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Fluoro and Amino-Functionalized Conjugated Polymers as Electron Transport Materials for Perovskite Solar Cells with Improved Efficiency and Stability Li Tian, Zhicheng Hu, Xiaocheng Liu, Zixian Liu, Peipei Guo, Baomin Xu, Qifan Xue, Hin-Lap Yip, Fei Huang, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19036 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Fluoro

and

Amino-Functionalized

Conjugated

Polymers as Electron Transport Materials for Perovskite Solar Cells with Improved Efficiency and Stability Li Tian1, Zhicheng Hu1,2, Xiaocheng Liu1, Zixian Liu1, Peipei Guo1, Baomin Xu3, Qifan Xue1, Hin-Lap Yip1,* Fei Huang1,2,* and Yong Cao.

1Institute

of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China

2South

3

China Institute of Collaborative Innovation, Dongguan, 523808, PR China

Department of Materials Science and Engineering, Southern University of Science and

Technology, Shenzhen, Guangdong Province 518055, PR China 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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KEYWORDS electron transport materials, hydrophobic polymer, fluoro- and amino- side chains, perovskite solar cells, conjugated polymers

Abstract

We report here novel bifunctionalized electron transport materials (ETMs) that can improve the efficiency and stability of perovskite solar cells (PVSCs) simultaneously. By functionalizing n-type conjugated polymers with fluoro- and amino-side chains, PN, PNF25% and PN-F50% with varied content of fluoro- and amino-side chains are prepared. It is found that the amino side chains in ETMs efficiently improve the interface contact and electron collection of PVSCs, with improved PCE from 14.0% for PC61BM-based devices to over 17% for PN and PN-F25% based devices. Moreover, the fluoro side chains endow these polymers with excellent hydrophobic properties, which largely enhance their water-resistance capability. ETMs with increased content of fluoro side chains can substantially improve the water resistance of perovskite layer, with significant 2


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improvement of stability in PVSCs. Our results indicate the fluoro and aminobifunctionalized strategy is a promising method to design ETM for high-performance and stable PVSCs.

Introduction

Perovskite solar cells (PVSCs) have received wide attention in the past several years due to their great advantages of low-cost processing and high efficiency.1 Hybrid organometal trihalide perovskite (CH3NH3PbX3, X=Cl, Br, I) materials possess fascinating properties of high absorption coefficient2, long electron/hole diffusion length3 and high charge carrier mobility4, all of which are tailored for photovoltaic application. Moreover, perovskite materials can be processed from vacuum deposition5 and solution processing 6,7, among which the latter is compatible for mass production of large–area solar cell modulation.

Two major types of device architecture are adopted to fabricate high-performance PVSCs.2,5 The conventional n-i-p type PVSCs employ mesoporous/compact TiO2 as ETM 3



Page 4 of 48

to support the growth of perovskite layer.8 The inverted p-i-n PVSCs with different device polarity employ hole transport layer as substrate, upon which perovskite layer are fabricated by solution protocols or vacuum deposition. 9 In particular, the inverted planarheterojunction PVSCs have attracted increasing attention due to advantages of lowtemperature solution-processable electron/hole transport layers recently.10,11

Interface engineering shows great importance to the performance of PVSCs.12 Insertion of electron/hole transport materials (ETMs/HTMs) between the perovskite layer and electrodes can improve electron/hole collection and extraction, reduce interface recombination and hysteresis, improve the performance of PVSCs.13-15 A wide range of interface materials have been developed to optimize the interface contact between perovskite layer and electrode. For example, conjugated small molecules16-19 (such as 2,2’,7,7’-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’-spirobifluorene, spiro-OMeTAD16 ) and conjugated polymers20,21 are widely reported HTMs for PVSCs. For the inverted PVSCs,

HTMs

such

as

poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate

4


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(PEDOT:PSS)3,15, water/alcohol soluble conjugated polymers22,23 and inorganic semiconductors24,25 are widely reported.

Compared to the wide range of HTMs for PVSCs, ETMs are less explored. In conventional perovskite solar cells, compact or mesoporous TiO2, ZnO26, Ta-WOx, and SnO2 etc27-28 can support the formation of high-quality perovskite films. Fullerene derivatives

29-34,

conjugated small molecules35-41 and polymers42-44 with high electron mobility are regarded as efficient ETMs to improve electron collection from perovskite layer for inverted PVSCs. These organic ETMs can be fabricated upon perovskite layer via solution processing techniques. Moreover, amino-functionalized ETMs showed enhanced capability to reduce the interface resistance and improve electron collection of PVSCs.4547

It was reported that amino-functionalized ETMs can passivate the surface traps of

perovskite47, reduce the work function of the metal cathode48-50, resulting in improved electron collection and device performance.

5



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Stability is another important criteria that limit the development of PVSCs.51-54 Coating a thin layer of hydrophobic polymers upon perovskite layer/cathode is proved to be an efficient strategy to improve the stability of PVSCs.55-60 Hydrophobic polymers possess good water resistance capability and solution processability.61-62 However, the insulating properties of the reported hydrophobic polymers for PVSCs limited their thickness in perovskite devices, whereas a thicker hydrophobic layer are potentially better candidate to improve the stability of PVSCs. Moreover, the additional one processing step for hydrophobic polymers is not beneficial for the fabrication of PVSCs. Therefore, it is of great urgency to develop bifunctional ETMs with interface modification and water resistance capabilities for high-performance PVSCs.

Herein, we reported novel bifunctional amino and fluoro functionalized conjugated polymers that act as ETM and improve the efficiency and stability of CH3NH3PbI3-xClx PVSCs

simultaneously.

By

functionalizing

n-type

conjugated

polymers

with

perfluorooctane (C8F17-) and amino side chains, PN, PN-F25% and PN-F50% with varied content of fluoro- and amino side chains are obtained. The amino-functionalized 6


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conjugated polymer can efficiently improve the interface contact and electron collection of PVSCs, with improved PCE of 14.0% for PC61BM-based devices to over 17.3% for devices with amino-functionalized ETMs. Moreover, we found that ETMs with increased content of fluoro side chains can substantially improve the water resistance of perovskite layer, with significant improvement in stability in perovskite device. These results indicate the fluoro- and amino- functionalized strategy is a promising method to design ETMs for high-performance and stable PVSCs.

Experimental section

Synthetic procedure for polymers

Polymer PN, PN-F25% and PN-F50% were prepared using direct C−H arylation polymerization according to the reported reference.63 Generally, dibromide monomer (M8, 0.12 mmol, 87.0 mg) and dithiophene monomer (M2, 0.12 mmol, 66.6 mg; M5, 0.12 mmol, 170.8 mg) were dissolved in dry chlorobenzene (1 mL) followed by the addition of pivalic acid (0.12 mmol) and K2CO3 (0.30 mmol). The mixture was degassed for two times

7



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to remove the dissolved oxygen. Catalyst Pd2(dba)3 (5 mg) was added, the mixture was then degassed for another two times and stirred under 110 °C for 24 h. After cooling to room temperature, the mixture was then poured into 200 mL of methanol. The crude polymer was precipitated into methanol as blue solid, which was then extracted using Soxhlet extraction successively from methanol, hexane, and chloroform. The chloroform fraction was then concentrated and precipitated into methanol again. The blue precipitate was then collected and dried under vacuum for 12 h to get the target polymer.

PN, yield (110.2 mg, 88%). 1H NMR (500 MHz, CDCl3, δ ppm) 8.92-8.70 (m, 2H), 7.777.36 (m, 10H), 4.25-4.10 (m, 4H), 2.88-2.45 (m, 12H), 2.13-2.01 (m, 4H), 1.77-1.04 (m, 48H), 0.86-0.76 (m, 6H), 0.76-0.65 (m, 4H). GPC(CHCl3), Mn=12.9 KDa, Ð=2.0.

PN-F25%, yield (145.7 mg, 82%).1H NMR (500 MHz, CDCl3, δ ppm) 8.93-8.70 (m, 2H), 7.79–7.05 (m, 10H), 4.32-4.02 (m, 4H), 3.66-3.57 (m, 2H), 3.37-3.28 (m, 2H), 2.62–0.90 (m, 64H), 0.82-0.58 (m, 4H);19F NMR (471 MHz, CDCl3) δ -80.72, -80.74, -113.42, 121.69, -121.91, -122.70, -123.61, -126.08. GPC(CHCl3), Mn=20.7 KDa, Ð=2.9.

8


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PN-F50%, yield (180.4mg, 78%). 1H NMR (500 MHz, CDCl3, δ ppm) 8.90-8.79 (m, 2H), 7.80-7.05 (m, 10H), 4.30-4.06 (m, 4H), 3.70-3.52 (m, 4H), 3.42-3.23 (m, 4H), 2.93-2.52 (m, 12H), 2.43-2.22 (m, 4H), 2.16-1.95 (m, 4H), 1.80-1.00 (m, 40H), 0.71 (m, 4H);19F NMR (471 MHz, CDCl3) δ -80.74, -113.41, -121.70, -121.91, -122.70, -123.61, -126.09. GPC (CHCl3), Mn=12.4 KDa, Ð=2.2.

Characterization of monomers and polymers

The 1H NMR,

13C

NMR and

19FNMR

spectra of monomers and polymers were recorded

on a Bruker AVANCE Digital NMR workstation operating at 500, 126 and 471 MHz, respectively. The molecular weights of the three polymers were collected from a Waters GPC 2410 where chloroform was used as mobile phase. The UV-vis absorption spectra of these polymers were measured using a HP 8453 spectrophotometer. The reduction and oxidation potentials of these polymers were estimated by electrochemical cyclic voltammetry (CV) on a CHI66E electrochemical workstation. ITO slides coated with polymer films were used as working electrodes, a saturated calomel electrode (SCE) was

9



chosen as the reference electrode and a platinum wire electrode was used as the counter electrode. The CV tests were conducted in a 0.1 M acetonitrile solution of tetrabutylammonium hexafluorophospate (Bu4NPF6) under the protection of nitrogen. The photoluminescence (PL) spectra was collected in FLS920 spectrofluorimeter. The perovskite layers were excited at a wavelength of 515 nm. Electron spin resonance (ESR) spectroscopies of the polymers in solid states were conducted on a JEOL JES-FA200 ESR spectrometer (300 K, 9.063 GHz, X-band) at room temperature. The water contact angle measurements of these polymers and PC71BM were performed using a DATAPHYSICS OCA40 Micro surface tension tester. The work function of Ag electrodes with/without a thin layers of polymers were measured using the scanning Kelvin probe measurement (SKP 5050, KP Technology).

O

N PN

N

N

F F

F

F

F F

F F

F F F F

O 3(

F F F F

F F F

F F F

F F F N

O

O

3

(

O S

F F

F F

(

C8H17

F F

O S

S

n

O

O

N

O

O

C8H17 S

N

N

C8H17

S

N

O

N

S N

F F

F F F F

F F

F F

F

F

F F

F F

F F F F

O 3( O

0.5

O

F

F F F F

F F F

F F F

N

O 3

O

(

C8H17

F F

(

N

F

(

F F F

(

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 48

0.5

S

n

O

O

N PN-F25%

N

O S

N

n

O

N PN-F50%

10


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Scheme 1 Chemical structures of PN, PN-F25% and PN-F50%

Fabrication and characterization of PVSCs

PVSCs with architecture of ITO/NiOx/Perovskite/ETM/Ag (shown in Figure 1) were fabricated according to our published reference,64 where NiOx was used as hole transport layer. The thickness of perovskite layer is 350 nm. ETMs were dissolved in CB and spincoated upon perovskite layer with thicknesses of 80 nm. The J-V characteristics for PVSCs were measured under AM 1.5G illumination (100 m W/cm2) using a solar simulator (Oriel model 91192). The external quantum efficiency (EQE) were measured with a monochromator (Newport, Cornerstone 130) linked to the same xenon lamp and a lock-in amplifier (Stanford Research Systems, SR 830) coupled to a light chopper. The long-term stability of unencapsulated PVSCs with different ETMs was monitored in air under ambient conditions (temperature ≈25 °C, humidity ≈40%).

11



Ag

80nm

PCBM/PN/PN-F25%/PN-F50%

80nm

CH3NH3PbI3-xClx

350nm

Page 12 of 48

NiOx ITO Glass

Figure 1 The device architecture of PVSCs used in this study

Results and discussion

Synthesis and characterization

PN, PN-F25% and PN-F50% were synthesized using direct C−H arylation polymerization according to the reported reference.63 The fluoro- and amino-based monomers were firstly synthesized using multi-step reactions (experimental details and characterization are shown in Support information). The direct C−H arylation polymerizations of these polymers from dibromide monomers and dithiophene monomers were performed in chlorobenzene with Pd2(dba)3 as catalyst and the existence of pivalic acid and K2CO3.

12


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The chemical structures of these polymers were confirmed by 1H NMR and 19F NMR. The resulted polymers possess good solubility in common non-polar or low-polar solvents (such as chloroform, chlorobenzene and THF), which render their good processability and good film-forming capability. The number-average molecular weights (Mns) and weightaverage molecular weights (Mws) of these polymers were obtained from GPC where CHCl3 as mobile phase. As shown in Table 1, the number-average molecular weight of PN, PN-F25% and PN-F50% is 12.9 kDa, 20.7 kDa and 12.4 kDa, respectively.

Figure 2 UV-vis absorption spectra of PN, PN-F25% and PN-F50% in chloroform (a) and as thin-film state (b). 13



Page 14 of 48

Optical properties and electrochemical properties

The optical absorption spectra of these polymers in solution and as thin film state were investigated using UV-vis absorption spectroscopy. As shown in Figure 2, it can be found that the three polymers showed almost the same absorption spectra in chloroform. These polymers yield two absorption bands with peaks at around 370 and 610 nm in solution, of which the form could be attributed to characteristics of π−π* transition in polymer backbones. The latter broad absorption bands located from 500 to 750 nm are the characteristic

of

donor-to-acceptor

intramolecular

charge

transfer

inside

their

backbones.65 In the thin film of these polymers, very similar but slightly red-shifted absorption bands to those in solution can be observed. Form the thin-film absorption edges of thin films, the optical band gaps of PN, PN-F25% and PN-F50% can be calculated from the absorption edges of their thin films, which are determined to be 1.68, 1.69 and 1.70 eV, respectively.

14


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The energy levels of the polymers were estimated by cyclic voltammetry (CV) analysis with a saturated calomel electrode as the reference and ferrocene/ferrocenium (Fc/Fc+) reference as an internal standard in 0.1 mol/L Bu4NPF6 acetonitrile solution. The oxidation and reduction potentials of these polymers are presented in Figure S1 and summarized in Table 1. The oxidation potentials of PN, PN-F25% and PN-F50% were estimated to be 1.14, 1.18 and 1.16 V, respectively. Accordingly, the highest occupied molecular orbital energy level (HOMO) energy levels of them were calculated to be -5.65, -5.69 and -5.67 eV, respectively. The reduction potentials of these polymers were -0.64, -0.63 and -0.60 V for PN, PN-F25% and PN-F50%, respectively, indicating lowest unoccupied molecular orbital energy level (LUMO) energy levels of -3.87, -3.88 and -3.91 eV, respectively. These results indicate that the side chains engineering showed limited impact on the optoelectronic properties of the polymers. The almost identical absorption spectra and energy levels of these polymers provide great convenience to study the effect of fluoroand amino-side chains on the photovoltaic performance with ignorance of conjugated backbones. 15



(a)

86.3°(b)

80.4° (c)

Page 16 of 48

104.3° (d)

138.4°

Figure 3 Water contact angles of PC61BM(a), PN(b), PN-F25%(c) and PN-F50% films(d).

Water contact angles

Fluoro-based polymers share features of hydrophobic properties and are widely used as water repellent materials.66 Due to the hydrophobic features, fluoro-based side chains could possess excellent water-repellent properties, which potentially render their application in perovskite with improved stability. Here, we firstly investigated the watercontact angles of thin films based on these polymers (Figure 3). For comparison, the common used ETM-PC61BM was also tested, which delivered a water contact angle of

16


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86.0°. For polymer PN with solely amino-side chains, the water contact angle were determined to be 80.8°, which is similar to that of PC61BM. After introducing fluoro-side chains, PN-F25% and PN-F50% showed apparently enlarged water contact angles of 104.4° and 138.1°, respectively, indicating gradual increasing hydrophobic properties for polymers with more fluoro side chains. The improved hydrophobic properties of PN-F25% and PN-F50% based ETMs will potentially improve the water-resistance of PVSCs.

Stability of perovskite layer

The fluoro-functionalized ETMs are endowed with excellent hydrophobic properties. To examine the water-resistance properties of fluoro-functionalized ETM, perovskite layers protected by a thin layer of 80-nm PC61BM, PN, PN-F25% and PN-F50% were fabricated (ITO/NiOx/Perovskite/ETM) and tested under humidity of 98%. UV−vis absorption spectroscopy was used to identify the absorption spectra evolution of perovskite layers as a function of time. As shown in Figure 4a, obvious decline in the absorption of perovskite layer with a PC61BM protection layer can be observed. After 5 and 10 h

17



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testing, the absorption intensity remains only 81% and 58% (calculated in wavelength of 550 nm) of its original spectrum. Similar phenomenon can be also observed in perovskite layer with a PN protection layer. This is because that the amine groups in the side chains of PN showed poor hydrophobic properties. However, with the protection of fluoropolymers PN-F25% and PN-F50%, the stability of perovskite layers were much improved. As shown in Figure 4c, with the protection of PN-F25%, the absorption of perovskite layer/PN-F25% can maintain 93% and 87% after 5 and 10 h test. Surprisingly, with the protection PN-F50%, it can be observed that the absorption spectra of perovskite layer/PN-F50% showed almost no changes after 10 h storing. These results revealed that fluoro-polymers could effectively prevent moisture penetrating to the perovskite layer, which potentially can improve the stability and extend the lifetime of PVSC.

Table 1 Molecular weights, absorption, optical band gaps and energy levels of PN, PNF25% and PN-F50%

18


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Mn(KDa)

λabs (nm)

λabs (nm)

Eg

Eox

Ere

LUMO

HOMO

in solution

in film

(eV)

(V)

(V)

(eV)

(eV)

12.9(2.0)

383 624

388 631

1.68

1.14

−3.87

-5.65

20.7(2.9)

374 602

388 612

1.69

1.18

-3.88

-5.69

12.4(2.2)

383 601

388 631

1.70

1.16

-3.91

-5.67

ELM (

PN

PNF25% PNF50%

Ð)

0.64 0.63 0.60

19



Page 20 of 48

Figure 4 UV-Vis-NIR absorption spectrum of perovskite layers protected by PC61BM (a), PN (b), PN-F25% (c) and PN-F50% (d) films.Photovoltaic performance

To investigate the photovoltaic performance of PVSCs with these polymers as ETL, devices with architecture of ITO/NiOx/perovskite/ETM (≈80 nm)/Ag were fabricated. The perovskite layer were prepared by dissolving the mixture of CH3NH3I, PbI2 and PbCl2 (molar ratio 4:1:1) at a total concentration of 40wt% and spin-coated atop of NiOx with thicknesses of 350 nm. The devices were tested under a simulated AM 1.5G illumination at 100 mW/cm2. The current density–voltage (J-V) curves were shown in Figure 5 and the device parameters were summarized in Table 2. For comparison, PVSCs with PC61BM as ETM was also fabricated, which delivered an open-circuit voltage (Voc) of 1.05 V, a short-circuit current density (Jsc) of 21.5 mA/cm2, and a fill factor (FF) of 0.615, respectively. Consequently, a PCE of 14.0% was achieved, which is comparable to those with similar device structure in the literature. PVSCs with polymeric ETM all showed enhanced photovoltaic performance in comparison with PC61BM-based PVSCs. The PCEs of PN and PN-F25% functionalized PVSCs are enhanced to 17.3% and 17.5%, 20


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respectively. And the J–V curves of the best device with PN-25% was measured under reversed voltage bias. The hysteresis property was tested by sweeping the J–V measurement from both forward and reversed direction (Figure S2).We can find that the hysteresis effect of our device is insignificant. The time dependence of the power output of the devices with different ETM were further measured by tracking their efficiencies under maximum power‐point conditions (Figure S3). All of the device showed a highly stable power output. Compared with PCBM, PN, PN-F25% and PN-F50% showed similar PL quenching efficiency of perovskite layer (Figure S4), then we further tested the timeresolved PL decay transients for each devices with different ETM (Figure S5). The samples were pumped at 505 nm and probed at 756 nm, indicating amino-functionalized ETMs possess efficient electron extraction capability.67 Noted that the fill factors (FFs) in PN and PN-F25% based devices were much improved than that in PC61BM-based devices. This can be attributed to the enhanced electron collecting properties enabled by the insertion of amino–functionalized ETMs. Amino functionalized polymers possess selfdoped behaviours, as indicated by the electron spin resonance results in Figure S6. In 21



Page 22 of 48

addition, they are proved to be able to modify the metal electrode, leading to lowered work function, which can enhanced electron collection in the cathode. 49, 68 Moreover, the amino group can also enable good interface contact with perovskite layer by reducing the contact resistance and promoting ohmic contact formation69, which resulted in a reduction of series resistance (Rs) of the solar cells. The Rs of PN, PN-F25%, PN-F50% and PCBM based devices are 8.5 Ω.cm2, 9.0Ω.cm2, 8.3Ω.cm2 and 11.2 Ω.cm2, respectively. The current density of PN and PN-F25% based devices are more than 22 mA/cm2, slightly higher than that in PC61BM-based devices. The external quantum efficiency (EQE) spectra of these devices were also collected. As shown in Figure 5b, it can be observed that EQE responses in PN and PN-F25% based devices are higher than that in PC61BMbased devices, corresponding well to the higher Jsc in PN and PN-F25% based devices.

22


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Figure 5 The J–V characteristics (a) and EQE curves (b) of PVSCs with PC61BM, PN, PN-F25% and PN-F50% as ETMs.

Unfortunately, PN-F50% based PVSCs showed slightly reduced efficiency and FF than PN and PN-F25% based devices, indicating that ETM with excessive fluoro side chain could result inferior electron collection capability. The electrode modification capability of Ag electrode coated with PN, PN-F25% and PN-F50% was evaluated by Kelvin probe measurement. It can be observed that the work function of Ag electrode can be efficiently reduced from 4.61 eV (for bare Ag) to 4.25 and 4.35 eV for PN and PN-F25% modified Ag electrodes. However, for the PN-F50% modified Ag electrode, the work function was only slightly reduced to 4.55 eV, indicating that PN-F50% possesses weak electrode 23



Page 24 of 48

modification capability even though it possesses amino chains in its side chains. Regrading to the electrical properties of the ETMs, the measured SCLC electron mobilities of the PN, PN-F25%,PN-F50% are 8.8 × 10 −5 cm 2 V −1 s −1, 8.8 × 10 −5 cm 2 V −1

s

−1

and 2.0 × 10

−5

cm

2

V

−1

s

−1,

respectively, which are at the same order of

magnitude. Therefore, we attribute that the less effective workfunction modification of the cathode with increasing content of the fluoro-side chains might be the main reason for the decrease in device performance70. Nevertheless, our results indicate that careful regulation on the ration of amino and fluoro-side chains (such as PN-F25%) can result excellent ETMs for PVSCs with improved performance.

To give an intuitive observation of the stability improvement using fluoro-functionalized ETM, we tested the stability of PVSC devices in water. As shown in Figure 6a, the colour of PC61BM-based devices was immediately changed from black to yellow when immersed in water. This is because that water molecules can be easily permeated into perovskite layers and destroy its crystal structure. However, PN-F25% based device showed no change in the colour after 5 min, indicating that the insertion of fluoro-functionalized ETM 24


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can efficiently prevent the permeation of water molecules into the perovskite layer. These results indicated that fluoro- and amino-functionalized conjugated polymer not only act as an efficient ETMs to improve the performance of PVSCs, but also can enhance the device stability by insulating perovskite layer from water/humidity. Table 2 device parameters of PVSCs with PC61BM PN, PN-F25% and PN-F50% as ETMs.

ETM

Voc(V)

Jsc(mA/cm2)

FF (%)

PCE (%)

PC61BM

1.05

21.5

61.5

14.0

PN

1.07

22.4

72.1

17.3

PN-F25%

1.10

22.1

71.8

17.5

PN-F50%

1.08

21.6

68.1

15.9

To study the impact of these ETMs on the long-term stability of perovskite solar cell, we test the device performance with different storage time. As shown in Figure 6b, the PC61BM based devices showed gradually decreased PCE in the initial storage less than 20 h. Moreover, after 300 h storage the PCE of PC61BM-based device was dropped to

25



Page 26 of 48

less than 40% of its initial PCE. Similar trend can be also observed in PN-based devices. Surprisingly, PVSCs with fluoro-functionalized ETM showed improved device stability. In the initial storage time less than 20 h, PN-F25% and PN-F50% based devices showed much reduced PCE loss, while after 300 h storage, 73% and 78% of the initial PCE can be maintained for PN-F25% and PN-F50% based devices. These results indicate that fluoro-functionalized ETM can efficiently promote the stability of PVSCs, implying that designing fluoro-based conjugated polymers is a novel strategy to prepare ETMs for stable PVSCs.

26


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Figure 6 (a) pictures of perovskite devices immersed in water; left: perovskite devices with PC61BM as ETMs, right: perovskite devices with PN-F25% as ETMs; (b) Long-time stability of perovskite solar cells with PC61BM, PN, PN-F25% and PN-F50% as ETMs.

Conclusions

In summary, we have designed a series of amino and fluoro-functionalized n-type conjugated polymers for the application of electron transport materials in PVSCs. It was found that the amine-side chains can endow these polymers with excellent interface modification capability, while the fluoro-side chains endow these polymers with hydrophobic properties. Consequently, the bifunctional conjugated polymers successfully enable improved performance (17.5%) of PVSCs, which is higher than that in PC61BM based devices (14.0%). Moreover, the bifunctional ETMs also result in greatly improved stability of PVSCs. Our results indicate the fluoro- and amino-bifunctionalized strategy is a promising method to design ETMs for high-performance and stable PVSCs.

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author Fei Huang * e-mail: [email protected],

Hin-Lap Yip * e-mail: [email protected]

ACKNOWLEDGMENT

This work was financially supported by the Natural Science Foundation of China (No. 21634004, 21490573, 91633301, 51803060 and 21761132001), the Science and Technology Program of Guangzhou, China (No. 201607020010), the Foundation of Guangzhou Science and Technology Project (No. 201707020019), and Peacock Team Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. KQTD2015033110182370).

Support information

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Experimental details, 1H NMR,

13C

NMR,

19F

NMR of monomers and polymers; cyclic

voltammetry curves of polymers; steady state PL, spectra hysteresis, steady-state current density, PL decay and SCLC results of devices.

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