To testify the EL properties of the exciplex, we fabricated the simple devices with structure of ITO/MoO
3 (10 nm)/m-MTDATA (65 nm)/m-MTDATA:BPhen (
x, 40 nm)/BPhen (65 nm)/LiF (1 nm)/Al, where
x is the molar ratio of m-MTDATA and BPhen, which is 2:1 (device 1), 1:1 (device 2) and 1:2 (device 3). Here we used m-MTDATA as the hole transporting layer and BPhen as the electron transporting layer. The large energy level difference of 1.3 eV between the HOMO levels of m-MTDATA and BPhen, and 0.9 eV between LUMO levels of them makes sure that the injected carriers will be well confined in the emitting layer. Although the PL quantum efficiency of the m-MTDATA:BPhen blend film is only 8.6% at room temperature, which is rather lower than 20% (m-MTDATA:t-Bu-PBD film), 26% (m-MTDATA:3TPYMB film) [
9] and 28.5% (m-MTDATA:PPT film) [
10], the
EQE of OLED based on m-MTDATA:BPhen blend is yet over 5%. As shown in Fig. 3(a), the maximum
EQE of the three devices are 6.5%, 7.0% and 7.3%, all of which exceed the theoretical limit of fluorescent devices. For our fluorescent OLEDs with 8.6% PL efficiency, the maximum
EQE should be no more than 0.43% (Eq. (1)). Even all the triplets convert to the singlets through TTA, the maximum EQE will still be no more than 1.1% (Eq. (2)). So we ascribe the high
EQE in our devices to the back energy transfer of triplet state to the singlet state of the exciplex, as proven above. Furthermore, we found that the device 3 shows the highest
EQE, meaning that the charge carrier balance in emission layer (EML), thus determining the efficiency of exciton recombination and generation, is also very important in the device efficiency. As we know, m-MTDATA shows a hole mobility in the order of 10
−4 cm
2/(V·s) [
11] and BPhen has an electron mobility of around 10
−4 cm
2/(V·s) [
12], too, indicating a good charge balance can be achieved in the devices. For the case of device 1 with the molar ratio of 2:1 corresponding a weight ratio of m-MTDATA and BPhen to about 4.8:1, the electrons will be blocked at the interface region between the EML and the electron transporting layer, resulting in narrower recombination region and higher exciton annihilation, thus low efficiency. As the ratio of BPhen in the m-MTDATA:BPhen blend increases, the injected electrons will be transported into the more wide region in EML, thus increasing the recombination width and reducing the exciton annihilation, leading to higher efficiency. The achievement of high efficiency in device 3 can also be further proved by the reduction of efficiency roll-off. The resulted
EQE of device 2 and 3 can be well fitted by the TTA model [
13], as shown in Fig. 3 (solid line). The well fittings indicate that the efficiency roll-off is TTA predominant, which should be originated from the large contribution of the triplet state in the EL emission through a back energy transfer process. The higher critical current density (
J0) of 83.3 mA/cm
2 for device 3 than that of 38.3 mA/cm
2 for device 2 also indeed means that the TTA in device 3 is greatly improved and the more triplet excitons are used for the emission. As shown in Fig. 3(b), the device 3 also shows higher power efficiency (
PE), the maximum
PE reaches 25 lm/W.