of Ravishankar University–B, 34 (1), 01-08 (2021)
Modeling of Abnormal
Hysteresis in CsPbBr3 based Perovskite Solar Cells
*B GopalKrishna, Sanjay Tiwari
Photonics Research Laboratory, School of
Studies in Electronics & Photonics
Pt. Ravishankar Shukla University, Raipur, India
Author Email: firstname.lastname@example.org
27 December 2020; Revised: 18 March 2021; Accepted: 22 March 2021]
Abstract: Perovskite solar cells are emerging photovoltaic devices
with PCE of above 25%. Perovskite are
suitable light absorber materials in solar cells with excellent properties like
appropriate band gap energy, long carrier lifetime and diffusion length, and
high extinction coefficient. Simulation study is an important technique to
understand working mechanisms of perovskites solar cells. The study would help
develop efficient, stable PSCs experimentally.
In this study, modeling of perovskite solar cell was carried out through
Setfos software. The optimization of different parameters of layer structure of solar
cell would help to achieve maximum light absorption in the perovskite layer of
solar cell. Simulation study is based
drift-diffusion model to study the different parameters of perovskite solar
cell. Hysteresis is one of the factors in the perovskite solar cell
which may influence the device performance.
The measurement of abnormal hysteresis can be done by current-voltage
curve during backward scan during simulation study. In backward scan, the
measurement starts from biasing voltage higher than open circuit voltage and
sweep to voltage below zero. The numerical simulation used to study the various
parameters like open circuit voltage,
short circuit current, fill factor, power conversion efficiency and hysteresis.
The simulation results would help to understand
the photophysics of solar cell physics which would help to fabricate highly
efficient and stable perovskite solar cells experimentally.
Keywords- Perovskite solar cells, Transient photo-current,
Hysteresis, Efficiency, Setfos software.
Perovskite solar cells have achieved a
reported efficiency of over 25% in recent years and can be future solar
technology for energy generation for human race. Perovskite materials are
semiconductors with properties like tunable band gap, efficient light
absorption and high charge mobility for fabricating efficient solar cells. The
performance of perovskite depends on its structural order which is temperature
dependent even in typical solar cell operating conditions. Perovskite/silicon, perovskite/CIGS, and
perovskite/perovskite tandem solar cells are the next PV technologies for
fabricating highly efficient PV devices.
Perovskite/silicon tandem solar
cells have reported to achieve a record efficiency of 26.7%. The power
conversion efficiency of perovskite/silicon tandem solar cells could reach
beyond 30% (Quiroz et al., 2018; Altazin et al. 2018). The physics and operational mechanism of perovskite solar cells
are very difficult to understand by simple analytical formulations. Numerical
study of the operational mechanism gives deeper knowledge of the underlying
device physics of PSCs.Drift diffusion model can be helpful to simulate
different solar cell parameters such as voltage, current, carrier density, and charge
recombination. The simulation of PSC will provide the J-V curve characteristics
to know the evolution of hysteresis in the device. The phenomena of hysteresis
are assumed to occur due to internal capacitance and polarization inperovskite
solar cell. The hysteresis caused by internal capacitance and polarization of
perovskite solar cell can be studied under fast and slow scanning rates
respectively. In this simulation study, the scanning rate was being kept at
slow process.There are different device simulations modelswhich are used to
study parameters like open-circuit voltage, photocurrents, series resistance, shunt resistance and fill factor(Snaith
et al.,2014).Many models based on mobile ions neglect RC effects which are
important to understand transient or frequency-domain experiments and used for
solar cell simulation to get errors in results. Therefore, simulation model
should include series resistance, frequency and transientparametersto get a
better knowledge of influence of mobile ions in layer structure of perovskite
solar cell. In this study, we present various parameter measurements of a
planar CsPbBr3 perovskite solar cellthrough simulations using
drift-diffusion model. The simulation model incorporating mobile ions and
charge traps gives the good information about the dependence of the
open-circuit voltage on the light intensityand photocurrent. The underlying
device physics of PSC can be described properly by the device model including
inert mobile ions and traps(Krishna et
al., 2020). The parameter analysis of PSC by simulation is importance to
determine which factors limit the device performance and will be taken care of
during development of PSC(Snaith et al.,2014; Perez et al. 2014).
In this research, numerical simulation of CsPbBr3 perovskite
solar cell using Setfos software will be done to optimize the different
parameters like layer thickness, hysteresis and other factors for getting
better device performance.
The numerical modeling was done by using simulation
software Setfos 5.0 from Fluxim. Different parameters used for simulation as
shown in Table 1.The CsPbBr3 perovskite solar cell structure has a p-i-n
architecture as shown in Figure 1. Perovskite solar cell structure consists of CsPbBr3
perovskite as light absorbing layer, TiO2 as electron transporting
layer (ETL), Spiro-OMeTAD as hole transporting layer (HTL). The architecture of
perovskite solar cell is FTO/TiO2 /CsPbBr3 / Spiro-OMeTAD / Au. The layer
thicknesses of TiO2, Spiro-OMeTAD, perovskite were taken such as 80
nm, 100 nm and 450 nm respectively.
Structure of the CsPbBr3 based Perovskite Solar Cell
When reversed biased voltage is applied to the PSC, the
properties of the device changes due to
the development of smaller value
of electric current in the device. The phenomena can be measured by I-V curve.
In backward scan, the measurement starts from biasing voltage higher than open
circuit voltage and sweep to voltage below zero. The numerical simulation used
to study the hysteresis which gives the proper understanding of charge
The efficiency of the solar cell can be influenced
by charge carrier doping because it leads to extra charge inside the bulk which
may produce screening effect. In case of high doping density,the screening
effect influences the electric field whichhinders the charge extraction. This
would lead to decrease in photocurrent. In case of low doping density, the
short-circuit current is decreased and fill-factor is increased (Tress et
al.,2015; Walsh et al., 2015).
PSC device, the photon-to-charge conversion efficiency may be reduced due to
reduced light absorption or hindered exciton dissociation. Exciton dissociation
is hindered due to the roughness in phase-mixing in perovskite solar cell. The different parameters for the materials used in the
solar cell for simulation were summarized in Table 1.
Table 1 Showing the Parameters of the light absorption material,
ETM and HTM used of the PSC
The J-V characteristics of perovskite solar
cell structure measured under reverse voltage. The simulated values of open-circuit voltage VOC, short-circuit
current Jsc, fill factor FF, EQE and PCE, are 0.62 V, 15.6 mA/cm2
, 0.79, 80% and 18 %, respectively as shown in Figure 3, 4 and 5. There is a strong influence of bandgap and
electron affinity of HTL and ETL on open circuit voltage Voc. There is an
increase in open circuit voltage Voc with increase in band gap of HTM because
there is a decrease in valance band energy of HTM. The value of open circuit
voltage Voc remains constant with increase in band gap of ETM because there is
no change in the conduction band energy of ETM. There is an increase in open
circuit voltage Voc with increase in electron affinity of HTM because there is
a decrease in valance band energy of HTM. There is a decrease in open circuit
voltage Voc with increase in electron affinity of ETM because there is a
decrease in conduction band energy of ETM. There is a increase in short circuit
current Jsc due to easy transport of holes to the anode(Eames et
al., 2015; Lee et al. 2017).
Transient opto-electrical measurements were performed
to analyze the current-voltage characteristics of CsPbBr3 solar
cells (PSCs) through Setfos software. The current voltage curves were measured
at constant voltage under slow scan-rate. The simulation was based on
drift-diffusion having different distributions of ions get IV curves.The
dynamic response measurements and numerical simulations gave the required
information of working mechanisms and characterization of perovskite solar
cells in terms of J-V curve hysteresis and EQE as shown in Figure 2 and 3.
Current-voltage characteristics for hysteresis effect
EQE measurement of the PSC investigated
Under fast scanning rate, there is negligible change in the polarization
density of perovskite material in the I-V curve. The scanning process is
forward scan plus a backward scan. In
this study, the scanning rate was slow to measure the hysteresis effect in the
PSC. The measurement is done until a steady state was reached. Therefore, the
effect of hysteresis changes with different scanning rates and thickness.
Hysteresis effect could be observed when scanning rate equals to the changing
rate of the polarization. There was no hysteresis effect observed when the same
voltage is applied between forward and backward scan.
The IV curve shape of a perovskite solar cell
depended on voltage ramp speed and direction. The hysteresis curve have been
observed at the time of simulation, but would remain a subject of intense
debate. The hysteresis curvewas occurred on the same time scale as the voltage
ramps used and as related to slow processes.The hysteresis observed in the
architecture of PSC was small for optimized thickness of 450 nm for perovskite
layer and PCE was about 18% as shown in
figure 2 and 4.
Power conversion efficiency of the Perovskite Solar Cell
The shape of the IV curve depends on the
slope speed and direction. The IV curve hysteresis measured with a slow
sinusoidal voltage with step time of 2 x 10-04sec.The hysteresis
measurement was performed under illumination with AM1.5. External quantum
efficiency (EQE) of the PSC was about 0.8 as shown in Figure 3.
Effect of front and back electrode on Solar Cell Performance
The front and back contact of the perovskite
solar cell are FTO and gold respectively. The device performance would be
improved by using FTO as front contact. The workfunction of gold is 5.3 eV due
to which it is a better element to be
used as a back contact. The performance of the perovskite is enhanced with FTO
as front contact and gold as back electrode under illumination condition(G.
Richardson et al., 2016; Courtier et al., 2018). The photovoltaic response and
stability of device can be improved by using textured FTO surface which would influence the light
absorption by perovskite layer as compared to the unetched substrate because
there would be an improvement in the scattered light in the perovskite layer.
Back contact gold make good chemical bond with itself and other materials,
highly stable at high temperatures, corrosion resistive during the fabrication
of metal electrode for perovskite solar
cells. The device performance would improve with the use of gold as back
contact. FTO and gold are front and back contacts in this simulation
study(Reenen et al. 2015).
Effect of Thickness of perovskite layer
thickness of perovskite plays an important part in the enhancement of the
device performance because there is a decrease in the light absorption with
decrease in perovskite thickness. This would result in low power conversion
efficiency of the device(Tiwari et al., 2017). The optimization of absorber
layer thickness is necessary to improve power conversion efficiency of the
device(Tiwari et al., 2018). The optimized layer thickness of perovskite, TiO2
and Spiro-OMeTAD were as 450 nm, 80 nm and 100 nm respectively.
Current-Voltage Curve for different thickness of perovskite layer
ion migration, charge trapping and capacitive effects are major factors for
hysteresis in perovskite solar cells. The scanning rate or electric poling
under electric field is also the major factors for hysteresis. The
ferroelectric effects dominate the other effects, then there is a delay in the
ferroelectric domains alignment. This delay in ferroelectric domains alignment
influence hysteresis phenomenon. When ion migration or charge trapping effects
dominates other effects, then there is a delay in charge extraction or related
recombination processes. This delay in charge extraction or related
recombination processes influences hysteresis phenomenon. The cationic
displacement in the perovskite would induce lattice or structural distortion due
to defect concentrations. The low defect concentrations may eliminate
hysteresis and enhance perovskite stability. The reduced dipole moment of
cations and trap or defect states or improved processing techniques would
eliminate hysteresis phenomenon. Hysteresis free solar cell could be possible
by selecting proper charge selective contact materials which decrease electrode
polarization at the interfaces of electrode and layer structure and enhance
charge carrier transport characteristics. The decrease in charge accumulation
would decrease hysteresis. The study of
hysteresis phenomena is very important to determine the accurate value of power
conversion efficiency of solar cell(Elumalai et al. 2016). Therefore, ferroelectricity,
ion migration, charge trapping and capacitive effects are the important factors
which influence net resultant I-V response of the solar cell. The proper
understanding of device operation is possible by convincingly explaining
hysteresis phenomenon. The use of sophisticated characterization techniques is
important to measure the hysteresis experimentally in perovskite solar cells.
of perovskite layer
performance of the perovskite solar cell can be determined by the light
absorption property of perovskite layer and photogenerated charge carriers. It
is seen that the absorption of light of particular radiation depends upon the
volumetric ratio of CsPbBr3. The increase in volumetric ratio of
CsPbBr3 would increase the absorption edge of the perovskite. The
varying volume of ABX3 leads to shift in the absorption edge in the
absorption spectra. The amount of wavelength of particular light penetrated
into perovskite material depends on the absorption coefficient of the
perovskite (Tiwari et al., 2018). The equation for absorption coefficient is
where A and t are absorbance and thickness of perovskite
lower value of absorption coefficient of perovskite film means poor absorption
of light. The absorption coefficient of semiconductor materials has a sharp
edge due the light does not excite the electron from valence band to conduction
band below its energy band gap(Yakhmi et al., 2018).
Optical parameters like refractive index (n) and extinction coefficient (k) can be calculated by the
Where n, R, k,
α and λ are the refractive index, reflectance, extinction
coefficient, absorption coefficient, wavelength respectively. The relationship
between parameters like dielectric constant, refractive index and extinction
coefficient is given by
The real and imaginary parts of dielectric constants are
represented by respectively.
The phenomena of screening effect of different charge
carriers and CsPbBr3 molecular dipoles depend on the values of
dielectric constant of the perovskite material. Uncombined state of CsPbBr3
perovskite has less imaginary dielectric values. The potential barrier of
charge would be decreased to pull down the scattering of free carriers due to
higher values of dielectric constants.
Recombination models are suggested to understand the electronic
structure, lifetime and mobility of CsPbBr3perovskite. These models
include parameters like dielectric constant, effective mass, band bonding
character, and band dispersion to understand the J-V characteristics and
hysteresis. From the Drude free electron model, the dependence of real and
imaginary parts of the dielectric constants on the wavelength can be related
with the equation
Here ,N ,,e,, c and are high
frequency dielectric constant, optical carrier concentration in the conduction
band, relaxation time, electronic charge, permittivity of free space, velocity
of light and effective mass of free carrier respectively.
The optical conductivity is the optical excitation
without any external electric field in the perovskite material as shown by
Optical conductivity (8)
The increase in photon energy would increase the optical
conductivity of the perovskite material.
In this paper, CsPbBr3 perovskite solar cell was simulated at
STC conditions. The simulation study would help in selecting proper materials
for the HTL and ETL to have high FF and PCE for fabrication of the solar cell.
IV characteristics of CsPbBr3 solar cells were done to find the PCE,
EQE and hysteresis effects through simulation.Drift-diffusion model used to
understand the influence of mobile ions and optical parameters on IV curve.The
simulation results showed that hysteresis effect would be evident by studying
I-V curve. The phenomena might be caused due to internal polarization and
capacitance in perovskite solar cell. The causes for hysteresis must be studied
experimentally. Hysteresis effect can be studied by understanding the
conductance at the interfacial ETM/perovskite layer and HTM/perovskite layer
and measuring I-V curve. It is possible that the polarization lags behind the
voltage which causes hysteresis effect. In future, the numerical model with ion
migration will allow simulating full transient experiments. In this way a
complete model could be presented to describe charge transport in perovskite
solar cells from microseconds to minutes after excitation.
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