J Mater Sci: Mater Electron (2021) 32:5353–5360
Impact of drying temperature on the photovoltaic
performance and impedance spectra of hole transport
material free air processed perovskite solar cells
Ifra Sardar1, Muhammad Hassan Sayyad1,*
Syed Afaq Ali Shah2, and Zhongyi Guo2,3
, Syeda Ramsha Ali1, Mehreen Akhtar1,
1
Advanced Photovoltaic Research Labs (APRL), Faculty of Engineering Sciences, Ghulam Ishaq Khan Institute of Engineering
Sciences and Technology, District Swabi, Topi 23640, Khyber Pakhtunkhwa, Pakistan
2
School of Electrical Engineering & Intelligentizaion, Dongguan University of Technology, Dongguan 523808, China
3
School of Computer and Information, Hefei University of Technology, Hefei 230009, China
Received: 14 October 2020
ABSTRACT
Accepted: 4 January 2021
Due to the rapid increase in power conversion efficiency (PCE) of organic–
inorganic perovskite solar cells (PSCs) and exceeding the PCE achieved in
conventional single-junction silicon solar cells this technology has become the
focus of research. The quality of perovskite film plays a vital role in developing
the high performance PSCs and depends upon many factors, such as, composition of the perovskite, growth method, drying temperature, etc. In this work,
hole transport material free (HTM-free) glass/FTO/c-TiO2/m-TiO2/m-ZrO2/
Carbon electrode based PSCs are fabricated. Effect of prevoskite drying temperature on the photovoltaic performance and impedance spectra of these
devices is studied by varying temperature from 50 to 70 °C. The photovoltaic
and impedance spectra of the devices are observed to be highly dependent on
the drying temperature. The best power conversion efficiency is obtained for
drying temperature of 60 °C. These results show that determining the optimum
drying temperature is crucial to ensure formation of perovskite crystals, highest
surface coverage of perovskite, highest light harvesting and successful charge
extraction from the fabricated devices in order to achieve highest performance.
Published online:
24 February 2021
Ó
The
Author(s),
under
exclusive licence to Springer
Science+Business Media, LLC
part of Springer Nature 2021
1 Introduction
Organometal halide perovskites (OMHPs) have
attracted considerable attention due to their direct
band gap, band gap tunability, high absorption
coefficient and high carrier mobility [1–3]. Using
these materials as absorbers in single-junction architectures, power conversion efficiency (PCE) of the
solar cells has drastically improved from 3.8% in 2009
[4] to 25.2% in 2020 [5]. Efficiencies up to 29.1% [5]
were reported when these OMHPs were used in
tandem architecture with silicon, outperforming the
Address correspondence to E-mail: hsayyad62@gmail.com; sayyad@giki.edu.pk
https://doi.org/10.1007/s10854-021-05240-x
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single-junction silicon solar cells. Hence, it is safe to
say that currently perovskite solar cells are the fastest-advancing photonic energy harvesting technology
[6]. For most of the techniques aiming at developing
low cost and high efficiency PSCs, the perovskite
precursor is deposited either by drop casting or spin
coating, followed by heat treatment of the perovskite
film to remove any excess solvent and facilitate the
formation of perovskite crystal structure [7]. The
formation and characterization of the perovskite film
has drawn much attention for optimizing the performance in the standard perovskite solar cells. Many
recent works focused on analyzing the effect of the
drying temperature on the perovskite film morphology using different characterization techniques such
as ultraviolet visible (UV–Vis) absorption spectroscopy, X-ray diffraction (XRD), atomic force
microscopy (AFM), scanning electron microscopy
(SEM), photoluminescence (PL) and impedance
spectroscopic measurements [7–9]. The results of
these studies have been correlated with the photovoltaic performance and working mechanisms of the
standard PSCs [7–9].
The XRD patterns reported in previous studies
revealed that at optimum drying temperature pure
MAPbI3 is formed. However, drying at the lower and
higher temperatures showed incomplete conversion
of the perovskite precursor and decomposition of the
perovskite, respectively [9]. In the SEM images the
gradual increase in the perovskite grain size has been
seen with the increasing drying temperature. Trap
states in the crystallite matrix and interfaces act as
centers for charge carriers recombination [10]. The
increase in grain size reduces the number of grains
and increases grain boundary resulting a decrease in
trap states, hence significantly reducing charge carriers recombination and facilitating the efficient
transport of electrons from the provskite layer into
TiO2. PL spectra of the perovskite films on TiO2 layer
revealed reduction in the photo luminescence intensity with increase in drying temperatures. This can be
correlated with the increase of grain size, enhancement of grain boundaries and decrease of carrier
trapping sites which result in the increase in carrier
transport mobility in the perovskite thin films and
efficient electron injection from the perovskite layer
into TiO2 [9, 11–13]. The AFM images further confirmed the positive effects of elevated drying temperatures on the MAPbI3 grain size [9]. The
electrochemical impedance spectroscopy (EIS)
J Mater Sci: Mater Electron (2021) 32:5353–5360
measurements performed under dark showed gradual decrease of recombination of charge carriers
within MAPbI3 thin films with increasing drying
temperatures due to increase of MAPbI3 grain size
[9].
Despite obvious advantages of PSCs they also face
some difficulties like expensive fabrication technique
for the deposition of gold electrodes. To resolve this
issue Etgar et al., were the very first group to report
hole transport material free (HTM-free) perovskite
solar cells [14]. Recently, several different materials
like carbon and graphene have been proposed to
replace Au electrode because of their similar work
function. The glass/FTO/c-TiO2/m-TiO2/CH3NH3PBI3/ZrO2/C type is the most commonly used HTMfree perovskite solar cell structure in which c-TiO2 is
compact layer of titanium oxide, m-TiO2 is mesoporous titanium oxide layer, ZrO2 is zirconia dioxide
spacer layer and CH3NH3PBI3 is the light absorbing
perovskite layer. Its structure is shown in Fig. 1. In
order to obtain higher efficiencies and optimize the
performance, deep understanding of this low cost
carbon based HTM-free perovskite solar cell is
required [15]. In this architecture CH3NH3PBI3
behave as hole transport layer (HTL) as well as
absorber layer at the same time [16]. The absence of
HTM simplifies and lowers manufacturing cost of
PSCs which is fundamental for commercializing this
technology.
The formation of perovskite crystals, perovskite
film morphology, grain size, number of grains,
number of traps and ions migration play a major role
in photovoltaic characteristic of PSCs. The drying
temperature of the absorbing perovskite layer influences all these factors and ultimately the performance
of the PSCs, hence it became necessary to optimize
the drying temperature for the effective harvesting of
Fig. 1 Structure of monolithic HTM-free PSC
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solar energy. In this work, effect of perovskite drying
temperature on photovoltaic performance and
impedance spectra of the glass/FTO/c-TiO2/mTiO2/m-ZrO2/Carbon electrode based solution processable full printable air processed HTM-free PSCs
is reported.
2 Experimental
For the device fabrication, printed glass/FTO/cTiO2/m-TiO2/m-ZrO2/Carbon type monolithic electrodes, a pre-mixed organometal halide perovskite
precursor solution (CH3NH3PbI3) prepared by mixing lead iodide, methylammonium iodide and
5-aminovaleric acid hydroiodide in c-butyrolactone
as solvent and polyimide adhesive masks were purchased from Solaronix SA (Aubonne, Switzerland).
The substrates were heated at 400 °C for 30 min and
then allowed to cool down at room temperature.
Then polyimide adhesive mask were applied on the
electrodes in order to prevent spreading of the solution which was to be deposited on the carbon layer in
the next step. The perovskite deposition was performed by drop-casting 5.6 lL of perovskite precursor solution onto the carbon layer by using a
micropipette. Then the devices were dried at 50, 60
and 70 °C in dark for one hour to let the growth of the
perovskite layer. The effective area of each monolithic
device was 1.5 cm2. Different steps of fabrication of
monolithic perovskite solar cells are illustrated in
Fig. 2.
The photovoltaic and impedance spectroscopic
measurements were performed by using Keithley
4200 semiconductor characterization system (Keithley Instruments, USA) under dark and AM 1.5 simulated illumination (OAI TriSOL, AM1.5G Class
AAA, USA) conditions. By using Newport Oriel PV
reference cell system (Model 91150 V) power calibration was made and irradiance was set at 100 mW
cm-2. Impedance spectra were recorded by applying
a 30 mV AC signal from 5 to 100 kHz.
3 Results and discussion
3.1 Photovoltaic properties
To investigate the effect of drying temperature on the
HTM-free PSC performance, devices were fabricated
with CH3NH3PbI3 perovskite as absorber and dried
at 50, 60 and 70 °C in dark for one hour. The current–
density (J–V) characteristics measured on the carbon
based HTM-free monolithic TiO2/CH3NH3PbI3
heterojunction perovskite solar cells under standard
AM1.5G illumination (100 mW cm-2) are compared
in Fig. 3. The characteristic photovoltaic parameters
required for comparison of the cells can be obtained
from these curves. The short-circuit current Isc is the
current that flows through the external circuit when
the electrodes of the solar cell are short circuited. The
short-circuit current of a solar cell depends on the
photon flux density incident on it. Therefore, often
the short-circuit current density is used to describe
the maximum current delivered by a solar cell.
The open-circuit voltage is the voltage at which no
current flows through the external circuit. It is the
maximum voltage that a solar cell can deliver. The
relation between the open-circuit voltage (Voc) and
light generated current density (JL) is given by:
kB T
JL
ln þ 1
Voc ¼
ð1Þ
q
Jo
where Jo is the saturation current, q is the electron
charge, V is the applied voltage, and kB is the Boltzmann constant.
The fill factor corresponds to the largest rectangular area that can fit in the J–V curve and can be calculated using the following relation:
FF ¼
Jmax Vmax
Jsc Voc
ð2Þ
where Vmax and Imax represent the voltage and current at the point of maximum power output of the
cell, respectively.
The efficiency (g) of a solar cell is defined as the
ratio of output power to total power incident on the
cell and can be determined by the following relation:
g¼
Pmax
Jsh Voc FF
100 ¼
100
Pin
Pin
ð3Þ
where Jsh is the short circuit current density, Voc is the
open circuit voltage, Pin is the intensity of incident
light.
The current–voltage characteristics are influenced
by the series (Rs) and shunt (Rsh) resistance [17].
Higher Rs reduces the fill factor, thereby lowering the
maximum power output; therefore, it is an important
factor for optimizing the performance of the solar
cells. The shunt resistance (Rsh) provides an alternate
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J Mater Sci: Mater Electron (2021) 32:5353–5360
Fig. 2 Schematic diagram for the preparation of monolithic perovskite solar cell
-2
Current Density (mA.cm )
10
o
50 C
o
60 C
o
70 C
5
0
0.0
0.5
Voltage (V)
1.0
Fig. 3 J–V characteristics of CH3NH3PbI3/TiO2 heterojunction
type HTM-free monolithic perovskite solar cell under standard
AM1.5G illumination (100 mW cm-2)
current path for the photo-generated current and is
related to the electron recombination occurring at the
TiO2/CH3NH3PbI3 interface due to the crystal defects
in the semiconductor material [18, 19]. Its low value
causes significant solar cell power loss. The values of
series and shunt resistances can be obtained from the
J–V curves using the relations (4) and (5), respectively
[20].
Rs ¼
dV
dJ J ¼ 0
dV
Rsh ¼
dJ V ¼ 0
ð4Þ
ð5Þ
The important parameters of the fabricated solar
cells like short-circuit photocurrent density and open-
circuit photovoltage, fill factor, power conversion
efficiency, series and shunt resistance are determined
and listed in Table 1. A clear correlation is observed
between the drying temperature of the perovskite
and the photovoltaic performance of the devices. The
device that was dried at 60 °C delivered the highest
efficiency (g) of 3.29%, as a result of the highest short
circuit current density (Jsc) and fill factor (FF) of
8.11 mA cm-2 and 51.94, respectively. Previously, it
has been reported that the formation of perovskite
crystals, perovskite film morphology, grain size,
number of grains, number of traps and ions migration play an important role in photovoltaic performance of the devices [7, 9]. At lower drying
temperatures, the photovoltaic study revealed lower
values of the g, Jsc and FF and can be attributed to the
incomplete conversion of the perovskite precursor to
the perovskite material, leading to lower light harvesting [7, 9]. When the drying temperature
increased to 70 °C, the values of g, Jsc and FF were
decreased. This decrease at higher drying
Table 1 Effect of perovskite drying temperature on the
photovoltaic parameters of the TiO2/CH3NH3PbI3 heterojunction
HTM-free monolithic perovskite solar cell under standard
AM1.5G illumination (100 mW cm-2)
Temperature
( °C)
JSC
(mA cm-2)
VOC
(mV)
FF
(%)
g
(%)
RS
(X)
50
60
70
7.29
8.11
7.22
820
780
800
51.30
51.94
49.55
3.07
3.27
2.88
27.08
26.24
29.42
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3.2 Impedance spectroscopy
Impedance spectroscopy (IS) is one of the most
widely used, reliable and well-established AC characterization technique for investigating solid-state
devices and electrochemical systems [25]. However,
using impedance spectroscopy for characterizing
emerging photovoltaics having complex device
architectures, such as, DSSCs and PSCs, presents new
challenges related to the interfacial degradation [26],
unusual material properties [25], and testing of the
simulated data [27].
In IS performance, for a given frequency (f) a
voltage (V) sweep is applied across the solar cell. The
applied voltages and frequencies change the
dynamics of the device and these changes can be read
and interpreted from the impedance spectra [28].
Therefore, to gain insight into charge kinetics of the
PSC which can help optimize various interfaces and
components of the device for its better and
stable performance study of the V and f dependent
impedance spectra is necessary. In this work, impedance spectroscopy was used to investigate the parallel capacitance–voltage (CP–V), series capacitance–
voltage (CS–V) and series resistance–voltage (RS–V)
characteristics of glass/FTO/c-TiO2/m-TiO2/mZrO2/Carbon electrode based HTM-free PSCs cell.
3.2.1 Parallel capacitance–voltage (CP-V) characteristics
Figure 4 shows the parallel capacitance–voltage (CP–
V) curves recorded at a 10 kHz frequency in the dark
on the CH3NH3PbI3/TiO2 heterojunction type HTMfree monolithic PSC dried at the temperatures of 50,
60 and 70 °C. A direct relationship is noticed between
the drying temperature of the perovskite and the CP–
V characteristics of the cells. The device that was
dried at 60 °C exhibited the highest capacitance
resulting highest surface coverage of perovskite and,
hence, provided the highest PCE (Fig. 3).
At lower drying temperatures, the parallel capacitance–voltage plots revealed lower value of the
capacitance and can be attributed to the incomplete
conversion of the perovskite precursor to the perovskite material, leading to lower surface coverage of
perovskites [7, 9]. When the drying temperature was
increased to 70 °C, the value of capacitance was
decreased. A similar trend of capacitance variation
with sweep voltage has also been observed in another
study [29]. This decrease at higher drying temperature may be attributed to several reasons such as,
decomposition of CH3NH3PbI3, decrease in the surface coverage of perovskites, degradation of the
absorber quality, development of larger pinhole sizes,
reduced current density and disorder in the semiconducting layer [21, 22, 29].
Parallel capacitance (nF)
temperature may be attributed to several reasons
such as, decomposition of CH3NH3PbI3, decrease in
the surface coverage of perovskites, degradation of
the absorber quality, development of larger pinhole
sizes [21, 22]. A lower value of Jsc as compared to the
other efficient monolithic HTM-free mesoscopic
CH3NH3PbI3/TiO2 heterojunction PSCs may be
attributed to the narrow depletion region formed
between TiO2 and perovskite layers where the electron hole pairs cannot be separated efficiently [23].
The quality of perovskite crystals depend on the
drying temperature. At the optimum temperature the
crystallization very large crystals are formed while at
higher temperatures the crystallization is very fast
and a non-uniform perovskite layer is formed with
many pin holes.
A relatively lower PCE and FF of our fabricated
device compared to the best reported values of this
type of PSC [24] may be due to the relatively large
area and the hindrance in transport of charge carriers
through the active layer and junction due to higher
series resistance (RS) of the device [23], respectively.
100
o
50 C
o
60 C
o
70 C
90
80
70
60
50
-1.0
-0.5
0.0
0.5
Voltage (V)
1.0
Fig. 4 Parallel capacitance–voltage (CP–V) curves recorded at a
10 kHz frequency in the dark on the CH3NH3PbI3/TiO2
heterojunction type HTM-free monolithic perovskite solar cells
dried at the temperatures of 50, 60 and 70 °C
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J Mater Sci: Mater Electron (2021) 32:5353–5360
3.2.2 Series capacitance–voltage (CS–V) and series
resistance–voltage (RS–V) characteristics
the MS plot furthermore suggests a p-type CH3NH3PbI3 perovskite layer.
Figure 5 shows the series capacitance–voltage (CS–V)
and series resistance–voltage (RS–V) characteristics.
When the value of biased voltage is varied to higher
value, the capacitance and resistance have shown
voltage dependency. As can be seen from Fig. 5,
when a low voltage bias was applied, the capacitance
and resistance remain unchanged. However, as the
bias increased, the spectra exhibit strong voltage
dependency. Many factors are believed to play role in
this voltage dependency of IS of solar cells such as,
traps, interfaces, density of states, restrictions upon
minority carriers density, charge blocking resulted by
slow charge injection/extraction dynamics at the
contacts, charge kinetics, migration of ions, device
degradation, etc. [29–33]. A number of other optoelectronic devices and structures are also reported to
have the same kind of behavior [26, 34–42].
As it has been previously described [42], at zero
applied bias, the device is fully depleted, but as the
forward bias reaches around 0.5 V, the depletion
width shrinks causing increase in the cell capacitance.
4 Conclusion
We demonstrated correlation between drying temperature of the perovskite and the photovoltaic performance and impedance spectra of the solution
processable full printable air processed carbon based
HTM-free PSCs. By drying the device at 60 °C the
highest power conversion efficiency (PCE) of 3.29%,
as a result of the highest short circuit current density
(JSC) and fill factor (FF) of 8.11 mA cm-2 and 51.94,
respectively, has been achieved. We attributed this to
the complete formation of perovskite crystals, highest
capacitance resulting highest surface coverage of
perovskite perovskite film morphology and, hence,
highest light harvesting. At lower drying temperature of 50 °C, the lower photovoltaic performance
was observed, which we attributed due to the
incomplete conversion of the perovskite precursor to
the perovskite material, leading to lower surface
coverage of perovskites and lower value of the
capacitance. This decrease observed at higher drying
temperature of 70 °C may be attributed to several
reasons such as, decomposition of CH3NH3PbI3,
decrease in the surface coverage of perovskites,
degradation of the absorber quality, development of
larger pinhole sizes, reduced current density and
disorder in the semiconducting layer. Making use of
the impedance spectroscopy, the voltage dependent
impedance spectra of the full printable HTM-free
monolithic perovskite solar cell are presented and
interpretation is provided. Our study emphasizes the
importance of optimum drying temperature in the
complete of conversion of perovskite precursor
3.2.3 Mott–Schottky analysis
100
1.0
Series Resistance (: )
Fig. 5 Series capacitance–
voltage (CS–V) and series
resistance–voltage (RS–V)
curves recorded at a 10 kHz
frequency in the dark on the
TiO2/CH3NH3PbI3
heterojunction type HTM-free
monolithic perovskite solar
cells dried at the temperatures
of 50, 60 and 70 °C
Series capacitance (PF)
To further clarify the highest performance of the cell
dried at 60 °C, Mott–Schottky (MS) analysis of devices were conducted (Fig. 6). From the X-intercept of
the linear regime in the Mott–Schottky plot (Fig. 6),
the built-in potential at the TiO2/CH3NH3PbI3 contact can be obtained [43, 44]. The built-in potential of
the device dried at 60 °C is around 13 mV and 41 mV
higher than those of the devices dried at 50 °C and
70 °C, respectively, and is in agreement with best
performance of the device dried at 60 °C. The slope of
o
50 C
o
60 C
o
70 C
0.8
0.6
0.4
0.2
0.0
-1.0
-0.5
0.0
0.5
Voltage (V)
1.0
90
80
70
o
50 C
o
60 C
o
70 C
60
50
-1.0
-0.5
0.0
0.5
Voltage (V)
1.0
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J Mater Sci: Mater Electron (2021) 32:5353–5360
6.
7.
8.
Fig.
6 Mott–Schottky
analysis
of
TiO2/CH3NH3PbI3
heterojunction type HTM-free monolithic perovskite solar cells
dried at the temperatures of 50, 60 and 70 °C
9.
solution into perovskite material and provides a
practical guideline for developing high performance
low cost and stable perovskite solar cells. The impedance spectra presented in this work may be useful to
further deeply investigate the influence of different
perovskite layers on interfacial kinetics, charge
transport and degradation of PSCs required for their
high-performance designs.
10.
11.
12.
Acknowledgements
We acknowledge the financial support extended by
the HEC Pakistan and US National Academy of Sciences, under the PAK-US Science and Technology
Cooperative Program, Phase-V, Project Number
5-530/PAK-US/HEC/2013/193.
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