Self-powered, ultraviolet-visible perovskite photodetector based on TiO2 nanorods

Article abstract of DOI:10.1039/c5ra27840f

A self-powered, ultraviolet-visible perovskite photodetector based on TiO₂ nanorods/CH₃NH₃PbI₃ heterojunction was reported. We found that the device showed good photovoltaic properties with a short-circuit current density of 17.83 mA cm^-2, an open-circuit voltage of 0.76 V and a fill factor of 51.34%, leading to a PCE of 6.95%. Based on the wide band gap and the perovskite supporting part of the TiO₂ nanorods, the device showed good ultraviolet-visible photo-response characteristics with the responsivity at zero bias reaching ～0.26 and 0.85 A W^-1 at 364 and 494 nm, respectively. These results present potential applications of TiO₂/perovskite photodetectors in ultraviolet and visible regions.

Full text of DOI:10.1039/c5ra27840f

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Self-powered, ultraviolet-visible perovskite
photodetector based on TiO2 nanorods
Cite this: RSC Adv., 2016, 6, 6205
Hai Zhou,a Zehao Song,a Pan Tao,a Hongwei Lei,b Pengbin Gui,a Jun Mei,a
a
Hao Wang and Guojia Fangb
*
A
self-powered, ultraviolet-visible perovskite photodetector based on TiO2 nanorods/CH3NH3PbI3
heterojunction was reported. We found that the device showed good photovoltaic properties with
a short-circuit current density of 17.83 mA cmꢀ2, an open-circuit voltage of 0.76 V and a fill factor of
51.34%, leading to a PCE of 6.95%. Based on the wide band gap and the perovskite supporting part of
the TiO2 nanorods, the device showed good ultraviolet-visible photo-response characteristics with the
responsivity at zero bias reaching ꢁ0.26 and 0.85 A Wꢀ1 at 364 and 494 nm, respectively. These results
present potential applications of TiO2/perovskite photodetectors in ultraviolet and visible regions.
Received 28th December 2015
Accepted 6th January 2016
DOI: 10.1039/c5ra27840f
www.rsc.org/advances
Nowadays, self-powered nanodevices and nanosystems have
attracted increasing attentions due to its self-concient charac-
I. Introduction
teristic, which can function well without an external power
source.10,11 For most of the traditional PDs, they need an external
electric eld to drive the photogenerated carriers to generate
photocurrent. As a novel self-powered PD based on the photovol-
taic effect, it can be operated at zero bias without consuming
external power. So it is highly desirable to meet the demands of the
energy shortage. In this work, CH3NH3PbI3/TiO2 nanorods self-
powered PDs were fabricated by a facile and low-cost process
and their ultraviolet (UV)-visible detection characteristics were
investigated. Herein, the TiO2 nanorods are prepared by hydro-
thermal synthesis method on FTO substrate directly without a seed
layer, which simplies the fabrication process of the device. And
therein, the TiO2 nanorods are important for two aspects: one is
can be a ultraviolet (UV) response layer for their wide band gap
semiconductor (ꢁ3.2 eV). The other is a supporting part for the
perovskite.
Organic–inorganic hybrid materials, particularly the perov-
skite family, have shown great promise for use in eld-effect
transistors, light-emitting diodes, sensors, and photodetec-
tors for more than a decade.1 Recently, with the power
conversion efficiency (PCE) of lead halide perovskite
(CH3NH3PbX3, X ¼ Cl, Br, I)-based thin lm photovoltaic
devices reaching more than 19.3%,2 organic–inorganic
perovskites have attracted more and more attention as light-
harvesting materials for mesoscopic solar cells with
enhanced performance due to their suitable band gap, high
absorption coefficient (>104 cmꢀ1), double carrier transport
properties and good tolerance for impurities and defects.3–7
For photodetectors, the organic–inorganic perovskites are
also very good for the visible detection with the simple
preparation process and low power consumption. Y. Yang
reported the perovskite photodetector based on CH3NH3-
PbI3ꢀxClx, the device showed exhibit a large detectivity
approaching 1014 Jones, which is about one order of magni-
tude higher than the detectivity of a Si photodetector in the
same spectral region.8 The perovskite photodetector based on
exible ITO coated substrate was also reported and its photo-
responsivity reaches to 3.49 and 0.0367 A Wꢀ1 with an
external quantum efficiency of 1.19 ꢂ 103%, 5.84% at 365 nm
and 780 nm with a voltage bias of 3 V, respectively.9
II. Experiment
The synthesis of TiO2 nanorods
The FTO substrates were initially cleaned with acetone, ethanol
and deionized water, respectively, and then blown dry with dry N2.
Then TiO2 nanorods were grown on FTO substrates without a TiO2
seed layer by hydrothermal synthesis method. The details of the
hydrothermal conditions for obtaining TiO2 nanorods are below:
in brief, 8 ml of deionized water was mixed with 8 ml of concen-
trated hydrochloric acid (36.5–38% by weight) and the mixture was
stirred in the air for 5 min. Aer that, 0.3 ml of titanium butoxide
was added and the solution was stirred for another 5 min, then it
was placed in a Teon-lined stainless steel autoclave. Then, the
FTO substrate was placed in. The hydrothermal synthesis was
conducted at 150 ꢃC for 3 h in an electric oven.
aHubei Collaborative Innovation Center for Advanced Organic Chemical Materials,
Hubei Key Laboratory of Ferroelectric and Dielectric Materials and Devices, Faculty
of Physics and Electronic Science, Hubei University, Wuhan 430062, China. E-mail:
nanoguy@126.com
bKey Lab of Acoustic and Photonic Materials and Devices of Ministry of Education,
Department of Electronic Science
& Technology, School of Physical Science &
Technology, Wuhan University, Wuhan, 430072, P. R. China
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Perovskite device fabrication
Organohalide lead perovskite (CH3NH3PbI3) was synthesized
using the method described by Lee et al. CH3NH3I was synthe-
sized rstly. Generally, hydroiodic acid (30 ml, 0.227 mol, 57
wt% in water, Aldrich) and methylamine (27.8 ml, 0.273 mol,
40% in methanol, TCI) were mixed and stirred in the ice bath
for 2 h. Aer stirring at 0 ꢃC for 2 h, the resulting solution was
evaporated at 50 ꢃC for 1 h and generated chemicals (CH3NH3I).
The precipitate was washed three times with diethylether, dried
under vacuum and used without further purication. 1 M PbI2
ꢃ
(dissolved in N,N-dimethylformamide (DMF) at 70 C for 12 h)
was spin coated onto the TiO2 nanorods at a speed of 2000 rpm
for 60 s and was heated at 70 ꢃC for 30 min. Subsequently,
the lm was dipped into a solution of CH3NH3I (10 mg mlꢀ1
,
dissolved in isopropanol) for 2 min. Then, the lm was ther-
ꢃ
mally annealed at 70 C in the air for 30 min. Aer the depo-
sition of perovskite materials, a hole transport layer (HTL)
solution, which consisting of 55 mM TBP, 26 mM Li-TFSI, and
68 mM spiro-OMeTAD dissolved in acetonitrile and chloro-
benzene (1 : 10, v/v), was coated on the TiO2 lm by spin-coating
at a speed of 2000 rpm for 30 s. Finally, the electrode was
deposited by thermal evaporation of gold under a pressure of
5 ꢂ 10ꢀ5 Torr. The active area was 0.09 cm2.
Fig. 1 The top (a) and cross-sectional (b) views of the SEM photograph
of the as-prepared TiO2 nanorods on FTO substrate. (c) The top view
of the SEM photograph of the CH3NH3PbI3 on TiO2 nanorods/FTO. (d)
The absorbance of the TiO2 nanorods and the CH3NH3PbI3 on TiO2
nanorods/FTO.
nanorods and the CH3NH3PbI3 on TiO2 nanorods/FTO. The
absorption spectrum of the CH3NH3PbI3 on TiO2 nanorods/FTO
shows a strong absorption from the near-IR region to 400 nm,
which mainly comes from CH3NH3PbI3. And the absorption in
UV region is mainly attributed to TiO2 nanorods.
The morphology was observed by Sirion eld emission
scanning electron microscopy (SEM) (Philips XL30). The PV
characteristics were measured with a Keithley 236 source-
measurement unit under 100 mW cmꢀ2 using an AM 1.5G
standard Newport #96000 Solar Simulator. The photosensitivity
was performed by using 66984 Xe arc source (300 W Oriel) and
Oriel Cornerstone™ 260 1/4 m Monochromator. The sample
was under illumination directly (parallel with the nanorods)
and the optical power of light was measured by a UV-enhanced
Si detector. All the I–V characteristics were measured by
a Keithley 4200 electrometer.
Fig. 2(a) shows the J–V characteristics of the device. The cell
delivered a short-circuit current density (Jsc) of 17.83 mA cmꢀ2
,
an open-circuit voltage (Voc) of 0.76 V and a ll factor (FF) of
51.34%, leading to a very high PCE of 6.95%. We attribute
this to the well-aligned band structure of the device (i.e., the
III. Results and discussion
In our experiment, the SEM images of TiO2 nanorods are shown
in Fig. 1(a) and (b), which are the top and cross-sectional views
of the as-prepared TiO2 nanorods on FTO substrate, respec-
tively. From the photographs, a 400 nm TiO2 lm was seen
between the TiO2 nanorods and FTO substrate. Also, the TiO2
nanorods grow vertically on the TiO2 lm and the gap between
TiO2 nanorods is very big, and the average diameter and length
of these TiO2 nanorods are around 200 nm and 2 mm. The
existence of the TiO2 lm may be attributed to the hydrothermal
synthesis method. Before the growth of the TiO2 nanorods, the
TiO2 seed layer will be formed rst under the hydrothermal
conditions. Then with the increase of time, the TiO2 nanorods
begin to grow on the TiO2 lm. Herein, the TiO2 lm will
prevent from perovskite contacting with the FTO, which will
result in a big photo-current. The top view of CH3NH3PbI3 lm
is shown in Fig. 1(c). From the photograph, we can see the
CH3NH3PbI3 lm is very at and uniform, also we can see the
CH3NH3PbI3 lm is completely lled and the ZnO nanorods
Fig. 2 (a) Current density versus voltage (J–V) plots of the perovskite-
based photovoltaic device. (b) Energy band diagram of the perovskite-
can not be seen. Fig. 1(d) shows the absorption of the TiO2 based solar cell.
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uorine-doped tin oxide (FTO), TiO2, CH3NH3PbI3, HTM and
Au electron-affinities and Fermi levels are closely co-ordinated)
(see Fig. 2(b)). This allows rapid charge-carrier separation at the
interface, preventing photo-excited electrons in TiO2 from
recombining with holes at the p–n junction.
R
D* ¼
(2)
1=2
ð2qJdÞ
here, Iph is the photo-current, S is the illuminated area, Popt is
the light density, Jd is the dark current and the shot noise from
the dark current is the major contribution to the noise. The
spectral responsivity and detectivity curves obtained under the
biases of 0 V are presented in Fig. 4(a) and (b), respectively.
From the curve of responsivity, our perovskite photodetector
shows UV-visible response. The UV responses should be
attributed to the TiO2 due to its wide band gap, and the
responses in visible region should be caused by the perovskite.
Also we can see the responsivity shows two peaks located at
364 and 494 nm, respectively, which is the value of ꢁ0.26 and
0.85 A Wꢀ1 at 364 and 494 nm, respectively. Moreover, from
800 to 494 nm, the value of responsivity curve shows
a continued increasing trend and reaches to the biggest at 494
nm, which is consistent with the report by Gao, et al.:12 perov-
skite materials show strong light-capturing ability over the
spectral region of 300–800 nm and in particular around 500 nm
they can absorb more than 90% of incident light. Due to the
low dark current of the device at zero bias, the perovskite
photodetector shows good detection performance, which is
shown in Fig. 4(b). We can see the peak detectivity of the
perovskite photodetector is above 7.8 ꢂ 1010 cm Hz1/2 Wꢀ1 at
494 nm. The value of detectivity is slightly slow compared with
the report,8 we think the main reason is that the dark current of
the device is a lot big and our following work will focus on the
method of decreasing the dark current, such as an insertion
layer applied in perovskite photodetector. From above, we
conclude that the TiO2 nanorods/CH3NH3PbI3 heterojunction
can work as a self-powered, ultraviolet-visible PD.
To investigate the self-powered photo-response character-
istic of the TiO2 NRs/CH3NH3PbI3 heterojunction device, more
detailed characteristics of the devices are shown in Fig. 3(a)–(d).
Fig. 3(a) shows the I–V curves of the devices measured at dark
and under illumination of 365 nm UV light, respectively. It
shows that our diodes have large photocurrent under UV light
with the current of ꢁ15 mA at zero bias. The photoresponse
behavior of the TiO2 NRs/CH3NH3PbI3 heterojunction is char-
acterized by measuring the current as a function of time at
zero bias when the light was periodically turned on and off, as
shown in Fig. 3(b). From the gure, we can see that for each
cycle, the “on”- and “off”-state currents remain the same within
the noise level, which indicates the reversibility and stability
of our PD. We can see that upon 365 nm UV illumination with
light intensity of 17.2 mW cmꢀ2, the current increases quickly
from ꢁꢀ5 to ꢁꢀ422 nA at 0 V, which indicates good UV pho-
toresponse characteristic. And upon 546 nm visible-light illu-
mination with light intensity of 37.9 mW cmꢀ2, the current
increases to ꢁꢀ750 nA at 0 V. And the ratio of photocurrent to
dark current is as high as 150 at 546 nm. The spectral responses
of Voc and Isc shown in Fig. 3(c) and (d) give another important
characteristic of the self-powered PD. Both Voc and Isc display
spectral responses from 300 to 800 nm, indicating that the
device can be used as UV-visible PD.
The responsivity (R) and detectivity (D*) are the important
parameters to characterize the quality of the optimized PD.
Based on the following equations
Iph
R ¼
(1)
SPopt
Fig. 3 (a) I–V characteristics of the perovskite-based PD under dark
and 365 nm UV illumination. (b) The dependence of photocurrent on
operating time for the PDs at zero bias. (c) The dependence of Voc on
operating wavelength for the perovskite-based PDs. (d) The depen- Fig. 4 (a) The spectral responsivity curve obtained under zero bias; (b)
dence of Isc on operating wavelength for the perovskite-based PDs.
the spectral detectivity curve under zero bias.
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The TiO2 nanorods/CH3NH3PbI3 heterojunction photode-
tector shows good self-powered, ultraviolet-visible photo-
response characteristics, the reason can be understood based
on the energy level diagram (shown in Fig. 2(b)). It is clear that
higher energy gaps and favorable energy alignment of TiO2
and CH3NH3PbI3 made them suitable candidates for photo-
response application. Under irradiation, the electron–hole
pairs are generated when photon energy exceeds the energy
band gap (hy > Eg). Photogenerated electrons ow from TiO2 to
the cathode. Although the perovskites have double carrier
transport ability, the photogenerated electrons can not move
from CH3NH3PbI3 to the anode due to the block of the HTM,
and the holes move from CH3NH3PbI3 to the anode. These
processes will contribute to a photocurrent at zero bias. With
the increase of the absorption, more photons will be captured
and generated more electron–hole pairs, leading to bigger
current and better photoresponse characteristics. Due to the
strong light-capturing ability of CH3NH3PbI3 lm in particular
around 500 nm, the device shows the biggest value of respon-
sivity near this wavelength. Although the absorption will
increase with the decrease of the wavelength, responsivity will
not increase, and the reason is that the shorter wavelengths
are beyond its wavelength of the best photoelectric trans-
formation efficiency due to the band-to-band transition of
different electrons excited by different energy photons.13
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IV. Conclusion
In conclusion, a self-powered, ultraviolet-visible perovskite
photodetector based on TiO2 nanorods/CH3NH3PbI3 hetero-
junction was fabricated. The device showed good photovoltaic
properties with a short-circuit current density of 17.83 mA
cmꢀ2, an open-circuit voltage of 0.76 V, a ll factor of 51.34%
and a very high PCE of 6.95%. Also it displayed good ultraviolet-
visible photo-response characteristic at zero bias, respectively.
Acknowledgements
This work is supported in part by the National Nature Science
Foundation of China (No. 51372075), Research Fund for the
Doctoral Program of Higher Education of China (RFDP, No.
20124208110006), Natural Science Foundation of Hubei Prov-
ince (No. 2014CFB538).
13 H. Zhou, J. Mei, P. Gui, P. Tao, Z. Song, H. Wang and
G.-J. Fang, The investigation of Al-doped ZnO as an
electron transporting layer for visible-blind ultraviolet
photodetector
based
on
n-ZnO
nanorods/p-Si
heterojunction, Mater. Sci. Semicond. Process., 2015, 38, 67–
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6208 | RSC Adv., 2016, 6, 6205–6208
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