A Simple Carbazole-Triphenylamine Hole Transport Material for

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C: Energy Conversion and Storage; Energy and Charge Transport

A Simple Carbazole-Triphenylamine Hole Transport Material for Perovskite Solar Cells Xuepeng Liu, Xiaoqiang Shi, Cheng Liu, Yingke Ren, Yunzhao Wu, Weng Yang, Ahmed Alsaedi, Tasawar Hayat, Fantai Kong, Xiaolong Liu, Yong Ding, Jianxi Yao, and Songyuan Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08168 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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The Journal of Physical Chemistry

1

A Simple Carbazole-Triphenylamine Hole Transport Material for

2

Perovskite Solar Cells

3 4

Xuepeng Liu,a Xiaoqiang Shi,a Cheng Liu,a Yingke Ren,a Yunzhao Wu,a Weng Yang,a

5

Ahmed Alsaedi,c Tasawar Hayat,c,d Fantai Kong,b* Xiaolong Liu,a Yong Ding,a Jianxi

6

Yao,a Songyuan Dai,a*

7 8

a

9

University, Beijing, 102206, P. R. China. E-mail: [email protected]

Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power

10

b

11

Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences,

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Hefei, Anhui, 230088, P. R. China. E-mail: [email protected]

13

c

14

Abdulaziz University, Jeddah, 21589, Saudi Arabia

15

d

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of

NAAM Research Group, Department of Mathematics, Faculty of Science, King

Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

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Abstract

18

In this work, a simple carbazole-based hole transport material with triphenylamine

19

moieties, termed LD22, has been used in perovskite solar cells (PSCs). It is noted that

20

LD22 exhibits a proper HOMO level of -5.27 eV, high hole mobility of 1.65 × 10-5

21

cm2V-1s-1, and relatively high glass-transition temperature of 132 oC. When LD22 was

22

used in PSCs, pristine LD22-based PSCs show a power conversion efficiency (PCE) of

23

13.04%. When LD22 is doped, the PCE improves to a promising 17.18%. More

24

importantly, the concentration of LD22 has little influence on the PSCs performance

25

regardless of the existence of dopants, which shows good repeatability. As a reference,

26

the device with doped 2,2’,7,7’-tetrakis(N,N-bis(4-methoxyphenyl)amino)-9,9’-

27

spirobifluorene (spiro-OMeTAD) shows a PCE of 17.73%. On the other hand, the

28

laboratory synthesis cost of LD22 is much lower than that of spiro-OMeTAD. 1

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1

Therefore, the results indicate that the simple carbazole-triphenylamine compounds

2

own the potential to be doped-free HTM and LD22 could be a promising HTM

3

candidate for high-performance PSCs due to its simple structure.

4 5

Introduction

6

Since the first report by Miyasaka et al. in 2009,1 the organic-inorganic hybrid

7

perovskite perovskite solar cells (PSCs) have attracted considerable attention in the past

8

few years. The photoelectric conversion efficiency (PCE) of PSCs over 22% has been

9

achieved till now.2,3 The conventional PSCs are composed of electron-transport layer,

10

perovskite light harvester, hole transport materials (HTM) and counter electrode. HTM

11

plays a crucial role in facilitating hole transfer from perovskite to counter electrode and

12

suppressing

13

pmethoxyphenylamino)-9,9’-spirobifluorene (Spiro-OMeTAD) is the most commonly

14

used HTMs for high-performance PSCs. However, the complicated synthesis steps and

15

difficult purification process of spiro-OMeTAD hinder its practical applications.5,6

16

Moreover, spiro-OMeTAD must be sufficiently thick to achieve high PSCs, which

17

needs more materials usage.7,8 Therefore, developing new substitutes for spiro-

18

OMeTAD is urgently necessary. At present, small organic molecule HTMs have been

19

widely studied in PSCs9,10 because of their simple structure, easily synthesis and smaller

20

batch-to-batch variation compared to polymers.11,12 Among the reported HTMs,

21

carbazole derivatives have drawn much attention because of the low-cost and good

22

charge-transport ability of carbazole unit,13-16 which have provided a wide range of

23

HTMs by simple synthetic procedures, such as carbazole-containing unit linked by

24

tetraphenylethylene (W217), benzene and its derivates (V886,6 V885,18 X51,19 SGT-

25

405,20,21 SGT-411,22 HBZ-71,23 SYN1,24 et al.), benzothiadiazole (JY-625), bipyridine

26

(F3326), thiophene derivates (DDOF,27 V86228), dibenzofuran (BF00229), pyrene (Dly-

27

230) or the HTMs with carbazole core and different terminal groups (CZ-TA, 31 V950,

28

32

29

carbazole-based HTMs are still unsatisfactory compared to spiro-OMeTAD or large

X25,

33

HL2,

recombination.4

charge

34

CMO,

35

2,2’,7,7’-Tetrakis

(N,N-di-

CMT36 et al.). However, the PSCs with the reported

2


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amounts of dopants (lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) or 4-tert-

2

butylpyridine (TBP)) should be used to improve the performance, as well as suffering

3

complicated molecular structures.37,38

4

In this work, we employ a new simple carbazole-based HTM with triphenylamine

5

moieties, termed LD22 (the chemical structure is shown in Figure 1), in perovskite solar

6

cells. Experimental measurements show that LD22 own appropriate HOMO level

7

matched well with perovskite materials, high hole mobility and relatively high glass-

8

transition temperature. Even without dopants, PSCs with LD22 as HTM layer shows a

9

maximum PCE of 13.04%. When LD22 was doped with Li-TFSI, TBP and FK209, the

10

champion PCE is improved to 17.18%, which is even comparable to doped spiro-

11

OMeTAD. Moreover, the concentration of LD22 had little effect on the PSCs

12

performance. Therefore, LD22 has the promising potential to replace expensive spiro-

13

OMeTAD for high-performance PSCs.

14 15

Figure 1. Chemical structures of LD22 and spiro-OMeTAD.

16 17

Experimental Section

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Device fabrication

19

The PSC devices fabrication procedures were mainly similar to our reported paper.39,40

20

Fluorine doped tin oxide (FTO) glass substrates were sequentially washed with the

21

detergent solution, deionized, and ethanol. and then treated with O3 for 15 minutes.

22

Then a compact TiO2 layer was deposited on the cleaned FTO substrates in air by spray

23

pyrolysis of 1 mL Titanium diisopropoxide bis(acetylacetonate) in 10 mL anhydrous

24

isopropanol. Mesoporous TiO2 layer were spin coated at 4000 rpm for 15 s, using a 3



1

diluted particle paste (Dyesol 30 NR-D) in ethanol solution (TiO2/ethanol = 1/5). Then

2

the obtained films were sintered at 500 oC for 30 min on a hot plate. The lead excess

3

(FAPbI3)0.85(MAPbBr3)0.15 precursor solutions were prepared by dissolving PbI2 (1.12

4

M), FAI (1.1 M), PbBr2 (0.2 M) and MABr (0.2 M) in the mixed solvent of

5

DMF:DMSO = 4:1 (V:V) in N2-filled glove box. The perovskite precursor solution was

6

deposited on above substrate by spin coating with a two-step program at 1500 rpm for

7

15 s and 5000 rpm for 30 s. ~ 90 µl chlorobenzene was dropped on the spinning

8

substrate during the second spin-coating step 15 s prior to the end of the procedure. The

9

films were then annealed at 100 oC for 90 min in the dry air flow glove box. After

10

cooling down, HTMs were deposited on the perovskite substrate s by spin coating at

11

4000 rpm for 20 s. The HTM was dissolved in anhydrous chlorobenzene (LD22: 20-50

12

mg mL-1, spiro-OMeTAD: 73 mg mL-1) without or with Tert-butylpyridine (tBP) and

13

tris(bis(trifluoromethylsulfon-yl)imide) (Li-TFSI). The molar ratio of additives for

14

HTM was 0.5 and 3.3 for Li-TFSI and TBP. Finally, an Au back electrode (~ 60 nm)

15

was deposited by thermal evaporation.

16 17

Characterization

18

The used characterizations can be found in our previous reports,8,40 and the test methods

19

are also shown as follows. The absorption spectrum of HTMs were performed on a UV-

20

vis spectrophotometer (U3900H, Hitachi, Japan). The steady-state PL were obtained on

21

a fluorescence detector (Fluorolog-3, HORIBA, USA) with a 450 W xenon CW lamp.

22

Cyclic voltammetry was recorded on a CHI660d electrochemical analyzer (CH

23

Instruments, Inc., China). The three electrode system was used and consist of a Pt wire

24

counter electrode, a calomel reference electrode (SCE) in saturated KCl solution, and a

25

Pt working electrode and. The 0.1 M tetrabutylammonium hexafluorophosphate in

26

CH2Cl2 acted as supporting electrolyte for CV measurements at 50 mV s-1 (vs. Fc/Fc+

27

as external reference). The glass transition temperature (Tg) of HTM was performed by

28

differential scanning calorimetry (Q2000, TA Instruments-Waters LLC, USA) with a

29

rate of 20 oC min-1 under N2 flow. A field-emission scanning electron microscope

30

(SU8020, Hitachi, Japan) was used to study the morphologies of the samples. TRPL 4


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were recorded on a Fluorescence Detector (QM400, Photo Technology International,

2

USA) with a pulsed nitrogen laser. The J-V characteristics of the cells were performed

3

under AM 1.5 (100 mW cm-2) illumination with a digital source meter (2420, Keithley,

4

USA) and a 3A grade solar simulator (94043A, Newport, USA). The IPCE was

5

recorded on QE/IPCE measurement kit (Newport, USA).

6 7

Results & discussion

8

Photophysical, electrochemical, hole mobility, conductivity, thermal stability

9

properties

10

The optical characteristics of LD22 was examined by UV-vis absorption and

11

photoluminescence spectroscopy in CH2Cl2 solution. The spectra are shown in Figure

12

2. The related data are summarized in Table 1. The absorption peak for LD22 is centered

13

at 381 nm, and the adsorption edge is shown at around 418 nm, leading to an optical

14

bandgap of around 2.97 eV (Eg =1240/λedge). Weak absorption of LD22 in the visible

15

region indicates the influence of it on the light absorption of perovskite layer is

16

negligible.41 The photoluminescence (PL) emission spectrum of LD22 shows a

17

maximum peak at 450 nm, with a relatively large Stokes shift of 69 nm.

18

19 20

Figure 2. Absorption and photoluminescence spectra of LD22 and spiro-OMeTAD.

21 22

Table 1 Summary of the optical, electrochemical, hole mobility and thermal properties 5



1

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of LD22 and spiro-OMeTAD. HTM

λmax

λPL

λedge

Eg

HOMO

LUMO

Tg

μ

σ

(nm)

(nm)

(nm)

(eV)

(eV)

(eV)

(oC)

(cm2V-1s-1)

(S cm-1)

LD22

381

450

418

2.97

-5.27

-2.30

132

1.65×10-5

9.04×10-6

Spiro-

386

428

414

2.99

-5.14

-2.15

122

1.31×10-5

6.23×10-6

OMeTAD 2 3

To estimate the electrochemical properties of LD22, cyclic voltammetry (CV)

4

measurements were performed in CH2Cl2 solution. As shown in Figure 3, the redox

5

peak of LD22 is highly reversible, implying its superior electrochemical stability. The

6

HOMO level of LD22 evaluated from the CV data is -5.27 eV, which is lower than that

7

of spiro-OMeTAD (-5.14 eV). As the valence band of (FAPbI3)0.85(MAPbBr3)0.15 is -

8

5.65 eV,42 illustrating that LD22 owns the favor for hole transfer (Figure 3b). In the

9

meantime, the LUMO level of LD22 calculated from the measured Eg is -2.30 eV,

10

which is significantly higher than the conduction band of perovskite, which can

11

efficiently prevent the electron transfer from perovskite to counter electrode.

12

13 14

Figure 3. (a) Cyclic voltammograms of LD22 and spiro-OMeTAD. (b) energy level

15

diagram with LD22 and spiro-OMeTAD.

16 17

The hole mobility and conductivity of LD22 are important factors that impact the

18

performance of PSCs. The hole mobilities of LD22 and spiro-OMeTAD were

19

determined by using space- charge-limited current (SCLC) method according to 6


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1

previous reports.43 The conductivity of them are measured with a device of

2

FTO/HTM/Au. The thickness of LD22 and spiro-OMeTAD for SCLC (conductivity)

3

measurements are 39 (42) and 40 (45) nm, respectively. The obtained results are shown

4

in Figure 4. The intrinsic hole mobility and conductivity with dopants of LD22 are

5

1.65×10-5 cm2V-1s-1 and 9.04×10-6 S cm-1, which is similar to those of reference spiro-

6

OMeTAD (1.31×10-5 cm2V-1s-1 and 6.23×10-6 S cm-1, respectively). As shown in Figure

7

S1, the LD22 better planarity compared with spiro-OMeTAD, which may be beneficial

8

to π-π stacking. Moreover, the UV-Vis absorption of LD22 and spiro-OMeTAD in film

9

and solution state are performed, as shown in Figure S2. The onset points of them are

10

red-shifted. It can be found that the red-shifts of LD22 (9 nm) is higher than that of

11

spiro-OMeTAD (3 nm), implying that LD22 owns the potential to be more favorable

12

for intermolecular packing.44,45 Accordingly, LD22 exhibits higher charge carrier

13

mobility than spiro-OMeTAD.

14

15 16

Figure 4. (a) J-V curves (ITO/PEDOT:PSS/HTM/Au devices) from SCLC

17

measurements; (b) J-V curves (FTO/HTM/Au devices) for LD22 and spiro-OMeTAD,

18

respectively.

19 20

The thermal stability of LD22 is studied by differential scanning calorimetry

21

measurements. As shown in Figure 5, LD22 shows a glass transition temperature (Tg)

22

of 132 oC, which is higher than that of spiro-OMeTAD (122 oC).46 The higher Tg value

23

suggests the more stable amorphous state, which is good for the stability of

24

corresponding PSC devices.47 7



1

2

Figure 5. Differential scanning calorimetry curve of LD22.

3 4 5

PL measurements and device performances

6

To investigate hole transfer ability of LD22 in perovskite/HTM interface, the TIPL

7

(steady-state photoluminescence) and TRPL (time-resolved photoluminescence) were

8

carried out and compared with spiro-OMeTAD. LD22 and spiro-OMeTAD were doped

9

as same as device fabrication (50 mg mL-1). Normally, the TIPL quenching and TRPL

10

decay time (τe) reducing in bilayer films are resulted from the hole migration from the

11

VB of perovskite to the HOMO level of HTM layer. As shown in Figure 6a, the

12

quenching values of perovskite/HTM film are 84% and 88% for LD22 and spiro-

13

OMeTAD. From the TRPL curves (Figure 6b), the fitted τe values are around: 15.6 ns

14

for pristine perovskite, 4.9 ns for perovskite/LD22 (doped), 4.6 ns for perovskite/spiro-

15

OMeTAD. Therefore, LD22 shows comparable hole transfer ability with spiro-

16

OMeTAD.

17

8


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1 2

Figure 6. (a) TIPL spectra, excitation wavelength: 473 nm, and (b) and TRPL spectra

3

of perovskite (P), P/LD22 (doped) and P/spiro-OMeTAD (doped). Excitation at 445

4

nm, monitored at 765 nm.

5 6

To explore the possibility of LD22 acting as HTMs for PSCs, we fabricated the

7

conventional mesoscopic (FAPbI3)0.85(MAPbBr3)0.15-based devices (FTO/bl-TiO2/mp-

8

TiO2/perovskite/HTM/Au) by a one-step solvent engineering method according to

9

previous reports.39 The detailed PSCs fabrication procedure is illustrated in

10

experimental section. To get comprehensive results, we fabricate PSCs with pristine

11

and doped LD22 (dopants are Li-TFSI, TBP and FK209) as HTMs, as well as the

12

influence of concentration of LD22 on the performance.

13

14

9



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1 2

Figure 7. (a) J-V characteristics under different scan direction; (b) The PCE change

3

along with the concentration variation of LD22; (c) IPCE spectra of the best-

4

performing devices using doped LD22 and doped spiro-OMeTAD as HTMs; (d) light-

5

soaking effect of pristine LD22-based devices.

6 7

Table 2 Photovoltaic parameters of best-performing PSCs with different HTMs and

8

measured through reverse and forward scans. HTM

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

pristine

Reverse

1.03

19.60

65

13.04

LD22

Forward

1.01

19.61

61

12.08

doped LD22

Reverse

1.06

21.12

77

17.18

Forward

1.03

21.13

71

15.40

doped spiro-

Reverse

1.09

21.63

75

17.73

OMeTAD

Forward

1.06

21.62

72

16.44

9 10

Figure 7a shows the current density-voltage (J-V) curves from the PSCs under

11

reverse (from Voc to Jsc) and forward (from Jsc to Voc) scans under AM 1.5 illumination

12

(100 mW cm-2), and the corresponding PCE, open-circuit voltage (Voc), short-circuit

13

current (Jsc) and fill factor (FF) are listed in Table 2.

14

Under reverse scan, the best-performing PSC employing pristine LD22 shows a

15

PCE of 13.04 %, together with a Jsc of 19.60 mA cm-1, a Voc of 1.03 V and an FF of

16

65%. Upon the addition of additives, the best-performing LD22 exhibits a high PCE of

17

17.18%, with a Jsc of 21.12 mA cm-1, a Voc of 1.06 V and an FF of 77%. The effect of 10


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1

LD22 deposition amounts on the PSCs performance were also studied. The

2

corresponding thickness of LD22 when used in PSCs are around 41, 71 and 104 nm for

3

20, 35 and 50 mg mL-1, respectively. As shown in Figure 7b, we found that the

4

concentration of LD22 show little effect on PSC performance in the presence or

5

absence of dopants, which demonstrates high reproducibility of LD22 acting as HTM

6

for PSCs. Figure 7a also shows the J-V curves of PSCs with doped spiro-OMeTAD as

7

the reference. The maximun PCE of doped spiro-OMeTAD-based device is 17.73%,

8

which is comparable with that of doped LD22. It can be found that the doped LD22

9

based devices show lower Voc than doped spiro-OMeTAD, implying that LD22 own

10

slightly inferior charge extraction ability (as shown in PL study). It can be predicted

11

that the PCE of LD22-based device close to 20% is attainable after the extensive

12

optimization of PSC fabrication. Moreover, it is noted that the hysteresis behavior in

13

the devices employing pristine or doped LD22 is similar with that of doped spiro-

14

OMeTAD (Figure 7a and Table 2). The stabilized power output at the maximum power

15

point for the doped LD22 and spiro-OMeTAD based devices to evaluate corresponding

16

performance judiciously. As shown in Figure S3, the steady-state efficiency of the best-

17

performing device with LD29 and spiro-OMeTAD measured at a bias of 0.84 V for 150

18

s are approximately 19.41 and 20.12 mA cm-2, leading to a stabilized output power of

19

16.3 and 16.9%, respectively. The incident photo-to-current conversion efficiency

20

(IPCE) spectrum of the best-performing devices employing doped LD22 and spiro-

21

OMeTAD are shown in Figure 7c. Promisingly, doped LD22 shows similar IPCE value

22

in the range of 400 – 700 nm compared with that of spiro-OMeTAD, which is also in

23

agreement with J-V measurements.

24

It should be stated that light-soaking plays an important role in the performance

25

enhancement of pristine LD22-based devices. As shown in Figure 7d and Table S1, the

26

performance of pristine LD22-based device gradually improved and is saturated after

27

about 5 minutes with prolonged light soaking. As the light soaking show minor effect

28

on the conductivity of LD22 (Figure 4b) and the same phenomenon (continuous light

29

soaking improves PCE) appears after the pristine LD22-based device was stored under

30

dark condition for 24 hours, the photoinduced doping of LD22 upon light exposure 11



1

could be the main reason for above results.48,49 Besides, light soaking does not show

2

any additional effect on the performance of doped LD22-based device.

3

As shown in Figure S1, the aging tests of the devices with different HTMs were

4

also performed. The devices were stored in a dark drying oven (around 15% relative

5

humidity at R.T.) Promisingly, the doped LD22 and spiro-OMeTAD show similar

6

stability. Interestingly, pristine LD22-based devices show gradually improved PCE

7

with 100 h (Figure S4), however, which shows more obvious reduction in PCE after

8

rising to the maximum PCE. The above results may be due to the longer exposure time

9

on light of each J-V test for pristine LD22-based devices than that of doped HTMs,

10

which suffer more illumination decomposition. On the other hand, the estimated the

11

synthesis and purification cost of LD22 is much lower than that of spiro-OMeTAD

12

(Table S3-5), which further exhibit its application potential in PSCs.

13 14

Conclusions

15

In summary, we reported a simple carbazole-triphenylamine compound (LD22) as

16

HTM for PSCs. LD22 owns the proper HOMO level, and higher hole mobility and

17

thermal stability compared with spiro-OMeTAD. Photoluminescence measurements

18

demonstrate that LD22 own good charge extraction ability equal to spiro-OMeTAD.

19

The PSCs with pristine LD22 achieve a PCE of 13.04%. When LD22 was doped, the

20

PCE improved to a promising 17.18%, which is comparable to that of spiro-OMeTAD.

21

Moreover, the concentration of LD22 has minor effect on the PSCs performance

22

regardless of the existence of dopants.

23 24

Associated content

25

Supporting Information

26

Optimized structures, absorption spectra in film and solution of LD22 and spiro-

27

OMeTAD, detailed photovoltaic parameters of PSCs based on various HTMs,

28

stabilized power output, stability figure, detailed synthetic route, hysteresis behavior,

29

and materials cost. 12


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1

Author Information

2

Corresponding Authors

3

*E-mail: [email protected]. Tel.: +86 1061772268.

4

*E-mail: [email protected]. Tel.: +86 55165593222.

5

Notes

6

The authors declare no competing financial interest.

7 8

Acknowledgements

9

This work was supported by the National Key Research and Development Program of

10

China (No. 2016YFA0202400), the National Key Basic Research Program of China

11

(973 Program) (No. 2015CB932201), and the National Natural Science Foundation of

12

China (No. 51702096 and 61705066).

13 14

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