The relation between carrier mobility and device efficiency

In this part, I would like to discuss the relation between carrier mobility and device. In this work six polymers were chosen to study the properties of transport and EL efficiency.

Two high efficiency polymers, LUMATION BP105 and S-Y, and the other four less efficient polymers, including PFO, TFB, DPOC10PPV, and MEH-PPV were studied.

Bipolar devices were made to test the EL efficiency of each polymer, hole–only and electron-only devices were made to obtain the carrier mobility. The fabrication process is described in page 24. Figure 3.11 shows the current efficiency. BP105 and S-Y were well-known high efficiency polymers.

FIG. 3.11: Current efficiency and external quantum efficiency of polymers in the device structure of ITO/PEDOT/polymer/LiF/Ca/Al. (a) high efficiency polymers, BP105 (down triangle) and S-Y (rhombus) (b) low efficiency polymers, PFO (square), MEHPPV (circle), DPOC10PPV (up triangle) and TFB (star).

In fact BP105 is the blue polymer with the highest EL efficiency, but its PL quantum efficiency is only 30% which is smaller than that of PFO (40%). However, its EL current efficiency is three times larger than PFO at 8 V. In Fig. 3.10 (b), these three polymers all have high PL quantum efficiency, but their EL efficiency is low. Therefore, PL quantum efficiency cannot explain why BP105 and S-Y have such high EL efficiency. To clarify

how BP105 and S-Y are special, electron-only and hole-only devices are used to get individual electron and hole currents as functions of voltage and the results are shown in Figure 3.12.

FIG. 3.12: Comparison of calculated currents and measured currents, including calculated electron currents (dashed line), calculated hole currents (solid line) ,measured electron currents (solid triangle) and measured hole currents (solid circle).

The hole currents of MEH-PPV, DPOC10PPV, and TFB are all larger than electron currents at least by one order of magnitude. For PFO electron current is slightly larger than hole current at low electric field, but hole current immediately exceeds electron

current as voltage increases. Therefore, the hole currents are all far larger than electron currents for all these low EL efficiency polymers under normal PLED operation voltage.

However, the electron current is far larger than hole current of BP105 for all voltage range. This is unusual since most polymers are generally p-type resulted from background p-doping in synthesis and other fabrication process, besides electron traps are known to prevail in PLED.[14] Although the electron current of S-Y is smaller than hole current at first, it soon catches up at higher voltage. So the remarkable correlation here is that the electron current is larger than or comparable to hole current for high EL efficiency polymers. The uni-carrier currents were fitted by the previous model to get carrier mobilities, as shown in Figure 3.12. The reason for the mismatch of experimental and calculated results at low voltages in BP105 and TFB is possibly that we have not included the trap-assisted tunneling of carriers from the contact into polymers.[65,66] At low electric fields and high barriers, carriers are injected from the contact into localized states in the energy gap and hop to band edge instead of being directly injected into the band edge. This effect becomes insignificant and can be neglected at higher fields. Note TFB has a particularly large electron injection barrier. The fitting at low voltage is not given because there is a linear region caused by the background doping and conducting filament12 which is not taken into account in the model. The results of calculated mobilities are shown in Figure 3.13.

FIG. 3.13: Fitted electron and hole mobility of polymers. (a) polymers whose electron and hole mobility are in the same order at normal operating voltages. (b) polymers whose electron and hole mobility are different at normal operating voltages.

In Fig. 3.13 (a), hole mobility was about the same as electron mobility for BP105. For S-Y and TFB electron mobilities approached to hole mobilities at high fields. In Fig. 3.13

(b), hole mobilities were several orders higher than electron mobilities in PFO, MEH-PPV and DPOC10PPV, which indicates that strong carrier imbalance exists in high PL but low EL efficiency polymers. The imbalanced mobility in PFO is consistent with previous reports.[59,67] The current eficiency, quantum efficiency, electron mobility and hole mobility for various polymers are summarized in Table 3.3.

TABLE 3.3: Maximal current efficiency, external quantum efficiency (EQE) and corresponding electron and hole mobility of polymers in this work.

Polymer Maximal current

By comparing with the carrier mobilities with EL efficiencies, the electron transport is shown to dominate the EL efficiency rather than high PL quantum efficiency. In the highly efficient polymers, BP105 and S-Y, the carrier mobilities are about the same magnitude at normal operating voltage range. The electron currents are larger than hole currents because of higher hole barrier. Interestingly the carrier mobilities are comparable in TFB, but the electron barrier is too large for electrons to inject and the EL efficiency is low. In general the electron currents are more crucial than hole currents. For the low efficiency polymers, PFO, MEH-PPV and DPOC10PPV, hole mobilities are several orders of magnitude higher than electron mobilities. The mobility difference yields the

carrier imbalance in device and give low EL efficiency. As the above discussion goes, there are two conditions to achieve high EL efficiency, high electron mobility and low electron barrier.

Even though the uni-polar devices provide useful comparisons between the electron transport properties of different materials, the electron and hole currents in Fig. 3.12 can not be simply summed up to give the total current in the bipolar LED. In general the bipolar current is very different from the sum of the two uni-polar currents experimentally.

The electron or hole current are determined by the effective injection barrier, the electric field distribution, as well as the mobility. All the three factors are strong functions of the space-charge distribution which is in turn determined by the presence of the carriers with the opposite charge. The barrier is lowered by the image-charge effect and the mobility depends on both the field exponentially and the energy disorder caused by the opposite charge⁵⁶. For example, in Fig. 3.12 the electron current for S-Y is small at low voltage in uni-polar device. In bipolar LED, the electron current can be much higher because of the presence of the holes will screen the field near the anode and cause a much higher field near the cathode. Because of the strong dependence of the electron mobility on the field as shown in Fig. 3.13, the electron current in LED can be much higher than the unipolar current at a given voltage to achieve good balance. For PFO, there is a large hole injection barrier. The hole current therefore depends sensitively on the field near the anode due to image force barrier lowering. In LED the field could be much higher due to the screening near the cathode, the hole current is therefore expected to be much larger than shown in Fig. 3.12 and dominate the electron current.

The purity of polymer is very important in getting high electron mobility. Many impurities play the role of electron traps, including inorganic impurities Cl, Na, K or organic impurities induced in synthesis process and even end-groups of polymers and absorbed molecular oxygen.[27,68] For BP105 bipolar device the electron current is free

of such obstacles[68], as suggested by the larger electron current than hole current shown in uni-polar devices. In addition to low cathode injection barrier, high purity, large molecular weight, and air stability for BP105 and S-Y are believed to be responsible for the desired high electron current and high efficiency.

In addition to charge transport and PL quantum efficiency there are a few other factors which might also influence the EL efficiency. There has been reports that the singlet exciton formation probability is higher than 1/4 in some polymers.[69]However more recent experiments suggest that the probability is 1/4 as the case of small molecules.[70,71] The difference in EL efficiency is therefore not due to the exciton spin consideration. The cathode quenching of the excitons is known to reduce the EL