Low-Temperature Solution-Processed Tin Oxide as an Alternative
Electron Transporting Layer for Efficient Perovskite Solar Cells
Weijun Ke,
†,‡
Guojia Fang,*
,†
Qin Liu,
†
Liangbin Xiong,
†
Pingli Qin,
†
Hong Tao,
†
Jing Wang,
†
Hongwei Lei,
†
Borui Li,
†
Jiawei Wan,
†
Guang Yang,
†
and Yanfa Yan*
,‡
†
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, School of Physics and Technology,
Wuhan University, Wuhan 430072, People’s Republic of China
‡
Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of
Toledo, Toledo, Ohio 43606, United States
*
S
Supporting Information
ABSTRACT: Lead halide perovskite solar cells with the
high efficiencies typically use high-temperature processed
TiO
2
as the electron transporting layers (ETLs). Here, we
demonstrate that low-temperature solution-processed
nanocrystalline SnO
2
can be an excellent alternative ETL
material for efficient perovskite solar cells. Our best-
performing planar cell using such a SnO
2
ETL has
achieved an average efficiency of 16.02%, obtained from
efficiencies measured from both reverse and forward
voltage scans. The outstanding performance of SnO
2
ETLs
is attributed to the excellent properties of nanocrystalline
SnO
2
films, such as good antireflection, suitable band edge
positions, and high electron mobility. The simple low-
temperature process is compatible with the roll-to-roll
manufacturing of low-cost perovskite solar cells on flexible
substrates.
O
rganic−inorganic lead halide perovskite solar cells have
attracted enormous attention in recent years. The power
conversion efficiency (PCE) of perovskite solar cells has rapidly
increased from 3.8% to 20.1% (certified) in just 6 years.
1−10
Such a rapid increase in efficiency is largely attributed to the
superior photovoltaic properties of lead halide perovskites, such
as the extremely high optical absorption coefficient and very
long carrier lifetime.
11−14
High-efficiency perovskite solar cells
typically use electron transporting layers (ETLs)/hole blocking
layers and hole transporting layers (HTLs)/electron blocking
layers to separate and collect photogenerated charge carriers
produced in perovskite absorbers. These layers are critical for
achieving high-efficiency cells because they prevent severe
carrier recombination at interfaces, which may dictate the open-
circuit voltages (V
oc
’s) and fill factors (FFs) of solar cells.
Perovskite solar cells without ETLs and/or HTLs have
exhibited lower efficiencies as compared to the cells with
ETLs and HTLs.
15,16
The electrical and optical properties of
ETLs and HTLs can significantly affect the performance of
perovskite solar cells. Perovskite solar cells use either regular or
inverted architectures.
17−23
So far, the record efficiency cells
have the regular architecture, in which light enters from the
ETL and compact TiO
2
is used as the ETL material. Though
the record efficiency cells use TiO
2
ETLs, the optical and
electronic properties of TiO
2
still exhibit some shortfalls,
making it not the ultimate ETL material. For example, the
electron mobility of TiO
2
is not high enough. Zhou et al.
showed that Y-doping can increase the electron mobility and
electrical conductivity of TiO
2
and therefore improve the
efficiencies for perovskite cells.
7
However, doping may not be
able to completely overcome the intrinsic low electron mobility
issue. Moreover, Snaith et al. reported that perovskite solar cells
using mesoporous TiO
2
are sensitive to ultraviolet (UV)
illumination.
24
There exist other transparent metal oxides, such
as ZnO, In
2
O
3
, and SnO
2
, that exhibit similar or even better
electrical and optical properties as compared to TiO
2
.
Especially, these oxides exhibit a much higher electron mobility
than TiO
2
.
25
Recently, Liu et al. reported that a planar
perovskite solar cell using a low-temperature solution-processed
nanoparticle (ZnO) ETL achieved a high PCE of 15.7%.
26
The
results suggest that metal oxides other than TiO
2
can be good
ETL materials for high-efficiency perovskite solar cells. SnO
2
is
a metal oxide that has not only a much higher electron mobility
but also a wider band gap than TiO
2
.
25,27
Because ETLs absorb
photons with energies higher than the band gap but do not
contribute to photocurrents, such absorptions cause only a
small current loss. Therefore, SnO
2
should lead to a smaller
ETL-induced current loss than TiO
2
. For ultra-high-efficiency
cells, every potential energy loss should be eliminated.
Moreover, SnO
2
, with a wider band gap, is more stable than
TiO
2
under UV illumination.
25
Fluorine-doped SnO
2
(FTO) is
a robust transparent conducting electrode that has been widely
used in the thin-film solar cell industry. Gelled SnO
2
nanoparticles have been used as ETLs for polymer-based
solar cells.
28
Dye-sensitized solar cells using high-temperature
prepared mesoporous SnO
2
particles coated with TiO
2
and
MgO have achieved high efficiencies.
27
However, there is no
report on efficient perovskite solar cells using SnO
2
as both
ETLs and antireflection films.
Here, we report on low-cost and low-temperature solution-
processed SnO
2
as an ETL material for achieving highly
efficient planar perovskite solar cells. The best-performing
planar cell using a SnO
2
ETL has achieved PCEs of 17.21% and
14.82% when measured under reverse and forward voltage
scans, respectively. The perovskite solar cells using SnO
2
ETLs
Received: February 23, 2015
Published: May 19, 2015
Communication
pubs.acs.org/JACS
© 2015 American Chemical Society 6730 DOI: 10.1021/jacs.5b01994
J. Am. Chem. Soc. 2015, 137, 6730−6733