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1
Using microgels to control the morphology and optoelectronic
properties of hybrid organic-inorganic perovskite films
Chotiros Dokkhana, Muhamad Z. Mokhtara, Qian Chena, Brian R. Saundersa,*
a School of Materials, University of Manchester, Manchester, M13 9PL, U.K.
Nigel W. Hodsonb
b BioAFM Facility, Faculty of Biology, Medicine and Health, Stopford Building, University of
Manchester, Oxford Road, Manchester, M13 9PT, U.K.
Bruce Hamiltonc
c Photon Science Institute, University of Manchester, Alan Turing Building, Oxford Road,
Manchester, M13 9PL, U.K.
Corresponding author:
Brian R. Saunders: brian.saunders@manchester.ac.uk
2
ABSTRACT
Microgels (MGs) are crosslinked polymer colloid particles that swell in a good solvent. Although
MGs have been studied for over 80 years their ability to control the morphology and optoelectronic
properties of composite films containing photoactive materials is in its infancy. Solution
processable hybrid organic-inorganic perovskites such as CH3NH3PbI3-zClz have attracted enormous
fundamental and applied interest because of their outstanding optoelectronic properties. There is
considerable interest in establishing methods to control perovskite film morphology, for example,
using micropatterning. Here, hydrophilic poly(N-vinylformamide)-based MGs were dispersed in
perovskite precursor solution which was then spin coated to deposit CH3NH3PbI3-zClz/MG films for
the first time. Remarkably, the CH3NH3PbI3-zClz/MG composites formed disordered inverse opal
(DIO) films. The CH3NH3PbI3-zClz/MG composition ranges which gave DIO films are identified
using a phase diagram. The pore wall thickness is shown to be controlled by the volume fraction of
MGs used and a simple model is presented to explain this behaviour. The MGs not only caused
CH3NH3PbI3-zClz to be more efficiently deposited but also increased light absorption and
photoluminescence intensity. Demonstration solar cells constructed containing the DIO
CH3NH3PbI3-zClz/MG films had an average conversion efficiency of 6.58 ± 0.81 %. A mechanism
for DIO film formation is discussed. The principles established in this study wherein MGs control
the morphology and properties of CH3NH3PbI3-zClz/MG films should also apply to other
perovskite/MG composites.
3
INTRODUCTION
Microgels (MGs) are crosslinked polymer colloid particles that swell in a thermodynamically good
solvent1-7. They have been studied for at least 80 years8 and were used more than 20 years ago to
form surface coatings in the automotive industry9. MGs have unique rheological and (reversible)
space-filling properties as a consequence of the tendency of these particles to (reversibly) swell1.
They are also good film formers. Hydrophilic (water-swellable) or hydrophobic (solvent-swellable)
MGs can be prepared using scalable free-radical polymerisation methods by selecting the
appropriate monomers1. In recent years the potential of MGs to form composite films with
photoactive materials (such as conjugated polymers) has begun to be explored10, 11. Polystyrene
MGs were blended with the conjugated polymers used for the hole transport matrix (HTM) in
hybrid organic-inorganic perovskite solar cells (PSCs) and enabled a more efficient use of those
expensive polymers11. However, there have not been any studies reported where MGs have been
included within the photoactive layer of PSCs. Here, MAPbI3-zClz/MG composite films (where MA
is CH3NH3+) are prepared and investigated for the first time. The aim of the work was to study the
role of the MGs in controlling the morphology and optoelectronic properties of these new
photoactive composite films.
Hybrid organic-inorganic perovskites continue to attract enormous research attention as photoactive
layers within solar cells12-25, photodetectors26 and light emitting diodes27. Perovskites can be solution
processed and have panchromatic absorption28, low exciton binding energies29, high exciton
diffusion lengths30 and are defect tolerant31. PSCs may well play an important role in future
renewable energy generation32. An emerging area of perovskite research is pattering the material to
achieve potentially useful geometries, such as fibres33. Here, we introduce and investigate a new
one-step deposition approach that creates (and allows control of) pores within perovskite films. The
study also presents demonstration PSCs where the MGs are included within the perovskite
photoactive layer for the first time. In our earlier work hydrophobic polystyrene MGs were included
in the HTM of PSCs. They were blended with hydrophobic conjugated polymer HTM and were not
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mixed with perovskite. Here, we show that hydrophilic poly(N-vinylformamide-co-2-(N-
vinylformamido)ethyl ether) (PNVF-NVEE) MGs cause micropatterning of MAPbI3-zClz to form
disordered inverse opal (DIO) morphology. Fig. S1 (ESI†) shows how the PNVF-based MGs used
in this study differ in composition, hydrophilicity and location in the PSCs to the polystyrene MGs
used earlier11.
Inverse opal (IO) films are porous materials containing monodisperse pores with long range order
and have been widely studied for optical and electronic applications34-36. IO perovskite films have
been prepared using multi-step approaches involving colloidal particle templates which are
subsequently removed37, 38. The IO perovskite films have exhibited highly ordered morphologies
with tuneable reflectivity and good PCEs37, 38. However, the existing methods used to prepare IO
films require delicate and time-consuming assembly procedures37-39. In contrast disordered inverse
opal (DIO) films have a lower degree of long-range order and are scalable40. Such films can provide
strong light scattering40, 41. Here, we investigate the physical chemistry principles governing the
formation of DIO perovskite films using MG micropatterning for the first time.
We selected PNVF-NVEE MGs for this study because these hydrophilic particles swell in dimethyl
sulfoxide (DMSO), which is a polar solvent that is used widely for perovskite film deposition42.
Scheme 1 depicts the approach used to prepare our DIO MAPbI3-zClz/MG films. The MGs were
synthesised by dispersion polymerisation which is a scalable method (Scheme S1, ESI†). NVF is an
amide and is structurally similar to dimethylformamide (DMF), which is also a widely used solvent
for perovskite deposition. DMF forms complexes with the PbI2 precursor43 and the adducts affect
perovskite crystallisation. One aim of this study was to investigate whether the MGs provide a
template around which MAPbI3-zClz crystallises. As shown in Scheme 1 the MGs were dispersed in
DMSO containing the perovskite precursors (PbCl2 and MAI) and then spin coated onto a
mesoporous TiO2 scaffold (mp-TiO2). The latter method is commonly used for the preparation of
PSCs12, 44. The code used to identify the MAPbI3-zClz/MG films is shown in the Scheme 1 caption.
5
MAPbI3-zClz (MP)meso-TiO2
bl-TiO2
ITO
Glass
MG+ PbCl2 & MAI
NHO
NO
O
NO
DIO film (Side view)
MAPbI3-zClz walls
Code: MPxMGy
MGsPNVF-NVEE
DMSO
(i) Spincoat
(ii) Toluene
(iii) Anneal
meso-TiO2/MAPbI3-zClz
MAPbI3-zClz/MG
(Top
vie
w)
Scheme 1. Diagram showing the preparation of disordered inverse opal (DIO) MAPbI3-zClz/MG
films. The code used to denote the films is MPxMGy where x and y are the nominal concentrations
of MAPbI3-zClz and MG used to prepare the films. The architecture used is suited to optoelectronic
studies including PSC measurements. MA, NVF, meso, bl and ITO are CH3NH3+, N-
vinylformamide, mesoporous, blocking layer and indium tin oxide, respectively. During spin
coating some of the MAPbI3-zClz fills the pores in the meso-TiO2 layer and the remainder covers
that surface as a MAPbI3-zClz/MG capping layer. Toluene antisolvent is added during spin coating to
accelerate MAPbI3-zClz crystallisation.
After first characterising the MGs, the morphologies of the MPxMGy films are studied using SEM
and AFM and a phase diagram is constructed. The morphology and optoelectronic properties of the
MPxMGy films are then studied and the effects of the MGs on these properties established. We also
demonstrate that operational PSCs containing DIO MAPbI3-zClz/MG films can be prepared. Those
PSCs had higher power conversion efficiency (PCE) than the control devices prepared without
MGs. The study demonstrates that the MGs provide micropatterning of MAPbI3-zClz films with
tuneable wall thickness. These behaviours are new for both for MGs and perovskite films. A
mechanism is proposed to explain the DIO film formation. It is highly likely that DIO films could
be prepared with other MGs and perovskites using our new MG micropatterning approach.
6
EXPERIMENTAL DETAILS
Materials
NVF (N-vinylformamide, 98%), azoisobutyronitrile (AIBN, 98%), potassium-tert-butoxide (95%),
bis(2-bromoethyl)ether (BBE, 95%), dicyclohexyl-18-crown-6 (98%), anhydrous tetrahydrofuran
(THF, 99.9%), and ethanol (99.9%), poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP-co-PVA, Mn
~50,000 g/mol) , anhydrous sodium sulphate (100%), chloroform (99.9%), NaCl (100%), toluene
(99.8%), chlorobenzene (CBZ, 99.8%), isopropanol (IPA, anhydrous, 99.5%), 4-tert-butylpyridine
(TBP, 96%) and lithium bistrifluoromethanesulfonimidate (LiTFSI, 99.95%) were all purchased
from Aldrich and used as received. Methylamine solution (33 wt.% in absolute ethanol) and HI (57
wt.%), titanium diisopropoxide bis(acetylacetonate) (TDB) (75 wt % in IPA), PbCl2 (98%) and
DMSO (99.7%) were also purchased from Aldrich and used as received. MAI was synthesised and
purified using the method previously reported45. Titania paste (TiO2, 18 NRT) was purchased from
Dyesol and used as received. Spiro-MeOTAD (Spiro, N2,N2,N2’,N2’,N7,N7,N7’,N7’- octakis(4-
methoxyphenyl) - 9,9’ – spirobi [9H-fluorene] - 2,2’,7,7’-tetramine, Fenglin Chemicals, 99.5%)
was also used as received. Water was of ultrahigh purity and de-ionised. NVEE was synthesised
and characterised using the methods described previously46.
Microgel synthesis
PNVF-NVEE MGs were prepared by non-aqueous dispersion polymerisation following a method
described previously46. Briefly, a mixture of NVF (6.00 g, 85.5 mmol), PVP-co-PVA (1.80 g) and
NVEE (1.79 g, 8.28 mmol) were added to ethanol (EtOH, 86 mL) in a round bottomed flask
equipped with overhead stirrer, nitrogen supply and a reflux condenser. The solution was heated to
70 °C and stirred vigorously. Then, AIBN (0.240 g, 1.45 mmol) in EtOH (2.0 mL) was added and
the polymerisation allowed to continue for 1 h. The dispersion was filtered after cooling to 0 °C
with a 50 μm mesh filter and then purified by repeated centrifugation and re-dispersion in EtOH. To
7
transfer the MGs from EtOH to DMSO, the MGs in EtOH were centrifuged and then redispersed in
DMSO. The particles were centrifuged once more and re-dispersed in DMSO.
MAPbI3-zClz/MG composite film formation
ITO-coated glass substrates (20 Ω/sq) were cleaned by ultrasonication in a 1.0 wt% Hellmanex
solution, rinsed with water, followed by IPA, NaOH (2.5 M) and then rinsed with water and dried.
A TiO2 hole blocking layer (bl-TiO2) (60 nm) was spin-coated at 2000 rpm for 60 s onto the ITO
using TDP solution in 1-butanol (0.15 M) and subsequent heating at 125 ºC for 5 min. The process
was repeated using a 0.30 M TDP solution. After that, TiO2 paste (1:5 in EtOH) was spin coated at
5000 rpm for 30 s onto the cleaned ITO substrate to form a mesoporous scaffold (meso-TiO2). The
meso-TiO2 film (thickness ~ 250 nm) was annealed at 500 ºC for 30 min and cooled to room
temperature. Then a MAPbI3-zClz precursor solution in DMSO (100 L) containing MGs was spin-
coated onto ITO/bl-TiO2/meso-TiO2 substrate at 4000 rpm for 25 s. The MAPbI3-zClz/MG films are
denoted as MPxMGy where x and y are, respectively, the concentrations of MAPbI3-zClz (assuming
100% conversion) and MG in the mixed solution used for spin coating. The MAPbI3-zClz solution
contained the precursors MAI and PbCl2 at a 3:1 molar ratio. During the spin coating process,
toluene (500 L) was dripped onto the surface at a uniform rate over the last 15 s. The nominal
compositions of the MPxMGy films studied are shown in Fig. 2. The films were dried at 100 ºC for
45 min and then stored in a desiccator over P2O5 in the dark until investigation.
Physical Measurements
Dynamic light scattering (DLS) measurements were conducted using a Malvern Zetasizer Nano ZS
instrument (via cumulants analysis) and provided the z-average diameter (dz) and polydispersity
index (PDI) for the dispersed MGs. The top view SEM images were obtained using a Philips XL30
FEGSEM. The cross-section SEM images and EDX spectra were obtained using a Carl Zeiss Sigma
FE-SEM and an AG-ULTRA 55. The samples were coated with Au or Pd. AFM images were
obtained using either a Bruker Multimode 8 or a Bruker Catalyst. AFM images were captured in
ScanAsyst™ (Peak Force Tapping) mode. UV-visible spectra were recorded using a Perkin Elmer
8
Lamda 25 UV-Vis spectrometer. Film thickness measurements were conducted using a Dektak 8
Stylus Profilometer (Bruker). XRD patterns were conducted using a Bruker D8 Advance
diffractometer (Cu-Kα). Films were scanned with a step size of 0.02o. The films were prepared
under a nitrogen atmosphere and measured using an airtight holder. Photoluminescence (PL)
spectra were obtained using an Edinburgh Instruments FLS980 spectrometer. The beam was
incident on the film surface side and an excitation wavelength of 480 nm was used.
Solar cell construction and power conversion efficiency measurements
Demonstration solar cells were prepared using MP37.5MG3.0 and compared with control MP37.5
devices. The procedure to prepare the ITO/bl-TiO2/meso-TiO2/MPxMGy films was described
above. CBZ was used as the HTM solvent at room temperature. LiTFSI (4.8 μL, 520 mg/ml) and
TBP (8.0 μL) were also added to the Spiro solution following our method reported earlier11. The
Spiro HTM films (200 nm) were formed by spin coating at 4000 rpm for 20 s onto the MPxMGy
films. The devices were coated with a gold layer (70 nm) using thermal evaporation. The geometry
of these PSCs is depicted in Fig. 9. The ITO/bl-TiO2/meso-TiO2/MP37.5/Spiro/Au devices did not
contain added MGs and were the control. Although both types of devices were not optimised for
efficiency, they were prepared using identical conditions and are, therefore, directly comparable.
The current density–voltage (J–V) characteristics were measured using a Keithley 2420
Sourcemeter and 100 mWcm2 illumination (AM 1.5G) and a calibrated NREL certified Oriel Si-
reference cell. An Oriel solar simulator (SOL3A) was used for these measurements. The active area
of the devices (0.025 cm2) was determined using a square aperture within a mask. The data shown
are from the reverse scan (Voc to Jsc) and the sweep rate was 100 mV s-1.
RESULTS AND DISCUSSION
Microgel characterisation
Our first attempts to include MGs within perovskite films employed hydrophobic polystyrene-based
MGs from our previous work11 (see Fig. S1, ESI†). However, those MGs did not swell in DMSO
9
which resulted in pronounced aggregation and poor quality MAPbI3-zClz/MG films. We next
prepared hydrophilic PNVF-NVEE MGs46 because the repeat unit is structurally similar to DMF,
which is a solvent for perovskite precursors42. The PNVF-NVEE MGs were colloidally stable in all
of the mixed solutions used and gave good quality films (see Fig. S2, ESI†). Hence, it was
important for preparing good quality composite films that the MGs both dispersed and swelled in
the MAPbI3-zClz solvent. This is because swollen MGs have enhanced dispersion stability1.
The MGs deposited from EtOH were spherical and had a number-average diameter (DSEM) of 913 ±
71 nm as determined by SEM (see Fig. 1A). Dynamic light scattering (DLS) measurements showed
the MGs had z-average diameters (dz) in EtOH and DMSO of 885 (PDI = 0.013) and 1125 nm (PDI
= 0.088), respectively (Fig. 1B). The former value agrees with DSEM. The dz value measured in
DMSO is larger than that for the MGs in EtOH because the particles swelled in the former solvent.
The volume swelling ratio for the MG particles in DMSO is estimated as 2.1 from these dz values.
This result shows that DMSO is a good solvent for PNVF-NVEE. The deposited MGs had a
tendency for hexagonal close packing when deposited from DMSO as can be seen from Fig. 1C.
AFM data measured for isolated MGs deposited from DMSO gave the average diameter and height
as ~ 1200 and 400 nm, respectively (see Fig. 1D). The AFM data show that the MGs substantially
flattened after deposition. This behaviour is typical for MGs deposited from a good solvent10, 47, 48
and is due to solvent removal from adsorbed swollen MGs which favours shrinkage in the z-
direction.
10
100 1000 10000
Inte
nsity
/ %
(AU
) EtOH
Diameter / nm
DMSO
B
C D
0 1000 2000 3000 40000
500
Hei
ght /
nm
Distance / nm
A
Figure 1. (A) SEM of MGs deposited from EtOH. (B) Size distribution measured using dynamic
light scattering (DLS) for the MGs dispersed in EtOH and DMSO. (C) SEM of MGs deposited
from DMSO. Their tendency to form hexagonally close packed clusters is shown. (D) AFM image
and a line profile for two well separated MGs deposited from DMSO. Scale bars: 1000 nm.
Using microgels to control the morphology of perovskite films
The MGs dispersed in DMSO containing the MAPbI3-zClz perovskite precursors were spin coated
onto an ITO/bl-TiO2/meso-TiO2 substrate (see Scheme 1). The MPxMGy films were black which
shows that perovskite formation occurred in the presence of the MGs (see Fig. S2, ESI†). SEM was
used to construct a morphological phase diagram (see Fig. 2). The pure MG films (i.e., MGy) had
isolated MG clusters at low y values which became more uniform films as y increased (see vertical
series with x = 0). The pure MAPbI3-zClz films (i.e., MPx) are the horizontal sequence of images
with y = 0. MP25 had an appearance similar to that of the meso-TiO2 because there was insufficient
perovskite to fill the meso-TiO2 layer and produce a significant capping layer. In contrast MP37.5
and MP45 had large crystals in the capping layer as well as large gaps between the crystals. These
large gaps are termed pinholes. Remarkably, the MPxMGy films from the blue shaded region of the
phase diagram had a porous honeycomb-like DIO morphology. These films are the first examples of
(disordered) IO-like perovskites deposited in a single step to the best of our knowledge.
11
Figure 2. Morphology phase diagram for MPxMGy films. The six films studied in detail are shown
in red. The blue shaded area shows the region of the phase diagram where DIO films were formed.
Scale bars: 5 µm.
The DIO films are considered in more detail. We compare SEM images for the MP25MG3.0,
MP37.5MG3.0 and MP45MG2.0 DIO films (Fig. 3A-C) with the respective MG-free MP25,
MP37.5 and MP45 systems (Fig. 3D – F). (Lower magnification SEM images recorded for these
films are shown in Fig. S3, ESI†) Hexagonal close packing of the pores is clearly evident
(highlighted in Fig. 3A, B and C). Such packing was also present for deposited MGs (see Fig. 1C).
Hence, the SEM data show that the MGs acted as a micropatterning species for MAPbI3-zClz.
12
Figure 3. SEM images obtained for (A) MP25MG3.0, (B) MP37.5MG3.0, (C) MP45MG2.0, (D)
MP25, (E) MP37.5 and (F) MP45 films. Hexagonal packing is illustrated in A - C. SEM images are
also shown for (G) MP30MG2.0, (H) MP30MG3.0 and (I) MP30MG4.0. The arrows in (G) show
that small DIO islands formed around MG clusters. Scale bars: 5 m.
The SEM images for the MPxMG3.0 films (x = 25 and 37.5) show that the fraction of surface
covered by the DIO film (DIO) increased with increasing x (see Fig. 3A and 3B). DIO also increased
for MP30MGy (y = 2.0, 3.0 and 4.0) as y increased as shown in figures 3G – I. (See also the lower
magnification SEM images in Fig. S3, ESI†). When these images are compared to those for the
control films prepared without added MGs (Fig. 3D to 3F) it can be seen that the MGs increased the
proportion of surface covered by perovskite. It follows that the MGs decreased the proportion of
precursor ejected during film deposition which should reduce material waste as up to 90% can be
lost49 during spin coating. This result can be explained by two phenomena: (i) MGs are well known
13
to increase the viscosity of fluids50 and (ii) the film thickness of spin coated films increases with
increasing fluid viscosity51. The former occurs because MGs occupy an increased effective volume
fraction when swollen and act rheologically as a thickening agent.
The pore diameter (DPore) and wall heights (H) of the DIO films were measured using SEM. The
former values were obtained by averaging more than 100 pore diameters from three to four different
sample areas. The average DPore values for the MP30MGy (y = 2.0, 3.0 and 4.0) films were 922 ±
83, 866 ± 79 and 812 ± 69 nm, respectively (see Table S1, ESI†). These data provide evidence that
DPore decreased with increasing y. AFM images and line profiles were measured for MP25MG3.0,
MP37.5MG3.0 and MP45MG2.0 films (See Fig. S4, ESI†). The average H values for the pores are
estimated from these line profiles as 400, 650 and 900 nm, respectively. Whilst the data set used to
estimate H was limited to these profiles we note that the value for MP37.5MG3.0 agrees well with
the heights of the three walls evident from the SEM cross-section image (Fig. S6, ESI†, blue
arrows). The data for MPxMG3.0 (x = 25 and 37.5) films suggest that H increased with increasing
x. Hence, DIO, DPore and H can be tuned using both x and / or y. We show below that the pore wall
thickness (W) can also be tuned using x and y.
An important mechanistic observation from the SEM image for MP30MG2.0 (Fig. 3G) is that DIO
formed in regions containing deposited MGs. Higher magnification images are shown in Fig. S5,
ESI†). The pore walls were located at the edges of the MGs. It follows that the MAPbI3-zClz film
formed at the surfaces of the MGs. These data provide strong evidence that there was an attractive
interaction between MAPbI3-zClz and the MGs. The MGs appeared to act as a template for MAPbI3-
zClz crystallisation.
Fig. 4A and B show a higher magnification SEM image measured for MP37.5MG3.0. Several pores
show MGs (arrows) located approximately at their centre. A cross-section for the film (Fig. 4C)
shows a pore clearly. The walls between the pores comprise MAPbI3-zClx grains that extend from
the top of the capping layer to the meso-TiO2/MAPbI3-zClx layer. The image also clearly shows a
14
flattened MG (yellow arrow). We examined the compositional changes of C, N, Pb and I for cross-
sections of a pore (Fig. 4D) and a wall (Fig. 4E) using EDX. (See Fig. S6, ESI†, for an additional
SEM image of this region.) The scanned regions are the orange dotted lines. These data show the
concentration of Pb and I increases in the capping layer (i) compared to the mp-TiO 2/MAPbI3-zClz
layer (ii), as expected. There is also a spike in the C concentration when the thin, flattened, MG is
traversed (see asterisk in Fig. 4D). Such a spike was not present for the control wall scan (Fig. 4E).
These data confirm the composite nature of MP37.5MG3.0.
B MG
A
MP37.5MG3.0, scale bar 300 nm
(i) MAPbI3-zClz (ii) mp-TiO2 / MAPbI3-zClz(iii) bl-TiO2 (iv) ITO
(ii)
(i)
(iii)(iv)
C
D
E
Carbon Nitrogen Lead Iodine
(i)
(ii)
(ii)
(i)
Counts / cps
*
Counts / cps
Figure 4. SEM images recorded for the MP37.5MG3.0 DIO film showing (A) several pores and (B)
two pores with MGs visible in the centre of the pores. (C) SEM image for a cross-section with a
pore evident and a flattened MG. SEM and EDX spectra are shown for a pore (D) and a wall (E).
The orange dotted lines show the regions scanned. The scale bars are 300 nm. The yellow, red and
blue arrows highlight MGs, pores and walls, respectively. The asterisk in (D) coincides with the
flattened MG (see text).
15
To better understand the influence of the MGs on the DIO films the following equation was used to
calculate the volume fraction of the MG in the collapsed state in the MPxMGy composite film (MG)
ϕ MG=1
( xy )( ρMG
ρMP)+1
(1)
where MG and MP are the densities of PNVF-NVEE and perovskite, respectively. These values
were taken as 1.0 g/cm3 and 4.1 g/cm3, respectively52. The MG values for the DIO films varied from
0.154 for MP45MG2.0 to 0.353 for MP30MG4.0 (see Table S1, ESI†). The variations of W
(measured from SEM using Image J and at least 100 walls) and DPore with MG are shown in Fig. 5A
and B, respectively. Figure 5A shows that W decreased by ~ 55% from 529 ± 184 nm to 226 ± 56
nm as MG increased from 0.154 to 0.353. The maximum and minimum DPore values were 988 ± 60
nm and 657 ± 87 nm respectively and differed by ~ 35%. Hence, the relative changes were greatest
for W. Both W and DPore decreased with increasing MG because the average inter-MG spacing
decreased.
0.1 0.2 0.3 0.40
200
400
600
800
1000
W /
nm
MG
Thry D = 913 nm Thry D = 625 nm Experiment
0.1 0.2 0.3 0.40
200
400
600
800
1000
Dpore
/ nm
MG
BA
(i) MP45MG2.0(ii) MP30MG2.0(iii) MP37.5MG3.0(iv) MP30MG3.0(v) MP25MG3.0(vi) MP30MG4.0
(i)(ii) (iii) (iv) (vi)
(i)(ii) (iii) (iv) (vi)
(v)
(v)
Figure 5. Variation of average (A) experimental and theoretical wall thicknesses (W) and (B) pore
diameter for the DIO films with MG volume fraction (see text). The legend in (B) also applies to
(A).
A simple geometric model was used to derive equation (2) which describes the variation of W for
the DIO films in terms of MG. (See ESI† for derivation.)
16
W =D{( ϕMG (hcp−2 D)
ϕ MG)
12−1} (2)
MG(hcp-2D) is the volume fraction of hexagonally closed packed spheres in 2-dimensions. The value
for D represents the MG diameter when the perovskite matrix crystallised. The derivation of
equation (2) is based on a hexagonal arrangement of the MGs within a continuous perovskite phase
(see Fig. S7, ESI†). It is assumed for simplicity that the height of the pore walls is the same as the
MG diameter (D). The MGs are assumed to have flattened after MAPbI3-zClx crystallisation. Fig.
5A shows calculated values for W from equation (2) using D = 625 and 913 nm. The former value
is the optimum value to fit the experimental W values; whereas, the latter value is DSEM. The fact
that D < DSEM gave an optimum fit is due to the simplifications used in deriving this model.
Nevertheless, equation (2) generally captures the physics of DIO formation wherein W decreases as
MG increases. It follows from equations (1) and (2) that W should decrease as x decreases or as y
increases as both of these changes increase MG. These trends are apparent from the data shown in
Fig. 5A. It follows that the formation of the DIO films occurs by uniform spreading of the MGs in a
locally ordered hexagonal arrangement with MAPbI3-zClx filling the spaces between MGs.
The effect of the MGs on the MAPbI3-zClx structure was investigated by obtaining X-ray diffraction
data for the M25MG3.0, MP37.5MG3.0 and MP45MG2.0 DIO films as well as the respective MG-
free films (see Fig. 6 and Table S2, ESI†). The presence of the MGs increased the intensity of the
MAPbI3-zClz peaks. Furthermore, their average width at half-maximum decreased when MGs were
included. The grain sizes for the films were calculated from the Scherrer equation53, 54 and are
shown in the figure. Whilst the average values were higher for the MPxMGy films there is
considerable spread in the full width at half maximum values used to calculate the grain sizes
(Table S2, ESI†). We conclude that there is some evidence that the grain size was increased for the
MP37.5MG3.0 and MP45MG2.0 DIO films; however, more work is required to confirm this point.
17
MP25MG3.0MP25
MP37.5MG3.0
MP37.5
MP45MG2.0MP45
0
10
20
30
40
50 MPxMGy MPx
Gra
in s
ize
/ nm
10 15 20 25 30 35
Inte
nsity
/ A
U
2 / o
(001)PbCl2
(110)(220) (222),
(310)
MP45MG2.0
MP45
MP37.5MG3.0
MP37.5
MP25MG3.0
MP25
Figure 6. XRD patterns for various films (indicated). The inset shows the variation of the average
grain size determined from the application of the Scherrer equation to the diffraction peaks. The
method used for this analysis is given elsewhere53, 54.
Using microgels to control the optoelectronic properties of perovskite films
UV-visible spectra for the MPxMG3.0 (x = 0, 25, 30 and 37.5) films were measured to probe the
effect of x on light absorption (see Fig. 7A). The MG particles did not absorb light in the
wavelength range examined as shown by the data for MG3.0. The spectra show that as x increased
the absorbance also increased. (See also the inset for Fig. 7A). This is due to more perovskite being
deposited (see Fig. 3A and B). The effect of y on the UV-visible spectra can be seen from Fig. 7B
which shows spectra for MP30MGy (y = 0, 2.0, 3.0 and 4.0). Including MGs strongly increased the
18
light absorbed by the MP30MGy films for y > 2.0 (see inset). The reason for this effect can be
found from the measured values for DIO (see Fig. 7C). (The latter values were measured from SEM
images using Image J.) The MP30MGy film with the lowest absorbance values was MP30MG2.0
which had a low DIO of 0.36. The data in Fig. 7C show that a minimum DIO value of 0.75 (for
MP30MG3.0) is required to obtain highly absorbing films.
0.1 0.2 0.3 0.40.0
0.2
0.4
0.6
0.8
1.0
DIO
MG0.1 0.2 0.3 0.4 0.5 0.60
1
2
3
4
Abs
orba
nce
A480 A740 A800
MP
20 25 30 35 400
1
2
3
4
Abs
x / %
480 740 800
0 1 2 3 40
1
2
3
4
Abs
480 740 800
y / %
500 600 700 8000
1
2
3
4
Abs
orba
nce
Wavelength / nm
MP37.5MG3.0 MP30MG3.0 MP25MG3.0 MG3.0
500 600 700 8000
1
2
3
4
Abs
orba
nce
Wavelength / nm
MP30MG4.0 MP30MG3.0 MP30MG2.0 MP30
A B
C
(i) MP45MG2.0(ii) MP30MG2.0(iii) MP37.5MG3.0(iv) MP30MG3.0(v) MP25.0MG3.0(vi) MP30MG4.0
D(i)
(ii)
(iii)
(iv)
(vi)
(v)
(i)
(v)(ii)
(iv) (vi)(iii)
Figure 7. UV-visible spectra for (A) MPxMG3.0 and (B) MP30MGy films. The insets for (A) and
(B) show the effects of x and y, respectively, on the absorbance measured at 480, 740 and 800 nm.
(C) Variation of fractional surface coverage of DIO (DIO) with MG. (D) Variation of absorbance
with fractional surface coverage of MAPbI3-zClz (MP). The latter was calculated using equation (4).
The simple geometric model used to derive equation (2) was extended for light absorption by the
DIO films (see ESI for the derivation and Fig. S7†). It is assumed that the total absorbance (Abs) is
the sum of the absorbance from the DIO capping layer (Abscap) and the underlying meso-TiO2/
MAPbI3-zClz layer (Absmeso). For a fixed H the Abscap value will be proportional to the fraction of the
surface covered by perovskite (MP). The key equations are:
19
|¿|α MPθMP H
2.303+|¿meso|¿ (3)
θMP=θMP (DIO)θDIO (4)
In equation (3) MP is the absorption coefficient for MAPbI3-zClz. For equation (4) MP(DIO) is the
fraction of the DIO film covered by MAPbI3-zClz and can be calculated using
θMP (DIO)=1− π2 √ 3 ( 1
1+ WD )
2
(5)
The measured values used for W are shown in Table S1 (ESI†). For simplicity, it was assumed that
D was equal to DPore, i.e., that the pores are the negative image of the MGs. The DIO values and
calculated MP(DIO) values (see Table S1, ESI†) were used to calculate MP using equation (4). Fig.
7D shows the absorbance values measured at 480, 740 and 800 nm plotted as a function of MP.
Linear relationships are evident. The parameters from the fits (Table S3, ESI†) can be compared to
equation (3). Using the gradient of 3.71 at a wavelength of 480 nm and the capping layer thickness
for MP37.5MG3.0 (700 nm) for H then MP = 1.2 x 107 m-1. The latter corresponds to an absorption
depth of ~ 80 nm which is reasonably close to the reported value of 100 nm30. We stress that the
purpose of this simplified model is to increase the understanding of how the MGs control light
absorption and it should be considered a semi-quantitative guide to trends. Equations (3) – (5) show
how the MPxMGy film absorbance is controlled by the MG-dependent parameters W and D.
To further probe the optoelectronic properties PL spectra were measured. The effect of x was
investigated for the MPxMG3.0 (x = 25, 30 and 37.5) films (see Fig. 8A). The PL intensity reached
a plateau when x = 30 (see inset), which corresponds to extensive DIO film formation (DIO = 0.75
from Fig. 7C). The effect of y was investigated for the MP30MGy (y = 0, 2.0, 3.0 and 4.0) films
(see Fig. 8B). The PL intensity increased strongly when y = 3.0 (see inset) due to extensive DIO
film formation (see Fig. 7C). Thus, increasing x or y increased DIO which, in turn, increased the PL
intensity. A high PL intensity indicates decreased quenching. As DIO increases more MAPbI3-zClz is
20
located further from the meso-TiO2 and is less able to be quenched55. Accordingly, we interpret the
increased PL intensity as being the result of MG micropatterning. In support of this proposal the
maximum PL intensity for all of the systems are plotted againstMP (see Fig. 8C) The PL
intensities for the films with MP > 0.21 are a factor of two or more higher than those with lower MP
values. The former films correspond to major DIO film coverage, i.e., DIO > 0.74.
0 1 2 3 40
1x1062x1063x1064x1065x106
Max
. Int
ens.
y / %20 25 30 35 40
0
2x106
4x106
Max
. Int
ens.
x / %
700 750 800 8500
1x106
2x106
3x106
4x106
5x106
Inte
nsity
/ C
ount
s
Wavelength / nm
MP37.5MG3.0 MP30MG3.0 MP25MG3.0
700 750 800 8500
1x106
2x106
3x106
4x106
5x106
Inte
nsity
/ C
ount
s
Wavelength / nm
MP30MG4.0 MP30MG3.0 MP30MG2.0 MP30
A B
0.1 0.2 0.3 0.4 0.5 0.6 0.7
1x106
2x106
3x106
4x106
5x106
PL in
tens
ity /
Cou
nts
MP
(i) MP45MG2.0(ii) MP30MG2.0(iii) MP37.5MG3.0(iv) MP30MG3.0(v) MP25.0MG3.0(vi) MP30MG4.0
(i)
(ii)
(iii)(v)
(vi)
(iv)
DIO < 0.37
DIO > 0.74C
Figure 8. PL spectra measured for (A) MPxMG3.0 and (B) MP30MGy films. The insets show the
effects of MP and MG concentration, respectively, on the maximum PL intensity. (C) Maximum PL
intensity as a function of MP. The PL intensities are grouped into two regions that depend on DIO as
shown.
An interesting question is what effect the MG particles have on the ability of MAPbI3-zClz to
function as photoactive layer in a PSC. This question was addressed by constructing preliminary
solar cells of MP37.5MG3.0 and MP37.5. The device architecture for the MP37.5MG3.0 device is
shown in Fig. 9. The aim of these measurements was to evaluate the effect of MG micropatterning
21
on the device performance. Neither MP37.5MG3.0 or the MP37.5 control devices were optimised.
The representative J-V data demonstrate that operational MP37.5MG3.0 PSCs were prepared. The
average PCE for the MP37.5MG3.0 devices (6.58 ± 0.81 %) was significantly higher than that for
the control MG37.5 devices (4.86 ± 0.75 %). It is noted that these PCE values are low compared to
PSCs prepared using processing that focus on achieving PCE values greater than 20%56. The
presence of pinholes (Fig. 3) is likely a major cause for the relatively low PCE values obtained in
these preliminary devices. Nevertheless, the measured performance data are encouraging because
they demonstrate that MGs can not only micropattern perovskite films, but that such DIO MAPbI3-
zClz films provide operational PSCs. Device optimisation is planned for future work.
0 200 400 600 8000
5
10
15
20 MP37.5MG3.0 MP37.5
Cur
rent
den
sity
/ (m
A c
m-2
)
Voltage / mV
Au
HTM
MAPbI3-zClz
meso-TiO2
bl-TiO2
ITO
GlassMG
Device MP37.5MG3.0 MP37.5
Jsc (mA/cm-2) 19.88 ± 3.89 16.21 ± 4.41
Voc (mV) 834 ± 25 795 ± 43
FF (%) 40.5 ± 5.8 39.6 ± 9.5
PCE (%) 6.58 ± 0.81 4.86 ± 0.75
Device architecture
Table of performance parameters
Figure 9. Device architecture used for MP37.5MG3.0 and representative current-voltage curves for
the MP37.5MG3.0 and MP37.5 PSCs. The table shows the average performance parameters. All of
the PSC data measured are tabulated in Table S4 (ESI†).
Finally, a mechanism for the DIO film formation based on the data shown above is proposed. The
key assumptions are that (i) the MG particles deposit onto the surface initially in a swollen state and
(ii) the MGs flatten considerably after MAPbI3-zClz crystallisation is complete. Spin coating of the
mixed DMSO solution containing the MAPbI3-zClz precursors and the MGs caused MG deposition
onto the meso-TiO2 surface. Some of these precursors infiltrated the meso-TiO2 scaffold. Excess
precursor crystallised in the capping layer. Crystallisation began as the DMSO was removed by
evaporation together with addition of the anti-solvent. Toluene is a poor solvent for the MGs and
MAPbI3-zClz and promotes both MG collapse and perovskite crystallisation57, 58. The results suggest
22
that crystallisation was completed before the MGs flattened. The perovskite phase covered the MGs
(which acted appeared to act as nucleation templates) and the thickness of this layer was smallest at
the top of the MGs and largest between the MGs. As the MGs flattened the thin perovskite “roof”
collapsed (if fully formed) to reveal a pore and the flattened MG (see Fig. 4B). The crystallised DIO
MAPbI3-zClz pores can be thought of as the negative image of the MGs before they flattened.
CONCLUSIONS
In this study we included amide-based MGs in the solution used to prepare MAPbI3-zClz films and
demonstrated that the MGs directed formation of a DIO capping layer for the first time. We
identified the region of the MAPbI3-zClz/MG phase diagram where DIO films formed. A simple
geometric model was able to capture the relationship between MG and the pore wall width. The
MGs also increased the light absorption and PL intensity by promoting DIO formation. The light
absorption was shown to be proportional to the fraction of MAPbI3-zClz in the capping layer which
was, in turn, controlled by MG. Preliminary device data showed that MGs provided PSCs with
improved PCE values compared to control MG-free devices. Overall, the MGs had the role of a
mesoscopic-scale micropatterning additive which controlled both the morphology and
optoelectronic properties of these new photoactive MAPbI3-zClz/MG composite films. These
behaviours occurred because the MGs were colloidally stable in the deposition solvent and also
deswelled and flattened after perovskite crystallisation. Indirect evidence was presented for an
interaction between the MGs and perovskite. Following a recent study involving poly(vinyl
pyrrolidone) stabilised MAPbI3-zClz59 we speculate that the attractive interaction originated from
hydrogen bonding between the amide carbonyl group and the amine proton of MA+ which promoted
crystallisation at the surface of the MGs and helped direct film formation. Because both repeat units
of the MG contain such carbonyl groups (Scheme S1, ESI†) they could have both contributed to
this effect. Further work is required to verify the exact mechanism responsible for the interaction
between the MGs and MAPbI3-zClz apparent from this study. The new one-step deposition method
introduced in this study to prepare DIO MAPbI3-zClz/MG films should be applicable to other
23
perovskites and MGs. In our future work we plan to decrease the size of the MGs to reduce the
absorbance of the photoactive layer with the aim of establishing semitransparent PSCs where the
DIO perovskite photoactive layer is deposited in one step.
CONFLICTS OF INTEREST
Some of the results in this study have been submitted as a patent application (GB 1808461.6).
ACKNOWLEDGMENTS
The authors would like to thank The Royal Thai Government and National Science and Technology
Development Agency (NSTDA), Thailand for scholarship support. The authors thank the Wellcome
Trust for equipment grant support to the EM Core Facility. We also thank Mr Eric Whittaker and
Mr Mu Chen for their technical assistance with the work. We also thank Yun Xiao for conducting
the initial polystyrene MG study.
24
References
1. B. R. Saunders, and B. Vincent. Adv. Coll. Interf. Sci. 1999, 80, 25.
2. V. Nerapusri, J. L. Keddie, B. Vincent, and I. A. Bushnak. Langmuir 2007, 23, 9572-9577.
3. B. Brugger, and W. Richtering. Adv. Mater. 2007, 19, 2973-+.
4. S. Nayak, and L. A. Lyon. Angew. Chem. Int. Ed. 2005, 44, 7686-7708.
5. R. Pelton. Adv. Coll. Interf. Sci. 2000, 85, 1-33.
6. J. Musch, S. Schneider, P. Lindner, and W. Richtering. J. Phys. Chem. B 2008, 112, 6309-
6314.
7. M. Karg, I. Pastoriza-Santos, B. Rodriguez-González, R. von Klitzing, S. Wellert, and T.
Hellweg. Langmuir 2008, 24, 6300-6306.
8. H. Staudinger, and E. Husemann. Ber. Dtsch. Chem. Ges. (A and B) 1935, 68, 1618-1634.
9. P. Bradna, P. Stern, Q. Quadrat, and J. Snuparek. Coll. Polym. Sci. 1995, 273, 324.
10. M. Chen, Z. Cui, S. Edmondson, N. Hodson, M. Zhou, J. Yan, P. O'Brien, and B. R.
Saunders. Soft matter 2015, 11, 8322-8332.
11. M. Chen, M. Z. Mokhtar, E. Whittaker, Q. Lian, B. Hamilton, P. O'Brien, M. Zhu, Z. Cui, S.
A. Haque, and B. R. Saunders. Nanoscale 2017, 9, 10126-10137.
12. J.-P. Correa-Baena, M. Saliba, T. Buonassisi, M. Grätzel, A. Abate, W. Tress, and A.
Hagfeldt. Science 2017, 358, 739-744.
13. M. A. Green, A. Ho-Baillie, and H. J. Snaith. Nat. Photon. 2014, 8, 506.
14. M. Alsari, O. Bikondoa, J. Bishop, M. Abdi-Jalebi, L. Y. Ozer, M. Hampton, P. Thompson,
M. T. Horantner, S. Mahesh, C. Greenland, J. E. Macdonald, G. Palmisano, H. J. Snaith, D.
G. Lidzey, S. D. Stranks, R. H. Friend, and S. Lilliu. Energy Env. Sci. 2018, 11, 383-393.
15. F. Bella, G. Griffini, J.-P. Correa-Baena, G. Saracco, M. Grätzel, A. Hagfeldt, S. Turri, and
C. Gerbaldi. Science 2016, 354, 203.
16. Z. Wang, D. P. McMeekin, N. Sakai, S. van Reenen, K. Wojciechowski, J. B. Patel, M. B.
Johnston, and H. J. Snaith. Adv. Mater. 2017, 29, 1604186.
25
17. W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E.
K. Kim, J. H. Noh, and S. I. Seok. Science 2017, 356, 1376-1379.
18. S. Chatterjee, and A. J. Pal. J. Mater. Chem. A 2018, 6, 3793-3823.
19. C. Eames, J. M. Frost, P. R. F. Barnes, B. C. O’Regan, A. Walsh, and M. S. Islam. Nat.
Commun. 2015, 6, 7497.
20. P. Calado, A. M. Telford, D. Bryant, X. Li, J. Nelson, B. C. O’Regan, and P. R. F. Barnes.
Nat. Commun. 2016, 7, 13831.
21. N. Aristidou, C. Eames, I. Sanchez-Molina, X. Bu, J. Kosco, M. S. Islam, and S. A. Haque.
Nat. Commun. 2017, 8, 15218.
22. P. Qin, A. L. Domanski, A. K. Chandiran, R. Berger, H.-J. Butt, M. I. Dar, T. Moehl, N.
Tetreault, P. Gao, S. Ahmad, M. K. Nazeeruddin, and M. Gratzel. Nanoscale 2014, 6, 1508-
1514.
23. F. Cai, J. Cai, L. Yang, W. Li, R. S. Gurney, H. Yi, A. Iraqi, D. Liu, and T. Wang. Nano
Energy 2018, 45, 28-36.
24. H. Tsai, R. Asadpour, J.-C. Blancon, C. C. Stoumpos, O. Durand, J. W. Strzalka, B. Chen,
R. Verduzco, P. M. Ajayan, S. Tretiak, J. Even, M. A. Alam, M. G. Kanatzidis, W. Nie, and
A. D. Mohite. Science 2018, 360, 67-70.
25. V. Stoichkov, N. Bristow, J. Troughton, F. De Rossi, T. M. Watson, and J. Kettle. Sol.
Energy 2018, 170, 549-556.
26. L. Dou, Y. Yang, J. You, Z. Hong, W.-H. Chang, G. Li, and Y. Yang. Nat. Commun. 2014,
5, 5404.
27. Z.-K. Tan, R. S. Moghaddam, M. L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A.
Sadhanala, L. M. Pazos, D. Credgington, F. Hanusch, T. Bein, H. J. Snaith, and R. H.
Friend. Nat. Nanotech. 2014, 9, 687.
28. S. Kazim, M. K. Nazeeruddin, M. Grätzel, and S. Ahmad. Angewandt. Chem. Int. Ed. 2014,
53, 2812-2824.
26
29. A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J. T.-W. Wang, S. D. Stranks, H. J.
Snaith, and R. J. Nicholas. Nat. Phys. 2015, 11, 582.
30. S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M.
Herz, A. Petrozza, and H. J. Snaith. Science 2013, 342, 341-344.
31. N. De Marco, H. Zhou, Q. Chen, P. Sun, Z. Liu, L. Meng, E.-P. Yao, Y. Liu, A. Schiffer,
and Y. Yang. Nano Lett. 2016, 16, 1009-1016.
32. W. Zhang, G. E. Eperon, and H. J. Snaith. Nat. Energy 2016, 1, 16048.
33. L. Qiu, J. Deng, X. Lu, Z. Yang, and H. Peng. Angew. Chem. Int. Ed. 2014, 53, 10425-
10428.
34. S.-H. Han, S. Lee, H. Shin, and H. Suk Jung. Adv. Energy Mater. 2011, 1, 546-550.
35. O. Sato, S. Kubo, and Z.-Z. Gu. Acc. Chem. Res. 2009, 42, 1-10.
36. Y. Wang, H. Chan, B. Narayanan, S. P. McBride, S. K. R. S. Sankaranarayanan, X.-M. Lin,
and H. M. Jaeger. ACS Nano 2017, 11, 8026-8033.
37. K. Meng, S. Gao, L. Wu, G. Wang, X. Liu, G. Chen, Z. Liu, and G. Chen. Nano Lett. 2016,
16, 4166-4173.
38. S. Zhou, R. Tang, and L. Yin. Adv. Mater. 2017, 29, 1703682.
39. X. Chen, S. Yang, Y. C. Zheng, Y. Chen, Y. Hou, X. H. Yang, and H. G. Yang. Adv. Sci.
2015, 2, 1500105.
40. N. R. Neale, B. G. Lee, S. H. Kang, and A. J. Frank. J. Phys. Chem. C 2011, 115, 14341-
14346.
41. M. El Harakeh, and L. Halaoui. J. Phys. Chem. C 2010, 114, 2806-2813.
42. W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, and S. I. Seok. Science 2015,
348, 1234-1237.
43. A. Wakamiya, M. Endo, T. Sasamori, N. Tokitoh, Y. Ogomi, S. Hayase, and Y. Murata.
Chem. Lett. 2014, 43, 711-713.
44. H. J. Snaith. J. Phys. Chem. Lett. 2013, 4, 3623-3630.
27
45. L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K. Nazeeruddin, and M.
Grätzel. J. Amer. Chem. Soc. 2012, 134, 17396-17399.
46. S. Thaiboonrod, C. Berkland, A. H. Milani, R. Ulijn, and B. R. Saunders. Soft matter 2013,
9, 3920-3930.
47. S. Schmidt, H. Motschmann, T. Hellweg, and R. von Klitzing. Polymer 2008, 49, 749-756.
48. S. Wellert, D. Kesal, S. Schon, R. von Klitzing, and K. Gawlitza. Langmuir 2015, 31, 2202-
2210.
49. J. Yin, Y. Lin, C. Zhang, J. Li, and N. Zheng. ACS. Appl. Mater. Interf. 2018, 10, 23103-
23111.
50. M. S. Wolfe, and C. Scopazzi. J. Coll. Interf. Sci. 1989, 133, 265-277.
51. C. J. Lawrence. Phys. Fluids 1988, 31, 2786-2795.
52. C. C. Stoumpos, C. D. Malliakas, and M. G. Kanatzidis. Inorg. Chem. 2013, 52, 9019-9038.
53. B. Jeong, I. Hwang, S. H. Cho, E. H. Kim, S. Cha, J. Lee, H. S. Kang, S. M. Cho, H. Choi,
and C. Park. ACS Nano 2016, 10, 9026-9035.
54. J. I. Langford, and A. J. C. Wilson. J. Appl. Cryst. 1978, 11, 102-113.
55. J. R. Poindexter, R. L. Z. Hoye, L. Nienhaus, R. C. Kurchin, A. E. Morishige, E. E. Looney,
A. Osherov, J.-P. Correa-Baena, B. Lai, V. Bulović, V. Stevanović, M. G. Bawendi, and T.
Buonassisi. ACS Nano 2017, 11, 7101-7109.
56. M. Saliba, J.-P. Correa-Baena, C. M. Wolff, M. Stolterfoht, N. Phung, S. Albrecht, D.
Neher, and A. Abate. Chem. Mater. 2018, 30, 4193-4201.
57. N. Sakai, S. Pathak, H.-W. Chen, A. A. Haghighirad, S. D. Stranks, T. Miyasaka, and H. J.
Snaith. J. Mater. Chem. A 2016, 4, 4464-4471.
58. M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach,
Y.-B. Cheng, and L. Spiccia. Angewandt. Chem. Int. Ed. 2014, 126, 10056-10061.
59. Y. Guo, K. Shoyama, W. Sato, and E. Nakamura. Adv. Energy Mater. 2016, 6, 1502317.
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