Diketopyrrolopyrrole or benzodithiophene-arylamine small-molecule hole transporting materials for stable perovskite solar cells

Abstract

Two simple small-molecular arylamine derivatives 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (OMeTPA-DPP) and 4,4′-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (OMeTPA-BDT) linked with diketopyrrolopyrrole or benzodithiophene moieties have been synthesized. The new compounds show better thermal stability than spiro-OMeTAD. The steady-state and time-resolved photoluminescence demonstrate that the new compounds have good hole extraction ability. The perovskite solar cells employing OMeTPA-BDT show a comparable power conversion efficiency with that of spiro-OMeTAD. After more than 200 hours of aging under one sun illumination, the residual efficiencies of the PSCs based on OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD are 8.69%, 11.15% and 9.08%, respectively. The results demonstrate that the newly-developed compounds can act as efficient hole transporting materials for stable perovskite solar cells.

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1. Introduction

Recently, organometallic halide perovskites (CH₃NH₃PbX₃, X = Cl, Br, I) have drawn great attention as light absorbing materials owing to their direct band gaps, large optical absorption, excellent ambipolar charge mobility and small exciton binding energy.^1–4 The breakthrough in perovskite solar cells (PSCs) occurred when Kim et al. reported solid-state devices using organic 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) as a hole transporting material (HTM) to replace the liquid electrolyte, leading to a promising power conversion efficiency (PCE) of 9.7%.¹ Since then, the solid-state PSCs have led to a remarkable PCE exceeding 20%.^5,6 Now HTMs are an important part of PSCs. Inorganic and organic HTMs have been applied in PSCs. However, inorganic HTMs usually exhibit relatively low efficiency in PSCs and are limited by materials selection (mainly Ni, Cu compounds).^7–11 Organic HTMs are widely used in PSCs for their superior performance. They are classified into small molecules and polymers.¹² The former mainly includes phenylamine derivatives,^13–19 thiophene derivatives,^20–24 phthalocyanine derivatives,^25–28 etc. The latter mainly includes tertiaryarylamine polymer (PTAA),⁵ aniline polymer,²⁹ diketopyrrolopyrrole and benzodithiophene polymers.^30–35 Among these compounds, spiro-OMeTAD is the most widely used HTM in PSCs. Nevertheless, it is expensive due to its complicated multi-step synthesis and difficult purification process. Moreover, the devices based on it show inferior stability. These defects limit the large-scale industrial production of cost-effective PSCs. Hence, developing new organic HTMs is necessary for cost reduction and improving the performance of the devices.

To develop cost-efficient and stable HTMs for PSCs, herein, we introduce low-cost DPP and BDT moieties as bridge into small-molecular compounds (OMeTPA-DPP, OMeTPA-BDT) and find these two compounds can act as HTM when applied in PSCs. The structures of them are shown in Fig. 1. Two molecules show better thermal stability than spiro-OMeTAD. The charge dynamics at the HTMs/perovskite interface are investigated by steady-state photoluminescence (PL) and time-resolved PL measurements. And the long-term stability of the PSCs based on new HTMs are superior to that of spiro-OMeTAD.

Fig. 1 Molecular structures of OMeTPA-DPP and OMeTPA-BDT.

2. Results and discussion

2.1 Optical electrochemical and thermal properties of the compounds

Fig. 2a shows the UV-Vis absorption and fluorescence emission spectra of OMeTPA-DPP and OMeTPA-BDT dissolved in dichloromethane (DCM), and the characteristic of them are listed in Table 1. OMeTPA-DPP and OMeTPA-BDT exhibit absorption peaks at 642 nm and 414 nm, respectively. The absorption spectrum of OMeTPA-DPP shows stronger red shift than OMeTPA-BDT due to more powerful absorbing capacity of functionalized diketopyrrolopyrrole configurations than benzodithiophene configurations. The fluorescence spectra of both the new compounds show large stokes shifts (64 nm and 69 nm), which suggest that they can occur relatively large structural change upon excitation. The absorption spectra confirm that the compounds don't significantly absorb the visible light. The optical band gap (E_g) of HTMs estimated from the intersection of the corresponding normalized absorption and fluorescence emission spectra are 1.82 eV and 2.74 eV, respectively.

Fig. 2 (a) Normalized UV-vis absorption and photoluminescence, (b) cyclic voltammogram with ferrocene as the reference in DCM and (c) thermogravimetric analysis (TGA) of OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD.

Table 1 Summary of the optical, electrochemical and thermal properties of new compounds

HTM	λmax^a/nm	λPL^a^,^b/nm	Eg^c/eV	HOMO^d/eV	LUMO^e/eV	Td^f/°C	Tg^g/°C	Tm^g/°C
a UV-vis absorption spectra and fluorescence spectra were measured in CH2Cl2 solution.b Excitation at λmax.c From the intersection of absorption and emission spectra.d From CV measurement and referenced to ferrocene.e ELUMO = EHOMO + Eg.f Decomposition temperature, from TGA.g Glass-transition temperature and melting temperature, from DSC.
OMeTPA-DPP	642	706	1.82	−5.13	−3.31	305	157	169
OMeTPA-BDT	414	483	2.74	−5.19	−2.45	399	158	215

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Cyclic voltammograms (CV) measurements (Fig. 2b) are carried out to get the electrochemical properties of new compounds and spiro-OMeTAD. The pair of redox peaks of OMeTPA-DPP and OMeTPA-BDT is highly reversible, indicating that they have excellent electrochemical stability. The highest occupied molecular orbital (HOMO) energy levels are calculated by the following equation:³⁶ E_HOMO = −5.1 − (E_ox.HTM vs. Fc/Fc⁺) (eV), where E_ox.HTM vs. Fc/Fc⁺ is onset of oxidation potential with reference to ferrocene as standard. The HOMO energy levels of them calculated from CV are −5.13 eV and −5.19 eV. Interesting, electron-withdrawing diketopyrrolopyrrole bridge do not lower HOMO level of OMeTPA-DPP compared to OMeTPA-BDT, and repeated CV data are shown in Fig. S2.† HOMO levels of new compounds are more positive than the conductive band of perovskite, indicating a favorable driving force that may generate for electron–hole separation at the HTMs/perovskite interface. And both of them are deeper than spiro-OMeTAD (−5.08 eV). It indicates that the compounds have appropriate HOMO levels relating to the valence band of perovskite and lead to higher open circuit voltage than that of spiro-OMeTAD. The lowest unoccupied molecular orbital (LUMO) levels calculated from HOMO and E_g are found to be −3.31 eV and −2.45 eV for OMeTPA-DPP and OMeTPA-BDT, respectively.

The thermal properties of the compounds are investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As demonstrated in Fig. 2c and S1,† the results show that these two HTMs exhibit more than 150 °C of glass-transition temperature (T_g) and more than 300 °C of decomposition temperature, respectively. The spiro-OMeTAD show a low T_g of 122 °C (shown in Fig. S1†), which is similar with the reported value (T_g = 126 °C).³⁷ The introduction of DPP or BDT moieties into these new molecules as cores can lower the twisting force and increase the π-conjugation ability than spiro-core. Moreover, the existence of heterocyclic aromatic DPP or BDT into molecular main chain could decrease mobility of chain segment and molecular flexibility of these two new compounds than spiro-OMeTAD, which lead to high T_g than spiro-OMeTAD.^38,39 And high thermal stability is beneficial to the long-term durability of PSCs.

2.2 Charge dynamics

To investigate the charge transfer dynamics at the HTM/perovskite interface, we measure charge generation of the perovskite film via steady-state and time-resolved photoluminescence (PL). Theoretically, the quenching of steady-state PL and reduction of PL lifetime indicate the efficient charge separation. From the steady-state PL spectra in Fig. 3a, we can find that both of OMeTPA-DPP and OMeTPA-BDT showed almost completely quenched (quenching efficiency 85% of OMeTPA-DPP and 96% of OMeTPA-BDT) compared with a perovskite film without any HTM, which indicate efficient charge transfer at HTM/perovskite interface.^40,41 The higher quenching efficiency of OMeTPA-BDT may be more beneficial for effective PCEs than that of OMeTPA-DPP. Moreover, the steady-state PL spectra of perovskite films coated with HTMs reveal a hypochromatic shift, which may result from the optical properties of the perovskite/HTM interface⁴² and the chemical interaction between perovskite surface and HTM.⁴³

Fig. 3 (a) Time-integrated PL spectra. Excitation wavelength: 600 nm. (b) Time-resolved PL spectra. Monitored at 765 nm, excitation at 445 nm.

Fig. 3b shows the time-resolved PL decay spectra of perovskite film coated with OMeTPA-DPP, OMeTPA-BDT and one without any HTM layer. In the absence of HTM layer, the excited perovskite has no other possibility than recombining with electron within perovskite layer, indicating a longer decay time. Pristine perovskite exhibits a lifetime of 3.77 ns, illustrates relatively slow carrier recombination in perovskite layer. However, when the HTMs exhibit, the perovskite can be quenched by the HTMs layer, resulting in a faster decay process. All the perovskite coated with HTMs show significantly reduced lifetime, meaning faster decay rates (1.99 ns for OMeTPA-DPP and 0.96 ns for OMeTPA-BDT). Combined with the steady-state PL spectra, the faster decay rates of perovskite coating with new HTMs show the dynamical possibility of the charge transfer at HTM/perovskite interface.

2.3 Device performance

To prove the potential of OMeTPA-DPP and OMeTPA-BDT acting as HTMs in PSCs, we compare the devices with new compounds and spiro-OMeTAD as HTM, together with a cell without HTM. The cross-sectional field-emission scanning electron microscopy (SEM) image of a conventional mesoscopic PSC (shown in Fig. 4a) shows typical structural configuration with clear interfaces. The thickness of mesoporous TiO₂ composed of perovskite and the HTM layer are about 500 nm and 150 nm. The device configuration of hybrid solar cells is shown in Fig. 4b. And Fig. 4c shows the energy level diagram of the materials used in our devices according to the positions of HOMO and LUMO levels of the new HTMs.

Fig. 4 (a) Cross-sectional SEM image of the perovskite solar cells. (b) Scheme of the PSCs configuration. (c) Energy level diagram of the corresponding materials used in devices.

Fig. 5a shows the current-density–voltage (J–V) curves of fore-mentioned devices. The photovoltaic parameters of the PSCs are summarized in Table 2. As the PSCs have exhibited a greatly improvement in PCE by increasing the hole-conductivity of HTMs via doping the additives such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpiridine (TBP). We fabricate the devices by doping the two additives into OMeTPA-DPP and OMeTPA-BDT, spiro-OMeTAD HTMs to enhance cells performance.

Fig. 5 (a) Current density–voltage of cells based on OMeTPA-DPP, OMeTPA-BDT, spiro-OMeTAD and a cell without any HTM. (b) Corresponding incident photo-to-current conversion efficiency (IPCE) spectrum of the devices.

Table 2 The current–voltage (J–V) characteristics of CH₃NH₃PbI₃-based devices with OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD as HTM and with any HTM for comparison measured under simulated AM 1.5G irradiation

HTM	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
No HTM	0.75	11.21	63.9	5.41
OMeTPA-DPP	0.92	14.87	63.0	8.63
OMeTPA-BDT	0.96	16.60	68.5	10.89
Spiro-OMeTAD	0.95	19.93	71.1	13.45

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After preliminary optimization of devices, the PCEs of the PSCs with OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD as HTM are 8.63%, 10.89% and 13.45%, respectively. The device fabricated without any HTM shows a PCE of only 5.41%. Obviously, the improved PCEs of devices with OMeTPA-DPP or OMeTPA-BDT than a device without HTM demonstrate that the new compounds have the potential to be efficient HTMs in PSCs. The short circuit current (J_sc) of the devices fabricated with OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD are 14.87, 16.60, and 19.93 mA cm⁻². The incident photon to current efficiency (IPCE) spectra represents the ratio of extracted electrons to incident photons at the electrode surface at a given wavelength. And corresponding IPCE spectra of the devices with OMeTPA-DPP, OMeTPA-BDT, spiro-OMeTAD and without HTMs is shown in Fig. 5b. Compared with hole-conductor-free device, the IPCE spectra of the PSCs with HTMs are significantly improved in 400-800 nm. The devices with new compounds show lower IPCE response in 400–800 nm than that of spiro-MeOTAD, which may result from lower HOMO level of them than spiro-OMeTAD. So there is lower driving force for electron injection efficiency between the perovskite layer and the OMeTPA-DPP/OMeTPA-BDT layer. In addition, the integrated IPCE spectra of the devices are in good agreement with the J_sc from each device. In perovskite solar cells, the open circuit voltage (V_oc) mainly depends on the HOMO energy level of HTMs and the Fermi level of TiO₂. The HOMO energy level of OMeTPA-BDT is lower than that of spiro-OMeTAD by about 150 mV. Therefore, the difference of 110 meV between them lead to the average V_oc of device based on OMeTPA-BDT are higher than that of spiro-OMeTAD. The hysteresis characteristics of the PSC devices with new HTMs and spiro-OMeTAD were evaluated from the forward and reverse scan directions, as shown in Fig. S3 and Table S1.† There are similar hysteresis behavior among the devices with new HTMs and spiro-OMeTAD.

To study the stability of devices base on different HTMs, we further optimize the device fabrication and gain a PCE of 10.05%, 12.81% and 14.20% for OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD, respectively (shown in Fig. S5†). Then we put these cells without encapsulation under one sun illumination. As shown in Fig. 6 and Table S5,† we noted that the devices with OMeTPA-DPP and OMeTPA-BDT show a 14% and 13% loss of PCE, however, the spiro-OMeTAD-based showed the most significant deterioration (36% loss of PCE) after 10 days of aging. The excellent stability of the OMeTPA-DPP and OMeTPA-BDT based cells than that of spiro-OMeTAD are attributable to excellent thermal stability and the existence of hydrophobic alkyl chains on them.

Fig. 6 Stability test for devices based on OMeTPA-DPP (■), OMeTPA-BDT (●), and spiro-OMeTAD (▲).

3. Conclusions

In summary, we develop two simple small-molecular hole transporting materials with high thermal stability by introducing DPP or BDT moieties into molecules. These two HTMs can be synthesized by short route with high-yielding. Photoluminescence study indicate that OMeTPA-DPP and OMeTPA-BDT can act efficient HTM in PSCs. After 10 days of aging, residual efficiency of the PSCs based on OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD are 8.69%, 11.15% and 9.08%, respectively. These HTMs are thus promising HTMs with the potential to replace spiro-OMeTAD due to their good performance and simple synthesis route. This work also provide a strategy for designing and developing new efficient and stable HTMs for PSCs.

4. Experimental section

4.1 Materials and reagents

All reagents and chemicals were purchased from Sigma-Aldrich, Alfa, Sinopharm Chemical Reagent Co., Ltd., TCI and Derthon Optoelectronic Materials Science Technology Co., Ltd. without further purification. FTO coated glass substrates with a sheet resistance of 15 Ω sq⁻¹ were purchased from TEC company.

4.2 Hole transporting materials synthesis

The synthesis of OMeTPA-DPP and OMeTPA-BDT are simple with high yields by well-know reaction (Suzuki cross-coupling reactions). The synthesis route of them is shown in Scheme 1. And the molecular structures were confirmed by ¹H and ¹³C NMR spectroscopy and mass spectrometry (MALDI-TOF). Detailed synthesis procedures for both HTMs are shown as follows.

Scheme 1 Synthetic routes to OMeTPA-DPP and OMeTPA-BDT.

Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (compound 2). Under the protection of Ar, compound 1 (3 mmol, 1.728 g), bis(pinacolato)diboron (3.75 mmol, 0.96 g), KOAc (9.36 mmol, 0.9 g), dry 1,4-dioxane were added into a 50 mL flask. Then Pd₂dba₃ (14 mg) and X-Phos (30 mg) were added to the system and stirring at 80 °C overnight. The reaction mixture was then cooled down to room temperature and extracted with CH₂Cl₂, remove the solvent. The residue mixture was purified by column chromatography (CH₂Cl₂/hexane = 1 : 1) to obtain the white solid (product 2, 1.10 g, 74%). ¹H NMR (400 MHz, DMSO) δ 7.47 (d, 2H), 6.93 (d, 4H), 6.70 (d, 4H), 6.68 (d, 2H), 3.76 (s, 6H), 1.26 (s, 12H). ¹³C NMR (100 MHz, CDCl₃) δ 156.23, 151.44, 140.43, 135.80, 127.20, 119.47, 118.65, 114.77, 83.41, 55.48, 24.89.

Synthesis of 3,6-bis(5-(4-(bis(4-methoxyphenyl)amino)phenyl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrole [3,4-c]pyrrole-1,4(2H,5H)-dione (OMeTPA-DPP). Compound 2 (1.2 mmol, 0.517 g), compound 3 (0.5 mmol, 0.34 g), 2 M solution of K₂CO₃ (8 mmol, 1.1 g) in H₂O, [Pd(PPh₃)₄] (0.05 mmol, 58 mg), DMF (20 mL) were added into a 100 mL Ar-protected flask. The reaction solution was kept with stirring at 90 °C overnight. The reaction mixture was then cooled down to room temperature and poured into cool water, extracted with CH₂Cl₂. The organic layer was dried with MgSO₄, and the solvent was removed by rotary evaporator. The residue mixture was purified by column chromatography (CH₂Cl₂/hexane = 1 : 1) to obtain the product as atropurpureus solid (OMeTPA-DPP, 415 mg, 73%). ¹H NMR (400 MHz, CDCl₃) δ 7.44 (d, 4H), 7.29 (d, 4H), 7.10 (d, 8H), 6.87 (t, 12H), 4.06 (m, 4H), 3.81 (s, 12H), 1.95 (s, 2H), 1.47–1.27 (m, 16H), 0.95–0.81 (m, 12H). ¹³C NMR (100 MHz, CDCl₃) δ 161.69, 156.38, 150.18, 149.40, 140.06, 139.49, 137.07, 127.34, 127.05, 126.78, 124.70, 122.87, 119.69, 114.84, 107.71, 55.49, 45.93, 40.93, 39.19, 30.30, 28.48, 23.08, 14.07, 10.60. HRMS (MALDI-TOF): [M − H] m/z calcd for C₇₀H₇₃N₄O₆S₂: 1130.4898; found: 1130.4796.

Synthesis of 4,4′-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (OMeTPA-BDT).

Compound 4 (0.6 mmol, 0.36 g), compound 2 (1.5 mmol, 0.65 g), 2 M solution of K₂CO₃ (8 mmol, 1.1 g) in H₂O, [Pd(PPh₃)₄] (0.05 mmol, 58 mg), DMF (20 mL) were added into a 50 mL Ar-protected flask and following the same procedure to make OMeTPA-DPP. The OMeTPA-BDT was obtained as a green solid (554 mg, 88%). ¹H NMR (400 MHz, CDCl₃) δ 7.60–7.46 (m, 6H), 7.10 (d, 8H), 6.95 (d, 4H), 6.85 (d, 8H), 4.18 (s, 4H), 3.81 (s, 12H), 1.82 (dd, 2H), 1.75–1.65 (m, 2H), 1.58 (dd, 4H), 1.53 (d, 2H), 1.40 (s, 8H), 1.02 (t, 6H), 0.93 (d, 6H). ¹³C NMR (100 MHz, CDCl3) δ 156.22, 148.93, 143.96, 143.36, 140.50, 132.60, 129.17, 127.08, 126.99, 126.21, 120.16, 114.83, 113.95, 75.92, 55.52, 40.71, 30.52, 29.26, 23.93, 23.19, 14.21, 11.38. HRMS (MALDI-TOF): [M − H] m/z calcd for C₆₆H₇₁N₂O₆S₂: 1052.4177; found: 1052.4655.

4.3 Device fabrication

The fluorine-coated SnO₂ glass substrate (FTO) with high transparency in the visible range (2 cm × 1.5 cm) were etched in specific pattern with zinc powder and 1 M aqueous solution of HCl for 20 seconds. Then glass substrates were washed using an ultrasonic bath for 30 min, rinsed with ethanol, annealed at 510 °C for 30 min.

The compact TiO₂ layer was deposited on the etched FTO substrate by aerosol spray-pyrolysis using O₂ as the carrier gas at 450 °C from a precursor solution of 1.2 mL titanium diisopropoxide and 0.8 mL bis(acetylacetonate) in 14 mL anhydrous isopropanol, and then sintered at 510 °C for 30 min. The mesoporous TiO₂ film was deposited by spin coating at 4000 rpm for 30 s with a ramp of 2000 rpm s⁻¹ from a diluted commercial TiO₂ paste in ethanol (Dyesol 18NR-T, 2 : 7, weight ratio), gradually heated to 510 °C, keep 15 min and cooled to room temperature in air. Then the TiO₂ films were treated in a 0.04 M aqueous solution of TiCl₄ at 70 °C for 30 min, rinsed with deionized water and following annealed at 510 °C for 30 min.

The PbI₂ in DMF solution (461 mg mL⁻¹) was dropped on the TiO₂/FTO substrate by spin-coated at 3000 rpm for 30 s, put in room temperature for 5 min, annealed on a hot plate at 70 °C for 30 min. After cooling down, the film was dipped in 9 mg mL⁻¹ CH₃NH₃I (prepared according to the previously reports⁴⁴) in 2-propanol solution for 30 s, spin coating at 3000 rpm for 30 s and dried at 70 °C for 30 min. The HTM layer was coated by spin-coating at 5000 rpm for 30 s. The concentration of new HTMs and spiro-OMeTAD are 70 mg mL⁻¹ in 1 mL of chlorobenzene with heating to70 °C overnight, then 28.8 μL of 4-tert-butylpyridine, 17.5 μL of lithium bis(trifluoromethylsulphonyl)imide (520 mg Li-TFSI in 1 mL of acetonitrile) were added to the solution. The HTMs were spin-coated on the CH₃NH₃PbI₃/TiO₂/FTO substrate at 4000 rpm for 30 s. All device fabrications were performed below 15% of relative humidity.

Finally, Au layer with a thickness of 60 nm was deposited on HTM layer by thermal evaporator to form the back contact. The active area of the device was defined by a black mask with a size of 0.09 cm² for all measurement.

4.4 Analytical measurement

¹H NMR and ¹³C NMR spectra were measured by a Bruker DPX 400 MHz spectrometer with the chemical shifts against tetramethylsilane (TMS). HRMS(MALDI-TOF) experiments were performed using a MS Bruker Daltonik Reflex III and Bruker solariX spectrometer. The absorption spectra of the compounds were recorded by a UV-Vis spectrophotometer (U-3900H, Hitachi, Japan). Cyclic voltammograms (CV) were carried out on a CHI660D (Shanghai Chenhua Device Company, China) in a three-electrode electrochemical cell with a Pt disk working electrode, an SCE reference electrode and a platinum wire sphere with supporting electrolyte of 0.1 mol L⁻¹ tetrabutylammonium hexafluorophosphate (TBAPF₆) in CH₂Cl₂ at a scan rate of 50 mV s⁻¹ using ferrocene as inner standard at the end of each measurement. The cross-sectional morphologies of the devices were observed with a field emission scanning electron microscopy (FE-SEM, sirion200, FEI Corp., Holland). The photocurrent–voltage (J–V) characteristics of the PSCs were measured under AM 1.5 (100 mW cm⁻²) illumination which was provided by an AAA grade solar simulator (Newport, USA, 94043A, calibrated with a standard crystalline silicon solar cell). The incident photon to current conversion efficiency (IPCE) was performed on QE/IPCE measurement kit (Newport, USA, CA). The steady-state photoluminescence (PL) emission and time-resolved PL decay measurements were carried out on a Fluorescence Detector (QM400 and LaserStrobe, Photo Technology International, USA). Time-integrated PL emission spectra of film were recorded by a standard 450 W xenon CW lamp. Time-resolved PL decay measurements were carried out on a pulsed nitrogen/dye laser. Differential scanning calorimetry of HTMs was recorded with scan rate of 20 °C min⁻¹ (DSC, TA Instruments-Waters LLC, USA, Q2000). The decomposition temperature of the compounds were carried out on a Thermal Gravimetric Analyzer (TGA, Q5000IR) at a heating rate of 10 °C min⁻¹ in a temperature range of 0 °C to 800 °C under nitrogen atmosphere.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (No. 2015CB932200), Natural Science Foundation of Anhui Province (No. 1508085SMF224).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18823k

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