Inkjet printing and instant chemical transformation of a CH3NH3PbI3/nanocarbon electrode and interface for planar perovskite solar cells

Article abstract of DOI:10.1002/anie.201408638

A planar perovskite solar cell that incorporates a nanocarbon hole-extraction layer is demonstrated for the first time by an inkjet printing technique with a precisely controlled pattern and interface. By designing the carbon plus CH₃NH₃I ink to transform PbI₂ in situ to CH₃NH₃PbI₃, an interpenetrating seamless interface between the CH₃NH₃PbI₃ active layer and the carbon hole-extraction electrode was instantly constructed, with a markedly reduced charge recombination compared to that with the carbon ink alone. As a result, a considerably higher power conversion efficiency up to 11.60% was delivered by the corresponding solar cell. This method provides a major step towards the fabrication of low-cost, large-scale, metal-electrode-free but still highly efficient perovskite solar cells.

Full text of DOI:10.1002/anie.201408638

Angewandte
Chemie
DOI: 10.1002/anie.201408638
Nanotechnology
Inkjet Printing and Instant Chemical Transformation of a CH NH PbI /
3
3
3
Nanocarbon Electrode and Interface for Planar Perovskite Solar
Cells**
Zhanhua Wei, Haining Chen, Keyou Yan, and Shihe Yang*
Abstract: A planar perovskite solar cell that incorporates
a nanocarbon hole-extraction layer is demonstrated for the first
time by an inkjet printing technique with a precisely controlled
pattern and interface. By designing the carbon plus CH NH I
solve these problems, some inorganic hole-transport materi-
[
18,19]
[20–26]
als
and hole-extraction materials
have been devel-
oped for perovskite solar cells. Among those materials,
carbon materials themselves stand out because of the suitable
Fermi level, earth abundance, low cost, and superior environ-
3
3
ink to transform PbI in situ to CH NH PbI , an interpene-
2
3
3
3
[
27–29]
trating seamless interface between the CH NH PbI active
mental stability.
In one type of carbon-based perovskite
3
3
3
layer and the carbon hole-extraction electrode was instantly
constructed, with a markedly reduced charge recombination
compared to that with the carbon ink alone. As a result,
a considerably higher power conversion efficiency up to
solar cells, a mesoporous TiO layer, a ZrO insulating layer,
2
2
and a carbon electrode are sequentially deposited, followed
by infiltration of a perovskite precursor solution, which
requires perfect pore filling to ensure high perfor-
[
25,27,28]
1
1.60% was delivered by the corresponding solar cell. This
mance.
In previous work, we demonstrated a new and
method provides a major step towards the fabrication of low-
cost, large-scale, metal-electrode-free but still highly efficient
perovskite solar cells.
simplified configuration for the carbon-based perovskite solar
cells, which embodies the two-step formation of the perov-
[30]
skite layer and the eliminating of the ZrO insulating layer.
2
The beauty of the design is that a pre-deposited PbI layer
2
I
n the last five years, hybrid organic–inorganic perovskite
plays the role of the aforementioned ZrO layer in separation
2
solar cells have experienced an explosive development, with
a power conversion efficiency (PCE) rising from the initial
the TiO and the carbon layer, but is transformed to the
2
perovskite layer at a later stage. The energy conversion
efficiency of such carbon based perovskite solar cells reached
values as high as 11.06%.
[1–11]
3
% to a certified 17.9%, and most recently, up to 19.3%.
What have transpired are the ideal photovoltaic properties of
the perovskite materials (CH NH PbI and
CH NH PbI Cl ), such as appropriate and tunable direct
The carbon-based perovskite solar cells appear to be quite
compatible with the powerful printing technology, which
would allow large-scale and low-cost production. However,
the printable perovskite solar cells of this type have not been
demonstrated until now, not to mention the chemical trans-
formation and the interface development associated with the
printing process. To tackle this problem, we have developed
an inkjet printing technique, which permits instant formation
of a conformable CH NH PbI /C bilayer with programmable
3
3
3
3
3
3Àx
x
band gap, high absorption coefficient, highly mobile electron
and holes, excellent (balanced) carrier transport diffusion
[12]
[
13]
length (100 to 1000 nm), and apparent tolerance of defects.
Furthermore, the perovskite materials are solution-process-
able and low-cost. With an in-depth understanding of their
working mechanism and judicious design and controllable
fabrication, over 20% PCE perovskite solar cells are
expected in the near future.
3
3
3
control. Importantly, this ink printing technique enables
a planar configuration, which is regarded as the configuration
of choice for perovskite solar cells because of the potential
benefits of large grain size and defect-free boundary, permit-
A typical perovskite solar cell consists of TiO (mesopo-
2
rous or compact), CH NH PbI , hole transport material
3
3
3
[
14,15]
(
HTM) and noble metal counter electrode.
Discourag-
[
6,31–37]
ingly, the conventional organic hole transporter materials
ting long range balanced electron–hole diffusion.
[
16,17]
(
such as spiro-OMeTAD) are expensive and unstable.
Another salient innovation is the formulation of the C +
The noble metal electrode is also costly and requires
demanding vacuum thermal evaporation deposition. To
CH NH I ink to simultaneously deposit the nanocarbon
3
3
electrode, transform PbI to CH NH PbI in situ, and create
2
3
3
3
an interpenetrating interface between CH NH PbI and C
3
3
3
electrode with minimal charge recombination. This inkjet
printed carbon-based planar perovskite solar cell has regis-
tered a considerably high efficiency up to 11.60%.
[
+]
[+]
[
*] Z. Wei, Dr. H. Chen, Dr. K. Yan, Prof. S. Yang
Department of Chemistry
William Mong Institute of Nano Science and Technology
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (China)
Figure 1 illustrates the strategies we used to prepare TiO2/
CH NH PbI /C planar solar cells by inkjet printing technique.
3
3
3
E-mail: chsyang@ust.hk
In step 1, a TiO compact layer was deposited on a FTO glass
2
+
by TiCl treatment. Then, a PbI thin film was deposited on
[
] These authors contributed equally to this work.
4
2
the TiO compact layer by multiple spin-coating runs (step 2).
[**] This work was supported by the HK-RGC General Research Funds
2
(
GRE No. HKUST 606511, 605710).
The conversion of PbI into CH NH PbI and the simulta-
2
3
3
3
neous deposition of a carbon hole-extraction layer were
accomplished by two different strategies were applied. One
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408638.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
Figure 1. Fabrication process flow for the instant inkjet printing of the
C/CH NH PbI planar perovskite solar cells. For comparison, a different
3
3
3
strategy was used to convert PbI into CH NH PbI using a separate
2
3
3
3
step 3 and step 4 (see text for details).
Figure 2. A) Cross-sectional SEM image of TiO compact layer grown
2
by TiCl treatment. B) Cross-sectional SEM image of TiO /PbI thin
4
2
2
film prepared by multiple spin-coating. C),D) Cross-sectional SEM
images and the corresponding diagrams of TiO /CH NH PbI /C solar
cells prepared by the inkjet printing technique using the C ink (C) and
strategy (black arrows) was to first print carbon black ink
carbon black dispersed in isopropanol solution) on the PbI2
layer (step 3), followed by immersion into a CH NH I bath to
2
3
3
3
(
3
3
the C+CH NH I ink (D).
3
3
obtain the final TiO /CH NH PbI /C planar solar cell (step 4).
2
3
3
3
As for the second strategy (gray arrows), step 3 and step 4 in
the first strategy were combined (namely, step 3 only here).
To this end, a reactive ink was prepared by mixing carbon
black and CH NH I in isopropanol together. Through the
ure S2) clearly reveals a rutile phase (JPCDS 011292) of the
TiO compact layer. In step 2, the PbI thin film was prepared
3
3
2
2
modified step 3, the PbI2 layer can be converted into
CH NH PbI instantly, resulting in improved interfaces of
by the multiple spin-coating method. The concentration of the
precursor solution and the spinning rate were carefully
controlled to obtain a fully covered and well-connected PbI2
thin film. As shown in Figure 2B, a circa 300 nm thick PbI2
thin film with a clear sheet structure covers well the TiO2
compact layer. For step 3, two different inks were used. For
preparing the C ink, carbon black (Supporting Information,
Figure S3) was dispersed in isopropanol at a concentration of
3
3
3
TiO /CH NH PbI /C. It is worth pointing out that a heating
2
3
3
3
process (1 hour) is needed for both strategies after carbon
electrode printing (ca. 5 seconds).
Through inkjet printing, the carbon electrode could be
precisely patterned in planar solar cells (the printer modifi-
cation can be found in the Supporting Information, Fig-
ure S1). As shown in Figure 1, the carbon electrodes with
À1
15 mgmL , while the preparation of the C + CH NH I ink
3
3
À1
“
HKUST” patterns were successfully and precisely achieved.
Moreover, it is clear that only the PbI2 covered by the
HKUST” pattern was converted to CH NH PbI for strat-
was accomplished by adding CH NH I (10 mgmL ) into the
3
3
above pristine C ink. Figure 2C represents the cross-sectional
SEM image of a TiO /CH NH PbI /C solar cell prepared by
“
3
3
3
2
3
3
3
egy 2, whereas all the PbI was converted into CH NH PbI
applying the C ink and the subsequent CH NH I bath. It is
3 3
2
3
3
3
for strategy 1, highlighting the patterning advantage of the
a clear layer by layer structure with a circa 350 nm thick
CH NH PbI thin film covered by a carbon layer. However,
combined ink of C + CH NH I. For a practical test, rectangle
3
3
3
3
3
carbon electrodes were printed on the perovskite thin films.
To the best of our knowledge, this is the first report of the
programmable fabrication of perovskites solar cells, and this
fast and controllable printing technique is a step forward for
developing commercial perovskite solar cells.
the contact between CH NH PbI and C is not very intimate
3 3 3
and some large pinholes are visible at the interface. By
comparison, the SEM image of the TiO /CH NH PbI /C solar
2
3
3
3
cell prepared by applying the C + CH NH I ink (Figure 2D)
3
3
presents a similar layer by layer structure but with a better-
defined crystallinity in the CH NH PbI thin film and more
To evaluate the interfacial connectivity of the TiO2/
CH NH PbI /C solar cells prepared by the two different
3
3
3
importantly, a better interface quality between the C and
CH NH PbI layers. The significantly improved C/
CH NH PbI interface with the C + CH NH I ink is presum-
3
3
3
inkjet printing strategies, we first turned to scanning electron
microscopy (SEM). Basically, a successful planar perovskite
3
3
3
3
3
3
3
3
solar cells requires a compact layer of TiO or ZnO with
ably a result of the in situ chemical transformation and the
carbon electrode deposition at the same time. More in-depth
discussion about the successful fabrication of planar perov-
skite solar cell can be found in the Supporting Information,
Figure S4. The improvement of the crystalline structure and
the interface is pivotal to the superior photovoltaic perfor-
mance, as will be shown below.
2
a sufficient roughness to block holes and to support/ensure
[
1,3,36–39]
a mechanically robust perovskite layer.
In step 1, the
TiO layer was deposited on an FTO glass by TiCl treatment.
2
4
As shown in Figure 2A, the as-prepared TiO layer is indeed
2
very compact and rough, with a thickness of about 150 nm.
The X-ray diffraction pattern (Supporting Information, Fig-
2
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Angewandte
Chemie
À2
1
4.40 mAcm , in good agreement with the J–V results. The
low IPCE values in the red light region (from 550 nm to
00 nm) should be attributed to the poor crystalline perov-
skite layer (see Figure 2C), would lead to weak light
8
[
1,25,28]
absorption in this wavelength region.
The solar cells
prepared with the C + CH NH I ink exhibit similar values at
3
3
less than 550 nm to those with the C ink, but significantly
higher values in the red light region. The integrated J is
sc
À2
estimated to be 16.80 mAcm , which is also consistent with
the J–V results. The improvement of IPCE in the red region
should be ascribed to the high quality crystalline perovskite
layer obtained with the C + CH NH I ink (see Figure 2D),
3
3
[
1,38]
which could improve charge transport and light scattering.
Besides, the improvement of the interconnectivity at the C/
CH NH PbI interface could also promote the photocurrent
3
3
3
owing to the enhanced charge transfer and suppressed charge
recombination. Figure 3E shows the PCE distributions of the
two sets of photovoltaic devices. Statistically, the overwhelm-
ing conclusion can be made that the C + CH NH I ink devices
3
3
(
(
average: 10.03%) considerably outperform the C ink devices
average: 7.55%). Finally, long-term stability testing has been
performed on the C + CH NH I ink devices and the result is
3
3
presented in Figure 3F. Significantly, the C + CH NH I ink
device retained about 90% of the highest PCE (measured one
day after the device was fabricated) even after storage for
3
3
Figure 3. Photovoltaic performance of the TiO /CH NH PbI /C solar
2
3
3
3
cells fabricated with different printing inks: A) The basic cell config-
uration, (B) energy level diagram, C) J–V curves, D) IPCE and E) PCE
distribution profiles of solar cells based on the C ink and the
C+CH NH I ink. F) Long-term stability testing result for the
1
2 days. Therefore, the obtained nanocarbon-based perov-
[29]
skite solar cells appear to be significantly more stable than
the conventional spiro-OMeTAD-based cells, for which the
environmental instability, especially in humid air, has been
3
3
C+CH NH I ink devices. For this experiment, un-encapsulated devices
3
3
were stored in ambient air (temperature: 258C, humidity: ca. 30%)
and tested every day.
[11,17]
widely recognized.
To gain a deeper insight into the improvement in Voc and
FF by C + CH NH I ink, we resorted to electrochemical
3
3
The final structure of the TiO /CH NH PbI /C solar cells
impedance spectroscopy (EIS), which is a powerful technique
for investigating charge transfer behaviors at the interfaces in
solar cells. The complex plane impedance plots for the two
solar cells at different forward applied biases (Vappl) (0.2 Vand
0.8 V) are presented in Figure 4A and B. Two obvious
semicircles are observed for both solar cells at a certain
Vappl, corresponding to two significantly different charge-
transfer or transport behaviors. As reported previously, two
such semicircles were also detected in perovskite solar cells
2
3
3
3
is indicated in Figure 3A. The energy level diagram and
charge transfer and transport processes in the solar cells are
illustrated in Figure 3B. When CH NH PbI is excited by light
3
3
3
illumination, electrons and holes are generated in the
conduction band (CB) and value band (VB) of CH NH PbI ,
3
3
3
respectively. With the appropriate energy level alignments,
electron injection happens at the TiO /CH NH PbI interface
2
3
3
3
from the CB (À3.86 eV) of CH NH PbI to the CB of TiO
3
3
3
2
[
40,41]
(
À4.00 eV), and the final collection is carried out by the
containing organic HTM,
and ascribed to the charge
FTO glass. In parallel, the holes are extracted by C through
transport in HTM at high frequencies and to the charge
transfer at the CH NH PbI /HTM interface at low frequen-
the C/CH NH PbI interface from the VB (À5.43 eV) of
3
3
3
3
3
3
CH NH PbI to C (Fermi level: À5.00 eV).
cies. In our case, therefore, the high-frequency semicircle may
be related to the charge transport in the C counter electrode,
while the lower-frequency semicircle may reflect the charge
transfer at the CH NH PbI /C interface. Clearly, the diameter
3
3
3
Figure 3C shows the J–V curves of the two TiO /
2
CH NH PbI /C solar cells. It can be seen that the solar cells
3
3
3
prepared with the C + CH NH I ink have an obviously higher
3
3
3
3
3
performance than that prepared by the C ink. Specifically, the
of the low-frequency semicircle strongly depends on Vappl, that
is, the higher the Vappl, the smaller the semicircle diameter,
testifying the attribution of the low-frequency semicircle to
charge transfer at the CH NH PbI /C interface. This is
À2
latter solar cells gives a V of 0.90 V, J of 15.00 mAcm , FF
oc
sc
of 0.63, and a PCE of 8.51%, while the former presents a V
oc
À2
of 0.95 V, Jsc of 17.20 mAcm , FF of 0.71, and a PCE of
1.60%. To specify the ratio of extracted current to incident
3
3
3
1
because the increase of Vappl upshifts the Fermi level of
photons at a given wavelength, the incident photon-to-
electron conversion efficiency (IPCE) spectra were recorded.
As shown in Figure 3D, the IPCE spectrum of the solar cells
prepared by C ink gives a high value (larger than 70%) before
CH NH PbI , promotes the electron transfer from
3
3
3
CH NH PbI to C (recombination reaction) and hence
3
3
3
[40]
lowers the recombination resistance (R ).
rec
To further understand charge transfer properties of the
5
50 nm but rapidly decreases above 550 nm and the cut off is
CH NH PbI /C interface, R was obtained by fitting the
3
3
3
rec
at about 800 nm; the integrated Jsc is estimated to be
complex plane impedance plots at different Vappl using the
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
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2
3
3
3
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3
3
NH I ink at different Vappl (0.2 V
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[
[
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3
3
3
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3
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3
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3
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[
Received: August 28, 2014
Published online: && &&, &&&&
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

.
Angewandte
Communications
Communications
Nanotechnology
Inkjet-printing solar cells: By designing
carbon plus CH NH I ink to transform
3
3
Z. Wei, H. Chen, K. Yan,
S. Yang*
PbI in situ to CH NH PbI , an inter-
2 3 3 3
&&&& — &&&&
penetrating seamless interface between
the CH NH PbI active layer and the C
3
3
3
Inkjet Printing and Instant Chemical
Transformation of a CH NH PbI /
Nanocarbon Electrode and Interface for
Planar Perovskite Solar Cells
hole-extraction electrode was instantly
constructed, resulting in a 11.60% effi-
cient, planar perovskite solar cell.
3
3
3
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!