Interface engineering of hybrid perovskite solar cells with poly(3-thiophene acetic acid) under ambient conditions

Article abstract of DOI:10.1039/c6cp00502k

The properties of methyl ammonium lead iodide (MAPbI₃) perovskite solar cells with poly(3-thiophene acetic acid) (P3TAA) as a hole transporting material (HTM) and a dense layer of ZnO nanoparticle film as an electron transporting material (ETM) are described using the conventional ZnO (n)/perovskite (i)/P3TAA (p) (n-i-p) architecture. The FT-IR spectra of a MAPbI₃/P3TAA mixture indicate a shift of the N-H stretching and the abolition of the N-H bending peak indicating the interaction between the components. UV-Vis spectra of the mixture exhibit a large red shift of the π-π? transition peak of the conjugated chain arising from the interaction causing an increase of the conjugation length. The cross-sectional SEM image of the device shows the sequence of the individual layers of ZnO, MAPbI₃, P3TAA and Ag, respectively. The current density (J)-voltage (V) curves obtained upon illumination with a light of 100 mW cm^-2 indicate the average PCE to be 7.38 ± 0.59% under ambient conditions. The IPCE values of these cells reach about 63% across a broad range of wavelength (300-800 nm). The HOMO and the LUMO of P3TAA are measured using cyclic voltammetry and the optical band gap and the relative energy level of the components explain the operation of photocurrent in the cell. For comparison purposes a device using poly(3-hexyl thiophene) (P3HT) as the HTM is fabricated under similar conditions and it exhibits a lower PCE (5.85 ± 0.51%) than that of the P3TAA based device. The longevity of the P3TAA based cell is also found to be better than that of the P3HT based cell for storing in air. The UV-Vis and impedance spectral results clearly explain the above results, signifying the influence of the interface on the performance of hybrid solar cells.

Full text of DOI:10.1039/c6cp00502k

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Page 1 of 27
Physical Chemistry Chemical Physics
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DOI: 10.1039/C6CP00502K
Interface Engineering of Hybrid Perovskite Solar Cells with Poly(3-
thiophene acetic acid) under Ambient Condition
*
Arnab Shit and Arun K. Nandi
Polymer Science Unit, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata ꢀ700 032, India
For Correspondence: A. K. Nandi email: psuakn@iacs.res.in
1

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Abstract:
3
The properties of methyl ammonium lead iodide(MAPbI ) perovskite solar cell with poly(3ꢀ
thiophence acetic acid)(P3TAA) as hole transporting material(HTM) and dense layer of ZnO
nanoparticles film as electron transporting material(ETM) is described using the conventional
3
ZnO(n)/Perovskite(i)/P3TAA(p) (nꢀiꢀp) architecture. The FTꢀIR spectra of MAPbI /P3TAA
mixture indicates shift of NꢀH stretching and abolition of NꢀH bending peak indicating
interaction between components. UVꢀVis spectra of the mixture exhibit large red shift of πꢀ
π* transition peak of the conjugated chain arising from the interaction causing increase of
conjugation length. The crossꢀsectional SEM image of the device shows sequence of the
individual layers of ZnO, MAPbI
3
, P3TAA and Ag, respectively. The current density(J)–
2
voltage(V) curves on illumination with light of 100 mW/cm indicate the average PCE to be
7.38±0.59% at ambient condition. The IPCE values of this cell reach about 63% across a
broad range of wavelength (300–800 nm). The HOMO and LUMO of P3TAA is measured
from cyclic voltammetry and optical band gap and the relative energy level of the
components explain the operation of photocurrent in the cell. For comparison purposes a
device using poly(3ꢀhexyl thiophene)(P3HT) as HTM is fabricated under similar condition
and it exhibits a lower PCE(5.85±0.51 %) than that of P3TAA based device. The longevity
of the P3TAA based cell is also found to be better than that of P3HT based cell for storing in
air. The UVꢀVis and impedance spectral results clearly explain the above results, signifying
the influence of interface on the performance of hybrid solar cells.
2

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Introduction:
3 3 3
Recently organolead halide perovskites of the general formula ABX (A= CH NH , B= Pb
and X= Cl, Br or I) have attracted significant attention as an excellent material for solar
energy conversion in the field of organicꢀinorganic hybrid solar cells. The initial report by
1
Miyasaka and coꢀworkers in 2009, demonstrated power conversion efficiency (PCE) of
~
4%. The exceptional optical and electronic properties of perovskite, for instance strong
2
,3
absorption bands that span over the whole visible region and large charge carrier diffusion
4
–6
lengths, make them ideal light absorbers towards photovoltaic application. Presently, the
organolead halide based perovskite solar cell devices have improved the PCE from the initial
1
,7–9
~
4% to certified efficiencies of ~ 20%.
Amongst the perovskite materials produced from
methyl ammonium iodide and lead iodide,(MAPbI ) has attracted significant attention
3
because of ease of preparation, easy solution processability, cost effectiveness and high
8
efficiency. The architecture of perovskites solar cell consist of a electron transfer material
(
ETM, n) / perovskites, a light absorbing material which is also an intrinsic semiconductor
i)/ hole transfer material (HTM, p) with nꢀiꢀp or the inverted pꢀiꢀn configuration. The
(
efficiency of the cell depends on suitable choice of each component, the morphology of the
10,11
material and on the interface between the layers.
Generally perovskite solar cells based on a mesoporous TiO
2 3
scaffold as ETM, a MAPbI
lightꢀabsorption layer and a 2,2′,7,7′ꢀtetrakisꢀ(N,Nꢀdiꢀpꢀmethoxyphenylamine)ꢀ9,9′ꢀspiroꢀ
12
bifuorene (spiroꢀOMeTAD) as HTM have achieved PCEs of ~15.0%, and several other
reports suggest that solar cells with efficiencies up to 20% can be achieved by fabricating
1
3,14
under controlled condition in glovebox.
2
The TiO perovskite solar cells require high
temperature processing technique which is not compatible with the industry produce. Hence,
high performance perovskite solar cells fabricated at low temperature is of special interest for
3

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15
the realistic application. Fullerene derivatives or other unstable organic materials as ETM
are quite expensive whereas stable inexpensive ZnO compact layers can be prepared easily
16
by spin coating of ZnO nanoparticles at room temperature without heat treatment steps. The
development of ZnO nanoparticle as ETM for MAPbI ꢀbased solar cells, has achieved a PCE
3
1
7
as high as 15.7% produced under controlled condition inside the glovebox.
In perovskite solar cells, the use of spiroꢀOMeTAD as HTM has been extensively studied as
it can infiltrate through the porous perovskite layer and afford a better contact for charge
17–19
transportation.
Alternatively less costly several conducting polymers such as
20
21,22
23
PEDOT:PSS,
grapheneꢀbased materials,
poly(3ꢀhexylthiopheneꢀ2,5ꢀdiyl) (P3HT),
polyꢀ(triarylamine),
and
24,25
as well as some small molecules such as peryleneꢀdiimide
2
6
27
28
derivatives,
pyreneꢀcore arylamine derivatives, diacetylideꢀtriphenylamine
and
29
inorganic material CuI have been used as HTM in perovskiteꢀbased solar cells. Amongst
the conducting polymer family, polythiophene is a good pꢀtype semiconductor for its high
crystallinity, charge mobility, conductivity and relative air stability and hence it has been
widely studied for use as HTM in perovskite solar cell. In an inverted pꢀiꢀn device,
ITO/polythiophene/MAPbI
electrochemical polymerization, a maximum PCE of 11.8 % is reported. Photovoltaic
devices fabricated through sequential vapor deposition of lead iodide and MAPbI with
undoped P3HT as a hole transporting layer exhibit a PCE of 13.7%. In a TiO2/
3
/C60/BCP/Ag, where polythiophene is produced from
30
3
11
31
CH
these cases the fabrication of the devices have been made under stringent dry condition in
glove box. Recently Namboopthiry and coꢀworkers have fabricated a cell with TiO as ETM,
CH NH as light absorbing material and P3HT as HTM under ambient atmospheric
condition and they have reported a maximum efficiency of 5.67%. In another
3 3 2
NH PbI Cl/ P3HT cell a maximum of 10.8% PCE is reported by Conings et al. In all
2
3
3 x
PbI3ꢀxCl
22
TiO
2
/CH
3
NH
3
PbI
2
Br/P3HT cell produced under ambient condition the best PCE is reported
4

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32
to be 6.64% indicating the fabrication condition has significant effect on the PCE of the
cell.
Interface between the different layers of the perovskite solar cell may have large effect on the
PCE and in an attempt here we chose an acid derivative of polythiophene eg. poly(3ꢀ
thiophene acetic acid) (P3TAA) where the –COOH groups have a possibility to interact with
perovskites through its NH
facilitating easy transfer of holes. In this article, a solution processed planar heterojunction
perovskite solar cell with MAPbI as light absorbing material, P3TAA as HTM and a dense
layer of ZnO nanoparticles film as ETM is described using the conventional ZnO
n)/Perovskite (i)/P3TAA HTM (p) (nꢀiꢀp) architecture. Here we have established that
3
group, that may provide a good mixing at the interface
3
(
P3TAA can be efficiently used as a HTM that can enhance the photovoltaic performance of
the perovskiteꢀbased solar cell and we have achieved an average PCE of 7.38%, a maximum
reported efficiency so far, using polythiophene systems as HTM under ambient atmospheric
condition with a satisfactory longevity for storing under ambient condition. In order to
establish the interfacial effect on the PCE of perovskite solar cell we have fabricated another
solar cell using poly(3ꢀhexyl thiophene) (P3HT) as HTM under identical condition and a
comparison of the results indicate that both PCE and longevity of the P3TAA cell are higher
than those of P3HT based solar cell. Probable explanation of the results has been attempted
through impedance spectroscopy.
Experimental:
Preparation of Ploy(3-thiophene acetic acid) (P3TAA):
3ꢀthiophene acetic acid (3ꢀTAA) was esterified with dry methanol by refluxing in presence of
concentrated H
2
SO
4
(0.05 ml) for 24 h to protect the oxidative decomposition of carboxylic
5

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acid group during polymerization. It was then polymerized by using anhydrous FeCl
chloroform. The molar ratio of FeCl and monomer is 4:1 and the mixture was stirred for 24 h
atmosphere. The polymer was precipitated into a large excess of methanol
3
in dry
3
o
33
at 0 C under N
2
to produce poly(3ꢀthiophene methyl acetate) (P3TMA). The precipitate was washed with
methanol followed by deionized water. Then P3TMA was hydrolyzed by refluxing with 2 M
o
NaOH solution for 24 h at 100 C. The waterꢀsoluble part was then neutralized with dilute
o
HCl and the precipitate was repeatedly washed with water and dried in vacuum at 60 C for 3
1
days. Each step was characterized by H NMR spectroscopy (Fig. S1). The regioregularity of
1
P3TAA was measured from the H NMR spectra presented in Fig. S1(b). The peak at 7.28
ppm is for α proton of HꢀH linkages and peak at 7.35 is for α proton of HꢀT linkages of
3
3
thiophene rings. From the ratio of peak area the % HꢀT linkage was calculated to be 42.3
1
mol%. The structures of P3TMA, and P3TAA were confirmed by H NMR spectra. [P3TMA
(Fig. S1a), (DMSOꢀd
6
) δ 7.35ꢀ7.28 ppm (m, thiophene ring proton, 1H), 3.68 ppm (s,
thiophene ring ꢀCH ꢀ, 2H), 3.64 ppm (s, ꢀCH , 3H); P3TAA (Fig. S1b), (DMSOꢀd ) δ 12.59
2
3
6
ppm (s, ꢀCOOH, 1H), 7.35ꢀ7.28 ppm (m, thiophene ring proton, 1H), 3.80 ppm (s, thiophene
ring ꢀCH ꢀ, 2H)]. P3HT is synthesized from the monomer 3ꢀhexylthiophene using the same
2
procedure of polymerization described above. After 24 h of polymerization, the reaction
mixtures were poured into methanol containing 10% HCl. The precipitate was reꢀdissolved in
CHCl , and the filtrate was dried at 60 °C in air and finally in vacuum. The regioregularity of
3
1
P3HT was measured from the H NMR spectra presented in Fig. S1(c) in a similar way as
above and the headꢀtail regioregularity was measured to be 81 mol %. The weightꢀaverage
w
molecular weight (M ) was estimated from gelꢀpermeation chromatography. It was
performed on a Waters instrument equipped with an ultraꢀstyragel column with polystyrene
standard as a reference using THF as an eluent at 35 °C at a flow rate of 0.6 ml/min. M and
w
6

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4
4
polydispersity were found as 1.1×10 and 1.83 for the P3TAA and 5.6×10 and 2.3 for P3HT,
respectively.
Synthesis of ZnO nanoparticles:
34,35
ZnO nanoparticles were prepared according to a procedure reported in literature.
Zinc
acetate dihydrate (2.95 g, 13.4 mmol) was dissolved in methanol (125 ml) with stirring at 65
o
C. A solution of KOH (1.48 g, 23 mmol) in methanol (65 ml) was then added dropwise at
o
o
6
0–65 C for 30 min. The reaction mixture was stirred for 2.5 h at 65 C. After cooling to
room temperature, the precipitate was collected by centrifugation. The precipitate was then
washed three times with methanol, dispersed in methanol and chloroform mixture (1:1 v/v) to
produce a ZnO nanoparticle solution with a concentration of 10 mg/ml.
Preparation of Methyl ammonium iodide (MAI):
The perovskite sensitizer methyl ammonium iodide was prepared according to a reported
36
procedure. An equimolar mixture of hydroiodic acid (57 wt% in water, SigmaꢀAldrich) and
methylamine (40% in water, Spectrochem) was stirred in an ice bath for 2 hours. After
o
stirring at 0 C for 2 hours, the resulting solution was evaporated using a rotary evaporator
and MAI was obtained. The precipitate was washed three times with diethyl ether and dried
in vacuum under dark conditions.
Solar cell fabrication:
Solar cells were fabricated on preꢀcleaned ITO–coated glass substrates with a sheet resistance
of 10 ꢁ/sq. First, a thin ZnO nanoparticle layer was spin coated onto the ITO surface at 3,000
r.p.m. for 30 s. The procedure was repeated two times to obtain a continuous smooth film. A
2
PbI solution (dissolved in N,Nꢀdimethyl formamide at a concentration of 460 mg/ml) was
then spin coated on top of the ZnO layer at 3,000 r.p.m. for 30 s. After short baking process at
7

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o
9
0 C for 20 minutes in air, the substrate was dipped into a solution of MAI in 2ꢀpropanol (10
o
mg/ml) for 30 s. The film was then annealed at 70 C in air for 15 min. On top of the
perovskite layer the hole transporting layer of P3TAA / P3HT was produced by spin coating
from their respective solution in THF (15 mg/ml) at 1500 r.p.m. for 1 min. Finally, a silver
ꢀ6
layer was deposited by thermal evaporation at a base pressure of 2×10 mbar. All device
fabrication steps were carried out under ambient conditions at the room temperature without
encapsulation.
Device Characterization:
The currentꢀvoltage (IꢀV) curves of solar cell devices were measured under ambient condition
using a Keithley 2401 source meter. The cells were illuminated by a 150 W Xeꢀlamp
2
equipped with an AM1.5G filter (Newport, 67005) at a calibrated intensity of 100 mW/cm ,
as determined by a standard silicon reference cell (91150V Oriel Instruments). The effective
2
area of the cell was defined to be 0.4 cm using a nonꢀreflective mask. The incident photonꢀ
toꢀcurrent conversion efficiency (IPCE) as a function of wavelength was measured with a
Newport 66902 150 Watt Xeꢀlamp in combination with an Oriel Cornerstone 130
monochromator and a Keithley 236 source measure unit controlled by Visual Basic software.
Electrochemical impedance (EIS) measurements were performed under a 0.92 V DC bias and
a 10 mV voltage perturbation conditions with Solartron SI1260 Impedance/Gainꢀphase
analyzer in the frequency range from 1MHz to 0.1 Hz.
Materials Characterization:
1
The H NMR spectra of the samples were made in DMSOꢀd
6
on a 500 MHz Bruker
instrument. The Xꢀray diffraction data are recorded on a Philips X′Pert PRO diffractometer
with CuꢀKα radiation. The data were collected with a 0.05 step size (2θ). Morphology and
micro structural images were acquired on a scanning electron microscope (ZEISS EVOꢀMA
8

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10). The FTIR spectra of the samples were taken using a Shimadzu FTIR instrument (model
8400S). UVꢀVis absorption spectra were acquired on a UVꢀVisꢀ spectrophotometer (Cary
8454, Agilent Technologies). The photoluminescence spectra of the solid films cast on
quartz plates was performed in a Fluoromaxꢀ3 instrument (Horiva Jovin Yvon). Cyclic
voltammetry experiments were performed by an electrochemical station (CHI6002E) in a
threeꢀelectrode cell configuration consisting of platinum working electrode, platinum wire
counter electrode, and Ag/AgCl quasi reference electrode with ferrocence/ferrocenium
+
(
Fc/Fc ) as the internal standard during the measurement at a scan rate of 100 mV/s. P3TAA
dissolved in THF and casted on the working electrode area followed by coating with Nafion.
The measurements were carried out with 0.1 mol/L tetrabutyl ammonium perchlorate (TBAP)
as supporting electrolyte in dry acetonitrile. All working solutions were thoroughly degassed
with oxygenꢀfree nitrogen and a nitrogen atmosphere was maintained above the solution
throughout the experiments.
Results and Discussion:
2
Fig.1 shows the Xꢀray diffraction (XRD) patterns of PbI (top panel) and that of perovskite
(
bottom panel) on ITO surface. The bottom panel XRD pattern of MAPbI corresponds to 2θ
3
signals at 14.1°, 24.5°, 28.4°, 40.5°, 43.1°, and 50.2° respectively representing the (110),
37,38
(202), (220), (314), and (404) diffraction planes of its tetragonal phase.
This XRD
pattern does not show any peak of PbI
2
(top panel) suggesting complete conversion of PbI to
2
3
MAPbI under the device preparation procedure.
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Fig. 1: XRD patterns for PbI films before and after dipping into MAI solution
2
In Fig. 2 the Fourier transform infrared (FTIR) spectra of MAPbI , P3TAA and the MAPbI /
3
3
P3TAA mixture in the ratio used for the cell fabrication is presented. In MAPbI
3
the
ꢀ1
characteristics peaks occur for NꢀH stretching vibration at 3088 cm , C–H stretching
ꢀ1
ꢀ1
ꢀ1
vibration at 2920 cm , N–H bending at 1571 cm , 1463 and 1419 cm is for C–H bending
3
9,40
ꢀ1
ꢀ1
vibration.
P3TAA shows characteristics peaks at 1705 cm for >C=O, 2922 cm for C–
3
3
H stretching, 1168 is for CꢀO stretching vibration. In the MAPbI / P3TAA mixture the NꢀH
3
ꢀ1
ꢀ1
ꢀ1
stretching peak at 3088 cm has shifted to 3126 cm and the N–H bending at 1571 cm is
almost abolished, indicating interaction between P3TAA with NH group of MAPbI
3
3
perovskite. Due to the use of very dilute concentration of P3TAA its characteristic vibration
peaks are not prominent in the FTIR spectra of the mixture. These results therefore indicate
the presence of interaction between the perovskite and P3TAA, which may cause a diffuse
interface between the components.
10

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MAPbI /P3TAA
3
1
459
376
3
126
1
627
2
923
1
P3TAA
MAPbI3
2922
1384
1168
1705
2920
1571
1419
3088
1463
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 2: FTIR spectra of MAPbI
3
3
, P3TAA and the MAPbI /P3TAA mixture
3 3
In Fig. 3a the UVꢀVis spectra at solid state of P3TAA, MAPbI , and MAPbI /P3TAA made
in a similar way to the device fabrication are compared. P3TAA shows a strong absorption
peak at 453 nm with a hump at 559 nm and a broad absorption peak at 800 nm. The peak at
453 nm corresponds to πꢀπ* transition of the conjugate chain and the hump at 559 nm
41
corresponds to the vibronic side chain band. The broad absorption peak at 800 nm may
appear for the vibronic coupling due to inter chain aggregation occurring at the solid state of
the conjugated main chain. The perovskite shows two characteristic weak absorption peaks at
4
92 and 747 nm. However, in the perovskite/P3TAA hybrid the πꢀπ* transition of the
conjugated polythiophene chain at 453 nm has shifted to 546 nm with shift of the hump from
59 nm to 612 nm. The broad peak at 800 nm becomes sharper and it shifts to 911 nm. This
5
spectral shift of P3TAA to lower energy region certainly indicates the interaction between the
perovskite and P3TAA causing the P3TAA chain to be more extended increasing its
conjugation lengths. This would certainly facilitate an easy movement of charge carriers
(holes) inside the cell.
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(a)
(b)
Fig. 3: UVꢀVis spectra of (a) MAPbI
MAPbI /P3HT layers at the solid state
To understand the state of P3HT in the P3HT/MAPbI
and P3HT/MAPbI are presented in Fig.3b. P3HT shows an absorption peak at 554 nm and a
3 3
, P3TAA and MAPbI /P3TAA (b) P3HT,
3
3
system the UvꢀVis spectra of P3HT
3
hump at 601 nm corresponding to πꢀπ* transition of the conjugate chain and vibronic side
chain band, respectively. In the hybrid these bands exhibit red shifts by 6 and 9 nm,
respectively which is significantly lower than those of perovskite/P3TAA hybrid system. This
result indicates that the P3HT chain remains at more coiled state than that of P3TAA chain in
the perovskite system. In other words, it may be surmised that the interface is sharper in the
3
P3HT/MAPbI system than that of perovskite/P3TAA hybrid system.
In Fig. 4(a) the surface SEM image of MAPbI
3
film is presented. The surface image shows
. A schematic presentation of the
the formation of packed crystallites of MAPbI
3
ITO/ZnO/MAPbI /P3TAA/Ag device structure is presented in Fig. 4(b) and the cross
3
sectional image of perovskite based device is shown in Fig. 4(c). From the Fig. 4(c) it is
apparent that the bottom layer is for ITO and the immediate upper layers sequentially indicate
that for ZnO, perovskite, P3TAA and Ag electrodes. The cross sectional SEM image of the
12

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device shows the thickness of the individual layers of ZnO, MAPbI
are measured to be 222, 228, 70 and 105 nm, respectively.
3
, P3TAA and Ag which
Fig. 4: (a) Surface SEM image of MAPbI
3
film, (b) schematic presentation of the
ITO/ZnO/MAPbI /P3TAA/Ag device structure, (c) cross sectional image of
3
perovskite based device.
Fig. 5(a) exhibits the JꢀV curves of ZnO/perovskite/P3TAA solar cell for the forward scan
(from short circuit to forward bias) and the reverse scan along with that at dark condition for
comparison purpose. It is apparent from the figure that compared to the dark current there is a
2
large hike in current on illumination with a light intensity of 100 mW/cm both in the forward
2
and reverse bias conditions. The short circuit current (Jsc) has increased to 12.57 mA/cm
2
under the forward scan, however, in the reverse scan it increases to 12.73 mA/cm . This sharp
increase of Jsc under the illumination condition certainly indicates a good performance of this
inorganicꢀorganic hybrid solar cells indicating ZnO is acting as a good ETM, and P3TAA as
a good HTM in the MAPbI perovskite solar cell produced under ambient condition
3
o
(
temperature 32 C and humidity 62%). It is noteworthy that in the reverse scan there is some
hysteresis which may cause an increase of fill factor (FF) and hence the PCE. In Table 1 the
photovoltaic parameters (eg. Jsc, FF, open circuit Voc and PCE) both for forward scan and
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reverse scan are presented taking a mean from eight cells with their standard deviations for
both the forward and reverse bias. The average value of efficiency is found to be 7.38±0.59%
which is higher than that of other perovskite solar cells produced at ambient conditions using
2
2,32
P3HT as HTM.
under similar condition as in P3TAA for comparison purposes and the PCE is found to be
.85±0.51 % (Fig. 5b). All the Jsc, Voc and FF (Table 1) are found to be lower in the P3HT
We have also fabricated a device using P3HT as HTM instead of P3TAA
5
based cell than those of the P3TAA based cell causing 1.53% lower efficiency. The PCE
value of the present P3HT based solar cell is comparable to the values reported earlier under
2
2
ambient condition. The probable reason for the large increase in PCE may be the good
interaction between perovskite layer and the P3TAA used as HTM under ambient condition.
The –COOH groups of P3TAA interact with MAPbI
3
through the lone pair of nitrogen atom
of NH group present in it as evident from FTIR spectroscopy. This causes the interface
3
between perovskite and HTM to become more diffused causing an easy injection of holes to
the P3TAA chains for its transport.
(a)
(b)
3
Fig. 5: JꢀV characteristics plots for the (a) ITO/ZnO/MAPbI /P3TAA/Ag (b)
ITO/ZnO/MAPbI /P3HT/Ag based perovskire solar cell device under dark and AM
3
2
1
.5G 100mW/cm illumination measured with forward and reverse scan direction.
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3
Table 1: Photovoltaic performance of ITO/ZnO/MAPbI /HTM/Ag based perovskire solar
2
cell device under AM 1.5G 100mW/cm illumination measured with forward and reverse
scan.
HTM
Applied bias
direction
Forward
Reverse
Jsc
Voc (V)
FF
PCE, η
(%)
Average
PCE (%)
2
(mA/cm )
12.57±0.4 0.90±0.02 0.61±0.02 6.90±0.56
P3TAA
P3HT
12.73±0.3 0.92±0.03 0.67±0.04 7.85±0.62 7.38±0.59
Forward
Reverse
11.71±0.3 0.85±0.02 0.58±0.02 5.77±0.45
5
.85±0.51
11.74±0.2 0.87±0.02 0.58±0.04 5.93±0.56
The incidentꢀphotonꢀtoꢀcurrent conversion efficiency (IPCE) spectrum (Fig. 6) for the
ZnO/MAPbI /P3TAA device shows a response in the region from the UVꢀvisible to near
3
infrared range (300–800 nm). The IPCE values of this cell reach about 63% across a broad
range of wavelength, indicating good lightꢀharvesting by the device.
Fig. 6: IPCE spectrum of ZnO/MAPbI /HTM based device
3
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3
The IPCE curve of ZnO/MAPbI /P3HT hybrid is also shown in the figure for comparison
purpose and it is interesting that both P3TAA and P3HT show response in the same
wavelength region but in the former case the IPCE values are higher than that of P3HT
supporting the PCE data presented above.
The operation of the device can be understood from the relative energy levels of the each
3
component of the device (Fig. 7). The excitons generated in the MAPbI layer can be
extracted or dissociated by transferring holes to the P3TAA layer and by transferring
electrons to the ZnO layer, attaining high charge transport efficiencies to both the electrodes.
4
2,43
Here, the energy levels for the ZnO and perovskite layer are taken from the literature.
The
corresponding HOMO and LUMO levels of P3TAA have been calculated from the onset
oxidation potential in the cyclic voltammetry experiment and the optical band gap obtained
from the Tauc’s plot to be ꢀ4.86 eV and ꢀ2.78 eV vs. vacuum, respectively (Fig. S2 a, b).
Fig. 7: Schematic representation of the band energy diagrams for
3
ITO/ZnO/MAPbI /P3TAA/Ag device.
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The conduction band of ZnO is located in the closer proximity of the perovskite layer and the
HOMO level of P3TAA is well matched for the separation of holes, which facilitates the
charge separation process. The band gap offsets between the different components facilitating
effective electron transport from the conduction band of MAPbI to ZnO and then to the ITO
3
electrodes and transport of holes from the valance band of the perovskite films to the P3TAA
and then to the Ag electrodes. Since the HOMO level (ꢀ4.86 eV) of P3TAA is comparable to
3
the valance band (ꢀ5.43 eV) of MAPbI , it facilitates the hole transfer to Ag electrode through
P3TAA. The relatively high LUMO energy (ꢀ2.78 eV) of P3TAA compared with that of the
conduction band of MAPbI (ꢀ3.93 eV) prevents the electron transfer through the perovskite
3
layer to the Ag electrode. This phenomenon suggests that P3TAA can be successfully used as
3
an effective HTM for the MAPbI based perovskite solar cell.
The longevity of the cell has been measured by keeping the cell under the ambient air
condition without encapsulation and the PCE is measured at different time interval. In Fig. 8
the normalized PCE is plotted with storage time and after 10 days 55% value of PCE is
retained. The longevity of P3HT based device made under similar condition as in P3TAA is
lower as evident from the Fig. 8 and here after 10 days only 27 % value of PCE is retained. .
This result indicates a fair stability of the P3TAA based cell under ambient atmospheric
3
8
condition and it may be attributed to the controlled PbI
2
passivation. The present system
also exhibits much better stability than that of TiO /MAPbI Cl /spiroꢀOMeTAD exposed to
2
3ꢀx
x
44
ambient air without encapsulation showing only 30 % PCE after 10 days of storing. The
better stability of P3TAA based cell that that of P3HT based cell may be due to mixing of
3
MAPbI with P3TAA causing diffuse interface.
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3
Fig. 8: Normalized PCE with respect to storage time for ZnO/MAPbI /HTM based
perovskite solar cells under ambient air condition without encapsulation.
Electrochemical impedance spectroscopy (EIS) is a non destructive powerful technique to
elucidate the charge transport process in the solar cells. The Nyquist plot of
3
ITO/ZnO/MAPbI /HTM/Ag cell is presented in Fig. 9. From the figure it is apparent that in
each case there are two semicircles characterizing the impedance spectra of the cell. The data
can be fitted in an equivalent resistance (R) – capacitance (C) circuit shown in Fig. 9 (Inset).
Fig. 9: Typical Nyquist plot of the hybrid perovskite solar cells based on
3
ZnO/MAPbI /HTM. The simulation result (line) is fitted to experimental data
(symbols). Inset is the equivalent circuit applied to fit the Nyquist plot.
18

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The semicircle at the higher frequency region corresponds to the chargeꢀtransfer resistance
HTM) at the hole transporting layer and the CHTM is the interfacial capacitance between
(R
2
9
electrode (Ag) and HTM. The semicircle at the low frequency region corresponds to the
recombination resistance (R ) of the ETM (ZnO)/perovskite interface and the CPE yields the
rec
29,45,46
chemical capacitance which is related to the Fermi level of ETM.
An additional
resistance (R ) appears in the equivalent circuit drawn from Zꢀview software and it may be
s
attributed to the additional contribution of the series resistance. The fitting parameters of EIS
data are presented in Table 2. The impedance analysis of the P3TAA based cell yields lower
RHTM and higher Rrec value compared to those of P3HT based cell. The lower RHTM value
indicates lower resistance experienced by the holes to flow along the HTM material and the
higher Rrec indicate a lower recombination of photoꢀinjected electrons. Thus the combined
effects of lower RHTM and higher Rrec value cause higher PCE in the P3TAA than that of
P3HT based solar cell. So, this result supports that the diffuse interface between the
components of perovskite solar cell yield higher efficiency due to easier transport processes.
3
Table 2: Fitting parameters for EIS data of ITO/ZnO/MAPbI /HTM/Ag based perovskire
solar cell device.
HTM
Rs
R
RHM
C
HTM
Rrec
CPEꢀP
CPEꢀT
Cµ*
(µF)
2
2
2
(
ꢁ/cm ) (ꢁ/cm )
(µF)
(ꢁ/cm )
ꢀ1 CPEꢀP
(
ꢁ s
)
ꢀ
5
5
P3TAA
P3HT
60.5
44.0
69.0
90.5
0.82
0.18
242.0
180.8
0.8
0.7
5.1×10
0.8×10
15.8
0.34
ꢀ
*
[Cµ is calculated from the CPEꢀT and CPEꢀP values using the equation.
1
(
CPEꢀT×Rrec )CPEꢀP
C =
]
ꢂ
Rrec
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To understand the charge recombination and dissociation process in the device
photoluminescence spectra (PL) of individual and mixed layers are presented in Fig. 10. The
3
PL spectra of MAPbI show emission peaks at 740 nm and 763 nm for excitation at 490 nm.
On addition of P3TAA the emission peak at 740 nm is fully quenched but that of 763 nm is
slightly quenched and similar reports are also observed in these types CH NH PbI Br/P3HT
3
3
2
32
systems where it is suggested that photoꢀgenerated holes are quickly extracted to the P3HT
5
layer before bulk recombination. On the other hand, addition of ZnO to MAPbI
3
the intensity
of all the emission peaks of MAPBI
3
are totally quenched. This is also true for the
ZnO/MAPbI /P3TAA hybrid system. The complete quenching of fluorescence intensity of
3
MAPbI by ZnO indicates ZnO is a very effective ETM for transfer of photoꢀgenerated
3
electrons by dissociation of the holeꢀelectron pair generated from white light illumination on
3
MAPbI system. Hence ZnO is a very effective ETM and P3TAA act as a good HTM for
perovskite solar cell application yielding a moderate average PCE of 7.38±0.59% under
ambient condition.
Fig. 10: Solid state photoluminescence spectra of indicated components and their
mixtures as present in the device.
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Conclusion:
In conclusion it may be inferred that P3TAA can act as an effective HTM in the
conventional ZnO (n)/Perovskite (i)/P3TAA (p) (nꢀiꢀp) solar cell even produced in air under
3
ambient condition. The FTIR spectra of MAPbI /P3TAA mixture indicate shifts of NꢀH
stretching peak and abolition of NꢀH bending peak suggesting an interaction between the
components. The UVꢀVis spectra of the mixture exhibit a large red shift of πꢀπ* transition
peak of the conjugated chain arising from the interaction causing increase of conjugation
3
length. These results suggest that the interface between the MAPbI /P3TAA is diffused
which facilitates the hole transfer process. The current density (J) – voltage (V) curves on
2
illumination with a light intensity of 100 mW/cm indicate a hysteresis in the reverse bias
than that of forward bias and the average PCE is measured to be 7.38±0.59%. The IPCE
values of this cell reach about 63% across a broad range of wavelength (300–800 nm). The
HOMO and LUMO energy levels of P3TAA are measured from cyclic voltammetry and
optical band gap and the relative energy levels of the components explain the operation of
3
P3TAA as HTM and ZnO as ETM. The Nyquist plot of the ITO/ZnO/MAPbI /P3TAA/Ag
cell show two semicircles and the recombination resistance is calculated to be 96.8 ꢁ. From
the photoluminescence spectra of individual and mixed layers it is proved that ZnO is a very
3
effective ETM and P3TAA acts as a good HTM for MAPbI hybrid perovskite solar cell
produced under ambient condition yielding an average PCE of 7.38±0.59%.We have also
fabricated a device using P3HT as HTM instead of P3TAA under similar condition for
comparison purposes and the PCE is found to be 5.85±0.51 % which is 1.53% lower than that
of P3TAA system. The longevity of the P3TAA based cell is found to be better that that of
P3HT based cell for storing under ambient condition in air. The UVꢀVis and impedance
spectral results clearly explain the influence of interface on the PCE value of the hybrid solar
cells.
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Electronic supplementary information (ESI) available:
1
H NMR spectra of P3TMA, P3TAA and P3HT, Cyclic voltammetry curve and Tauc’s plot
for P3TAA. This material is available free of charge via the Internet at http://pubs.rsc.org/.
Acknowledgements:
We gratefully acknowledge SERB (Grant No: SB/SI/OCꢀ11/2013), New Delhi and CSIR,
New Delhi (Grant No: 02(0051)/12/EMRꢀII) for financial support. A.S. acknowledge DSTꢀ
INSPIRE program for providing the fellowship.
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Table of Content
Interface Engineering of Hybrid Perovskite Solar Cells with Poly(3-
thiophene acetic acid) under Ambient Condition
*
Arnab Shit and Arun K. Nandi
Polymer Science Unit, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata ꢀ700 032, India
3
ZnO/MAPbI based perovskite solar cell with poly(3ꢀthiophene acetic acid) shows
higher efficiency(7.38%) and stability than P3HT based cell(5.85%) in air.
27