Side-Chain Polymers as Dopant-Free Hole-Transporting Materials for

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Side Chain Polymers as Dopant-Free Hole Transporting Materials for Perovskite Solar Cells – The Impact of Substituents’ Positions in Carbazole on Device Performance Jianchang Wu, Chang Liu, Bo Li, Fenglong Gu, Luozheng Zhang, Manman Hu, Xiang Deng, Yuan Qiao, Yongyun Mao, Wenchang Tan, Yanqing Tian, and Baomin Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07859 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Side Chain Polymers as Dopant-Free Hole Transporting Materials for Perovskite Solar Cells – The Impact of Substituents’ Positions in Carbazole on Device Performance Jianchang Wu‡a,b,c, Chang Liu‡d,a,c, Bo Lie, Fenglong Gue, Luozheng Zhangd,a,c, Manman Hua,c, Xiang Denga,c, Yuan Qiaoa,c, Yongyun Maoa,c, Wenchang Tanb,*, Yanqing Tiana,c*, Baomin Xua,c* aDepartment

of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province 518055, China

bSchool

of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen, Guangdong Province 518055, China

cShenzhen

Engineering Research and Development Center for Flexible Solar Cells, Southern University of Science and Technology, Shenzhen, Guangdong Province 518055, China

dAcademy

for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen, Guangdong Province 518055, China

eKey

Laboratory of Theoretical Chemistry of Environment, Ministry of Education; School of Chemistry & Environment of South China Normal University, Guangzhou 510006, China. (E-mails: [email protected], [email protected], [email protected]) ‡These authors contributed equally.

KEYWORDS: perovskite solar cells, hole-transporting material, dopant-free, side-chain polymer, substituents’ positions in carbazole

ABSTRACT Side chain polymers have the potential to be the excellent dopant-free hole transporting materials (HTMs) for perovskite solar cells (PSCs) due to their unique characteristics, such as tuneable energy levels, high charge mobility, good solubility and excellent film-forming ability. However, there were few researches focusing on side chain polymers for PSCs. Here, two side chain polystyrenes with triphenylamine substituents on carbazole moieties were designed and characterized. The properties of side chain polymers were tuning finely, including the photo-physical and electrochemical properties and charge mobilities, by changing the positions of triphenylamine substituents on carbazole. Owing to the higher mobility and charge extraction ability, the polymer P2 with the triphenylamine substituent on the 3,6-positions of carbazole unit showed higher performance with power conversion efficiency (PCE) of 18.45%, which was much higher than the 1

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PCE (16.78%) of the P1 with 2,7-positions substituted. These results clearly demonstrated the side chain polymers can act as promising HTMs for PSCs applications and the performance of side chain polymers could be optimized by carefully tuning the structure of the monomer, which provides a new strategy to design new kinds of side chain polymers and obtain high performance dopant-free HTMs.

INTRODUCTION Perovskite solar cells (PSCs) based on organic–inorganic metal halides have attracted significant attention since the first report in 2009.1 PSCs have been recognized as one of the most promising photovoltaic technologies because of their unique characteristics, such as large absorption coefficient, broad spectral absorption range, long exciton diffusion length and high charge carrier mobility.1-4 Recently, PSCs have achieved remarkable power conversion efficiencies (PCEs) of higher than 23%, which demonstrated great potential for commercialization.5 In a typical PSC device, the perovskite material was sandwiched by two selective layers of n-type/p-type semiconductors. The selective layers could help extract the electrons and holes from the interfaces between the perovskite and selective layers effectively.6-9 To date, the most commonly used hole transporting

material

(HTM)

for

highly

efficient

PSCs

is

2,2’,7,7’-tetrakis(N,N’-di-p-

methoxyphenylamine)-9,9’-spirobifluorene (Spiro-OMeTAD), which contains a spirobifluorene core and diphenylamine end group.10-13 The twist structure of spiro-core and diphenylamine groups ensure the high solubility in common organic solvent, which facilitates the process of device fabrication. However, the nonplanar configurations lead to large intermolecular distances and weak intermolecular interactions.14-17 This blocks the electron transporting between the molecules, resulting in the low charge mobility of those materials. In order to improve the charge mobility of those HTMs, several additives such as tert-butylpyridine (t-BP) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were added to the HTM Layer. In the HTM layer, LiTFSI normally functions as the p-dopant, which improves the hole conductivity of this layer; whereas t-BP enhances the hole extraction on the perovskite/HTM interface.18 However, these dopants not only make the device-fabrication process complicated but also decrease the longterm stability of the devices. The ratio of Li-TFSI, t-BP, and HTM should be tuned carefully, because the tBP-LiTFSI complexes could influence the performance and stability of the devices.18 Besides, 2


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the Li-TFSI is hygroscopic which can adsorb water and turns into liquid in seconds. This tends to expose the underlying perovskite layer to moisture, accelerating the degradation of perovskite.13, 19

Another additive, t-BP, can gradually evaporate at room-temperature, resulting pinholes in the

HTM layer, which will lead to direct contact of the perovskite layer to the metal electrode and decrease the open-circuit voltage (Voc) of PSC devices. To get stable and efficient PSCs, numerous dopant-free HTMs including small molecules20-23 and polymers24-26 have been vigorously explored. Suitable dopant-free HTMs should satisfy the following three characteristics: 1) suitable energy levels (The HOMO level of HTMs must be located above the ground-state level of the perovskite to shift the HOMO level towards the perovskite for resulting in an enhancement of Voc and the LUMO level should be higher enough than that of perovskite to block electron); 2) a high intrinsic hole mobility to deliver the photogenerated holes to the electrode effectively; and 3) high solubility in common organic solvents for device fabrication. Generally, there are several ways to get an HTM with proper frontier energy level effectively, such as extending the conjugated length,27-29 finely tuning the positions of functional groups,11 and substituting atoms with different electronegativity.30 On the other hand, hole mobility is mainly influenced by the π–π intermolecular interactions existing in the HTM layer. Constructing conjugated molecules with electron donor (D)-acceptor (A) scaffolds is an effective way to achieve high mobility via strong dipole-dipole interactions.22, 24, 26, 31-32 The PSC devices based on those D-A small molecules and polymers showed comparable performance with that of Spiro-OMeTAD. Another strategy to get HTM with high hole mobility is the introduction of fused rings, such as fused tetrathienoanthracene,27 truxene moiety,33 pyrene unit,34 and other polycyclic aromatic hydrocarbons35 to the molecules for increasing the planarity of the molecules and facilitating the π–π stacking of HTM molecules. However, the long conjugations of the rigid D–A repeating units of the conjugation polymers and/or the planar structures of fused rings make them difficult to form good thin films owing to their solubility problems.36 Recently, side chain polymers have attracted much attention and been demonstrated to be efficient and inexpensive HTMs in PSCs.36-39 In the side-chain polymers, the conjugated small molecule units are connected by the flexible non-conjugated C-C single bonds in the polymer chains. The flexible polymer chains not only ensure high solubility of the polymers and excellent 3



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film-forming ability but also induce close π–π stacking interactions between the small molecule units on the side-chains, which endows with good hole mobility. In addition, the energy level could be finely tuned by changing the structures of small molecule hole-transporting units to match the energy level of perovskites. Herein, as a continuous research on side-chain HTMs, we report the facile synthesis of two new side chain polymers based on polystyrene with carbazole (P1 and P2) and triphenylamine conjugations as the HTMs. The Carbazole with special rigid structure and large conjugated system has excellent photoelectrical properties.40-42 The difference between the two polymers are the connection positions of the triphenylamine on carbazole units. For P1, the triphenylamine segment was connected with carbazole on the 2,7-positions of carbazole; For P2, the triphenylamine segment was connected with carbazole on the 3,6-positions of carbazole unit. Thus, the conjugation lengths of the HTMs were manipulated to finely tune the energy levels. Further, the two HTMs can be synthesized in short synthetic routes (3 steps by using cheap starting materials, Scheme 1) to endow the high yields of the materials with low cost. By using the sidechain polymer strategy, the polymers possess excellent solubility in chlorobenzene and the nonchlorine green solvent of 2-methylanisole (2-MA) for achieving good film-forming ability for highly efficient devices. Also because of the hydrophobic characteristics of the two polymers, the perovskites were insulated from moisture. The devices based on the two polymers exhibited good stability when aged in ambient air of 30% relative humidity in the dark for 30 days.

RESULTS and DISCUSSIONS Materials syntheses Scheme 1 gave the detailed syntheses of the two polymers P1 and P2. Palladium-catalyzed Suzuki cross-coupling reaction was used to prepare the HTM moieties (compound 3 and 6) between 2,7dibromocarbazole (1) or 3,6-dibromocarbazole (5) and 4-[di(p-methoxyphenyl)amino]benzene-1boronic acid (2). Subsequently, substitution reaction with 4-vinylbenzylchloride (4) was utilized to obtain the monomers M1 and M2. The polymers of P1 and P2 were obtained by the free-radical polymerization of corresponding monomers with 2,2’-azobisisobutyronitrile (AIBN) as an initiator. The chemical structures of all the intermediates were confirmed by 1H and

13C

NMR and MS 4


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spectra. Polymers structures were also confirmed by 1H NMR, where the representative chemical shifts of the double bonds of the vinyl group of styrene (5.8 and 5.2 ppm) disappeared, showing the formation of polymers. The molecular weight of P1 and P2 were measured by gel permeation chromatography (GPC) to show Mn of 0.94 x 104 (Mw/Mn = 2.02) for P1 and Mn of 1.28 x 104 (Mw/Mn = 1.46) for P2. The higher Mn of P2 is mainly due to the less steric hindrance for vinyl in the monomer M2. Both target polymers exhibited good solubility in common organic solvents such as dichloromethane, chloroform, tetrahydrofuran, 2-methylanisole, chlorobenzene, and toluene. On the other hand, because of the easy synthesis of those polymers, the synthesis cost of P1 and P2 are very low as compared with the price of Spiro-OMeTAD. A detailed cost estimation for the synthesis of P1 and P2 was given in the electronic supplementary information (ESI) (Table S1†). It showed that the cost (including regents, solvents and other consumables) required for the synthesis of P1 and P2 were estimated to be only 11.4$/g and 8.9$/g (Table S1 and Table S2), respectively, which showed a promising scaled-up strategy for commercial production.

5



The synthesis of P1: H N Br ②

Br ⑦

O

O

HO B HO

+

(i)

N

O

H N

N

2,7-dibromocarbazole

1

2

O

O

3

Cl

4 n

O

O

N

N

N

yield 82%

O

(iii)

N

O

(ii) yield 90% O

N

N

N

yield 82% O

O

O

P1

The synthesis of P2:

O

HO B HO

+ ⑥ Br

O

H N

H N

Br ③

M1

(i)

N

O N

yield 80%

N

O

3,6-dibromocarbazole

2

5

O

6

O

O

Cl

(ii) yield 95%

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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4 n

N

N

(iii) yield 80%

O

O N

N

O

P2

O

O

N

N

O

M2

O

O

Scheme 1. The syntheses of polymers P1 and P2. Reagents and conditions: (i) 4-[di(pmethoxyphenyl)amino]benzene-1-boronic acid (2), Pd(PPh3)4, K2CO3, THF, reflux; (ii) NaH, 4vinylbenzylchloride (4), DMF; (iii) AIBN, THF, 85 C.

Photo-physical and Electrochemical properties UV–vis and fluorescence absorption spectra of P1 and P2 were shown in Figure 1a and the 6


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characteristic data were summarized in Table 1. In the absorption spectra, P1 showed a weak absorption peak at 291 nm and a strong one at 377 nm, whereas P2 revealed a weak shoulder peak at 310 nm and strong peak at 333 nm. UV-vis spectrum of P1 showed considerable bathochromic effect of 44 nm with respect to the spectrum of its counterpart P2, due to the triphenylamine moiety conjugated at the 2,7-positions of carbazole formed more effective and prolonged conjugation length than that of the substitution at the 3,6-positions. For the PL test, the emission maxima (λem, max) of P1 and P2 are 453 nm and 424 nm, respectively. The absorption and emission spectra of P1 and P2 thin films were also measured, the results were similar with that of in solution (Figure S1). The optical band-gap energies were determined to be 2.95 eV for P1 and 3.19 eV for P2, respectively, according to the corresponding absorption edge of P1 (453 nm) and P2 (424 nm). Cyclic voltammetry (CV) measurement was performed to determine the energy levels of HTMs experimentally. Figure 1b depicts the cyclic voltammograms (CV) of the HTMs in dichloromethane solution. The data were summarized in Table 1. As can be seen from this figure, P1 showed two reversible oxidation processes in the positive range, whereas there were three reversible oxidation peaks for P2. For P2 with substitution at the 3,6-positions of carbazole, it can form stable di-cations and tri-cations due to the delocalization of charges throughout the conjugation chain, but P1 with substitution at the 2,7-positions of carbazole can only transform into di-cations (Figure S2).43 The first oxidation potential of P1 and P2 were 0.14 eV and 0.11 eV, respectively. The smaller onset oxidation potential for P2 was caused by the fact that triphenylamine substituted at the 3,6positions of carbazole offered a stronger electron-donating ability than that of 2,7-positions. Highest occupied molecular orbitals (HOMO) of these compounds were determined from the onset oxidation (Eox onset) to be -5.24 eV for P1 and -5.21 eV for P2, which were both deeper than that of Spiro-OMeTAD (-5.01 eV)44. The deeper HOMO levels of HTMs could benefit the higher open-circuit voltage (Voc) of the device. The lowest unoccupied molecular orbital (LUMO) energies were calculated by adding the optical band-gap energies to the HOMO energies. P2 with higher LUMO energy levels (-2.02 eV) compared with P1 (-2.29 eV) could block electron flow into the metal electrode more effectively. The electronic density distributions of polymers’ frontier orbitals were calculated by using density functional theory (DFT). As the HOMOs of the side-chain polymer 7



Page 8 of 24

and the corresponding monomer were similar for CV measurement37, the optimized structures and frontier molecular orbital distribution of the monomers (M1 and M2) of P1 and P2 were studied by using DFT calculation (Figure S3). For M1 and M2, the HOMO levels show the π-bonding character, spreading over the triphenylamine and carbazole unit. For M1, LUMOs mainly distribute on the carbazole unit and styrene unit, while LUMOs of M2 mainly distribute on the styrene unit. The calculated HOMO energy levels of M1 and M2 are -4.78 eV and -4.77 eV, respectively, which shows the same trend with that measured by CV.

Figure 1 a) Normalized absorption and emission spectra of P1 and P2 in THF solution; b) Cyclic voltammograms of P1 and P2 in dry dichloromethane–TBAPF6 (0.1 M); scan speed: 100 mV/s, potentials vs. Fc/Fc+; c) TGA curves of P1 and P2; inset: DSC traces of P1 and P2; d) Double logarithmic J–V curves of HTMs based hole-only device

Table1. Spectroscopic, electrochemical and thermal data of P1 and P2. HTMs

P1

Td

Tg

λabs, onset

λabs, max

λem, max

Ega

EHOMOb

ELUMOc

[C]

[C]

[nm]

[nm]

[nm]

[eV]

[eV]

[eV]

440

188

420

291,377

453

2.95

-5.24

-2.29 8


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P2 aEstimated

436

191

389

310,333

424

3.19

-5.21

-2.02

from the absorption onset wavelength, Eg(eV)= 1240/λabs, onset (nm); bCalculated from

EHOMO = -( Eoxonset + 5.10) (ev) 45; cELUMO calculated by ELUMO=EHOMOb + Eg;

Thermal Properties Thermal properties of these HTMs were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (Figure 1c). Both P1 and P2 exhibited good thermal stability in TGA test with decomposition temperature (Td) around 400 C and glass phasetransition temperature (Tg) around 190 C. The data demonstrated that positions of the triphenylamine on carbazole did not affect polymers’ thermal properties too much. The high Tg could benefit the stable amorphous state of the HTM layer.

Charge mobility The hole-transporting properties of HTMs were evaluated by using the space charge limitation of current (SCLC) method under dark conditions, as shown in Figure 1d. The hole-only devices were fabricated

with

device

structure

of

Indium

tin

oxide

(ITO)

glasses/(poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS)/ P1 or P2 /Au. The mobility of P1 and P2 were determined to be 1.12 x 10-4 cm2ˑV-1ˑs-1 and 1.32 x 10-4 cm2ˑV-1ˑs-1, respectively, which are similar with a few other HTMs based on carbazole derivatives.36, 46 The higher mobility of P2 could be explained by that P2 can form stable di-cations and tri-cations when it received positive charges due to the delocalization of charges throughout the π-conjugation, whereas P1 can only form di-cations.43 Both the mobility of P1 and P2 were higher than that of non-doped SpiroOMeTAD (8.45 x 10-5 cm2ˑV-1ˑs-1),44 indicating a better hole-transporting capability.

Surface morphologies and charge transfer properties The surface topography and roughness of the HTMs films were investigated by atomic-force microscopy (AFM). Figure 2a and Figure 2b showed that both P1 and P2 could form smooth films with the roughness (Rq) of 1.2 nm and 1.1 nm, respectively. These smooth surfaces could not only protect the perovskite from moisture, but also provide excellent HTM/Au contacts, facilitating the 9



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charge transfer between HTMs and Au. The contact angle (see Figure S4) of P1 and P2 films were 83.7o and 79.6o, respectively, which were much higher than that of doped Spiro-OMeTAD film (14.4 o), indicating the hydrophobicity of the films.

Figure 2. a) and b) AFM images of P1 film and P2 film; c) Photoluminescence (PL) spectra of perovskite film, perovskite/P1 film, and perovskite/P2 film; d) Time-resolved PL (TRPL) spectra of perovskite film, perovskite/P1 film, and perovskite/P2 film. The charge transfers at the interfaces between the perovskite (Cs0.05FA0.81MA0.14PbI2.55Br0.45) and the hole-transporting materials were investigated by steady-state and time-resolved photoluminescence (PL) spectra. The intensities of the PL can reflect the yield of the radiative recombination and efficiency of the charge transfer at the interfaces between the perovskite and charge-selective layers. As shown in Figure 2c, P1 and P2 coated on perovskite films greatly quenched the PL emission of the perovskite films, indicating the excellent hole extraction ability. Compared with P1, P2 showed a more effective PL quenching, indicating the better hole-extraction capability of P2 than P1. From the Figure 2d, the lifetime of holes for ITO/perovskite was calculated to be 235 ns; whereas the lifetime of perovskite/HTM samples is much shorter. The decay time of perovskite/P1 and perovskite/P2 were 159 ns and 94 ns, respectively. This also indicates that P2 10


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has much better hole extraction ability than P1.

Perovskite solar cells Performance and stability The prepared polymers P1 and P2 were used as dopant-free HTMs in PSC structures, using Cs0.05FA0.81MA0.14PbI2.55Br0.45 as the absorber material. Planar PSC structure was used to test the performance of the synthesized polymers due to advantages of the fast fabrication and low temperature processing for devices. An optimized device (Figure 3a, Figure 3b) was composed of a thin tin oxide (SnO2), a perovskite layer, an HTM layer and finally a thermally evaporated Au layer. The current–voltage (J–V) characteristics of the champion PSCs that employed P1 and P2 as the HTMs were shown in Figure 3c, respectively and summarized in Table 2. The champion device based on P1 exhibited an open-circuit voltage (Voc) of 1.11 V, a short-circuit current density (Jsc) of 21.58 mA cm-2 and an FF of 0.695, resulting in a PCE of 16.78% in the reverse scan. In contrast, the P2-based device had Voc of 1.13 V, Jsc of 22.34 mA cm-2, FF of 0.727, and a PCE of 18.45%. This value is remarkably higher than that of the P1-based device, mainly owing to the higher Jsc and FF. For P2, the better hole-extraction capability facilitates the charge transfer from perovskite to HTM, and the higher hole mobility promotes the charge transport in the HTM film, which benefits the increase of Jsc and FF. In addition, P2-based device exhibited considerably weaker hysteresis behavior than that of P1 due to the improved hole mobility and the stronger electron blocking property by the higher LUMO energy level of P2. Besides using chlorobenzene to dissolve the HTMs, some non-halogen solvent, such as toluene and 2-methylanisole (2-MA) can also be used in the device optimization (Figure S5, Table S3). Using 2-MA as solvent, the PCE of the device based on P2 could achieve 16.42%, which is slightly lower than that of using chlorobenzene as solvent. The external quantum efficiency (EQE) spectra (Figure 3d) of the devices showed wide photoelectric responses to the solar spectrum with EQE over 80% among a broad spectral range of 350–750 nm. The integrated Jsc values calculated from the EQE spectra were at 20.66 and 21.87 mA cm−2, respectively, which were consistent with the experimental J−V results. To increase the performance of P1 and P2, tert-butylpyridine (t-BP) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were added to the HTM layer. Compared to the dopant-free P1 and P2, doping P1 and P2 both show better performance (Figure S6 and Table S4). 11



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The PCE increased from 16.78% to 18.77% for P1 and from18.45% to 19.67% for P2. This was mainly attributed to the higher Jsc and FF for doped HTMs which have the improved charge mobility. And owing to the higher intrinsic mobility of P2, the PCE improvement was not so significant compared with that of P1. We also prepared the devices based on the state-of-the art HTM, SpiroOMeTAD. The device based on Spiro-OMeTAD with dopant could achieve 20.4%, with Jsc of 23.7 mA cm-2, Voc of 1.14 V and FF of 75.78%. Hence, the performance of P1 was comparable with that of Spiro-OMeTAD. But for the Spiro-OMeTAD without dopant, the device performance was very low, only achieved 11.21%, which was much poorer than that of P1 (16.78%) and P2 (18.45%). This demonstrated side chain polymers, P1 and P2 with high mobility as promising HTMs for perovskite solar cells.

Figure 3. a) Energy level diagram of each components of PSCs device; b) Cross sectional images of devices fabricated with P1 (upper) and P2 (lower); c) Best-performing J–V scans collected under AM 1.5 simulated sunlight for P1 and P2 based PSCs; d) EQE of P1 and P2 based PSCs.

Table 2. Summary of the dopant-free devices’ parameters: open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and average efficiency (PCE). 12


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HTMs

Scan direction

Voc [V]

Jsc [mA cm-2]

FF [%]

PCE [%]

P1

Reverse

1.11

21.58

69.5

16.78

Forward

1.08

21.51

66.3

15.40

Reverse

1.13

22.34

72.7

18.45

Forward

1.10

21.95

73.2

17.67

P2

Figure 4a and Figure 4b display the time course of stead-state photocurrent and efficiency at maximum power point under AM 1.5G illumination. The corresponding stead-state photocurrents were measured by applying a constant bias voltage taken from the maximum power point. The stead-state efficiency was calculated by the applying voltage and the corresponding stead-state Jsc. Steady-state efficiency of 16.01% and 17.69% were obtained for the devices based on P1 and P2, respectively. All devices exhibited highly stable steady-state photovoltaic performances and thus proved the reliability of above devices performances. The histogram of the photovoltaic parameters was shown in Figure S7. It is clear that the devices show good reproducibility (by fabricating and analyzing over 20 devices) and the PSCs based on P2 exhibit a higher average PCE (17.5 ± 0.5%) than those based on P1 (16.0 ± 0.6%). The stability of PSCs is one of the most important factors for long term application. The stabilities of the PSCs devices with as-synthesized HTMs were investigated by exposing them to ambient air at around 30% relative humidity without encapsulation and measuring J-V curves per 24 hours. From Figure 4c, the PSCs devices with P1 and P2 both presented excellent long-term stability, maintaining over 80% after 30 days in ambient air with humidity of 30%. This is mainly attributed to the hydrophobic properties of synthesized polymers without doping additives. The hydrophobic nature of P1 and P2 protected the perovskite from moisture. To further understand the interfacial charge transfer and recombination processes in PSCs, electrical impedance spectroscopy (EIS) was measured at a bias of 0.6 V under simulated AM 1.5 illumination. The fitting results (Figure 4d) reveal that PSC based on P2 exhibited a larger recombination resistance (Rrec = 1290.2 Ω) than that based on P1 (Rrec =614.6 Ω). The larger Rrec value means a slower charge recombination rate, leading to the higher Jsc and FF.

13



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Figure 4. The steady-state current density and efficiency at the maximum power points for devices based on P1 (a) and P2 (b); c) The stability of the P1 and P2 based PSCs; d) Electrochemical impedance (EIS) spectroscope plots for PSCs based-on P1 and P2. The inset shows the equivalent circuit employed to fit the Nyquist plots.

CONCLUSIONS In conclusion, two novel side chain polymers were designed and synthesized by attaching carbazole-based hole-transporting unit to a polystyrene chain. Through tuning the triphenylamine substituent positions at carbazole, dopant-free HTMs, P1 (2,7-positions substituted) and P2 (3,6positions substituted) were obtained. The unique advantages of P1 and P2, such as good solubility, homogeneous film-formation abilities, suitable energy levels and high hole mobilities make them as the suitable dopant-free HTMs for PSCs. Owing to the higher mobility and charge extraction ability, devices based on P2 presented better performance, affording an impressive PCE of 18.45%. In addition, with the high solubility of the two polymers in various organic solvents, the devices based on the synthesized HTMs could be also processed with green-solvent, 2-MA, and achieving PCE of 16.42%. Those results clearly qualify side chain polymers, P1 and P2 with good solution processability and high mobility as promising HTMs for future low-cost, long-term stable, green14


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solvent-processable PSCs applications.

EXPERIMENTAL SECTION Materials 2,7-Dibromocarbazole (compound 1), 3,6-dibromocarbazole (compound 5), Pd(PPh3)4, 4vinylbenzylchloride (compound 4), and azodiisobutyronitrile (AIBN) were purchased from J&K Scientific Ltd. or Energy Chemical Ltd. and used without further purification. 4-[Di(pmethoxyphenyl)amino]benzene-1-boronic(compound 2) was purchased from Sdyano fine chemical Co. Ltd. The SnO2 solution was purchased from Alfa Aesar (tin (IV) oxide, 15 wt% in H2O colloidal dispersion). PbI2 (purity > 99.9985%) was purchased from Alfa Aesar. CsI (purity > 99.999%) was obtained from Sigma-Aldrich. All of the other salts and anhydrous solvents, including Li-TFSI salt, N,N-Dimethylformamide (DMF), ethanol, isopropyl alcohol, tertbutylpyridine (tBP), chlorobenzene, acetonitrile, and dimethyl sulfoxide (DMSO), were purchased from Sigma-Aldrich. All the above chemical products were used directly without further purification or treatment. Measurement and characterization Routine

Nuclear

Magnetic

Resonance

(NMR)

spectra

were

recorded

on

a

Brucker Avance III 400MHz (400 and 100 MHz for 1H and 13C NMR, respectively). Chemical shifts were reported as δ values (ppm) with tetramethylsilane (TMS) as the internal standard. The splitting patterns are designated as follows: singlet (s), doublet (d), triplet (t), and multiplet (m). UV-vis absorption spectra were recorded on a PerkinElmer Lambda650 spectrophotometer from dilute solutions in THF. Cyclic voltammetry (CV) was obtained in a tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 mol·L-1) in CH2Cl2 solution at room temperature using a CHI600E electrochemical workstation (CH Instrument Ins.) operated at a scanning rate of 100 mV s-1. A Pt wire (ϕ = 1.0 mm) embedded in Teflon column was used as the working electrode, and a Pt sheet and Ag/AgCl electrodes were served as the counter and reference electrodes, respectively. Ferrocene/ferrocenium was used as the internal reference to calibrate the redox potentials. Thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out with a Mettler-Toledo TGA STARe/DSC1 thermal analyzer under purified nitrogen gas flow with a 15



Page 16 of 24

10 oC min-1 heating rate. Time-resolved photoluminescence (TR-PL) was measured with Edinburgh Instrument FLS980 using a 375 nm laser as an excitation source derived from Nd:YAG laser. Photocurrent density-voltage (J-V) curves were measured under AM 1.5G One Sun Illumination (100 mW/cm2) with a solar simulator (Enlitech SS-F7-3A) equipped with 300 W Xenon lamp and a Keithley 2400 source meter. The light intensity was adjusted by an NREL-calibrated Si solar cell. The device active area was 0.08 cm−2. The J-V curves were recorded from −0.2 V to 1.2 V with 0.01 V steps, integrating the signal for 10 ms after a 10 ms delay. The external quantum efficiency (EQE) was measured with an EQE system (Enlitech QE-R) containing a Xenon lamp, monochromator, Si detector and dual-channel power meter.

Device Fabrication PSC devices having the structure of ITO/SnO2/perovskite Cs0.05FA0.81MA0.14PbI2.55Br0.45/HTL/Au were fabricated, where the Indium tin oxide (ITO) was the bottom layer. The process for fabricating the optoelectronic devices is summarized as follows. ITO glasses (Nippon Sheet Glass) were cleaned with detergent, deionized water, and acetone and sonicated with ethanol in an ultrasonic bath for 30 minutes. After that, ITO glasses were treated in a UV cleaner for 30 minutes. SnO2 aqueous colloidal dispersion (15 wt%) was diluted using deionized water to the concentration of 5 wt%. These solutions were stirred at room temperature for 2 h. The SnO2 electron transporting layer (ETL) were fabricated by spin-coating the above SnO2 solution on ITO glass at 4000 rpm for 30 s. After that, as-prepared SnO2 film were annealed on a 150 C hot plate for 60 minutes to remove residual solvent and surfactant. The perovskite layer was deposited on the ETL substrates using one-step solution process. The precursor solution of mixed perovskite consisting of 172 mg of FAI, 507 mg of PbI2, 22.4 mg of MABr and 73.4 mg of PbBr2 was prepared by dissolving the materials in 1 mL of a mixed solvent of DMF and DMSO with a volume ratio of 9:1. Then, a stock solution of 1.5 M CsI in DMSO was added to the mixed perovskite precursor to obtain the Cs0.05FA0.81MA0.14PbI2.55Br0.45 precursor solution. The perovskite films were then deposited onto the SnO2 substrates via a two-step spin-coating procedure. The first step was performed at 1000 rpm for 10 s with an acceleration of 500 rpm/s. The second step was performed at 4000 rpm for 35 s with a ramp-up of 2000 rpm/s. Chlorobenzene (~100 µL) was quickly dropped onto the 16


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spinning substrate during the second spin-coating step at the time point of 15 s before the end of the procedure. Afterward, the as-prepared films were heated at 100 C for approximately 1 h until their color changed to dark red. The hole-transporting layer (HTL) was prepared by spin-coating an HTM solution at 4000 rpm for 25 s. The HTM solutions were prepared by dissolving the HTMs in a chlorobenzene or other non-chlorine solvents with a 20 mg/mL concentration. Spiro-OMeTAD was dissolved in chlorobenzene (60 mM) with 28.8 mL tBP solution and 17.5 mL Libis(trifluoromethanesulfonyl) imide(Li-TFSI)/acetonitrile (520 mg/1 mL). Finally, an 80 nm thick Au layer was thermally evaporated at a rate of ∼0.05 nm/s under a vacuum of 4×10−5 Torr to complete the device fabrication.

Synthesis of 4,4'-(9H-carbazole-3,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (6). 3,6-Dibromocarbazole (5) (1.62 g, 5.0 mmol), 4-[di(p-methoxyphenyl)amino]benzene-1-boronic acid (2) (3.84 g, 11.0 mmol), and Pd(PPh3)4 (0.29 g, 0.25 mmol) were mixed in degassed THF (100 mL). Degassed K2CO3 (2.0 mol L-1 in H2O, 13 mL) aqueous solution was then added at the room temperature. The reaction mixture was refluxed for 24 h. The reaction mixture was cooled to RT and poured into CH2Cl2, organic materials were extracted with CH2Cl2 and washed with water. The CH2Cl2 layer was dried over sodium sulfate and filtered. The organic solvent was removed by rotary evaporation, and the residue was purified by column chromatography (silica gel, petroleum ether/CH2Cl2 = 1/2, Rf = 0.3) to give product as white solid (3.1 g, with a yield of 80 %). 1H NMR (400 MHz, THF-d8) δ 10.35 (s, 1H), 8.38 (s, 2H), 7.65 (d, J = 8.4 Hz, 2H), 7.58 (d, J = 8.6 Hz, 4H), 7.47 (d, J = 8.4 Hz, 2H), 7.08 (d, J = 8.9 Hz, 8H), 7.03 (d, J = 8.6 Hz, 4H), 6.88 (d, J = 9.0 Hz, 8H), 3.79 (s, 12H). 13C NMR (100 MHz, DMSO) δ 156.0, 147.4, 140.8, 139.7, 133.9, 131.3, 127.7, 126.8, 124.7, 123.7, 120.9, 118.2, 115.4, 111.8, 55.7. HRMS (EIS): m/z [M]+calcd for C52H43N3O4: 773.3248; found: 773.3237. Synthesis of 4,4'-(9-(4-vinylbenzyl)-9H-carbazole-3,6-diyl) bis(N,N-bis(4-methoxyphenyl)aniline) (M2). To a solution of 6 (1.0 g, 1.29 mmol) in dry DMF (10 mL) was added 60% NaH oil (77 mg, 1.94 mmol). The mixture was stirred at room temperature for 1 h. 4-Vinylbenzylchloride (0.24g, 1.55 mmol) was added to above solution by syringe. The mixture was heated at 60 C for 24 h. Water 17



Page 18 of 24

was added to quench the reaction. The organic solvent was extracted with CH2Cl2 and water to remove DMF. After the removal of CH2Cl2, the residue was directly purified by column chromatography on silica gel (Petroleum ether: CH2Cl2=1:1) to give a white solid (1.1 g, with a yield of 95%). 1H

NMR (400 MHz, THF-d8) δ 8.43 (s, 2H), 7.67 (d, J = 8.5 Hz, 2H), 7.58 (d, J = 8.6 Hz, 4H), 7.51 (d, J

= 8.5 Hz, 2H), 7.36 (d, J = 8.1 Hz, 2H), 7.20 (d, J = 8.0 Hz, 2H), 7.08 (d, J = 8.9 Hz, 8H), 7.03 (d, J = 8.5 Hz, 4H), 6.88 (d, J = 8.9 Hz, 8H), 6.69 (dd, J = 17.6, 10.9 Hz, 1H), 5.77 – 5.69 (m, 1H), 5.65 (s, 2H), 5.17 (d, J = 10.9, 1H), 3.79 (s, 12H). 13C NMR (100 MHz, DMSO) δ 156.0, 147.5, 140.8, 140.3, 140.1, 137.9, 136.7, 136.6, 133.5, 131.9, 127.8, 127.5, 126.8, 124.9, 123.6, 120.8, 118.5, 115.4, 114.7, 110.4, 55.7. HRMS (ESI): m/z [M]+calcd for C61H51N3O4: 889.3874; found: 889.3857. Synthesis of P2 The monomer M2 (0.5 g, 0.56 mmol) was polymerized in THF (1.5 mL) with 1 wt% AIBN (5 mg) as initiator under nitrogen at 85 C for 3 days. The polymerization was stopped by pouring the reaction mixture into methanol. The obtained yellow polymer was purified by repeated reprecipitation from methanol followed by drying under vacuum (0.40g, with a yield of 80%). Mn = 1.28 x 104; Mw/Mn = 1.46. Synthesis of 4,4'-(9H-carbazole-2,7-diyl) bis(N,N-bis(4-methoxyphenyl)aniline) (3) Compound 3 was prepared using the procedure of compound 6. Yield, 82%, yellow solid. 1H NMR (400 MHz, DMSO-d6) δ 11.27 (s, 1H), 8.09 (d, J = 8.2 Hz, 2H), 7.63 (s, 2H), 7.59 (d, J = 8.8 Hz, 4H), 7.39 (d, J = 8.2 Hz, 2H), 7.07 (d, J = 8.9 Hz, 8H), 6.94 (d, J = 9.0 Hz, 8H), 6.88 (d, J = 8.7 Hz, 4H), 3.75 (s, 12H). 13C NMR (100 MHz, DMSO) δ 156.2, 148.1, 141.4, 140.6, 137.9, 133.2, 128.0, 127.1, 121.6, 120.9, 120.3, 117.9, 115.4, 108.3, 55.7. HRMS (ESI): m/z [M+H]+calcd for C52H44N3O4: 774.3326; found: 774.3322. Synthesis of 4,4'-(9-(4-vinylbenzyl)-9H-carbazole-2,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (M1). M1 was prepared using the procedure of M2. Yield, 90%, yellow solid. 1H NMR (400 MHz, DMSOd6) δ 8.17 (d, J = 8.1 Hz, 2H), 7.81 (s, 2H), 7.61 (d, J = 8.7 Hz, 4H), 7.46 (d, J = 8.3 Hz, 2H), 7.36 (d, J = 8.2 Hz, 2H), 7.18 (d, J = 8.2 Hz, 2H), 7.05 (d, J = 8.9 Hz, 8H), 6.93 (d, J = 9.0 Hz, 8H), 6.87 (d, J = 8.7 Hz, 3H), 6.62 (dd, J = 17.6, 11.0 Hz, 1H), 5.78 (s, 2H), 5.71 (d, J = 17.9 Hz, 1H), 5.17 (d, J = 11.3 18


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Hz, 1H), 3.75 (s, 12H). 13C NMR (100 MHz, DMSO) δ 156.2, 148.2, 148.1, 141.8, 140.6, 138.2, 138.1, 136.6, 133.1, 128.1, 127.5, 127.0, 126.8, 121.3, 121.1, 120.3, 118.3, 115.4, 114.6, 107.1, 55.7. HRMS (ESI): m/z [M]+calcd for C61H51N3O4: 889.3874; found: 889.3860. Synthesis of P1 The monomer M1 (0.5 g, 0.56 mmol) was polymerized in THF (1.5 mL) with 1 wt% AIBN (5 mg) as initiator under nitrogen at 85 C for 3 days. The polymerization was stopped by pouring the reaction mixture into methanol. The obtained yellow polymer was purified by repeated reprecipitation from methanol followed by drying under vacuum (0.41g, with a yield of 82%). Mn = 0.94 x 104; Mw/Mn = 2.02.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: absorption and emission spectra of thin films, oxidation process, water contact angles, J–V curves with different solvents, statistics of the photovoltaic parameters, 1NMR,

13CNMR

and HRMS of

intermediates and polymers.

CONFLICTS OF INTEREST There are no conflicts to declare.

ACKNOWLEDGEMENTS This work was supported by the National Key Research and Development Project funding from the Ministry of Science and Technology of China (Grants No. 2016YFA0202400 and 2016YFA0202404), the Peacock Team Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. KQTD2015033110182370), the start-up funding of SUSTech (Grant No. Y01256114), the leading talents of Guangdong province program (Grant No. 2016LJ06S686), and the Shenzhen Engineering R&D Center for Flexible Solar Cells project funding from Shenzhen Development and Reform Committee (Grant No. 2019-126).

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