Photocurrent induced by conducting channels of hole transporting layer to adjacent photoactive perovskite sensitized TiO2 thin film: Solar cell paradigm

Article abstract of DOI:10.1021/la502398x

A high performance perovskite solar cell was fabricated using the distinguished morphology of polyaniline nanoparticles (PANI-NPs) as an efficient hole transporting layer (HTL) with methylammonium lead iodide perovskite (CH₃NH₃PbI₃) as sensitizer. PANI-NPs were simply synthesized by the oxidative chemical polymerization of aniline monomer at 0-5 °C. A reasonable solar-to-electricity conversion efficiency of ～6.29% with a high short circuit current (J_SC) of ～17.97 mA/cm² and open circuit voltage (V_OC) of ～0.877 V were accomplished by Ag/PANI-NPs/CH₃NH₃PbI₃/mp-anatase-TiO₂/bl-TiO₂/FTO perovskite solar cell. The transient photocurrent and photovoltage studies revealed that the fabricated solar cell showed better charge transport time, diffusion coefficient, diffusion length, and charge collection efficiency. Herein, the use of PANI-NPs as the HTL improved the charge carrier generation and the charge collection efficiency of the fabricated solar cell.

Full text of DOI:10.1021/la502398x

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Article
Photocurrent induced by conducting channels of hole transporting layer to
adjacent photoactive perovskite sensitized TiO2 thin film: Solar cell paradigm
Sadia Ameen, M. Shaheer Akhtar, Hyung-Kee Seo, and Hyung-Shik Shin
Langmuir, Just Accepted Manuscript • DOI: 10.1021/la502398x • Publication Date (Web): 08 Oct 2014
Downloaded from http://pubs.acs.org on October 16, 2014
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Photocurrent induced by conducting channels of hole transporting layer to adjacent
photoactive perovskite sensitized TiO2 thin film: Solar cell paradigm
Sadia Ameen†a, M. Shaheer Akhtar†b, Hyung-Kee Seoa** and Hyung-Shik Shin*a
aEnergy Materials & Surface Science Laboratory, Solar Energy Research Center, School of
Chemical Engineering, Chonbuk National University, Jeonju, 561ꢀ756, Republic of Korea
bNew & Renewable Energy Material Development Center (NewREC), Chonbuk National
University, Jeonbuk, Republic of Korea
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______________________________________________________________________________
Abstract
The high performance perovskite solar cell was fabricated using distinguished
morphology of polyaniline nanoparticles (PANIꢀNPs) as efficient hole transporting layer (HTL)
with methyl ammonium lead iodide perovskite (CH3NH3PbI3) as sensitizer. PANI NPs were
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simply synthesized by the oxidative chemical polymerization of aniline monomer at 0ꢀ5 C. The
reasonable solarꢀtoꢀelectricity conversion efficiency of ~6.29 % with high short circuit current
(JSC) of ~17.97 mA/cm2 and open circuit voltage (VOC) of ~0.877 V were accomplished by
Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO perovskite solar cell. The transient
photocurrent and photovoltage studies revealed that the fabricated solar cell showed better
charge transport time, diffusion coefficient, diffusion length and the charge collection efficiency.
Herein, the use of PANI NPs as HTL improved the charge carrier generation and the charge
collection efficiency of the fabricated solar cell.
Keywords: Polyaniline, nanoparticles, perovskite, thin film, solar cells, charge collection
efficiency
*Corresponding author. Tel: +82ꢀ63ꢀ270ꢀ2438, Fax: +82ꢀ63ꢀ270ꢀ2306
Eꢀmail address: hsshin@jbnu.ac.kr (H. S. Shin), **enterpride2@jbnu.ac.kr (H. K. Seo)
†Authors contributed equally to this work
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1. INTRODUCTION
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A new kind of inorganicꢀorganic solar cell based on organic halide perovskite materials have
recently gained a great deal of attention due to good electron and hole conductivity,1ꢀ3 high
carrier mobility (50 cm2/V.s),4 direct band gap (~1.55 eV) and high stability.5 Interestingly, the
perovskite as sensitizer displays a very strong absorption in the visible region to the near infrared
region6,7and their optical and electronic properties could be tuned by changing the chemical
compositions of perovskites.2,8 The light absorption in perovskites solar cell could be improved
by using light absorbing polymers as HTLs, which could improve the optical density of the
mesoporic thin films due to the advancement in the light harvesting by the holeꢀconductors with
the pores of thin films.9,10 The perovskites are highly versatile materials with spectral tunability
which efficiently improves the photoinduced electron transfer from the pꢀtype polymers to nꢀtype
metal oxides thin films.11,12
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In general, the hole transporting material, 2,2’,7,7’ꢀtetrakisꢀ(N,Nꢀdiꢀpꢀmethoxyphenylꢀ
amine)ꢀ 9,9’ꢀspirobifluorene (spiroꢀMeOTAD) is commonly used in various dye sensitized solar
cells and perovskite solar cells because it shows the efficient charge transport, low recombination
rates and improves the pore filling of TiO2 layer. Hitherto, the synthetic process of spiroꢀ
MeOTAD is excessively costly at the laboratory level and therefore, it is important to find cheap
and effective alternatives to spiroꢀMeOTAD for the fabrication of low cost perovskite solar cells.
So far, several conducting polymers such as polyꢀ[2,1,3ꢀbenzothiadiazoleꢀ4,7ꢀdiyl [4,4ꢀbis(2ꢀ
ethylhexyl)ꢀ4Hꢀcyclopenta [2,1ꢀb:3,4ꢀb`] dithiopheneꢀ2,6ꢀdiyl]] (PCPDTBT), (polyꢀ[[9ꢀ(1ꢀ
octylnonyl)ꢀ9Hꢀcarbazoleꢀ2,7ꢀdiyl]ꢀ2,5ꢀthiophenediylꢀ2,1,3ꢀbenzothiadiazoleꢀ4,7ꢀdiylꢀ2,5ꢀ
thiophenediyl]) (PCDTBT) and polyꢀtriarylamine (PTAA) have been used as HTLs or electronꢀ
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blocking layers for the fabrication of perovskite solar cells.13 A apart from various conducting
polymers, polyaniline (PANI) is one of the most extensively studied polymers owing to its ease
of synthesis, high conductivity, and the good environmental stability.14 PANI shows versatility in
nanostructures of nanofibers, nanorods, nanowires, nanotubes and nanoflakes with high surface
to volume ratio and low diffusional resistance.15 PANI nanomaterials are widely used for the
fabrication of efficient electronic and nanodevices such as FETs, sensors, catalysis, photovoltaics
etc. due to its improved optical, structural, electronic and electrical properties.16 Recently, Y.
Xiao et al. fabricated the solidꢀstate perovskiteꢀsensitized solar cell using dual function of PANI
with brachyplast structure, in which PANI structure was deposited by a twoꢀstep cyclic
voltammetry (CV) approach.17 Inspired by the aforementioned development, a highly efficient
solar cell is fabricated using organic halide perovskite (CH3NH3PbI3) as light harvester which is
coated on mesoporous TiO2 and the organic PANI NPs is applied as HTL. In this work, the
fabricated perovskite solar cell achieves reasonably high incidentꢀphotonꢀtoꢀcurrent efficiency
(IPCE) of ~51% in the wavelength range of ~450ꢀ700 nm and maximum overall solarꢀtoꢀ
electricity conversion efficiency of ~6.29 % under AM 1.5 illumination at an intensity of 100
mW cmꢀ2. The use of PANI NPs as HTL has substantially improved the charge carrier generation
and the charge collection efficiency of the fabricated perovskites solar cell.
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2. EXPERIMENTAL
2.1 Synthesis of PANI NPs by oxidative chemical polymerization
In a typical synthesis of PANI NPs, 3 mmol of aniline monomer (0.27 ml, SigmaꢀAldrich,
≥99.5%) was dissolved in 10 ml DI (deionized) water under constant stirring at the room
temperature. Thereafter, a separate solution of 10.9 mmol ammonium perꢀoxydisulphate (APS,
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2.5g, RiedelꢀdeꢀHaen, 97%) was prepared and added drop by drop into the prepared aniline
monomer solution using peristaltic pump. Finally, the reaction mixture was kept for 12 h under
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static condition at 0ꢀ5 C. The obtained greenish precipitates were centrifuged at ~4500 rpm for
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10 min and the desired product was washed with copious amount of DI water, methanol and
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dried in vacuum oven at 40 C. In order to achieve the optimized condition for PANI NPs, the
concentrations of aniline monomer were varied from 1 mmol ꢀ 6 mmol while the concentration
of ammonium perꢀoxydisulphate was kept constant to 10.9 mmol, as presented in the supporting
information.
2.2 Preparation of perovskite CH3NH3PbI3 solution
The methyl ammonium iodide (CH3NH3I) was synthesized as reported elsewhere.18 In
brief, the reactant mixture of methylamine (27.86 ml, CH3NH2, 40% in methanol, TCI
chemicals) and hydroiodic acid (30 ml of 57 wt% in water, HI, Aldrich, 99%) were taken in 200
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ml round bottom flask. The reaction mixture was continuously stirred at 0 C for 4 h and
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afterwards, the precipitates were recovered by the evaporation at 50 C for 1 h. The obtained
yellow product of CH3NH3I was repeatedly washed with diethyl ether ((C2H5)2O Alfa Aesar,
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99+% assay) and dried at 60 C in vacuum oven for 24 h. The synthesis of CH3NH3PbI3 was
carried out by mixing equimolar precursor solutions of CH3NH3I and lead iodide (PbI2, Aldrich,
99%) in γꢀbutyrolactone (C4H6O2, TCI, 99%) at 60 oC for 12 h.
2.3 Fabrication of perovskite solar cell (Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀ
TiO2/FTO)
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The fluorinated tin oxide glass (FTO glass, Hartford Glass Co., 8ꢁ/sq, 80% transmittance
in visible spectrum) substrates were partially etched by using zinc powder (Aldrich, 99%) and
2M hydrochloric acid (HCl, Samchun Chemicals, 35ꢀ37%). The etched FTO substrates were
cleaned by the ultrasonic bath using acetone, isopropanol and DI water. Thereafter, the etched
FTO substrates were coated with blocking layer (bl) using 0.1 M Ti(IV) bis(ethyl acetoacetato)ꢀ
diisopropoxide (CH3CH2OCOCH=C(Oꢀ)CH3]2Ti(OCH(CH3)2)2, Alfa Aesar, 75% in
Isopropanol) in 1ꢀbutanol (C4H9OH, Showa Chemicals, 99%) solution by spinꢀcoating method
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and then the substrates were heated at 500 C for 30 min. The mesoporous (mp) anataseꢀTiO2
layer was screenꢀprinted on the preꢀtreated FTO glass substrates using diluted TiO2 paste
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(Solaronix) and again sintered at 500 C for 30 min. The synthesized perovskite solution was
coated on the annealed TiO2 thin film through spin coating at the speed of ~2000 rpm for 40 s
using 0.45 µm pore PVDF membrane syringe filter (Jet Biofil) at ambient atmosphere.
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Thereafter, the thin films were annealed at 100 C for 30 min to achieve the CH3NH3PbI3/mpꢀ
anataseꢀTiO2/blꢀTiO2/FTO. PANIꢀNPs as HTL was spinꢀcoated on CH3NH3PbI3/mpꢀanataseꢀ
TiO2/blꢀTiO2/FTO substrate at ~3000 rpm for 30s using PANI solution in choloroform (15 mg/1
ml) with 13.6 µl Liꢀbis (trifluoromethanesulfonyl) imide (CF3SO2NLiSO2CF3, Liꢀ
TFSI)/acetonitrile (28.3 mg/1 ml, TCI, >98%) and ~6.8 µl TBP (C9H13N, Aldrich, 96%) as
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additives in the ambient condition and dried at 100 C for 15 min to obtain PANIꢀ
NPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO. Finally, Ag contact (thickness ~100 nm) was
made by the thermal evaporation to achieve Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀ
TiO2/FTO as fabricated perovskite solar cell.
2.4 Characterizations
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The field emission scanning electron microscopy (FESEM, Hitachi Sꢀ4700) and the
atomic force spectroscopy (AFM, Nanoscope IV, Digital Instruments, Santa Barbara, USA) were
used to investigate the morphology of mpꢀanataseꢀTiO2/blꢀTiO2/FTO, CH3NH3PbI3/mpꢀanatase
TiO2/blꢀTiO2/FTO and Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO thin films.
The line scan element mapping was analyzed by the FESEM coupled Energy dispersive Xꢀray
spectroscopy with the mapping mode. The crystalline nature and phases of CH3NH3PbI3/FTO
and CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO thin films were studied by Xꢀray powder
diffraction (XRD, Rigaku, Cu Kα, λ = 1.54178A˚) in the Bragg angle ranging between 20oꢀ80o.
The optical properties of the deposited thin film were demonstrated by UVꢀVis
spectrophotometer (JASCO, Vꢀ670, Japan) and photoluminescence (PL) spectroscopy (JASCO,
FPꢀ6500). The current density (J)ꢀvoltage (V) measurements were performed for elucidating the
performance of Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO perovskite solar cell
by using computerized digital multimeter (model 2000, Keithley) with a variable load under one
sun (1.5 AM at 100 mW/cm2). A 1000 W metal halide lamp supplied the light and the intensity
was adjusted to simulate 1.5 AM at 100 mW/cm2 using a Si photo detector fitted with a Kaꢀ5
filter as a reference, which was calibrated at NREL, USA. Before JꢀV measurements, a mask of
black tape was pasted on top of the cell exempting the active area. The operating temperature
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was maintained in the range of 20ꢀ30 C by a small cooling fan. The incident photonꢀtoꢀcurrent
conversion efficiency (IPCE) was carried out by specially designed IPCE system for solar cell by
PV measurements, Inc., USA. Before performing the IPCE measurements, the system was
calibrated with a silicon photodiode, which was calibrated using the NISTꢀcalibrated photodiode
G425 as standard. The IPCE results of Ag/PANI/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO
perovskite solar cell were collected as a function of wavelength from ~400ꢀ800 nm using 75 W
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Xe lamp as a light source for generating monochromatic beam at a low chopping frequency. The
charge collection efficiency and photoelectron density were revealed by the analysis using
intensityꢀmodulated photocurrent spectroscopy (IMPS) and intensityꢀmodulated photovoltage
spectroscopy (IMVS) using IVIUM technologies (CompactStat.e20250, USA).
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3. RESULTS AND DISCUSSION
The morphological investigations are examined by taking the surfaceꢀview and cross
section FESEM images of the fabricated perovskite solar cell, as shown in Figure 1. The surface
view (Figure 1(a)) of TiO2 exhibits the uniformly distributed mesoporous thin film of TiO2
nanoparticles. After the perovskite coating, the thin film possesses uniform grain structure of
microscale size which is completely penetrated into the mesoporous TiO2 thin film deposited on
FTO substrate (Figure 1(b)). The crossꢀsectional FESEM image (Figure 1(c) reveals the layered
structure of perovskite solar cell. From Figure 1(c), the thickness of overall fabricated perovskite
solar cell is estimated as ~1.5 µm, consisting of CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2
mesoporous layer (∼600ꢀ700 nm) which is uniformly capped by PANIꢀNPs HTL. A thin Ag
contact layer of ~100 nm could be clearly seen on top of the active layer of the fabricated
perovskite solar cell.
Figure 2 shows the topographic and three dimensional (3D) AFM images of the
synthesized PANIꢀNPs and PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2 thin films. Figure
2(a) displays spherical aggregated nanoparticles of PANI with the average size of ~20 nm. The
wellꢀmixed structure of perovskite and PANIꢀNPs is observed in Figure 2 (c), indicating the
good penetration of PANIꢀNPs into the perovskite thin film. From 3D AFM images (Figure 2(b,
d)), PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2 thin film shows the less rough surface
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compared to PANIꢀNPs thin film. The root mean roughness (Rrms) of films is calculated by the
AFM images of the thin films. PANIꢀNPs thin film obtains the higher Rrms value (~46.5 nm) than
PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2 thin film (~28.2 nm), indicating the good pore
filling of mesoporous TiO2 thin film along with the perovskite sensitizer.
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The elemental compositions of each layer in PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀ
TiO2/blꢀTiO2 thin film are estimated by the elemental mapping and line scanning of crossꢀ
sectional view through electron Xꢀrays dispersive spectroscopy (EDS), as depicted in Figure 3.
The mapping image (Figure 3(a)) displays that the thin film is majorly comprised of C, Ti, O, N,
I and Pb with atomic % of ~9.25%, ~28.76% ~56.26% ~3.74%, 0.51 and 1.48% respectively.
The similar patterns are seen in the line scanning profile, as shown in Figure 3(b). It has been
observed that the atomic % ratio of Pb to I is 1:3, which deduces the formation of PbI3 in the
perovskite sensitizer.
The Xꢀray diffraction patterns of CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO and
CH3NH3PbI3/FTO thin films are examined to investigate the crystalline nature of the perovskite
sensitizer deposited on TiO2 thin film. Figure 4 (a) shows the diffraction peaks at ~14.2o, ~19.6o,
~28.6o, ~32.1o, ~40.6o and ~43.3°, which correspond to (110), (112), (220), (310), (224), and
(314) crystal planes of the tetragonal perovskite structure.19,20 Whereas, the other peaks at ~26.6o,
~33.8o, ~37.7, ~51.4o, and ~54.6o belong to the FTO layer of the glass substrate.16 The XRD
pattern of CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO exhibits the visible TiO2 diffraction peaks
along with significant perovskite and the FTO diffraction peaks. Importantly, no impurity peaks
other than the deposited materials are observed, suggesting that CH3NH3PbI3/mpꢀanataseꢀ
TiO2/blꢀTiO2/FTO thin film possess pure phase of CH3NH3PbI3 and TiO2 materials.
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The ultravioletꢀvisible (UVꢀvis) and the room temperature photoluminescence (PL)
spectroscopy of CH3NH3PbI3/FTO and PANIꢀNPs/CH3NH3PbI3/FTO thin films are analyzed to
investigate the optical properties. Figure 5(a) shows a typical UVꢀvis absorption spectrum of
CH3NH3PbI3/FTO thin film. From the derived tauc plot, CH3NH3PbI3/FTO thin film exhibits the
optical bandgap (Eg) of ~1.69 eV, which is consistent with the other reports of CH3NH3PbI3.20,21
The deposited CH3NH3PbI3/FTO thin film absorbs a higher wavelength edge near ~700 nm.
However, after the deposition of PANIꢀNPs on CH3NH3PbI3/FTO thin film, a red shift occurs
and decreases the optical band gap. The increase in the peak intensity of UVꢀVis spectra of
PANIꢀNPs/CH3NH3PbI3/FTO thin film might ascribe to the selective interactions between
CH3NH3PbI3 and the quinoid ring of PANIꢀNPs, which might facilitate the charge transfer from
quinoid unit to CH3NH3PbI3 via highly reactive imine groups. The room temperature PL spectra
of CH3NH3PbI3/FTO and PANI/CH3NH3PbI3/FTO thin films are carried out with an excitation
wavelength of ~320 nm. Figure 5(b) shows that CH3NH3PbI3/FTO and PANIꢀ
NPs/CH3NH3PbI3/FTO thin films display similar emission bands in the blue and green regions.
Noticeably, PANIꢀNPs/CH3NH3PbI3/FTO thin film presents the lower PL intensity than
CH3NH3PbI3/FTO thin film due to the significant PL quenching which clearly indicates a contact
between pevoskite layer and PANI layer. The occurrence of PL quenching might facilitate the
chargeꢀcarriers generation in CH3NH3PbI3 at PANIꢀNPs/CH3NH3PbI3/FTO interface.22
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A schematic representation of the fabricated perovskite solar cell is depicted in Figure
6(a). The first layer of the fabricated solar cell is formed by TiO2 blocking layer followed by the
crystalline meosporous (mp) TiO2 layer on the FTO substrate. The crystalline CH3NH3PbI3 are
deposited on top of the mpꢀanataseꢀTiO2 by the solventꢀdrying during spinꢀcoating, followed by
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evaporation at 80 C under vacuum. The hole transport layer (PANIꢀNPs) is coated on the
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layered structure of the CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2 thin film in which HTL assists in
the hole extraction to the top deposited Ag electrode. Figure 6(b) shows the energy level diagram
of perovskite solar cell. Under light illumination, the photons are absorbed by CH3NH3PbI3 and
create the electronꢀhole pairs. Thereafter at the CH3NH3PbI3/mpꢀ anataseꢀTiO2/blꢀTiO2 interface,
the generated excitons dissociate and the photoexcited electron injects into the conduction band
of TiO2, and simultaneously hole reaches to HTL. During the operation of solar cell, some of the
remaining holes run across the perovskite layer before reaching to HTL. The exciton dissociates
and the charge transfers at CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2 and PANI/CH3NH3PbI3/mpꢀ
anataseꢀTiO2/blꢀTiO2 interfaces to generate the energy. The bottom section of perovskite solar
cell acts as an electron collector which is composed of ~100 nm thick holeꢀblocking compact
TiO2 film deposited on the FTO followed by the deposition of ~600ꢀ700 nm thick layer of
PANI/CH3NH3PbI3/mpꢀ anataseꢀTiO2/blꢀTiO2.
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The photovoltaic parameters of the fabricated Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀ
TiO2/blꢀTiO2/FTO perovskites solar cell are obtained by measuring the current densityꢀvoltage
(JꢀV) characteristics under a light intensity of 100 mWcmꢀ2 (1.5 AM). The JꢀV curve is shown in
the Figure 7(a) for the fabricated perovskite solar cells with and without PANI NPs. The overall
solarꢀtoꢀelectric conversion efficiency of ~6.29 % with high short circuit current density (JSC) of
~17.97 mA.cmꢀ2, open circuit voltage (VOC) of ~0.877 V and low fill factor (0.40) are accounted
by the fabricated Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO perovskite solar
cell. However, the inferior JSC of ~9.67 mA/cm2 and solarꢀtoꢀelectric conversion efficiency of
~1.95 % are observed for the fabricated solar cell without using PANIꢀNPs HTL, indicating that
the introduction of PANIꢀNPs as HTL is a crucial step to improve the photovoltaic parameters.
Importantly, the photocurrent and the photovoltage are much superior to dyeꢀsensitized solar
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cells based on PANI HTLs.23 The uniform covering of HTL over CH3NH3PbI3/mpꢀanataseꢀ
TiO2/blꢀTiO2/FTO thin film has reduced the recombination and the extraction of hole improves
the charge transfer rate.24The small spherical nanoparticles of PANI might help to achieve the
uniform surface coverage over CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO thin film, which
might reduce the occurrence of pinꢀholes (due to extraction of holes) and avoids the direct
contact of HTL with mpꢀTiO2 layer, and enhances the dark current leakages.4,25The performance
of Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO perovskite solar cell has been
quantified on a macroscopic level in terms of the incidence photon to current conversion (IPCE)
efficiency, as shown in Figure 7(b). The fabricated Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀ
TiO2/blꢀTiO2/FTO perovskite solar cell presents maximum IPCE value of ~51% in the
wavelength range of ~450ꢀ700 nm and drops at longer wavelengths. The obtained IPCE is
consistent with the JSC of the perovskite solar cells. Herein, PANIꢀNPs with high surface area
might significantly improve the light scattering capacities and provides the better interaction
between the photons and CH3NH3PbI3 sensitizer. In order to investigate series and charge
transfer resistances of the solar cells, the Nyquist plots are obtained from the fabricated
perovskite solar cells with and without PANIꢀNPs under a frequency range from 100 kHz ꢀ 1 Hz,
as shown in Figure 7. From impedance plots, the intercept of Zre at high frequency belongs to the
ohmic series resistance (RS) and the diameter of first semicircle at high frequency represents the
chargeꢀtransfer resistance (RCT) at PANI/CH3NH3PbI3 interface.26 The fabricated perovskite solar
cells with and without PANI shows large RS values of ~20.5 ꢁ and ~41.2 ꢁ respectively. It is
reported that the high RS effectively results to low fill factor and increases the recombination
sites.27 In our case, the fabricated perovskite solar cell with PANI obtains large RS which might
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considerably result to low FF. However, the high JSC is related to the improved transporting
ability for the electron and hole, as observed the low RCT values.
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The intensity modulation photocurrent spectroscopy (IMPS) and the intensity modulation
photovoltage spectroscopy (IMVS) measurements have been examined to elucidate the charge
transfer and the recombination processes in the fabricated perovskite solar cells. IMPS and
IMVS measurements are performed under the fixed light intensity at different voltages of light,
as shown in Figure 8. From Figure 8, the chargeꢀtransport time (τCT) and the electron
recombination time (τR) of the fabricated thin film solar cell are estimated using the minimum
frequencies of IMPS and IMVS plots respectively.28 The fabricated Ag/PANIꢀ
NPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO perovskite solar cell records the reasonable τCT
of ~25.2 ms and τR of ~146 ms, suggesting the high charge transport rate and lesser
recombination rate. Figure 9 (a,b) shows the plots of τCT and τR versus different photon flux of
the fabricated perovskite solar cell which are derived by IMPS and IMVS. The order of τCT and
τR value decreases with the increase of photon flux. The generation of electronꢀhole
recombination and the electron diffusion in perovskite solar cell are examined by evaluating the
electron diffusion length (DL) parameter i.e. DL = (Dn·τR)1/2, 29 where Dn is diffusion coefficient
obtained by IMPS plot. For the fabricated Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀ
TiO2/FTO perovskite solar cell, Dn value is estimated as ~3.83 x 10ꢀ6 cm2sꢀ1. The relatively low
DL value of ~2.36 µm is observed by the fabricated perovskite solar cell which represents the
probability of large electrons to enter from the CH3NH3PbI3/mpꢀ anataseꢀTiO2/blꢀTiO2/FTO thin
film layers to the top Ag layer electrode. The low FF might result from the low DL value.
Moreover, the charge collection efficiency could be calculated by the relation ηCC = 1–(τCT/τR)
using τCT and τR values of the fabricated perovskite solar cell.29 The high ηCC value is obtained
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for the fabricated Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO perovskite solar
cell, suggesting the high charge collection during the illumination and results to the high
electronꢀtransport rate and the photocurrent density. Thus, the introduction of PANIꢀNPs as HTL
might enhance the charge transfer time and the charge collection of the fabricated perovskite
solar cells and produces the high performance of the perovskite solar cell.
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4. CONCLUSIONS
In summary, a unique and wellꢀdefined morphology of PANIꢀNPs as efficient HTL is
employed to fabricate the high performance perovskite solar cell using methyl CH3NH3PbI3 as
sensitizer. The reasonable solarꢀtoꢀelectricity conversion efficiency of ~6.29 % with short circuit
current (JSC) of ~17.97 mA/cm2 and open circuit voltage (VOC) of ~0.877 V are obtained by
Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀTiO2/FTO perovskite solar cell. The IPCE
measurement reveals that the fabricated Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀanataseꢀTiO2/blꢀ
TiO2/FTO perovskite solar cell presents the maximum value of ~51% in the wavelength range of
~450ꢀ700 nm and drops at longer wavelengths. The transient photocurrent and photovoltage
studies elucidate that the fabricated solar cell exhibits a high ηCC value, suggesting the high
charge collection during the illumination due to good transport time and the low recombination
time which significantly results to high electronꢀtransport rate and the photocurrent density.
Hence, the use of PANIꢀNPs as HTL improves the charge carrier generation and the charge
collection efficiency of the fabricated perovskite solar cell.
ACKNOWLEDGEMENTS
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Dr. Sadia Ameen acknowledges the Research Funds of Chonbuk National University in
2012. This work is fully supported by NRF Project # 2014R1A2A2A01006525. This work is
also supported by “Leaders in IndustryꢀUniversity Cooperation Project”(2014), supported by the
Ministry of Education, Science & Technology (MEST) and the National Research Foundation of
Korea (NRF). We acknowledge the Korea Basic Science Institute, Jeonju branch, for utilizing the
research supported facilities.
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Supporting Information
This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure Captions
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Figure1. Surface FESEM images of (a) mpꢀTiO2/blꢀTiO2/FTO, (b) CH3NH3PbI3/mpꢀTiO2/blꢀ
TiO2/FTO and (c) the cross section of Ag/PANIꢀNPs/CH3NH3PbI3/mpꢀTiO2/blꢀTiO2/FTO
thin film.
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Figure 2. Topographic (a, c) and three dimensional (b, d) AFM images of synthesized PANI NPs
and PANIꢀNPs/CH3NH3PbI3/mpꢀTiO2/blꢀTiO2 thin film.
Figure3. Elemental mapping (a) and the line scanning profile (b) of PANIꢀ
NPs/CH3NH3PbI3/mpꢀTiO2/blꢀTiO2 thin film.
Figure 4. XRD patterns of (a) CH3NH3PbI3/FTO and (b) CH3NH3PbI3/mpꢀTiO2/blꢀTiO2/FTO
thin films.
Figure 5. (a) UVꢀVis spectra and (b) room temperature photoluminescence spectra of
CH3NH3PbI3/FTO and PANIꢀNPs/CH3NH3PbI3/FTO thin film. Inset shows the UVꢀVis
spectrum of PANI NPs.
Figure 6. A schematic representation (a) and energy level diagram (b) of Ag/PANIꢀ
NPs/CH3NH3PbI3/FTO perovskite solar cell.
Figure 7. JꢀV curves (a) and Nyquist plots (b) of fabricated perovskite solar cells with and
without PANIꢀNPs and (c) IPCE curve of the fabricated Ag/PANIꢀ
NPs/CH3NH3PbI3/FTO perovskite solar cell.
Figure 8. (a) IMVS and (b) IMPS measurement plots of the fabricated
Ag/PANI/CH3NH3PbI3/FTO perovskite solar cell.
Figure 9. The electron transport (a) and the recombination lifetime of electrons (b) of the
fabricated Ag/PANIꢀNPs/CH3NH3PbI3/FTO perovskite solar cell with respect to different
incident photon fluxes.
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Figure 1
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Figure 2
ACS Paragon Plus Environment

Langmuir
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Figure 3
ACS Paragon Plus Environment

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Figure 4
ACS Paragon Plus Environment

Langmuir
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Figure 5
ACS Paragon Plus Environment

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Figure 6
ACS Paragon Plus Environment

Langmuir
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Figure 7
ACS Paragon Plus Environment

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Figure 8
ACS Paragon Plus Environment

Langmuir
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Figure 9
ACS Paragon Plus Environment

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ACS Paragon Plus Environment