Wide bandgap materials design toward tunable bandgap and increased carries injection property: silane interrupting π-conjugation together with peripheralcarbazolyl substituents

Abstract

A series of wide bandgap materials constructed by silicon-interrupted backbone and non-conjugated carbazole pendants, namely SiPPF, SiPPFCz, SiPF and SiPFCz, are designed and synthesized. Their applications as hosts for triplet emitters in phosphorescent polymer light-emitting diodes (PhPLEDs) are systematically studied. The results show that their photophysical and carries injection properties could be finely tuned by regulating the silicon-confined conjugation length and the peripheral carbazole substituents, respectively. Among them, SiPFCz achieves a compromise of essential demands for ideal electrophosphorescent host materials, such as high triplet energy levels, small barriers for carries injection and homogeneous morphology as well as good miscibility to the dopants. Therefore, the performances of SiPFCz-based devices are superior to devices hosted by SiPPF, SiPPFCz and SiPF, which sheds light on the design strategy of wide bandgap host materials.

---

Introduction

Wide bandgap semiconducting polymers and oligomers have attracted intensive research efforts for their application in highly efficient electrophosphorescent devices through inexpensive solution processing technologies.^1–6 In such phosphorescent organic light-emitting diodes (PhOEDs), the phosphorescent dopants, which can harvest both singlet and triplet excitons simultaneously to achieve nearly 100% internal quantum efficiency, are slightly dispersed into appropriate wide bandgap polymeric or oligomeric hosts to suppress the detrimental effects such as triplet–triplet annihilation.^7,8 Therefore, the wide bandgap polymer and oligomer play crucial roles in PhOEDs.^9,10 Basically, the wide bandgap polymers should have higher triplet energy (E_T) level than that of the guests in order to prevent reverse energy transfer from the guests back to the hosts.^11–16 Besides, there are other inevitable demands for host materials, such as appropriate highest occupied molecular orbital levels (HOMOs) and lowest unoccupied molecular orbital levels (LUMOs), good thermal stability, etc.^17–19 However, due to the high LUMOs and/or low HOMOs as a consequence of the intrinsic wide bandgap, it is difficult to achieve the aforementioned requirement for wide bandgap host polymers and oligomers.^20–22 So it is quite a challenge to design wide bandgap polymers and oligomers capable of giving adequate E_T and good charge injection ability.

In the past decade, tremendous efforts have been devoted to developing tailor-made multifunctional wide bandgap polymeric and oligomeric host materials.^23–28 Non-conjugated poly(vinyl-carbazole) (PVK) as a typical example of polymeric host material has been widely used in PhPLEDs.^29–31 The N-pendent carbazole unit endows the polymer with a high E_T and good hole transporting properties. However, because of its non-conjugated backbone, PVK suffers from the problems of large resistivity and tendency to aggregation, which leads to high operating voltage and excimer formation. Poly(2,7-fluorene)s and oligomeric fluorene (F₃) are another kind of widely used hosts due to their high photoluminescence (PL) quantum efficiency and excellent bipolar conductivity.^32–36 However, the completely conjugated backbone of poly(2,7-fluorene)s and oligomeric fluorene leads to low-lying triplet energy state (2.15–2.30 eV), which are not good hosts for blue and green triplet emitters. Therefore, the conjugation length of backbone in poly(2,7-fluorene)s and oligomeric fluorene should be finely tailored to achieve high E_T, making them suitable for the blue and green guests. In addition, the fluorene-only compounds usually have a deep HOMO level at about −5.9 eV, and consequently, a large barrier for injection of holes from PEDOT:PSS. Several hybrid types of oligomers also prompted the development of host materials for solution processed phosphorescent OLEDs. These oligomers are usually made up of the combination of aromatic electron donor and acceptor units, such as oxadiazole and triazine modified carbazole derivitives,^37,38 CzOXD, TRZ-1Cz(MP)2, TRZ-3Cz(MP)2, and benzimidazol modified aromatic amine derivatives, TBBI and Me-TBBI.^39,40 Although through meta-linkage or methyl-induced twist linkage, these oligomers obtained moderate E_T and showed good devices performance for green triplet emitters, their E_T are still not high enough for blue triplet emitters. Recently, our group has demonstrated that the saturated Si center could effectively interrupt the π-conjugation to realize high E_T.^41–43 Herein, we also employed this manner to interrupt the conjugated backbone of poly(2,7-fluorene)s and oligomeric fluorene with additional non-conjugated carbazole as pendant, constructing a series of wide bandgap polymers and oligomer, namely SiPPF, SiPPFCz, SiPF and SiPFCz. First, the poly(2,7-fluorene)s and oligomeric fluorene were interrupted by silane-bridge to control the conjugation length, which can effectively modulate the triplet energy to be suitable for the electrophosphorescent doping devices.^44–48 Second, the peripheral carbazole substituents were attached to the polymeric backbone by non-conjugated alkyl group, which could improve the carries injection properties of the fluorene without changing the conjugation length of the backbone.⁴⁹ Third, the steric hindrance of the non-planar silicon moiety in the backbone could prevent the intermolecular interaction in solid state and thus enables formation of smooth and stable amorphous films.^50,51 Blue, green and red devices using these copolymers and oligomer as hosts doped with iridium complexes as the light emitting layers are fabricated. Due to its higher triplet energy and good carries injection properties, the SiPFCz-based devices show better performance than corresponding SiPPFCz- and SiPF-based devices. The luminous efficiencies of SiPFCz-based blue, green and red electrophosphorescent devices are 0.61 cd A⁻¹, 10.27 cd A⁻¹ and 2.02 cd A⁻¹, respectively.

Experimental section

Characterization and measurements

¹H and ¹³C NMR spectra were measured on AVANCZ 500 spectrometers at 298 K using CDCl₃ as solvent and tetramethylsilane (TMS) as standard in all cases. Elemental analysis was performed by Flash EA 1112, CHNS-O elemental analysis instrument. Molecular weight of the polymer was determined by gel permeation chromatography (GPC) with an HPLC Waters 510 pump using a series of low-polydispersity polystyrene standards in tetrahydrofuran (THF; HPLC grade, Aldrich) at 308 K. Thermal gravimetric analysis (TGA) was undertaken on a Perkin-Elmer thermal analysis system at a heating rate of 10 °C min⁻¹ and a nitrogen flow rate of 80 mL min⁻¹. Differential scanning calorimetry (DSC) was performed on a NETZSCH (DSC-204) unit at a heating rate of 10 °C min⁻¹ under nitrogen. UV-visible absorption spectra were recorded on a UV-3100 spectrophotometer. Photoluminescent (PL) spectra were carried out with a RF-5301PC fluorometer. EL spectra and Commission Internationale De L'Eclairage (CIE) coordination of these devices were measured by a PR650 spectrometer. The luminance–current density–voltage characteristics were recorded simultaneously with the measurement of the EL spectra by combining the spectrometer with a Keithley model 2400 programmable voltage–current source. All measurements were carried out at room temperature under ambient conditions.

Materials

Solvents were carefully dried and distilled from appropriate drying agents prior to use. The starting materials were purchased from Aldrich or Across company and used as received.

Bis(7-bromo-9,9-dihexyl-9H-fluoren-2-yl)diphenylsilane (M5). A solution of 2,7-dibromo-9,9-dihexylfluorene (1.97 g, 4 mmol) in dry THF was cooled to −78 °C. To the solution was added n-BuLi (2.5 M, 1.8 mL, 1.1 equiv.) dropwise via a syringe. After stirred for 2 h, dichlorodiphenylsilane (0.4 mL, 2 mmol) was added dropwise. The mixture was warmed to room temperature and stirred overnight. Then the solution was poured to water and extracted with chloroform. The organic phase was dried over anhydrous MgSO₄ before the solvent was evaporated. The crude product was purified by column chromatography to give the product as a white solid in 62% yield (1.25 g). ¹H NMR (CDCl₃, 500 MHz, ppm): δ 7.65–7.55 (10H, m, Ar–H), 7.47–7.43 (8H, m, Ar–H), 7.38–7.35 (4H, t, J = 7.9 Hz, Ar–H), 1.91–1.88 (8H, m, –CH₂–), 1.16–1.04 (24H, m, –CH₂–), 0.80–0.77 (12H, t, J = 7.3 Hz, –CH₃), 0.64–0.62 (8H, m, –CH₂–). ¹³C NMR (CDCl₃, 125 MHz, ppm): δ 153.75, 149.86, 141.99, 140.31, 136.74, 135.70, 135.23, 133.50, 131.23, 130.43, 130.05, 128.29, 126.64, 121.94, 121.77, 119.77, 55.83, 40.56, 31.95, 30.05, 24.23, 23.00, 14.46. Elemental anal. calcd for C₆₂H₇₄Br₂Si: C, 73.94%; H, 7.41%. Found: C, 74.13%; H, 7.21%.

SiPPF. In a 50 mL round flask, a mixture of M1 (168.6 mg, 0.34 mmol), M4 (200 mg, 0.34 mmol), [(PPh₃)₄]Pd(0) (2 mol%, 8 mg), 0.6 mL of toluene and 0.4 mL of K₂CO₃ (2 M) was degassed and stirred at 85 °C for 3 days under nitrogen atmosphere. After cooling down to room temperature, the mixture was extracted, concentrated, and the desired polymer was precipitated from methanol and obtained as a white solid in 72% (164 mg). ¹H NMR (CDCl₃, 500 MHz, ppm): δ 7.72–7.43 (16H, m, Ar–H), 7.41–7.29 (8H, m, Ar–H), 2.04–1.95 (4H, m, –CH₂–), 1.03–0.97 (12H, m, –CH₂–), 0.68–0.65 (10H, m, –CH₂–, –CH₃). ¹³C NMR (CDCl₃, 125 MHz, ppm): δ 151.76, 142.71, 140.32, 139.76, 136.95, 136.43, 132.84, 129.70, 128.79, 127.20, 126.62, 126.09, 121.51, 120.11, 55.29, 40.51, 31.47, 29.73, 23.83, 22.58, 13.98. M_w: 25 800, polydispersity index (PDI): 2.29.

SiPPFCz. The procedure for SiPPFCz was followed to produce SiPPF using M3 (148 mg, 0.25 mmol) and M6 (200 mg, 0.25 mmol) as a white solid in 76% (184 mg). ¹H NMR (CDCl₃, 500 MHz, ppm): δ 8.03–7.97 (4H, m, Ar–H), 7.76–7.60 (16H, m, Ar–H), 7.54 (2H, s, Ar–H), 7.48–7.35 (8H, m, Ar–H), 7.32–7.30 (2H, m, Ar–H), 7.29–7.08 (8H, m, Ar–H), 4.12–4.04 (4H, m, N–CH₂), 1.95–1.83 (4H, m, CH₂), 1.65–1.59 (4H, m, CH₂), 1.16–1.06 (8H, m, CH₂), 0.65–0.57 (4H, m, CH₂), ¹³C NMR (CDCl₃, 125 MHz, ppm): δ 142.98, 140.73, 140.25, 137.41, 136.87, 134.66, 133.36, 130.18, 128.44, 127.03, 126.68, 125.92, 123.13, 121.76, 120.67, 119.06, 108.99, 55.55, 43.21, 40.86, 30.16, 29.18, 27.25, 24.17. M_w: 20 600, PDI: 1.90.

SiPF. The procedure for SiPF was followed to produce SiPPF using M3 (117 mg, 0.20 mmol) and M6 (200 mg, 0.20 mmol) as a white solid in 73% (173 mg). ¹H NMR (CDCl₃, 500 MHz, ppm): δ 7.78–7.77 (1H, d, J = 7.63 Hz, Ar–H), 7.72–7.60 (12H, m, Ar–H), 7.51–7.49 (1H, d, J = 7.63 Hz, Ar–H), 7.46–7.36 (6H, m, Ar–H), 1.96–1.94 (4H, m, CH₂), 1.13–1.05 (12H, m, CH₂), 0.79–0.69 (10H, m, CH₂, CH₃), ¹³C NMR (CDCl₃, 125 MHz, ppm): δ 152.23, 150.53, 143.10, 142.65, 140.92, 140.56, 137.36, 136.84, 136.79, 135.64, 135.54, 134.67, 133.29, 133.04, 131.25, 130.11, 129.95, 128.38, 128.24, 127.04, 126.47, 121.92, 120.75, 119.81, 55.59, 40.74, 31.96, 30.13, 24.32, 23.00, 14.45. M_w: 30 300, PDI: 2.24.

SiPFCz. The procedure for SiPFCz was followed to produce polymers using M3 and M6 as a white solid in 61%. ¹H NMR (CDCl₃, 500 MHz, ppm): δ 8.03–8.02 (8H, d, J = 7.63 Hz, Ar–H), 7.73–7.65 (16H, m, Ar–H), 7.59–7.57 (2H, m, Ar–H), 7.52 (2H, s, Ar–H), 7.48–7.45 (2H, t, J = 7.32 Hz, Ar–H), 7.42–7.40 (4H, m, Ar–H), 7.37–7.34 (8H, m, Ar–H), 7.33–7.30 (2H, m, Ar–H), 7.26–7.22 (12H, m, Ar–H), 7.16–7.13 (8H, t, J = 7.32 Hz, Ar–H), 4.11–4.08 (8H, t, J = 7.32 Hz, N–CH₂–), 1.92–1.89 (8H, m, –CH₂–), 1.65–1.59 (8H, m, –CH₂–), 1.13–1.03 (16H, m, –CH₂–), 0.64–0.58 (8H, m, –CH₂–). ¹³C NMR (CDCl₃, 125 MHz, ppm): δ 151.46, 151.03, 143.02, 141.10, 140.74, 140.12, 137.39, 136.86, 134.67, 133.28, 130.16, 128.42, 127.64, 127.38, 127.02, 126.53, 125.93, 123.19, 123.14, 121.72, 120.69, 120.52, 120.29, 119.06, 109.01, 55.39, 43.25, 40.74, 30.09, 29.15, 27.22, 24.03. Elemental anal. found: C, 88.33%; H, 6.62%, N, 3.37%.

Device fabrication

Devices consisting of ITO/PEDOT:PSS/light emitting layer (LEL)/TPBI/LiF/Al were fabricated as follows: PEDOT:PSS was spin-coated onto the cleaned ITO-coated glass substrate from its aqueous solution and then heated at 120 °C for 20 minutes to remove the residual water solvent. Doped polymer was used as LEL, which was prepared by spin-coating its chlorobenze solution onto the top of the PEDOT:PSS layer. After the spin-coating, all devices were transferred into a vacuum chamber immediately without exposure to the atmosphere. Inside the chamber, 40 nm-thick TPBI, 0.5 nm-thick LiF, and 200 nm-thick Al were in sequence deposited by thermal evaporation at a pressure of 4.0 × 10⁻⁶ mbar. All processes after the preparation of PEDOT:PSS layer were performed in a nitrogen-filled glove box.

Results and discussion

Synthesis and characterization

The chemical structure and synthetic approach for the polymers and oligomer are illustrated in Scheme 1. The synthesis procedures of monomers M1–M4 are prepared according to our reported literatures.^20,21 M5 was achieved by monolithiation of 1,4-dibromobenzene with n-BuLi at −78 °C in diethyl ether substitution between the monolithiate and Ph₂SiCl₂. Finally, the palladium-catalyzed Suzuki coupling polymerization was employed to produce the desired polymers and oligomer in reasonable yields. The structures of the synthesized monomers and the resulting polymers were confirmed by ¹H and ¹³C NMR spectroscopy as well as elemental analysis. All the copolymers and oligomer are readily soluble in common organic solvent such as chloroform, THF, and toluene. The weight-average molecular weights (M_w) of SiPPF, SiPPFCz, SiPF, as determined through gel permeation chromatography (GPC) using THF as the eluent and polystyrene as standards, were 25 800, 20 600 and 30 300, respectively, with a polydispersity index (PDI) in the range of 1.90–2.44.

Scheme 1 Chemical structures and synthetic routes to the polymers and oligomer.

Thermal properties

Fig. 1 shows the thermal properties of the polymers and oligomer measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal data are also listed in Table 1. SiPPF, SiPPFCz, SiPF and SiPFCz exhibit decomposition temperatures (T_d: corresponding to 5% weight loss) as high as 420 °C, 418 °C, 449 °C and 442 °C, respectively. The DSC traces of all the materials exhibits distinct glass transition during the second heating scans, the glass transition temperature (T_g) of SiPPF, SiPPFCz, SiPF and SiPFCz are established to be 174 °C, 127 °C, 125 °C and 95 °C, respectively, which are much higher than the commonly used wide bandgap host materials, such as CBP and mCP. It is easily to understand that the rigid tetraphenylsilane or dimethyldiphenylsilane units in the polymer backbone enhanced the chain rigidity of the polymer and resulted in high values of T_g. As the temperature is increasing, no other crystallization and melting peaks are observed up to 300 °C, demonstrating that they are stable amorphous materials. The results show that they could form morphologically stable and uniform amorphous films upon solution-process which is a very desirable feature for their application in OLEDs since it has been demonstrated that high T_g materials lead to enhanced device stability and lifetime.

Fig. 1 (a) TGA curves of the polymers and oligomer with a heating rate of 10 °C min⁻¹ in nitrogen. (b) Second heating DSC traces of the polymers and oligomer with a heating rate of 10 °C min⁻¹ in nitrogen.

Table 1 Thermal and photophysical data of the polymers and oligomer

Compd	Mw (PDI)	Td/Tg (°C)	Solution		Film		Eg (eV)
			UV (nm)	PL (nm)	UV (nm)	PL (nm)
SiPPF	25 800 (2.29)	420/174	336	374, 393	333	403, 422	3.33
SiPPFCz	20 600 (1.90)	418/127	295, 331	373, 392	265, 297, 333	381, 412, 433	3.33
SiPF	30 300 (2.24)	449/125	303, 325	354, 371	323	358, 375, 390	3.47
SiPFCz	—	442/95	295, 320	352, 369	265, 297, 321	375, 412, 433	3.47

---

Electrochemical characterization

Cyclic voltammetry was employed to investigate the electrochemical behaviors of polymers and oligomer as well as to estimate the HOMOs and LUMOs of the materials. The oxidation and reduction process were performed in CH₂Cl₂ and DMF solution, respectively, using 0.1 M n-Bu₄NPF₆ as the supporting electrolyte and ferrocene (Fc) as the internal standard. As shown in Fig. 2, the oxidative onset potential from 1.39 V and 1.40 V for SiPPF and SiPF shifts negatively to 0.92 V and 0.89 V for carbazole substituted SiPPFCz and SiPFCz, respectively, which are attributed to the introduction of carbazoles. Thus HOMO energy levels of SiPPFCz and SiPFCz were calculated to be −5.52 and −5.49 eV. These values were 0.41 and 0.51 eV higher than SiPPF and SiPF, respectively. It is evident that SiPPFCz and SiPFCz have higher HOMO levels compared with SiPPF and SiPF due to the pendent carbazole unit as expected, suggesting smaller carries-injection barriers from neighboring layers or electrodes to these materials. In contrast, the reduction behaviors of SiPPFCz and SiPFCz are quite similar with their corresponded non-carbazole-containing molecules, SiPPF and SiPF, respectively.

Fig. 2 Cyclic voltammogram of the polymers and oligomer.

Optical properties

The normalized absorption (UV) and photoluminescence (PL) emission spectra of polymers and oligomer in dilute dichloromethane solution are shown in Fig. 3a. SiPPF and SiPF exhibit almost the same absorption spectra shape with a blue-shift for SiPF due to its shorter conjugated length. This broad absorbance band around 320 to 340 nm are assigned to the characteristic π–π* transitions of backbone conjugated segments. Due to the same polymer backbone, SiPPFCz and SiPFCz exhibit the similar lowest π–π* transitions with their counterparts, SiPPF and SiPF, respectively, but with an additional absorption peak at 295 nm, which originates from the π–π* transitions of peripheral carbazole units. The PL spectra of SiPPF and SiPPFCz are similar with two peaks at 374 and 393 nm. Because of the reduction of one phenyl unit in the backbone, these two peaks for SiPF and SiPFCz are blue-shifted to 354 and 371 nm. The band gaps of the polymers estimated from the onset of their absorption in film are 3.33 eV for SiPPF and SiPPFCz, and 3.47 eV for SiPF and SiPFCz, respectively. The optical band gaps of SiPF and SiPFCz are bigger than that of SiPPF and SiPPFCz, indicating the successful regulation of the effective conjugation lengths by inset different segments between the two δ-Si bond in polymer backbone. It is should be noted that the appended carbazole groups in SiPPFCz and SiPFCz have no effect on the conjugation of the polymer backbone.

Fig. 3 (a) The normalized absorption and PL spectra of the polymers and oligomer in dilute dichloromethane solution (10⁻⁵ M). (b) Normalized PL spectra of the polymers and oligomer films doped with 8 wt% FIrpic.

Being used as the host for the phosphorescent dye bis[(4,6-difluorophenyl)pyridyl]-iridium picolinate (FIrpic), a higher value of E_T for the host material is a provision allowing the effective confinement of triplet excitons on the guest. Fig. 3b compares PL spectra of polymers and oligomer doped with blue iridium complexes FIrpic in concentrations of 8 wt%. For SiPPF film, almost no FIrpic emission could be observed, indicating that the triplet energy level of SiPPF is lower than that of FIrpic. Although the SiPPFCz has the same backbone with SiPPF, a new emission around 472 nm stemming from FIrpic could be observed, which is due to the energy transfer from the peripheral carbazole to FIrpic. When the conjugated length is reduced for SiPF, obvious FIrpic emission could be observed around 472 nm beside the emission from the pristine SiPF. This means that the triplet energy of SiPF increased to be comparable with the FIrpic. As for the doped SiPFCz film, the emission of SiPFCz around 412 nm almost disappeared and only the emission from FIrpic is observed. This implies that energy transfers completely occur from the oligomer to phosphorescent guest and the back energy transfers are forbidden, which suggests that SiPFCz can be used as a host for FIrpic in blue phosphorescent PLEDs. Therefore, the E_T of these polymers and oligomer is higher than that of non-silane interrupted poly(2,7-fluorene)s and oligomeric fluorene (F₃), which didn't exhibit energy transfer from the host to the dopant, FIrpic, in their doping film.³⁵

Morphological stability

We further used atomic force microscopy (AFM) to investigate morphologies of these polymers and oligomer. The films were prepared by spin-coating their 10 mg mL⁻¹ CH₂Cl₂ solutions on quartz substrates. As displayed in Fig. 4, all the pristine polymers and oligomer could form smooth and featureless films at room temperature with a root-mean-square (rms) surface roughness less than 0.73 nm. When doped with 10 wt% FIrpic, they all still provided smooth and uniform surfaces, free of phase separation and particle aggregation.

Fig. 4 AFM topographic images of the polymers and oligomer. Top: the pristine films. Bottom: doped with 8 wt% FIrpic.

Electroluminescence properties

To evaluate the potential of these compounds as hosts for phosphorescent PLEDs, blue light emitting devices with a configuration of ITO/PEDOT:PSS (40 nm)/host:FIrpic (100 : 8, 30 nm)/TPBI (50 nm)/LiF/Al are fabricated, where PEDOT:PSS was used as a hole injection layer (HIL) and TPBI was used as an electron transporting layer (ETL). The low triplet energy level and high barrier to the hole injection of SiPPF precluded OLED studies using this material. Unfortunately, the SiPPFCz- and SiPF-based devices could not successfully obtain the good performance, this may be attributed to their E_T is similar to that of FIrpic, which cannot effectively confine the energy on FIrpic and quench its emission. The J–V–L characteristics and curves of luminous efficiency versus current density of SiPFCz-based device are shown in Fig. 5. The device using SiPFCz as the host shows a turn-on voltage (at 1 cd m⁻²) of 4.2 V. The maximum luminous efficiency and maximum brightness of 0.61 cd A⁻¹ and 276 cd m⁻² are achieved for this device, respectively.

Fig. 5 (a) The J–V–L characteristics and (b) efficiencies vs. current density for the blue device with a configuration of ITO/PEDOT:PSS/SiPFCz:FIrpic (8 wt%)/TPBI/LiF/Al. Inset: normalized EL spectra at 6.0 V of the devices.

Interpreting this behaviour to be attributed to that the SiPFCz possesses a compromise of high triplet energy level and good carries injection properties, which means that SiPFCz has shorter conjugated length (higher triplet energy level) than that of SiPPFCz and smaller barriers for hole injection than that of SiPF.

To test the applicability of these hosts as green and red phosphorescent emitters, we then fabricated devices using SiPPFCz, SiPF and SiPFCz as the hosts and the fac-tris(2-phenylpyridyl)iridium Ir(ppy)₃ as green dopant and bis(2-benzo[b]thiophen-2-yl-pyridine) (acetylacetonate)iridium (III) ((btp)₂Ir(acac)) as red dopant, with devices structure of ITO/PEDOT (40 nm)/hosts:dopants (100 : 8, 30 nm)/TPBI (50 nm)/LiF/Al. Table 2 summarizes the performance of all these doped devices. For the green phosphorescent devices, SiPPFCz- and SiPFCz-based devices show stable green emissions of Ir(ppy)₃, and no emission from the host was observed. While, the electroluminescent spectrum of the SiPF-based device features an emission around 400 nm from the pristine SiPF and a large exciplex emission peak at 572 nm. The turn-on voltages of SiPPFCz- and SiPFCz-based devices are 6.4 and 5.8 V, respectively, which are significantly lower than that of SiPF-based device (12.2 V). This suggests that the introduction of carbazole group can effectively decrease the turn-on voltage due to the low barrier for the holes injection into emission layer, which is consistent with the HOMOs determined by the CV measurement. The brightness and efficiencies of SiPFCz-based device are superior to that of SiPPFCz- and SiPF-based devices and the reason for this has been discussed above. The J–V–L characteristics and curves of efficiencies versus current density of SiPFCz-based device are showed in Fig. 6. The maximum luminous efficiency of 10.27 cd A⁻¹ is achieved for SiPFCz-based device. A normalized EL spectrum at 15.0 V is also shown in the inset of Fig. 6b, pure green emission is obtained in this doped device.

Table 2 The devices performance of ITO/PEDOT (60 nm)/hosts:dopants (100 : 8, 30 nm)/TPBI (50 nm)/LiF/Al

Host	Guest	Turn-on voltage (V)	Brightness (cd m^−2)	Luminous efficiency (cd A^−1)	Power efficiency (lm W^−1)	EL λmax (nm)	CIE
SiPFCz	FIrpic	4.2	276	0.61	0.40	470	0.15, 0.28
SiPPFCz	Ir(ppy)3	6.4	154	2.05	0.47	516	0.32, 0.59
SiPF	Ir(ppy)3	12.2	2.5	0.19	0.03	572	0.33, 0.60
SiPFCz	Ir(ppy)3	5.8	940	10.27	2.13	516	0.33, 0.61
SiPPFCz	(btp)2Ir(acac)	5.8	65	0.78	0.22	620	0.66, 0.33
SiPF	(btp)2Ir(acac)	8.8	—	—	—	620	0.66, 0.32
SiPFCz	(btp)2Ir(acac)	6.0	177	2.02	0.57	620	0.67, 0.33

---

Fig. 6 (a) The J–V–L characteristics and (b) efficiencies vs. current density for the green device with a configuration of ITO/PEDOT:PSS/SiPFCz:Ir(ppy)₃ (8 wt%)/TPBI/LiF/Al. Inset: normalized EL spectra at 18.0 V of the devices.

The trend of the performance of red devices is similar with the green devices. They all show stable red emissions of (btp)₂Ir(acac), and no emission from the host was observed, indicating the triplet energy level of these compounds are high enough to prevent the backward energy transfer from (btp)₂Ir(acac). The SiPPFCz- and SiPFCz-based devices also showed lower turn-on voltages of 5.8 and 6.0 V, respectively, while the turn-on voltage of SiPF-based device is 8.8 V. As expected, the SiPFCz-based device also exhibits the best performance with a maximum luminous efficiency of 2.02 cd A⁻¹ (Fig. 7). Normalized EL spectra of this device at 18.0 V is also shown in the inset of Fig. 7b, the emission exclusively from (btp)₂Ir(acac), no emission from the host was observed. Therefore, these results confirm that SiPFCz is a suitable host polymer for phosphorescent PLEDs, and more efficient devices are expected by further optimization of the device structure.

Fig. 7 (a) The J–V–L characteristics and (b) efficiencies vs. current density for the red device with a configuration of ITO/PEDOT:PSS/SiPFCz:(btp)₂Ir(acac) (8 wt%)/TPBI/LiF/Al. Inset: normalized EL spectra at 18.0 V of the devices.

Conclusions

In summary, we have developed a new molecular design strategy for wide bandgap material with a special molecular structure, using silane to interrupt π-conjugated in the main chain and alkyl to attach carbazole pendent group. Followed by this idea, a series of silane-containing polymers and oligomer, SiPPF, SiPPFCz, SiPF and SiPFCz, are synthesized. Through systematic investigation, we demonstrate that their conjugation could be effectively interrupted by the non-conjugated silane, which result in high E_T as compared with the non-silane polymers and oligomer (poly(2,7-fluorene)s, oligomeric fluorene (F₃)). Meanwhile, the carries injection barriers of the polymer and oligomer, SiPPFCz and SiPFCz, could be reduced by peripheral carbazole substituents compared with SiPPF and SiPF. Blue, green and red devices employing these materials doped with iridium complexes as the light emitting layer are also investigated. Among them, SiPFCz-based devices get best performance, with luminous efficiencies of 0.61 cd A⁻¹, 10.27 cd A⁻¹ and 2.02 cd A⁻¹, for blue, green and red dopants, respectively.

Acknowledgements

This work is financially supported by the National Basic Research Program of China (973 Program, 2013CB834701, 2013CB834800), National Science Foundation of China (Grant no. 51403063, 21174050, 91233113) and China Postdoctoral Science Found (Grant no. 2014M562174).

Notes and references

1. Y. Kawamura, S. Yanagida and S. R. Forrest, J. Appl. Phys., 2002, 92, 87.
2. P. T. Furuta, L. Deng, S. Garon, M. E. Thompson and J. M. J. Fréchet, J. Am. Chem. Soc., 2004, 126, 15388.
3. Y. C. Chen, G. S. Huang, C. C. Hsiao and S. A. Chen, J. Am. Chem. Soc., 2006, 128, 8549.
4. A. v. Dijken, J. J. A. M. Bastiaansen, N. M. M. Kiggen, B. M. W. Langeveld, C. Rothe, A. Monkman, I. Bach, P. Stössel and K. Brunner, J. Am. Chem. Soc., 2004, 126, 7718.
5. J. X. Jiang, Y. H. Xu, W. Yang, R. Guan, Z. Q. Liu, H. Y. Zhen and Y. Cao, Adv. Mater., 2006, 18, 1769.
6. A. Haldi, A. Kimyonok, B. Domercq, L. E. Hayden, S. C. Jones, S. R. Marder, M. Weck and B. Kippelen, Adv. Funct. Mater., 2008, 18, 3056.
7. Y. G. Ma, H. Y. Zhang, J. C. Shen and C. M. Chen, Synth. Met., 1998, 94, 245.
8. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
9. L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido, Adv. Mater., 2011, 23, 926.
10. L. Duan, L. Hou, T.-W. Lee, J. Qiao, D. Zhang, G. Dong, L. Wang and Y. Qiu, J. Mater. Chem., 2010, 20, 6392.
11. S. J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008, 20, 4189.
12. L. Xiao, S. J. Su, Y. Agata, H. Lan and J. Kido, Adv. Mater., 2009, 21, 1271.
13. J. S. Swensen, E. Polikarpov, A. Von Ruden, L. Wang, L. S. Sapochak and A. B. Padmaperuma, Adv. Funct. Mater., 2011, 21, 3250.
14. X. Yang, H. Huang, B. Pan, M. P. Aldred, S. Zhuang, L. Wang, J. Chen and D. Ma, J. Phys. Chem. C, 2012, 116, 15041.
15. Y. H. Son, Y. J. Kim, M. J. Park, H.-Y. Oh, J. S. Park, J. H. Yang, M. C. Suh and J. H. Kwon, J. Mater. Chem. C, 2013, 1, 5008.
16. C. Han, Z. Zhang, H. Xu, S. Yue, J. Li, P. Yan, Z. Deng, Y. Zhao, P. Yan and S. Liu, J. Am. Chem. Soc., 2012, 134, 19179.
17. A. Chaskar, H.-F. Chen and K.-T. Wong, Adv. Mater., 2011, 23, 3876.
18. H. H. Chou and C. H. Cheng, Adv. Mater., 2010, 22, 2468.
19. C. W. Lee and J. Y. Lee, Adv. Mater., 2013, 25, 5450.
20. T. Fei, G. Cheng, D. Hu, P. Lu and Y. Ma, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 4784.
21. D. Hu, G. Cheng, P. Lu, H. Liu, F. Shen, F. Li, Y. Lv, W. Dong and Y. Ma, Macromol. Rapid Comm., 2011, 32, 1467.
22. C. H. Chien, S. F. Liao, C. H. Wu, C. F. Shu, S. Y. Chang, Y. Chi, P. T. Chou and C. H. Lai, Adv. Funct. Mater., 2008, 18, 1430.
23. Y. Mo, R. Tian, W. Shi and Y. Cao, Chem. Commun., 2005, 4925.
24. K. L. Chan, S. E. Watkins, C. S. K. Mak, M. J. Mckiernan, C. R. Towms, S. I. Pascu and A. B. Holmes, Chem. Commun., 2005, 5766.
25. B. Ma, B. J. Kim, L. Deng, D. A. Poulsen, M. E. Thompson and J. M. J. Fréchet, Macromolecules, 2007, 40, 8156.
26. Y. Zhang, C. Zuniga, S.-J. Kim, D. Cai, S. Barlow, S. Salman, V. Coropceanu, J.-L. Brédas, B. Kippelen and S. Marder, Chem. Mater., 2011, 23, 4002.
27. S. Shao, J. Ding, T. Ye, Z. Xie, L. Wang, X. Jing and F. Wang, Adv. Mater., 2011, 23, 3570.
28. Z. Wu, Y. Xiong, J. Zou, L. Wang, J. Liu, Q. Chen, W. Yang, J. Peng and Y. Cao, Adv. Mater., 2008, 20, 2359.
29. J. Pina, J. Seixas de Melo, H. D. Burrows, A. P. Monkman and S. C. Navaratnam, Phys. Lett., 2004, 400, 441.
30. K. M. Vaeth and C. W. Tang, J. Appl. Phys., 2002, 92, 3447.
31. X. Gong, M. R. Robinson, J. C. Ostrowski, D. Moses, G. C. Bazan and A. J. Heeger, Adv. Mater., 2002, 14, 581.
32. X. W. Chen, J. L. Liao, Y. M. Liang, M. O. Ahmed, H. E. Tseng and S. A. Chen, J. Am. Chem. Soc., 2003, 125, 636.
33. X. Gong, W. L. Ma, J. C. Ostrowski, G. C. Bazan, D. Moses and A. J. Heeger, Adv. Mater., 2004, 16, 615.
34. S. P. Huang, T. H. Jen, Y. C. Chen, A. E. Hsiao, S. H. Yin, H. Y. Chen and S. A. Chen, J. Am. Chem. Soc., 2008, 130, 4699.
35. M. Sudhakar, P. I. Djurovich, T. E. Hogen-Esch and M. E. Thompson, J. Am. Chem. Soc., 2003, 125, 7796.
36. W. Wei, P. I. Djurovich and M. E. Thompson, Chem. Mater., 2010, 22, 1724.
37. Y. Tao, Q. Wang, C. Yang, K. Zhang, Q. Wang, T. Zou, J. Qin and D. Ma, J. Mater. Chem., 2008, 18, 4091.
38. L. Zeng, T. Y.-H. Lee, P. B. Merkel and S. H. Chen, J. Mater. Chem., 2009, 19, 8772.
39. Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T. Kajita and M.-a. Kakimoto, Chem. Mater., 2008, 20, 2532.
40. Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T. Kajita and M.-a. Kakimoto, Adv. Funct. Mater., 2008, 18, 548.
41. D. Hu, P. Lu, C. Wang, H. Liu, H. Wang, Z. Wang, T. Fei, X. Gu and Y. Ma, J. Mater. Chem., 2009, 19, 6143.
42. D. Hu, F. Shen, H. Liu, P. Lu, Y. Lv, D. Liu and Y. Ma, Chem. Commun., 2012, 48, 3015.
43. H. Liu, G. Cheng, D. Hu, F. Shen, Y. Lv, G. Sun, B. Yang, P. Lu and Y. Ma, Adv. Funct. Mater., 2012, 22, 2830.
44. X. Ren, J. Li, R. J. Holmes, P. I. Djurovich, S. R. Forrest and M. E. Thompson, Chem. Mater., 2004, 16, 4743.
45. Y. Y. Lyu, J. Kwak, O. Kwon, S. H. Lee, D. Kim, C. Lee and K. Char, Adv. Mater., 2008, 20, 2720.
46. J. K. Bin, N. S. Cho and J. I. Hong, Adv. Mater., 2012, 24, 2911.
47. S. Gong, Q. Fu, Q. Wang, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Mater., 2011, 23, 4956.
48. X.-H. Zhou, Y.-H. Niu, F. Huang, M. S. Liu and A. K.-Y. Jen, Macromolecules, 2007, 40, 3015.
49. M. Zhang, S. Xue, W. Dong, Q. Wang, T. Fei, C. Gu and Y. Ma, Chem. Commun., 2010, 46, 3923.
50. S. J. Yeh, M. F. Wu, C. T. Chen, Y. H. Song, Y. Chi, M. H. Ho, S. F. Hsu and C. H. Chen, Adv. Mater., 2005, 17, 285.
51. P.-I. Shih, C.-H. Chien, C.-Y. Chuang, C.-F. Shu, C.-H. Yang, J.-H. Chen and Y. Chi, J. Mater. Chem., 2007, 17, 1692.

---

Footnote

† Electronic supplementary information (ESI) available: ¹H NMR and ¹³C NMR of the polymers and oligomer; the PL spectra of polymers and oligomer in films. See DOI: 10.1039/c4ra16797j

---