Effect of tantalum doping in a TiO2 compact layer on the performance of planar spiro-OMeTAD free perovskite solar cells

Article abstract of DOI:10.1039/c7ta09193a

Perovskite solar cells (PSCs) are currently the most exciting solar photovoltaic technologies for future deployment. A conventional PSC device structure typically employs a titanium dioxide (TiO₂) electron transport layer. However, the low elec

Full text of DOI:10.1039/c7ta09193a

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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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Effect of tantalum doping in TiO
2
compact layer on the performance of planar
DOI: 10.1039/C7TA09193A
Spiro-OMeTAD free perovskite solar cells
Rahul Ranjan, Asit Prakash, Arjun Singh, Anand Singh,* Ashish Garg,*,‡ and
†
‡
‡
,§
Raju Kumar Gupta*,†,‼
†Department of Chemical Engineering
‡Department of Materials Science and Engineering
§Department of Chemistry
‼Center for Nanosciences and Center for Environmental Science and Engineering
Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India
*Corresponding author. Tel: +91-5122596972; Fax: +91-5122590104.
E-mail address: Anand Singh (anands@iitk.ac.in); Ashish Garg (ashishg@iitk.ac.in);
Raju Kumar Gupta (guptark@iitk.ac.in)
Abstract
Perovskite solar cells (PSCs) are currently the most exciting solar photovoltaic technologies
for future deployment. Conventional PSC device structure typically employs a titanium
dioxide (TiO
obstacle to PSC efficiency enhancement. In this paper, we report on the conductivity
enhancement of TiO by tantalum (Ta) doping and its effect on improving the device
performance. In contrast to commonly used mesoporous TiO , our work used planar PSC
device structure based on compact TiO layer with device structure being: FTO/compact-
/P3HT/Ag. Photovoltaic measurements show that Ta doping of compact
results in an improvement in the fill factor (FF) of the devices due to decrease in the
), attributed to improved charge transport and increase in the shunt
resistance (Rsh), due to reduced leakage paths. These changes were examined using Kelvin
probe force microscopy (KPFM) which indicated that the Fermi level of TiO shifts
downward upon Ta doping providing driving force for the electron transfer from perovskite
LUMO to TiO conduction band resulting in higher current density. Further, impedance
2
) electron transport layer. However, low electrical conductivity of TiO
2
is an
2
2
2
TiO
2
2
/CH
3
NH
3
PbI
3
TiO
series resistance (R
s
2
2
spectroscopy analysis of the devices suggests a decrease in the charge transfer resistance with
Ta-doping and increase in Rsh due to higher recombination resistance of doped film. PSC
1

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devices with Ta doping of 3.0% led to a 40% improvement in the overall efficiency as
DOI: 10.1039/C7TA09193A
compared to un-doped TiO
.94% .
2
with best device showing power conversion efficiency of ca.
9
Keywords: Doping, planar perovskite solar cells, compact TiO , impedance spectroscopy,
2
Spiro-OMeTAD free perovskite solar cells
1
. Introduction
Perovskite solar cells (PSCs) have emerged as an exciting solar photovoltaic technology due
to high absorption coefficient of perovskite compounds, high charge-carrier mobility, long
exciton diffusion length and a tunable band gap which allows the PSCs to offer a balance
between high power conversion efficiency (PCE) and potentially low cost fabrication.1-6 The
reported PCE of the perovskite based solar cells have rapidly increased from 3.9 % in 20097
8
to 22.1% in 2017, indicating significant progress within a short period of time. The most
common hole transport layer (HTL) used in regular structure are spiro-OMeTAD
2
2
2′ 2′
7
7
7′ 7′
(
N ,N ,N ,N ,N ,N ,N ,N -octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-
tetramine), P3HT (poly-3-hexylthiophene) and PTAA (poly(triaryl amine)). To further
enhance the conductivity of Spiro-OMeTAD, additives such as Li-TFSI
(bis(trifluoromethane)sulfonimide lithium salt) and t-BP (4-tert-butylpyridine) are commonly
used. Strategies such as use of C60 as electron transport layer with Spiro-OMeTAD have
achieved PCE of 19.1% and another group by use of triple cation perovskite and Spiro-
OMeTAD as HTL has got PCE of 21.1%.9-10 Researchers have also sought to replace
expensive as well as moisture sensitive hole transporting layer of Spiro-OMeTAD by
conjugated polymer P3HT, due to its hydrophobic nature and could prevent moisture from
reaching perovskite layer, thus rendering higher atmospheric stability. Guo et al. achieved
PCE of 12.4% with optimum doping of Li-TFSI and D-TBP (2,6-di-tert-butylpyridine) in
P3HT. Although hygroscopic nature of Li and corrosive nature of the pyridine derivative has
been found to deteriorate the perovskite, when these additives are used with P3HT, the
degradation of perovskite layer is reduced compared to cases in which Spiro-OMeTAD or
PTAA are employed.11
The emergence of perovskite materials with ambipolar properties has provided the
1
2
platform for the development of planar heterojunction (PHJ) perovskite solar cells. In PHJ
2

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DOI: 10.1039/C7TA09193A
perovskite, use of mesoporous TiO can be avoided leading to simpler and low thermal
2
budget processing. Here, characteristics of electron transport layer (ETL) are critical to PHJ
perovskite solar cell performance due to dual role of ETL as electron transport as well as hole
blocking layer. In PHJ PSCs, different metal oxides have been used as ETL. Researchers
13
have used various materials for use as ETL in PHJs such as ZnO, SiO
2
, ZrO . Liu et al. used
2
thin film of ZnO nanoparticles as an ETL in PSCs and achieved the efficiency of 15.7%.14
Tseng et al. achieved 15.9% efficiency of PSCs using sputtered ZnO thin film as an ETL
having device structure as ITO/ZnO/Perovskite/Spiro-OMeTAD/Ag. It was found that the
atmosphere of the sputtering chamber could tune the surface electronic properties (band
1
5
structure) of the ZnO thin film resulting in improved performance of the PSC. Recently,
ultrathin SnO was prepared using atomic layer deposition (ALD) method and was utilized
as ETL in PSCs.16
2
TiO
stability and hence is one of the most used ETL for PSCs. Though, the charge transport
property such as conductivity of the TiO compact thin film is rather poor and needs to be
improved for efficient charge extraction. One of the simple and effective ways to improve the
electrical and optical properties of TiO is its chemical doping using an appropriate ion.
There are several methods reported for doping in TiO such as Li doping by electrochemical
method, yttrium doping using hydrothermal method etc.17-18 In past, different dopants such as
Y, Zn, Mg, Nb have been used in compact as well as mesoporous TiO layer and have
resulted in improved electron transport properties of TiO layer; thereby improving the
photovoltaic performance of PHJ PSC devices consisting of TiO as ETL (doped or
undoped), CH NH
as absorber and Spiro-OMeTAD as HTM.19-24 The
improvements have been attributed to better energy level alignment leading to improved
2
has emerged as a promising candidate due to its low cost and high chemical
2
2
2
2
2
2
3
3
PbI3–xCl
x
2
3
electron transport and hole blocking upon Zn doping, positive shift in the flat-band potential
and increased conductivity upon Nb doping.21 Recently, in dye sensitized solar cells
(DSSCs), group V element Ta was used to dope TiO
2
(Ta-TiO ) electron transport layers
2
resulting in an enhanced photovoltaic performance.25-26 In addition to PCE improvements,
there are reports on reduced hysteresis too e.g. in device using Li-doped compact TiO
layer.27
2
Among various ions used for doping in TiO , Tantalum (Ta) appears as a good choice
2
5
+
4+
as Ta ion has an ionic radius of 0.64 Å which is comparable to Ti (0.61 Å), hence it can
substitute Ti4+ sites in the TiO
framework without issues such as strain or formation of
3
2

Journal of Materials Chemistry A
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secondary phases. It is shown to work better than Nb, another group V element 28 and has a
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DOI: 10.1039/C7TA09193A
negative formation energy and superior thermodynamic stability over N-doped TiO
.29
2
Although Ta doping results in an enhancement in current density of the devices, one study
attributed it to positive shift in the flat-band potential25 while another report showed
negligible shift in the flat-band potential.30 However, most of the works related to doping in
TiO
mesoporous structure. Therefore, there is a need to understand the role of Ta doping in
compact TiO layers on PSC device performance along with the use of cheaper and
2
are on devices consisting of Spiro-OMeTAD as HTL while TiO
2
has been used in
2
environmentally stable HTLs i.e. P3HT. Also, there exists a trade-off between time and
efficiency when one uses planar structure with P3HT as HTL. In this manuscript, we present
our results on compact Ta doped TiO
conventional planar perovskite structure of FTO/compact-TiO
device structure incorporating P3HT as HTL instead of routinely used Spiro-OMeTAD.
Literature reports various methods for the preparation of TiO film such as sol-gel, ALD,
electrochemical deposition and DC-magnetron sputtering. We have used sol-gel method for
the preparation of compact TiO layer. Further, we investigated the effect of different Ta-
TiO layers with varying stoichiometry (1.0-5.0 mol% doping) on device performance by
2
(Ta-TiO
2
) layer as an ETL in perovskite solar cells in
2
/CH NH PbI /P3HT/Ag with
3
3
3
2
3
1
2
2
conducting detailed electrical, optical and morphological characterization e.g. electrical
conductivity, absorption spectra, Fermi level and surface morphology. Charge carrier
dynamics studies on Ta-TiO
2
thin films using photoluminescence (PL) and KPFM show that
results in a larger
a shift in the Fermi level along with improved conductivity of Ta-TiO
2
driving force for electron transport and thus enhance the short circuit current and fill factor of
the devices rendering a maximum PCE of 9.94%.
2
2
. Experimental Details
.1 Materials
Methylamine solution (CH
water), lead acetate (99.999 trace metal basis)(Pb(OAc)
DMF), poly(3-hexylthiophene-2,5-diyl)(P3HT), tantalum(V) ethoxide, fluorine doped tin
oxide (FTO) were purchased from Sigma-Aldrich and were used without further purification.
3
NH
2
, 40 wt% in absolute ethanol), hydroiodic acid (HI, 57% in
2
), anhydrous dimethylformamide
(
2
.2 Synthesis of methylammonium iodide
Under nitrogen atmosphere, 10 mL HI was added dropwise while stirring to 24 mL
methylamine solution in 100 mL of ethanol at room temperature. Methylammonium iodide
4

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DOI: 10.1039/C7TA09193A
(
MAI) was obtained by concentrating the reaction mixture in vacuum. The white crystals
obtained were washed three times in diethyl ether and dried overnight in a vacuum oven.32
2
.3 Fabrication of solar cell devices
FTO layer on glass substrates was patterned by etching with Zn powder and 2M HCl
followed by cleaning with soap and then ultrasonication in DI water, acetone and isopropyl
alcohol respectively for 10 min with nitrogen blow drying after each step. Cleaned substrates
were UV-ozone treated for 15 min having pretreatment of UV only for 10 min and post
treatment of oxygen only for 10 min. Solution of depositing compact TiO
2
ETL layers was
prepared by dissolving Titanium diisopropoxide bis(acetylacetonate) solution in ethanol in
1
5
3
3
:9 ratio with mixing of tantalum (V) ethoxide in titanium precursor solution in 1.0, 3.0 and
.0 mol% for doped films. The solution was spin coated on UV-ozone treated substrates at
000 rpm for 1 min followed by annealing at 100ºC for 10 min on hot plate and at 500ºC for
0 min in a muffle furnace in air resulting in ~ 40 nm thin compact TiO
layer coated substrates were transferred to the glove box after cooling to room
temperature. For depositing perovskite layers, a solution was prepared by mixing methyl
ammonium iodide (CH NH I) and Pb(OAc) with 3:1 ratio in DMF to get a 40 wt% solution
which was kept in glove box for 2 h at 70ºC. The filtered perovskite solution was spin coated
on TiO coated substrates at 2000 rpm for 1 min followed by bench drying for 5 min leading
2
films. The compact
TiO
2
3
3
2
2
to the formation of a light brownish film and then kept at 100ºC for 10 min rendering the film
to become dark brownish. Thickness of final perovskite film was measured to be ca. 250 nm.
Subsequently, P3HT solution (20 mg/mL in chlorobenzene) was coated at 2000 rpm for 30 s
having thickness of ~ 60 nm, followed by thermal evaporation of an 80 nm thin Ag layer at
0
.5 Å/s to form the top electrodes.
2
.4 Thin film and device characterization
The conductivity of compact undoped TiO
400 source meter using two probe methods on device structure of FTO/TiO
structure analysis, X-ray diffraction (XRD) of the films was conducted on X’Pert Pro
PANanalytical) diffractometer. Chemical states were examined using X-ray photoelectron
2
and Ta-TiO
2
films was measured using Keithley
2
2
/Au. For
(
spectroscopy (XPS) using PHI 5000 (Versa Probe II, FEI Inc) while XPSPEAK 4.1 software
was used for analysing the spectra. Agilent technologies, Cary 7000 spectrophotometer was
used for absorbance measurements. Raman spectrometer (WiTec, using laser light of λ = 532
nm) was used for Raman studies. Morphological studies of the films were conducted on
Asylum MFP-3D AFM and field emission scanning electron microscope (FESEM, Quanta
5

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2
00, Zeiss). The surface potential was measured using Kelvin-Probe force microscopy
DOI: 10.1039/C7TA09193A
(KPFM) using Asylum MFP-3D AFM. Photovoltaic device characteristics were measured
using a Keithley 2400 source meter under dark and simulated solar light (AM1.5G) using a
Newport solar simulator. Bruker DektakXT stylus profilometer was used for measuring the
film thickness. Photoluminescence (PL) of the samples was measured using Horiba Jobin
Yvon Fluorolog-3 spectrofluorometer. External quantum efficiency (EQE) was measured
using Newport IQE200 system. Impedance spectroscopy measurements were carried out
using Potentiostat/Galvanostat (Autolab 302N, Metrohm, Netherlands).
3
. Results and Discussion
Initially, we fabricated a 250 nm perovskite thin film on FTO/TiO
2
substrate to study the
formation of perovskite film and its grain structure. Fig. S1 shows XRD pattern, absorbance
data and morphology of the perovskite film. The film exhibited a single perovskite phase
structure with good surface coverage. The absorbance spectra of the film showed a broad
range of absorbance from 400 to 760 nm suggesting that perovskite film has good absorption
in visible reason. Fig. S1 (d) shows the morphology of pinhole free compact TiO
2
film.
3
.1 Structural, morphological and optical characteristics of Ta-doped TiO
Fig. 1 (a) shows the XRD patterns of pristine TiO and Ta-TiO films having different mol%
of Ta. It can be deduced from the figure that all the samples possess anatase phase (ICDD file
No. 01-084-1285). No peak for elemental Ta or Ta could be detected in the XRD plots,
suggesting that Ta is doped uniformly in crystal lattice of TiO . Due to lower effective mass
of electron in anatase phase, anatase TiO shows higher electrical conductivity as compared
2
films
2
2
2
O
5
2
2
to rutile phase and hence is the desirable phase. However, there is also significant peak
broadening upon Ta doping as intensity of (101) reflection as well as of other peaks decreases
with the increase in Ta doping and the peaks shift towards the smaller 2θ as shown in Fig. 1
5
+
4+
33
(
b). Latter is because the ionic radius of Ta (0.64 Å) is greater than the Ti (0.61Å). As
suggested by the morphological studies (shown later), the peak broadening is not associated
with the decrease in the grain size and hence it could perhaps be related to decreased
crystallinity of the films overall.
6

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(101)
Pure TiO
DOI: 10.1039/C7TA09193A
2
(101)
(
a)
(b)
1
3
5
.0 mol% Ta-TiO
2
2
2
.0 mol% Ta-TiO
.0 mol% Ta-TiO
(
200)
(004)
(
105)(211) (204)
2
0
30
40
50
60
23
24
25
26
27
28
Angle 2 (degree)
Angle2( degree )
Fig. 1 XRD patterns of (a) pristine TiO
2
and various Ta-doped TiO
2
and (b) magnified
spectra of (101) anatase crystal plane.
E
Pure TiO
E
Pure TiO
2
1.0 mol% Ta-TiO
2
g1 (a)
2
g1
(b)
1
3
5
.0 mol.% Ta-TiO
2
2
2
3.0 mol% Ta-TiO
.0 mol.% Ta-TiO
.0 mol.% Ta-TiO
2
2
5.0 mol% Ta-TiO
E
g3
E
B
A
+B
g2
1g
1g 1g
1
00
200
300
400
500
600
700
1
00
120
140
160
180
200
-
1
Wavenumber (cm )
-1
Wavenumber (cm )
Fig. 2 Raman spectra of (a) pristine TiO
2
and various Ta-doped TiO samples and (b)
2
Normalized Eg1 vibration Raman peak.
While XRD data provided broad and average information over a large sampling volume,
Raman spectra can provide information at much smaller length scales and is sensitive to the
crystallinity. The Raman spectra of undoped TiO
2
and 1.0, 3.0 and 5.0 mol% of Ta doped
samples are shown in Fig. 2 (a). The well assigned Raman peaks at 144, 397, 521 and 639
-1
cm are observed in all the samples. Theses peaks are attributed to the vibration modes of
Eg1, B1g, A1g+ B1g and Eg3, respectively and thus confirm the presence of anatase phase of
-
1
TiO . The weak peak of anatase phase at 194 cm of Eg2 can also be noticed. In rutile phase,
2
four Raman peaks are observed due to oxygen atom vibrations which do not involve Ti-atom.
However, in anatase phase, six active Raman vibrations are observed. Thus, Raman spectra
7

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confirm the presence of anatase phase in compact TiO
2
layer, which is in agreement with
DOI: 10.1039/C7TA09193A
XRD results. Further, except B1g which shows only Ti-atom vibration, all others are mixture
3
4
of both O-atom and Ti-atom motions in the anatase phase. Fig. 2 (b) shows the normalized
g1 anatase vibration signal of the pure TiO and doped samples. The Raman shift as well as
the increment of full width half maxima (FWHM) is observed in the doped films. The E
peak in Raman Spectra represents the symmetric stretching vibration of O-Ti-O in TiO . If
E
2
g
2
4
+
doping occurs at substitutional position on the Ti , the Ti-O-Ti bond gets disturbed and new
Ta-O-Ti or Ta-O-Ta bonds form. The Raman active mode is affected due to this new Ta-O
bond which formed in place of Ti-O bond and results in shifting and widening of peaks.35
Thus, both Raman shift to higher wave number and widening of peaks suggest towards
incorporation of Ta doping in the TiO
2
layer.29
The absorption spectra of pristine TiO
2
and various Ta-doped in TiO films are shown in Fig.
2
3
(a). The absorption spectra of doped thin films show red shift upon Ta doping as suggested
by the band gap calculations of pure TiO and doped TiO thin films, calculated using Tauc
plots (Fig. 3 (b)). The band gap of various tantalum doped compact TiO films decreased
with the increase of Ta doping percentage. The calculated value for pristine TiO was 3.59
eV and remains nearly the same for 1.0 % Ta-TiO film at ~3.58 eV. At increased doping
levels, band gap decreased from 3.55 eV (3.0 mol% Ta-TiO ) and 3.47 eV (5.0 mol% Ta-
TiO
), in agreement with the previously reported values. 36
2
2
2
2
2
2
2
8

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DOI: 10.1039/C7TA09193A
2
000
(
b)
Pure TiO
Pure TiO
2
2
(
a)
1
3
5
.0 mol% Ta-TiO
1.0 mol% Ta-TiO
2
2
2
2
.0 mol% Ta-TiO
.0 mol% Ta-TiO
3.0 mol% Ta-TiO
1500
2
2
5.0 mol% Ta-TiO
1
000
5
00
3
00
330
360
390
420
2.7
3.0
3.3
3.6
3.9
4.2
4.5
Wavelength (nm)
(h) (ev)
2
2
1
1
500
000
500
000
2
1
1
000 (c)
Pure TiO
2
(d)
5.0 mol% Ta-TiO
2
500
000
5
00
5
00
0
1
.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0
1
.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
h(eV)
h (ev)
Fig. 3 (a) The UV-Vis absorption spectra of pristine and various Ta doped thin films (b)
corresponding Tauc plot, (c) and (d) the band gap calculation using Tauc plot of pristine TiO
and 5.0 mol.% Ta doped thin film.
2
The interface between the active layer and the bottom electrode has an important effect on the
device performance. Therefore, it is critical to examine the roughness of TiO layer. Fig. 4
show AFM images for the surface of pure TiO and tantalum doped thin films deposited on
FTO coated glass substrate showing dense and compact grains in all the thin films. However,
the RMS roughness of the film slightly increases from 11.3 nm (pure TiO ) to 15.5 nm (5.0
mol.% Ta-TiO ) upon increase in Ta doping.
2
2
2
2
9

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DOI: 10.1039/C7TA09193A
Fig. 4 AFM images of (a) pure TiO
c) 3.0 mol.% Ta-TiO (RRMS:12.6 nm) (d) 5.0 mol.% Ta-TiO
made on FTO coated glass substrate respectively (RRMS: RMS roughness).
2
(RRMS:11.3 nm) (b) 1.0 mol.% Ta-TiO
2
(RRMS: 11.6 nm)
(
2
2
(RRMS:15.5 nm) compact layer
The surface roughness of tantalum doped TiO
electron transport as it affects the contact area between the perovskite active layers and Ta
doped TiO compact layer. Higher roughness could lead to an increase in the contact area
2
compact layer could play a significant role in
2
between active layer and the electron transport layer leading to improved charge
transportation.37-38 However, doping level beyond 3.0% results in other problems such as
formation of voids even though 5.0% Ta-TiO films show highest r.m.s. roughness, resulting
2
in lower conductivity as well as poor charge extraction as compared to 3.0 mol% doped thin
films as shown in subsequent sections.
To confirm the tantalum doping in the TiO
the electronic structure and chemical changes due to Ta doping. Fig. 5 (a) shows the full XPS
spectra of pure TiO and tantalum doped TiO thin films. The obtained spectra illustrate that
the pure TiO film shows the presence of only Ti and O peaks whereas the doped thin films
spectra consist of peaks from Ti, O and Ta elements. Fig. 5 (b) shows the characteristics
2
thin films, XPS analysis was performed to study
2
2
2
1
0

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binding energy peaks of Ta 4f at 26.4 eV and 28.3 eV for 4f7/2 and 4f5/2, respectively. Increase
DOI: 10.1039/C7TA09193A
in Ta amount is observed for 5.0 mol% Ta-TiO
while Ta is absent in the pristine TiO sample (Fig. 5c). The presence of Ta in the doped
thin film confirms the doping of Ta in pristine TiO
thin film.25, 39-40 Table S1 contains atomic
of various elements for undoped TiO and Ta-TiO thin film samples which shows that Ta
content increases with increase in the Ta doping. The high resolution XPS spectrum of Ti in
undoped TiO sample shows 2p3/2 and 2p1/2 characteristic peaks at binding energies of 459.1
2
compared to 3.0 mol% Ta-TiO sample
2
5
+
2
2
%
2
2
2
4
+
eV and 464.8 eV, respectively corresponding to Ti oxidation state (Fig. 5d). However, a
detectable peak at lower binding energy is observed than the peak appeared at 459 eV for 3.0
mol% Ta-TiO sample (Fig. 5e). This additional peak at 457 eV appears after the Ta doping
2
which refers to Ti3+ oxidation state due to incorporation of Ta5+ ions within the crystal
structure.41-43
1
1

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DOI: 10.1039/C7TA09193A
O1s
Ti2s
Undoped TiO2
(
a)
3
5
.0 mol% Ta-TiO2
.0 mol% Ta-TiO2
Ti2p3
Ti LMM1
Ta4f7
O KLL
Ta4d5
Ta4d3
Ta4p3
Ti3p
Ti3s
0
200
400
600
800
1000 1200
Binding Energy (eV)
4
f7/2
(c)
Ta 4f
(b)
4
f5/2
Ta 4f
5.0 mol% Ta-TiO
3.0 mol% Ta-TiO
2
2
4f5/2
4
f7/2
Undoped TiO
2
1
8
20
22
24
26
28
30
32
2
0
22
24
26
28
Binding Energy (eV)
30
32
Binding Energy (eV)
(e)
Ti(IV)2p
3
/2
Ti 2p
Ti(IV)2p
3
/2
(d)
Ti 2p
Ti(IV)2p
1
/2
Ti(IV)2p
1
/2
Ti(III)2p
3
/2
4
50
455
460
Binding Energy (eV)
465
470
475
453 456 459 462 465 468 471
Binding Energy (eV)
Fig. 5 (a) XPS spectra of undoped TiO
films prepared on FTO glass substrate; Ta4f spectra for (b) 3.0 mol.% Ta-TiO
undoped TiO , 3.0 mol.% Ta-TiO and 5.0 mol% Ta-TiO2; Ti2p spectra for (d) undoped TiO
and (e) 3.0 mol% Ta-TiO
2
, 3.0 mol.% Ta-TiO
2
and 5.0 mol% Ta-TiO
2
thin
2
and (c)
2
2
2
2
.
1
2

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DOI: 10.1039/C7TA09193A
To find out the role of Ta doping on TiO work function, which could have influence on the
2
migration of electrons to the cathode, we conducted Kelvin probe force microscopy (KPFM)
of the undoped and 3.0 mol% Ta-TiO film. KPFM is an atomic force microscopy technique
2
which measures the distributed contact potential difference (CPD) between the tip and the
sample, through equation 1, which can be used to assess the surface potential.44
φ  φ
tip
sample
V 
(1)
CPD
e
Here, e is the electronic charge and ϕtip and ϕsample are the work functions of the tip and the
sample, respectively. The scanned area of 5 µm× 5 µm was taken for KPFM imaging. As the
Fig. 6 shows, the Ta-doped TiO
undoped TiO film having CPD value of 89 mV. Thus decrease of ~128 mV of CPD could be
manifested in the shift of Fermi level of doped film. The ϕtip has value of 5.0 eV, thus the
2
film showed the CPD value of -39 mV in comparison to
2
4
5
Fermi level of sample can be calculated as Φ  Φ  eV . The calculated value for
sample
tip
CPD
undoped film was 4.9 eV. For 3.0% Ta-TiO , the value was 5.04 eV. The lower edge of
2
4
+
conduction band (CB) in TiO
2
consists of Ti 3d bands and upper edge of valance band (VB)
is made of O 2p bands. As it is known that n- or p-type dopant affects the CB or VB of
material but due to defect ridden nature in TiO , the doping affect the trap states. The doping
2
-
2
can either decrease the deep trap density which can shifts the CB and Fermi level upward. In
this case, Voc increased but Jsc decreased due to decrease in driving force for electron
injection. When doping contributes to the formation of deep trap reverse trend is generally
4
6
observed . In our case, due to Ta doping the CB and Fermi level shift downward which is in
2
5
agreement with the reports on Ta doped films in DSSC solar cell. This change in the Fermi
level can result in large gap between conduction band of TiO and perovskite LUMO (Fig.
(c)) which could improve electron injection as suggested by device characterization. 46
2
6
1
3

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DOI: 10.1039/C7TA09193A
(C)
-
3.0 eV
-
4.0 eV
-3.9 eV
128.5 mV
Undoped film
-
4.4eV
FTO
P3HT
-
4.7eV
Ta doped film
Ag
TiO2
7.59 eV
-
5.0 eV
-
5.4 eV
-
Fig. 6 KPFM image of (a) undoped TiO
2
film (b) 3.0 % mol Ta- TiO film showing surface
2
potential maps. (c) Schematic representation of energy band diagram of different layer.
1
4

Page 15 of 25
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DOI: 10.1039/C7TA09193A
Pure TiO
2
1
3
5
.0 mol% Ta-TiO
2
2
2
.0 mol% Ta-TiO
.0 mol% Ta-TiO
6
00
650
700
750
800
850
Wavelength (nm)
Fig.7 Photoluminescence (PL) spectra (excitation at 500 nm) of perovskite film on pristine
TiO and different mol% Ta-doped TiO thin films.
2
2
Further to analyse whether the Ta-doped TiO
or extraction at the interface, we carried out the photoluminescence measurements on the
doped and undoped TiO films. Fig. 7 shows the steady state photoluminescence spectra of
perovskite on pure TiO and different mol% doped TiO thin films deposited on FTO glass.
From the Fig. it is clear that as we doped tantalum in TiO film, photoluminescence of
perovskite films gets quenched as evident from the decrease in the intensity of the PL peak
with increasing Ta doping in TiO thin films with 3.0 mol% Ta doped thin films showing
maximum PL quenching due to the highest conductivity of the doped thin films. This
suggests that Ta doping in pure TiO films could be responsible for enhanced rate of photo-
generated charge carrier extraction at the compact layer and perovskite interface.
2
films affect the carrier recombination
2
2
2
2
2
2
3
.2 Electrical Characterization of undoped and Ta-doped TiO
One of the effects of Ta doping on TiO is to modify its electrical conductivity via promotion
of anatase phase. To analyse this variation upon Ta doping into TiO layer, we measured the
current–voltage (I-V) characteristics of device structure FTO/Ta doped TiO (0-5.0
2
films
2
2
2
1
5

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DOI: 10.1039/C7TA09193A
mol.%)/Au and the results are shown in Fig. 8. The dc conductivity (σ ) of the samples are
0
calculated using the eqn 2
-
1
I = σ
0
A L V
(2)
2
where A (9.0 mm ) is area of the devices and L (~ 68 nm) is the thickness of thin films. The
-3
conductivity of pure TiO was measured from eqn 2 as 0.73 ± 0.13 x 10 mS/cm. With 1.0%
2
-3
Ta-TiO
calculated conductivity was 3.29±0.18 x 10 mS/cm. The conductivity showed one order
increment from pure TiO (0.0 mol%) to 3.0 mol% Ta doping. However, at higher molar
2
the conductivity increased to 1.64 ± 0.1 x 10 mS/cm. For 3.0% doping the
-3
2
-3
concentration (5.0 mol%), conductivity decreased to 2.68 ± 0.5 x 10 mS/cm. It may be due
to formation of pinholes or higher roughness in the doped films.
1
1
500
000
Pure TiO
2
1
3
5
.0 mol% Ta- TiO
2
.0 mol% Ta- TiO
.0 mol% Ta- TiO
2
2
5
00
0
-
500
-
-
1000
1500
-
3
-2
-1
0
1
2
3
Volatge (V)
Fig. 8 Current density–voltage (J-V) characteristics of pristine and Ta doped TiO
2
thin films
having device structure FTO/ Ta-TiO
2
doped (0-5 mol%)/Au. Inset shows the device
structure.
1
6

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DOI: 10.1039/C7TA09193A
3
.3 Integration into Perovskite Solar Cells and Photovoltaic performance
The perovskite devices were fabricated using the device structure FTO/compact-
TiO /CH NH PbI /P3HT/Ag, as shown in Fig. 9(a), where, TiO layer was either in undoped
or Ta-doped form. As the photovoltaic characteristics are shown Table 1, the average power
conversion efficiency of the devices with un-doped TiO film was 4.89% in the forward
sweep direction with Voc ~ 0.85 V, Jsc ~ 18.4 mAcm , FF ~ 0.32. Generally, researchers have
used Spiro-OMeTAD with Li-TFSI and tBP as dopant and CH NH (i.e. mixture of
methyl ammonium lead iodide and chloride) as active layer. Having incorporated
mesoporous TiO they have achieved high efficiency. In this device, we have used planar
2
3
3
3
2
2
-2
3
3
PbI3–xCl
x
2
structure with no doping in P3HT which is the reason behind comparatively low efficiency of
pristine device. However, the performance is comparable with literature where researchers
have used P3HT as HTL having planar structure.47 Further, poor fill factor is mainly
attributed to higher series resistance, as both compact TiO and undoped P3HT have low
2
2
conductivity leading to high series resistance of 48 Ωcm . With the doping of 1.0 mol % Ta
in compact TiO
hence slight increase in Jsc can be noticed. But at the same time, Voc decreases slightly from
.85 V to 0.83 V which neutralises the enhancement in efficiency. With 3.0 mol % Ta-TiO
2
, due to increase in conductivity, series resistance decreases to 29 Ωcm2,
0
2
-2
doping, the efficiency increase to 6.81% due to increase in Jsc (20.6 mAcm ) and FF (0.41)
although there was again a little decrease in Voc from 0.85 V to 0.83 V compared to undoped
TiO
as conduction band to shift downward. As Voc is defined as the difference between the Fermi
level of TiO and HTM, shifting of Fermi level leads to a decreased Voc, as also suggested by
KPFM image in Fig. 6. However, at the same time due to larger difference between
perovskite LUMO and TiO
conduction band, electron injection and hence Jsc increases.46
These observations are in agreement with the previous data on DSSC solar cells using Ta
2
. These trends can be explained by the fact that Ta doping causes the Fermi level as well
2
2
3
9
doped TiO . In our case, the best device was with 3.0 % tantalum doping which exhibited a
2
-2
PCE of 9.24% with Jsc of 21.13 mAcm , Voc of 0.90 and FF of 0.49. For 5.0 mol% Ta
doping, Voc further decreases to 0.75 as explained above and efficiency decreases. We also
observe a decrease in the shunt resistance and increase in series resistance for 5.0% doping,
which can be correlated to the increase of roughness and voids in the film.48
The change in PCE and Jsc upon Ta doping of TiO
external quantum efficiency (EQE) characteristics, as shown in Fig. 9(b) where 3.0 mol% Ta-
TiO devices show the best external quantum efficiency. Fig. 9(c), 9(d) shows the J-V curve
2
are also in agreement with the changes in
2
1
7

Journal of Materials Chemistry A
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of undoped and doped (1.0-5.0%) device in (a) forward and (b) reverse sweep direction,
DOI: 10.1039/C7TA09193A
respectively. Similar trend in the performance was also evident in the reverse sweep
direction. The undoped device show an average efficiency of 6.63% which increased to 8.17
%
for 3.0 mol % Ta doped device (Table 1). It is interesting to note that Ta doping reduced
the hysteresis in the J-V curves as forward and reverse sweeps J-V characteristics show low
difference. Small amount of hysteresis at high doping concentrations may be due to
ferroelectric hysteresis of perovskite material or defects at the surface of these materials.49
The best device for 3.0 mole% in reverse sweep was 9.94% with FF of 0.57 and Jsc of 19.4
-2
mAcm . Table S2 summarizes PCE data from recent literature having planar device
structure. The obtained efficiency for our devices is at par or more than the reported planar
device structure having P3HT as HTL.
Fig. 10(a) shows the J-V curve of the best device in forward and reverse scan direction. Since
3
.0 mol% Ta-TiO
2
device has shown the best performance, effect of Ta doping in TiO on
2
charge transport was further analysed by impedance spectroscopy (IS). It was carried out in
dark with a bias of 0.6 V and 5 mV AC signal was applied within frequency range of 100 Hz
to 100 kHz. Fig. 10(b) shows the Nyquist plots of undoped and 3.0 mol% Ta-TiO
with the equivalent circuit (inset).50 The circuit consists of series resistance (R
), parallel
recombination resistance (Rrec) and constant phase element (CPE). The ohmic resistance is
represented by R whereas Rrec is associated with interface charge transport process. The non-
2
device
s
s
51
ideal behaviour of capacitor is associated with CPE. The CPE values are 0.97 and 0.95 for
undoped and 3.0 % Ta doped samples, respectively indicating that the interface is not
5
1
2
perfectly electrical. For undoped device, R
s
is 7.5 Ω.cm whereas for the doped device, R
s
2
reduces to 2.1 Ω.cm due to higher conductivity of Ta doped film. Further, recombination
2
2
resistance is also increased from 88.9 Ω cm for undoped device to 223 Ω cm for doped
device. This clearly confirms the effect of Ta doping in reducing the recombination losses in
the devices with Ta-doped TiO films.
2
1
8

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DOI: 10.1039/C7TA09193A
Table-1 Summary of average photovoltaic performance of undoped TiO and different Ta-
2
doped TiO
2
devices in forward and reverse sweep direction for 10 devices.
Ta
J
sc
V
oc
FF
η (%)
R
S
R
SH
doping
mol%
-
2
(Ωcm2)
(Ω cm2)
(
mA cm )
(V)
Forward sweep direction
Pure TiO
2
18.40 ± 1.5
0.85 ± 0.05
0.83 ± 0.04
0.83 ± 0.05
0.75 ± 0.06
0.32 ± 0.04
0.30 ± 0.04
0.41 ± 0.05
0.32 ± 0.07
4.89 ± 0.83
4.80 ± 0.92
6.81 ± 1.99
4.50 ± 1.33
48.3 ± 19.3
29.1 ± 2.70
18.5 ± 2.80
23.3 ± 1.50
108.1 ± 33.6
105.2 ± 19.6
220.0 ± 65.3
150.2 ± 70.1
1
3
5
.0 mol% 18.93 ± 2.7
.0 mol% 20.60 ± 1.7
.0 mol% 19.20 ± 2.1
Reverse sweep direction
Pure TiO
2
20.40 ± 1.4
0.91 ± 0.06
0.36 ± 0.04
0.35 ± 0.05
0.42 ± 0.04
0.37 ± 0.07
6.63 ± 0.54
45.7 ± 28.0
25.6 ± 3.1
17.4 ± 2.3
19.6 ± 1.4
164.0 ± 44.8
131.3 ± 32.7
276.6 ± 81.6
229.2 ± 30.3
1
3
5
.0 mol% 22.60 ± 1.2
0.85 ± 0.03
0.85 ± 0.04
0.80 ± 0.05
6.40 ± 0.47
8.17 ± 0.97
6.24 ± 0.76
.0 mol% 22.10 ± 1.2
.0 mol% 21.60 ± 2.6
1
9

Journal of Materials Chemistry A
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DOI: 10.1039/C7TA09193A
1
00
80
(
b)
(a)
6
4
2
0
0
0
0
Pure TiO
2
1
3
5
.0 mol% Ta-TiO
2
2
2
.0 mol% Ta-TiO
.0 mol% Ta-TiO
3
00
400
500
600
700
800
2
Wavelength(nm)
2
2
1
1
5
0
5
0
5
0
25
Pure TiO
PureTiO
2
(
c)
(d)
1
3
5
.0 mol% Ta-TiO
1.0 mol% Ta-TiO
3.0 mol% Ta-TiO
5.0 mol% Ta-TiO
2
2
2
2
2
2
20
15
10
5
.0 mol% Ta-TiO
.0 mol% Ta-TiO
0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Voltage (V)
Fig. 9 (a) Schematic of planar conventional perovskite solar cell, (b) EQE of undoped and Ta
1.0-5.0%) doped device, J-V curve of undoped and doped (1.0-5.0 %) device in (c) forward
and (d) reverse sweep direction.
(
2
2
1
1
50
00
50
00
50
2
2
1
1
5
0
5
0
5
0
(
a)
Reverse scan
Forward scan
Pure TiO
2
(b)
3.0 mole% Ta-TiO
CPE
2
Rs
Rrec
-
5
-
0.2
0.0
0.2
0.4
0.6
0.8
1.0
0
50
100
150
200
250
2
Voltage(V)
Z'( cm )
Fig. 10 (a) J-V curve of best device in forward and reverse sweep direction, (b) Nyquist
plots of undoped and 3.0 mol% Ta-TiO
2
device in dark. Inset shows the equivalent circuit of
Nyquist plot.
2
0

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DOI: 10.1039/C7TA09193A
4
. Conclusions
In summary, we have demonstrated the positive effect of Ta doping in compact TiO layer
2
resulting in substantial improvement in the device performance in Spiro-OMeTAD free
planar perovskite solar cells. Quenching of photoluminescence upon Ta doping suggests that
the electrons could be extracted more efficiently from perovskite film compared to undoped
TiO
higher shunt resistance in the devices. The device showed improved Jsc due to lowering of
Fermi level of TiO shifted upon Ta doping and hence a larger gap between conduction band
2
film. These effects resulted in an enhancement in FF due to lower series resistance and
2
and perovskite LUMO provides a greater driving force for electron transfer. Thus,
enhancement in current density was observed with Ta doping. The best device was achieved
for 3.0 mol% Ta doping of TiO
direction and 9.94% in reverse scan direction showing substantial improvement over device
with undoped TiO
2
with enhancement in efficiency to 9.24% in forward scan
2
.
ASSOCIATED CONTENT
Supporting Information
XRD pattern, absorbance data and morphology of the perovskite film, morphology of
compact TiO film. This material is available free of charge.
2
AUTHOR INFORMATION
Corresponding Author
*
E-mail: guptark@iitk.ac.in
Notes
The authors declare no competing financial interest.
Acknowledgements
We acknowledge the financial support from DST-RCUK Grant no SR/RC-UK/APEX Phase
II/2015 and DST Grant No. DST/TMD/CERI/C140(G) under Clean Energy Research
Initiative. RKG acknowledges financial assistance from Department of Science and
Technology (DST), India, through the INSPIRE Faculty Award (Project No. IFA-13 ENG-
5
7). DST support to the Center for Nanosciences is acknowledged.
2
1

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Page 25 of 25
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C7TA09193A
TOC
This work investigates the effect of tantalum doping in compact TiO
2
layer on the
performance of planar Spiro-OMeTAD free perovskite solar cells. 40% improvement in the
overall efficiency was obtained as compared to the device with undoped TiO2.