Electrospray technique in fabricating perovskite-based hybrid solar cells under ambient conditions

Article abstract of DOI:10.1039/c6ra27704g

In this work, we utilized the electrospray technique to deposit the solidified crystal precursors on the substrate to investigate the transition processes in forming an orthorhombic methylammonium lead iodide film for fabricating perovskite planar heterojunction solar cells. The formation of solidified crystal precursors by the electrospray technique improves the de-wetting of the perovskite film on the substrate. Judicious selection of the applied voltage in the electrospray process generates crystal precursors of the appropriate dimensions. These as-electrosprayed crystals are the solid-state reactants for the halogen exchange and they form a uniformly covered film on the substrate under suitable annealing conditions. The hybrid device prepared by the electrospray technique exhibits a power conversion efficiency of 9.3%, a short-circuit current of 19.71 mA cm^?2, an open-circuit voltage of 0.87 V, and a fill factor of 0.55. The electrospray technique with the solid-state reaction mechanism proposed in this paper would be ideal for the large-area coating of a perovskite active layer, and thus has potential for use in real mass production.

Full text of DOI:10.1039/c6ra27704g

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Electrospray technique in fabricating perovskite-
based hybrid solar cells under ambient conditions
Cite this: RSC Adv., 2017, 7, 10985
a
b
ac
Yaw-Shyan Fu, b Li-Chung Laid
*
*
Pei-Ying Lin, Yueh-Ying Chen, Tzung-Fang Guo,
and Chung-Kwang Leed
In this work, we utilized the electrospray technique to deposit the solidified crystal precursors on the
substrate to investigate the transition processes in forming an orthorhombic methylammonium lead
iodide film for fabricating perovskite planar heterojunction solar cells. The formation of solidified crystal
precursors by the electrospray technique improves the de-wetting of the perovskite film on the substrate.
Judicious selection of the applied voltage in the electrospray process generates crystal precursors of the
appropriate dimensions. These as-electrosprayed crystals are the solid-state reactants for the halogen
exchange and they form a uniformly covered film on the substrate under suitable annealing conditions.
The hybrid device prepared by the electrospray technique exhibits a power conversion efficiency of 9.3%,
a short-circuit current of 19.71 mA cmꢀ2, an open-circuit voltage of 0.87 V, and a fill factor of 0.55. The
electrospray technique with the solid-state reaction mechanism proposed in this paper would be ideal for
the large-area coating of a perovskite active layer, and thus has potential for use in real mass production.
Received 2nd December 2016
Accepted 1st February 2017
DOI: 10.1039/c6ra27704g
rsc.li/rsc-advances
organic photovoltaics (OPVs).10 Current perovskite-based solar
cells with a planar architecture have achieved a highest power
conversion efficiency (PCE) of approximately 20%.11–13
Introduction
The organic–inorganic hybrid organometal halide perovskites
have drawn much attention due to their high absorption coef-
cients, direct band gaps, and long diffusion lengths.1–5 Miya-
saka's group rst employed methylammonium lead iodide
(CH3NH3PbI3, MAPbI3) as a light sensitizer in dye-sensitized
solar cells in 2009,6 and since then researchers have made
many improvements to advance the photovoltaic parameters of
perovskite-based hybrid solar cells. Snaith and co-workers used
the mixed halide perovskite precursors CH3NH3PbI3ꢀxClx
deposited on “scaffold” Al2O3,7,8 and found that the perovskite
layer may transport electrons without n-type TiO2. Etgar et al.
Our previous work successfully demonstrated the fabrication
of efficient hybrid solar cells by the spin-coating technique to
form a perovskite layer in the PHJ conguration.10 Many recent
studies have reported processes to optimize device perfor-
mance.14–20 Some two-step approaches apply the pre-coated PbI2
to the substrate followed by a reaction with CH3NH3I (MAI)
vapor at 150 C in N2 atmosphere,14 or an inter-diffusion reac-
ꢁ
tion between MAI with PbI2 in a MAI/PbI2 double layer cong-
uration15,16 to form the perovskite layer. Other approaches apply
the mixed solvent to enhance the morphology of perovskite
lm17,18 or introduce the additives (1,8-diiodooctane, poly-
vinylpyrrolidone, NH4Cl or HI) into the precursor solution to
improve the crystallinity while casting the perovskite layer.19,20
However, during the lm forming process of the perovskite
layer, the de-wetting of the lm on the substrate causes super-
saturation of the solution at different times and positions. In
addition, the wet-coating approach may not be a suitable
process to prepare the perovskite layer on a large-area substrate,
which is needed for practical applications. Accordingly, we are
interested in developing an approach to deposit solidied
crystal precursors on the substrate at an early reaction stage and
then to convert the solid precursors into a perovskite layer
through a solid-state reaction at an elevated temperature, as
this process could be suitable for large-area coating.
reported the production of
a conductor-free mesoscopic
perovskite/TiO2 heterojunction solar cell.9 These studies indi-
cated that the perovskite may function as a compound semi-
conductor to absorb the light and transport electrons/holes in
the active layer. Based on this idea, various prototypes of planar
architecture have been developed. An earlier work reported on
the donor–acceptor contact interface for charge separation,
based on the hybrid CH3NH3PbI3 perovskite/fullerene (C60)
planar heterojunction (PHJ) with a device conguration of
aDepartment of Photonics, National Cheng Kung University, 70101 Tainan, Taiwan,
Republic of China. E-mail: guotf@mail.ncku.edu.tw
bDepartment of Greenergy, National University of Tainan, 70005 Tainan, Taiwan,
Republic of China. E-mail: ysfu@mail.nutn.edu.tw
cAdvanced Optoelectronic Technology Center, National Cheng Kung University, 70101
Tainan, Taiwan, Republic of China
dGreen Technology Research Institute CPC Corporation, 81126 Kaohsiung, Taiwan,
Solid state reactions have been applied to prepare light-
absorbing layers, such as Cu2ZnSnS4 (CZTS)21 and CuInSe2
(CIS),22 under non-vacuum conditions. We fabricated a CuInSe2
Republic of China
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thin lm via a low temperature solid state reaction from the as- substrate, the PEDOT:PSS (Baytron PVP Al 4083, ltered with
deposited individual binary precursor CuSe and InSe powders.23 0.45 mm PVDF lter) was spin-coated onto the ultraviolet ozone
It should be noted that nanocrystals are more chemically active treated glass/ITO substrate at 4000 rpm for 60 s, and then
than bulk materials in the solid-state reaction to convert the annealed in air at 150 ꢁC for 25 minutes. The perovskite
reactant into the product.24 The electrospray technique precursor was then deposited on the glass/ITO/PEDOT:PSS
produces size-selected and highly dispersive nanocrystals,25–27 under ambient conditions by electrospray deposition. The elec-
such as nanostructure oxide ceramic powders,28–30 for use in trospray setup is depicted in Fig. 1(a), and consisted of a syringe
light emitting diodes,31 fuel cells32,33 and solar cells.34,35 In other and needle (internal diameter 0.21 mm), a syringe pump
words, the electrospray method could be a suitable and easy (KdScientic, Model 200), a high positive voltage power supply
process to deposit small solidied crystals of the appropriate (You-Shang Technical Corp.), and an aluminium foil as the plate
dimensions for halide exchange, thus enabling phase trans- ground electrode. At the electrospray deposition, a syringe
formation in the solid-state reaction to prepare the perovskite attached to a syringe pump was lled with the as-prepared
active layer on the substrate.
CH3NH3PbI3ꢀxClx solution. The ow rate was set at 0.5 mL
In this paper, we apply an electrospray technique to make the hꢀ1. The electrospray started by applying a high positive voltage
solidied crystals that are then used as precursors. Later, of 20 kV at the electrode (nozzle head). The distance between the
through solid-state diffusion induced by heat treatment, the nozzle head and the ground electrode (aluminium foil) was
solid precursors convert to an orthorhombic CH3NH3PbI3 layer 9 cm. The deposition time was 80 s, and the substrate was held
for the fabrication of hybrid perovskite/C60 PHJ solar cells. We at 70 ꢁC by a hotplate. The perovskite lms were then annealed
adjust the applied voltages in the electrospray process to at 90, 110, 130 ꢁC for 60 minutes under ambient conditions.
modulate the dispersion density of the solidied crystals, which Aerward, the samples were transferred to a thermal evaporator
is the key parameter to determining the surface coverage of the to deposit the fullerene (C60) (30 nm) (>99.0%, Solenne), bath-
precursors for the subsequent solid reaction. The orthorhombic ocuproine (BCP) (10 nm) (Sigma-Aldrich), and Al electrode
CH3NH3PbI3 perovskite lm exhibits a suitable morphology for (100 nm) in sequence under a vacuum of 6 ꢂ 10ꢀ6 Torr. The
use in device fabrication. The resulting cell presents decent active area of the device was 0.06 cm2.
photovoltaic parameters, with an open-circuit voltage (VOC) of
0.87 V, a short-circuit current (JSC) of 19.71 mA cmꢀ2, and a ll
factor (FF) of 0.55, corresponding to a PCE of 9.3%. The elec-
trospray technique with the solid reaction mechanism proposed
in this paper would be ideal for the large-area coating of
Characterization
The device characteristics, the current density–voltage (J–V)
curves, were measured in a nitrogen-lled glove box using
a Keithley 2400 SourceMeter under standard 1 sun AM 1.5G
a perovskite layer for real mass production.
simulated irradiation (100 mW cmꢀ2) from a Newport 91160A
300W Solar Simulator (Class A). The light intensity for the
Experimental
Material and sample preparation
illumination can be tuned by placing a neutral density lter in
front of the device during the measurement. The scan rate was
0.1 V sꢀ1. The simulated solar irradiance was corrected by
Methyl ammonium iodide (CH3NH3I) was synthesized by
a Schott visible-color glass-ltered (KG5 color lter) Si diode
reacting 14 mL of methylamine (CH3NH2, 40 wt% in methanol,
Alfa Aesar) and 15 mL of hydroiodic acid (HI, 57 wt% in water,
Showa) in a 100 mL round-bottom ask at 0 ꢁC under nitrogen
atmosphere for 2 h with stirring. Aer the reaction, the solvent
of the solution was evaporated using a rotary evaporator and the
solvents were carefully removed at 50 ꢁC. A white colored
powder, CH3NH3I, was formed aer the reaction. The precipi-
tate was washed with diethyl ether (99.7%, Sigma-Aldrich) three
times and dried at 60 ꢁC in a vacuum oven overnight. Precursor
solution CH3NH3PbI3ꢀxClx was synthesized by mixing CH3NH3I
and lead chloride (PbCl2) (98%, anhydrous, Merck) in a 3 : 1
molar ratio with a total concentration of 100 mg mLꢀ1 dissolved
in N,N-dimethylformamide (DMF) ($99.8%, Fisher Scientic) at
ꢁ
60 C with stirring for 12 h.
Device fabrication
Prior to device fabrication, the pre-patterned glass/indium-tin-
oxide (ITO) (15 U ,ꢀ1, RITEK Corp.) was sequentially cleaned
by ultrasonic treatment in detergent, de-ionized water, acetone and
isopropyl alcohol. For the cell fabricated on a glass/ITO/poly(3,4-
crystals on a glass/ITO/PEDOT:PSS substrate by the electrospray
ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS) technique, and (b) a schematic of the formation of solidified crystals.
Fig. 1 (a) A schematic of the setup we used to prepare the solidified
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(Hamamatsu S1133). The incident photon to electron conver-
sion efficiency (IPCE) spectra were measured by a 300 W xenon
light source (Oriel), Oriel Cornerstone 130 1/8 m mono-
chromator, Keithley 2400 sourcemeter, a chopper (SR540,
Stanford Research System Inc.) and a Lock-in amplier (SR510,
Stanford Research System Inc.). The crystallographic properties
of perovskite on the glass/ITO/PEDOT:PSS substrate were
characterized by X-ray diffraction (XRD) measurements
(Simatsu, XRD6000, Japan), as recorded using a Cu Ka (l ¼
0.15418 nm) radiation source that was operated at 40 kV and
100 mA. The X-ray diffractogram was obtained at a scan rate of
0.03ꢁ sꢀ1 for 2q values between 10ꢁ and 60ꢁ. The surface
coverage of the perovskite images was obtained using an optical
microscope (Olympus BX51M, Japan) and analyzed using the
ImageJ soware package. The particle images were obtained
using a scanning electron microscopy (SEM) system (Hitachi,
4200A, Japan) operating at 10 kV.
Results and discussion
The solidied crystals were prepared on a glass/indium-tin-
oxide (ITO)/poly(3,4-ethylenedioxythiophene) poly(styrene-
sulfonate) (PEDOT:PSS) substrate by the electrospray tech-
nique described above. Since the electrostatic force overcomes
the surface tension of the spray solution at a sufficiently high
electric bias, the solution arriving at the nozzle head directly
nebulizes as a jet of liquid droplets. Eventually, these droplets
are accelerated by the electric eld toward the grounded plate
and deposited on the glass/ITO/PEDOT:PSS substrate.
Fig. 1(b) shows a schematic of the formation of solidied
crystals on the glass/ITO/PEDOT:PSS substrate. There are four
steps that occur as the droplets are accelerated toward the
grounded plate in the electrospray process: solvent evaporation,
coulombic explosion, solidication and deposition. Liquid
droplets in a jet carry charges when they leave the nozzle head.
The droplets are dispersed due to the repulsive force of the
charges on the droplets. “Solvent evaporation” then occurs and
raises the surface charge density due to a decrease in the droplet
volume and the surface area. As electrostatic force overcomes
the surface tension to reach the limit of the charge-mass ratio
(the Rayleigh limit36), the droplets further disrupt into smaller
Fig. 2 OM images of the films on glass/ITO/PEDOT:PSS substrates as
deposited by different applied voltages: (a) 9, (b) 10, (c) 14, (d) 15, (e) 16,
(f) 18, (g) 20, and (h) 22 kV.
identical and set at 100 mg Lꢀ1, 80 s, and 70 ꢁC, respectively. As
illustrated by the OM images in Fig. 2, the surface of the
substrate for the samples deposited at the biased voltages
between 9 to 14 kV is composed of many large patterns.
Presumably, the electrostatic force at these biases did not reach
the Rayleigh limit on the droplets to induce sequential
coulombic explosions, so the droplet size did not decrease. The
large droplets were thus directly deposited on the substrate,
resulting in a very rough surface morphology of the lms on the
substrate. Further increasing the applied voltages to a magni-
tude of 15–22 kV markedly improved the coverage of the solid
lms on the substrates, as depicted in Fig. 2. It should be noted
that the sample prepared using a 20 kV applied voltage is
densely covered with a solid lm, and this would thus be the
optimal condition in this study. However, the high voltage, 22
kV, has a negative effect on the deposition of the lm. As shown
in Fig. 2, there are many dark patterns on the OM image of the
sample deposited at 22 kV. At the high applied voltage the
droplets easily explode at an early stage due to the fact that
the Coulomb force is greater than the surface tension in
the droplet,37 which can lead to non-uniform deposition of the
solids on the substrate.
droplets (satellite droplets),
a process which is termed
“Coulombic explosions”. The droplets continuously go through
the above “solvent evaporation” and “Coulombic explosions”
processes several times while traveling between the nozzle head
and substrate. Finally, the evaporation of solvent precipitates
the solutes from the tiny droplets and the solidied solutes are
deposited on the substrate. The deposition of the solidied
crystals or powders is correlated with the tiny droplet sizes,
which can be modulated by the applied voltage on the nozzle
head and the distance from the substrate. Accordingly, the size-
selected and highly dispersive nano solids can be deposited on
the glass/ITO/PEDOT:PSS substrate by the electrospray process.
Fig. 2 shows the optical microscope (OM) images of the lms
on glass/ITO/PEDOT:PSS substrates as deposited by different
applied voltages. The concentration of the electrospray solu-
tion, deposition time, and the substrate temperature were all
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Fig. 3 shows the X-ray diffraction (XRD) patterns for the as- patterns, with a full width at half maximum (FWHM) of 0.22ꢁ,
deposited lm prepared at 20 kV on glass/ITO/PEDOT:PSS suggesting the relatively high crystallinity of perovskite lm,
substrates and the lms annealed at 110 ꢁC for 0, 20 and 60 and the peak at the 15.6ꢁ diffraction angle assigned to MAPbCl3
minutes. As shown in Fig. 3, the peaks at 14.1ꢁ, 28.4ꢁ, 43.2ꢁ, and disappears, as shown in the inset of Fig. 3(b). This observation
58.9ꢁ are assigned to the (110), (220), (330), and (440) planes, for the thermal treatment of orthorhombic CH3NH3PbI3
respectively, for the orthorhombic crystal structure of CH3- perovskite herein agrees with the results of previous research.41
NH3PbI3.38 For the as-deposited lm without 110 ꢁC annealing, In this study, annealing the lm prepared by electrospraying at
the peaks at 12ꢁ and 16.8ꢁ (marked with “*” sign) are assigned 110 ꢁC for 60 minutes can form an orthorhombic CH3NH3PbI3
to XRD patterns by a Pb-complex39,40 composed of Pb halide and lm on the glass/ITO/PEDOT:PSS substrate with the appropriate
the organic species. We observe a small shi in preferred crystal phases for device fabrication.
orientation (220) peaks toward the higher diffraction angles
with the d-spacing changes in Fig. 3(b), indicating lattice perovskite lms produced in this study is correlated with the
distortion by the substitution of the Cl with I as the atom radius annealing temperature. Fig. shows scanning electron
It should also be noted that surface morphology of the
4
of Cl (0.167 nm) atoms is much smaller than I (0.206 nm). The microscopy (SEM) images with a magnication of 5kꢂ for
excess Cl ions in the solid lm react with CH3NH3PbI3 crystallite perovskite lms on glass/ITP/PEDOT:PSS substrates annealed at
and form a CH3NH3PbI3:Cl intermediate at early reaction stage. 70, 90, 110, and 130 ꢁC for 60 minutes. For the sample annealed
Through the thermal treatment to anneal the lm at 110 ꢁC for at 70 ꢁC, as presented in Fig. 4(a), the SEM image shows that the
20 minutes, the magnitudes of the (100) and (200) peaks surface morphology is composed of many solid particles with
increase markedly and the peaks at 12ꢁ and 16.8ꢁ, as shown in dimensions of around 1.5–2.0 mm. This indicates that the
the XRD pattern of the as-deposited sample disappear, indi- temperature of 70 ꢁC is too low to initiate the interdiffusion
cating the transformation of precursors to form orthorhombic process of the solid to form a continuous lm on the substrate.
ꢁ
CH3NH3PbI3. The prolonged thermal treatment (110 C for 60 Fig. 4(b) presents an SEM image of the sample annealed at 90 ꢁC
minutes) further increases the relative magnitude of XRD for 60 minutes. Under this annealing condition, we observe the
growth of perovskite lm on the substrate as induced by the
diffusion of the solid at the elevated temperature, but there are
still many voids with dimensions larger than a few mm. The
number and density of voids should thus be reduced for better
coverage of the lm on the substrate. Further increasing the
temperature to 110 ꢁC markedly improves the morphology of
the perovskite lm and decreases the surface roughness, as
illustrated in Fig. 4(c). This result is correlated with XRD
measurement in Fig. 3, which shows that the lm exhibits
a good crystal structure when annealed at 110 ꢁC for 60 minutes.
However, annealing the sample at 130 ꢁC dramatically changes
the surface morphology. As shown in Fig. 4(d), the SEM image
reveals many large crystals on the substrate. The lm is discrete,
with many big voids between grains, and thus is not applicable
as an active layer for the fabrication of good photovoltaic cells.
Fig. 3 XRD patterns for the as-deposited film prepared at 20 kV on Fig. 4 SEM images of the perovskite films on glass/ITP/PEDOT:PSS
glass/ITO/PEDOT:PSS substrates and the films annealed at 110 ꢁC for substrates annealed at (a) 70, (b) 90, (c) 110, and (d) 130 ꢁC for 60
0, 20 and 60 minutes.
minutes.
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Fig. 5(a) presents the schematic device structure and the photon to electron conversion efficiency (IPCE) spectrum of the
current density–voltage (J–V) curves with forward (hollow icons) cell fabricated by electrospraying having a peak IPCE above 80%
and reverse scans (solid icons) of hybrid perovskite solar cells of at approximately 420–500 nm. The photocurrent integrated
glass/ITO/PEDOT:PSS/orthorhombic CH3NH3PbI3 perovskite/ from the overlap of the IPCE spectrum with the AM 1.5G solar
C
60/bathocuproine (BCP)/Al conguration under the standard irradiation gives a current density of 18.35 mA cmꢀ2, in which
1 sun AM 1.5G simulated solar irradiation. As plotted in Fig. 5, the integrated photocurrent is correlated with a magnitude of
the hybrid cell made of a CH3NH3PbI3ꢀxClx active layer by the JSC, as depicted in Fig. 5(a). Fig. 6 shows the cross-sectional
electrospray method reported in this work exhibits a VOC of image of the hybrid device in this study. We observe
0.87 V, a JSC of 19.71 mA cmꢀ2, and an FF of 0.55, corresponding a compact perovskite active layer with a thickness of approxi-
to a PCE of 9.3%. These photovoltaic parameters are compa- mately 380–400 nm. The electrospray technique developed
rable to those of a hybrid cell with a CH3NH3PbI3ꢀxClx active herein is thus a suitable approach to deposit perovskite mate-
layer prepared by the conventional spin-coating process, which rials for fabricating hybrid solar cells. For the devices made of
exhibits a VOC of 0.94 V, a JSC of 17.74 mA cmꢀ2, and an FF of 0.6, an orthorhombic CH3NH3PbI3 active layer annealed at 90 ꢁC
corresponding to a PCE of 10%. Fig. 5(b) shows the incident and 130 ꢁC instead of 110 ꢁC for 60 minutes, we observe the
photovoltaic parameters of a VOC of 0.81 V, a JSC of 15.23 mA
cmꢀ2, and an FF of 0.44, corresponding to a PCE of 5.4% for the
90 ꢁC device, and a VOC of 0.85 V, a JSC of 13.57 mA cmꢀ2, and an
FF of 0.30, corresponding to a PCE of 3.5% for the 130 ꢁC device.
The relatively low device performance is due to the non-uniform
coverage or incomplete reaction of perovskite lm on the
substrate. As seen in the SEM image in Fig. 4(d), the voids
between grain boundaries cause electric shunting between the
electrodes, markedly reducing the FF, JSC, and PCE of the solar
cells. In addition, the J–V curve for the hybrid cell made of
a
CH3NH3PbI3 active layer prepared by electrospraying
CH3NH3I and PbI2 (molar ratio of 1 : 1) in the DMF solution is
plotted in Fig. 5 for comparison. The device exhibits a VOC of
0.63 V, a JSC of 5.77 mA cmꢀ2, and an FF of 0.25, corresponding
to a PCE of 0.9%. The low device performance is attributed to
the too fast formation of CH3NH3PbI3 crystals during the elec-
trospray process. CH3NH3I instantly reacts with PbI2 solids to
form CH3NH3PbI3 perovskite, and thus the lm does not
uniformly cover the substrate, and the thermal treatment also
did not improve the morphology of the CH3NH3PbI3 lm. On
the other hand, the formation rate of CH3NH3PbI3ꢀxClx is
relatively slow compared to that of CH3NH3PbI3,42 thus causing
better coverage of the lm on the glass/ITO/PEDOT:PSS
substrate. The surface morphology of the CH3NH3PbI3 lm is
too rough, as can be seen by the naked eye, and is not able to
make a uniform lm for fabricating decent cells. Accordingly,
the morphology of the perovskite lm prepared by
Fig. 5 (a) The schematic device structure and the current J–V curves
with forward (hollow icons) and reverse scans (solid icons) for hybrid
perovskite solar cells of glass/ITO/PEDOT:PSS/orthorhombic CH3-
NH3PbI3 perovskite/C60/BCP/Al fabricated by electrospraying and
then annealed at 90 ꢁC (P/;), 110 ꢁC (,/-), and 130 ꢁC (B/C) for
60 minutes and fabricated by conventional spin coating (>/A), and
the glass/ITO/PEDOT:PSS/CH3NH3PbI3 perovskite/C60/BCP/Al (O/:)
configuration under the standard 1 sun AM 1.5G simulated solar irra-
diation. (b) The IPCE spectra and the integrated photocurrent calcu-
lated under AM 1.5G solar irradiation of the cells of glass/ITO/
PEDOT:PSS/orthorhombic CH3NH3PbI3 perovskite/C60/BCP/Al fabri- Fig. 6 Cross-sectional image of the hybrid device produced in this
cated by electrospraying then annealed at 110 ꢁC for 60 minutes study with
glass/ITO/PEDOT:PSS/orthorhombic CH3NH3PbI3
perovskite/C60/BCP/Al configuration.
a
(,/-) and fabricated by conventional spin coating (>/A).
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electrospraying in our current study is still the key component
to determining the photovoltaic parameters of the cells, which
can be modulated by the precise control of the applied voltage,
solution concentration, annealing temperature and time, as
well as other parameters. Note that the stability of the materials
and solar cell performance are important issues that have been
commonly shown in perovskites made by spin-coating and
vacuum evaporation methods. The stability of the unencapsu-
lated perovskite solar cell devices made by our electrospray
method was further tested during exposure to 70% RH at room
temperature for about 200 hours. The device PCE was reduced
to approximately 50% of the initial values due to signicant
decay of the JSC and FF. Although the device fabrication of
perovskite solar cells can be realized in ambient conditions for
real mass production, the long-term stability of the photovoltaic
performance should be further improved through encapsula-
tion or interface engineering in the future.
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In conclusion, in this work we successfully demonstrated an 11 H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan,
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rhombic CH3NH3PbI3 active layer for fabricating perovskite- S. I. Seok, Science, 2015, 348, 1234.
based hybrid solar cells. Depositing the solids by electro- 13 N. Ahn, D.-Y. Son, I.-H. Jang, S. M. Kang, M. Choi and
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elevated temperature improves the crystallinity and coverage of 16 K. M. Boopathi, M. Ramesh, P. Perumal, Y.-C. Huang,
the perovskite lm on the substrate. A hybrid cell made of the
active layer produced by the electrospray process exhibits a VOC
C.-S. Tsao, Y.-F. Chen, C.-H. Lee and C.-W. Chu, J. Mater.
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of 0.87 V, a JSC of 19.71 mA cmꢀ2, and an FF of 0.55, corre- 17 N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and
sponding to a PCE of 9.3%, which is comparable to the
S. I. Seok, Nat. Mater., 2014, 13, 897.
performance of a cell prepared by the conventional spin-coating 18 H. Ishihara, W. Chen, Y.-C. Chen, S. Sarang, N. D. Marco,
process. The results indicate that the electrospray technique
developed in this work would be ideal to achieve large-area
O. Lin, S. Ghosh and V. C. Tung, Adv. Mater. Interfaces,
2016, 3, 1500762.
coating of a perovskite active layer with the potential for real 19 P.-W. Liang, C.-Y. Liao, C.-C. Chueh, F. S. Zuo, T. Williams,
mass production. We understand that the morphology of the X.-K. Xin, J. Lin and A. K.-Y. Jen, Adv. Mater., 2014, 26, 3748.
deposited lm is still the key component to determining the 20 Y. Guo, K. Shoyama, W. Sato and E. Nakamura, Adv. Energy
photovoltaic parameters of the resulting cells. Intensive work to
Mater., 2016, 6, 1502317.
further advance the device performance is currently in progress. 21 R. A. Wibowo, W. H. Jung, M. H. Al-Faruqi, I. Amal and
K. H. Kim, Mater. Chem. Phys., 2010, 124, 1006.
22 W. K. Kim, S. Kim, E. A. Payzant, S. A. Speakman, S. Yoon,
Acknowledgements
R. M. Kaczynski, R. D. Acher, T. J. Anderson, O. D. Crisalle,
The authors would like to thank the National Science Council
(NSC) (now Ministry of Science and Technology) in Taiwan 23 K. C. Hsu, Y. S. Fu, P. Y. Lin, I. T. Tang and J. D. Liao, Int. J.
(NSC102-2628-M-006-001-MY3), (MOST105-2119-M-024-001), Photoenergy, 2013, 2013, 156964.
the Asian Office of Aerospace Research and Development 24 D. Son, S. M. Hughes, Y. D. Yin and A. P. Alivisatos, Science,
S. S. Li and V. Craciun, J. Phys. Chem. Solids, 2005, 66, 1915.
(AOARD-14-4012), and the CPC Corporation, Taiwan
2004, 306, 1009.
(MEA0300012) for nancially supporting this research.
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