Work supported in part by US Department of Energy contract DE-AC02-76SF00515
1
Effects of molecular interface modification in hybrid organic-inorganic photovoltaic
cells
a)
94305
ABSTRACT
We have systematically investigated the effects of surface modification of titania
(TiO
2
) in hybrid TiO
2
/regioregular poly(3-hexylthiophene) (P3HT) photovoltaic cells.
By employing a series of para-substituted benzoic acids with varying dipoles and a series
of multiply-substituted benzene carboxylic acids, the energy offset at the TiO
2
/polymer
interface and thus the open circuit voltage of devices can be tuned systematically by 0.25
V. Transient photovoltage measurements showed that the recombination kinetics were
dominated by charge carrier concentration in these devices and were closely associated
with the dark current. The saturated photocurrent of TiO
2
/P3HT devices exhibits more
than a two-fold enhancement when molecular modifiers with large electron affinity were
employed. The ability of modifiers to accept charge from polymers, as revealed in
photoluminescence quenching measurement with blends of polymers, was shown to be
correlated to the enhancement in device photocurrent. A planar geometry
photoluminescence quenching measurement showed that TiO
2
substrates modified by
these same molecules that accept charge quenched more excitons in regioregular P3HT
than bare TiO
2
surfaces. An exciton diffusion length in P3HT as large as 6.5 – 8.5 nm
July 2007
Department of Materials Science & Engineering, Stanford University, Stanford, CA
Chiatzun Goh, Shawn R. Scully and Michael D. McGehee
Submitted to Journal of Applied Physics
SLAC, Stanford University, Stanford, CA 94025
SLAC-PUB-12695
2
was extracted. By measuring the external quantum efficiency (EQE) of working devices,
it was found that all of the excitons that were quenched were accountable as extracted
photocurrent. EQE was effectively increased from 5% to 10 – 14% with certain surface
modifiers; consequently exciton harvesting was more than doubled. The use of
Ruthenium (II) sensitizing dyes with good exciton harvesting property coupled with
suppression of the recombination kinetics improved the efficiency of optimized bilayer
TiO
2
/P3HT devices from 0.34 % to 0.6 % under AM 1.5 solar illuminations. The
implication of this work is directly relevant to the design of nanostructured bulk
heterojunction inorganic-organic cells, in which efficient exciton harvesting and control
of the recombination kinetics are key to achieving high efficiency.
_____________________________
a) Electronic mail: mmcgehee@stanford.edu
Subject-matter keywords: photovoltaic cells, organic, hybrid organic-inorganic, poly(3-
hexylthiophene), P3HT, titania, TiO
2
, molecular, surface, interface, modification,
recombination, dipoles, protonation, dyes, bilayer, bulk heterojunctions.
3
I. INTRODUCTION
There has been tremendous development in the field of organic photovoltaic (PV)
cells
1-7
and dye-sensitized solar cells (DSSCs)
7-10
as part of a continuous effort to realize
low-cost solar cells. These excitonic solar cells
11
rely on the offset band energy
1-3
at a
donor-acceptor heterojunction to split excitons.
12,13
Due to the small exciton diffusion
length in typical organic semiconductors of 3 – 15 nm,
14-18
the most efficient organic
solar cells have been based on blending the donor acceptor phases intimately together in
so called “bulk heterojunction” structure,
2,3
such that all excitons can reach the interface
before undergoing geminate recombination. Once the excitons are split, the charge
carriers need to be collected by the external electrodes before they recombine at the
interface. In bulk heterojunction PV cells, the large interfacial area between the donor-
acceptor pair inherently increases the cross section for charge recombination and
consequently increases device dark current. There is increasing evidence of the
formation of geminate pairs upon exciton separation in some devices,
19-21
which requires
engineering of the interface energetics to improve the yield of fully separated electron
and hole carriers under operating condition. By optimizing the interface energetics, the
exciton splitting can be engineered efficient, while the back recombination of split
carriers can be suppressed. Therefore, the study and application of interface engineering
is essential towards improving excitonic solar cells performance.
Interface modification can have many effects on a PV cell. Molecular interface
modifiers (IM) can impart a dipole at the donor-acceptor interface and shift the interface
energy offset upon attachment.
22-24
Besides affecting the energy levels of the donor-
acceptor pair, the IMs’ molecular orbitals form electronic states at the interface, which
4
may block or mediate forward charge transfer or reduce back charge recombination.
Furthermore, IMs are known to passivate inorganic surface states by chemically
interacting with surface dangling bonds,
25,26
thus changing the surface energetics. By
modifying the physical and chemical properties of a surface, the IMs also affect the
interfacial interaction with a material, for example giving rise to different growth modes
or morphologies of organic semiconductors.
27,28
Lastly, IMs can act as energy acceptors
if their absorption spectrum overlaps with the photoluminescence of an emitter in close
proximity.
29
The combination of metal oxide (e.g. titania, tin oxide, zinc oxide) and conjugated
polymer is an attractive donor-acceptor pair candidate in excitonic solar cells.
30-33
In
these hybrid inorganic-organic PV cells, the inorganics can be individually structured
before the organic phase is incorporated to form bulk heterojunction PV cells.
34,35
The
flexibility to pattern these metal oxides separately makes it easier to fabricate ordered
bulk heterojunction
36-39
and conveniently allows the interface modification step to be
included before the organics are deposited. The ability to tune the interfacial properties
while keeping the same donor-acceptor materials provides for a means to systematically
study the effect of interface modification. Modification of organic-inorganic interfaces
using molecular materials is fitting, as the surface of metal oxides guarantees well-
situated reactive sites for molecular attachment. Although substantial effort has been
directed towards improving the efficiency of hybrid PV cells, the focus was on the
combination of different materials as donor-acceptor pair and how best to optimize
nanoscale phase separation,
31,32,35,38,39
while little work has been done to investigate the
donor-acceptor interface.
22,38,40
5
For this study, we modified the interface in titania (TiO
2
)/regioregular poly(3-
hexylthiophene) (P3HT) PV cells by attaching molecules to TiO
2
that bind via
carboxylate bonds. We tuned the interfacial energy level offsets by forming interfacial
dipoles with both a series of para-substituted benzoic acids (Fig. 1a) and a class of
benzene carboxylic acid molecules (Fig. 1b). The change in energy level offset resulted
in a correlated change in the dark current of the PV device and the open circuit voltage
(V
oc
). Transient photovoltage (TPV) measurements revealed the recombination kinetics
in these devices, which explains the difference in dark current. IMs with large electron
affinity tend to yield larger photocurrent by at least two-fold compared to an unmodified
cell. Photoluminescence (PL) quenching measurement of blends of IMs with P3HT
showed a clear correlation of the IM’s ability to quench excitons in P3HT and the
enhancement in device photocurrent. A planar geometry PL quenching measurement
showed that TiO
2
substrates modified by these same molecules that possess large electron
affinity quenched more excitons in P3HT than bare TiO
2
surface. This enhanced exciton
harvesting results in an extracted exciton diffusion length (L
d
) of 6.5 – 8.5 nm for P3HT.
The measured external quantum efficiency (EQE) of working devices accounted for all
the photocurrent predicted based on the number of quenched excitons, with EQE
increasing from 5% to 10 – 14% with surface modifications. We further employed a
class of red Ruthenium (II) dye molecules (Fig. 1c) with good exciton harvesting
property and concomitantly controlled the interfacial recombination kinetics to improve
the efficiency of optimized bilayer TiO
2
/P3HT devices from 0.35 % to 0.6 % under AM
1.5 solar illuminations. An amphiphilic Ru (II) dye showed great promise as an IM
