Enhancing Performance and Uniformity of Perovskite Solar Cells via a

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Enhancing Performance and Uniformity of Perovskite Solar Cells via a Solution-Processed C70 Interlayer for Interface Engineering Yaqing Zhou, Baoshan Wu, Guanhua Lin, Yang Li, Dichun Chen, Peng Zhang, Mingyu Yu, Binbin Zhang, and Daqin Yun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08429 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

Enhancing Performance and Uniformity of Perovskite

Solar

Processed

C70

Cells

via

Interlayer

a

Solution-

for

Interface

Engineering Ya-Qing Zhou,#,† Bao-Shan Wu, #,‡ Guan-Hua Lin,† Yang Li,† Di-Chun Chen,§ Peng Zhang,† Ming-Yu Yu,† Bin-Bin Zhang† and Da-Qin Yun*,†

†

College of Energy, Xiamen University, Xiamen 361005, P. R. China.

‡

State Key laboratory for Physical Chemistry of Solid Surfaces, iChEM

(Collaborative Innovation Center of Chemistry for Energy Materials), Department of

Chemistry, College of Chemistry and Chemical Engineering, Xiamen University,

Xiamen 361005, P. R. China.

§

Xiamen Branch of Luyang Ship Material Research Institute, Xiamen, 361006, China.

ABSTRACT: Although some kinds of semiconductor metal oxides (SMOs) have been

applied as electron selective layers (ESLs) for planar perovskite solar cells (PSCs),

1

ACS Paragon Plus Environment


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electron transfer is still limited by low electron mobility and defect film-formation of

SMO ESLs fabricated via low-temperature solution process. Herein, the C70 interlayer

between TiO2 and (HC(NH2)2PbI3)x(CH3NH3PbCl3)1-x is prepared by spin-coating and

low-temperature annealing for planar n-i-p PSCs. The resultant TiO2/C70 ESL shows

good surface morphology, efficient electron extraction and facilitation of high-quality

perovskite film formation, which can be attributed to the suitable nano-size and the

superior electronic property of C70 molecules. In comparison with pristine TiO2-based

PSCs, the efficiency and hysteresis index are respectively enhanced 28% and reduced

76% by adding C70 interlayer between TiO2 and perovskite on the basis of statistical

data of more than 50 cells. With the main advantages of low-temperature process and

optimized interface, the champion efficiency of PSCs on flexible substrates could

exceed 12% in contrast with above 18% on rigid substrate.

KEYWORDS: perovskite solar cell, electron selective layer, low-temperature solution

process, fullerene, flexible solar cell.

1. Introduction

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In virtue of the unique photoelectric properties of organic-inorganic hybrid halide

perovskite, perovskite solar cell (PSC) is one of the most promising photovoltaic devices.1 Since the device efficiency has been improved to be above 22%,2 the next

big issues confronting the commercialization are several challenges (e.g. scaled-up fabrication, stability, flexible module, toxicity and current hysteresis).3 On the one

hand, low-temperature solution process is urged to reduce the total material cost in scaled-up fabrication on flexible substrates.4 On the other hand, fullerenes can be used

as electron selective layer (ESL) materials to limit or eliminate current hysteresis of PSCs in comparison to those with TiO2 ESL.5

Among all kinds of semiconductor metal oxides (SMOs), titanium dioxide (TiO2) is widely applied as ESL material in mesoporous and planar PSCs.6 In recent years,

low-temperature solution process of TiO2 ESL has been developed for high-efficiency planar PSCs.7-8 However, two disadvantages usually exhibited in planar PSCs

equipped with low-temperature solution-processed TiO2 ESL. For one thing, the

electron mobility of TiO2 ESL is not good enough for electron transfer from perovskite to ESL in comparison with fullerenes,9-10 resulting in interfacial charge

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accumulation near the TiO2 ESL.11 For another, oxygen vacancies as deep-level traps

could be generated while TiO2 ESL is exposed to UV irradiation, which should be responsible for the severe recombination loss.12-13

Some researchers have focused on modification of conventional SMO ESLs (e.g.

TiO2 and SnO2) for better electron extraction and low recombination loss (e.g. increasing contact area, doping, adding a fullerene modifier).14 Beside of wide application of fullerenes as ESL materials in p-i-n planar PSCs,15 fullerenes as an

interlayer between SMO ESL and perovskite are popularly used to promote the

interfacial electron transfer and passivate the interfacial trap states, consequently enhancing the performance (e.g. low current hysteresis and high efficiency).16-22 Because of the high cost of synthesis and purification for fullerene derivatives,23

pristine fullerenes (e.g. C60 and C70) without further modification are more suitable

for ESL material in commercial PSCs. However, pristine fullerene ESLs prepared by

low-temperature solution process were seldom reported for high-efficiency PSCs,

even though fabricated via high-cost thermal evaporation in a vacuum environment.24-27

4


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As is well-known, C70 fullerene films possess higher electron conductivity than the one of C60 fullerene in dark as well as under the illumination of AM 1.5 G,28 but C70 molecules have been rarely used as ESL material for planar PSCs.24, 27, 29 Based on the

above

consideration,

we

fabricated

PSCs

with

a

configuration

of

ITO/TiO2/(HC(NH2)2PbI3)x(CH3NH3PbCl3)1-x/Spiro-OMeTAD/Ag, and modified TiO2

ESL with C70 interlayer via spin-coating and low-temperature annealing. Both

morphology of ESLs and electronic property at ESL/perovskite interface were

characterized and analyzed to interpret results of J-V measurements. Further, the

prototype of flexible PSCs was demonstrated and tested.

2. Experimental

2.1 Materials

Lead (II) iodide (99.999%) was purchased form Alfa-Aesar. Titanium (IV)

isopropoxide (TTIP) (97%), formamidine acetate salt (99%), hydroiodic acid (HI) (57

wt.% in H2O), anhydrous dimethyformamide (DMF) (99.8%), methylamine

hydrochloride (MACl), anhydrous dimethylsulfoxide (DMSO) (99.9%), acetonitrile

(99.8%), C70 (99%), anhydrous dichlorobenzene (99%), anhydrous chlorobenzene 5



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(99.8%), 4-tertbutyl pyridine (4-tBP, 96%), and bis(trifluoromethane) sulfonimide lithium salt (Li-TFSI) (99.95%), were purchased from Sigma-Aldrich. 2,2',7,7'tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spiro-bifluorene (Spiro-OMeTAD) was

received from Ossila. Tris(2-(1H- pyrazol-1-yl)- 4-tert-butylpyridine)-cobalt(III)

tris-(bis(trifluoromethylsulfonyl)imide)) (FK209) was purchased from Dyenanmo AB.

Other reagents and chemicals were used as received without further purification

unless otherwise noted.

2.2 Synthesis of annealing-free nanocrystalline anatase TiO2 nanoparticles (NPs)

TiO2 NPs were prepared by a hydrolytic sol-gel method based on a modified procedure from the previous report.30 Briefly, TTIP (35 mM) mixed with isopropyl

alcohol was slowly added dropwise to 120 mL deionized water mixed with nitric acid

under continuous vigorous stirring for 30 min at room temperature. Subsequently,

white colloidal solution of TiO2 NPs was formed after hydrolysis and the solution was

heated to 80 ºC and stirred vigorously for 4 h to yield a nanocrystalline TiO2 NPs solution with a concentration of about 20 mg mL-1. Before use, the TiO2 NPs solution

was filtered through a 0.45 µm PVDF syringe filter. 6


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2.3 Fabrication of solar cells

Planar heterojunction PSCs with a configuration of ITO/ESLs/(HC(NH2)2PbI3)x-

(CH3NH3PbCl3)1-x/Spiro-OMeTAD/Ag by employing TiO2 or TiO2/C70 as ESLs. A

patterned ITO glass substrate was cleaned by sequential ultrasonic treatment in

detergent, deionized water, acetone and isopropyl alcohol. The as-prepared TiO2 NPs

were spin-coated onto the ITO substrate at 4000 rpm for 50 s, then these samples were

annealed at 120 ºC for 60 min in air under ambient pressure. The following coating

steps were performed under argon atmosphere inside a glovebox. The TiO2-coated

samples were spin-coated with C70 at 3000 rpm for 50 s, and then were annealed on a

hot plate at 120 ºC for 20 min. The C70 solution was prepared by dissolving 10 mg C70

in 10ml dichlorobenzene at room temperature. For the perovskite layer, the preparation of PbI2 (DMSO) complex was similar to the previous report31 and

formamidinium iodide (NH2CH=NH2I,FAI) was synthesized according to the previous method.32 The 1.50 M PbI2(DMSO) complex solution in DMF was

preheated at 70 ºC and the solution was spin-coated on top of the C70 layer at 2000 rpm for 45 s. Subsequently, a 70 mg mL-1 mixed solution of FAI and MACl in

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2-propanol was spin-coated on the top of the PbI2 (DMSO) film at 2000 rpm for 40 s.

The coated substrates were then annealed on a hot plate at 120 ºC for desired period

of time. After cooling to room temperature, Spiro-OMeTAD was then deposited on

top of the perovskite layer by spin-coating at 2000 rpm for 40 s. The Spiro-OMeTAD

solution was prepared by dissolving Spiro-OMeTAD in chlorobenzene (60 mM), with the addition of 30 mM Li-TFSI (520 mg mL-1 in acetonitrile), 200 mM 4-tBP, and 1.8 mM FK209 (300 mg mL-1 in acetonitrile). Finally, 100 nm of silver was deposited by

thermal evaporation using a shadow mask to pattern the electrodes at a base pressure of 4 ×10-4 Pa, and the active area of this electrode was fixed at 0.12 cm2. To explore

the role of C70 interlayer, the control samples without C70-coating were prepared

under the same conditions. For the fabrication of flexible PSCs, the above process

was applied on the patterned ITO-PEN substrates.

2.4 Characterisation

The current density-voltage (J-V) characteristics of photovoltaic cells were performed

using a 300 W Xenon solar simulator (Newport Oriel Solar Simulators) with a source meter (Keithley 2420, USA) under the illumination of AM 1.5 G (100 mW cm-2, 8


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calibration with a Si-reference cell). For the sake of accuracy, the J-V curves were

measured by the forward scan (from short circuit to open circuit) and reverse scan

(from open circuit to short circuit). The step voltage was fixed at 100 mV. In order to

quantify the hysteresis effect on current output, the hysteresis index was calculated according to the previous reported formula.33 The external quantum efficiency (EQE)

spectrum was measured with a power source (66920, Newport, 300 W Xenon lamp)

with a monochromator (Cornerstone 260, Newport) and an optical power meter

(2936-R, Newport). Stabilized PCEs and photocurrent densities of PSCs were also

measured by an electrochemical workstation (CHI660E, Chenhua, Shanghai) as

maximum power point tracking method under the continuous illumination of 100 mW cm-2. The electrochemical impedance spectroscopy (EIS) was measured by an

electrochemical workstation (VersaSTAT3, Princeton Applied Research, USA) in dark

condition. AC 20 mV perturbation was applied with the frequency ranged from 1

MHz to 0.1 Hz under 0.2 V DC bias. The obtained impedance spectra were fitted with

Z-View software (Scribner Associates, USA). All the devices were characterised in air

(relative humidity ranging from 40% to 50% and temperature at about 22 ºC).

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High-resolution transmission electron microscopy (TEM) images were taken on a

JEOL JEM2100 at 300 kV. The diluted TiO2 NPs solution was dropped onto the

copper mesh and dried, after which the sample was loaded into the TEM chamber for

measurement. The morphologies were investigated by field-emission scanning

electron microscopy (SEM) (SUPRA 55). The X-ray diffraction (XRD) spectra of

TiO2 NPs were measured using Rigaku Ultima IV X-ray diffractometer. The

transmission

spectra

were

recorded

using

an

ultraviolet-visible

(UV-vis)

spectrophotometry (UV-2600, Shimadzu). To check the effect of TiO2 and TiO2/C70

layer, the photoluminescence (PL) spectra of perovskite films deposited on ITO,

ITO/TiO2 and ITO/TiO2/C70 was excited by 530 nm wavelength light and recorded

using Hitachi F-7000 spectrofluorometer equipped with a 150 W Xenon lamp as an

excitation source. Atomic force microscopy (AFM, Dimesion EDGD) was used to

characterize the roughness of TiO2 and TiO2/C70 ESLs. AFM scans were performed with Dimension Icon on 3 × 3 µm2 surface areas in tapping mode. Water and DMF

contact angles were measured on a contact angle measuring system (SDC-100,

Dongguan Shengding Precision Instruments Co. Ltd) at ambient environment.

10


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3. Results and Discussion

Figure 1a presents XRD patterns of as-synthesized TiO2 NPs and simulated one. All

the diffraction peaks of TiO2 sample match well with the pure anatase phase

(PDF#21-1272), as well as the published results in which the sample is natural single crystalline without the presence of any other polymorphs.34 The consistency indicates

that TiO2 NPs were already crystalline in the colloidal solution without further

annealing. The TiO2 sample was also characterized by high-resolution TEM as shown

in Figure 1b. It is obvious that the observed particle size is below 5 nm and the typical 0.35 nm distance of lattice fringes is consistent with the anatase 101 plane.35 Further,

the electron diffraction pattern (inset of Figure 1b) is applied to verify the

nano-crystalline nature of anatase phase, and thus the TEM results correspond well with the XRD results.34 Although the conductivity of TiO2 NPs increases with the

high quality of crystalline, the conductivity of un-doped TiO2 is typically lower than that of fullerenes, such as C60 and C70.9, 36

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Figure 1. (a) Experimental and simulated XRD patterns of TiO2. (b) High-resolution

TEM image of TiO2 NPs with inset showing selected area electron diffraction pattern.

Beside of the crystalline nature of TiO2, both the mechanical integrity of the film and

the electronic conduction between particles are crucial to the conductivity of TiO2 ESLs.33 In order to investigate the morphology of selected ESLs, TiO2 and TiO2/C70

films were produced by spin-coating and thermal annealing at 120 ºC (seeing

experimental section). Top-view SEM images of TiO2 and TiO2/C70 ESLs in Figure 2a

and 2b indicate that the TiO2/C70 ESL is obviously smoother in contrast to the TiO2

ESL, resulting from the suitable nano-size of C70 molecules for the meso-pore on the

TiO2 ESL. Also, the surface roughness was further measured by AFM as shown in

Figure 2c and 2d. The values of root-mean-square (rms) surface roughness were

respectively 1.258 ± 0.319 nm and 0.601 ± 0.200 nm for TiO2 and TiO2/C70 ESLs,

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which is in accord with the results of SEM measurement. On the basis of the high conductivity of C70 molecular37 and the compact mechanical integrity of TiO2/C70

ESL, the modified ESL should possess higher conductivity than the one of the pristine

TiO2 ESL.

Figure 2. Morphology of selected ESLs on ITO/glass. Top-view SEM images of (a)

TiO2 and (b) TiO2/C70 ESLs. AFM height images of (c) TiO2 and (d) TiO2/C70 ESLs.

In consideration of the influence of surface wetting on solution coating process, the

contact angle measurement was applied to characterize the wettability on TiO2 ESLs

and TiO2/C70 ESLs as shown in Figure S1. As we expected, the contact angle of water

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droplets increased from 50º on TiO2 ESL to 89º on TiO2/C70 ESL in accord with the

hydrophobic property of C70 molecules. Since the PbI2 (DMSO) complex solution in

DMF was spin-coated on ESL, the contact angles of DMF droplets were also tested

on TiO2 ESL and TiO2/C70 ESL, showing an increase of 72% from 22º on TiO2 ESL

to 38º on TiO2/C70 ESL. According to the research on the correlation between the

substrate surface wetting capability and the perovskite grain morphology, the higher

contact angle of DMF droplets on TiO2/C70 ESL could increase nucleus spacing by

suppressing heterogeneous nucleation and facilitate grain boundary migration in grain growth by imposing less drag force.38

Figure 3. (a) UV-vis spectra of ITO, ITO/TiO2 and ITO/TiO2/C70. (b) The

steady-state PL spectra of perovskite film on ITO, ITO/TiO2 and ITO/TiO2/C70.

14


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It is well known that good ESLs for n-i-p PSCs should keep balance between the electronic and the optical conductivities.39 The optical properties of TiO2 and TiO2/C70

ESLs fabricated on ITO/glass were evaluated by UV-vis spectrophotometry as shown

in Figure 3a. The C70 molecule coating on TiO2 NPs results in a slight decrease of

transmittance between 350 nm and 700 nm. In consideration of the absorption of

ITO/glass, the transmittance of TiO2/C70 ESL should be beyond 85 % within the main

absorption wavelength of (FAPbI3)x(MAPbCl3)1-x from 450 nm to 850 nm. In previous

report, fullerenes (derivative) are useful interfacial modification materials to passivate the deep traps on TiO2 and the defects on perovskites.18,

40

The charge transfer

between perovskite and ESL was further investigated by steady-state PL of perovskite

film on ITO, ITO/TiO2 and ITO/TiO2/C70 in Figure 3b. The PL intensity of

ITO/perovskite was reduced by a half via insertion of TiO2 ESL, meaning that a significant fluorescence quenching of perovskite happened.7 The slight decreased

fluorescence intensity was further observed by adding the interlayer C70 between TiO2

ESL and perovskite, indicating that C70 modification could further facilitate the more efficient electron extraction than pristine TiO2 ESL did.20, 41

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Figure 4. (a) Diagram of energy levels (relative to the vacuum level) and (b)

cross-sectional SEM image showing inner structure of the PSC based on TiO2/C70

ESL.

In order to demonstrate that the C70 interlayer is beneficial to the interfacial charge

transfer, we fabricated PSCs with a configuration of ITO/ESLs/(FAPbI3)x-

(MAPbCl3)1-x/Spiro-OMeTAD/Ag by employing TiO2 or TiO2/C70 as ESL. Based on

the analysis of morphology of ESLs and electron transfer at the interface between

ESL and perovskite, the detailed energy levels and cross-sectional morphology of

PSCs were further presented in Figure 4 (non-colorful cross-sectional morphology as

shown in Figure S2a). According to previous reports, the efficient charge extraction

from perovskite to ESL relies on many factors, such as the appropriate conduction

16


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band of ESLs,3, 39 the high electrical conductivity/mobility of ESLs,7, 42-43 the low interfacial traps8, 44 and the optimized interfacial dipole.45-47 As shown in Figure 4a,

the lowest unoccupied molecular orbital (LUMO) energy level of C70 layer (4.2 eV48-49) is similar to the conduction band minimum of TiO2 layer (4.2 eV18, 40, 50-52).

As mentioned above, C70 fullerene layer should not only possess higher electrical conductivity/mobility than TiO2 layer,9, 28 but can also passivate the surface traps of perovskite and TiO2.18, 22, 53 In consideration of LUMO energy level and electronic

property for C70 molecules, the C70 interlayer could facilitate the electron extraction

from perovskite to TiO2 based, which is in accord with the result of steady-state PL

spectra. By the comparison of the thickness between TiO2 ESL and TiO2/C70 ESL in

Figure S2b and S2c, the thickness of C70 interlayer is about 7 nm, similar to the optimum thickness of the C60 layer in the p-i-n perovskite solar cell architectures.54

Moreover, the thickness of TiO2/C70 ESLs is abut 50 nm as thin as TiO2 ESLs in

Figure S3, which is thin enough to keep the bulk resistance of ESLs low and enhance the fill factor (FF) of PSCs.26, 55-56 Figure 4 (b) also displays that the perovskite grains

were grown through the upper and lower, and the size is about 700 nm in vertical

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direction and nearly 1 µm in horizontal direction. According to previous studies, the

interface electron traps at the grain boundaries are of crucial importance with regard to suppressing trap-assisted recombination, as well as non-radiative recombination.57

On account of the quality of the perovskite grains as the primary factor for charge

carrier transporting and recombination, the grain size plays a key role in short-circuit current density (JSC).58 With the benefit of compact perovskite film without pinholes, the open-circuit voltage (VOC) of PSCs should exceed 1 V.59-61 However, we also

characterized the cross-sectional morphology of TiO2-based PSCs in Figure S2. In

contrast with TiO2/C70-based PSCs, the perovskite grain size of TiO2-based PSCs

varied from 300 nm to 600 nm with higher grain boundary area, which might be

resulted from high surface roughness and low wettability of TiO2 ESLs as discussed

above.

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Figure 5. (a) J-V curves of champion PSCs based on TiO2/C70 ESL in forward/reverse

scanning direction. (b) EQE plots of PSCs based on TiO2 ESL and TiO2/C70 ESL. (c)

Stabilized PCEs and photocurrent densities of PSCs based on TiO2/C70 ESL measured

at 0.85 V bias with inset showing statistical deviation of PCEs and photocurrent

densities. (d) Nyquist plots of PSCs based on TiO2 ESL and TiO2/C70 ESL in dark at

0.2 V bias with inset showing a magnification of the high-frequency region.

Figure 5a and S4 show the J-V curves of champion PSCs based on TiO2 or TiO2/C70

as ESLs under standard one sun AM 1.5G simulated solar irradiation. The TiO2-based

PSCs exhibits power conversion efficiency (PCE) of 12.12% (forward scanning 19



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direction) or 15.18% (reverse scanning direction) in Figure S4a. By inserting an C70

interlayer, the PCE attains to 16.76% (forward scanning direction) or 18.28% (reverse

scanning direction) in Figure 5a. Figure S4b displays the enhancement of champion

TiO2/C70-based PSCs’ performance, obviously ascribed to the improvement of VOC,

JSC and FF as shown in Table 1. The EQE spectrum of PSCs based on selected ESLs

was further measured as presented in Figure 5b. The integrated current densities estimated from EQE spectrum are respectively 20.32 mA cm-2 and 21.57 mA cm-2 for

TiO2-based and TiO2/C70-based PSCs, which basically agree with the current densities

in Table 1. The enhancement of JSC could be interpreted by efficient electron transfer,

low recombination loss and high perovskite grain quality with the C70 modification as

analyzed in Figure 2-4, especially for the enhancement from 450 nm to 800 nm in

Figure 5b. In consideration of the accuracy of J-V measurements for TiO2/C70-based

PSCs, Figure 5c shows the stabilized PCE of above 18% measured by the maximum

power point tracking method. During the continuous one sun illumination for 1200 s

without any isolation from the ambient environment, the current density (~21.73 mA cm-2) slightly fluctuated at a stable bias of 0.85 V as shown in Figure 5c-inset. Thus,

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the stabilized PCE is basically consistent with the calculated value from the measured

J-V curves. To further confirm the influence of C70 interlayer on photovoltaic

performance, EIS characterization was applied to evaluate the interfacial dynamics (charge recombination losses and charge transfer properties) as shown in Figure 5d.62

During EIS measurement, a small AC voltage of 20 mV was applied to measure the

impedance with a range of frequency from 1 MHz to 0.1 Hz at 0.2 V DC bias under

dark condition. Figure 5d presents the Nyquist plots (imaginary vs real part of the

impedance) of TiO2-based and TiO2/C70-based PSCs. The impedance plots have two

distinct features; a small arc at high frequency originated from the charge transfer

kinetics, and a large arc at low frequency originated from the charge recombination kinetics.39 Figure S5a shows the fitted Nyquist plots based on the fitted equivalent

circuit as shown in Figure S5b, and the fitted values are listed in Table S1. The fitted

equivalent circuit model contains three parts; series resistance (RS), transfer resistance

(Rtr) paralleled with a constant phase angle element (CPEtr) and recombination resistance (Rrec) paralleled with another constant phase angle element (CPErec).63 Rtr

and Rrec are ascribed to high-frequency arc and low-frequency arc, respectively.

21



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Owing to the identical perovskite/HTL interface in both types of PSCs, Rtr and Rrec

mainly reflect charge transfer and recombination kinetics at ESL/perovskite interface,

respectively. Although the values of Rtr are close to each other for two types of PSCs,

Rrec of TiO2/C70-based PSCs is nearly twice as high as the one of TiO2-based PSCs, meaning the decrease of charge recombination loss by insertion of C70 interlayer.56

The suppression of charge recombination reflects the interfacial effect of C70

interlayer at ESL/perovskite interface in accordance with the enhancement of VOC and FF in J-V measurement,63-64 as well as the results of steady-state PL measurements.

22


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Figure 6. Histograms of the parameters based on the statistics of 100 PSCs

employing TiO2 or TiO2/C70 as ESLs. (a) open-circuit voltage (VOC), (b) short-circuit

current density (JSC), (c) fill factor (FF) and (d) Power conversion efficiency (PCE).

Moreover, the reproducibility and uniformity of PSCs’ performance were revealed by

characterizing 100 cells, and the statistical data was displayed in Table 1 and Figure 6.

The stable enhancement of all the parameters from J-V measurement is ascribed to

faster interface electron transfer, lower charge recombination loss and higher

perovskite quality by adding the C70 interlayer between TiO2 and perovskite as is

mentioned above. In order to further verify the influence of C70 interlayer on PSCs’

performance, the current hysteresis was evaluated by hysteresis index as shown in

Figure S6. With the advantage of C70 interlayer, the average value of hysteresis index

was 76% reduced in compare with TiO2-based PSCs. Beside of the influence of

perovskite grain boundary, the application of fullerene as an interfacial modification

layer between SMO ESL and perovskite can passivate the interfacial trap states to reduce the hysteresis and enhance PCE.18-19, 22 With the benefit of low-temperature

solution-processed TiO2/C70 ESLs, high-efficiency flexible PSCs were fabricated on

23



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Page 24 of 40

ITO/PEN substrates. Figure 7a presents J-V curve of the flexible PSCs. The champion performance was attained by Voc=1.05 V, Jsc=19.19 mA cm-2, FF=61.71% and

PCE=12.42%. As shown in Figure 7b, a prototype of flexible PSCs is demonstrated to

be bended freely.

Figure 7. (a) J-V curves of the flexible PSCs based on a TiO2/C70 ESL coated on a

ITO/PEN substrate. (b) Photograph of a prototype of flexible PSCs.

Table 1. Photovoltaic parameters of PSCs based on selected ESLs.

ESLs

Scan direction

VOC

JSC

FF

PCE

[V]

[mA cm-2]

[%]

[%]

F (Champion)

0.93

22.78

57.35

12.12

R (Champion)

0.99

22.61

67.66

15.18

F (Champion)

1.05

23.13

69.32

16.76

R (Champion)

1.07

23.10

73.80

18.28

TiO2

R (Average)

0.98

21.98

62.36

13.48

TiO2 / C70

R (Average)

1.06

22.60

71.88

17.24

TiO2

TiO2 / C70

* F and R indicate forward scanning direction and reverse scanning direction, respectively. 24


Page 25 of 40

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4. Conclusions

In summary, we demonstrated the low-temperature solution-processed TiO2/C70 ESLs

for high-efficiency planar n-i-p PSCs. The modification with C70 interlayer was used

to optimize the interface between TiO2 ESL and perovskite, in which the regulations

of surface flatness, electronic conductivity, interfacial electron transfer, recombination

loss and perovskite grain quality were achieved via interface engineering. Finally, the

efficiency was enhanced 28% and current hysteresis was suppressed 76%, resulted

from the insertion of C70 interlayer. With the high reproducibility and uniformity of

PSCs’ performance, the champion efficiencies of PSCs were archived by 18.28% and

12.42% on rigid substrates and flexible substrates, respectively. This research work is

believed to shed light on the further boost of the commercialization for

high-efficiency flexible PSCs. ASSOCIATED CONTENT

Supporting Information

Images of water and DMF droplet contact angles on TiO2 ESLs and TiO2/C70 ESLs.

Cross-sectional SEM images of inner structure of TiO2/C70-based PSC and TiO2

25



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-based PSC. Cross-sectional SEM images of TiO2 and TiO2/C70 ESLs on ITO-glass.

J-V curves of champion PSCs based on TiO2 ESL in forward/reverse scanning

direction, and TiO2 ESL or TiO2/C70 ESL in reverse scanning direction. The measured

and fitted Nyquist plots of PSCs based on TiO2 or TiO2/C70 ESLs in dark at 0.2 V bias.

The equivalent circuit model for PSCs in EIS measurements. Histograms of hysteresis

index based on the statistics of 50 PSCs employing TiO2 or TiO2/C70 as ESLs. Fitted

data of EIS measurements of PSCs based on TiO2 or TiO2/C70 ESLs.

AUTHOR INFORMATION

*Corresponding author: Da-Qin Yun

E-mail: [email protected]

Author Contributions #

Ya-Qing Zhou and Bao-Shan Wu contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

26


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The Fundamental Research Funds for the Central Universities (2013121031) are

acknowledged for ﬁnancial support. Dr. Yun is grateful to the China Scholarship

Council for funding. Prof. Nan-Feng Zheng is acknowledged for his assistance in the

EQE measurements. Prof. Su-Yuan Xie is also gratefully acknowledged for his

assistance in the J-V characteristic measurements.

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Enhancing Performance and Uniformity of Perovskite Solar Cells via a

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