The relation between fabrication environment and device efficiency

to the low electron mobility. However at normal operation voltages above 4 V the recombination zone becomes rather homogeneous throughout the film thickness.[72]

Since the comparison of the efficiencies of the devices are based on its maximal value at higher voltage the difference in the cathode quenching is not expected to have a major effect. Similarly due to the homogeneous recombination zone the difference in the light out-coupling efficiency can be neglected. The importance of electrode quenching also depends on the exciton diffusion length which in turns depends on the exciton lifetime.

For polymers with particularly long lifetime there might be an enhanced quenching effect.

Transient photoluminescence experiments however see no major difference in the exciton lifetime. Indeed most of the luminescent polymers have lifetime around 0.5 ns.[73,74]

3.5. The relation between fabrication environment and device efficiency

In this work, the relation between the fabrication environment and device efficiency has

been compared. Six bipolar devices and two electron-only devices based on were fabricated. The hole transport and emissive polymers were spin-coated in air for some devices and in the glove box for other devices. For bipolar devices made in air, A was ITO/ PEDOT:PSS/ PFO/ LiF/ Ca/ Al, B was ITO/ PEDOT:PSS/ TFB/ PFO/ LiF/ Ca/ Al, C was ITO/ PEDOT:PSS/ PFO/ CsF/ Al, D was ITO/ PEDOT:PSS/ TFB/ PFO/ CsF/ Al.

For bipolar devices made in the glove box, E was ITO/ PEDOT:PSS/ PFO/ CsF/ Al and F was ITO/ PEDOT:PSS/ TFB/ PFO/ CsF/ Al. For electron-only devices, G was Ag/

PEDOT:PSS/ PFO/ Ca/ Al made in air and H was Ag/ PEDOT:PSS/ PFO/ Ca/ Al made in glove box. The fabrication process is described in page 26. Fig. 3.14 shows the results of devices A, B, C and D, and compares the cathodes LiF/Ca/Al and CsF/Al in single layer and bilayer structures.

FIG. 3.14: The performances of blue PLED fabricated in air: device A (open square), device B (solid square), device C (open circle) and device D (solid circle). (a) The current efficiency. Inset are the EL spectra. (b) The luminance. Inset is the current density.

For single layer devices, the maximum efficiency was 0.96 cd/A for device A with LiF and 1.29 cd/A for device C with CsF. Both Cs and Li are believed to be liberated at the organic/metal interface during evaporation.[75] The work function of Cs (2.1 eV) is lower than Li (2.5 eV), and therefore more efficient electron injection is provided by the CsF/Al cathode. That is why the efficiency and luminance of device A were higher than those of

device C. Despite of the small electron mobility, the ohmic contact at the CsF cathode seems to make the single layer devices electron-dominated as the current is largely contributed by the cathode. For TFB/PFO bilayer devices, a better charge balance compared to single layer devices has been achieved. The efficiency of bilayer PLED were 1.29 cd/A for device B with LiF and 1.63 cd/A for device D with CsF. The advantages of adding TFB layer are four-fold. First, TFB plays the role of HTL because of the high hole mobility and the IP of TFB at 5.3 eV between PEDOT:PSS (5.2 eV) and PFO (5.8 eV).

Holes can be injected and transported to PFO layer more easily. Second, TFB is also EBL due to its lower EA (2.3 eV) than PFO (3.0 eV). Electrons injected and transported in PFO are blocked by the TFB layer instead of reaching the anode. Third, the recombination is shifted away from the cathode and concentrates near the TFB/PFO interface to reduce quenching by liberated Cs atoms. Forth, the TFB layer prevents the degradation of PFO layer by the acid PEDOT:PSS.[76] The maximum luminance was 1038 cd/m² (8V) for device A and 2001 cd/m² (10V) for device B, 1377 cd/m² (8V) for device C and 2528 cd/m² (10V) for device D. The currents of bilayer devices were smaller than those of single layer devices because the electron current is blocked by the TFB layer. The spectra of the four devices, shown in Fig. 3.14, are similar, slight difference in the green shoulder may reflect the various recombination zones where the ketone defect levels differ. CsF/Al cathode is clearly superior to LiF/Ca/Al presumably due to more efficient electron injection. In addition to injection, electron mobility is also important to the electron current. One way to enhance the electron mobility is to reduce the oxygen adsorption by polymer because oxygen would cause electron traps.[77,78] For this purpose, we compared the polymer spin-coated in air and in glove box with oxygen level about 1 ppm. Fig. 3.15 shows the results of device C, D, E and F, to compare single layer and bilayer structures with CsF/Al cathode.

FIG. 3.15: The performances of blue PLED with CsF/Al cathode: device C (open circle), device D (solid circle), device E (open triangle) and device F (solid triangle). (a) The current efficiency. Inset shows the EL spectra. (b) The luminance. Inset is the current density.

Compared with single-layer device C in air, the maximum efficiency of the device E in glove box was slightly enhanced from 1.18 cd/A (7V) to 1.37 cd/A (4V). However, the efficiency decreases rapidly at higher voltages, probably because without oxygen adsorption the electron current rises too much. Due to the HTL, the bilayer devices are

likely to be hole dominated so the enhancement of electron mobility by coating in glove box is expected to have a more pronounced effect than single-layer devices. The current of bilayer device F was smaller than that of single-layer device E, indicating that electron blocking by TFB. Oxygen reduces the current in single-layer devices (C versus E) but enhances the bilayer devices (D versus F). This might be due to another competing effect of electron traps near the anode which cause a dipole layer and help the hole injection through the large barrier.[65,79]The spectra for the devices made in the glove box are similar to those made in the air, also shown in Fig. 3.13. Among all devices the best is F with both bilayer structure and spin-coating in the glove box. Its peak luminance was 1760 cd/m² and peak current efficiency was 2.5 cd/A, corresponding to EQE of 2 % at deep blue with Commission Internationale de L’Eclairage (CIE) coordinate at (0.15, 0.14).

The efficiency is not far from the best proprietary polymers[21,68] and is quite remarkable for polyfluorene with low molecular weight (Mw = 71000) and moderate purity (metal purity = 14.2 ppm). In fact such polymers are usually considered as models for scientific inquiry rather than practical applications. These results demonstrate that with proper design of the device structure and fabrication procedure, large-scale application can be realized using commonly available polymers which are easy to synthesize and does not need to satisfy strict material specifications.

Finally in order to confirm the effect of oxygen on electron mobility, two electron-only devices are made, device G in air and device H in glove box. The result is shown in Figure 3.16.

2 3 4 5 6 7 8 9 10

FIG.3.16: Comparison of electron currents of devices fabricated in air and in glove box for device G (air, open square) and device H (glove box, solid square).

The electron current of device H was about one order of magnitude higher than that of device G, which is consistent with our assumption of electron trapping effect of oxygen.

Electron mobility was fitted using space-charge-limited current voltage-current relation

3

J is the current density, εis the permittivity of the polymer, μe is the electron mobility, V is driving voltage, Vbi is the built-in voltage, and L is the polymer thickness. The fitted electron mobility is 5×10⁻⁷ cm²/Vs in air and 5×10⁻⁶ cm²/Vs in glove box, both of them smaller than the hole mobility around 10⁻⁵ cm²/Vs.[67,80]