An annealing-free aqueous-processed anatase TiO2 compact layer for efficient planar heterojunction perovskite solar cells

Article abstract of DOI:10.1039/c7cc01104k

A facile aqueous-based fabrication scheme is developed for producing annealing-free anatase TiO₂ (AF-TiO₂) films that exhibit efficient electron transport properties in planar heterojunction perovskite solar cells (PSCs). AF-TiO₂ films are fabricated by spin coating on a substrate a colloidal solution of anatase TiO₂ nanoparticles (NPs) prepared via a low temperature hydrolytic sol-gel method. The resultant AF-TiO₂ films show low electrical resistance, high transmittance in the visible and near-infrared regions and facilitation of high-quality perovskite film formation, which can be attributed to their homogeneous surface morphology and nanocrystallinity. The AF-TiO₂ based PSCs achieve a power conversion efficiency (PCE) of 18.29 ± 0.18%, showing significant improvement compared to the control PSCs (PCE = 11.33 ± 0.32%) based on TiO₂ films made by high-temperature annealing of amorphous TiO₂ (HTA-TiO₂).

Full text of DOI:10.1039/c7cc01104k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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DOI: 10.1039/C7CC01104K
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Annealing-free aqueous-processed anatase TiO compact layer for
2
efficient planar heterojunction perovskite solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
a
a
Chengwu Yang, Mingyu Yu, Dichun Chen, Yaqing Zhou, Wei Wang, Yang Li,
*
c
*a
DOI: 10.1039/x0xx00000x
Tung-Chun Lee and Daqin Yun
www.rsc.org/
A facile aqueous-based fabrication scheme is developed for carriers to the designated electrodes. Various types of materials
producing annealing-free anatase TiO (AF-TiO ) films that exhibit have been employed in carrier transport layers, such as
efficient electron transport properties in planar heterojunction PEDOT:PSS, NiO
perovskite solar cells (PSCs). AF-TiO films are fabricated by spin tetrakis-(N,N-di-p-methoxyphenylamine)9,9 -spirobifluorene (Spiro-
coating on a substrate a colloidal solution of anatase TiO OMeTAD) as hole transport layers, and ZnO, SnO , CsCO , PCBM
nanoparticles (NPs) prepared by a low temperature hydrolytic sol- and TiO Among these ETLs,
gel method. The resultant AF-TiO films show low electrical TiO is considered a promising electron transport material
resistance, high transmittance in the visible and near-infrared because of its environmental-friendly nature, wide band gap,
2
2
'
'
x
, CuSCN, poly(triaryl amine) (PTAA) and 2,2 ,7,7 -
'
2
2
2
3
7,15-21
2
as electron transport layers.
2
2
2
2-24
regions and facilitation of high-quality perovskite film formation, high electron mobility and good stability.
which can be attributed to their homogeneous surface Recently, several vacuum deposition approaches have
morphology and nanocrystallinity. The AF-TiO based PSCs achieve been developed for fabricating low-temperature processable
a power conversion efficiency (PCE) of 18.29 ± 0.18%, showing TiO
significant improvement compared to the control PSCs (PCE = electron-beam evaporation, and magnetron sputtering.
1.33 ± 0.32%) based on TiO films made by high-temperature Compared to vacuum methods, however, solution-based
annealing of amorphous TiO (HTA-TiO
2
2
5
2
ETLs for PHJ PSCs, such as atomic layer deposition,
2
6
14
1
2
2
2
).
techniques are generally more cost-effective and scalable.
Among them, sol-gel methods are arguably the most widely
Solution-processable perovskite solar cells (PSCs) have recently
attracted significant attention as a promising candidate for
adopted routes for depositing TiO
2
films owing to ease in
Nevertheless, as-prepared
films via the sol-gel methods are generally amorphous
and therefore exhibit poor electrical conductivity, which incurs
4,11,27-29
fabrication and low cost.
TiO
1-6
making high efficiency and flexible photovoltaic devices. To
date, numerous PSCs using TiO mesoporous structures have
shown high power conversion efficiencies (PCEs) of over 20%.
2
2
7-
1
1,30
negative impacts to the performance of PSCs.
have been developed to enhance the electrical properties of
TiO films via optimising their morphology, doping and
composition; these strategies include surface modification of
Strategies
10
However, mesoporous TiO
temperature annealing process of over 450 C, which limit
their use in flexible PSCs. As an alternative to mesoporous TiO
2
layers requires a high-
o
2
2
3
1-32
PSCs, planar heterojunction (PHJ) PSCs made by low-
TiO
2
with TiCl
4
treatment, ozone (O
3
) treatment,
and
o
temperature (< 150 C) processing have been investigated
doping modification of TiO
2
film with appropriate substitution
+
2+
3+
3+
5+ 4,18,33-35
extensively due to their simple device structures and their
compatibility with flexible polymeric substrates as well as
of Li , Mg , Al , Y , and Nb .
of crystalline TiO at a low temperature is considered one of
the most direct and efficient strategies to improve the
In particular, formation
2
1
1-14
inverted device designs.
In general, the p-i-n type PHJ PSC
2
9,30
structure consists of an electron transport layer (ETL), a
perovskite absorber layer and a hole transport layer (HTL). The
ETL and the HTL are responsible for extracting photo-generated
charge carriers from the perovskite layer and transporting these
electrical characteristics for highly efficient PSCs.
context, considerable effort has been devoted to prepare TiO
nanocrystals (NCs) through low-temperature non-hydrolytic
In this
2
1
6,32,36
sol-gel method for PSC applications.
However, it remains
NCs via
a challenge to synthesise high-quality anatase TiO
2
a.
low-temperature hydrolytic sol-gel method for efficient PSCs.
Herein, we report a facile, efficient and environmental-
friendly approach for preparing ultrathin anatase TiO film via
2
College of Energy, Xiamen University, Xiamen 361002, China.
E-mail: dqyun@xmu.edu.cn
Xiamen Branch of Luoyang Ship Material Research Institute, Xiamen, 361006, China.
Department of Chemistry and Institute for Materials Discovery, University College
London (UCL), United Kingdom. E-mail: tungchun.lee@ucl.ac.uk
Electronic Supplementary Information (ESI) available: Experimental details including
b.
c.
an aqueous-based and annealing-free fabrication scheme. The
resultant annealing-free anatase TiO (AF-TiO ) films can act as
†
2
2
synthesis, experimental procedure and supporting data. See
DOI: 10.1039/x0xx00000x
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film shows good optical transparency, with transmittance
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DOI: 10.1039/C7CC01104K
greater than 85% in the entire visible region (Fig. 1b). For
2
comparison, the effects of anatase TiO film thicknesses on
transmittance performance are presented in Fig. S2.
To gain further insights into the nanoscale morphology and
nanocrystal structures, high-resolution transmission electron
microscopy (HRTEM) has been performed. The HRTEM image
(
2
Fig.2a) reveals that the TiO NPs are around 15 – 25 nm in
Fig. 1. (a) Powder X-ray diffraction pattern of AF-TiO
2
nanoparticles. Diffraction peaks of each crystal plane are
diameter. By analysing the diffraction-contrast image and the
fast-Fourier transform FFT images of selected areas, individual
crystal domains within a single NP are identified to have a
domain size of 2.5 – 5.5 nm (Fig. 2b,c), consistent with that
obtained by Scherrer peak width analysis. The selected area
labelled. (b) Transmittance spectra of AF-TiO
deposited on glass substrate. Inset showing
representative SEM image of AF-TiO deposited on a
glass/ITO substrate, exhibiting a smooth morphology.
2
films
a
a
2
an efficient electron transport layer for planar heterojunction electron diffraction (SAED) pattern (Fig.2a inset) displayed
PSCs. The nanocrystalline anatase TiO
nanoparticles (NPs) typical electron diffraction rings corresponding to (101), (004),
prepared by this low-temperature hydrolytic sol-gel method (200), and (105) lattice planes of anatase structure of TiO
can form stable nanocolloidal solutions in aqueous media, consistent with the powder XRD results (Fig. 1a). These results
enabling the preparation of a smooth film with high optical indicate that the TiO NPs indeed contain a high proportion of
transmittance. The resultant AF-TiO
film shows better electron nanocrystalline anatase phase, implying that they have a
extraction ability than the conventional TiO
films prepared in reduced number of surface defect traps and could therefore
organic solvents and treated by high-temperature annealing enhance charge extraction on the interface between the TiO
processes, abbreviated as HTA-TiO
below. Moreover, the AF- layer and the perovskite layer.
TiO compact layer was also found to facilitate the formation
To investigate the performance of the AF-TiO
of high-quality perovskite films.
A general drawback of hydrolytic sol-gel method is the cross-sectional SEM image of typical device architecture is
2
2
,
2
2
2
2
2
2
2
films as ETLs
in PHJ PSCs, a complete PSC device has been fabricated. A
amorphous nature of the resultant TiO
2
NPs and thus the shown in Fig. 3a. The electrodes of indium tin oxide (ITO) and
requirement of post-synthetic high-temperature (over 450 C) Ag were used as the bottom cathode and the top anode,
o
annealing step to induce crystallisation. In our case, low- respectively. The thicknesses of the ETL (TiO
temperature hydrolysis of titanium(IV) isopropoxide (TTIP) is OMeTAD) and the (FAPbI (MAPbCl 1-x perovskite absorber
performed via a sol-gel method in the presence of HNO
as the layer are approximately 60 nm, 260 nm, and 600 nm,
hydrolysing agent, resulting in the formation of nanocrystalline respectively. It is noted that the smooth morphology of AF-TiO
anatase TiO
procedures), consistent with other similar systems in the perovskite films (See Fig. S1, S3–S5 for details).
2
), the HTL (Spiro-
3
)
x
3
)
3
2
NPs (See ESI for detailed experimental films can facilitate the formation of highly crystalline and compact
2
3
7
literature. In this method, the fast initial nucleation step
induces the formation of small electrostatically-stabilised TiO
Fig. 3b shows the current-density-voltage (J-V) curves of the
perovskite solar cells based on the AF-TiO
2
compared to devices
2
o
NPs. The subsequent ageing step at 80 C triggers the made of the HTA-TiO
2
. The photovoltaic parameters of the
formation of small nanocrystalline anatase domains within a
o
TiO
2
NP. The mild reaction conditions (80 C, 1 atmosphere
pressure) and surfactant-free aqueous solvent systems make
this method a promising way to produce high-quality anatase
TiO
2
NPs for PSCs and other applications.
The as-synthesised anatase TiO NPs can be readily dispersed
in water to form nanocolloidal solutions that are stable for
2
-
1
months (inset of Fig. 1a, concentration = 20 mg mL ). Powder X-
ray diffraction (XRD) measurements of this sample reveal typical
diffraction peaks of anatase TiO
2
(Fig.1a). In particular,
o
o
o
o
diffraction peaks centred at 25.40 , 38.28 , 47.46 , 54.26 , and
o
6
2.68 , were assigned to (101), (004), (200), (105) and (204)
diffractions, respectively. This result verifies that the as-
synthesised TiO NPs indeed contain a significant amount of the
2
2
2
Fig. 2. (a) Bright-field HRTEM image of TiO
deposited on a carbon film. The inset shows a select area
electron diffraction of TiO NPs. Diffraction rings of the
101), (004), (200) and (105) planes are observed. (b)
Zoom-in of the region of interest denoted by the blue
square in (a). Selected adjacent nanocrystal domains are
highlighted and numbered. Domain size: 2.5 – 5.5 nm. (c)
FFT images of selected nanocrystal domains in (b),
verifying the difference in crystal orientation.
2
nanocrystals
anatase domains. Scherrer peak width analysis reveals the
average size of the nanocrystalline domains to be approximately
2
6
.1 nm. Inset of Fig. 1b shows a top-view scanning electron
microscopy (SEM) image of AF-TiO film on a glass/ITO substrate
prepared by spin coating. It was observed that AF-TiO can form
a dense, pinhole-free, and uniform continuous film compared to
that of HTA-TiO (ESI Fig. S1). Additionally, the resultant AF-TiO
(
2
2
2
2
2
| J. Name., 2012, 00, 1-3
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Table 1. Parameters of the PSCs, showing TiO
power conversion efficiency (PCE) and series resistance (R
2
layer thickness, short-circuit current density (Jsc), open-circuit voltage (Voc), fill fa
V
c
ieto
w
r
A
(
r
F
tic
F
l
)
e
,
Online
DOI: 10.1039/C7CC01104K
S
). Statistical analysis (average ± standard deviation) is based on measurement of
1
0 individual devices for both AF-TiO
2
and HTA-TiO
2
films. “Champion” refers to the measurement data of the device with the highest PCE.
-2
2
Compact layer
Annealing temperature)
Thickness*
(nm)
J
sc (mA cm )
V
oc (V)
FF (%)
PCE (%)
R
S
(Ω cm )
(
champion
average
22.41
1.10
74.87
18.50
4.49
o
AF-TiO
2
(25 C)
58 ± 2
61 ± 2
22.27 ± 0.19 1.11 ± 0.01 73.63 ± 0.86 18.29 ± 0.18 4.79 ± 0.20
18.85 0.99 62.74 11.70 11.38
18.75 ± 0.11 0.98 ± 0.01 61.47 ± 1.44 11.33 ± 0.32 12.56 ± 1.34
champion
average
o
HTA-TiO (500 C)
2
*
Thickness of the TiO
2
films was measured by a surface profilometer, consistent with the thickness determined by SEM.
light on the development of highly efficient and flexible PSCs
without relying on mesoporous TiO structures.
2
Typical external quantum efficiency (EQE) data of the
devices was shown in Fig. 3c. The integrated Jsc of the devices
-
2
-2
calculated from EQE data was 21.63 mA cm and 17.45 mA cm
for the AF-TiO and HTA-TiO , respectively, which match well
2
2
with the J-V measurement data (Table 1). The enhancement of
EQE from 500 nm to 800 nm can be attributed to the smaller
R
s
and the improved morphology and crystallisation of the
perovskite film facilitated by the AF-TiO ETL (Fig. S1, S3–S5).
The J-V curves and parameters of the AF-TiO device
2
2
measured by reverse and forward scan directions were shown
in Fig. 3d and Table S2. Notably, the device does not exhibit
distinct hysteresis, which is an indicator of inefficient charge
Fig. 3. (a) Cross-section SEM image of a completed PSC
device, with the structure of glass/ITO/TiO /(FAPbI
MAPbCl 1-x/Spiro-OMeTAD/Ag. (b) J-V curves of typical
devices using AF-TiO and HTA-TiO as an ETL. The
measurements were performed under simulated AM 1.5 G
transfer at the TiO
2
/perovskite interface, ferroelectric effects,
2
3 x
)
ionic displacement and the trapping/detrapping of charge
3
8-41
(
3
)
carrier at the perovskite interface.
performance of TiO ETLs, we have investigated the effects of the
ETL thickness and that of the additional low-temperature annealing
step after the deposition of anatase TiO NPs by spin coating.
In attempt to optimise the
2
2
2
-
2
sunlight 100 mW cm . (c) EQE spectra of representative
devices comprising AF-TiO and HTA-TiO as an ETL. (d)
Forward-scan (from 0 V to 1.2 V) and reverse-scan (from 1.2
V to 0 V) J-V curves of a typical AF-TiO based device.
2
2
2
Device-level measurement data based on 10 PSCs per configuration
is presented in the ESI (Fig. S2, S6–S12 and Table S1–S4).
2
Electrical impedance spectroscopy (EIS) was performed to
devices are summarised in Table 1. Data in Table 1 shows that further study interfacial charge transport property of the
1
4,42
the solar cell of HTA-TiO
2
exhibits considerably low short-circuit PSCs.
Nyquist plots of the devices were measured at a DC
current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) bias of 0 – 0.8 V in the dark and the equivalent circuit model
values. In contrast, the overall PCE, including Jsc, Voc, and FF, for the perovskite solar cells is shown in Fig. 4a (See Fig. S12 for
significantly increased when AF-TiO
We propose that the notably high PCE of PSCs based on AF-TiO
ETL is caused by the high content of nanocrystalline anatase resistance (Rrec
phase within the compact layer and the highly homogeneous of electrons at the AF-TiO
morphology, which together favour electron extraction by larger Jsc, Voc and FF for these solar cells.
reducing the rate of carrier recombination, compared to that of
In conclusion, we have developed
In particular, the dramatic difference in environmental-friendly method for fabricating nanocrystalline
) could be attributed to the presence of anatase TiO films as compact electron transport layers for PHJ
domains that enhance electron transport PSCs. TiO NPs with small (2.5 – 5.5 nm) anatase domains was
and therefore improving the electrical characteristics. As a- successfully prepared by a low temperature hydrolytic sol-gel
result, the devices of AF-TiO
exhibit a Jsc of 22.27 ± 0.19 mA cm method. The aqueous TiO nanocolloids can form via spin
a Voc of 1.11 ± 0.01, and a FF of 73.63 ± 0.86 %, corresponding coating continuous thin films that show superior electron
2
NPs was used as the ETL. details). Compared to HTA-TiO
devices exhibit significantly smaller
indicating more efficient extraction and transport
/perovskite interface, thus resulting in
2
-based devices, AF-TiO
2
based
R
s
and larger recombination
2
)
,
2
a
facile and
1
1,21,32
the HTA-TiO
series resistance (R
nanocrystalline TiO
2
.
s
2
2
2
2
2
2
2
1
,
to a PCE of 18.29 ± 0.18 %, which shows significant extraction capability, high optical transmittance and can
improvement compared to the control PSCs (PCE = 11.33 ± facilitate the formation of high-quality perovskite films. The
0
.32%) using HTA-TiO
that planar structure photovoltaic devices based on AF-TiO
indeed achieve a highly efficient power conversion, which sheds green, efficient and flexible PSCs designs and beyond.
2
films as the ETL. This result demonstrates PSC based on AF-TiO
2
achieves a PCE of 18.29 ± 0.18 %. This
can powerful and generic strategy offers a novel way towards
2
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Acknowledgements
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