Hole-transporting materials based on thiophene-fused arenes from sulfur-mediated thienannulations

Abstract

Dithienonaphthalene and dithienochrysene derivatives that bear diarylaminophenyl groups were prepared in two steps from commercially available compounds via a Sonogashira coupling, followed by a simple sulfur-mediated thienannulation. Owing to their relatively narrow HOMO–LUMO gaps, the thus obtained compounds showed bathochromically shifted UV-Vis absorption and fluorescence spectra with higher quantum yields compared to the molecules without the diarylaminophenyl groups. Moreover, the suitable energy levels of their HOMOs and reversible redox properties permit applications for these compounds in hole-transporting layers in perovskite solar cells.

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Introduction

Thiophene-fused π-conjugated systems have attracted significant attention in materials science on account of their outstanding charge-transporting properties, which originate from their highly delocalized electronic structures, strong intermolecular π–π interactions, and suitable energy levels of their highest occupied molecular orbitals (HOMOs). Owing to these promising characteristics, a variety of thiophene-fused π-conjugated molecules have been developed and used as semiconductors in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs).^1–4 Moreover, thiophene and thiophene-fused molecules that bear triphenylamine moieties have been employed as core structures in the development of donor–π–donor-type hole-transporting materials (HTMs) for perovskite solar cells.^5–12

Thiophene-annulation reactions (thienannulations) have been widely employed to prepare a variety of thiophene-fused molecules from arenes and polycyclic aromatic hydrocarbons (PAHs). In general, arenes and PAHs with two functional moieties such as halogen, alkynyl, or sulfur-containing groups are necessary to form a fused thiophene ring on the aromatic cores,^13–15 even though there are a few other examples.^16–20 In any case, conventional thienannulations usually require multiple steps to generate thiophene-fused cores.

Very recently, we have reported a sulfur-mediated thienannulation of alkynylated PAHs (Fig. 1a).²¹ This reaction requires merely one arylethynyl group to create a thiophene ring. Moreover, amino groups are tolerated under the applied reaction conditions. We anticipated that this reaction could provide a step-economical route to thiophene-fused PAHs that contain triphenylamine moieties. Herein, we report the synthesis of naphtho[2,1-b:3,4-b′]dithiophene and chryseno[5,6-b:11,12-b′]dithiophene derivatives that bear 4-(di-p-anisylamino)phenyl groups (1 and 2; Scheme 1) via a Sonogashira coupling reaction followed by a thienannulation. Photophysical measurements and DFT calculations revealed the effect of the diarylaminophenyl groups on the HOMO energy levels of the thus obtained molecules. The hole-transporting mobilities of the thiophene-fused PAHs were examined, and the device performance of perovskite solar cells based on 1 and 2 as the hole-transporting layer was evaluated.

Fig. 1 Sulfur-mediated thienannulation reaction.

Scheme 1 Synthesis of 1 and 2, as well as structures of reference compounds 8, 9, and spiro-OMeTAD.

Results and discussion

Thiophene-fused PAHs 1 and 2 were synthesized from 1,4-dibromonaphthalene (3) or 6,12-dibromochrysene (4) with N,N-di-p-anisyl-4-ethynylaniline (5) (Scheme 1). A Pd-catalyzed Sonogashira coupling reaction²² of 3 with 5 furnished dialkynylated naphthalene 6 in 87% yield, which was subsequently subjected to sulfur-mediated thienannulation reaction conditions (3.9 equiv. of S₈, DMF, 140 °C, 48 h)²¹ to afford 1 in 37% yield. Similarly, 7, synthesized by a Sonogashira coupling of 4 and 5, was converted into 2 (67% yield). As expected, the diarylaminophenyl groups of 6 and 7 remained unaffected under the thienannulation reaction conditions. In the thienannulation, 7 exhibited higher reactivity, which is consistent with previous findings that chrysene derivatives are more susceptible to such thienannulations than naphthalene derivatives.²¹1 and 2 were characterized by ¹H and ¹³C NMR spectroscopy, as well as by high-resolution MALDI-TOF mass spectrometry measurements. The structure and packing mode of 2 in the solid state was confirmed by a single-crystal X-ray diffraction analysis (Fig. 2). Single crystals of 2 were obtained from a recrystallization in CH₂Cl₂/EtOH, and contained one molecule of CH₂Cl₂ per unit cell. In the packing mode of 2 (Fig. 2b), efficient π–π stacking was not observed.

Fig. 2 (a) X-ray structure of 2 (thermal ellipsoids set to 50% probability). All hydrogen atoms and solvent molecules are omitted for clarity. (b) Packing structure of 2. Red, green, and blue lines represent the a, b, and c axes of the unit cell, respectively.

The photophysical properties of 1 and 2 were investigated. The UV-Vis absorption and fluorescence spectra of 1 and 2, as well as of reference compounds 8 and 9²¹ are shown in Fig. 3. As summarized in Table 1, 1 and 2 exhibit absorption maxima at 424 and 446 nm, respectively. The absorption spectra of 1 and 2 showed bathochromically shifted absorptions relative to those of 8 and 9, which should be attributed to the π-extension of the core structures and the electron-donating properties of the diarylaminophenyl groups. The effect of the diarylaminophenyl groups on the HOMO levels of 1 and 2 are also supported by DFT calculations (Table 1). The fluorescence maxima of 1 and 2 appeared at 497 and 523 nm, respectively, and 1 and 2 displayed large Stokes shifts of 73 and 77 nm, respectively, presumably because of the polarization in the excited states owing to the donor–π–donor-type structures. In addition, the fluorescence quantum yields of 1 and 2 were higher than those of 8 and 9, respectively. In contrast to the solution state, no fluorescence was observed from 1 and 2 in the solid state.

Fig. 3 UV-Vis absorption (solid lines) and fluorescence (dashed lines) spectra of CH₂Cl₂ solutions of 1 (green), 2 (blue), 8 (gray), and 9 (gray).

Table 1 Photophysical properties and HOMO energy levels of 1, 8, 2, and 9

	λ abs [nm]	λ em [nm]	Φ F	KS-HOMO^c [eV]	HOMO^e [eV]
a Emission maxima upon excitation at 446 nm (1), 380 nm (8), 445 nm (2), and 330 nm (9). b Absolute fluorescence quantum yields determined by a calibrated integrating sphere system (error: ±3%). c Kohn–Sham (KS)-HOMOs calculated at the B3LYP/6-31G(d) level of theory. d For the calculations, n-Bu and n-hex groups were replaced with Me groups. e HOMO levels were estimated from the onset potentials of the first oxidation peaks in the cyclic voltammogram [HOMO = −(Eonset + 5.1 eV)].
1	424	497	0.55	−4.42	−5.14
8	381, 397	417, 438	0.26	−5.06^d	—
2	446	523	0.22	−4.43	−5.00
9	345, 370	392, 413	0.08	−4.97^d	—

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Cyclic voltammetry (CV) measurements were carried out to determine the electrochemical properties and the HOMO energy levels of 1 and 2. As shown in Fig. 4, both 1 and 2 exhibit good electrochemical stability and undergo three- and four-step oxidations, respectively. Considering the higher current of the first peak than those of the second and third peaks of 1, the first oxidation of 1 should be simultaneous two-electron oxidation and 1 exhibits four-electron oxidation similar to 2. The HOMO energy levels of 1 (−5.14 eV) and 2 (−5.00 eV), calculated from the onset of their oxidation potentials relative to Fc/Fc⁺, are higher than the valence-band level of perovskite (−5.44 eV).²³

Fig. 4 Cyclic voltammograms of 1 (green) and 2 (blue) in CH₂Cl₂ (1.0 mM); scan rate: 100 mV s⁻¹; working electrode: Pt; reference electrode: Ag/AgNO₃; supporting electrolyte: [n-Bu₄N][PF₆]; Fc/Fc⁺ = ferrocene/ferrocenium.

Furthermore, we employed 1 and 2 to fabricate OFETs in order to measure their hole mobilities (μ_h). Initially, we fabricated top-contact/bottom-gate OFETs on Si/SiO₂ substrates by a drop-cast method using toluene solutions of 1 and 2, followed by evaporative deposition of a gold electrode. The amorphous thin films (see ESI† for detail) of 1 and 2 worked as p-type semiconductors under an inert atmosphere with mobilities (μ_h) of 8.6 × 10⁻⁴ and 2.5 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively, which are higher than those of thiophene derivatives bearing (di-p-anisylamino)phenyl groups.⁶

Given their suitable HOMO energy levels and the good hole mobilities, 1 and 2 were applied as hole-transporting layers (HTLs) for the fabrication of perovskite solar cells with an FTO/meso-TiO₂/CH₃NH₃PbI₃/HTM/Au configuration.²⁴ In addition, a solar cell with spiro-OMeTAD as the HTM was prepared, together with a cell that did not contain any HTM. Fig. 5 shows the current–density–voltage (J–V) curves of these devices and the photovoltaic performance parameters of the perovskite solar cells are summarized in Table 2. After optimization of the devices, the power conversion efficiency (PCE) of the solar cells increased by incorporating the two additives lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (TBP) into the HTMs.²⁴ The PCEs of devices using 1 (6.7%) and 2 (5.7%) are higher than that of the device that did not contain any HTM (3.1%). The PCE of the control device using spiro-OMeTAD showed a better performance (12.3%) than the devices with thiophene-fused HTMs.

Fig. 5 Current-density–voltage characteristics for perovskite solar cells using 1 (green), 2 (blue), and spiro-OMeTAD (black) as HTMs, as well as of a device without HTM (gray); all measurements were recorded using a mask (0.04 cm²) under a photon flux of 100 mW cm⁻² (AM1.5G; solid lines) or in the dark (dashed lines).

Table 2 Photovoltaic performance parameters of perovskite solar cells using 1, 2, or spiro-OMeTAD as HTMs, as well as of a device without a HTM

	J sc (mA cm^−2)	V oc (V)	FF	PCE (%)
a Composition of the solutions for the deposition of HTLs: HTM (0.060 M), TBP (0.098 M), and Li-TFSI (0.158 M) in chlorobenzene. b HTM (0.015 M), TBP (0.098 M), and Li-TFSI (0.158 M) in chlorobenzene.
1	14.9	0.83	0.54	6.7
2	17.3	0.75	0.44	5.7
Spiro-OMeTAD^a	18.2	1.02	0.66	12.3
without HTM	11.4	0.74	0.35	3.1

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Conclusions

We have synthesized dithienonaphthalene and dithienochrysene derivatives that contain diarylaminophenyl groups (1 and 2) in two steps using a Sonogashira reaction followed by a simple sulfur-mediated thienannulation. 1 and 2 exhibit relatively narrow HOMO–LUMO gaps compared to the corresponding compounds without diarylaminophenyl groups, which resulted in bathochromic shifts in their UV-Vis absorption and fluorescence spectra. Given their reversible electrochemical properties and the suitable HOMO energy levels of 1 and 2, these were used to fabricate OFETs and perovskite solar cells to demonstrate their potential utility as hole-transporting materials. Our concise synthetic route to thiophene-fused π-systems should provide a new strategy to develop organic semiconductors of the donor–π–donor type.

Experimental section

General information

Unless otherwise noted, all materials including dry solvents were obtained from common commercial suppliers and used without further purification. All reactions were performed using standard vacuum-line and Schlenk techniques. Work-up and purification procedures were carried out with reagent-grade solvents under air. N,N-Di-p-anisyl-4-ethynylaniline (5)²⁵ was synthesized according to previously reported procedures. Analytical thin-layer chromatography (TLC) was performed using E. Merck silica gel 60 F254 pre-coated plates (0.25 mm). The developed chromatograms were analyzed using a UV lamp (254 nm). Flash column chromatography was performed with E. Merck silica gel 60 (230–400 mesh). High-resolution mass spectra (HRMS) were recorded using a Bruker Daltonics Ultraflex III TOF/TOF (MALDI-TOF-MS). ICP-MS was performed using a Shimadzu ICPM-8500. Nuclear magnetic resonance (NMR) spectra were recorded using a JEOL ECA 600II spectrometer (¹H 600 MHz, ¹³C 150 MHz) with an UltraCool probe. Chemical shifts in the ¹H NMR spectra are expressed in parts per million (ppm) relative to CHDCl₂ (δ 5.32 ppm) or DMF-d₆ (δ 8.00 ppm). Chemical shifts in the ¹³C NMR spectra are expressed in ppm relative to CD₂Cl₂ (δ 53.8 ppm) or DMF-d₇ (δ 53.98 ppm). Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, dt = doublet of triplets, m = multiplet), coupling constant (Hz), and integration.

Synthesis of 6

A 50 mL Schlenk flask containing a magnetic stirring bar was charged with 3 (140 mg, 490 μmol, 1.0 equiv.), Pd(PPh₃)₂Cl₂ (7.0 mg, 1.0 μmol, 2 mol%), PPh₃ (5.1 mg, 19 μmol, 4 mol%), and CuI (3.0 mg, 16 μmol, 3 mol%). The flask was filled with argon before Et₃N (10.5 mL) and 5 (400 mg, 1.22 mmol, 2.5 equiv.) were added. The mixture was stirred for 24 h at 85 °C. Then, the mixture was cooled to room temperature, and concentrated in vacuo. The residue was subjected to chromatography on silica gel (CHCl₃/hexane = 1 : 2) to afford 6 (330 mg, 87% yield) as a yellow solid. ¹H NMR (600 MHz, CD₂Cl₂) δ 8.45 (dd, J = 6.0 Hz, J = 3.0 Hz, 2H), 7.67 (s, 2H), 7.63 (dd, J = 6.0 Hz, J = 3.0 Hz, 2H), 7.42 (d, J = 9.0 Hz, 4H), 7.11 (d, J = 9.0 Hz, 8H), 6.88 (d, J = 9.0 Hz, 4H), 6.86 (d, J = 9.0 Hz, 8H), 3.80 (s, 12H); ¹³C NMR (150 MHz, CD₂Cl₂) δ 157.2, 149.8, 145.5, 133.5, 133.0, 130.0, 127.9, 127.6, 127.1, 122.0, 119.2, 115.4, 113.9, 97.4, 86.8, 56.0; HRMS (MALDI-TOF) m/z calcd for C₅₄H₄₂N₂O₄ [M]⁺: 782.3139, found: 782.3131.

Synthesis of 7

A 50 mL Schlenk flask containing a magnetic stirring bar was charged with 4 (188 mg, 490 μmol, 1.0 equiv.), Pd(PPh₃)₂Cl₂ (7.0 mg, 1.0 μmol, 2 mol%), PPh₃ (5.1 mg, 19 μmol, 4 mol%), and CuI (3.0 mg, 16 μmol, 3 mol%). The flask was filled with argon before Et₃N (10.5 mL) and 5 (400 mg, 1.22 mmol, 2.5 equiv.) were added. The mixture was stirred for 24 h at 85 °C. Then, the mixture was cooled to room temperature and concentrated in vacuo. The residue was subjected to chromatography on silica gel (CHCl₃/hexane = 2 : 1) to afford 7 (400 mg, 93% yield) as a yellow solid. ¹H NMR (600 MHz, CD₂Cl₂) δ 8.98 (s, 2H), 8.83 (d, J = 8.4 Hz, 2H), 8.62 (d, J = 8.4 Hz, 2H), 7.81–7.74 (m, 4H), 7.51 (d, J = 9.0 Hz, 4H), 7.13 (d, J = 9.0 Hz, 8H), 6.90 (d, J = 9.0 Hz, 4H), 6.89 (d, J = 9.0 Hz, 8H), 3.81 (s, 12H); ¹³C NMR (150 MHz, CD₂Cl₂) δ 157.3, 149.9, 140.5, 133.1, 132.1, 130.5, 128.1, 128.0, 127.7, 127.5, 125.8, 123.9, 121.8, 119.3, 115.4, 113.9, 96.6, 87.3, 56.0; HRMS (MALDI-TOF) m/z calcd for C₆₂H₄₆N₂O₄ [M]⁺: 882.3452, found: 882.3452.

Synthesis of 1

A 10 mL Schlenk tube containing a magnetic stirring bar was charged with 6 (312 mg, 399 μmol) and S₈ (400 mg, 1.50 mmol, 3.9 equiv.). The tube was filled with argon before DMF (4.0 mL) was added and the mixture was stirred for 48 h at 140 °C. Then, the mixture was cooled to room temperature and concentrated in vacuo. The residue was subjected to chromatography on silica gel (CHCl₃/hexane = 1 : 2) to afford 1 (125 mg, 37% yield) as an orange solid. ¹H NMR (600 MHz, DMF-d₇) δ 8.63 (dd, J = 6.0 Hz, J = 3.0 Hz, 2H), 8.58 (s, 2H), 7.79 (d, J = 9.0 Hz, 4H), 7.67 (dd, J = 6.0 Hz, J = 3.0 Hz, 2H), 7.15 (d, J = 9.0 Hz, 8H), 6.99 (d, J = 9.0 Hz, 8H), 6.91 (d, J = 9.0 Hz, 4H), 3.81 (s, 12H); ¹³C NMR (150 MHz, DMF-d₇) δ 156.8, 149.3, 142,5, 140.3, 135.8, 129.9, 127.8, 127.5, 127.0, 126.3, 125.5, 125.0, 119.4, 118.0, 115.2, 55.3; HRMS (MALDI-TOF) m/z calcd for C₅₄H₄₂N₂O₄S₂ [M]⁺: 846.2581, found: 846.2576.

Synthesis of 2

A 10 mL Schlenk tube containing a magnetic stirring bar was charged with 7 (176 mg, 199 μmol) and S₈ (200 mg, 781 μmol, 3.9 equiv.). The tube was filled with argon before DMF (2.0 mL) was added and the mixture was stirred for 48 h at 140 °C. Then, the mixture was cooled to room temperature and concentrated in vacuo. The residue was subjected to chromatography on silica gel (CHCl₃/hexane = 2 : 1) to afford 2 (120 mg, 67% yield) as an orange solid. ¹H NMR (600 MHz, DMF-d₇) δ 9.45 (d, J = 7.8 Hz, 2H), 8.88 (d, J = 7.2 Hz, 2H), 8.75 (s, 2H), 7.93–7.86 (m, 4H), 7.91 (d, J = 9.0 Hz, 4H), 7.18 (d, J = 9.0 Hz, 8H), 7.01 (d, J = 9.0 Hz, 8H), 6.96 (d, J = 9.0 Hz, 4H), 3.83 (s, 12H); ¹³C NMR (150 MHz, DMF-d₇) δ 156.9, 149.5, 144.6, 140.3, 137.4, 133.0, 129.1, 128.6, 127.7, 127.5, 127.2, 126.9, 125.3, 125.0, 124.4, 119.5, 115.2, 55.3; HRMS (MALDI-TOF) m/z calcd for C₆₂H₄₆N₂O₄S₂ [M]⁺: 946.2894, found: 946.2905.

X-ray crystallography for 2. Details of the crystal data and a summary of the intensity data collection parameters are listed below. In each case, a suitable crystal, covered in mineral oil, was mounted on a glass fiber and transferred to the goniometer of a Rigaku PILATUS diffractometer. Graphite-monochromated Mo Kα radiation was used. The structures were solved by direct methods with (SIR-97)²⁶ and refined by full-matrix least-squares techniques against F² (SHELXL-2014/7)²⁷ using the Yadokari-XG software package.²⁸ The intensities were corrected for Lorentz and polarization effects. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed using AFIX instructions. The crystal data for 2·CH₂Cl₂ are as follows.

Empirical formula: C₆₃H₄₈Cl₂N₂O₄S₂; formula weight: 1032.05; triclinic; space group: P1̄; a = 10.1597(7) Å; b = 11.6731(8) Å; c = 22.4151(13) Å; α = 96.0864(15)°; β = 91.8662(17)°; γ = 108.699(2)°; V = 2497.6(3) ų; Z = 2; D_calc = 1.372 g cm⁻³; μ = 0.268 mm⁻¹; F(000) = 1076; crystal size: 0.25 × 0.20 × 0.15 mm³; θ range: 3.004°–25.000°; reflections collected: 76 385; independent reflections: 8768; R_int = 0.0300; parameters: 690; GOF: 1.035; R₁ = 0.0456, wR₂ = 0.1161 (I > 2σ(I)); R₁ = 0.0497, wR₂ = 0.1197 (for all data).

Photophysical measurements

Dilute solutions in degassed spectral grade CH₂Cl₂ in a 1 cm square quartz cell were used for all measurements. UV-Vis absorption spectra were recorded using a Shimadzu UV-3510 spectrometer with a resolution of 0.5 nm. Emission spectra were measured using a F-4500 Hitachi or Shimadzu RF-6000 spectrometer with a resolution of 0.4 nm. Absolute fluorescence quantum yields (Φ_F) were determined by a Shimadzu RF-6000 using a calibrated integrating sphere system (207-21460-41).

OFET fabrication

OFETs were fabricated in a top-contact/bottom-gate configuration on a heavily doped n⁺-Si (100) wafer with a 300 nm-thick thermal silicon oxide (SiO₂) dielectric layer. Prior to use, the Si/SiO₂ substrates were cleaned by consecutive sonication in acetone and 2-propanol, followed by drying. Thin films of 1 and 2 were grown on Si/SiO₂ substrates using the drop-cast method. For that purpose, saturated toluene solutions of 1 or 2 were used at room temperature under a vapor of toluene. Subsequently, gold films (∼200 nm) were deposited through a shadow mask as drain and source electrodes. The transfer characteristics of the devices were measured at room temperature under a nitrogen atmosphere using a Keithley 4200-SCS. The field-effect hole mobility (μ_h) was calculated in the saturation regime (V_DS = −60 V) of the I_DS using the following equation: I_DS = (WC_i/2L)μ_h(V_GS − V_th)², where W and L are the width and length of the drain–source channel, respectively, C_i is the capacitance of the SiO₂ insulator, and V_GS and V_th are the gate and threshold voltages, respectively.

Perovskite solar cells²⁴

(1) Preparation of mesoporous TiO₂ layers. Patterned transparent conducting oxide substrates (FTO; 25 mm × 25 mm; Asahi Glass Co., Ltd, Japan) were cleaned with ultrasound for 10 min using (i) a 1 wt% neutral aqueous detergent solution, (ii) acetone, (iii) 2-propanol, and (iv) distilled water. Then, the substrates were subjected for 15 min to an O₃/ultraviolet treatment. The FTO substrates were covered with a compact TiO₂ layer by spray pyrolysis of an ethanol solution (0.05 M) of titanium diisopropoxide bis(acetylacetonate) (75 wt% in 2-propanol; Tokyo Chemical Industry Co., Ltd (TCI), Japan) at 450 °C. The resulting compact TiO₂ layer was treated with an aqueous solution (100 mL) of TiCl₄ (440 μL; special grade; Wako Pure Chemical Industries Ltd, Japan) at 70 °C for 30 min, followed by two rinses with distilled water. The substrate was sintered at 500 °C for 20 min. Subsequently, mesoporous TiO₂ layers (thickness: 200 nm; average particle size: ∼20 nm) were deposited on the plates by spin-coating (slope 5 s, 5000 rpm, 30 s, slope 5 s) of a suspension of TiO₂ paste (PST-18NR; JGC Catalysts and Chemicals Ltd) in ethanol (paste/ethanol = 1/5, w/w), followed by sintering at 500 °C for 30 min. The obtained substrates were subjected for 15 min to an O₃/ultraviolet treatment, before being used for the fabrication of the perovskite layers.

(2) Preparation of the perovskite layers. In a glove box filled with an inert gas, a 1.1 M solution of PbI₂ (L0279 for perovskite precursor, TCI) and CH₃NH₃I (TCI) in DMSO (room temperature) was deposited onto the mesoporous TiO₂ films by spin-coating (slope 1 s, 1000 rpm, 40 s, slope 1 s, 0 rpm, 30 s, slope 5 s, 5000 rpm, 20 s, slope 5 s); during the last 5 s of spin-coating, 0.5 mL of toluene were slowly dropped onto the rotating substrate. The resulting transparent films were annealed on a hot plate (100 °C, 10 min) to form the perovskite layers (250 nm).

(3) Preparation of the hole-transporting layers and the Au electrodes. A mixture of the hole-transporting material (15–60 mM) was dissolved in a solution containing chlorobenzene, 4-tert-butylpyridine (TBP; 40–98 mM; Aldrich), and LiTFSI (40–158 mM; Wako Pure Chemical Industries Co., Ltd). After 30 min, the resulting suspension was filtered through a membrane filter (Cosmonice filter S, Nacarai Tesque, Inc.). The hole-transporting layer was deposited onto the perovskite layer by spin-coating (slope 5 s, 4000 rpm, 30 s, slope 5 s) of the obtained filtrate. The resulting film was dried on a hot plate (70 °C, 30 min). Finally, a gold layer (80 nm) was thermally deposited on the hole-transporting layer.

Theoretical study

The Gaussian 09 program²⁹ running on a SGI Altix4700 system was used for optimizations at the B3LYP/6-31G(d) level of theory.³⁰ All structures were optimized without any symmetry assumptions. Unless otherwise noted, zero-point energy, enthalpy, and Gibbs free energy values at 298.15 K and 1 atm were estimated from the gas-phase studies. Harmonic vibration frequency calculations were performed at the same level to verify that all stationary points exhibit no imaginary frequency and are hence local minima.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the JST ERATO program (JPMJER1302 to K. I.), the MEXT Funding Program for KAKENHI (JP16K05771 to Y. S. and JP26288093 to A. W.), and a grant-in-aid for Scientific Research on Innovative Areas “π-Figuration” (JP17H05149) from JSPS. Calculations were performed using the resources of the Research Center for Computational Science, Okazaki, Japan. ITbM is supported by the World Premier International Research Center (WPI) Initiative, Japan.

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Footnote

† Electronic supplementary information (ESI) available: Computational studies, and spectral data for all compounds. CCDC 1562381 (2). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qm00473g

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