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

4

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

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