Published: March 18, 2011
r
2011 American Chemical Society
2084 dx.doi.org/10.1021/cm103221a
|
Chem. Mater. 2011, 23, 2084–2090
Air Stable, Efficient Hybrid Photovoltaic Devices Based on
Poly(3-hexylthiophene) and Silicon Nanostructures
Fute Zhang,
†
Baoquan Sun,*
,†
Tao Song,
†
Xiulin Zhu,
‡
and Shuittong Lee
§
†
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM),
and
‡
Key Laboratory of Organic Synthesis of Jiangsu Province, School of Chemistry, Chemical Engineering and Materials Science,
Soochow University, 199 Ren’ai Road, Suzhou, 215123, China
§
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong
Kong, Hong Kong SAR
b
S Supporting Information
’ INTRODUCTION
Composites of conjugated polymers and inorganic nanostruc-
tures have attracted wide interest because they can combine the
desirable properties of both materials to produce efficient hybrid
photovoltaic (PV) devices.
14
Conjugated polymers are readily
processed to form films of large area, and their absorption coefficients
are large so that only thin films are required for light harvesting in PV
devices. The electronic and optical properties of inorganic nano-
structures can be tuned simply by changing their dimensional shape
and size, leading to efficient light harvesting and charge transport. The
bandgap and offset of conjugated polymers and inorganic nano-
structures are such that charges are separated across the organic/
inorganic interface. The effects of the shape of the inorganic semi-
conductor nanocrystals, choice of conjugated polymer, and fabrica-
tion process on device performance have been investigated in detail,
leading to a current state-of-the-art power conversion efficiency
(PCE) of 3.1%.
5
A multitude of PV structures have been developed
by combining conjugated polymers with various inorganic nano-
structures such as CdSe,
5,6
TiO
2
,
7,8
ZnO,
9
PbS,
10
PbSe,
11
Ge
12
and
Si
1317
to achieve highly efficient charge separation/transport.
Si nanow ire arrays (SiNWs) showing highly ordered orienta-
tion are interesting one-dimensional nanostructures with specific
optical and electrical properties.
1830
The unique structu re of
SiNWs with a large surface area to volume ratio produces efficient
light harvesting and charge separation/transport properties, which
have been used in PV devi ces.
1725,31
SiNWs offer an optimum
optical path for efficient light harvesting along the long axial
direction, while generated carriers are transported in the short
radial direction (ca. micrometers), which decreases the velocity
of charge recombination.
32
In addition, SiNWs display extended
near-infrared absorption and enhanced light trapping capabilities
over a wide range of incident angles compared with bulk mat-
erials.
19
SiNWs may allow high performance PV devices to be realized
using low-purity Si with the short minority carrier diffusion length
that is associated with thin sheet absorbers.
33
They also provide new
opportunities to improve the PCE and to reduce the cost
of PV devices. Indeed, a theoretical model has shown that, in
Received: November 9, 2010
Revised: March 6, 2011
ABSTRACT: Efficient, stable hybrid photovoltaic (PV) devices
based on poly(3-hexylthiophene) (P3HT) and silicon nanowire
arrays (SiNWs) are reported. A two-step, chlorination/methy-
lation procedure is used to convert SiH bonds into Si Cones
to reduce the velocity of charge recombination at the silicon
surface as well as achieve a favorable alignment of band-edge
energies. In addition, Pt nanodots (PtNDs) are deposited onto
the surface of the SiNWs to further tune the band-edge
alignment and passivate nonmethylated silicon sites. Methy-
lated silicon surfaces modified with PtNDs possess a favorable
internal electric field in accord with expectations based on the
electron affinity (∼3.7 eV) and net positive surface dipole measured on such surfaces by ultraviolet photoemission spectroscopy.
This attests to the degree of chemical control tha t can be exerted over the internal electric field in such systems by surface
functionalization. In concert with methyl termination and decoration with PtNDs, hybrid PV devices based on composites of SiNWs
and P3HT achieve an extern al quantum efficiency (EQE) of 76% at 800 nm and a power conversion efficiency (PCE) of 5.9% under
simulated air mass 1.5 solar irradiation at 100 mW cm
2
. Moreover, these devices exhibit stable performance for more than 1200 h.
In contrast, devices based on composites of hydrogen-terminated planar silicon and P3HT display an EQE of 0.19% at 560 nm and
PCE of 0.006%. The ∼800 times enhancement in device performance and improvement in stability are assigned to the facile
derivatization of the surface of silicon nanostructures.
KEYWORDS: silicon nanowire arrays, poly(3-hexylthiophene), photovoltaic device, surface modification, charge recombination