Study of Arylamine-Substituted Porphyrins as Hole-Transporting

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Study of Arylamine Substituted Porphyrins as Hole Transporting Materials in High Performance Perovskite Solar Cells Song Chen, Peng Liu, Yong Hua, Yuanyuan Li, Lars Kloo, Xingzhu Wang, Beng S. Ong, Wai-Kwok Wong, and Xunjin Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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ACS Applied Materials & Interfaces

Study of Arylamine Substituted Porphyrins as Hole Transporting

Materials

in

High

Performance

Perovskite Solar Cells Song Chen,a‡ Peng Liu,b‡ Yong Hua,b* Yuanyuan Li,c Lars Kloo,b* Xingzhu Wang,d Beng Ong,a Wai-Kwok Wong,a* Xunjin Zhua* a

Institute of Molecular Functional Materials, Research Centre of Excellence for Organic

Electronics, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China. b

Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, School

of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden c

Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, Chemical

Science and Engineering, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden. d

College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China.

KEYWORDS: porphyrin, arylamine substituents, hole transporting material, hole mobility, perovskite solar cells

ACS Paragon Plus Environment

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ABSTRACT: To develop new hole transporting materials (HTMs) for efficient and stable perovskite solar cells (PSCs), 5,10,15,20-tetrakis{4-[N,N-di(4-methoxylphenyl)amino-phenyl]}porphyrin has been prepared in gram scale through the direct condensation of pyrrole and 4[bis(4-methoxyphenyl)amino]benzaldehyde. Its Zn(II) and Cu(II) complexes exhibit excellent thermal and electrochemical stability,

specifically, high hole mobility and very favorable

energetics for hole extraction that render them a new class of HTMs in organometallic halide perovskite solar cells (PSCs). As expected, ZnP as HTM in PSCs affords competitive power conversion efficiency (PCE) of 17.78%, which is comparable to the most powerful HTM of Spiro-MeOTAD (18.59%) under the same working conditions. Meanwhile, the metal centers affect somewhat the photovoltaic performances that CuP as HTM produces a lower PCE of 15.36%. Notably, the perovskite solar cells employing ZnP show much better stability than that of Spiro-OMeTAD. Moreover, the two porphyrin-based HTMs can be prepared from relatively cheap raw materials with a facile synthetic route. The results demonstrate that ZnP and CuP can be a new class of HTMs for efficient and stable perovskite solar cells. To the best of our knowledge, this is the best performance for porphyrin-based solar cells with PCE > 17%.

1. Introduction It becomes very emergent to develop cost-effective, high-efficient solar cells and meet the increasing need of energy and decreasing amount of fossil fuel, meanwhile relieve the environmental crisis. Organic-inorganic hybrid perovskites solar cells (PSCs) are believed to be a promising and low-cost technology to replace those traditional energy sources due to the inspiring conversion efficiencies acheived.1-6 As a new class of solution-processing light absorbing material, organometal halide perovskites have attracted an increasing attention for


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their broad absorption in the solar spectrum, excellent charge carrier mobility and low charge recombination due to their broad absorption in the visible regions,7 high absorption coefficient,8 high charge carrier mobility9 and long diffusion length.10 This kind of material as photosensitizer produced a power conversion efficiency (PCE) of 3.8% in liquid electrolyte-based dye-sensitized solar cells (DSSCs).1 To solve the dissolution of the perovskite in the liquid electrolyte, hole transport materials (HTMs) was used in perovskite solar cells, leading to impressive PCE values.11-14 At the same time, some polymeric HTMs have been reported among which polytriarylamine (PTAA) showed the highest PCE of 20.2% in mesoscopic TiO2-based devices.4, 15-18

Compared to the polymeric HTMs, small molecular HTMs have advantages such as

convenient purification, defined molecular structures, relatively high efficiency and less batchto-batch variation.19 Till now, the 2,2′,7,7′-Tetrakis(N,N-di-pmethoxyphenylamine)-9,9′spirobifluorene (Spiro-OMeTAD) is still working the best as HTM in PSCs although a variety of other organic HTMs have been developed. Specifically, only a few examples of small molecules can be facilely synthesized and worked with efficiencies higher higher than 16% in PSCs.20-21 It’s known that the Spiro-OMeTAD suffers from its synthetic difficulty and negative effect on the long-term stability of the device, which might block its wide application in perovskite solar cells.22-23 Moreover, due to the low conductivity and hole mobility of Spiro-MeOTAD in the pristine form, dopants and additives are necessary to improve the device performance, such as tert-butylpyridine (t-BP), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and cobalt complexes (FK209) as p-type dopants.24-26 Therefore, it is highly desirable to develop costeffective and highly efficient new class of HTMs as alternatives to Spiro-OMeTAD. Porphyrins and derivatives have been extensive studied as photosensitizers in DSSCs and donor materials in bulk heterojunction organic solar cells (BHJ OSC) because of their excellent


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thermal stability, efficient light-harvesting ability, and high susceptibility in electron-transfer reactions. We have reported that the use of 5,15-substituted porphyrin-based small molecules as donor materials in organic solar cells that facilitates delocalized π-conjugation and π-stacking in the solid state, and large planar π-conjugation macrocycles can ensures high carrier mobility and efficient intermolecular charge transfer in the π-stacking direction.27-29 Since the photophysical and photochemical properties of porphyrin molecules can be facilely tuned through functionalization of the periphery (meso and β-positions) and variation of the metal center,30 porphyrins have also been applied as HTMs in PSCs.31-32 During the period of our study on these new porphyrin-based HTMs, Yeh’s group designed two symmetric ethynylaniline-substituted porphyrins and used them as HTMs in PSCs with PCEs reaching 16%. However, the synthesis of those HTMs still involves many steps such as condensation of aldehyde and pyrrole for porphyrin core, bromination, and Sonogashira coupling.33 On the contrast, the symmetrical meso-aryl substituted porphyrins can be one-pot synthesized in propionic acid, which exhibits a similar structure and energetics as Spiro-OMeTAD and other HTMs with high performances in PSCs, might be suitable as a new class of HTMs in PSCs. MeO

OMe N OMe

MeO N

N M

N N

N N

MeO

OMe N MeO

OMe ZnP M = Zn2+ CuP M = Cu2+

Figure 1. Structures of ZnP and CuP. Herein, 5,10,15,20-tetrakis{4-[N,N-di(4-methoxylphenyl)aminophenyl]}porphyrin and its


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Zn(II) and Cu(II) complexes of (ZnP and CuP) (Figure 1) were prepared and their applications as HTMs in PSCs were then investigated. Impressively, the PSC devices based on ZnP deliver the highest power conversion efficiency of 17.78% under ambient atmosphere conditions, which is comparable to the device employing the well-known Spiro-OMeTAD (18.59%) under the same working conditions. The device based on CuP gave a slight lower PCE of 15.36%. Notably, the facile synthesis of ZnP based on commercially starting materials makes it a promisng new class of HTMs with excellent stability, high hole mobility and appropriate energetics. It was believed that porphyrin derivatives as HTMs will play a more important role in PSCs upon further optimizing the molecular structure and engineering the device.

Scheme 1. Synthetic route for ZnP and CuP. 2. Results and Discussions The synthetic route is shown in Scheme 1 and the experimental details are given in the Supporting Information (SI). The porphyrin (P) was facilely prepared through the direct condensation of 4-[bis(4-methoxyphenyl)amino]benzaldehyde and pyrrole according to Adler’s methods with cheap starting materials,34 and the two porphyrinate complexes, referred to as ZnP and CuP, were prepared quantitatively by refluxing the porphyrin free base with excess zinc acetate and copper acetate in CHCl3 for overnight, respectively.35 The bulky and twisted structure of methoxy substituted triphenylamine group can provide high solubility in common


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organic solvents, which is propitious to be spin-coated forming very smooth HTM film.36 The following solvent evaporation and further recrystallization in CHCl3/CH3OH afforded the pure products for direct use as HTMs in PSCs, which is potential for the mass production with easily separation techniques, such as water scrubbing and pulping. The new compounds were characterized by spectral methods and MALDI-TOF, and the commercially available SpiroOMeTAD will be used as a standard HTM for comparison. 1.2

CuP ZnP

1.0

0.8

Absorbance

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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0.6

0.4

0.2

0.0 300

400

500

600

700

Wavelength (nm)

Figure 2. Optical absorption spectrum of ZnP and CuP in CHCl3. Table 1. Optical and electrochemical data for ZnP and CuP. λmax/nm HTMs

Eoxa EHOMO ELUMO

ERe

Hole Mobility

(ɛ/105 M−1 cm−1)

[V]

[eV]a

[eV]

[meV]

[cm2·V−1·s−1]

CuP

404 (0.92), 444 (1.33), 549 (0.19), 592 (0.13)

0.35

−5.37

−3.40

374

2.89×10−4

ZnP

404 (0.88), 445 (2.38), 563 (0.21), 610 (0.25)

0.27

−5.29

−3.35

303

3.06×10−4

SpiroOMeTAD20-21

388 (0.60)

0.20

−5.22

−2.28

495

1.58×10−4

The absorption spectra of ZnP and CuP in CHCl3 solution were measured on quartz


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substrates (Figure 2), and their photophysical data is summarized in Table 1. Due to the interaction of unpaired electron in ݀௫ మ ି௬మ with porphyrin π system, CuP shows a splitting Soret bands (404 and 444 nm) and blue-shifted Q bands (549 and 592 nm), while ZnP shows a Soret band (445 nm) with a vibrational shoulder (404 nm) and two Q bands (563 and 610 nm). In addition, ZnP shows higher intensity Soret bands and bathochromic shifted Q bands as compared to CuP, which could be explained by Gouterman’s four orbital modes.37-39 The absorption spectra of TiO2/perovskite films with and without porphyrin-based HTMs were also recorded and there is no significant difference for the absorption from 400 to 800 nm due to the saturated absorption by the perovskite film in this range (Figure S3). Therefore, the porphyrinbased HTMs have very limited effect in terms of the light harvesting competition with perovskite. In addition, their thermal stability was evaluated by thermal gravimetric analysis (TGA) and both exhibit excellent thermal stability with 5% weight loss temperature (Td5) calculated to 449 °C and 436 °C, respectively.

Figure 3. (a) Scheme of the device. (b) Energy level diagram of the perovskite and hole transporting materials. To characterize the redox energies of the HTMs, the electronic properties of CuP and ZnP (Figure S5) were measured by cyclic voltammetry in CH2Cl2 solution containing 0.1 mM


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tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte in a three-electrode system and the data are presented in Table 1. The energy level of the highest occupied molecular orbitals (HOMO) can be estimated according to the literatures (assuming that the energy level of Fc+/Fc = −5.1 eV), while the lowest unoccupied molecular orbital (LUMO) was calculated by adding the optical band-gap energies to the HOMO energies.40-41 The HOMO energy levels of CuP and ZnP were estimated to be −5.37 and −5.29 eV, respectively. The HOMOs of both CuP and ZnP lie above that of perovskite (−5.65 eV), which should ensure sufficient driving force for the hole injection from perovskite into the gold counter electrode. Compared with ZnP, CuP exhibits a deeper HOMO energy level which is beneficial for achieving higher open circuit voltage (VOC), but smaller gap between the HOMOs energy levels of CuP and perovskite might cause a hole-injection barrier.42 At the same time, the LUMO levels of both CuP (−3.40 eV) and ZnP (−3.35 eV) are higher than that of perovskite (−4.05 eV), which should guarantee blocking the electron transport from perovskite to Au counter electrode and hence suppressing the carrier recombination.43-45

Figure 4. Frontier orbitals of MP (M = Cu2+, Zn2+).


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To gain further understanding on the 3D geometry and molecular orbitals of the two porphyrin complexes, Density Functional Calculations (DFT) were performed using the Gaussian 09 program package at the B3LYP/6-31G(d)* level of theory. In the optimal structural conformations shown in Figure 4, four meso-substituted triarylamines in MP are closely perpendicular to the porphyrin ring because of the steric hindrance, leading to a twisted nonplanar dimensional molecular structure. And the HOMOs electron density distribution for both ZnP and CuP are mainly located at the porphyrin core and meso-substituted triarylamine, while the LUMOs are largely localized on the porphyrin ring. The calculated hole reorganization energies (ER) of ZnP (303 meV) and CuP (374 meV) are much higher than Spiro-OMeTAD (495 meV), indicating that ZnP and CuP have a great potential for hole-transporting materials in perovskite-based solar cells.

1/2 -1 Square Root of Current Density (A . cm )

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


0.6

0.5

0.4

0.3

0.2

Spiro-OMeTAD CuP ZnP

0.1

0.0 0.0

0.5

1.0

1.5

2.0

2.5

Applied Bias (V)

Figure 5. J−V plots of the hole-only devices based on CuP, ZnP and Spiro-OMeTAD. The charge transport properties of the two porphyrinate complexes were determined by the space-charge limited current (SCLCs) to quantitatively compare with Spiro-OMeTAD.46-47 As shown in Figure 5, both CuP and ZnP

show higher hole mobility (CuP: 2.89 × 10−4


9


cm2·V−1·s−1, ZnP: 3.06 × 10−4 cm2·V−1·s−1) than doped Spiro-OMeTAD (1.58 × 10−4 cm2·V−1·s−1), which indicates that both CuP and ZnP as HTMs are capable to collect holes generated spontaneously. This result also coincides with the computational study that both CuP and ZnP display smaller calculated ER which indicates a faster hole-transport rate. (a)

24

-2

Current density (mA cm )

20

16

12

CuP Reverse CuP Forward ZnP Reverse ZnP Forward Spiro-OMeTAD Reverse Spiro-OMeTAD Forward

8

4

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

(b) 100

80

60

IPCE (%)

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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CuP ZnP Spiro-OMeTAD

40

20

0 400

500

600

700

800

Wavelength (nm)

Figure 6. (a) J-V curves measured under reverse and forward voltage scanning with AM1.5G illumination, and (b) IPCE spectra of PSCs with ZnP, CuP and Spiro-OMeTAD as HTMs, respectively.


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Table 2. The parameters of PSC performances with ZnP, CuP and Spiro-OMeTAD as HTMs. HTMs

Scan direction

JSC (mA cm–2)

Integral JSC (mA cm–2)

VOC (V)

FF (%)

PCE (%)

Reserve

21.609

20.526

1.072

66.3

15.36

Forward

21.567

1.072

58.8

13.59

Reserve

22.689

1.099

71.3

17.78

Forward

22.459

1.080

65.9

15.98

Reserve

22.418

1.108

74.9

18.59

Forward

21.678

1.074

69.3

16.13

CuP 21.754

ZnP SpiroOMeTAD

21.732

The PSCs were fabricated with ZnP and CuP as HTMs under a conventional device structure of FTO/Compact TiO2/Mesoporous TiO2 (mp-TiO2)/Pervoskite/HTMs/Au (Figure 3a), and worked under AM 1.5 illumination. For comparison, the devices with well-known doped Spiro-OMeTAD as HTM were also fabricated and tested, and details are shown in Experimental Section. The current–voltage (J–V) curves of these PSCs are shown in Figure 6(a) and the relevant photovoltaic parameters are tabulated in Table 2. The ZnP-based PSCs provided a final PCE of 17.78% with a short-circuit current density (JSC) of 22.68 mA cm–2, an open-circuit voltage (VOC) of 1.099 V and a fill factor (FF) of 71.3%, while CuP-based PSCs showed a slightly lower PCE of 15.36% with a JSC of 21.60 mA cm–2, a VOC of 1.072 V and a FF of 66.3%. The integrated photocurrents from the Incident Photon to Current Efficiency (IPCE) spectra in Figure 6(b) match the measured JSC well (Table 2). Previously, a natural chlorophyll derivative (Chl-1) have also been applied as HTMs in perovskite solar cells, but showed a lower efficiencies of 11.44%.31 In contrast, both CuP and ZnP exhibit deeper HOMO energy levels than the Ch1-1, which is responsible for the much higher VOC (CuP: 1.07, ZnP: 1.09 V)


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achieved, in comparison with 0.83 V for Chl-1 in PSCs. On the other hand, CuP shows a lower VOC than ZnP in PSCs, though the former have a slight deeper HOMO level than the latter. This is mainly due the lower solubility of CuP than ZnP in chlorobenzene. As a result, the film spincoated from ZnP solution shows a smoother and larger surface coverage than that of CuP in the SEM images (vide infra, Figure 7), leading to an increased shunt resistance and reduced parasitic current loss.48 At the same time, some pinholes observed on CuP film cause fast charge recombination, consist with its lower efficiency in PSC.49 For comparison, the cell based on Spiro-OMeTAD achieved a PCE of 18.59% with a JSC of 22.41 mA cm–2, VOC of 1.108 V and FF of 74.9% under the same fabrication conditions. The hysteresis behavior of the devices were measured through forward and reserve scans, as shown in Figure 6. Under the same conditions, CuP, ZnP and Spiro-OMeTAD exhibited performances with PCE of 13.59%, 15.98% and 16.13% for the forward scan, respectively. Compared to the CuP and Spiro-OMeTAD, a slighter hysteresis phenomenon was observed for ZnP in during forward and reverse scans, which indicates ZnP can more effectively suppress the hysteresis behavior in the PSCs.

Figure 7. SEM images (top view) of the surface morphology of the mixed-ion perovskite (a), Spiro-OMeTAD (b), CuP (c) and ZnP (d).


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Distribution of mixed-ion perovskite materials (FAPbI3)0.85(MAPbBr3)0.15 nanocrystals over the surface of TiO2 photoanode without and with HTMs was imaged by high-resolution scanning electron microscope (SEM). As depicted in Figure 7(a), the pristine perovskite film covers the TiO2 substrate with rough perovskite nanocrystals, and Figure 7(b) show that Spiro-OMeTAD films present a very homogeneous surface where no special features could be distinguished within a micrometer to nanometer range. On the contrary, some aggregation was identified in either the CuP or ZnP layer with domains different in size and shape (Figure 7(c), 7(d)), mainly because of their large and planar π-conjugated systems, as well as moderate solubility in common organic solvents. Currently, we have no proofs whether these structures might be beneficial or detrimental for their performance in PSCs. In the future, we are going to improve their solubility in common organic solvents through structural modification, and then investigate the effect on the topology of HTM layers, as well as the total performances in PSCs.

12

Perovskite (P) P+CuP P+ZnP P+Spiro-OMeTAD

10

PL intensity (a.u.)

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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6

4

2

0 650

700

750

800

850

Wavelength (nm)

Figure 8. Steady state photoluminescence spectra of glass/perovskite/HTMs based on CuP, ZnP and Spiro-OMeTAD. In order to further investigate the charge dynamics of hole extraction and transport for these


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three HTMs, we measured the steady-state photoluminescence (PL) of the perovskite’s in HTM/perovskite/glass devices (Figure 8).50 Clearly, the PL of the HTM/perovskite/glass films were significant quenched within 760–780 nm as compared to that of the pristine perovskite film. This result definitely confirms the efficient hole extraction and transfer from the perovskite film into the HTLs. Impressively, the ZnP shows a similar quenching property as the SpiroOMeTAD when coated in the perovskite film, suggesting a comparable ability of charge separation at perovskite interface for ZnP. 12

CuP ZnP Spiro-OMeTAD

10

8

Counts

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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6

4

2

0 14

15

16

17

18

19

PCEs (%)

Figure 9. Histogram data of PCEs To confirm the reproducibility of device performance, a series of devices were fabricated using CuP, ZnP and Spiro-OMeTAD as HTMs. The histograms of PCEs and statistical average of parameters are shown in Figure 9, which demonstrated good reproducibility for those PSCs using porphyrin-based HTMs. The average PCEs of the devices based on ZnP shows >75% of the devices (among 29 devices) having higher efficiency than 17%, which is comparable to those of the devices with Spiro-OMeTAD with high reproducibility, indicating that the porphyrinate complexes as HTM materials have great potential for the future industrialization.


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20

16

Efficiency (%)

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


12

8

4

Spiro-OMeTAD ZnP

0 0

5

10

15

20

25

30

Time / Day

Figure 10. Stability test of the PSCs employing ZnP and Spiro-OMeTAD without encapsulation, stored under ambient conditions (humidity about 40~45% and temperature of 20~25℃). Furthermore, the stability of the PSCs employing ZnP and benchmark Spiro-OMeTAD as HTM, respectively, was carried out within 30 days for each under ambient conditions (humidity about 40~45% and temperature of 20~25 °C). After 30 days, the average PCE of the ZnP based device retains around 85% of the initial PCE, while the average PCE of the Spiro-OMeTAD based devices retains only 45% of the initial PCE under the same conditions (Figure 10). The ZnP based devices show much better stability than the Spiro-OMeTAD based devices. 3. Conclusion In summary, we have built up two symmetrical meso-arylamine substituted porphyrin complexes as new HTMs in PSCs. For their suitable energy level, efficient exciton dissociation and charge transfer in HTM/perovskite films, an impressive power conversion efficiency of 15.36% (JSC = 21.60 mA cm–2, VOC = 1.072 V, FF = 66.3%) and 17.78% (JSC = 22.68 mA cm–2, VOC = 1.099 V, FF = 71.3%) has been achieved for CuP and ZnP, respectively. This represents the highest PCE among those ever reported for porphyrin- and phthalocyanine-based HTMs in PSCs. Notably, the


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perovskite solar cells employing ZnP show much better stability than that of Spiro-OMeTAD. Moreover, the two porphyrin-based HTMs can be prepared from relatively cheap raw materials with a facile synthetic route. The results demonstrate that ZnP and CuP can be a new class of HTMs for efficient and stable perovskite solar cells. 4. Experimental Section Materials and Reagents 4,4'-Dimethoxytriphenylamine were synthesized according to literature methods.51 All air and water-sensitive reactions were performed under nitrogen atmosphere. Organic solvents used in this work were purified using standard process. All of the chemicals were purchased from Dieckmann Chemical Ltd, China. Solvents and other chemicals were also commercial available. The synthetic routs of two HTMs CuP and ZnP are outlined in Scheme 1 and the details are depicted below. 4-(Bis(4-methoxyphenyl)amino)benzaldehyde To the stirred solution of 4-methoxy-N-(4methoxyphenyl)-N-phenylbenzenamine (2.6 g, 7.8 mmol) in DMF (30 mL), POCl3 (1.17 mL, 12.6 mmol) was added dropwise at 0 °C under N2. The reaction was performed at 90 °C under stirring for 3 h. After rotary evaporation of the solvent, the residual was dissolved in CHCl3 and washed with a saturated CH3COONa aqueous solution. The organic layer was dried over Na2SO4. After removal of the solvent, the crude product was purified on the silica gel column (CH2Cl2/n-Hexane = 1/1) to afford the target compound (2.59 g, 92% yield). 1H NMR (400 MHz, DMSO, δ): 9.70 (s, 1 H), 7.65 (d, 2 H), 7.21 (d, 4 H), 7.00 (d, 4 H), 6.68 (d, 2 H), 3.76 (s, 6 H).


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meso-Tetra-4-[N,N-di(4-methoxylphenyl)amino]phenylporphyrin (P) An 2.0 g of 4-(bis(4methoxyphenyl)amino)benzaldehyde was added to the 20 mL of stirring propionic acid, and the resulting solution was brought to reflux. A 0.42 mL of freshly distilled pyrrole in 3 mL of propionic acid was added for more than 15 mins and the mixture refluxed for 2 h with stirring. The tarry solution was distilled to remove propionic acid and purified via chromatography (CH2Cl2/n-Hexane). Then to give the product by recrystallization (CH2Cl2/CH3OH) in 22% yield. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.97 (s, 4H), 8.33 (s, 3H), 8.28 (d, J = 8.8 Hz, 3H), 8.01 (d, J = 8.4 Hz, 4H), 7.31–7.41 (m, 20H), 7.28 (d, J = 8.8 Hz, 6H), 6.96–7.03 (m, 16H), 3.86 (d, J = 13.2 Hz, 24H), −2.64 (s, 2H). ZnP Free base porphyrin (P) (304 mg, 0.20 mmol) and zinc acetate(II) dihydrate (219 mg, 1.00 mmol) were refluxed in CHCl3 (20 mL) for 4 h. The solvent was washed by water and the organic phase was concentrated via rotary evaporator. Then to give the product by recrystallization (CHCl3/CH3OH) in quantitative yield. 1H NMR (400 MHz, CDCl3) δ (ppm): 9.08 (8 H, s, pyrrolic-H), 8.01 (d, J = 8.4 Hz, 8 H, Ar–H), 7.37 (d, J = 8.8 Hz, 16 H, Ar–H), 7.31 (d, J = 8.4 Hz, 8 H, Ar–H), 6.96 (d, J = 9.2 Hz, 16 H, Ar–H), 3.85 (s, 24 H, OCH3); 13C NMR (400 MHz, CDCl3) δ (ppm): 156.0, 150.4, 148.1, 141.2, 135.3, 134.9, 131.8, 126.9, 121.1, 118.3, 114.9, 55.5; (MALDI–TOF, m/z) calculated for C100H80N808Zn: 1586.5404; found: 1586.5420. CuP Free base porphyrin (P) (304 mg, 0.20 mmol) and copper(II) acetate monohydrate (199 mg, 1.00 mmol) were refluxed in CHCl3 (20 mL) for 4 h. The solvent was washed by water and the organic phase was concentrated via rotary evaporator. Then to give the product by recrystallization (CHCl3/CH3OH) in quantitative yield. (MALDI-TOF, m/z) calculated for C100H80N808Cu: 1584.5431; found: 1584.5403.


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Device fabrication and Characterization. The methods and procedures in details were provided in Supporting Information. ASSOCIATED CONTENT Supporting information The supporting information is available including the synthesis and characterization of all intermediates, details of device fabrication and characterization. AUTHOR INFORMATION Corresponding Author * [email protected] (X. Zhu); [email protected] (W.-K. Wong); [email protected] (Y. Hua); [email protected] (L. Kloo). Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the supports from Hong Kong Research Grants Council (HKBU 22304115), Hong Kong Baptist University (FRG2/16-17/024, FRG1/15-16/052, SDF/03-17-096) and the Areas of Excellence Scheme, University Grants Committee, Hong Kong SAR ([AoE/P-03/08]), the Swedish Research Council, the Swedish Energy Agency and the Knut & Alice Wallenberg Foundation, and the Natural Science Foundation of China (21120102036 and 91233201).


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Study of Arylamine-Substituted Porphyrins as Hole-Transporting

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