Dopant-free and low-cost molecular “bee” hole-transporting materials for efficient and stable perovskite solar cells

Xicheng Liu a, Fei Zhang bc, Zhe Liu a, Yin Xiao bc, Shirong Wang bc and Xianggao Li *bc
aQufu Normal University, School of Chemistry and Chemical Engineering, China
bSchool of Chemical Engineering and Technology, Tianjin University, China. E-mail: lixianggao@tju.edu.cn; Tel: +86 22 27404208
cCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), China

Received 29th August 2017 , Accepted 11th October 2017

First published on 12th October 2017


With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PSCs), the device lifetime has attracted extensive research interest and concern. To enhance device durability, developing dopant-free hole-transporting materials (HTMs) with a high performance is a promising strategy. Herein, three new HTMs with a N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD) core: TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz are designed and synthesized, showing suitable energy levels and excellent film-formation properties. PCEs of 15.28% were achieved based on pristine TPD-4MeOTPA as the HTM, which is a little lower than that of the p-doped 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD)-based device (17.26%). Importantly, the devices based on the new HTMs show relatively improved stability compared to devices based on spiro-OMeTAD when aged under ambient air with 30% relative humidity in the dark.


1. Introduction

Recently, a new class of solid-state heterojunction solar cells based on solution-processable organometal halide perovskite absorbers have attracted extensive interest.1–8 Owing to unique characteristics, such as the high charge carrier mobility, large absorption coefficient, broad spectral absorption range, and long diffusion length,9–11 the power conversion efficiency (PCE) of solid-state perovskite solar cells (PSCs) has quickly increased to over 20%.12–14

Due to the advantages of promoting hole migration, preventing internal charge recombination and enhancing the stability of cells, hole-transporting materials (HTMs) play a key role in PSCs.15–18 Many kinds of HTMs have been applied in PSCs, such as inorganic p-type semiconductors, conducting polymers, and small molecule hole conductors.19–30 Owing to the advantages of convenient purification, controllable molecular structures and relatively high efficiency,31–33 small molecular HTMs have been widely used in PSCs. Among these, 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) is the most studied. Due to the relatively tedious synthesis and high price, numerous alternative HTMs have been explored to replace spiro-OMeTAD.34–37 However, the PCEs of dopant-free PSCs which are consistently lying between 10% and 13%, few are over 15%.38–42

N,N′-Diphenyl-N,N′-di(m-tolyl)benzidine (TPD) is a promising hole-transporting material for application in the organic photoelectric field. However, an unbefitting energy level, poor solubility and thermal stability limit its application.31–33 In this paper, we designed and synthesized three novel TPD-based HTMs as shown in Fig. 1. The energy levels of the HTMs were tuned by substituting the TPD-core with different electron-donating groups through olefinic bonds. Owing to structure readjustment and increasing molecular weight, the solubility and thermal stability were effective improved. The devices based on [(FAI)0.85(PbI2)0.85(MABr)0.15(PbBr2)0.15], fabricated with doped-free HTMs, achieve the highest PCE of 15.28% under AM 1.5 G (100 mW cm−2) illumination. This result is a little lower than the well-known p-doped spiro-OMeTAD (17.26%) obtained under the same conditions. Moreover, the devices based on the as-synthesized HTMs present an improved stability compared with the device based on spiro-OMeTAD under ambient air with 30% relative humidity without encapsulation after 600 h in the dark.


image file: c7tc03931j-f1.tif
Fig. 1 Molecular structures of the as-synthesized TPD-based HTMs.

2. Results and discussions

2.1 General information

HTMs were obtained by Wittig reaction with cheap starting materials and a simple synthesis process. These three new HTMs were fully characterized by 1H NMR spectroscopy and mass spectrometry. All the analytical data are consistent with the proposed structures (ESI, Fig. S1). We roughly estimated the synthetic cost of 1 g as-synthesized HTM according to a previous report43–45 (ESI, Tables S1–S3). The estimated synthesis costs are 83.4 $ g−1, 96.7 $ g−1 and 44.5 $ g−1 for TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz, respectively, which are much cheaper than that of spiro-OMeTAD (581.52 $ g−1).46 The glass transition temperatures (Tg) and decomposition temperatures (Td) of HTMs were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively (ESI, Fig. S2). As shown in Table 1, the outstanding thermal stability of the as-synthesized HTMs are confirmed by the high Tg and Td.
Table 1 The optical, electrochemical, thermal properties and energy levels of HTMs
λ onset [nm] λ abs [nm] E g [eV] HOMOc [eV] LUMOd [eV] T g [°C] T d [°C]
a Absorption spectra in the 1.0 × 10−5 mol L−1 THF solution. b Optical energy gaps calculated by the absorption thresholds (λonset) from the UV-vis absorption spectra of the films. c Measured by photoelectron yield spectroscopy (PYS, ESI, Fig. S3). d |LUMO| = |HOMO| − |Eg|. e Decomposition temperatures, measured by TGA at a heating rate of 10 °C min−1 under N2. f Measured by DSC at a heating rate of 10 °C min−1 under N2 according to the heat–cool–heat procedure. g Absorption peaks of solid films.
TPD-4MeTPA 454 305, 407/309, 408g 2.68 −5.33 −2.65 145 432
TPD-4MeOTPA 459 299, 408/303, 423g 2.59 −5.28 −2.69 150 416
TPD-4EtCz 442 301, 394/304, 397g 2.76 −5.34 −2.58 138 452


Ultraviolet-visible (UV-Vis) absorption spectra of the as-synthesized HTMs in dilute tetrahydrofuran (THF) solutions (1.0 × 10−5 mol L−1) and solid films are shown in Fig. 2. The peaks at a short wavelength (∼300 nm) can be assigned to the n–π* transition of the TPA moiety, while peaks at a longer wavelength (∼400 nm) are due to the intramolecular charge transfer (ICT) of π–π*.47 These three compounds showed similar n–π* transition peaks, while the ICT peaks showed a bathochromic shift from TPD-4EtCz to TPD-4MeTPA and TPD-4MeOTPA due to extension of π–π conjugation.48,49 The no obvious change in absorption spectra for thin films indicates the amorphous structure of the spin-coated films, which were further identified by XRD measurements (ESI, Fig. S4).50 A slight bathochromic shift (1 nm, 15 nm and 3 nm for TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz, respectively) and a broadening of the ICT bands in the solid films can be observed which indicate the existence of slight intermolecular interactions in the solid state.51


image file: c7tc03931j-f2.tif
Fig. 2 Normalized UV-vis absorption spectra of the new HTMs (a) THF solution (c = 1.0 × 10−5 mol L−1); (b) solid films.

The optimized molecular structures and frontier molecular orbital distribution of the new HTMs were studied by DFT calculation.31 The highest occupied molecular orbital (HOMO) levels, lowest unoccupied molecular orbital (LUMO) energy levels and the energy gap (Eg) are shown in Fig. 3. From Fig. 3, we found that the HOMOs distribute on the whole molecular skeleton. The LUMOs mainly distribute on the TPD branches. As shown in Fig. 3, similar to spiro-OMeTAD, the TPD center of the as-synthesized HTMs adopt a spiro conformation because of the larger terminal branches. The measured angles between two terminal branches are 62.72°, 64.87° and 61.77° for TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz, respectively. Compared to 89.94° between the two fluorine rings in spiro-OMeTAD,50 the spiro conformation of the as-synthesized HTMs will prevent stronger intermolecular stacking and form an amorphous solid film. The energy levels agreed well with the tendency determined by photoelectron yield spectroscopy (PYS, ESI, Fig. S3) and optical measurements (Table 1). The HOMO levels of the as-synthesized HTMs matched well with the valence band level of organometal halide perovskite (−5.5 eV) to favor hole migration. The LUMO levels are much higher than the conduction band level (−3.9 eV) and block the electron back-transfer from perovskite to Au electrode (Fig. 4a).15–18


image file: c7tc03931j-f3.tif
Fig. 3 Distributions of HOMOs and LUMOs and optimized molecular structures of the new HTMs.

image file: c7tc03931j-f4.tif
Fig. 4 (a) Energy level diagram of the perovskite solar cells based on different HTMs; (b) cross-sectional SEM image of the PSC with the configuration of FTO/compact TiO2 (40 nm)/mesoporous TiO2 (200 nm)/perovskite (480 nm)/HTM (160 nm)/Au (80 nm), the scale bar is 200 nm; (c) J–V hysteresis curves of PSCs comprising champion devices with HTMs measured starting with the backward scan and continuing with the forward scan, and (d) IPCE spectra of the devices based on the new HTMs without additives/dopants and spiro-OMeTAD with additives/dopants.

The time-of-flight (TOF, ESI, Fig. S5) transient hole-current measurement was applied to measure the hole mobilities of the HTMs.52 At room temperature, the hole mobilities are 2.14 × 10−4, 4.92 × 10−4 and 1.27 × 10−4 cm2 V−1 s−1 for TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz, respectively, compared to 1.42 × 10−4 cm2 V−1 s−1 for spiro-OMeTAD at the applied electric field of 1.5 × 105 V.

2.2 Application in the mesoporous structured PSCs

The HOMO levels of the as-synthesized HTMs (−5.28 to −5.34 eV) matched well with the valence band level of organometal halide perovskite (−5.5 eV) to favour hole migration. The LUMO levels (−2.69 to −2.58 eV) are much higher than the conduction band level (−3.9 eV) and block the electron back-transfer from perovskite to the Au electrode (Fig. 4a).15–18 The perovskite layer was obtained by using the anti-solvent method as described in the literature.53 The device structure can be clearly seen from a high-resolution cross-sectional scanning electron microscopy (SEM) image (Fig. 4b). As can be seen from the image, the perovskite penetrates into the mp-TiO2 and forms an overlayer. Similarly, the HTMs blend into the pores in the TiO2/perovskite layer and form a thin capping layer on the top.

We evaluated the photovoltaic performance of the PSCs based on the three new HTMs and spiro-OMeTAD with or without additives and dopants. The photocurrent density–voltage (J–V) curves under an AM 1.5 G irradiation of 100 mW cm−2 are presented in Fig. 4c, and the photovoltaic parameters are summarized in Table 2. The lower performance by TPD-4EtCz is mainly related to the lower FF caused by the poor film-forming ability shown in ESI, Fig. S6.54 As expected, TPD-4MeTPA and TPD-4MeOTPA give a higher open-circuit voltage than spiro-OMeTAD for the device, which corresponds with their lower HOMO levels. The best device based on TPD-4MeOTPA affords a Voc of 1.099 V, a short-circuit current density (Jsc) of 20.84 mA cm−2, and a fill factor (FF) of 0.667, leading to a PCE of 15.28% under AM 1.5 G (100 mW cm−2) illumination. This result is a little lower than that of spiro-OMeTAD (17.26%) doped with lithium bis(trifluoromethyl-sulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP). However, devices based on the new HTMs doped with LiTFSI and tBP exhibit a lower photovoltaic performance, especially in terms of FF and Jsc (ESI, Table S4). This is partly because the additives and dopants which work well with spiro-OMeTAD may not be suitable for the new HTMs.55,56 Moreover, the dopants seem to have a negative impact on the film morphology, which is further confirmed by the SEM image in ESI, Fig. S6. Similar behaviour was observed in other reports.42,54,57–60 In the absence of additives and dopants, spiro-MeOTAD-based devices generated a PCE of only 5.43% owing to significant lowering of the Voc and FF compared to the doped devices.61,62 Only a small hysteresis was observed in the J–V curves. The measured PCE differences [(PCEbackward − PCEforward)/PCEaverage × 100%] are 1%, 9%, 3% and 1% and the stabilized power outputs are 14.31%, 15.02%, 11.28% and 17.09% for devices based on TPD-4MeTPA, TPD-4MeOTPA, TPD-4EtCz and spiro-OMeTAD, respectively (ESI, Fig. S7), consistent with the obtained PCE.54

Table 2 JV curves of HTMs and the spiro-OMeTAD based device under different scan directionsa
HTMsb J sc (mA cm−2) V oc (V) FF PCE (%)
a Bias step of 5 mV. b ++ = samples include LiTFSI and tBP additives.
TPD-4MeTPA Backward 20.601 1.089 0.637 14.33
Forward 20.502 1.099 0.625 14.13
TPD-4MeOTPA Backward 20.835 1.099 0.667 15.28
Forward 20.800 1.118 0.599 13.94
TPD-4EtCz Backward 20.310 1.104 0.523 11.73
Forward 20.271 1.134 0.493 11.34
Spiro-OMeTAD++ Backward 21.612 1.074 0.714 17.26
Forward 21.593 1.089 0.696 17.04


The incident photon-to-electron conversion efficiency (IPCE) spectra of the cell with the four different HTMs are presented in Fig. 4d. The integrated current densities estimated from the IPCE spectra (19.90 mA cm−2, 20.30 mA cm−2, 19.45 mA cm−2, and 20.83 mA cm−2 for TPD-4MeTPA, TPD-4MeOTPA, TPD-4EtCz, and spiro-OMeTAD, respectively) are in good agreement with the Jsc values obtained from the J–V curves. We fabricated batches of 10 cells each using different HTMs and demonstrate in Fig. 5 excellent reproducibility by the narrow statistical distribution of the photovoltaic metrics.


image file: c7tc03931j-f5.tif
Fig. 5 Statistical deviation of the photovoltaic parameters for 10 different solar cells using different HTM: (a) Voc, (b) Jsc, (c) FF, and (d) PCE.

The time-resolved photoluminescence (PL) spectra are shown in Fig. 6, which are obtained by fitting the data with a bi-exponential decay function (ESI, Table S5). For the TiO2/CH3NH3PbI3 film without a HTM layer, the pristine perovskite film gave a short-lived lifetime τ1 = 22.14 ns and a long-lived lifetime τ2 = 171.70 ns. When coated with a thin HTM layer on the surface of the perovskite, the lifetime significantly decreases to (τ1 = 8.50 ns, τ2 = 74.53 ns), (τ1 = 7.55 ns, τ2 = 63.70 ns), (τ1 = 12.26 ns, τ2 = 92.58 ns) and (τ1 = 3.59 ns, τ2 = 30.26 ns) for TPD-4MeTPA, TPD-4MeOTPA, TPD-4EtCz and the spiro-OMeTAD based film, respectively. The PL decay lifetime for the new HTMs are somewhat longer compared with doped spiro-OMeTAD, but significantly shorter than that of the HTM-free device, which means that the new HTMs can extract the holes from the perovskite efficiently.


image file: c7tc03931j-f6.tif
Fig. 6 Normalized time-resolved PL spectra of the devices based on the as-synthesized HTMs without additives/dopants and spiro-OMeTAD with additives/dopants.

2.3 Stability and lifetime of PSCs

The stability of the PSCs is a hindrance for application, especially in an arbitrary environment. We compared the stability of the PSCs with the as-synthesized HTMs and spiro-OMeTAD by exposing them to ambient air at 30% relative humidity without encapsulation. The operation stability was investigated in the following way: the J–V curves were measured after storage under an atmospheric environment per 24 hours. The time evolution of the photovoltaic metrics is shown in Fig. 7.
image file: c7tc03931j-f7.tif
Fig. 7 The stability of the new HTMs and spiro-OMeTAD-based PSCs in ambient air without encapsulation (■ TPD-4MeTPA; ● TPD-4MeOTPA; ▲ TPD-4EtCz; ▼ spiro-OMeTAD).

The PCE based on the three HTMs only decreased <8% after 600 h storage in ambient air without encapsulation (Fig. 7). In the first 300 h, a slight increase of Voc can be observed in non-doped PSCs with the as-synthesized HTMs. For the long storage time (300–600 h), the decrease of Jsc in PSCs based on doped spiro-OMeTAD is more obvious. This indicates that the non-doped PSCs based on the as-synthesized HTMs have promising long-term stability at room temperature, which was mainly attributed to their lack of doping additives and less pinholes in the HTM layer.12,63 The contact angles (ESI, Fig. S8) of TPD-4MeTPA (89.4°), TPD-4MeOTPA (94.8°), and TPD-4EtCz (89.1°) are larger than that of doped spiro-OMeTAD (66.9°) reported in our recent paper.60 Larger contact angles indicate higher hydrophobicity of the film. The more hydrophobic nature of the as-synthesized HTMs help to expel moisture away from the perovskite film, which could also lead to enhanced device stability.

3. Experimental section

3.1 Materials

N,N′-Diphenyl-N,N′-di(m-tolyl)benzidine (TPD) was purchased from Heowns Biochemical Technology Co., Ltd, Tianjin, China. PbI2 and PbBr2 was purchased from Tokyo Chemical Industry Co., Ltd, N,N′-dimethylformide (DMF) from Alfar Aesar, hydroiodic acid (AR, 45 wt% in water) and methylamine (AR, 27% in methanol) from Sinopharm Chemical Reagent Co. Ltd. 2,2′,7,7′-Tetrakis (N,N-di-p-methoxy-phenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) was from Luminescence Technology Corp., Taiwan, China. Tetrahydrofuran was distilled before using, all the other agents were directly used without further purification. The substrates were FTO conducting glass (Pilkington, thickness: 2.2 mm, sheet resistance: 14 Ω per square). The patterned FTO glass was first cleaned with mild detergent, rinsed several times with distilled water and subsequently with ethanol in an ultrasonic bath, and finally dried under an air stream.

3.2 Measurements

The as-synthesized compounds were identified by nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectroscopy (HRMS). The NMR was obtained on a Bruker AVANCE III 400 MHz spectrometer, with the chemical shifts reported in ppm using tetramethylsilane (TMS) as an internal standard. HRMS was recorded on a SolariX maldi-FTMS mass spectrometer. Ultraviolet-visible absorption (UV-vis) spectroscopy was obtained using a Thermo Evolution 300 UV-visible spectrometer. Decomposition temperature (Td) and glass transition temperature (Tg) were determined by thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) on a TA Q500 for thermo gravimetric analysis and a TA Q20 for thermal analysis under a nitrogen atmosphere. The photoelectron yield spectroscopy (PYS) was carried out on the Sumitomo PYS-202 ionization energy detection system. X-ray diffraction (XRD) was measured with a Rigaku Miniflex 600 X-ray diffractometer. Scanning electron microscopy (SEM) was performed with a Hitachi S-4800.

Current–voltage characteristics (J–V) were measured on a Keithley 2602 SourceMeter under AM 1.5 irradiation (100 mW cm−2) from an Oriel Solar Simulator 91192. A mask with a window of 0.16 cm2 was clipped on the TiO2 side to define the photoactive area of the cells. Incident-photon-to-current conversion efficiency (IPCE) was measured by the direct current (DC) method using a lab-made IPCE setup under 0.3–0.9 mW cm−2 monochromic light illumination without bias illumination. Time resolved PL spectra was recorded on a PL spectrometer, Edinburgh Instruments, FLS 900, excited with a picosecond pulsed diode laser (EPL-445), and measured at 775 nm after excitation at 445 nm. The time-of-flight (TOF) measurement was recorded on a TOF401 measurement system, Sumitomo Heavy Industries Ltd. Samples were prepared through spin-coating with the structure of the ITO/as-synthesized compounds (∼1 μm)/Al (100 nm) with a working area of 3 × 3 mm2.

3.3 Synthesis

The designed synthetic routes for the as-synthesized compounds are depicted in Scheme 1, which were synthesized by the Wittig reaction using formyl replaced TPD (2) and Wittig reagents (3) in three steps from commercially available and relatively inexpensive starting reagents. The structures of the as-synthesized compounds were confirmed via1H NMR and MS, which agreed well with the proposed molecular structure (see ESI, Fig. S1).
image file: c7tc03931j-s1.tif
Scheme 1 Synthetic route for the three new HTMs.
4,4′-([1,1′-Biphenyl]-4,4′-diylbis((4-formylphenyl)azanediyl))bis(2-methylbenzaldehyde) (2). TPD (5.0 g, 9.7 mmol) and imidazole (5.1 g, 61.5 mmol) were added into a two neck 250 mL round bottom flask, followed by 90 mL of acetonitrile. Trifluoroacetic anhydride (17.3 mL, 123.0 mmol) was dropped under a nitrogen atmosphere (N2). Then the above mixture was refluxed until TPD was consumed completely (monitored by thin-layer chromatography). The reaction solution was poured into 1 L water to precipitate a yellow powder. The filter cake was washed with water until the filtrate became colorless, and compound (1) was obtained: yellow solid (15.0 g, 99.1%): mp 243–245 °C, 1H NMR (400 MHz, CDCl3) δ (ppm): 2.63 (s, 6H), 6.89 (s, 4H), 7.04–7.24 (m, 14H), 7.53 (d, J = 8.4 Hz, 4H), 7.38 (d, J = 8.5 Hz, 4H), 6.74 (d, J = 10.1 Hz, 8H), 7.27 (s, 4H); ESI-MS (m/z): [M] calcd for C66H40F24N10O8, 1556.3; found 1556.9.

The polyimidazoline product ((1), 10 g, 6.4 mmol) was dissolved in 200 mL THF and then pumped with HCl gas (100 mL, 2.5 mol L−1 – prepared by adding 21.0 mL of concentrated HCl to 79.0 mL H2O). The reaction solution was refluxed for 12 h. The reaction solution was cooled to room temperature, then an orange solid was formed. The reaction mixture was filtered and recrystallized from diethyl ether, and compound (2) was obtained (3.8 g, 93.0%): mp 197–199 °C, IR: 2803.99, 2722.76, 1693.86; 1H NMR (400 MHz, CDCl3) δ (ppm): 10.15 (s, 2H), 9.91 (s, 2H), 7.80 (d, J = 8.5 Hz, 4H), 7.74 (d, J = 8.4 Hz, 2H), 7.62 (d, J = 8.4 Hz, 4H), 7.32–7.21 (m, 8H), 7.19–7.04 (m, 4H), 2.60 (s, 6H); ESI-MS (m/z): [M + H]+ calcd for C42H32N2O4, 628.2; found, 629.5.

Wittig regents (3). Wittig reagents (3) were synthesized according to the literature.31
N,N-Di(phenyl)-N′,N′-di(4-(4-N,N-di(4-(4-methoxy-phenyl)amino)phenyl)ethenyl)-1,1′-biphenyl-4,4′-diamine (TPD-4MeTPA). 4-[N,N-Di(p-tolyl)amino]benzyl(triphenyl)phosphonium bromide (W1, 2.64 g, 4 mmol) and compound (2) (0.31 g, 0.5 mmol) were added into a 100 mL round-bottom flask under N2. Anhydrous THF (40 mL) was added to the above flask, and cooled down to 0 °C. The THF solution of t-BuOK (16 mmol, 0.8 mol L−1) was added dropwise to the above flask, stirred for 30 min at 0 °C, followed with stirring at room temperature until compound (2) was consumed completely (monitored by thin-layer chromatography). The reaction was terminated with ice water. The crude product was heated under reflux for 8 h in THF with a catalytic amount of iodine. Then the remaining iodine was removed using sodium hydroxide (NaOH) solution (Wt = 10%, 100 mL) by stirring for 2 h. After that, the product was purified by chromatography on a silica gel column (petroleum ether[thin space (1/6-em)]:[thin space (1/6-em)]ethyl acetate = 50[thin space (1/6-em)]:[thin space (1/6-em)]1 as eluent) to give the title compound as a pure E stereoisomer TPD-4MeTPA (0.59 g, 69%): 1H NMR (400 MHz, CDCl3) δ (ppm): 7.67 (dd, J = 11.9, 7.9 Hz, 2H), 7.58–7.42 (m, 6H), 7.40–7.28 (m, 10H), 7.20–6.76 (m, 60H), 2.30 (q, J = 5.0 Hz, 30H). HRMS (m/z): calcd for C124H108N6, 1705.86690; found 1705.86589.

TPD-4MeOTPA and TPD-4EtCz were synthesized with W2 (4-[N,N-di(p-methoxyphenyl)amino]benzyl(triphenyl)phosphoniumbromide), W3 (3-[(9-ethyl)-carbazole]methyl(triphenyl)phosphonium bromide) and compound 2 using the same method.

TPD-4MeOTPA: (0.63 g, 73%): 1H NMR (400 MHz, CDCl3) δ (ppm): 7.53–7.43 (m, 6H), 7.37 (d, J = 8.3 Hz, 4H), 7.31 (t, J = 7.2 Hz, 8H), 7.10 (dd, J = 35.0, 7.7 Hz, 26H), 6.91 (s, 16H), 6.83 (d, J = 8.6 Hz, 18H), 3.79 (s, 24H), 2.33 (d, J = 11.3 Hz, 6H). HRMS (m/z): calcd for C126H108N6O8, 1833.82622; found 1833.82536.

TPD-4EtCz: (0.51 g, 65%): 1H NMR (400 MHz, CDCl3) δ (ppm): 8.15 (dd, J = 24.0, 17.1 Hz, 6H), 7.73–7.63 (m, 4H), 7.59 (t, J = 9.7 Hz, 2H), 7.56–7.27 (m, 22H), 7.22–6.92 (m, 14H), 4.34 (t, J = 12.2 Hz, 8H), 2.39 (d, J = 25.3 Hz, 6H), 1.50–1.35 (m, 12H). HRMS (m/z): calcd for C102H84N6, 1393.67910; found 1393.67831.

3.4 Quantum chemical calculation

Quantum chemical calculation was performed on the Gaussian 03 program with the Beck's three-parameter exchange functional and the Lee–Yang–Parr's correlation functional (B3LYP) using 6-31G(d) basis sets.64

3.5 Solar cell fabrication details

A 30 nm-thickness compact TiO2 layer, 200–300 nm mesoporous TiO2 layers and a perovskite layer ([(FAI)0.85(PbI2)0.85(MABr)0.15(PbBr2)0.15]) were prepared according to the reported methods.53 The HTM layers were spin-coated on top of the TiO2/perovskite films at 3000 rpm for 20 s with a concentration of 20 mg mL−1. For comparison, perovskite solar cells based on spiro-OMeTAD were also fabricated by using a chlorobenzene solution doped with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP) under the same conditions. All the above fabrication processes were carried out in air. Finally, an 80 nm-thickness Au photocathode was deposited by thermal evaporation.

4. Conclusions

We synthesized three TPD-core HTMs (TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz) with simple synthetic procedures and low cost. The PSC based on dopant-free TPD-4MeOTPA as the HTM affords an impressive PCE of 15.28%, which is a little lower than that obtained employing the well-known p-doped spiro-OMeTAD. The devices based on as-synthesized HTMs obtained a higher stability than the device based on spiro-OMeTAD after 600 h at room temperature in ambient air with 30% relative humidity without encapsulation. The introduction of these three novel HTMs with improved synthesis and excellent performance highlights their potential use in the future deployment of PSCs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Key R&D Program of China (2016YFB0401303), the National Natural Science Foundation of China (21676188) and Key Projects in the Natural Science Foundation of Tianjin (16JCZDJC37100). The calculation in this work was supported by the High Performance Computing Center of Tianjin University, China.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc03931j
X. C. Liu and F. Zhang have equivalent contribution.

This journal is © The Royal Society of Chemistry 2017