Interface Engineering by Thiazolium Iodide Passivation Towards Reduced Thermal Diffusion and Performance Improvement in Perovskite Solar Cells

Article abstract of DOI:10.1002/adfm.201910561

Interface engineering has become one of the most facile and effective approaches to improve solar cells performance and its long-term stability and to retard unwanted side reactions. Three passivating agents are developed which can functionalize the surface and induce hydrophobicity, by employing substituted thiazolium iodide (TMI) for perovskite solar cells fabrication. The role of TMI interfacial layers in microstructure and electro-optical properties is assessed for structural as well as transient absorption measurements. TMI treatment resulted in V_OC and fill factor enhancement by reducing possible recombination paths at the perovskite/hole selective interface and by reducing the shallow as well as deep traps. These in turn allow to achieve higher performance as compared to the pristine surface. Additionally, the TMI passivated perovskite layer considerably reduces CH₃NH₃ ⁺ thermal diffusion and degradation induced by humidity. The un-encapsulated perovskite solar cells employing TMI exhibit a remarkable stability under moisture levels (≈50% RH), retaining ≈95% of the initial photon current efficiency after 800 h of fabrication, paving the way towards a potential scalable endeavor.

Full text of DOI:10.1002/adfm.201910561

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Interface Engineering by Thiazolium Iodide Passivation
Towards Reduced Thermal Diffusion and Performance
Improvement in Perovskite Solar Cells
Manuel Salado, Michael Andresini, Peng Huang, Mohd Taukeer Khan, Fulvio Ciriaco,
Samrana Kazim, and Shahzada Ahmad*
1. Introduction
Interface engineering has become one of the most facile and effective
Hybrid
inorganic–organic
perovskite
approaches to improve solar cells performance and its long-term stability and
to retard unwanted side reactions. Three passivating agents are developed
which can functionalize the surface and induce hydrophobicity, by employing
substituted thiazolium iodide (TMI) for perovskite solar cells fabrication. The
role of TMI interfacial layers in microstructure and electro-optical properties
is assessed for structural as well as transient absorption measurements.
TMI treatment resulted in VOC and fill factor enhancement by reducing pos-
sible recombination paths at the perovskite/hole selective interface and by
reducing the shallow as well as deep traps. These in turn allow to achieve
higher performance as compared to the pristine surface. Additionally, the TMI
passivated perovskite layer considerably reduces CH3NH3+ thermal diffu-
sion and degradation induced by humidity. The un-encapsulated perovskite
solar cells employing TMI exhibit a remarkable stability under moisture levels
(≈50% RH), retaining ≈95% of the initial photon current efficiency after 800 h
of fabrication, paving the way towards a potential scalable endeavor.
solar cells (PSCs) have emerged as prom-
ising photovoltaic (PV) material, yielding
solar-to-electric power conversion effi-
ciencies of 25.2% in just a decade.[1] To
optimize the performance, substantial
progress has been made through compo-
sitional engineering.[2] Although methyl
ammonium-based perovskite is the most
studied material, the mixed organic cat-
ions and halide anions gave stable and
efficient solar cells.[3,4] For instance, the
use of triple cations with inorganic cesium
cation suppresses impurities and induces
uniformity. In spite of such advantages,
the cesium-based absorber showed phase
segregation[5] and poor atmospheric toler-
ance,[6] especially in high humidity condi-
tions (80% RH). Additionally, due to the
layered structure of the devices, interfaces
(physical interaction as well as energy
Dr. M. Salado, M. Andresini, Dr. P. Huang, Dr. M. T. Khan, Dr. S. Kazim,
level matching) between the different layers play a vital role in
the final performance.[7] The fabrication of efficient and stable
devices requires the strategy of having high open circuit voltage
(VOC) without sacrificing short circuit current (JSC). Subse-
quently, efficient devices can be fabricated by minimizing the
defect concentration associated with recombination processes
in the material itself or by lowering the defect states at the
interfaces.[8] Taking this in to account, the use of an engineered
interface between the charge selective layer and perovskite has
led to an improvement in the holes extraction as well as for
the defects passivation on the perovskite surface.[9] The use of
excess lead iodide (PbI2) for the perovskite deposition resulted
into the formation of a passivation layer that smoothened the
Prof. S. Ahmad
BCMaterials-Basque Center for Materials
Applications and Nanostructures
Martina Casiano
UPV/EHU Science Park, Barrio Sarriena s/n, Leioa 48940, Spain
E-mail: shahzada.ahmad@bcmaterials.net
M. Andresini
Department of Pharmacy—Drug Sciences
University of Bari “A. Moro” Via E. Orabona 4, Bari 70125, Italy
Dr. M. T. Khan
Department of Physics
Faculty of Science
Islamic University of Madinah
Prince Naifbin Abdulaziz
Al Jamiah
top surface of absorber and led to an increment in the VOC
,
Madinah 42351, Kingdom of Saudi Arabia
however, the excess of PbI2 was found to promote phase segre-
gation. Due to the weak structure of perovskites, iodide anions
are prone to volatility, while its diffusion induces irreversible
degradation and destabilize the crystal structure. The inclusion
of imidazolium iodide between perovskite and the hole selec-
tive contact to passivate the iodide vacancies in the crystal lat-
tice was found to be an effective strategy to improve charge
transport properties,[10] and consequently, PV performance and
durability was significantly improved. With an aim to develop
new efficient passivating molecules to passivate the defect on
Dr. F. Ciriaco
Department of Chemistry
University of Bari “A. Moro” Via E. Orabona 4, Bari 70125, Italy
Dr. S. Kazim, Prof. S. Ahmad
IKERBASQUE
Basque Foundation for Science
Bilbao 48013, Spain
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201910561.
DOI: 10.1002/adfm.201910561
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the surface and interface between the perovskite/HTM as well
as hole extraction enhancement in PSCs, we rationally design
three different substituted thiazolium iodide (TMI) salts, where
the presence of sulfur atom is supposed to play a pivotal
role in interaction with perovskite’s iodine atoms to enhance
the perovskite–HTM interfacial contact and thus improves
hole extraction.[11] Subsequently, we fabricated the PSCs using
mixed cation/halide anion based perovskite and evaluated the
PV performance by employing TMI-based passivating agent.
We elucidated the influence of passivated surface on PV
parameters by modulating the passivating layer thickness
as well as by changing TMI ring’s substitution. In particular,
mono- or bi-substitution of N-methyl thiazolium iodide
hydrogen atoms with methyl group was explored, with the
aim to modulate electron-donating properties of the layer and
conferring hydrophobicity. Subsequently, it allowed to modulate
hydrophillic character of the passivating layer, improving the
intrinsic stability and thermal durability of PSCs.
2. Results and Discussion
Three different substituted TMIs (Figure 1) have been syn-
thesized as passivating agents with the aim to improve the
thermal stability and opto-electrical properties. Due to the
interaction between the thiazolium and the perovskite surface.
The three organic salts N-methyl thiazolium iodide (MIC2),
3,4-dimethyl thiazolium iodide (MIC3), and 3,4,5-trimethyl
thiazolium iodide(MIC4) were synthesized through nitrogen
methylation with iodomethane starting from the commercially
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available cost effective thiazoles (details in Experimental
Section).
the importance of k-points density, binding energies for MIC2,
MIC3, and MIC4 on MAPbI3 were calculated setting k-points
density to (2.2.1) and (4.4.1). The resulting energies were found
to be consistent, proving that k parameter set to (2.2.1) was suf-
ficient for a correct modeling of the systems. We additionally
ran calculations changing the cut-off energy for the plane-wave
basis set from the default one (340) to 400 and 450 eV. While
calculated binding energies with 340 eV and 400 eV cut-off
energies were found to be substantially different, we did not
observe significant differences between 400 and 450 eV cut-off
energy outputs. The estimated binding energy of MIC2 was
found to be the largest (−0.654 eV, i.e., 63.1 kJ mol−1), while
MIC3 and MIC4 showed slightly lower interaction energies
(respectively −0.338 and −0.257 eV). Therefore, the presence
of a growing number of methyl substituents seems to affects
the TMI/perovskite interaction, while the sulfur atom plays the
pivotal role in coordinating perovskite ions. (Figure 1)
3. Structural Models and Ab-Initio Computation
We ran density functional theory (DFT) B3LYP calculations for
the determination of the energy of frontier orbitals of the salts
with the aim to predict the efficacy of the passivation layer in
the extraction of positive charge from the perovskite and the
subsequent transmission to the Spiro-OMeTAD layer. As a
result, HOMO–LUMO levels of the three salts calculated with a
6–311G basis set were found to be suitable for charge extraction
purposes. Calculated HOMO levels were respectively −5.55 eV
(MIC2), −5.41 eV (MIC3), and −5.23 eV (MIC4), while LUMO
levels were −1.99 eV (MIC2), −1.92 (MIC3), and −1.85 (MIC4)
(Figure 1), depicting that these TMI salts present a large
HOMO–LUMO energy gap (approximately greater than 3.3 eV).
The promising results signal that TMI could efficiently extract
holes from the perovskite layer while, simultaneously, high
LUMO levels could act as an electron barrier between anode
and perovskite layer. With the aim to have an estimation of the
binding energies of TMI salts adsorbed on perovskite surface,
we also run the DFT Perdew–Burke–Ernzerhof (PBE) calcula-
tions based on the projector-augmented wave (PAW) method.
In particular, TMI-perovskite binding energies were obtained
by comparing the resulting energy of the TMI adsorbed on
perovskite surface with the sum of the resulting energies of
perovskite and TMI calculated with the same parameters. For
MAPbI3, a 2 × 2 × 3 supercell slab was generated with two sur-
face cut along the (001) direction and a vacuum head space of
20 Å. Figure 2 illustrates the abbreviated settings adopted for
the upper and lower surface. The optimized geometries of
MIC2, MIC3, and MIC4 from the previous calculations were
used as input for the PAW calculations. In order to investigate
4. Characterization of the Perovskite Films
To elucidate the crystal structure of the passivating molecules,
and possible changes induced in the methylammonium lead
triiodide due to passivation, we carried out X-ray diffraction
(XRD) experiments on the perovskite samples with and without
TMI treatment (Figure 3a,b). Expectedly, no noticeable changes
in the main diffraction peaks (14.6° and 28.8°) of methylammo-
nium lead triiodide with or without passivating treatment was
found.[12] However, new diffraction peaks appeared after the
passivation treatment. To further investigate whether the new
peaks arise from passivating molecules crystallizing over the
surface or from the formation of an thiazolium salt based perov-
skite, the diffractograms of TMI was also recorded (Figure S2,
Supporting Information). The assessment of the diffraction pat-
terns TMI and the new diffraction maxima generated after TMI
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Figure 3. a) XRD patterns of perovskite thin film and with passivation layer (MIC2, MIC3, and MIC4), b) zoom-in of the low angle peaks after passiva-
tion (red rectangle in 3a) indicating the main reflection peaks of the different passivation layers, and c) diffractograms at 140 °C (red line indicates the
presence of PbI2 characteristic peak). X-ray pattern of MIC2PbI3 is a reference from Cambridge Structural Database.
passivation treatment, suggest the formation of TMIz-PbxIy
mixed type structure atop of MAPbI3. Furthermore, we also
discarded a symmetry reduction of the MAPbI3 cubic structure
due to partial substitution of MA by TMI cation. The peaks
associated to the rhombic perovskite are not in accordance with
the one generated after the deposition of TMI atop of MAPbI3
layer. By discarding these two possibilities, the experimental
evidences pointing towards the surface modification induced by
the presence of TMI.
The information elucidated from the new diffraction
maxima generated after the TMI deposition is not sufficient
for ab-initio determination of the passivated crystal structure.
In order to gain further insights, we performed a full search
of thiol-imidazolium bearing lead iodide structures in the Cam-
bridge Structural Database, with a unique compound matching
the thiazole ammonium iodide characteristics.[13] Nevertheless,
the simulated diffraction pattern derived from the 1D crystal
structure reported is not in accordance with the position of the
passivation layer maxima generated by the addition of thiazole
ammonium iodide. Arguably, the XRD analysis suggests, the
TMIz-PbxIy passivation layers obtained after MIC-2, MIC-3, and
MIC-4 usage have different crystal structures. Specifically in the
case of MIC-3 and MIC-4, the presence of additional diffraction
patterns suggests the possibility of layered type (2D) structures
with interlayer spacing of 8 and 9 Å, respectively. This is in
agreement with an expanded interlayer space due to the bulkier
MIC-4 intercalation, in comparison to MIC-3, between the PbI2
layers foreseen in layered 2D perovskites. For MIC-2 sample,
the presence of several new peaks at low angles suggests a
lower symmetric TMI-PbI3 passivation layer structure.
as aforementioned, the interaction with the perovskite would
entail a better charge extraction, due to the interactions with the
electron-rich sulfur atom from the thiazolium salts.
Arguably, the use of a passivation layer at the perovskite/HTM
interface improves the charge extraction from the perovskite
and prolong the lifetime of the devices.[10,16] In the past,[10,17] an
improvement in the VOC and the fill factor (FF) of perovskite-
based devices was reported, due to fine energy level alignment
between layers that improves the charge extraction. Remarkably
the stability was also improved with the utilization of fluorinated-
based compounds which improves the hydrophobicity of the
perovskite surface, repelling moisture, and increasing its envi-
ronmental tolerance.[18,19] However, in operational conditions,
besides humidity, high temperature represents another main
factor that entails a fast degradation process.[20] CH3NH3PbI3
crystals are reported to suffer from thermal diffusion at 135 °C,
when methyl ammonium cation scatters from the crystalline net-
work.[21] This effect results into an irreversible process, towards
the formation of the initial precursors (PbI2 + CH3NH3I). Thus,
thermal stability of the passivated perovskite layers was moni-
tored by temperature dependence X-ray diffractograms. For this,
the samples were heated at varying temperatures (30, 50, 100,
120, 140, and 150 °C) and cooled down (Figure S3, Supporting
Information) to investigate irreversibility in the degradation. It
can be deducted from Figure 3c, TMI passivated devices showed
higher thermal stability, even at 150 °C in the case of MIC4
usage. This behavior is in accordance with the thermal measure-
ments of substituted TMI (Figure S4, Supporting Information)
which are found to be thermally stable above 220 °C. Perovskite
microstructure was studied by scanning electron microscopy
(SEM, Figure S5, Supporting Information). We found that
TMI salts cover the perovskite surface, particularly, the highly
substituted TMI covered the grain boundaries entirely. TMI
passivating layer reduces the surface roughness (visualized
from the SEM images) between the grain boundaries and the
surface. Depending on the composition of the perovskite, the
We ascribed this to the formation of a layered perovskite,
which in literature is also termed as 2D perovskite atop
of 3D perovskite surface, due to the large size of C3H4NS
(rA,eff = 320 ppm).[14] Such type of layered perovskite[15] will
improve thermal durability and will induce an effective passiva-
tion effect, rather than the inclusion of TMI layer. Additionally,
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hydrophobic character can reduce the degradation by water
inclusion in the perovskite layer. Mixed perovskites with cesium
cation in their compositions, presents higher stability than
methyl ammonium-based ones. However, phase segregation in
mixed perovskites is still unclear, that can limit its up-scaling
purposes.[5] To evaluate the hydrophobic property induced by
passivating layer, contact angle measurements were carried out
on perovskite films treated with the TMI salts. Expectedly, the
deposition of the passivation layer induces an increase in the
contact angle value (>80°) compared to reference sample (70°)
(Figure S6, Supporting Information). Thus, a better interface
protection against humidity can be conferred. Optical behavior
was analyzed by UV–vis absorbance (Figure 4a) to study possible
changes in the optical bandgap for the passivated perovskite. We
noted the optical absorption spectra of TMI-treated perovskites
are similar to the pristine MAPbI3, signaling that TMI salts have
negligible effect on light harvesting properties. The strong pho-
toluminescence (PL) emission band centered at 773 nm was
observed for pristine MAPbI3 deposited on glass and excited
at 460 nm from glass side. A significant decrease in PL inten-
sity was noted for TMI-treated perovskite surface. Expectedly,
MAPI3 treated with TMI shows blue shifting in PL peak from
773 to 769 and 768 nm for MIC2 and MIC3, respectively. The
PL quenching of the passivated perovskites suggest their assis-
tance in hole extraction by these TMI salt, as indicated by their
higher HOMO level compared to the MAPI3 valence band. In
addition to this, higher FF factor were obtained for the solar
cells fabricated with MIC2- and MIC3-treated perovskite.[22,23]
The blueshift of the PL emission in TMI passivated perovskites
can be ascribed to the reduced number of trap density of state,
as a result of passivating trap states in the bulk as well as on
the surface/grain boundaries of the perovskite. These results
are in accordance with admittance spectroscopy measurement,
which depicts an increment in the recombination resistance of
the device at VOC, as shown by the density of states (DOS) value
calculated using temperature-dependence admittance spectros-
copy. These results show that the alkyl chain of TMI can fill the
voids (defects) on the grain boundaries surface and interface
of perovskite and hole transport layers (HTL) layer, thus passi-
vating the perovskite surface. Similar blueshifting behavior was
also observed for other organic molecules used for passivating
the perovskite.[10,24–29]
In order to investigate the charge carrier dynamics in TMI
passivated perovskite, thin film was deposited on quartz sub-
strate and femtosecond-transient absorption spectroscopy
(fs-TAS) was performed. The lower pump excitation energy
density ≈4 µJ cm−2 was used to avoid auger recombination
and excitation energy density was constant for all the sam-
ples having the same perovskite thickness, ensuring that a
similar charge carrier density is generated. Figure 4b,c shows
the fs-TAS spectra of pristine MAPI3 and MAPI3 treated with
MIC2 film using pump excitation wavelength of 510 nm and
probed at different delay time by a probe pulse of white light
continuum. Three different spectral regions were observed, a
negative photo induced bleaching peak was observed at 760 nm
and two positive photo-induced absorption peaks were observed
at both sides of the 760 nm band, i.e., below 720 and > 780 nm.
The photo-induced negative bleaching peak (PB) observed at
760 nm (position of Eg, optical bandgap) can be allotted to the
ground state bleaching due to the excitons absorption, which
leads to band filling,[30,31] i.e., the presence of band gap car-
riers or excitons. The photo-induced positive peak (PB) below
Figure 4. a) Absorption spectra and steady-state PL emission measured at 460 nm excitation of pristine and TMI treated perovskites, b,c) fs-TAS of
pristine MAPbI3 perovskite and MIC2 passivated perovskite film excited at 510 nm with the excitation energy 4 µJ cm−2 at pump-probe delay from 0 to
1 ps, and d) kinetic delay for photo bleaching probed at 760 nm of the perovskite film without and with TMI treatment.
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Figure 5. a) Cross-section SEM image, b) schematic process for deposition of passivation layer, c) J–V curves of surface passivated and pristine device
and d) EQE and integrated current density of devices. The device structure was FTO/SnO2/(FAPbI3)0.97(MAPbBr3)0.03/Spiro-OMeTAD/Au.
720 and >780 nm can be attributed to the absorption of the
excited transient species.[32] As reported in our other reports,
the observed bleaching band on a time scale of a few picosec-
onds (ps) may corresponds mostly to the exciton dynamics. As
mentioned before, we used 510 nm excitation, which is not
sufficient to excite charge carriers in the second valence band.
Consequently, the hole cooling is absent and therefore the
rising of the bleach at 760 nm (τ = 0.14 ps) reflects directly the
exciton formation. Electron cooling is reflected by the decay of
transient around 815 nm (0.32 ps) and the growth of the posi-
tive contribution at wavelengths below 720 nm (0.2–0.3 ps).
Figure 4d shows the comparison of kinetics profile of 760 nm
bleaching peak at ≈4 µJ cm−2. The observed exciton dynamics
in the first 5 ps is similar in the pristine and TMI-treated perov-
skite, which exhibited the same TAS decay lifetime of 0.14 ps
by fitting the single exponential function. We suggest that the
dynamics of free carriers are an intrinsic property of the perov-
skite and therefore, these processes are not influenced by the
passivation layer. Moreover, PL quenching as well as TA meas-
urements indicate that TMI treatment suppresses the surface
non-radiative recombination defects.
140 nm, respectively) (Figure 5a). The dependence of TMI on
the device performance by the usage of MIC2, MIC3, and MIC4
(Table S1a, b, Supporting Information) was competitive and
represents significantly increased power conversion efficiency
(PCE) when the perovskite was treated with a substituted TMI
(MIC3) as compared to control sample. Devices fabricated with
MIC4 as passivating agent showed an increment in the series
resistance (Figure 5c). We speculate that due to the methyl
substitutions, complete solubility in 2-propanol was difficult to
achieve, resulting into aggregates which interacts with Spiro-
OMeTAD to act as recombination sites. Due to this, the device
performance drops with higher substituted TMI salts even
though the recombination rate was low. (Figure 6).
In order to evaluate the beneficial effect of the TMI passivation
in devices, we tested different perovskite compositions (MAPbI3,
5. Device Fabrication and Characterization
Considering that TMI can efficiently passivate the surface
defects in the perovskite layer, we fabricated devices with
fluorine doped tin oxide (FTO)/b-TiO2/meso-TiO2/MAPbI3,
Cs0.05MA0.15FA0.85Pb(I0.85Br0.15 3 and (FAPbI3)0.97(MAPbBr3)0.03/
)
Spiro-OMeTAD/Au structure. The thicknesses of the electron
transport layer TiO2, the perovskite layer and the hole transport
layer Spiro-OMeTAD was in total about 0.8 µm, (150, 500, and
Figure 6. Recombination resistance versus voltage obtained at room
temperature from impedance spectroscopy experiments.
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Figure 7. a) Evolution of PCE with time under atmospheric humidity conditions (∼60% RH) over 800 h and b) photostability at MPP under continuous
illumination [1 sun AM 1.5 (1000 W m−2)] for un-encapsulated devices.
Cs0.05MA0.15FA0.85Pb(I0.85Br0.15)3, (FAPbI3)0.97(MAPbBr3)0.03) and
different architectures {mesoscopic (TiO2) and planar (SnO2)}
to estimate their PV properties. After optimization of the TMI-
treated device, we obtained the best device with an efficiency of
18.93%, which gave a VOC of 1048 mV, JSC of 22.75 mA cm−2,
and an FF of 79.37% (Figure 5a). The integrated current density
from the external quantum efficiency (EQE) was 21.5 mA cm−2
(Figure 5b), which is in agreement with data obtained from the
measurement of J–V curves reported. As previously reported,[10]
even in this case the optical bandgap remain unchanged after
deposition of the passivation layer, and it can be stated that the
low-dimensional perovskite formed atop of the 3D perovskite
does not affect the optical properties of the material. Further-
more, EQE decreases to half the peak value and the differen-
tial of the EQE (Figure S7a, Supporting Information) suggests
that the absorption threshold is 1.61 eV. This value is consistent
with the result obtained from the differential of absorption
(Figure S7b, Supporting Information). The PCE statistics vali-
dates the reproducibility of our results suggesting the improve-
ment in the performance of PSCs (Figures S8 and S9, Sup-
porting Information).
Charge accumulation and hysteresis behavior in PSCs have
been attributed to the interactions between layers.[33] Although
the identification of the process involved can be complex, it
is possible to explain the processes occurring in the bulk of
perovskite with the use of admittance spectroscopy.[34] Recently,
the importance of conduct analysis to identify recombination
and transport processes in the devices was highlighted.[35] We
have employed electrochemical impedance spectroscopy (EIS),
to analyze the devices with different TMI salts and MAPbI3
as examples to avoid complications in the analysis of EIS data
(Figures S11–S13, Supporting Information). According to the
microstructure, (XRD and SEM) and opto-electrical (J–V, EQE,
and TA) characterization, a clear interaction between the pas-
sivation material and perovskite was noted, which will have
its bearing in the charge recombination and transport inside
the devices. Figure 6 illustrates the recombination resistance
at different photovoltages. A plain influence of the passiva-
tion layer on the recombination process due to filling of grain
boundaries as well as defects could be observed, which is in
accordance with previous reports.[16] Recombination resistance,
close to the open circuit voltage, is higher when MIC4 was
used as passivation agent, which suggests a reduction of the
non-radiative recombination defects. We calculated the ideality
factors obtained from the slope Rrec versus V indicating a dif-
ferent recombination mechanism (from bulk β∼0.5–0.6 to sur-
face β < 0.5).[36] According to the figure, non-passivated samples
showed a trap-state recombination, meanwhile a progressive
change in the slope was observed when substituted TMI salt
were used as passivating agent.
We suggest that the improved thermal resistance together
with the hydrophobic character of the passivation layer, induce
a better long-term stability of the perovskite devices.[37] We
assessed the long-term device stability for 800 h (Figure 7a). In
particular, we periodically tested the devices which were kept
under dark conditions and relative humidity (RH∼50–60%). It is
worthy to note, that the control device (non-passivated) presented
a decay in the efficiency as well as in the EQE (Figure S10, Sup-
porting Information). On the contrary, TMI-treated samples
showed reduced losses in efficiency. Moreover, we investigated
the device stability under continuous light soaking at the max-
imum power point (25 °C, 100 mW cm−2) and we found that
there is no obvious decrease in device performance (Figure 7b),
except for the unpassivated ones, where a loss of almost 20% of
the initial efficiency was observed. Our results suggest that the
long-term stability of TMI-based devices is competitive and suit-
able for applications under real operation conditions.
6. Defect Spectroscopy
To analyze the effect of TMI passivation on distribution of trap
density in MAPbI3 absorbing layer, we performed temperature-
dependence admittance spectroscopy and Mott–Schottky anal-
yses (Figure 8). We first measured the temperature dependent
capacitance as a function of frequency. From the derivative of
the capacitance spectrum (Figure S16, Supporting Informa-
tion), we determined the frequency of peak emission (fpeak) rate
of electrons from a trap state located below ET to the LUMO
edge ELUMO as depicted schematically in Figure 9. From the
fpeak
T2
1
T
ln
vs.
slope of Arrhenius plot,
, the value of activation
energy EA, and attempt to escape frequency υ0 were deduced
through the relation:[38]
ν0
T
EA




(1)
ln
= ln β −
( )
2
kBT
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Figure 8. a) Mott–Schottky analysis and b) trap density of states (NT) of devices at 300 K.
Mott–Schottky plot of fabricated devices at 10 kHz is shown
in Figure 8a. The values of Vbi was extracted from the intercept
on bias axis and then N was calculated from the slope of the
linear region using Equation (2). The depletion width W cor-
responding to zero bias was calculated from the equation
2εε0Vbi
where β is a proportionality constant, T is the temperature, and
kB is the Boltzmann constant. The calculated activation energy
for each device are presented in Table 1. The defect activation
energy of all the devices have similar value and found to be
close to 90 meV.
The Mott–Schottky analyses was made to measure the value
of built-in-potential Vbi, and depletion layer width W at perov-
skite/hole selective layer interface. A Mott–Schottky plot is a
capacitance–voltage (C–V) graph at a frequency where device
capacitance is frequency-independent. In the depletion region,
the total charge density is determined only by the density of
immobile dopants N and can be considered an ideal parallel
W =
. We can deduct from Table 1 that the depletion
qN
width of perovskite layer was increased from 59 nm for device
without any passivation layer to 63 nm for MIC3 passivation
and 69 nm for MIC4 passivation layer. Due to the increase in
depletion width, the built-in potential of passivated devices
was improved from 0.75 V for device without any passivation
layer to > 1.1 V for passivated devices. The increased Vbi value
enhance the charge transfer at the perovskite/hole selective
layer interface.
plate capacitor. The width of the depletion layer is related to
εε0 A
W
the capacitance C through the relation C =
. The built-in
potential (Vbi), doping density of immobile ions N at depletion
region in perovskite side is related to the capacitance through
the relation:[39]
The trap density profile were calculated from the equation[40]


dC Vbi
(3)
N = − f


T




1
2
kBT
q
df qWk T


B
(2)
=
V −V −


bi
C2 εε qN
0
and are presented in Figure 8b. Eω represents a demarcation
2


βT
ω
energy and was calculated from the equation Eω = k T ln
.




B
It can be observed from Figure 8b that the traps distributed
through a Gaussian curve, and TMI passivation significantly
reduced the overall trap density of states in the bandgap of
perovskite layer (highlighted by dotted ellipse). The peak value
of DOS was also reduced from 1.44 × 1018 (eV cm3)−1 for refer-
ence device to 6.08 × 1017 (eV cm3)−1 for MAPI-MIC4. More-
over, the trap density profile of reference device showed an
Table 1. Device parameters deduced from admittance spectroscopy and
Mott–Schottky plot.
Parameters
MAPI-MIC2 MAPI-MIC3 MAPI-MIC4
MAPI
90.58
59
Activation energy EA (meV)
Depletion layer width W (nm)
Built-in-potential Vbi (V)
92.72
90.73
63
89.00
69
–
–
1.18
1.14
0.75
Doping density (N cm−3
)
1.0 × 1017
1.47 × 1018
7.54 × 1017
1.29 × 1018
6.08 × 1017 6.08 × 1017
7.72 × 1017 1.44 × 1018
Figure 9. Band diagram of a p-i-n junction, where ϕ is the band bending
at the junction, Ec is the conduction band, EF is the Fermi level, W is the
width of the depletion layer, Vbi is the built-in field, and Eω is the demar-
cation energy; separates traps that respond to Vac at frequency ω from
those that do not.
Peak value of traps DOS
(eV cm3)−1
Peak position of traps
density (eV)
0.151
0.157
0.159
0.161
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Table 2. Electrical parameters extracted from SCLC measurements.
additional peak around 0.1 eV which corresponds to shallow
traps with a density of 2.46 × 1017 (eV cm3)−1. This peak is
almost diminished in the passivated devices, further confirming
that TMI passivation represents an effective strategy to reduce
the shallow as well as deep traps at the interface of perovskite
absorbing layer and HTL.
σ [S cm−1
]
μ [cm2 V−1 s−1
9.86 × 10−4
7.25 × 10−4
9.59 × 10−4
1.11 × 10−3
]
nc [cm−3
]
ntrap [cm−3
]
Device
4.59 × 10−7
2.60 × 10−7
3.49 × 10−7
3.76 × 10−7
2.91 × 1015
2.24 × 1015
2.28 × 1015
2.12 × 1015
1.17 × 1016
1.30 × 1016
1.29 × 1016
1.48 × 1016
MAPbI3-MIC2
MAPbI3-MIC3
MAPbI3-MIC4
MAPbI3
7. SCLC Results
was evaluated through the relation: nc = σ/eµ and for pristine
devices, it was estimated to be 2.12 × 1015 cm−3, whereas it
shows enhancement for the devices with TMI interfacial layers.
This might be due to decrease of trap density in these devices
which results in increase of free carrier density.
Charge carrier mobility (µ) and density of traps (ntrap) in the
fabricated devices were extracted via evolution of the space
charge limited current (SCLC). At low applied bias (Figure 10),
the J–V characteristics follow Ohm’s law (red line). The elec-
trical conductivity (σ) was evaluated from the ohmic region
through the relation: σ = JL/V, where L is thickness of thin
films. The conductivity of pristine device was calculated to be
3.76 × 10−7 S cm−1, while the device with MAPbI3-MIC2 gave
conductivity value of 4.59 × 10−7 S cm−1. On further increase
of applied bias, the injected carrier starts to fill the traps and
when applied bias reached at the onset voltage VTFL, all traps
states filled from injected carriers and J–V characteristics starts
To conclude, substituted TMIs have been employed as pas-
sivating agent to allow interface engineering and minimizing
the defect concentration by means of passivation. We noted
improved thermal stability and opto-electrical properties due to
the interaction between the thiazolium salts and the perovskite
surface. The reported simplistic method towards passivation of
the perovskite surface allowed a significant reduction in defects
and suppression of recombination phenomena. Moreover, the
straightforward and cost-effective synthesis of thiazolium-based
compounds, together with their integration in device towards
improved charge extraction, is presented. Deposition of substi-
tuted N-methyl thiazolium iodide salts resulted into improved
performance as compared to the control devices. Additionally,
passivating layer inclusion reduced long-term instability of
un-encapsulated devices. Simulations studies suggest that the
presence of a higher number of methyl substituents affect the
TMI–perovskite interaction, while the sulfur atom plays the
pivotal role in coordinating perovskite ions. We elucidated the
underlying mechanism for a rational passivation layer devel-
opment, which could facilitate further investigations towards
performance enhancement of perovskite-based materials for
energy application.
to follow Child’s law (orange line) as represented by equation:
[41–43]
9
V2
(4)
J = εε0µ
3
8
L
where ε is the dielectric constant of perovskite layer and ε0 is
the permittivity of free space. From Child’s region, we extracted
the mobilities of all devices and presented in Table 2. All devices
exhibits similar values of mobilities of 9.86 × 10−4 cm2 V−1 s−1 for
MIC2, 7.25 × 10−4 cm2 V−1 s−1 for MIC3, 9.59 × 10−4 cm2 V−1 s−1
for MIC4, and 1.11 × 10−3 cm2 V−1 s−1 for reference device. The
trap density ntrap was estimated using following relation:
2εε0VTFL
(5)
ntrap
=
eL2
From these results, it is evident that the devices with TMI
passivation layers showed fewer defects as compared to ref-
erence device. The free carrier concentration in perovskites
8. Experimental Section
Solar Cell Device Fabrication: PSCs were fabricated on FTO
(NSG10) substrates previously cleaned by
a sequential treatment
MAPbI3-MIC2
in the ultrasonicator bath in a 2% Hellmanex solution, acetone and
isopropanol, followed by UV ozone treatment for 20 min. The samples
were then heated to 500 °C and a compact blocking layer of TiO2 was
deposited onto the FTO glass substrate by spray pyrolysis, using a
dilution 1/19 mL of titanium (IV) diisopropoxide bis(acetylacetonate)
solution in ethanol keeping them for 30 min. After the samples attained
room temperature, a mesoporous dispersion of TiO2 nanoparticles
(30 NRD from Dyesol) in ethanol was spun-coated at 2000 rpm
(1000 rpm s−1 acceleration) for 30 s, followed by a progressive heating
step till 500 °C for 30 min. Stoichiometric precursor solutions (1.25 m)
were prepared, by mixing MAI (Sigma–Aldrich) and PbI2 (TCI) in N,N’-
dimethylsulfoxide (DMSO) and kept under stirring at 70 °C overnight
in order to dissolve completely PbI2. The perovskite and HTM films
were prepared inside an argon filled glove box under moisture and
oxygen controlled conditions (H2O level:1 ppm and O2 level:10 ppm).
The perovskite layers were fabricated by using a two-steps spin-coating
process, first step 1000 rpm for 10 s; and second step 3500 rpm for 30 s.
During the second step, 110 µL of chlorobenzene were poured onto the
films 10 s prior to the end of the program, and then substrates were
annealed at 100 °C during 40 min. For planar PSCs, the SnO2 layers as
Figure 10. Dark J–V characteristics of devices with various TMI-based
passivating agents at 300 K.
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ETMs were fabricated by spin-coating on the pre-cleaned ITO substrate
at 2500 rpm for 30 s using the diluted SnO2 aqueous colloidal dispersion
with the concentration of 2.5 wt%. The SnO2 films were annealed in air
at 150 °C for 15 min. The perovskite films were obtained by the two-
step deposition method. In short, PbI2 (1.3 m) was dissolved in 1 mL of
N,N-dimethylformamide and DMSO (ratio = 9.5:0.5) solution at 70 °C.
Then, the solution was spin-coated on the SnO2 substrate at 2000 rpm
for 30 s, and a mixture solution of FAI:MABr:MACl (60:10:10 mg in 1 mL
IPA) was spin-coated on the PbI2 substrate at 2000 rpm for 30 s. The
as-prepared films were annealed at 150 °C for 15 min under 30–40%
relative humidity conditions. Once the samples were cool down, TMI
salt were spin coated on perovskite layers at 3500 rpm for 30 s and
kept at 100 °C for 10 min and left to cool down. Spiro-OMeTAD was
then spun coated at 4000 rpm for 30 s by dissolving 72.3 mg of Spiro-
OMeTAD in 1 mL of chlorobenzene; 28.8 mL of 4-tert-butylpyridine was
added to the solution as dopants. Finally, 70 nm of gold was deposited
by thermal evaporation.
Device Characterization: Current density–voltage (J–V) curves were
recorded using an Oriel solar simulator (Newport) producing 1 sun AM
1.5 (1000 W m−2) 3A sunlight. Current–voltage curves were measured in
air with a potentiostat (Keithley 2604). J–V measurement were performed
at 100 mV s−1 scan rate (pre-sweep delay: 10 s) and a black metal mask
(0.16 cm2) was used over the square solar cell active area to reduce the
influence of scattered light. Incident PCE measurements were carried out
using a 150W Xenon lamp attached to with Bentham PVE300 motorized
1/4m monochromator as the light source. Impedance spectroscopy
(IS) measurements were carried out altering the positions of the Fermi
level by a white LED source. During the measurements, samples were
kept inside a faradaic chamber to avoid external interferences. A 20 mV
perturbation in the range 2 MHz–1 mHz was used to obtain the spectra.
After measurement, data were fitted by Z-view software and analyzed in
order to extract characteristic parameters of the cells.
Synthesis of Thiazole Salt: 4-methylthiazole and 4,5-dimethylthiazole.
In a round bottom flask, 239 mg of thiazole (2,8 mmol, 1 equiv.)
were dissolved in 1 mL of MeOH, then 2.28 g of MeI (16 mmol, 5.7
equiv.) were added and the solution was stirred at room temperature
for 36 h under dark. The product was precipitated with 5 mL of ethyl
acetate (AcOEt) and the filtrated was washed with 2 × 10 mL of AcOEt.
The powder was dried under reduced pression, affording the desired
product as a white solid {yield = 78.8% (MIC2), 61% (MIC3), 93%
(MIC4)}.1H in DMSO-d6 and 13C NMR in CD3OD spectra of compounds
MIC2- MIC3-MIC4 were recorded and are presented in Figure S1
(Supporting Information).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work has received funding from the European Union H2020
Programme under European Research Council Consolidator grant
(MOLEMAT, 726360), the authors gratefully acknowledge Advanced
Research Facilities (SGIker) of the University of Basque Country/Euskal
Herriko Unibertsitatea for NMR measurements and transient
spectroscopy measurements. M.S. thanks the National Research grant
“Juan de la Cierva” (FJCI-2017-31761). The authors thank A. Longarte and
R. Montero with TA equipment (Basque Government, IT1162-19) support.
1
N-methylthiazolium Iodide (1): H NMR (500 MHz, DMSO) δ 10.10
(m, 1H), 8.46 (dd, J = 3.6, 1.2 Hz, 1H), 8.30 (dd, J = 3.6, 2.6 Hz, 1H),
4.20 (s, 3H). 13C NMR (125 MHz, MeOD) δ 160.2 (s), 139.5 (s), 127.1
(s), 42.6 (s).
3,4-dimethyl thiazolium iodide (2): 1H NMR (500 MHz, DMSO) δ
10.05 (d, J = 2.4 Hz, 1H), 7.97 (m, 1H), 4.08 (s, 3H), 2.51 (s, overlapping
DMSO, 3H). 13C NMR (125 MHz, MeOD) δ 160.1 (s), 148.5 (s), 122.2
(s), 40.9 (s), 13.5 (s).
1
3,4,5-trimethyl thiazolium iodide (3): H NMR (500 MHz, DMSO) δ
9.92 (s, 1H), 4.07 (s, 3H), 2.49 (s, 3H), 2.40 (s, 3H). 13C NMR (125 MHz,
MeOD) δ 156.0 (s), 144.0(s), 134.4 (s), 41.4 (s), 12.4 (s), 11.6 (s).
Thin Film Characterization: For structural characterization, thin films
were prepared by spin-coating of MAPbI3 and MAPbI3 + thiazolium
solutions onto glass. X-ray diffractograms were recorded using a D8
Advance diffractometer from Bruker (Bragg–Brentano geometry, with an
X-ray tube Cu Kα, λ = 1.5406 Å). A scan range of 3–80° was selected with
an acquisition time of 1° per min. For XRD-temperature dependence,
perovskite film was deposited on an aluminium substrate to facilitate a
homogeneous distribution of the temperature. The absorption spectra
were registered with an UV–vis–IR spectrophotometer (Varian Cary
50 UV/Vis Spectrophotometer). PL steady-state measurements were
recorded with a spectrophotometer (PerkinElmer Instrument LS55).
The femtosecond transient absorption measurements is described
elsewhere.[44] Ultrashort laser pulses were generated in an oscillator-
regenerative amplifier laser system (Coherent, Mantis-Legend)
that provides a 1 KHz train of 35 fs pulses at 800 nm. Pump pulse
wavelength in the range 500–650 nm is obtained by means of sum
frequency of the fundamental 800 nm and the signal beam of an optical
parametric amplifier (Coherent, TOPAS), while the 400 nm is generated
as the second harmonic of the fundamental beam. The white light
continuum probe is produced by focusing ( f = 100 mm) a small fraction
of the amplifier output on a 2 mm thick CaF2 window. Pump probe delay
is achieved with a translation stage (Thorlabs, DDS220) that allows a
maximum range of 1 ns. Transient absorbance (TA) was measured with
a fiber-coupled spectrometer (Avantes, Avaspec) as a function of the
pump probe delay. Samples were continuously scanned across the focal
plane to avoid thermal effects on the sample. The spot radii of the pump
and probe pulses (0.55 and 0.2 mm) at sample surface were measured
by means of a complementary metal–oxide–semiconductor camera.
Pump pulse energies were varied in the range 20–200 nJ.
Conflict of Interest
The authors declare no conflict of interest.
Author Contributions
M.S. and M.A. contributed equally to this work. M.S.M. made the
experiments, devices, impedance spectroscopy, and collected the data,
M.A. synthesized the thiazolium salts, M.A. and F.C. ran the ab-initio
computational studies, P.H. fabricated effective performance devices,
T.K. perform admittance spectroscopy and data analysis, and S.K.
performed electro-optical and spectroscopy measurements. S.A.
supervised and directed the research. All authors contributed to the first
draft and commented to the completion of the final document.
Keywords
charge transport, opto-electrical properties, passivation, perovskites
solar cells, thin film photovoltaics
Received: December 19, 2019
Published online:
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