Bipolar and Unipolar Silylene-Diphenylene σ-π Conjugated Polymer Route for Highly Efficient Electrophosphorescence

σ-π conjugated polymer strategy is proposed for designing electroluminescent host polymers with silylene-diphenylene as the backbone repeat unit giving a high triplet energy (ET = 2.67 eV). By incorporation of high ET (3.0 eV) electron (oxadiazole, OXD) and hole (triphenyl amine, TPA) transport moieties, or TPA alone (in this case, the main chain acts as electron transport channel) as side arms on the silylene, the high ET bipolar and unipolar polymers are formed, allowing a use of iridium green phosphor (Ir(ppy)2(acac), Ir-G) (ET = 2.40 eV) as the dopant. The matching of energy levels of the dopant with the hosts, leading to charge trapping into it; and singlets and triplets of the exciplex and excimer can be harvested via energy transfer to the dopant. Using these host-guest systems as the emitting layer, chlorinated indium-tin-oxide (Cl-ITO) as the anode, and benzimidazole derivative (TPBI) as the electron transport layer, this two-layer device gives the high luminance efficiency 80.1 cd/A and external quantum efficiency 21.2%, which is the best among the report values for polymer light emitting diode (PLED) in the literatures. This example manifests that σ-π conjugated polymer strategy is a promising route for designing polymer host for efficient electrophosphorescence.

its lower current density (also below 0.1 A/cm 2 ) 9 . Very recently, alternative-current-driven devices with PVK as host were developed, exhibiting the high luminance efficiency 110 cd/A from a four-layer device; but, due to the higher operating voltage (± 75 V), its corresponding maximum power efficiency was only 29.3 lm/W 10 .
Unlike the conventional π -π conjugated polymers, The σ -π conjugated polymer is a promising structure to give high triplet energy. In the past, the reports for σ -π conjugation phenomena usually focus on their energy change in photoluminescence and absorption spectra 11,12 , and the change in electrochemical oxidation potential 13 . Moreover, the studies of σ -π conjugated polymers in optoelectronics are very rare. The reported σ -π conjugated materials consist of oligosilylene (σ -part) and π -conjugated (π -part) moieties, and exhibit both properties of them 14 . The oligosilylene part possesses great hole mobility (~10 −4 cm V −2 s −1 ) 15,16 and rather high band gap (> 3.5 eV) 17 , while the π -conjugated part can facilitate electron transport ability. However, the Si-Si bonding of silylene is unstable under UV irradiation 17 , and longer π -conjugated length also gives lower E T . Thus, for an ideal σ -π conjugated material applied to optoelectronics, it should be avoided to introduce Si-Si bonding and π -conjugated length should be limited to two phenyl rings or less (biphenyl gives reasonably high E T 2.85 eV) 18 while p-terphenyl gives an E T of 2.55 eV, which is too low for blue phosphor emitter guest.
Here, we design the σ -π conjugated polymers, by incorporation of electron (oxadiazole, OXD) and hole (triphenyl amine, TPA) transport moieties for bipolar polymer and TPA only for unipolar polymer as side arms on silylene in the silylene diphenylene backbone. Upon doping with green emission Ir-complex, (Ir(ppy) 2 (acac), Ir-G), along with the cholorinated indium-tin-oxide (Cl-ITO) 19,20 glass plate as the anode, 2,2′ ,2″ -(1,3,5-benzi netriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) as the electron transport layer, the two-layer-only device gives the extremely high performance with maximum luminance efficiency 80.1 cd/A (EQE 21.2%), corresponding power efficiency 62.9 lm/W) and maximum brightness 25452 cd/m 2 , which is the best among the report values for PLED in the literatures. For both unipolar and bipolar polymers, two separated channels are permitted for electron and hole transport toward the dopant by charge trapping. For the former, electrons and holes are transport via OXD and TPA moieties, respectively; while for the later, electrons are via main chain and holes via TPA. Furthermore, the formation of TPA-OXD exciplex in the bipolar polymer and that of TPA excimer in the unipolar polymer can also provide an additional route for transferring exciton energy to Ir-G. For comparison of the present results with those reported in the literatures described above, all these results are summarized in Table 1.

Results and Discussion
σ-π conjugated polymers: synthesis and optical/semiconductor characterization. For demonstrating the present σ -π conjugated molecular design strategy for use as host in electroluminescence, we synthesize poly(dibutylsilylene-diphenylene) [Si(dBu)] as the main chain analogue (see Fig. 1), in which its backbone is the same as the proposed polymers but the transport moieties attached to Si are replaced by butyl groups. Compared to the absorption peak of biphenyl molecule at 242 nm, that of Si(dBu) shows red-shift to 271 nm by 29 nm (Supplementary Fig. 1a), which is resulted from a hyper-conjugative manner between benzene π orbitals and silicon σ orbitals, which forms a low-lying σ π (σ π *) orbitals to stabilize the structure 21,22 . Furthermore, in single carrier measurement ( Supplementary Fig. 1b), its hole mobility of 2.76 × 10 −7 cm 2 V −1 s −1 and electron mobility of 8.47 × 10 −5 cm 2 V −1 s −1 are determined by applying the space-charge limited current (SCLC) method 23,24 . The charge mobilities at the level of 10 −5~1 0 −7 cm 2 V −1 s −1 indicate that the σ -π conjugated silylene-diphenylene backbone possesses semiconducting characteristic and can provide as one of the carriers transport pathways.
Herein, we synthesized four transport moieties modified silylene-diphenylene polymers containing two unipolar materials, Si(doTPA) and Si(dtOXD), and two bipolar materials Si(tOXD)(oTPA) and Si(tOXD) (tTPA), where TPA is triphenyl amine, OXD is oxadiazole, and "d" denotes the di-substitution, and "t" and "o" denote tert-butyl and alkoxy, respectively. The chemical structures of these host polymers, guest Ir(ppy) 2 (acac) and electron transport layer material TPBI are given in  electron-rich TPA and electron deficient OXD moieties, which gives lower energy emission than its corresponding excimer emissions ( Supplementary Fig. 3c,d). While the four host materials are doped with 8 wt% Ir-G, only green emissions with a peak at 520 nm (Supplementary Fig. 6) are observed, indicating an occurrence of highly efficient energy transfer from the host to the guest.
Triplet state measurements. From delayed phosphorescence measurements, all the triplet emission spectra and their characteristic values of the silylene-diphenylene polymer in the dilute solution (5 × 10 −5 M) at 77 K are shown in Fig. 2a and Table 2, respectively. The detailed measurement procedures are shown in Supplementary Notes. Si(dBu) exhibits phosphorescence peaks at 465(shoulder (sh)), 493 and 533(sh) nm possessing an E T 2.67 eV (calculated from the peak 465 nm). Compared to that of molecular biphenyl 2.85 eV, the lower E T is resulted from the σ -π hyper-conjugation 21,22 . For the unipolar polymers, the phosphorescence spectrum of Si(doTPA) with the peaks at 465(sh), 493 and 533(sh) nm and that of Si(dtOXD) with the peaks at 468, 503 and 533(sh) nm are quite similar to that of Si(dBu). Also, for the unipolar polymers, by enlarging the spectra in the range 390-450 nm, a peak of Si(doTPA) at 405 nm and two weak peaks of Si(dtOXD) at 416 and 446 nm are observed, which are nearly identical to their corresponding isolated TPA at 405 nm and OXD at 414 and 446 nm 25,26 , indicating that the occurrence of triplet energy transfer from the side arm TPA or OXD moieties to the backbone silylene-diphenylene is efficient but still incomplete. As the side arm moieties can form excimer or exciplex, their triplet emissions should also appear in the broad peaks from 450 to 700 nm in addition to that from the main chains. We further performed the spectral deconvolutions by fitting each spectrum with the spectral distributions of Si(dBu) and molecular complex in Fig. 2a, and found that the phosphorescence of the unipolar polymers Si(doTPA) and Si(dtOXD) possess 2.17% contribution from TPA excimer and 33.14% contribution from OXD excimer emissions, respectively. Up to this point, we found that the present σ -π polymers containing multiple triplet states, contributing from: isolated transport moieties, molecular complexes, and main chain. Thus the E T s of Si(doTPA) are assigned as 3.07 eV (referred to the peak 405 nm) for side arm isolated TPA, 2.67 eV (referred to the shoulder 465 nm) for silylene-diphenylene backbone and 2.52 eV (referred to the shoulder 494 nm) for TPA excimer ( Supplementary Fig. 7a). Also, E T s of Si(dtOXD) are 2.98 eV (referred to the peak 416 nm) for side arm isolated OXD, 2.66 eV (referred to the peak 468 nm) for silylene-diphenylene backbone and 2.28 eV (referred to the peak 546 nm) for OXD excimer (the presence of OXD excimer is revealed in Supplementary Fig. 7b). For the bipolar polymers, both Si(tOXD)(oTPA) and Si(tOXD)(tTPA) show the same phosphorescence peaks at 467(sh), 503 and 536 nm. By enlarging their spectra in the range 390-450 nm, only Si(tOXD)(tTPA) shows peaks at 405 and 424 nm contributed by both isolated TPA and OXD moieties, but Si(tOXD)(oTPA) shows no observable peak. This difference is probably due to the more steric hindrance of tTPA than oTPA in forming exciplex with tOXD, leading to less isolated tOXD and TPA in Si(tOXD)(oTPA) than in Si(tOXD)(tTPA); such that after triplet energy transfer from side arms to backbone, Si(tOXD)(tTPA) has more amount of isolated OXD and TPA than Si(tOXD)(oTPA). On the other hand, by subtracting the normalized Si(dBu) spectrum from the normalized Si(tOXD)(oTPA) and Si(tOXD)(tTPA) spectra individually at the first peak of Si(dBu) 465 nm, both the residual spectra are very similar ( Supplementary Fig. 8) with the peaks at 513 and 547 nm resulting from TPA-OXD exciplex triplet emissions, which gives an E T of 2.42 eV referred to the peak 513 nm. By such deconvolution, the fractions of triplet exciplex emission are 59.27% and 44.44% for Si(tOXD)(oTPA) and Si(tOXD)(tTPA), respectively. This result also in agreement with no residual triplet emission from tOXD and oTPA in Si(tOXD)(oTPA) discussed above.
As the present σ -π polymers show multiple triplet states, the energy transfer routes among them are shown in Fig. 2b and depicted below. The side arm isolated moieties TPA (3.07 eV) and OXD (2.98 eV) can efficiently transfer their E T to silylene-diphenylene backbone (referred to 2.67 eV of Si(dBu)), and the TPA-OXD exciplex forms upon photoexcitation in which one component of the pair is originally in the photo-excited state and the other is in the ground state. Thus, it is forbidden for directly energy transfer from the side arm moieties to TPA-OXD  exciplex (2.42 eV) since the exciplex exists only under excitation but is dissociative in the ground state 27,28 . As the system doped with the green phosphor (2.40 eV), the silylene-diphenylene backbone and TPA-OXD exciplex can transfer their triplet energies to the phosphor. Though the triplet states of TPA-OXD and the green phosphor are close, no back energy transfer from the latter to the former could occur. The reason is that the TPA-OXD exciplex is an excited state species rather than ground state species and can not receive the triplet energy from excited Ir-G, which results in highly efficient exciton harvesting for light emission.
Phosphorescence PLED device performance. To obtain highly efficient PLED, it is required to fully utilize singlet and triplet excitons generated in the device as in OLED. This can be done by use of host/guest system with phosphor as dopant and high triplet energy materials as host 29,30 . Usually, the host material is so designed such that it can generate high energy singlet and triplet excitons for transferring energy to the guest 2 . In this work, for the bipolar polymer with TPA and OXD, it permits two separated channels for electron and hole that leads to a formation of singlet and triplet excitons in the dopant by charge trapping; for the unipolar polymer Si(doTPA), such charge trapping also occurs but electrons transport via main chain and holes via TPA moiety as shown in Fig. 3a. In addition, the formation of excimer and exciplex allows an additional route for transferring exciton energy to phosphor dopant except for Si(dtOXD) due to its lower excimer E T (2.28 eV) than Ir-G. To simplify the device structure by limiting to the two layers (the host/guest active layer and ETL), we introduce chlorinated indium tin oxide (Cl-ITO) as the anode 19,20 (Fig. 3b), in which the out-layer of ITO is chlorinated for adjusting its work function yet retaining the original high conductivity of the ITO. Here, by appropriate extent of chlorination, Cl-ITO with the work function 5.58 eV was obtained as determined by UPS ( Supplementary Fig. 4), which is slightly higher than the HOMO level of TPA 5.   Table 3. The thickness of the emitting layer is optimal at this 90 nm (Supplementary Fig. 9). For the bipolar host systems, it takes both advantages of efficient charge injections due to no hole injection barrier (from Cl-ITO 5.6 eV to TPA 5.3 eV) and low electron injection barrier of 0.3 eV (from TPBI 2.7 eV to OXD 2.4 eV), and unipolar charge transport along each transport moiety. The optimal performances of the devices with Si(tOXD)(oTPA) and Si(tOXD)(tTPA) are better than those device with unipolar polymers owing to better balanced current densities. For Si(tOXD)(oTPA) device, the maximum brightness and luminescence efficiency are 25452 cd/m 2 and 80.1 cd/A (corresponding power efficiency 62.9 lm/W and external quantum efficiency 21.2%), respectively. And for Si(tOXD)(tTPA) device, the corresponding values are 18750 cd/m 2 and 73.5 cd/A (42.7 lm/W and 19.5%). Based on the energy level diagram in Fig. 3b, the charge injections and transport behaviors are revealed below. Since the work function of Cl-ITO is 5.58 eV, there is no barrier for hole injection to the host polymers with the hole transport moiety TPA. For the unipolar host system Si(doTPA) (dual oTPA moieties), though it has no electron transport moiety, its current density profile is higher than those of the two bipolar devices, Si(tOXD)(oTPA) and Si(tOXD)(tTPA). In this device, the electron injection barrier (0.8 eV) from TPBI (2.7 eV) to TPA (1.9 eV) is rather high, thus electrons are mainly injected to the main chains as its LUMO (2.33 eV) is much closer to that of TPBI giving a lower injection barrier of 0.37 eV. The luminescence efficiency increases from 57.4 cd/A (at 1 cd/m 2 ) to 66.5 cd/A (at 300 cd/m 2 ) and remains steady up to 1000 cd/m 2 (with 63.3 cd/A) (Fig. 3d). In the practical luminescence operating range: 100-1000 cd/m 2 , the unipolar host system with TPA is significantly better than the bipolar host systems. However, the poor device performance is observed when using Si(dtOXD) as the host, its maximum brightness and efficiency are lower, only 4087 cd/m 2 and 10.3 cd/A, respectively, due to the large hole injection barrier from Cl-ITO to OXD moiety (0.5 eV) and even higher barrier to the main chain (0.8 eV).
As indicated in the PL spectra of the bipolar polymers ( Supplementary Fig. 3), Si(tOXD)(tTPA) and Si(tOXD) (oTPA), the incorporation of tert-butyl group to replace hexyloxy group in TPA can reduce the exciplex formation with OXD due to steric hindrance provided by the bulky tert-butyl group as reflected in the higher fraction of photoexcited exciplex emission 59.2% of the latter than 44.4% of the former (Fig. 2), which is in agreement with the higher EQE of the latter 21.2% than the former 19.5% (Table 3). These results indicate that the triplet energy transfer from the exciplex to the phosphor also play a significant role in addition to charge trapping. It should be mentioned here, although the E T of the exciplex 2.42 eV is very close to that of Ir-G 2.40 eV, the back energy transfer from Ir-G to exciplex would not occur.
Emission mechanism of phosphorescence PLED device. To examine the occurrence of charge trapping, we performed time-resolved electroluminescence (TREL) measurements within 0-400 ns at a narrow voltage pulse (1 MHz, pulse width 300 ns at 10 V) with different Ir-G contents (0.1, 3 and 8 wt%) in Si(tOXD)(oTPA) (Fig. 4); the experimental setup and condition are provided in Supplementary Fig. 10. The extra emission at 490 nm from the host in the case of 0.1 wt% indicates a presence of exciplex singlet emission close to those by photoexcitation 476 nm (Supplementary Table 2). The exciplex emission not only can be observed in the 0.1 wt% dopant system, but also in higher dopant contents (weak at 3 wt% and further weaker or nearly disappeared at 8 wt%). This result supports that the emitting mechanism is mainly by charge trapping of holes and electrons from the host to the guest phosphor dopant, Ir-G, which then form triplet exciton and emit green phosphorescence.

Conclusion
We have demonstrated that σ -π conjugated polymers with silylene-diphenylene as the backbone repeat unit incorporated with the high triplet energy (3.0 eV) hole transport moiety (triphenyl amine, TPA) and/or electron transport moiety (oxadiazole, OXD) as side arms on silylene exhibit multiple triplet states, including those of main chain as well as transport moieties and molecular complexes between them. The unipolar and bipolar polymer hosts when doped with 8 wt% Ir-G can be used as emitting layer, along with Cl-ITO as anode and TPBI as ETL, to compose of a two-layer-only LED. The device with the polymer host Si(tOXD)(oTPA) exhibits high performance with the maximum luminance efficiency 80.1 cd/A (EQE 21.2%), power efficiency 62.9 lm/W and brightness 25452 cd/m 2 , which is the highest record among the reported phosphorescence green emission PLED. For the unipolar polymer host Si(doTPA), the main chain acts as electron transport channel with low injection barrier and TPA as efficient hole transport channel, which gives high performance with very steady efficiency up to 1000 cd/m 2 . In addition, the present two-layer-only device provides significant lower fabrication cost than the Host V on a (V) L max (cd/m 2 ) η L at L = 100 cd/ m 2 (cd/A) η L at L = 1000 cd/ m 2 (cd/A) Max. η L , η P , and EQE b (cd/A, lm/W, %)  3~5 layers OLED. Note also that the σ -π conjugated backbone with the silylene linkage for direct incorporation of charge transport moieties without spacer into the polymer as side arms provides a more effective and flexible route for tuning charge injection/transport toward charge balance, yet maintaining higher triplet energy relative to the conventional conjugated polymers. The σ -π conjugated polymer strategy for high triplet energy polymer host is deserved for further investigation for high electro-phosphorescence efficiency in PLED.

Methods
Determination of energy levels. The HOMO/LUMO energy levels are determined from CV, absorption spectroscopy and UPS measurements as shown in the Supplementary Information. For Ir(ppy)2(acac) and TPBI, the energy levels are taken from the literatures 31,32 .
Device fabrication. For device preparation, an indium tin oxide (ITO) glass substrate was exposed to oxygen plasma at a power of 50 W and a pressure of 193 mTorr for 5 min, followed by 10 minutes UV treatment in a Pyrex Petri dish with 0.2 ml of o-dichlorobenzene to form chlorinate ITO (Cl-ITO), and finally treated with extra 3 minutes UV ozone. All the silylene-diphenylene polymers with 8 wt% Ir(ppy) 2 (acac) (Ir-G) as the green emission dopant were dissolved in chlorobenzene (CB) to make the polymer solutions, and then used them to prepare the solid film by spin-coating on Cl-ITO. TPBI layer used as electron transporting and hole/exciton blocking layer was deposited on top of the polymer layer by thermal evaporation in a vacuum of 2 × 10 −6 Torr. (where TPBI is 2,2′ ,2″ -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)). Finally, a thin layer of CsF or LiF (about 1 nm) covered with aluminum (100 nm) for bipolar device was deposited in a vacuum thermal evaporator through a shadow mask at a vacuum of 2 × 10 −6 Torr. The fabrication procedure for single carrier device was similar to that for the bipolar device.