Phosphonic Acid Modification of Indium-Tin Oxide Electrodes: Combined Phosphonic Acid Modification of Indium-Tin Oxide Electrodes: Combined XPS/UPS/Contact Angle StudiesXPS/UPS/Contact Angle Studies

Sergio A. Paniagua, Peter J. Hotchkiss, Simon C. Jones, Seth R. Marder, Anoma Mudalige, F. Saneeha Marrikar, Jeanne E. Pemberton, Neal R. Armstrong

XPS Studies PM-IRRAS / Contact Angle Measurements UPS / Summary

Thrust 2: Light Sources and Organic Electronics, STA 4: Organic electronics and energy harvesting devices, Project 4.2: Organics for portable power generation

PAs used in this project:

XPS shows the benefits of OP treatment for activation towards modification.

The expected monolayers are obtained.

XPS spectra acquired without charge neutralizer:

Analysis of the modified surfaces is necessary to explain device behavior

P

FF

FF

FF

F

F

FF

FF

F

FF

FF

F

HPA ODPA FHOPA PFBPA

O

HO

OHP

O

HO

OHP

O

HO

OHP

O

HO

OH

TFBdiPA

FF

F F

PO

HOOH

PO

OH

HO

HPA DSC / OP ITO

= 4.5 eV = 4.5 eV

0.1 eV

PFBPAFHOPA TFBdiPA

= 4.9 eV = 4.5 eV

0.2 eV

= 5.1 eV

0.2 eV

ECBM

EF

EVAC

-0.7 eV -0.1 eV -0.6 eV -0.8 eV

3.0 eV

= 5.1 eV

0.2 eV

ODPA

- + - + - + - +-+ -

EVBM

relative to DSC ITO (+0.4 eV)

0.1 eV

0

10

20

30

40

50

60

70

80

DSC ITO OP ITO HPA ODPA FHOPA PFBPA TFBdiPA

Treatment

Surf

ace

Ener

gy (m

J/m

2 )

PM-IRRAS gives insight into the binding mode of the PAs to the oxide surface.

Reaction scheme proposed for the modification protocol used:

•Surface energy components for each treatment are obtained from contact angle data probing with water and hexadecane

•By changing the surface modifier, the polar component of the surface energy can be changed while keeping the dispersive component relatively the same

Surface band energy diagrams constructed from UPS data arising from the monolayer-ITO surface system. The high work function Ф of OP treatment is maintained with FHOPA and to a lesser degree with PFBPA. Significant surface dipoles are seen for the modifications with alkyl PAs and TFBdiPA.

Upper column→ Polar Lower column→ Dispersion

ODPApowder

ODPAmodifiedOP ITO

MO

OM

OM

OHO

O

OH

O

P

R

HO OOH

MO

OM

OM

OHO

O

OH

OM

O

OM

OM

OHO

O

O

O

P

RHO O

O OHO P

R

-H2O MO

OM

OM

OHO

O

O

O

O

OP

R

OH +H+

-H2O

predominant product probable byproductsome monodentate might be left

+

One of the most common bottom contact electrodes in organic PVs is indium-tin oxide (ITO), whose hydrophilic nature makes it incompatible with the usual nonpolar organic layers used as hole-transport layers, with the consequence of reduced charge collection, and device instability due to delamination of the organic layer.

ITO

Glass

N N

>100 nm

~20 nm

~40 nm

~10 nm

~100 nm

Illustration of an excitonic planar bilayer heterojunction cell.

Schematic view of a detergent/solvent “cleaned” (DSC) ITO surface; contamination reduces work function, electroactivity, and homogeneity.

Oxygen plasma (OP) treatment can be used to “activate” the ITO surface. Surface modification with phosphonic acid (PA) monolayers following this activation may result in a more favorable interaction with the hole-transport layer through better surface energy matching, and a more homogeneous contact while retaining some of the large work function increase seen in OP-ITO.

MoleculeFull monolayer coverage of corresponding thiol on Au (x1014 cm-2)

Coverage relative to full monolayer (from contact angle data)

Change in Ф (eV) relative to OP-treated ITO. Values in brackets ( ) are estimated relative to DSC-treated ITO

SD (eV) relative to OP ITO

Modifier-induced SD (eV) relative to a hypothetical dipole free OP ITO

FHOPA 3.4 0.86 0.0 (0.6) 0.2 0.6

PFBPA not found not determined -0.2 (0.4) -0.1 0.3

ODPA 5.5 0.91 -0.6 (0.0) -0.7 -0.3

HPA 5.5 0.90 -0.6 (0.0) -0.6 -0.2

TFBdiPA not found not determined -0.6 (0.0) -0.8 -0.4

HO

In

O

O

In

HO

O

In

O

In

OH

O

Sn

O

In

O

OO

O

In

O

In

O

O

Sn

O

In

O

In

O

In

O

In

Contamination

In2O3 /SnO2(ITO bulk)

"InOOH" +In2O3 /SnO2

(COx, H2O,CxHx, etc)

HO

In

O

O

In

HO

O

In

O

In

OH

O

Sn

O

In

O

OO

O

In

O

In

O

O

Sn

O

In

O

In

O

In

O

In

DSC/OP ITO

FHOPA modified DSC/OP ITO

DSC/OP ITO

FHOPA modified DSC/OP ITO

O

In

OH

O

In

O

O

In

OH

In

O

O

Sn

O

In

O

OO

O

In

O

In

O

O

Sn

O

In

O

In

O

In

O

In

O O O OHO

In

OH

O

In

O

O

In

OH

In

O

O

Sn

OH

In

OH

OO

O

In

O

In

O

O

Sn

O

In

O

In

O

In

O

In

O O O OH

Surface modification with phosphonic acids

Phosphonic Acid Modified DSC/OP ITO

TFBdiPApowder

TFBdiPA modified OP ITO

The Secondary electron edge (SEE) and Valence band maximum (VBM) of the ITO can be tuned by choosing different phosphonic acids

This research was made possible through a grant from the NSF Science and Technology Center of Materials and Devices for Information Technology Research, No. DMR - 0120967