A Simple Carbazole-Triphenylamine Hole Transport Material for

Article Cite This: J. Phys. Chem. C 2018, 122, 26337−26343

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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*,† †

Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing, 102206, P. R. China Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230088, P. R. China § NAAM Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia ∥ Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

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‡

S Supporting Information *

ABSTRACT: In this work, a simple carbazole-based hole transport material with triphenylamine moieties, termed LD22, has been used in perovskite solar cells (PSCs). It is noted that LD22 exhibits a proper HOMO level of −5.27 eV, high hole mobility of 1.65 × 10−5 cm2 V−1 s−1, and relatively high glass-transition temperature of 132 °C. When LD22 was used in PSCs, pristine LD22-based PSCs showed a power conversion efficiency (PCE) of 13.04%. When LD22 is doped, the PCE improves to a promising 17.18%. More importantly, the concentration of LD22 has little influence on the PSC performance regardless of the existence of dopants, which shows good repeatability. As a reference, the device with doped 2,2′,7,7′-tetrakis(N,N-bis(4-methoxyphenyl)amino)-9,9′spirobifluorene (spiro-OMeTAD) shows a PCE of 17.73%. On the other hand, the laboratory synthesis cost of LD22 is much lower than that of spiroOMeTAD. Therefore, the results indicate that the simple carbazole-triphenylamine compounds own the potential to be doped-free HTM and LD22 could be a promising HTM candidate for highperformance PSCs due to its simple structure.

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unit,13−16 which have provided a wide range of HTMs by simple synthetic procedures, such as a carbazole-containing unit linked by tetraphenylethylene (W217), benzene derivates (V886,6 V885,18 X51,19 SGT-405,20,21 SGT-411,22 HBZ-71,23 SYN1,24 etc.), benzothiadiazole (JY-625), bipyridine (F3326), thiophene derivates (DDOF, 27 V862 28 ), dibenzofuran (BF00229), pyrene (Dly-230), or the HTMs with a carbazole core and different terminal groups (CZ-TA,31 V950,32 X25,33 HL2,34 CMO,35 CMT,36 etc.). However, the PSCs employing the reported carbazole-based HTMs are still unsatisfactory compared to spiro-OMeTAD. Moreover, large amounts of dopants (lithium bis(tri fluoromethanesulfonyl)imide (LiTFSI) or 4-tert-butylpyridine (TBP)) should be used to improve the performance, and which also suffer complicated molecular structures.37,38 In this work, we employ a new simple carbazole-based HTM with triphenylamine moieties, termed LD22 (the chemical structure is shown in Figure 1), in perovskite solar cells. Experimental measurements show that LD22 owns appropriate HOMO level, which matched well with the valence band of

INTRODUCTION Since the first report by Miyasaka et al. in 2009,1 the organic− inorganic hybrid perovskite perovskite solar cells (PSCs) have attracted considerable attention in the past few years. The photoelectric conversion efficiency (PCE) of PSCs over 22% has been achieved until now.2,3 The conventional PSCs are composed of an electron-transport layer, a perovskite light harvester, hole transport materials (HTMs), and a counter electrode. HTM plays a crucial role in facilitating hole transfer from perovskite to counter electrode and suppressing charge recombination.4 2,2′,7,7′-Tetrakis (N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is the most commonly used HTM for high-performance PSCs. However, the complicated synthesis steps and difficult purification process of spiro-OMeTAD hinder its practical applications.5,6 Moreover, spiro-OMeTAD must be sufficiently thick to achieve high PSCs, which needs more materials usage.7,8 Therefore, developing new substitutes for spiro-OMeTAD is urgently necessary. At present, small organic molecule HTMs have been widely studied in PSCs9,10 because of their simple structure, easy synthesis, and smaller batch-to-batch variation compared to polymers.11,12 Among the reported HTMs, carbazole derivatives have drawn much attention because of the low-cost and good charge-transport ability of the carbazole © 2018 American Chemical Society

Received: August 22, 2018 Revised: October 13, 2018 Published: October 29, 2018 26337

DOI: 10.1021/acs.jpcc.8b08168 J. Phys. Chem. C 2018, 122, 26337−26343

Article

The Journal of Physical Chemistry C

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

Cyclic voltammetry was recorded on a CHI660d electrochemical analyzer (CH Instruments, Inc., China). The threeelectrode system was used and consists of a Pt wire counter electrode, a calomel reference electrode (SCE) in saturated KCl solution, and a Pt working electrode. The 0.1 M tetrabutylammonium hexafluorophosphate in CH2Cl2 acted as a supporting electrolyte for CV measurements at 50 mV s−1 (vs Fc/Fc+ as external reference). The glass transition temperature (Tg) of HTM was performed by differential scanning calorimetry (Q2000, TA Instruments-Waters LLC, USA) with a rate of 20 °C min−1 under N2 flow. A fieldemission scanning electron microscope (SU8020, Hitachi, Japan) was used to study the morphologies of the samples. Time-resolved photoluminescence (TRPL) was recorded on a fluorescence detector (QM400, Photo Technology International, USA) with a pulsed nitrogen laser. The J−V characteristics of the cells were performed under AM 1.5 (100 mW cm−2) illumination with a digital source meter (2420, Keithley, USA) and a 3A grade solar simulator (94043A, Newport, USA). The IPCE was recorded on a QE/IPCE measurement kit (Newport, USA).

perovskite materials. Moreover, LD22 also shows high hole mobility and relatively high glass transition temperature. Even without dopants, PSCs with LD22 as the HTM layer show a maximum PCE of 13.04%. When LD22 was doped with LiTFSI TBP, the champion PCE is improved to 17.18%, which is even comparable to doped spiro-OMeTAD. Moreover, the concentration of LD22 had little effect on the PSC performance. Therefore, LD22 has the promising potential to replace expensive spiro-OMeTAD for high-performance PSCs.

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EXPERIMENTAL SECTION Device Fabrication. The PSC device fabrication procedures were mainly similar to our reported paper.39,40 Fluorine doped tin oxide (FTO) glass substrates were sequentially washed with the detergent solution, deionized, and treated with ethanol and then treated with O3 for 15 min. Then, a compact TiO2 layer was deposited on the cleaned FTO substrates in air by spray pyrolysis of 1 mL of titanium diisopropoxide bis(acetylacetonate) in 10 mL of anhydrous isopropanol. Mesoporous TiO2 layers were spin coated at 4000 rpm for 15 s, using a diluted particle paste (Dyesol 30 NR-D) in ethanol solution (TiO2/ethanol = 1/5). Then, the obtained films were sintered at 500 °C for 30 min on a hot plate. The lead excess (FAPbI3)0.85(MAPbBr3)0.15 precursor solutions were prepared by dissolving PbI2 (1.12 M), FAI (1.1 M), PbBr2 (0.2 M), and MABr (0.2 M) in the mixed solvent of DMF:DMSO = 4:1 (V:V) in a N2-filled glovebox. The perovskite precursor solution was deposited on the above substrate by spin coating with a two-step program at 1500 rpm for 15 s and 5000 rpm for 30 s. An ∼90 μL portion of chlorobenzene was dropped on the spinning substrate during the second spin-coating step 15 s prior to the end of the procedure. The films were then annealed at 100 °C for 90 min in the dry air flow glovebox. After cooling down, HTMs were deposited on the perovskite substrates by spin coating at 4000 rpm for 20 s. The HTM was dissolved in anhydrous chlorobenzene (LD22, 20−50 mg mL−1; spiro-OMeTAD, 73 mg mL−1) without or with tert-butylpyridine (tBP) and tris(bis(trifluoromethylsulfon-yl)imide) (Li-TFSI). The molar ratio of additives for HTM was 0.5 and 3.3 for Li-TFSI and TBP. Finally, a Au back electrode (∼60 nm) was deposited by thermal evaporation. Characterization. The used characterizations can be found in our previous reports,8,40 and the test methods are also shown as follows. The absorption spectra of HTMs were taken on a UV−vis spectrophotometer (U3900H, Hitachi, Japan). The steady-state PL were obtained on a fluorescence detector (Fluorolog-3, HORIBA, USA) with a 450 W xenon CW lamp.

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RESULTS AND DISCUSSION Photophysical, Electrochemical, Hole Mobility, Conductivity, and Thermal Stability Properties. The optical characteristics of LD22 were examined by UV−vis absorption and photoluminescence spectroscopy in CH2Cl2 solution. The spectra are shown in Figure 2. The related data are summarized in Table 1. The absorption peak for LD22 is centered at 381 nm, and the adsorption edge is shown at around 418 nm, leading to an optical bandgap of around 2.97 eV (Eg = 1240/λedge). Weak absorption of LD22 in the visible

Figure 2. Absorption and photoluminescence spectra of LD22 and spiro-OMeTAD. 26338


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Table 1. Summary of the Optical, Electrochemical, Hole Mobility, and Thermal Properties of LD22 and Spiro-OMeTAD HTM LD22 spiro-OMeTAD

λmax (nm) 381 386

λPL (nm) 450 428

λedge (nm) 418 414

Eg (eV) 2.97 2.99

HOMO (eV) −5.27 −5.14

LUMO (eV) −2.30 −2.15

Tg (°C) 132 122

μ (cm2 V−1 s−1) −5

1.65 × 10 1.31 × 10−5

σ (S cm−1) 9.04 × 10−6 6.23 × 10−6

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

Figure 4. (a) J−V curves (ITO/PEDOT:PSS/HTM/Au devices) from SCLC measurements. (b) J−V curves (FTO/HTM/Au devices) for LD22 and spiro-OMeTAD, respectively.

intrinsic hole mobility and conductivity with dopants of LD22 are 1.65 × 10−5 cm2 V−1 s−1 and 9.04 × 10−6 S cm−1, which is similar to those of reference spiro-OMeTAD (1.31 × 10−5 cm2 V−1 s−1 and 6.23 × 10−6 S cm−1, respectively). In Figure S1, the better planarity of LD22 compared with spiro-OMeTAD is shown, which may be beneficial to π−π stacking. Moreover, the UV−vis absorption of LD22 and spiro-OMeTAD in film and solution state is performed, as shown in Figure S2. The onset points of them are red-shifted. It can be found that the red-shift of LD22 (9 nm) is higher than that of spiroOMeTAD (3 nm), implying that LD22 owns the potential to be more favorable for intermolecular packing.44,45 Accordingly, LD22 exhibits a higher charge carrier mobility than spiroOMeTAD. The thermal stability of LD22 is studied by differential scanning calorimetry measurements. As shown in Figure 5, LD22 shows a glass transition temperature (Tg) of 132 °C, which is higher than that of spiro-OMeTAD (122 °C).46 The higher Tg value suggests the more stable amorphous state, which is good for the stability of corresponding PSC devices.47 PL Measurements and Device Performance. To investigate the hole transfer ability of LD22 in the perovskite/HTM interface, the TIPL (steady-state photoluminescence) and TRPL (time-resolved photoluminescence) were carried out and compared with spiro-OMeTAD. LD22 and spiro-OMeTAD were doped the same as device fabrication (50

region indicates the influence of it on the light absorption of the perovskite layer is negligible.41 The photoluminescence (PL) emission spectrum of LD22 shows a maximum peak at 450 nm, with a relatively large Stokes shift of 69 nm. To estimate the electrochemical properties of LD22, cyclic voltammetry (CV) measurements were performed in CH2Cl2 solution. As shown in Figure 3, the redox peak of LD22 is highly reversible, implying its superior electrochemical stability. The HOMO level of LD22 evaluated from the CV data is −5.27 eV, which is lower than that of spiro-OMeTAD (−5.14 eV). The valence band of (FAPbI3)0.85(MAPbBr3)0.15 is −5.65 eV,42 illustrating that LD22 owns the favor for hole transfer (Figure 3b). In the meantime, the LUMO level of LD22 calculated from the measured Eg is −2.30 eV, which is significantly higher than the conduction band of perovskite, which can efficiently prevent the electron transfer from perovskite to counter electrode. The hole mobility and conductivity of LD22 are important factors that impact the performance of PSCs. The hole mobilities of LD22 and spiro-OMeTAD were determined by using the space-charge-limited current (SCLC) method according to previous reports.43 The conductivities of them are measured with a device of FTO/HTM/Au. The thicknesses of LD22 and spiro-OMeTAD for SCLC (conductivity) measurements are 39 (42) and 40 (45) nm, respectively. The obtained results are shown in Figure 4. The 26339


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PCE of 17.18%, with a Jsc value of 21.12 mA cm−1, a Voc value of 1.06 V, and a FF of 77%. The effect of LD22 deposition amounts on the PSC performance was also studied. The corresponding thickness of LD22 when used in PSCs is around 41, 71, and 104 nm for 20, 35, and 50 mg mL−1, respectively. As shown in Figure 7b, we found that the concentration of LD22 shows little effect on PSC performance in the presence or absence of dopants, which demonstrates high reproducibility of LD22 acting as HTM for PSCs. Figure 7a also shows the J−V curves of PSCs with doped spiro-OMeTAD as the reference. The maximun PCE of the doped spiro-OMeTADbased device is 17.73%, which is comparable with that of doped LD22. It can be found that the doped LD22 based devices show a lower Voc than doped spiro-OMeTAD, implying LD22’s own slightly inferior charge extraction ability (as shown in PL study). It can be predicted that a PCE of the LD22-based device close to 20% is attainable after the extensive optimization of PSC fabrication. Moreover, it is noted that the hysteresis behavior in the devices employing pristine or doped LD22 is similar to that of doped spiroOMeTAD (Figure 7a and Table 2). The stabilized power outputs at the maximum power point for the doped LD22 and spiro-OMeTAD based devices are measured to evaluate the device performance judiciously. As shown in Figure S3, the steady-state efficiencies of the best-performing device with LD29 and spiro-OMeTAD measured at a bias of 0.84 V for 150 s are approximately 19.41 and 20.12 mA cm−2, leading to a stabilized output power of 16.3 and 16.9%, respectively. The incident photo-to-current conversion efficiency (IPCE) spectra of the best-performing devices employing doped LD22 and spiro-OMeTAD are shown in Figure 7c. Promisingly, doped LD22 shows a similar IPCE value in the range 400−700 nm compared with that of spiro-OMeTAD, which is also in agreement with J−V measurements. It should be stated that light-soaking plays an important role in the performance enhancement of pristine LD22-based devices. As shown in Figure 7d and Table S1, the performance of a pristine LD22-based device gradually improved and is saturated after about 5 min with prolonged light soaking. As the light soaking shows a minor effect on the conductivity of LD22 (Figure 4b) and the same phenomenon (continuous light soaking improves PCE) appears after the pristine LD22based device was stored under dark conditions for 24 h, the photoinduced doping of LD22 upon light exposure could be the main reason for the above results.48,49 Besides, light

Figure 5. Differential scanning calorimetry curve of LD22.

mg mL−1). Normally, the TIPL quenching and TRPL decay time (τe) reducing in bilayer films result from the hole migration from the VB of perovskite to the HOMO level of the HTM layer. As shown in Figure 6a, the quenching values of the perovskite/HTM film are 84 and 88% for LD22 and spiroOMeTAD. From the TRPL curves (Figure 6b), the fitted τe values are around 15.6 ns for pristine perovskite, 4.9 ns for perovskite/LD22 (doped), and 4.6 ns for perovskite/spiroOMeTAD. Therefore, LD22 shows comparable hole transfer ability with spiro-OMeTAD. To explore the possibility of LD22 acting as a HTM for PSCs, we fabricated the conventional mesoscopic (FAPbI3 )0.85(MAPbBr 3) 0.15 -based devices (FTO/bl-TiO 2/mpTiO2/perovskite/HTM/Au) by a one-step solvent engineering method according to previous reports.39 The detailed PSC fabrication procedure is illustrated in the Experimental Section. To get comprehensive results, we fabricate PSCs with pristine and doped LD22 (dopants are Li-TFSI and TBP) as HTMs. We also study the influence of concentration of LD22 on the device performance. Figure 7a shows the current density−voltage (J−V) curves from the PSCs under reverse (from Voc to Jsc) and forward (from Jsc to Voc) scans under AM 1.5 illumination (100 mW cm−2), and the corresponding PCE, open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) are listed in Table 2. Under reverse scan, the best-performing PSC employing pristine LD22 shows a PCE of 13.04%, together with a Jsc of 19.60 mA cm−1, a Voc of 1.03 V, and a FF of 65%. Upon the addition of additives, the best-performing LD22 exhibits a high

Figure 6. (a) TIPL spectra, excitation wavelength 473 nm, and (b) TRPL spectra of perovskite (P), P/LD22 (doped), and P/spiro-OMeTAD (doped). Excitation at 445 nm, monitored at 765 nm. 26340


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Figure 7. (a) J−V characteristics under different scan directions. (b) The PCE change along with the concentration variation of LD22. (c) IPCE spectra of the best-performing devices using doped LD22 and doped spiro-OMeTAD as HTMs. (d) Light-soaking effect of pristine LD22-based devices.

HOMO level and higher hole mobility and thermal stability compared with spiro-OMeTAD. Photoluminescence measurements demonstrate that LD22 has good charge extraction ability equal to spiro-OMeTAD. The PSCs with pristine LD22 achieve a PCE of 13.04%. When LD22 was doped, the PCE improved to a promising 17.18%, which is comparable to that of spiro-OMeTAD. Moreover, the concentration of LD22 has a minor effect on the PSC performance regardless of the existence of dopants.

Table 2. Photovoltaic Parameters of Best-Performing PSCs with Different HTMs and Measured through Reverse and Forward Scans Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

reverse forward reverse forward reverse

1.03 1.01 1.06 1.03 1.09

19.60 19.61 21.12 21.13 21.63

65 61 77 71 75

13.04 12.08 17.18 15.40 17.73

forward

1.06

21.62

72

16.44

HTM pristine LD22 doped LD22 doped spiro-OMeTAD

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08168.

soaking does not show any additional effect on the performance of the doped LD22-based device. As shown in Figure S1, the aging tests of the devices with different HTMs were also performed. The devices were stored in a dark drying oven (around 15% relative humidity at R.T.) Promisingly, the doped LD22 and spiro-OMeTAD show similar stability. Interestingly, pristine LD22-based devices show gradually improved PCE with 100 h (Figure S4), however, which shows a more obvious reduction in PCE after rising to the maximum PCE. The above results may be due to the longer exposure time on light of each J−V test for pristine LD22-based devices than that of doped HTMs, which suffer more illumination decomposition. On the other hand, the estimated synthesis and purification cost of LD22 is much lower than that of spiro-OMeTAD (Tables S3−S5), which further exhibits its application potential in PSCs.

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Optimized structures, absorption spectra in film and solution of LD22 and spiro-OMeTAD, detailed photovoltaic parameters of PSCs based on various HTMs, stabilized power output, stability figure, detailed synthetic route, hysteresis behavior, and materials cost (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 1061772268. *E-mail: [email protected]. Phone: +86 55165593222. ORCID

Fantai Kong: 0000-0002-9548-6781 Jianxi Yao: 0000-0002-5472-9337 Songyuan Dai: 0000-0001-5710-9208

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CONCLUSIONS In summary, we reported a simple carbazole-triphenylamine compound (LD22) as a HTM for PSCs. LD22 has the proper

Notes

The authors declare no competing financial interest. 26341


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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (No. 2016YFA0202400), the National Key Basic Research Program of China (973 Program) (No. 2015CB932201), and the National Natural Science Foundation of China (Nos. 51702096 and 61705066).

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A Simple Carbazole-Triphenylamine Hole Transport Material for

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