmorphology

60 °C 90 °C

120 °C 150 °C

T6 on Au at different deposition temperatures

Relationship morphology-crystallinity: 3D growth on increasing temperature

PENTACENE on Si/SiOx

layer by layergrowth

bulk growth

1ML=15Å

1) Phase recognition in thin films (lattice phonon spectra)2) Crystal growth by ML deposition and phase homogeneity3) Control of semiconductor layer (Raman mapping) of

actual devices

Fast, visual and efficient test (quality “test” for deposition)

fromBULK CRYSTALS

toTHIN FILMS

andTEST PATTERNS FOR ELECTRONIC DEVICES

Confocal Raman mapping of devices based on T6 thin films grown on micro-fabricated test patterns

10 nm(4ML)

25 nm(10ML)

60 nm(25 ML)

The electrode-channel steps actas a template for deposition of

molecules on the substrate

Above ~20 ML the template effect given by the patterned electrodes is lost and T6 growth proceeds in a homo-epitaxial mode

“Quality test”for deposition

Adv. Funct. Mater. 17, 3119 (2007)

H4T6 (tetrahexyl-sexithiophene)/PS composite as active layer in a FET device

showspolymorphism and

phase mixing

The relative amount of the twopolymorphs for a series of devices wasrelated to the transistor efficiency.

Yellow-phase

Red-phase

050504-1Y+R

1

In the transistors where H4T6 metastable “red phase” is the largest, the device response and the charge mobility are comparable to those of sexithienyl thin films grown by high vacuum sublimation.

red

yellow

working transistor

YES NO

Langmuir 23, 2030 (2007)

Dithiophene-Tetrathiafulvalene (DT-TTF)

High-Performance Single Crystal OrganicFET based on two DT-TTF polymorphs

Device with graphite source (S) and drain (D) electrodes and SiO2 as dielectric.

Device with gold source (S) and drain (D) electrodes and Parylene C as dielectric.

2

Adv. Mater. 22, 4198 (2010)

Blend of organic source and drain electrodes withsemiconducting organic single crystal attive layer(DT-TTF) in field-effect transistors

from toluene from PhCl

3

J. Mater. Chem. 22, 16011 (2012)

From tolueneLower blending

High contact resistence

From PhClHigher blending

Low contact resistence

CONCLUSIONSCONCLUSIONSChemical purity and physical purity (phase homogeneity) are both required for optimal device response, effectively ensuring that performance parameters are due to the intrinsic properties of the material.

1. Lattice phonon confocal Raman mapping as ideal tool (fast, versatile, in situ, non-destructive) to monitor crystal phases and phase mixing in organic semiconductors.

2. Especially useful when crystal morphology cannot assist phase recognition.

3. Application to thin film yields information on crystallinity, phaserecognition and homogeneity of the deposited material.

4. Control of semiconducting layer in organic electronic devices: “quality test” of deposition.

Tommaso SalzilloElisabetta VenutiRaffaele Guido Della Valle

Former postdoc: Ivano Bilotti

Acknowledgements

Major funding: EU Integrated Project NAIMO (No NMP4-CT-2004-500355) 2004-2008EU Large Project One-P (FP7-NMP-2007-212311) 2009-2012

Alberto Girlando

UNIV. PARMA

Matteo Masino

Acknowledgements

Major funding: EU Integrated Project NAIMO (No NMP4-CT-2004-500355) 2004-2008EU Large Project One-P (FP7-NMP-2007-212311) 2009-2012

Tommaso SalzilloElisabetta VenutiRaffaele Guido Della Valle

Alberto Girlando

UNIV. PARMA

Matteo Masino

Horiba JY (Massimo Placidi)

Thanks foryour

attention