Surface Modification of Indium-Tin Oxide Electrodes Surface Modification of Indium-Tin Oxide Electrodes With Gold Nanoparticles and Its Effect on Organic With Gold Nanoparticles and Its Effect on Organic

Photovoltaic PerformancePhotovoltaic PerformanceDiogenes Placencia and Neal R. Armstrong -- University of Arizona

Overview

Oxide Problems: Issues with ITO as an oxide bottom contact arise from its surface and electrical heterogeneity. We have recently shown that this electrical activity at the oxide surface is not uniform, and ITO can be considered a “partially blocked” electrode. Several investigators have postulated that patches of substoichiometric SnxOy sites in the ITO surface region could serve as electroactive regions.

Impact on Device Performance: Series Resistance (Rs) in an organic solar cell is composed of several resistances, one of which is the charge-transfer resistance (Rs,CT). Partially blocked electroactive sites contribute to the increase in charge-transfer resistance, leading to an increase in overall series resistance, leading to the degradation of both fill factor (FF) and Voc.

VR

P

RS

n

J

Jph

JPJ

Rs = Rs,B + Rs,CT + RS,E + Rs,ITO

A New Approach to OPV Optimization: Gold nanoparticles are known to improve the catalytic efficiency of reactions such as CO oxidation, but have not been explored for their potential impact on charge transfer resistance on TCO surfaces. We have optimized the solution and gas-phase deposition of gold nanoparticles on ITO to i) minimize loss in transparency in the oxide; ii) increase the electron-transfer rates of solution probe molecules and iii) improve in planar heterojunction solar cell performance.

Methods

MOx SubstrateHAuCl4

Basic pH

Heating

Stirring

Rinse

∆

Gold-Doped MOx

+

Al BCP C60 TiOPc

100nm 10nm 40nm 18nm

N N

TiOPcTiOPc CC6060 BCPBCP

600

400

200

x103

94 92 90 88 86 84 82

4000

3600

3200

2800

3400

3200

3000

2800

Inte

nsit

y (

Cts

/s)

Pre-Annealing: mainly AuxOy and Au(OH)z species

Sn

O

O

In

HO

In

O

O

In

-O

O

Sn

O

In

O

OO

O

In In

O

Sn In

O

In

O

In

O

O

Au

OO

Au

Au Au

O

O

O

O

Au

Au

O

Au

OH

O

AuOH

O

O

AuO

O

AuO

Au

O

O

AuO

AuO

OH

O

Au

OH

HO

Au

OH

OH

Au

AuHO

OH

O

O

Au

O

Au

O

HO

Au

O

O

Au

Au

OAu

HO

O

Au

OH

OH

Au

OHOH

HO

OH

HO

HO

XPS Analysis

A sol-gel type pre-cursor is proposed as the predominate Au species at the ITO surface prior to annealing. Annealing produces reduced gold; a final

O2-plasma etch produces a mixture of metallic and oxide

species.

Organic Photovoltaic Molecules

90

88

86

84

82

80% T

ran

sm

itta

nce

900800700600500400Wavelength

Control

Au-Doped ITO

Binding Energy (eV)

Au/ITOkS = 1.2E-2 cm/sec >>kS = 8.4E-4

cm/sec

Optical Characterization

Sol-gel Precursor

Electrochemical InvestigationThe optical properties of the ITO showed little decrease in transparency after gold NP

deposition. Solution electrochemistry of dimethyl

ferrocene as a probe molecule showed that Ks can be varied

through concentration, approaching values of gold at high loading and low values at

lower loadings.

Effects on OPV PerformanceGold NP Optimization Results

-10

-8

-6

-4

-2

0

2

4

-0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9

Bias (V)

J (m

A/cm

2)

Control ITO

Optimized Time

Optimized Time + pH

Parameters Control Time Time + pH

Voc 0.62 0.62 0.64

       

Jsc (mA/cm2) -7.24 -7.28 -8.72

       

FF 0.49 0.53 0.54

       

Efficiency (%) 2.22 2.39 3.01

TiOPc/C60 planar heterojunction OPVs showed a significant

improvement in device efficiency as Au-NPs

were added to the ITO surface: Both exposure

time to the Au-precursor solution, and solution pH

during this exposure, were important in this

optimization.

Annealed:

Au°

OP-etched

Surface Composition; Optical/Electrochemical Properties

Research Support: NSF-Chemistry and NSF-STC (CMDITR); Office of Naval Research; Department of Energy/Basic Energy Sciences

0.35

0.30

0.25

0.20

0.15

0.10

0.05

Ab

sorb

an

ce (

A.U

.)

11001000900800700600500400Wavelength (nm)

Control ITO - 18nm TiOPc

Modified ITO - 18nm TiOPc

TiOPc deposited on Au-doped ITO electrodes showed an increase in

absorbance in the NIR. Phase I TiOPc (Q-band ca. 750nm) decreases in

absorbance while Phase II TiOPc ( Q-band ca. 820nm) increases and is

thought to play a role in the improved performance of the OPV

device.

Future DirectionsFocus will be placed on obtaining

HRTEM images of the NPs and subsequent increase in NP loading due to phase II Pc present on the surface. Further investigation will

include photocurrent-action analysis.

Incident Photon to Current

0

10

20

30

40

50

60

400 450 500 550 600 650 700 750 800 850 900

Wavelength (nm)

IQE (

%)

0

2

4

6

8

10

12

14

16

EQ

E (

%)

IQE

EQE

1. M. Haruta; S. Tsubota; T. Kobayashi; H. Kageyama; M. Genet; B. Delmon. Journal of Catalysis 144, 175 (1993)2. J. Guzman; B. C. Gates. Journal of the American Chemical Society 126, 2672, (2004)

1

2

ConclusionsGold NPs were found not to affect series resistance. However, it was discovered that there was a change in the phase of the TiOPc (from phase I to Phase II), which led to the increase in photocurrent and

overall increase in device performance .