Dongcheng
Chen
*,
Binbin
Li
,
Lin
Gan
,
Xinyi
Cai
,
Yuguang
Ma
,
Yong
Cao
and
Shi-Jian
Su
*
State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. E-mail: mschendc@scut.edu.cn; mssjsu@scut.edu.cn
First published on 27th November 2017
Electroluminescence originating from layer-to-layer charge transfer (LLCT) in organic light-emitting diodes (OLEDs) was early observed, whereas, the inferior performances limit their practical applications. In this work, we demonstrated an efficient approach to improve the overall performances of LLCT-based planar heterojunction OLEDs by simply diluting an n-type electron transport material into a carbazole-based matrix material to suppress excited-state quenching. As a result, an optimized device with a peak current efficiency of 41 cd A−1 (corresponding to a maximum external quantum efficiency of 12.6%) exhibited significantly enhanced efficiencies and much higher brightness at high current densities in contrast to the control devices without a mixture layer. We illustrated that even with a similar narrow recombination region, it is feasible to achieve excellent fluorescent OLEDs with light emission originating from LLCT. We believe that this work should pave the way for developing high-performance LLCT-based OLEDs with a high efficiency, reduced efficiency roll-off and sufficiently large maximum brightness.
Recent works by Adachi et al. demonstrated that exciplex-based OLEDs with a mixed donor:acceptor (D:A) EML that exhibits thermally-activated delayed fluorescence (TADF) could be theoretically highly efficient due to the capacity of harvesting both singlet and triplet excitons for radiative utilization.13,14 Inspired by these works, there have been many reports about efficient exciplex OLEDs employing a binary-mixed EML.15–19 In the view of the microcosmic level, LLCT exciplex emission from a bilayer planar heterojunction may be analogous to the case of exciplex emission originating from a D:A mixture layer, since both processes occur bi-molecularly based on a similar intermolecular charge transfer mechanism. Thereby, OLEDs with light emission originating from LLCT might show comparable performances in contrast to the exciplex OLEDs using an independent D:A mixture EML. However, in practice, the performances of the former are dramatically lower than the latter, even though they use the same D and A materials. LLCT-based OLEDs typically show low efficiencies and low maximum brightness. The efficiency drawback is more remarkable when the device is at a high current density, which makes the efficiencies at higher luminance extremely low.
The macroscopic physical morphology of LLCT systems based on stacked, planar heterojunctions is totally different to the case of binary-mixed counterparts with a bulk heterojunction configuration. This difference in layer construction should result in variation of the condensed state properties, which might be relevant to the carrier and excitonic behaviours. There is still a lack of investigation of key factors leading to the inferior performances of LLCT-based OLEDs. Our previous results showed that selection of appropriate p- and n-type material systems should significantly affect the resulting device performances, suggesting that progress of materials, especially n-type materials, and their combination strategies should help to improve performances of LLCT-based devices.20 High efficiency yellow and green planar pn heterojunction OLEDs with a peak EQE value above 10% were demonstrated based on an extremely simplified device structure. However, efficiencies at high current densities and the maximum luminance are still moderate for the resulting devices.20 In terms of device physics, there is a common viewpoint that a narrow carrier recombination and exciton generation region for these kinds of devices should be fatal to the EL performances.21–23 Compared with the conventional OLEDs with an independent EML, in which the excited state could be widely dispersed within the EML, the carriers and excitons can only aggregate at a very thin interface because of the planar contact heterojunction configuration. The narrow carrier recombination region is generally, empirically regarded as the intrinsic nature limiting the overall performances of LLCT-based EL devices.
In this work, we demonstrated efficient planar heterojunction OLEDs with light emission originating from LLCT, through introducing a carbazole-based matrix material into the n-type transport layer to suppress the excited-state quenching effect. Based on this strategy, high performance fluorescent OLEDs were fabricated with a peak current efficiency (CE) of 41 cd A−1, a maximum external quantum efficiency (EQE) of 12.6% and a greatly enhanced efficiency at higher current densities in contrast to the control devices without a mixture layer. The improved device performance can be attributed to the suppression of the excited-state quenching effect. From this prototype investigation, we proposed that even with a narrow carrier recombination region, it is feasible to achieve excellent fluorescent OLEDs with light emission originating from LLCT with an approximate device configuration. We believe that this should be a widely applicable and effective method for enhanced efficiency, especially at higher current densities, and maximum brightness to construct efficient planar heterojunction OLEDs with light emission originating from LLCT.
Fig. 2 (a) PL emission spectra of neat films and binary-mixed films (1:1, molar ratio); (b) fluorescent and phosphorescent emission spectra of the materials in toluene. |
The schematic device configurations are shown in Fig. 1b. Detailed device structures are as follows: ITO/TAPC (45 nm)/TCP:x mol% TmPyTZ (x = 0, 5, 10, 20, 30, 40, 50, 65, 80, or 100, 20 nm)/TmPyTZ (75 nm)/LiF (1 nm)/Al (120 nm), where these devices are denoted as D1, D2, D3, D4, D5, D6, D7, D8, D9, D10, respectively. D1 and D10 without a mixture layer play a role as the control devices. TCP was used as the diluting spacer matrix material. The concentration of TmPyTZ in the mixture layer was increased from 0% (pure TCP) to 100% (pure TmPyTZ) with a corresponding reduction in the concentration of TCP, aiming at probing the concentration-dependent effect. Fig. 3a displays their J–V characteristics. It can be seen that as the doping ratio of TmPyTZ in the binary mixture film gradually increases from 0% to 30%, the injection current density dramatically increases. While the mixing proportion is further increased above 30%, the increase of current densities tends to be slow. A similar changing trend of current densities as a function of doping ratio was also observed by Ha et al, while they investigated current densities of the devices employing a TCTA–BmPyPB mixture.25 From their energy levels (Fig. 1c, for details to determine the energy level data refer to Fig. S2, ESI†), it can be deduced that electron transport in the mixture layer should be preferred on the TmPyTZ channel because of its higher electron affinity (EA) in contrast to that of TCP (2.96 vs. 2.65 eV). The dependence of current densities on the mixing ratio of TmPyTZ can be explained by the percolation transport theory.25–27 With an increase in doping ratios of TmPyTZ, an intersecting network for electron transport can be formed. Thereby, percolating materials can govern the charge transport in a wide range of concentrations. As the doping fraction exceeds 30%, the transport capacities of the percolating materials in the mixture layer get close, thus, the current densities in the devices with a mixing ratio of TmPyTZ above 30% tend to be analogous. At a high doping ratio of TmPyTZ, the electron flux can be well transported within the mixture layer. This can eliminate layer-to-layer electron injection barriers between the mixture layer and the pure TmPyTZ ETL layer, and make electron carriers be captured at the interface between the mixture layer and the HTL. As for the cases of low doping ratios, there are two approaches for carrier capture and exciton generation. The electron carriers can migrate by the TmPyTZ channel formed in the mixture layer, leading to exciton generation at the interface between the mixture layer and the HTL. On the other hand, holes can also be injected from the TAPC layer into the TCP channel via overcoming the layer-to-layer energy barriers under high electric fields, leading to exciton generation with the encounter of electrons from the TmPyTZ channel. Whereas, for a device with a pure TCP layer, electron carriers should accumulate at the TCP/ETL interface, because of a rather big electron injection barrier from TmPyTZ to TCP, and then holes are injected from TAPC into TCP, which leads to exciton generation at the TCP/ETL interface.
Fig. 3 (a) J–V and (b) L–V characteristics of the devices from D1 to D10 as a function of different TmPyTZ ratios in the binary mixture layer. |
These analyses can be directly proved by the luminance–current density (L–V) characteristics (Fig. 3b) and the EL spectra results under various current densities (Fig. 4). The emissions of D1 with a pure TCP layer mainly arise from the TCP/TmPyTZ interface. This confirms that holes are injected across the TCP layer and recombined with electrons at the TCP/TmPyTZ interface. The large injection barriers from TAPC to TCP render D1 with the largest driving voltages among all these devices. Deep blue emission was observed for D1, indicating that deep blue OLEDs based on the LLCT mechanism should be possible. The EL spectra of D2 show two obviously distinct emission bands. The high-energy band peaking at around 430 nm should be attributed to LLCT emission due to the TCP/TmPyTZ interactions, while the low-energy emission band should arise from that of the TAPC/TmPyTZ system. The high-energy band tends to be dominant at high current densities. This can be explained by the fact that more hole carriers can be injected into the TCP channels in the mixtures under large electric fields and thus provide more emission arising due to the TCP/TmPyTZ interactions. As the TmPyTZ proportions in the mixture layer are gradually enhanced, the high-energy emission band of the resulting devices dramatically reduces, meanwhile, the driving voltages also simultaneously decline. While in the devices having a mixture layer with TmPyTZ concentrations equal to or above 30% (D5–9), EL emission due to the TCP/TmPyTZ interactions almost disappears. This is because most of the electrons migrate to the TAPC/mixture interface by percolating the TmPyTZ channel formed in the mixture layer, thus offering nearly pure LLCT emissions due to TAPC and TmPyTZ interactions, which are identical to the emission of the device without TCP (D10). Owing to the avoidance of layer-to-layer injection barriers, the devices from D5 to D9 also exhibit similar driving voltages with respect to those of D10. These results are consistent with the previous analyses based on their energy levels and J–V characteristics. According to the above results, TCP should not be involved in carrier transport and the exciton generation process in the devices with a high fraction of TmPyTZ (D5–9), i.e., TCP plays the role of a diluting spacer without being involved in the carrier and exciton processes in these devices.
Fig. 4 EL emission spectra under various current densities of the devices (a) D1, (b) D2, (c) D3, (d) D4, (e) D5, (f) D6, (g) D7, (h) D8, (i) (D9) and (j) D10. |
CE, power efficiency (PE) and EQE data are, respectively, plotted in Fig. 5a–c as a function of current densities, and their key performance parameters are summarized in Table 1. D1 exhibits a maximum EQE of 0.34%. The low efficiency is attributed to the inferior luminescent property of the TCP/TmPyTZ exciplex system. With the increasing doping fractions of TmPyTZ in the mixture layer, the efficiencies of the devices are significantly increased, owing to the enhanced contribution from the TAPC/TmPyTZ emission. The maximum EQE of D5 reaches 11.0%. For D5, D6, D7, their maximum EQEs remain nearly constant, while the peak efficiencies of D8–10 show a decreasing trend. D10 without any TCP component merely possesses a maximum EQE of 7.13%, which is around 30% lower than the efficiencies of D5, D6 and D7. These results demonstrated that through diluting TmPyTZ into the TCP matrix with an appropriate fraction the efficiency of the resulting devices could be significantly enhanced. Meanwhile, the maximum luminance values of D5–D7 are 3625, 3278, 3542 cd m−2, respectively, which are about 25 fold larger than that of D10 (122.4 cd m−2). Prior results show that the thickness of the transport layer can affect their device performances obviously,20 thus, we further optimized the devices by tuning the thickness of the TAPC layer. It was observed that the thickness of the TAPC layer obviously affects the device performances, probably because of modulation of the carrier balance.20 Accordingly, on the basis of a 75 nm-thin TAPC layer as a HTL, the performance of the device employing a TmPyTZ:50% TCP layer can be further improved, with maximum EQE, CE, PE and luminance of 12.6%, 41.0 cd A−1, 53.7 lm W−1 and 3578 cd m−2, respectively (Table 1, for detailed performance data of these devices refer to Fig. S3, ESI†).
Fig. 5 Efficiency performances of the developed devices: (a) CE–J, (b) PE–J and (c) EQE–J characteristics. |
Devices | V on | Maximum efficiency | L max (cd m−2) | CIEx,yb | λ peak (nm) | ||
---|---|---|---|---|---|---|---|
CE (cd A−1) | EQE (%) | PE (lm W−1) | |||||
a Driving voltage where the electroluminescence is 1 cd m−2. b At a current density of 1 mA cm−2. | |||||||
D1 | 4.0 | 0.36 | 0.34 | 0.28 | 199.7 | (0.173, 0.124) | 430 |
D2 | 3.5 | 1.59 | 0.60 | 1.38 | 333.0 | (0.211, 0.384) | 426, 502 |
D3 | 3.1 | 5.15 | 1.78 | 5.05 | 1331 | (0.231, 0.501) | 426, 510 |
D4 | 2.6 | 21.0 | 6.66 | 25.3 | 2948 | (0.259, 0.556) | 516 |
D5 | 2.1 | 37.1 | 11.0 | 48.5 | 3625 | (0.303, 0.590) | 526 |
D6 | 2.1 | 37.7 | 11.0 | 49.3 | 3278 | (0.318, 0.592) | 530 |
D7 | 2.1 | 37.6 | 11.1 | 49.2 | 3542 | (0.316, 0.590) | 526 |
D8 | 2.1 | 30.8 | 9.00 | 40.3 | 3161 | (0.338,0.591) | 532 |
D9 | 2.1 | 31.2 | 9.10 | 40.9 | 2832 | (0.335, 0.591) | 530 |
D10 | 2.1 | 24.2 | 7.13 | 34.6 | 122.4 | (0.360, 0.584) | 536 |
D7 (55 nm TAPC) | 2.2 | 38.8 | 11.9 | 50.8 | 2981 | (0.376, 0.572) | 540 |
D7 (65 nm TAPC) | 2.2 | 39.8 | 12.1 | 52.1 | 3010 | (0.387, 0.568) | 544 |
D7 (75 nm TAPC) | 2.2 | 41.0 | 12.6 | 53.7 | 3578 | (0.399, 0.561) | 550 |
We note that the exciton recombination regions for these devices are extremely narrow, in the consideration of the light emission originating from LLCT at a planar heterojunction. Compared with the control device, D10, the exciton recombination areas for the devices with a mixture diluting layer should be even smaller, because the TmPyTZ molecules that are in contact with the TAPC layer to offer LLCT excited states are greatly reduced by mixing with TCP. Thus, the origins of the improvement of the maximum efficiency, reduced efficiency roll-off at high current densities and maximum luminance for the devices here should not be ascribed to the broadened carrier recombination region. These results indicate that even with a narrow carrier recombination region, it is feasible to achieve excellent fluorescent OLEDs with light emission originating from LLCT, provided that the device configuration and material systems are well modulated.
In order to clarify the origin of enhanced performances of the devices with a diluting spacer component, transient PL experiments were performed. Fig. 6a displays the transient PL decay curves of TAPC (8 nm)/TmPyTZ (10 nm) and TAPC (8 nm)/TCP:50% TmPyTZ (10 nm) films. The thin thickness of the layers facilitates the energy transfer from the local excited states to the charge transfer state. The decay curves are fitted by the following function:
I(t) = A1e−t/τ1 + A2e−t/τ2, |
We further conducted transient EL experiments based on the devices employing TAPC/TmPyTZ (D10) and TAPC/TCP:50% TmPyTZ (D7). As shown in Fig. 6b, D7 exhibited a much longer decay lifetime in contrast to that of D10. The prolonged EL decay lifetime of D7 should also be ascribed to the reduced excited-state quenching. This trend is consistent with the transient PL decay results. Through the above transient PL and EL measurement results, we concluded that reducing the excited-state quenching effect should be the main factor to improve the LLCT device performance. We note that this strategy might be widely applicable to enhance the device performance based on the LLCT mechanism, and this methodology might also be promoted to design p- or n-type materials to develop high-performance LLCT-based devices, e.g., chemically coupling the diluting spacer units to the compounds used for LLCT emission.
Footnote |
† Electronic supplementary information (ESI) available: Transient PL decay curves, ultraviolet photoelectron spectra and absorption spectra, performance figures of optimized devices. See DOI: 10.1039/c7tc04459c |
This journal is © The Royal Society of Chemistry 2018 |