Post on 11-Dec-2015
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 .