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N, N-Di-p-Methylthiophenylamine-Substituted (2Ethylhexyl)-9H-Carbazole: A Simple, Dopant-Free HoleTransporting Material for Planar Perovskite Solar Cells Linjun Xu, Peng Huang, Jing Zhang, Xuguang Jia, Zhijie Ma, Yue Sun, Yi Zhou, Ningyi Yuan, and Jianning Ding J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04469 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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The Journal of Physical Chemistry

N, N-di-p-Methylthiophenylamine-Substituted (2-Ethylhexyl)-9H-Carbazole: A Simple, Dopant-free Hole-Transporting Material for Planar Perovskite Solar Cells Linjun Xu1, Peng Huang2, Jing Zhang1*, Xuguang Jia1, Zhijie Ma1, Yue Sun1, Yi Zhou2*, Ning-Yi Yuan1*, Jian-Ning Ding1*

1

School of Material Science & Engineering, Jiangsu Collaborative Innovation Center of Photovoltaic Science &

Engineering, Changzhou University, Changzhou 213164, Jiangsu, China. Email: [email protected]; [email protected]; [email protected] 2

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials

Science, Soochow University, Suzhou 215123, China. E-mail: [email protected]

ABSTRACT A

dopant-free

hole-transporting

material

(HTM);

with

(2-ethylhexyl)-9H-carbazole as core and N,N-di-p-methylthiophenylamine as end groups, termed CMT, has been designed and synthesized by a simple method. For the first time, four methylthiol groups have been introduced, rather than methoxy groups, at the para-position of the diphenylamine. Under AM 1.5 illumination at 100 mW cm-2, perovskite solar cells based on CH3NH3PbI3 with pristine CMT as the HTM achieved a power conversion efficiency of 13.05%, with a short circuit current density of 21.82 mA cm-2, an open circuit voltage (VOC) of 1.03 V, and a fill factor of 58.23%. The value of VOC is comparable to that of the device based on Spiro-OMeTAD, which was 1.02 V.

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Introduction Recently, solid-state organic-inorganic halide perovskite solar cells (PSCs) have aroused a great deal of attentions, due to their high power conversion efficiency (PCE) and low cost.1-5 Tsutomu Miyasaka et al. were first to use organic-inorganic halide perovskite as a light absorber in solar cells and a PCE of 3.8% was achieved.6 To date, the PCE of perovskite solar cells have been improved to 22.1%.7 Hole-transporting materials (HTMs) are essential to create organic-inorganic halide PSCs with high PCE.8-13 State-of-the-art PSCs utilize Spiro-OMeTAD (2,2’,7,7’-tetrakis (N, N-di-p-methoxy-phenylamino)-9-9’-spirobifluorene) as the HTM. However, the high synthetic and purification costs of Spiro-OMeTAD is one of the major barriers to commercialization

of

PSCs.

Moreover,

dopants

such

as

lithium

bis(trifluoromethanesulfonyl)imide (Li-TFSI) or 4-tert-butylpyridine (TBP) are usually needed obtain high hole transport characteristics with Spiro-OMeTAD.14, 15 Such doping not only increases the cost of device fabrication, but also decreases device stability. Li-TFSI absorbs moisture from the air, which breaks down the perovskite layer.16 Also, TBP reacts with the perovskite layer to form [PbI2.TBP] complexes or with iodine to form an iodopyridinate complex.17, 18 In this regard, it is highly desirable to develop novel HTMs that are easy to synthesize and can improve the efficiency and stability of PSCs. Among the reported organic HTMs, small organic molecules have been widely studied; since they are easy to purify, specific in structure, and demonstrate no batch to batch variation compared


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with the polymers alteratives. Moreover, they also show a better infiltration into the nanostructure of the PSC, which increases the repeatability of the device.9 However, most small organic molecules reported for solution-processed PSCs required doping with ion additives to increase the hole mobility and PCEs. Few HTMs in their pristine form can compete with the doped Spiro-OMeTAD PSCs. Limited PCEs can be obtained with dopant-free HTMs, such as a pentacene-based system (11.8%),19 a new spiro-dioxepine (12.7%),20 and triphenylamine-based materials (11–13%).21-24 Therefore, the development of dopant-free and low-cost HTMs to replace Spiro-OMeTAD is still needed. In this work, we report a dopant-free HTM, (2-ethylhexyl)-9H-carbazole substituted by N, N-di-p-methylthiophenylamine at the 2, 7-positions (CMT), which was synthesized by a simple method. It is well known that the electro-donating ability of sulfur is weaker than oxygen.25 Moreover, for conjugated photovoltaic polymers, the one with alkyllthio substituent usually shows a lower highest occupied molecular orbital (HOMO) energy level than the one with alkoxy substituent.26 The addition of four methylthiol groups rather than methoxy groups at the para-positions of the diphenylamine was intended to lower the highest occupied molecular orbit (HOMO) energy level of CMT. The planar PSC devices based on CH3NH3PbI3 were fabricated with the n-i-p architecture. Even without additives, planar PSC devices with CMT serving as the HTM layer gave a PCE as high as 13.05%, with a short circuit current density (JSC) of 21.82 mA cm-2, an open circuit voltage (VOC) of 1.03 V, and a fill factor (FF) of 58.23%, under AM 1.5 illumination of 100 mW cm-2.



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Experimental All reagents were obtained from Aldrich, and TCI Chemical Co, and used as received. The detailed synthetic processes of Compound CMT are as follows. Bis(4-(methylthio)phenyl)amine 1: To a flask with two necks, 4-(methylthio)aniline (1.67 g, 1.2 equiv), sodium tert-butoxide (1.34 g, 1.5 equiv), Pd2(dba)3 (183 mg, 0.02 equiv) and DPPF (166 mg, 0.03 equiv) were added in 100 ml anhydrous 1,4-dioxane and stirred at 60 °C for half an hour. Then (4-bromophenyl)(methyl)sulfane (2.03 g, 10 mmol) was added and stirred at 90°C for 20h. After cooling down, the mixture was filtered the solid out, diluted with ethyl acetate and washed by brine for three times. The organic phase was combined and dried over MgSO4. After the solvent was removed, the residue was purified by neutral alumina column using petroleum ether/ethyl acetate (15:1~10:1, v/v) as elute to afford compound 1 (1.82 g, 69.73%). 1

H NMR (500 MHz, DMSO-d6) ppm: 8.22 (s, 1H), 7.21 (m,4H), 7.02 (m, 4H), 2.41 (s,

6H). C14H15NS2 [M+ ] Exact Mass (261.02), MS (EI) (261.0). Compound

CMT:

A solution

of

compound

1

(1.305

g,

2.5

equiv),

2,7-dibromo-9-(2-ethylhexyl)-9H-carbazole (0.874 g, 2 mmol), Pd2(dba)3 (36.6 mg, 0.02 equiv), tri-tert-butylphosphonium tetrafluoroborate (17.34 mg, 0.03 equiv) and sodium tert-butoxide (0.576 g, 3 equiv) were added in anhydrous toluene (35 mL) and the solution was refluxed under argon atmosphere for 20h. After cooling to room temperature, reaction mixture was filtered through Celite, 100 mL of distilled water were added and extraction was done with. The organic layer was dried over anhydrous MgSO4, filtered and solvent evaporated. The crude product was purified by


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column chromatography using 15: 1 (v/v) petroleum ether/ethyl acetate as an eluent. The obtained product was recrystallized from acetone into methanol. The precipitate was filtered off and washed with methanol to collect CMT as a faint yellow solid (0.993 g, 62.3 %). 1H NMR (500 MHz, Acetone-d6), ppm: 7.94 (d, 2H), 7.23 (m, 8H), 7.12 (d, 2H), 7.05 (m, 8H), 6.90 (q, 2H), 3.97 (d, 2H), 2.47 (s, 12H), 1.75 (m, 1H), 1.13 (m, 8H), 0.77 (t, 3H), 0.71 (t, 3H). 13C NMR (500 MHz, Acetone-d6), ppm: 11.16, 14.31, 16.54, 23.56, 25.05, 31.68, 40.17, 47.21, 106.03, 117.43, 119.73, 121.31, 125.12, 129.14, 132.62, 143.22, 146.16, 146.63. C48H51N3S4 Exact Mass (797.30), MS (MADI-TOF) (797.0). Results and discussion The synthetic route to CMT is shown in Figure 1. First, diphenylamine substituted by methylthio groups in the para-positions was synthesized by the Buchwald-Hartwig reaction using inexpensive starting materials. CMT was obtained with a yield of over 60%, and then was fully characterized (Figure S1-S4). CMT is readily soluble in most common organic solvents, which is an important characteristic for solution-processing techniques. The thermal stability of CMT was investigated by thermogravimetric analysis (TGA). The temperatures at which a 5% weight loss was 387°C, as shown in Figure 2, indicating that the stability of CMT is sufficient for its application in PSCs.



Figure 1. synthetic routes for CMT.

100 95

Weight (%)

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

387 °C

90 85 80 75 70

CMT

65 60 0

200

400

Temperature (°C) Figure 2. TGA plot of CMT with scan rate of 10°C min-1 under N2 atmosphere.

Figure 3a shows the UV-vis transmittance and absorption of a CMT film. The absorption edge at 425 nm corresponds to an optical band gap of 2.92 eV, and it is nearly transparent in the visible region of 400–700 nm. Figure 3b shows the cyclic voltammograms of CMT. From the equation: EHOMO = -e (Eoxonset + 4.71) (eV),27 the HOMO energy level of CMT is determined to be -4.90 eV, which matches the valence band level of the CH3NH3PbI3 perovskite (-5.43eV).28 And the HOMO energy level of CMT with methylthiol groups is 0.12 eV lower than that of the one with methoxyl groups which will be reported in our other work, demonstrating the methylthiol groups at the para-position of diphenylamine can lower the HOMO energy level effectively. The LUMO energy level of CMT is calculated to be -1.98 eV from the


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equation: LUMO = HOMO + Egopt ,27 which is more positive than that of CH3NH3PbI3 (-3.93 eV).29 The large energy barrier (~1.95eV) between the LUMO levels of CH3NH3PbI3 and CMT could efficiently resist the electrons and greatly reduce the chance of recombination. Thus, this material can not only act as a hole-transporting layer but also as an electron-blocking layer in PSCs.

1.0

100

a

0.8

80

0.6

60

0.4

40

CMT film absorbance CMT film transmittance

0.2

0.0

20

0 300

400

500

600

Wavelength (nm)

700

Transmittance (%) Current

Normalized 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


b

CMT -0.2

0.0

0.2

0.4

0.6

+

Potential (V vs Ag/Ag )

Figure 3. (a) UV-vis transmittance and absorbance of CMT film. (b) Cyclic voltammograms of CMT on Pt electrode in an acetonitrile solution of 0.1 mol L-1 Bu4NPF6 (Bu = butyl) with a scan rate of 100 mV s-1. Water contact angles on (c) Spiro-OMeTAD, (d) PEDOT:PSS, and (e) CMT films.

The hydrophobicity of the hole transport layer is also important for PSCs. Figure 3c shows that Spiro-OMeTAD exhibits a water contact angle (WCA) of 66°, while that of PEDOT:PSS in Figure 3d is smaller at 14.5°. In contrast, Figure 3e shows that CMT exhibits a much larger WCA of 88.5°. This is due to the 2-ethylhexyl group attached to the nitrogen atom of carbazole that, as a nonpolar, flexible chain, modulates the CMT film surface energy. The WCA indicates that CMT can protect



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perovskite from moisture in the air better than Spiro-OMeTAD and PEDOT:PSS which would improve the stability of the PSCs. Table 1 summarizes the key properties of CMT.

Table 1. Summary of the key properties for CMT. λfilm

λedge

(Egopt)film

ELUMO

EHOMO

Tg

µ

(nm)

(nm)

(eV)

(eV)

(eV)

(oC)

(cm2 V-1 s-1)

294,339,396

418

2.92

-1.98

-4.90

Compounds

CMT

325

2.6×10-5

Planar PSC devices were fabricated with an FTO/compact-TiO2/CH3NH3PbI3/ CMT/Au structure, as shown in Figure 4a. Compared to mesoscopic PSCs, the planar PSC device is easier and more economical to fabricate. The device consists of a layer of CH3NH3PbI3 (∼400 nm) covered with CMT (∼50 nm). The compact TiO2 (∼50 nm) was used as an electron transport layer, and Au (∼80 nm) was used as the back contact. The energy diagram for each layers is shown in Figure 4b. PSC devices with CMT as the HTM were measured under AM 1.5 (100 mW cm-2) simulated light illumination. The current–voltage curves of photovoltaic devices made with different HTM concentrations are shown in Figure 4c, and the corresponding photovoltaic parameters are summarized in Table 2. The best performance obtained with 22.52 mg ml-1 CMT dissolved in chlorobenzene, gave a Voc of 0.95 V, Jsc of 21.22 mA cm-2, and an FF of 63.69%, resulting in a PCE of 12.91%. Therefore, this is the optimum concentration for PSC device performance. The results indicate that the concentration of the HTMs has a significant impact on PSC device performance. We also investigated the impact of the HTM spin-coating speed on the performance of


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the PSC device. The current–voltage curves of photovoltaic devices fabricated with different HTM spin-coating speeds are shown in Figure 4d, and the corresponding photovoltaic parameters are summarized in Table 3. It is clear that the appropriate spin-coating speed was 5000 rpm; the PSC device made under this condition exhibited a PCE as high as 13.05%, with a VOC of 1.03 V, JSC of 21.82 mA cm-2, and an FF of 58.23% during the reverse scan. A slighter hysteresis phenomenon was observed, which the PCE of the device reach 12.21%, with a VOC of 1.02 V, a JSC of 21.60 mA cm-2, and a FF of 55.17%, during the forward scan (Figure S6, Table S1). A PSCs device with a HTM of Spiro-OMeTAD (72.3 mg mL-1) doped with TBP (28.8 µL) and Li-TFSI/acetonitrile (17.5 µL, 520 mg mL-1) was fabricated under the same conditions. The current–voltage curve of this photovoltaic device is shown in Figure S5; the PCE reached to 16.7%, with a JSC of 22.32 mA cm-2, VOC of 1.02 V, and an FF of 72.75%. Comparing the performance of the PSC device that based on the dopant-free CMT gave a lower PCE than the Spiro-OMeTAD based device, but a comparable VOC. We also fabricated a PSCs device employing doped CMT (22.52 mg mL-1)

as

the

HTM

(doping

with

4-tert-butylpyridine

(28.8

µL)

and

Li-TFSI/acetonitrile (17.5 µL, 520 mg mL-1)). But the PCE of this device can only reach 10.55%, with a VOC of 0.98 V, a JSC of 20.79 mA cm-2, and a FF of 51.62% (Figure S7, Table S1).



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Figure 4. (a) Diagram of photovoltaic device structure. (b) Energy level diagram for each layers of photovoltaic device. (c) J−V curves of photovoltaic device with different concentrations of HTM. (d) J−V curves of photovoltaic device with different spin coating speed of HTM. Table 2. Photovoltaic parameters of PSCs with different concentrations of CMT as HTM Density (mg ml-1)

Jsc (mA cm-2)

Voc (V)

FF (%)

PCE (%)

19.52

0.87

20.45

67.59

11.96

22.52

0.95

21.22

63.69

12.91

23.52

0.87

17.80

63.95

9.93

24.52

0.85

16.25

61.68

8.52

Table 3. Photovoltaic parameters of PSC with different spin coating speed of HTM Spin coating speed (rpm)

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

4000

0.89

21.10

58.67

11.08

5000

1.03

21.82

58.23

13.05

6000

0.87

21.80

63.69

12.15

To investigate the reason of the lower PCE for CMT further, the hole transport mobility of CMT was measured by the space-charge-limited-current (SCLC)


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technique.

First,

an

ITO/PEDOT:PSS/CMT/MoO3/Al-structured

device

was

fabricated, as shown in Figure 5a, then the hole mobility of CMT was measured under dark conditions, as shown in Figure 5b. The values were calculated using the Mott– Gurney law: J

9

8

The hole transport mobility of the CMT film, averaged over four measurements, was 2.6×10-5 cm2·V-1·S-1. This is lower than that of Spiro-OMeTAD (between 10-5 and 10-4 cm2·V-1·S-1)30, which could influence negatively the charge transport from perovskite to the back contact, then hinder the PCE of the PSCs.

Figure 5. (a) Device structure of ITO/PEDOT:PSS/CMT/MoO3/Al for SCLC. (b) J1/2-V curves of ITO/PEDOT:PSS/CMT/MoO3/Al.

The morphology of the CMT layers on the top of perovskite layer was also explored as well. When the concentration of CMT was 22.52 mg ml-1 in chlorobenzene, the CMT films with spin-coating speed 4000 rpm shows some aggregation and rough surface (Figure 6a), which was negative for the charge transport. The root-mean-square (RMS) roughness value was 13.7 nm. As the spin-coating speed



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increased, the film became more uniform. The RMS values were 4.88 nm for the 5000 rpm spin-coating speed CMT film (Figure 6b) and 5.38 nm for the 6000 rpm one (Figure 6c), respectively. When the concentration of CMT increased to 24.52 mg ml-1, the CMT film with 5000 rpm spin-coating speed became rougher (Figure 6d, RMS is 15.3 nm). The results of the AFM morphology for the CMT under different fabrication conditions were corresponding well with the PCE results of the PSC devices.

Figure 6. AFM images (5 µm × 5 µm) of CMT layers under different fabrication conditions on the top of perovskite layer: a, 4000 rpm, 22.52 mg ml-1; b, 5000 rpm, 22.52 mg ml-1; c, 6000 rpm,


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22.52 mg ml-1; d, 5000 rpm, 24.52 mg ml-1.

Good device reproducibility is also important for PSCs. The efficiency histogram (Figure 6) of the PSC devices fabricated using CMT as the HTM shows that over 85% of all devices (of the 28 tested) had a PCE higher than 11%.

Figure 7. Histograms of 28 device PCE based on 22.52 mg ml-1 CMT with 5000 rpm spin coating speed.

Furthermore, the stability investigation of the PSCs devices employing CMT and Spiro-OmeTAD as HTM without encapsulation was proceeded in a high humility (70% RH) environment at 60℃ for 24h, under AM 1.5 illumination of 100 mW cm-2, as shown in Figure 8. It can be seen that the PCE of the device based on Spiro-OMeTAD was reduced to 0 after 20h, but the device based on CMT can still retain 30% of the initial PCE after 20h and the complete decomposition of CH3NH3PbI3 occurred at about 24h. During the test process, even the PCE of the devices decreased to 0 after 24 h, the PSCs device based on CMT retained higher ratio of the initial PCE than the ones based on Spiro-OMeTAD in most of time, demonstrating that the CMT-based PSCs devices have better stability. And the reason of the better stability is mainly due to the CMT layer can protect the perovskite layer better than Spiro-OmeTAD layer



which is in accord with the better hydrophobicity of CMT.

Normalized PCE (%)

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

100 80 60 40

Spiro-OMeTAD CMT

20 0 0

4

8

12

16

20

24

Time (h) Figure 8. E℃ciency evolution of the PSC devices in a high humidity (70 RH) environment at 60℃ without encapsulation for 24h, under AM 1.5 illumination of 100 mW cm-2.

Conclusions In conclusion, we report a dopant-free HTM, (2-ethylhexyl)-9H-carbazole substituted by N,N-di-p-methylthiophenylamine at the 2,7-positions (CMT), which was synthesized by a very simple method. It is the first demonstration of four methylthiol groups rather than methoxy groups, at the para-positions of the diphenylamine, which is a much simpler structure than Spiro-OMeTAD. The planar PSC devices based on CH3NH3PbI3 were fabricated with the n-i-p architecture. PSCs with pristine CMT as the HTM achieved a PCE as high as 13.05%, with a Voc of 1.03 V, Jsc of 21.82 mA/cm2, and an FF of 58.23%. While the lower hole transport mobility of CMT limits the PCE relative to that attainable with Spiro-OMeTAD, the VOC is competitive against the 1.02 V achieved with the Spiro-OMeTAD-based device.

Supporting Information Figure S1-7, table S1 and the fabrication of the planar PSC devices.

Acknowledgment


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This work was supported by the National Natural Science Foundation of China (51572037, 51603021, 51673139), 333 Project of Jiangsu Province (BRA2017353) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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(30) Poplavskyy,

D.;

Nelson,

J.

Nondispersive

Hole

Transport

in

Amorphous

Methoxy-Spirofluorene-Arylamine Organic Compound. J. Appl. Phys. 2003, 93, 341-346.

TOC Graphic


Films

of

9H-Carbazole - ACS Publications - American Chemical Society

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