Solution-processed organic light-emitting diodes with enhanced efficiency by using a non-conjugated polymer doped small-molecule hole-blocking layer

Abstract

The small-molecule hole-blocking material SPPO13 has been doped with a non-conjugated polymer to act as the hole-blocking layer in solution-processed OLEDs. Such a doping strategy can significantly improve the electron injection in devices, resulting in an enhanced luminous efficiency and a reduced turn-on voltage.

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Organic light-emitting diodes (OLEDs) have attracted much attention in recent years due to their potential applications in next-generation flat-panel displays and solid-state lighting. OLEDs can be fabricated either through a vacuum evaporation method or through a solution-processed one. For the former technique route, a multilayer structure and co-evaporation of organic dyes are usually adopted which make the device manufacturing process complicated. Furthermore, the vacuum evaporation process is time and energy consuming. As a result, OLEDs fabricated from the latter technique route, that is solution processing, have gradually attracted increasing research enthusiasm both in scientific and industrial communities due to their many unique advantages such as simple device structures, low-cost and facile manufacturing process, compatibility with flexible substrates, and easy processability over large-areas by spin-coating, ink-jet printing or roll-to-roll coating.^1–3 However, the progress of solution-processed OLEDs is still severely hindered by the relatively low device efficiency arising from the limitation of effective carrier injection and transport.^4,5

In order to achieve high-efficiency OLEDs, efficient and balanced injection of electrons and holes from electrodes and their transport in the semiconductor layer are necessary. Since hole mobility is often larger than electron mobility for most conjugated polymers, hole-blocking layer (HBL) is introduced into OLEDs to move the recombination zone away from the cathode and avoid quenching of excitons by the cathode.⁶ The HBLs, composed of small-molecule materials, are usually fabricated by vacuum thermal deposition, leading to increased fabrication complexity and energy consumption. Therefore, many efforts have been made to fabricate HBLs with solution-processing method. OLEDs with solution-processed conjugated small molecules,^7–10 doped or intrinsic non-conjugated polymers^6,11,12 and some n-type metal oxides^13,14 as HBLs were developed. Nevertheless, when the HBL was deposited, the injection barrier between the cathode and the HBL may hinder electron injection. As a consequence, low work function metals, electrode modification layer made of alkali salts, or electron injection layer (EIL) are frequently used in order to overcome this drawback.^15,16 Unfortunately, the low work function metals, such as Ba and Ca, are very sensitive to oxygen and moisture. In addition, the deposition of alkali salts layer and EIL is usually achieved by vacuum thermal evaporation method which is time and energy consuming. Deposition of electrode modification layer or EIL onto the HBL by solution-processing seems difficult because the solvent used in the next layer may damage the previous layer. One way to overcome this limitation is to use doped structure in HBL to enhance device performance. We noticed that Cs doped phenanthroline derivates,¹⁷ Li doped benzimidazole derivates,¹⁸ and Cs doped phenyldipyrenylphosphine oxide¹⁹ were introduced into OLEDs as HBL in order to improve the device performance. Unfortunately, these doped HBLs are still deposited through a vacuum thermal evaporation method. As a result, a solution-processed HBL which can both confine electrons in the emissive layer and facilitate the injection of electrons from the cathode is highly desired.

In this work, we show that the electron injection ability of solution-processed HBL composed of 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13) can be dramatically improved by doping with a non-conjugated polymer polyethylene glycol (PEG). By using this kind of HBL in solution-processed OLEDs, luminous efficiency was enhanced from 18.40 to 23.86 cd A⁻¹ and turn-on voltage was reduced from 7.6 to 5.6 V.

We fabricated a series of OLEDs with non-conjugated polymer PEG 6000 (mean mol mass: 5000–7000) doped SPPO13 as the HBL to improve the device efficiency. To make a comparison, OLED with the undoped HBL was also fabricated as a control device. The device structure used in this study is ITO/PEDOT:PSS (40 nm)/PVK:OXD-7:FIrpic (80 nm, with a weight ratio of 100 : 40 : 10)/HBL (70 nm)/Al (100 nm) (shown in Fig. S1†), where poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is used as the hole injecting layer (HIL), poly(N-vinylcarbazole) (PVK) serves as the polymer host, and 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) is doped into the PVK host to improve electron transport in EML. The emitter, sky-blue phosphorescent complex iridium(III) bis(4,6-difluorophenylpyridinato-N, C2′) picolinate (FIrpic), was mixed into the PVK host to achieve blue-light emission. SPPO13, which is either doped with PEG or existed as the neat film, is used as the HBL.

Firstly, we investigated PEG as a dopant in SPPO13 with different concentrations to act as the HBL in OLEDs. The weight concentrations of PEG in the mixed thin films were 10%, 20%, 30% and 40%, respectively. After deposition by spin-coating, HBLs with different PEG concentrations were annealed at 60 °C for 10 min to remove the solvent. The performances of such OLEDs are summarized in Table 1.

Table 1 Performances of OLEDs with different PEG concentrations in HBL

PEG concentration	Von^a (V)	ηmax^b (cd A^−1)	η1000^c (cd A^−1)	Lmax^d (cd m^−2)
a Turn-on voltage which is defined as the voltage at a brightness of 1 cd m^−2.b Maximum luminous efficiency.c Luminous efficiency at a brightness of 1000 cd m^−2.d Maximum luminance.
0%	7.6	18.40	15.24	4938
10%	6.8	20.75	20.37	9093
20%	6.4	21.23	21.20	11 036
30%	5.6	23.86	23.08	13 138
40%	6.4	15.23	13.38	3909

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The electroluminescent (EL) spectra, current density–luminance–voltage (J–L–V) and luminous efficiency–current density (η–J) characteristics of these OLEDs are shown in Fig. 1. Because the emissive layers are same, the EL spectra of devices with doped and undoped SPPO13 as HBLs all exhibited sky-blue light emissions. As shown in Fig. 1a, the emission peaks at 475 and 500 nm originated from the emission of FIrpic. From Fig. 1b, it can be found that devices with doped HBL showed higher current densities than the undoped one under the same driving voltage, except the one with 40% PEG. Among them, the doped HBL with a PEG concentration of 30% showed the lowest turn-on voltage of 5.6 V. Such a turn-on voltage is much lower than that of undoped device (7.6 V). What's more, the devices with the doped HBL showed a significantly higher brightness than the undoped one. Meanwhile, devices with the doped HBL (PEG: ≤30%) showed obviously increased luminous efficiency than the undoped one. As shown in Fig. 1c, the device with a doped HBL (30% PEG) exhibited a maximum luminous efficiency of 23.86 cd A⁻¹ which was 29.7% larger than that of the undoped one. With the weight concentration of PEG varying from 10% to 30%, the luminous efficiency was gradually improved. However, further increasing the PEG concentration to 40%, the luminous efficiency began to decline. In order to explain this phenomenon, atomic force microscopy (AFM) was used to investigate the microstructure of SPPO13 thin films with PEG concentrations of 30%, 40% and without doping (Fig. 2). The AFM images showed that both the undoped film and the 30% PEG doped film exhibited smooth surfaces with root-mean-square roughness (R_q) of 0.439 and 0.322 nm, respectively. However, when its concentration increased to 40%, PEG separated out from the SPPO13 film, which caused a poor film quality with R_q of 16.1 nm. As a more smooth and homogeneous morphology of the film in the OLED device could decrease the appearance of film defects, it could facilitate the transporting and recombination of charge carriers, while suppressing the current leakage.²⁰ The rough surface may be the reason why device with the HBL doped by 40% PEG showed poor performances.

Fig. 1 (a) Normalized EL spectra of OLEDs. (b) J–L–V characteristics and (c) η–J characteristics of devices with different PEG concentrations in HBL. The filled symbols stand for current density and empty symbols for brightness in (b).

Fig. 2 AFM images (5 μm × 5 μm scale) of the surface morphology of the HBL without doping (a) and with PEG-doping by either a 30% (b) or a 40% (c) concentration.

Unexpectedly, we observed that performances of devices with various HBL annealing temperatures existed huge differences. To explore the influence of temperature on HBL, OLEDs with different HBL annealing temperatures were fabricated which were 100, 80, 60 and 45 °C, respectively. Device without thermal annealing was also fabricated to serve as the control device. The annealing time and PEG doping concentration for every device were all fixed at 10 min and 30%. The performances of such OLEDs are summarized in Table 2. The corresponding J–L–V and η–J curves are shown in Fig. 3.

Table 2 Performances of OLEDs with different annealing temperatures applied on the HBL

Annealing temperature (°C)	Von (V)	ηmax (cd A^−1)	η1000 (cd A^−1)	Lmax (cd m^−2)
100	6.4	17.01	9.92	15 536
80	7.2	17.56	17.37	13 166
60	5.6	23.86	23.08	13 138
45	5.2	13.54	10.21	5527
None	6.0	14.34	11.02	2648

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Fig. 3 (a) J–L–V characteristics and (b) η–J characteristics of devices based on the 30% PEG-doped HBL without annealing (none) and with different annealing temperatures (45–100 °C). The filled symbols stand for current density and empty symbols for brightness in (a).

Obviously, the device with an annealing temperature of 60 °C showed the best device performance (23.86 cd A⁻¹). It can be found from Fig. 3a that, compared with the none-annealing device, devices with annealed HBLs showed higher brightness and current density under a same driving voltage. Among them, devices with annealing temperatures of 80 and 100 °C exhibited higher current density than other ones under the same voltage. Meanwhile, with the annealing temperature varying from 60 to 100 °C, the brightness of devices under a same voltage was slightly increased. However, unfortunately, the turn-on voltage has also risen. For example, the turn-on voltage of 80 °C annealed device was as high as 7.2 V. Furthermore, the devices treated with a relatively high annealing temperature (≥60 °C) showed higher luminous efficiency than the none-annealing one, as shown in Fig. 3b. So with different annealing temperature for HBLs, devices can exhibit notable difference in performance. When the annealing temperature increased from 45 to 60 °C, the device performance was dramatically improved. Nevertheless, when the annealing temperature was further increased, the device efficiency decreased. We think that below 60 °C, the improvement of device performance originated from the removal of the residual solvent in solution-deposited films. However, the relatively low efficiency of devices with higher annealing temperature can be attributed to the melt and aggregation of PEG at 80 or 100 °C. Because the commercially available HBL material SPPO13 has a relatively high glass transition temperature (125 °C) and it is thermally durable enough,²¹ the thermal degradation of the HBL mainly depends on the thermal property of PEG. In Fig. S2,† the thermal property of PEG 6000 was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). It exhibited a high thermal decomposition temperature (T_d, corresponding to 5% weight loss) of 365 °C. However, the DSC curve showed PEG 6000 had a melting point of only ∼61 °C. The root-mean-square roughness (R_q) value of the HBL film without annealing, and with annealing temperature of 45 and 60 °C was 0.35, 0.39 and 0.32 nm, respectively. We can see that they possessed relatively smooth and homogeneous surface morphology. While the high temperature annealed HBLs (80 and 100 °C) suffer severely from the precipitated phase in the films, as shown in Fig. S3.† Due to the melting point of PEG 6000 being as low as about 61 °C, it is unavoidable to separate out from the HBL film under such a high temperature, resulting in rough surface morphology and poor film-forming property. The precipitated phase of PEG from HBL film will damage the film morphology and may lower the device performance severely.

Obviously, when PEG with an appropriate concentration was doped into HBL, the device efficiency was dramatically improved and the turn-on voltage was decreased (see Table 1). It is easy to understand that the decline of turn-on voltage should benefit from the improvement of the carriers injection ability. Therefore, we can assume that doping PEG will facilitate the electron injection, then increase the device efficiency. To verify whether PEG-doped HBL can facilitate the electron injection or not, photovoltaic (PV) measurements were performed to obtain open-circuit voltage (V_oc) across devices with undoped HBL and the 10%, 20%, 30%, 40% PEG doped ones. The value of the open-circuit voltage is related directly to built-in potential between anode and cathode, thus V_oc is primarily influenced by the effective work function of the cathode and can reflect the electron injection ability in OLEDs.²² As shown in Fig. 4, the V_oc of the devices with undoped HBL and the 10%, 20%, 30%, 40% PEG doped ones is 0.35, 0.50, 0.67, 0.70, and 0.60 V, respectively. The enhancement of V_oc (from 0% to 30% doped one) indicates that the electron injection ability of SPPO13 HBL was improved upon PEG doping. When the doping concentration was increased to 40%, the V_oc was lower than that of the 30% doped device. This was mainly attributed to the poor film quality mentioned above. The larger V_oc values in PEG doped devices than that in the undoped one indicated that the corresponding work function of the cathode was lowered, so the energy barrier for electron injection was reduced, thus improving the electron injection ability. The change of V_oc was accorded with the changing trend of the turn-on voltage (V_on) of devices with different doping concentration, indicating that the improved electron injection ability could reduce the turn-on voltage of devices. As a result, we can make the conclusion that doping non-conjugated polymer PEG into SPPO13 HBL gives rise to the increase in electron injection ability and thus improve device performance.

Fig. 4 Photovoltaic characteristics of devices with undoped HBL and 10%, 20%, 30%, 40% PEG doped ones.

In addition, we have fabricated a series of devices using PEG with different molecular weights (PEG 2000, PEG 6000, PEG 10000, and PEG 20000) to investigate if the molecular weight of PEG will affect the performance of devices. As depicted in Fig. S4,† the maximum luminous efficiency of devices based on PEG 2000 (21.37 cd A⁻¹) were a little lower than that of the devices based on PEG 6000 (25.11 cd A⁻¹), PEG 10000 (25.07 cd A⁻¹), and PEG 20000 (25.38 cd A⁻¹). When the molecular weight is higher (from ∼6000 to ∼20 000), the maximum luminous efficiencies of the devices are nearly same. In Fig. S2,† the PEG with different molecular weight all exhibit relatively high T_d (>350 °C), while the melting point of PEG 2000 (53 °C) is lower than that of PEG 6000 (61 °C), PEG 10000 (62 °C), and PEG 20000 (63 °C). As the annealing temperature of these HBL is 60 °C, it is obvious that the performance of the PEG 2000-based device will be affected and it shows relatively lower luminous efficiency than others. When the molecular weight becomes higher, the melting temperatures of PEG show very little difference and are all above the annealing temperature 60 °C. Thus the removal of the residual solvent in HBLs leads to the similarity of luminous efficiencies in the device with higher molecular weight. The different performance of the devices after the PEG doping can be resulted from the changing of electron mobility and electron injection. Some previous researches have proved the little impact of PEG on the transport of carriers,²³ so we have mainly studied the effect of electron injection on the devices. As shown in Fig. S5,† the V_oc of the devices based on the 30% PEG-doped HBL with PEG 2000, PEG 6000, PEG 10000, and PEG 20000 is 0.55, 0.70, 0.65, and 0.65 V, respectively. This also indicates the electron injection ability of PEG 6000, PEG 10000, and PEG 20000-based device are better than that of PEG 2000-based device.

In conclusion, solution-processed small-molecule hole blocking material SPPO13 has been doped with non-conjugated polymer PEG to act as a HBL in OLEDs. Such a doping strategy can significantly improve the electron injection ability from the cathode to HBL, resulting in an enhanced luminous efficiency and a reduced turn-on voltage in OLEDs compared with those using the undoped HBL. The influences of PEG doping concentration and annealing temperature on the device performance have also been investigated. It was found that the devices with a moderate PEG concentration of 30% and a moderate HBL annealing temperature of 60 °C showed the best device performance.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51273020, 51373022), and Research Fund for the Doctoral Program of Higher Education of China (20130006110007).

References

1. K. T. Kamtekar, A. P. Monkman and M. R. Bryce, Adv. Mater., 2010, 22, 572–582.
2. H. Wu, L. Ying, W. Yang and Y. Cao, Chem. Soc. Rev., 2009, 38, 3391–3400.
3. L. Ying, C. Ho, H. Wu, Y. Cao and W. Wong, Adv. Mater., 2014, 26, 2459–2473.
4. J. H. Zou, H. Wu, C. S. Lam, C. D. Wang, J. Zhu, C. M. Zhong, S. J. Hu, C. L. Ho, G. J. Zhou, H. B. Wu, W. Choy, J. B. Peng, Y. Cao and W. Y. Wong, Adv. Mater., 2011, 23, 2976–2980.
5. Y. Tian, X. Xu, J. Wang, C. Yao and L. Li, ACS Appl. Mater. Interfaces, 2014, 6, 8631–8638.
6. C. Hsiao, A. Hsiao and S. Chen, Adv. Mater., 2008, 20, 1982–1988.
7. K. S. Yook, S. E. Jang, S. O. Jeon and J. Y. Lee, Adv. Mater., 2010, 22, 4479–4483.
8. B. Zhang, G. Tan, C. Lam, B. Yao, C. Ho, L. Liu, Z. Xie, W. Wong, J. Ding and L. Wang, Adv. Mater., 2012, 24, 1873–1877.
9. B. Zhang, L. Liu, G. Tan, B. Yao, C.-L. Ho, S. Wang, J. Ding, Z. Xie, W.-Y. Wong and L. Wang, J. Mater. Chem. C, 2013, 1, 4933–4939.
10. T. Earmme and S. A. Jenekhe, Adv. Funct. Mater., 2012, 22, 5126–5136.
11. J. Chen, C. Shi, Q. Fu, F. Zhao, Y. Hu, Y. Feng and D. Ma, J. Mater. Chem., 2012, 22, 5164–5170.
12. Y. Zhao, L. Duan, D. Zhang, L. Hou, J. Qiao, L. Wang and Y. Qiu, Appl. Phys. Lett., 2012, 100, 083304.
13. T. Yang, W. Cai, D. Qin, E. Wang, L. Lan, X. Gong, J. Peng and Y. Cao, J. Phys. Chem. C, 2010, 114, 6849–6853.
14. L. P. Lu, D. Kabra and R. H. Friend, Adv. Funct. Mater., 2012, 22, 4165–4171.
15. Y. Li, D. Zhang, L. Duan, R. Zhang, L. Wang and Y. Qiu, Appl. Phys. Lett., 2007, 90, 012119.
16. R. Grover, R. Srivastava, M. N. Kamalasanan and D. S. Mehta, J. Lumin., 2014, 146, 53–56.
17. Y. Kishigami, K. Tsubaki, Y. Kondo and J. Kido, Synth. Met., 2005, 153, 241–244.
18. T. Rabe, S. Hamwi, J. Meyer, P. Görrn, T. Riedl, H. H. Johannes and W. Kowalsky, Appl. Phys. Lett., 2007, 90, 151103.
19. T. Oyamada, H. Sasabe, C. Adachi, S. Murase, T. Tominaga and C. Maeda, Appl. Phys. Lett., 2005, 86, 033503.
20. K. S. Yook and J. Y. Lee, Adv. Mater., 2014, 26, 4218–4233.
21. S. O. Jeon and J. Y. Lee, Sol. Energy Mater. Sol. Cells, 2011, 95, 1102–1106.
22. C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T. Fromherz, M. T. Rispens, L. Sanchez and J. C. Hummelen, Adv. Funct. Mater., 2001, 11, 374–380.
23. C. C. D. Wang, W. C. H. Choy, C. Duan, D. D. S. Fung, W. E. I. Sha, F.-X. Xie, F. Huang and Y. Cao, J. Mater. Chem., 2012, 22, 1206–1211.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, chemical structures, and additional AFM images. See DOI: 10.1039/c5ra23371b

‡ These authors contributed equally to this work.

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