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Solution-Processed Hybrid Light-Emitting Devices Comprising TiO 2 Nanorods and WO 3 Layers as Carrier-Transporting Layers

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ABSTRACT

The goal of this research is to prepare inverted light-emitting devices with improved performance by combining titanium dioxide (TiO2) nanorods and tungsten trioxide (WO3) layer. TiO2 nanorods with different lengths were established directly on the fluorine-doped tin oxide (FTO) substrates by the hydrothermal method. The prepared TiO2 nanorods with lengths shorter than 200 nm possess transmittance higher than 80% in the visible range. Inverted light-emitting devices with the configuration of FTO/TiO2 nanorods/ionic PF/MEH-PPV/PEDOT:PSS/WO3/Au were constructed. The best device based on 100-nm-height TiO2 nanorods achieved a max brightness of 4493 cd/m2 and current efficiency of 0.66 cd/A, revealing much higher performance compared with those using TiO2 compact layer or nanorods with longer lengths as electron-transporting layers.

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Luminance decay curve of the device based on 100-nm-height TiO2 nanorods as a function of time
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Fig9: Luminance decay curve of the device based on 100-nm-height TiO2 nanorods as a function of time

Mentions: Inverted light-emitting devices with the configuration of FTO/TiO2/ionic PF/MEH-PPV/PEDOT:PSS/WO3/Au were fabricated and evaluated, using TiO2 as electron-transporting layer, ionic PF as wetting layer, MEH-PPV as active layer, PEDOT:PSS as hole-transporting layer, and WO3 as hole-injecting layer. The brightness-voltage (B-V), current efficiency-current density (E-J) characteristics, and EL spectra of all devices are depicted in Fig. 7a–c. The overall device performance based on different types of TiO2 is summarized in Table 1. The max brightness and current efficiency of the device based on TiO2 nanoparticles reached 333 cd/m2 and 0.08 cd/A, respectively. By using TiO2 compact layer as an ETL, the device performance was promoted with max brightness of 790 cd/m2 and current efficiency of 0.15 cd/A. The reason to the improved performance is due to full coverage of FTO surface by the TiO2 compact layer that reduces charge trapping and quenching on FTO anode. Turing to TiO2 nanorods, the max brightness achieved 4493, 2589, and 1090 cd/m2 for the devices based on 100, 200, and 300-nm-height nanorods, respectively. Furthermore, the current efficiency was decreased from 0.66 to 0.05 cd/A as the length of TiO2 nanorods was increased. The reason to this phenomenon can be explained as follows. First, TiO2 nanorods with length of 100 nm own the highest transmittance among three lengths of nanorods, which is beneficial for light output. Second, 100-nm TiO2 nanorods provide shorter pathways for carrier injection to the active layer to generate light. We also notice that the devices based on TiO2 nanorods revealed much better device performance than others using TiO2 nanoparticles or compact layer as an ETL. This is due to higher mobility of TiO2 nanorods that is favored for carrier transport than the other two TiO2 nanomaterials, as described in the previous section. Figure 7c shows the original EL spectra of the inverted devices based on TiO2 nanomaterials operated at 12 V. The emission maximum wavelength and shoulder emission are located at 588 and 630 nm, respectively, revealing an orange-red light. A very bright inverted light-emitting device based on 100-nm-height TiO2 nanorods under driving bias of 15 V is shown in Fig. 7d. Figure 8a shows the relationship between the transmittance of TiO2 nanorods and the brightness of devices. The value of transmittance was chosen at 588 nm, since the device emitted light at the same wavelength. It is seen that TiO2 nanorods with higher transmittance bring higher brightness. To clarify the relationship between nanorod length and transmittance/brightness, the characteristics transmittance-nanorods and length-brightness are depicted and shown in Fig. 8b. As the nanorod length is increased, both transmittance and brightness are decreased. It is noted that the transmittance as well as the brightness of the devices in the whole visible region should be taken into account to completely understand the transmittance effect on the brightness of the devices, not only at 588 nm. Figure 9 shows the luminance decay curve as a function of time for the device based on 100-nm-height TiO2 nanorods. The device was monitored at a constant voltage of 10 V in ambient environment without encapsulation. The lifetime of the device is defined as the time when the luminance is decreased to a half of its initial luminance, which is determined about 40 h. The stability test of inverted OLEDs is seldom reported in the literature. G. He et al. reported the inverted OLEDs with the configuration of ITO/Cs2CO3/Bphen/Alq3/NPB/MoO3/Au [35]. The device lifetime was only 20 h when using Cs2CO3 as the EIL. By inserting a thin layer of aluminum between ITO and Cs2CO3, the luminance of the device decayed to 80% of its initial value during an operation time of 70 h. In our case, the usage of TiO2 nanorods as the ETL can provide moderate stability in ambient condition. To the best of our knowledge, this is the first demonstration of inverted light-emitting devices using TiO2 nanorods as ETL. These results suggest that TiO2 nanorods may possess potential use in light-emitting applications.Fig. 7


Solution-Processed Hybrid Light-Emitting Devices Comprising TiO 2 Nanorods and WO 3 Layers as Carrier-Transporting Layers
Luminance decay curve of the device based on 100-nm-height TiO2 nanorods as a function of time
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121115&req=5

Fig9: Luminance decay curve of the device based on 100-nm-height TiO2 nanorods as a function of time
Mentions: Inverted light-emitting devices with the configuration of FTO/TiO2/ionic PF/MEH-PPV/PEDOT:PSS/WO3/Au were fabricated and evaluated, using TiO2 as electron-transporting layer, ionic PF as wetting layer, MEH-PPV as active layer, PEDOT:PSS as hole-transporting layer, and WO3 as hole-injecting layer. The brightness-voltage (B-V), current efficiency-current density (E-J) characteristics, and EL spectra of all devices are depicted in Fig. 7a–c. The overall device performance based on different types of TiO2 is summarized in Table 1. The max brightness and current efficiency of the device based on TiO2 nanoparticles reached 333 cd/m2 and 0.08 cd/A, respectively. By using TiO2 compact layer as an ETL, the device performance was promoted with max brightness of 790 cd/m2 and current efficiency of 0.15 cd/A. The reason to the improved performance is due to full coverage of FTO surface by the TiO2 compact layer that reduces charge trapping and quenching on FTO anode. Turing to TiO2 nanorods, the max brightness achieved 4493, 2589, and 1090 cd/m2 for the devices based on 100, 200, and 300-nm-height nanorods, respectively. Furthermore, the current efficiency was decreased from 0.66 to 0.05 cd/A as the length of TiO2 nanorods was increased. The reason to this phenomenon can be explained as follows. First, TiO2 nanorods with length of 100 nm own the highest transmittance among three lengths of nanorods, which is beneficial for light output. Second, 100-nm TiO2 nanorods provide shorter pathways for carrier injection to the active layer to generate light. We also notice that the devices based on TiO2 nanorods revealed much better device performance than others using TiO2 nanoparticles or compact layer as an ETL. This is due to higher mobility of TiO2 nanorods that is favored for carrier transport than the other two TiO2 nanomaterials, as described in the previous section. Figure 7c shows the original EL spectra of the inverted devices based on TiO2 nanomaterials operated at 12 V. The emission maximum wavelength and shoulder emission are located at 588 and 630 nm, respectively, revealing an orange-red light. A very bright inverted light-emitting device based on 100-nm-height TiO2 nanorods under driving bias of 15 V is shown in Fig. 7d. Figure 8a shows the relationship between the transmittance of TiO2 nanorods and the brightness of devices. The value of transmittance was chosen at 588 nm, since the device emitted light at the same wavelength. It is seen that TiO2 nanorods with higher transmittance bring higher brightness. To clarify the relationship between nanorod length and transmittance/brightness, the characteristics transmittance-nanorods and length-brightness are depicted and shown in Fig. 8b. As the nanorod length is increased, both transmittance and brightness are decreased. It is noted that the transmittance as well as the brightness of the devices in the whole visible region should be taken into account to completely understand the transmittance effect on the brightness of the devices, not only at 588 nm. Figure 9 shows the luminance decay curve as a function of time for the device based on 100-nm-height TiO2 nanorods. The device was monitored at a constant voltage of 10 V in ambient environment without encapsulation. The lifetime of the device is defined as the time when the luminance is decreased to a half of its initial luminance, which is determined about 40 h. The stability test of inverted OLEDs is seldom reported in the literature. G. He et al. reported the inverted OLEDs with the configuration of ITO/Cs2CO3/Bphen/Alq3/NPB/MoO3/Au [35]. The device lifetime was only 20 h when using Cs2CO3 as the EIL. By inserting a thin layer of aluminum between ITO and Cs2CO3, the luminance of the device decayed to 80% of its initial value during an operation time of 70 h. In our case, the usage of TiO2 nanorods as the ETL can provide moderate stability in ambient condition. To the best of our knowledge, this is the first demonstration of inverted light-emitting devices using TiO2 nanorods as ETL. These results suggest that TiO2 nanorods may possess potential use in light-emitting applications.Fig. 7

View Article: PubMed Central - PubMed

ABSTRACT

The goal of this research is to prepare inverted light-emitting devices with improved performance by combining titanium dioxide (TiO2) nanorods and tungsten trioxide (WO3) layer. TiO2 nanorods with different lengths were established directly on the fluorine-doped tin oxide (FTO) substrates by the hydrothermal method. The prepared TiO2 nanorods with lengths shorter than 200 nm possess transmittance higher than 80% in the visible range. Inverted light-emitting devices with the configuration of FTO/TiO2 nanorods/ionic PF/MEH-PPV/PEDOT:PSS/WO3/Au were constructed. The best device based on 100-nm-height TiO2 nanorods achieved a max brightness of 4493 cd/m2 and current efficiency of 0.66 cd/A, revealing much higher performance compared with those using TiO2 compact layer or nanorods with longer lengths as electron-transporting layers.

No MeSH data available.


Related in: MedlinePlus