This document is downloaded from DR-NTU, Nanyang Technological
University Library, Singapore.
Title
High efficiency electrospun TiO2 nanofiber based hybrid
organic–inorganic perovskite solar cell
Author(s)
Dharani, Sabba; Mulmudi, Hemant Kumar; Yantara,
Natalia; Thu Trang, Pham Thi; Park, Nam Gyu; Graetzel,
Michael; Mhaisalkar, Subodh; Mathews, Nripan; Boix,
Pablo P.
Citation
Dharani, S., Mulmudi, H. K., Yantara, N., Thu Trang, P.
T., Park, N. G., Graetzel, M., Mhaisalkar, S., Mathews,
N., & Boix, P. P. (2014). High efficiency electrospun TiO2
nanofiber based hybrid organic–inorganic perovskite
solar cell. Nanoscale, 6(3), 1675-1679.
Date
2014
URL
http://hdl.handle.net/10220/20226
Rights
© 2014 Royal Society of Chemistry.This is the author
created version of a work that has been peer reviewed
and accepted for publication by Nanoscale, Royal Society
of Chemistry. It incorporates referee’s comments but
changes resulting from the publishing process, such as
copyediting, structural formatting, may not be reflected in
this document. The published version is available at:
http://dx.doi.org/10.1039/c3nr04857h.
Journal Name
RSCPublishing
ARTICLE
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
High efficiency electrospun TiO 2 nanofiber based
hybrid organic-inorganic perovskite solar cell.
Sabba Dharani1,2, Hemant Kumar Mulmudi1,2, Natalia Yantara1,2, Pham Thi Thu
Thrang1,2, Nam Gyu Park3, Michael Graetzel4, Subodh Mhaisalkar1,2, Nripan
Mathews1,2,5*, Pablo P. Boix1*,
DOI: 10.1039/x0xx00000x
www.rsc.org/
The good electrical and morphological characteristics of TiO 2 nanofibers and the high
extinction coefficient of CH 3 NH3PbI 3 perovskite are combined to obtain a solar cell with
power conversion efficiency of 9.8 %. The increase of the film thickness dramatically
diminishes the performance due to the reduction in porosity of TiO 2 nanofiber framework. The
optimum device (~413 nm film thickness) is compared to a planar device, where the latter
produces higher V oc but lower J sc, and consequently lower efficiency at all measured light
intensities.
Introduction
Solid state perovskite (CH3NH3PbI3) solar cells have invoked
tremendous amount of scientific and commercial interest as
they have several advantage such as ease of fabrication, cost
effectiveness and high efficiency. 1-5 Generally, a mesoscopic
film of TiO2 nanoparticles (acting as an electron transporter) is
used as a host to load the perovskite pigments. Other
approaches employ Al 2O3 and ZrO2 which primarily act as
scaffold and don’t indulge in electron transport to replace TiO 2
nanoparticles. 4, 6 Replacing TiO2 nanoparticles with one
dimensional (1-D) nanostructures 7 has been shown to facilitate
the pore filling of viscous materials. 2, 8 Moreover, onedimensional nanostructures were reported to exhibit better
charge transport and lower recombination than the
nanoparticles in liquid dye-sensitized solar cells (DSC). 7, 9, 10
Electrospinning is a simple, inexpensive technique by which
one dimensional nanofibers can be synthesized. 10-14 Such long,
interconnected nanofibers have been utilized as a substitute for
the mesoporous nanoparticles in DSC. 15-17 Though the
nanofibers exhibited several advantages over nanoparticles in
terms of charge transport and reduced charge recombination, 10,
18
they suffered from being limited by the film thickness when
spun directly on conducting substrates. Thicker films (>2 µm)
undergo shrinkage leading to delamination from the substrate.
Additionally, the formation of cracks is predominant when the
composite film is sintered to remove the organic precursors and
to attain the crystalline semiconductor. In contrast, DSC with
thinner electrospun films suffer from poor absorption cross-
This journal is © The Royal Society of Chemistry 2013
section due to the low surface area, which leads to poor light
harvesting when conventional dyes with low extinction
coefficients are employed.19, 20 In order to satisfy the light
harvesting criterion, two approaches can be employed. One is
to increase the film thickness. For this purpose, several
researchers employed additional steps to make pastes from the
electrospun nanofibers and then obtain thick photoanodes by
either screen printing or by spincoating. 21 Though this method
improves the light harvesting, it does not benefit from the
charge transport advantages associated with the 1-D
nanofibers.22 The second approach employs sensitizers with
high light absorption coefficients. In this sense, CH 3NH3PbI3
(an organic-inorganic halide perovskite)is a suitable sensitizer
whose absorption coefficient is 1.5 X 10 4 cm-1 at 550 nm (an
order of magnitude higher than the conventional Ru based
dyes).23 Thus, coupling the high absorption coefficient of
perovskite material and the good charge transport properties
and porous structure of the nanofibers is a route towards high
photovoltaic performance through a non-expensive process.
More recentIy, the implementation of the sequential deposition
protocol (loading PbI 2 within the pores and subsequent
transformation into CH 3NH3PbI3)1 results in a higher loading of
the absorber. As a consequence, this ensures an increase of the
light harvesting efficiency.
In this paper TiO2 nanofibers directly electrospun on TiO 2
compact layer are combined with a sequentially deposited
perovskite to fabricate solar cells with efficiencies of 9.8 %. To
the best of our knowledge, this is the highest efficiency
J. Name., 2013, 00, 1-3 | 1
ARTICLE
obtained with such electrospun TiO 2 nanofibers. The effect of
different nanofiber film thickness is investigated in contrast to
a planar device.
Experimental
Fluorine doped tin oxide (FTO, <14 ohm/square, 2.2 mm thick)
substrates were first etched with Zn powder and hydrochloric
(HCl) acid (4 M) to form the desired pattern, which was
subsequently cleaned with decon soap solution and ethanol
respectively. Then a thin compact layer of TiO 2 referred to as
the blocking layer, which is about 80 - 100 nm, was deposited
by aerosol spray-pyrolysis at 450 oC using ambient air as carrier
gas.24 Also these substrates were immersed in 40 mM of TiCl4
solution for 30 min at 70 oC, followed by rinsing with deionized
water and ethanol. Some of these substrates were used for
fabricating the cells and are referred to as “Planar” devices. For
spray-pyrolysis a solution of titanium diisopropoxide
bis(acetylacetonate) (75 wt. % in isopropanol) and absolute
ethanol was used in the ratio 1:9 by volume. Then the substrates
were calcined at 500 oC for 30 min. For the synthesis of
nanofibers (NF), a sol-gel solution comprising of 0.8 g PVP
(Mw = 1300000), 4 g titanium (IV) butoxide (97 %), 1.18 g
acetyl acetone (≥99 %) in 10 mL methanol was prepared and
was electrospun at 25 kV with a feed-rate of 0.3 mL/h using
NANON (MECC) electrospinning setup. The nanofibers were
collected on the blocking layer deposited FTO substrates which
were placed on a metallic collecting plate of electrospinning
setup. Then the composite mat of nanofibers was calcined at
450 oC in a box furnace for 5 h to remove the organic
components and to get crystalline TiO 2 nanofibers. Then the
nanofibers were subjected to 0.1 M TiCl 4 treatment for about
12 h at 40 oC followed by sintering at 500 oC for 30 min.
An organic-inorganic perovskite (CH 3NH3)PbI3 was deposited
by sequential method as reported in literature. 1 Lead iodide
(1M) was dissolved in N,N-dimethylformamide overnight
under stirring conditions at 70 oC and was spincoated on the
planar substrates as well as on the nanofiber substrates at 6000
r.p.m for 5 s. These substrates were then dried for 30 mins at 70
o
C. Subsequently the films were dipped in 10 mg/mL solution
of CH3NH3I in 2-propanol for approx. 20 min. Then the films
were rinsed with 2-propanol and dried at 70 oC for 30 min.
An organic hole conductor namely spiro-OMeTAD (2, 2’, 7, 7’tetrakis-(N,
N-di-p-methoxyphenylamined)
9,
9’-
2 | J. Name., 2012, 00, 1-3
Journal Name
spirobifluorene) was dissolved in chlorobenzene (120 mg/mL)
and spincoated on these substrates. Additives like Li (CF 3SO2)2
N, tert.-butylpyridine and FK102 dopant were added to the
above solution.1, 25 The masked substrates were placed in a
thermal evaporator for gold (Au) deposition via shadow
masking. Thickness of the Au electrode was about 80 nm and
the active area was defined by the overlap of TiO 2 and Au (0.2
cm2). During measurements, samples were masked using a
black tape with 0.25 cm2 in dimension, to prevent the stray
currents generated away from the active area.
Characterization
Top view and cross-sectional images were recorded by Field
Emission Scanning Electron Microscope (FESEM, JEOL, JSM7600F, 5 kV). Photocurrent-voltage measurements were
measured using San-EI Electric, XEC-301S under AM 1.5 G.
For light intensity dependence studies, UV Fused Silica
Metallic Neutral Density Filters (Newport Corporation) were
used. Incident photon to current conversion efficiency (IPCE)
was determined using PVE300 (Bentham), with dual
Xenon/quartz halogen light source, measured in DC mode and
no bias light is used. Incident light intensity was calibrated
using a photodiode detector (silicon calibrated detector,
Newport).
Results and discussion
The device architecture of the nanofiber-based perovskite cell is
illustrated in Fig. 1. In this device configuration, the
mesoporous nanofibers replace the mesoporous nanoparticles
used in conventional perovskite solar cells.1 The
electrospinning conditions, such as solution viscosity and
deposition voltage, have been carefully tailored in order to
synthesize TiO2 nanofibers with suitable diameter to achieve
both a good perovskite loading and optimum charge transport
properties.
This journal is © The Royal Society of Chemistry 2012
Journal Name
ARTICLE
Fig. 2: Effect of different nanofiber film thicknesses on the photovoltaic
performance.
The optimal thickness of nanofiber film to achieve best cell
performance is about 413 nm. With thicker films of 844 nm and
1215 nm, Voc dropped to 0.78 V and 0.74 V respectively. This
is probably due to a higher recombination rate in the thicker
films, due to the increase in surface area and the consequent
increase of trap assisted recombination in TiO 2 films.2, 27 This
higher recombination is also expressed as an early onset of the
dark current for thicker TiO 2 films. However the most dramatic
reduction is in the short circuit current densities, which drop
drastically for increasing TiO 2 thicknesses. This is in contrast to
the classical DSC behavior where an increase of the film drives
an increase of the current. 27, 28 Overlapping of the nanofibers in
thicker films leads to a closure of the pores (Fig. S2). This
results in the lower loading of the perovskite within thick films,
leading to significantly lower light absorption (Fig. S3).
Fig. 1: (a) Schematic illustrating the various components of nanofiber
based perovskite solar cell. FESEM of (b) the annealed nanofibers film
at 450 oC for 5 h and (c) of TiCl4 treated rough nanofibers, which were
employed in the perovskite solar cells.
A diameter of 120 - 200 nm presents good electrical
advantages10 and an excellent porous network (see Fig. S1b), in
contrast to smaller diameter ones which are less continuous
(Fig. S1a), and larger diameter ones which form more closed
structures (Fig. S1c) resulting in lesser absorber loading.
In addition to porosity, the total thickness of the formed film
has also a critical effect on the cell performance. 1, 2, 26 In this
respect, the photovoltaic characteristics for different nanofiber
film thickness are illustrated in Fig. 2 and tabulated in Table 1.
This journal is © The Royal Society of Chemistry 2012
Table 1: Effect of film thickness on j-V parameters of nanofiber based
solar cell.
Film
Jsc (mA/cm2)
Voc (V)
FF
η (%)
Thickness (nm)
15.88
0.98
0.63
9.82
413
6.41
0.78
0.66
3.32
844
5.14
0.74
0.66
2.49
1215
Given the limitation of the technique and the diameter of the
optimum fibers, homogenous films thinner than 413 nm are not
viable. Therefore, planar devices were fabricated to compare its
performance.
When spun on the films, PbI 2 adopts the nanofibers’ porous
and rough topography (Fig. 3(a)). The cross-section image
(inset of Fig. 3a) reveals that the top surface with PbI 2 is rough.
However it is also evident from the inset of Fig. 3(a) as well as
from EDX mapping (Fig. S4), that the PbI2 has infiltrated
throughout the depth of the fiber film.
J. Name., 2012, 00, 1-3 | 3
ARTICLE
Journal Name
In contrast, PbI2 loaded directly on top of the TiO 2 compact
layer to form the planar device (described in experimental
section, Fig. 3 (b)), forms a smooth and compact topography. In
this case, from the cross-section image which is shown in the
inset, it is difficult to confirm the distribution of PbI 2 on the
blocking layer. When the PbI2 loaded nanofibers are immersed
in CH3NH3I solution, CH3NH3I was able to seep through the
pores and have contact with PbI 2 leading to complete
transformation to (CH 3NH3) PbI3 within the photoanode. In case
of the planar device, it was observed that the formation of
perovskite is incomplete as the insertion of CH 3NH3I might be
hampered by the compactness of the PbI 2 film (Fig. S5 (b)).
This may call for the need to immerse the planar devices for
longer time in the IPA solution, but it was observed that such
action only resulted in the dissolution of the lead iodide in the
solution itself. This phenomenon has also been observed by
Burschka et al.1
In Fig. 3(c), the cross-sectional view of the complete nanofiber
device is shown, with a film thickness of 413 nm. Owing to the
rough morphology of the fibers, the rough surface topography
required a higher concentration of spiro-OMeTAD (120
mg/mL) to avoid short-circuiting of the device. For
comparative study same conditions have been followed for the
planar device, From Figs. 3 (c & d), it is observed that under
similar spiro deposition conditions, the spiro overlayer
thickness is different in both the nanofiber and planar devices.
The thinner spiro overlayer in case of the nanofiber can be
attributed to a higher roughness and wetting, which results in a
better coverage of spiro through the mesoporous nanofiber film
forming a good contact between the TiO 2/perovskite/spiro
interfaces.
The photovoltaic characteristics (j-V plots) of the devices
fabricated on nanofiber and planar are presented in Fig. 4 (a).
The photovoltaic parameters as obtained are tabulated in Table
2. Clearly the nanofiber based perovskite cell has exceedingly
high efficiency (9.82 %) compared to the planar (η = 3.11 %)
device. The 413 nm thick mesoporous nanofibers’ film enables
more loading of perovskite material as compared to the
compact structure of the planar device. This effect is reflected
in terms of the higher J sc: 15.88 mA/cm2 for the nanofiber
device while the planar device exhibits only 4.02 mA/cm 2. The
Voc of the nanofiber devices dropped in comparison to the
planar devices. This is expected since the trap-assisted
recombination will be lower for the latter, following the same
trend observed in the nanofibers devices with different film
thickness. Additionally, direct injection of charges from the
perovskite to the compact layer can reduce the voltage drop as
has been seen in non-injecting devices.29
The high Jsc of the nanofiber devices is also validated by the
IPCE action spectra shown in Fig. 4 (b). The J sc calculated from
the IPCE data concurs well with the J sc calculated from the j-V
plots, tabulated in Table 2, and the shape of the IPCE spectrum
4 | J. Name., 2012, 00, 1-3
Fig. 3: FESEM images showing: (a) top-view of PbI2 loaded TiO2
nanofibers (inset is the cross-section view), (b) top-view of PbI2 coated
TiO2 blocking layer (inset is the cross-section view), (c) cross-section
view of the full device with nanofibers as the photoanode and (d)
cross-section view of the full device without the nanofiber scaffold. The
Au detachment is an after effect of the mechanical cleaving.
for nanofiber devices is in good agreement with the shapes
reported for nanoparticle devices in the literature. 1
However, there is a slight discrepancy with the shape of IPCE
spectrum for planar devices in the wavelength region of 350650 nm as seen in Fig. 4 (b), where there is depression instead
of a peak. This depression could be due to light absorption by
the untransformed PbI 2 (Fig. S5 (b)). The photovoltaic
parameters (Jsc, η) of nanofiber and planar devices as a function
This journal is © The Royal Society of Chemistry 2012
Journal Name
ARTICLE
of light intensity are shown in Fig. 4(c & d). Short circuit
current density showed linear dependence on light flux and this
behavior is in agreement with literature. 7 The linear behavior of
short circuit current density with light intensity demonstrates
that charge collection efficiency is independent of light
intensity.
the more open structure provided by the planar devices, poor
PbI2 to CH3NH3PbI3 conversion hinders the efficiencies.
Acknowledgements
This work was supported by National Research Foundation
(NRF) under the Competitive Research Program (CRP) and the
SinBeRISE CREATE program. N.G.P. acknowledges the
financial support from the National Research Foundation of
Korea (NRF) grant funded by the Ministry of Science, ICT &
Future under contract No. NRF-2010-0014992.
Notes and references
Fig. 4: (a) Current density versus voltage plots, (b) IPCE action spectra
for nanofiber (represented by squares) and planar devices (represented
by circles). Effect of different light intensities on (c) Jsc and (d)
efficiency, for planar and nanofiber cells.
The nanofibers devices systematically show higher short circuit
current densities and higher power conversion efficiencies
compared to planar devices at all light intensities. Remarkably,
the nanofiber cell yielded 11.79 % photo electrochemical
conversion efficiency at 0.1 sun while the planar device yielded
3.03 %. Further avenues to improve the efficiency could
involve the use of better dopants such as FK209 which could
further reduce charge recombination. 1, 25
Table 2: j-V characteristics of planar vs fiber.
Sample
Jsc
(mA/cm2)
Voc
(V)
FF
η
(%)
Jsc from
IPCE
(mA/cm2)
Fiber
15.88
0.98
0.63
9.82
15.5
Planar
4.02
1.06
0.73
3.11
3.93
Conclusions
In conclusion, we have employed electrospun nanofibers as
photoanode for efficient perovskite solar cells. Efficiencies of
9.8 % (11.79 % at 0.1 Sun) are obtained which to our
knowledge is the first report in this configuration. The
efficiencies are mainly determined by the open porosity of the
electrospun nanofiber network which varies with TiO 2
nanofiber photoanode thicknesses and fiber diameters. Despite
This journal is © The Royal Society of Chemistry 2012
1. Energy Research Institute @NTU (ERI@N), Research Techno Plaza,
X-Frontier Block, Level 5, 50 Nanyang Drive, Singapore 637553.
2. School of Materials Science and Engineering, Nanyang Technological
University, Nanyang Avenue, Singapore 639798.
3. School of Chemical Engineering and Department of Energy Science,
Sungkyunkwan University, Suwon 440-746, Korea.
4. Laboratory of Photonics and Interfaces, EPFL, Lausanne, Switzerland.
5. Singapore-Berkeley Research Initiative for Sustainable Energy, 1
Create Way, Singapore 138602, Singapore.
* Correspondence to: [email protected], [email protected]
References
1. J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K.
Nazeeruddin and M. Grätzel, Nature, 2013.
2. H. S. Kim, J. W. Lee, N. Yantara, P. P. Boix, S. A. Kulkarni, S.
Mhaisalkar, M. Grätzel and N. G. Park, Nano Letters, 2013,
13, 2412-2417.
3. L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K.
Nazeeruddin and M. Grätzel, Journal of the American
Chemical Society, 2012, 134, 17396-17399.
4. D. Bi, L. Haggman, G. Boschloo, L. Yang, E. M. J. Johansson and A.
Hagfeldt, RSC Advances, 2013.
5. H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S.
J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M.
Grätzel and N. G. Park, Scientific Reports, 2012, 2.
6. J. M. Ball, M. M. Lee, A. Hey and H. J. Snaith, Energy and
Environmental Science, 2013, 6, 1739-1743.
7. M. Law, L. E. Greene, J. C. Johnson, R. Saykally and P. Yang, Nat
Mater, 2005, 4, 455-459.
8. M. Y. Song, D. K. Kim, K. J. Ihn, S. M. Jo and D. Y. Kim, Synthetic
Metals, 2005, 153, 77-80.
9. B. H. Lee, M. Y. Song, S.-Y. Jang, S. M. Jo, S.-Y. Kwak and D. Y.
Kim, The Journal of Physical Chemistry C, 2009, 113, 2145321457.
10. D. Sabba, N. Mathews, J. Chua, S. S. Pramana, H. K. Mulmudi, Q.
Wang and S. G. Mhaisalkar, Scripta Materialia, 2013, 68, 487490.
11. A. Greiner and J. H. Wendorff, Angewandte Chemie International
Edition, 2007, 46, 5670-5703.
12. D. Li, J. T. McCann and Y. Xia, Small, 2005, 1, 83-86.
J. Name., 2012, 00, 1-3 | 5
ARTICLE
Journal Name
13. D. Li and Y. Xia, Advanced Materials, 2004, 16, 1151-1170.
14. T. Subbiah, G. S. Bhat, R. W. Tock, S. Parameswaran and S. S.
Ramkumar, Journal of Applied Polymer Science, 2005, 96,
557-569.
15. D. Hwang, S. M. Jo, D. Y. Kim, V. Armel, D. R. MacFarlane and S.
Y. Jang, ACS Applied Materials and Interfaces, 2011, 3, 15211527.
16. W. Zhang, R. Zhu, F. Li, Q. Wang and B. Liu, The Journal of
Physical Chemistry C, 2011, 115, 7038-7043.
17. S. H. Ahn, D. J. Kim, W. S. Chi and J. H. Kim, Advanced Materials,
2013, n/a-n/a.
18. P. S. Archana, R. Jose, C. Vijila and S. Ramakrishna, The Journal of
Physical Chemistry C, 2009, 113, 21538-21542.
19. P. Péchy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P.
Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia,
G. B. Deacon, C. A. Bignozzi and M. Grätzel, Journal of the
American Chemical Society, 2001, 123, 1613-1624.
20. T. Horiuchi, H. Miura, K. Sumioka and S. Uchida, Journal of the
American Chemical Society, 2004, 126, 12218-12219.
21. R. Jose, A. Kumar, V. Thavasi and S. Ramakrishna, Nanotechnology,
2008, 19.
22. K. Mukherjee, T.-H. Teng, R. Jose and S. Ramakrishna, Applied
Physics Letters, 2009, 95, 012101-012103.
23. J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park and N.-G. Park,
Nanoscale, 2011, 3, 4088-4093.
24. L. Kavan and M. Grätzel, Electrochimica Acta, 1995, 40, 643-652.
25. J. H. Noh, N. J. Jeon, Y. C. Choi, M. K. Nazeeruddin, M. Gratzel and
S. I. Seok, Journal of Materials Chemistry A, 2013, 1, 1184211847.
26. J. M. Ball, M. M. Lee, A. Hey and H. J. Snaith, Energy &
Environmental Science, 2013, 6, 1739-1743.
27. S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet,
P. Comte, M. K. Nazeeruddin, P. Péchy, M. Takata, H. Miura,
S. Uchida and M. Grätzel, Advanced Materials, 2006, 18,
1202-1205.
28. S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Grätzel, M. K.
Nazeeruddin and M. Grätzel, Thin Solid Films, 2008, 516,
4613-4619.
29. M. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J.
Snaith and J. Henry, Science (New York, N.Y.), 2012, 643, 1-7.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012