High efficiency green InP quantum dot light-emitting diodes by balancing electron and hole mobility

Upon modifying cadmium-free InP quantum dot emitting layer by passivation with 1,4-butanediamine and zinc iodide, we successfully decrease electron mobility and enhances the hole transport in the InP QLED. As a result of optimizing the electrons and hole injection, it leads to green 545 nm InP QLED with a maximum quantum efficiency (EQE) of 16.3% and a current efficiency of 57.5 cd/A.

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The industrialization of quantum dots (QDs) light-emitting diodes (QLEDs) requires the use of less hazardous cadmium-free quantum dots, among which ZnSe-based blue, InP-based green and red quantum dots have received considerable attention. In comparison, however, the development of InP-based green QLEDs is lagging behind.

In this regard, one member of this research team, Dr. Tseng from Unique Materials Co., Ltd, (Taipei, Taiwan), has been deeply involved in the QDs field for years and has developed green InP/ZnSe/ZnS quantum dots with a diameter of 8.6 nm, which consistently maintain their photoluminescence quantum yield (PLQY) of 86 ± 2 %. In a series of tests, we found that an InP QDs passivation method by addition of 1,4-butanediamine (BDA) and zinc iodides, is very efficient in harnessing carrier mobility and importantly there is no need to carry out further purification process upon modification.

In general, when metal oxides are used as electron transport materials, the mobility of electrons will be much faster than that of holes. Therefore, in order to achieve the balance of electron and the hole injection, the simplest way is to unidirectionally suppress the ability of electron injection, such as adjusting the ratio of Zn/Mg in ZnMgO, or adding a barrier layer between the electron injection layer and the light-emitting layer. It is worth noting that if appropriate materials can be found, the light-emitting layer materials can be modified to move the energy levels of QDs to appropriate positions, so as to achieve the goal of slowing down the injection rate of electrons and increasing the injection rate of the holes simultaneously. Eventually, the injection of the electrons/holes will tend to balance, enhancing the efficiency of exaction recombination.

In this study, after BDA passivation we have successfully reduced the mobility of electrons and increased the mobility of holes. The decrease in electron mobility is due to the movement of the vacuum energy level caused by the surface dipole, which leads to a mismatch between the conduction band and the electron transport layer. At the same time, the correlation between the current density and voltage of the hole device reveals that the hole injection capability of the BDA-modified InP GQD is slightly enhanced. Evidently, after BDA modification, the decrease of electron mobility as opposed to the increase of the hole mobility makes more balanced the electron and hole injection.

A further improvement can be achieved by adding zinc halide using ZnI2 as an additive after the BDA modification. The results show that the hole injection mobility has been further increased, while the change in electron mobility is not so significant. The use of ZnI2 has two advantages. On the one hand, it can be rationalized by filling vacancies with zinc iodide salt, further reducing the hole traps. On the other hand, the weak surface interaction as well as relatively small amount of the passivated iodide does not change the surface dipole and hence the band levels remain relatively unchanged. As a result, the maximum of external quantum efficiency (EQEmax) of the green 545 nm InP QLED is as high as 16.3%, and the current efficiency is 57.5 cd/A.

The industrialization of quantum dots (QDs) light-emitting diodes (QLEDs) requires the use of less hazardous cadmium-free quantum dots, among which ZnSe-based blue, InP-based green and red quantum dots have received considerable attention. In comparison, however, the development of InP-based green QLEDs is lagging behind.

In this regard, one member of this research team, Dr. Tseng from Unique Materials Co., Ltd, (Taipei, Taiwan), has been deeply involved in the QDs field for years and has developed green InP/ZnSe/ZnS quantum dots with a diameter of 8.6 nm, which consistently maintain their photoluminescence quantum yield (PLQY) of 86 ± 2 %. In a series of tests, we found that an InP QDs passivation method by addition of 1,4-butanediamine (BDA) and zinc iodides, is very efficient in harnessing carrier mobility and importantly there is no need to carry out further purification process upon modification.

In general, when metal oxides are used as electron transport materials, the mobility of electrons will be much faster than that of holes. Therefore, in order to achieve the balance of electron and the hole injection, the simplest way is to unidirectionally suppress the ability of electron injection, such as adjusting the ratio of Zn/Mg in ZnMgO, or adding a barrier layer between the electron injection layer and the light-emitting layer. It is worth noting that if appropriate materials can be found, the light-emitting layer materials can be modified to move the energy levels of QDs to appropriate positions, so as to achieve the goal of slowing down the injection rate of electrons and increasing the injection rate of the holes simultaneously. Eventually, the injection of the electrons/holes will tend to balance, enhancing the efficiency of exaction recombination.

In this study, after BDA passivation we have successfully reduced the mobility of electrons and increased the mobility of holes. The decrease in electron mobility is due to the movement of the vacuum energy level caused by the surface dipole, which leads to a mismatch between the conduction band and the electron transport layer. At the same time, the correlation between the current density and voltage of the hole device reveals that the hole injection capability of the BDA-modified InP GQD is slightly enhanced. Evidently, after BDA modification, the decrease of electron mobility as opposed to the increase of the hole mobility makes more balanced the electron and hole injection.

A further improvement can be achieved by adding zinc halide using ZnI2 as an additive after the BDA modification. The results show that the hole injection mobility has been further increased, while the change in electron mobility is not so significant. The use of ZnI2 has two advantages. On the one hand, it can be rationalized by filling vacancies with zinc iodide salt, further reducing the hole traps. On the other hand, the weak surface interaction as well as relatively small amount of the passivated iodide does not change the surface dipole and hence the band levels remain relatively unchanged. As a result, the maximum of external quantum efficiency (EQEmax) of the green 545 nm InP QLED is as high as 16.3%, and the current efficiency is 57.5 cd/A.

Figure 1. (a) The schematic diagram of InP GQDs passivated by the synergistic effect of BDA combined with zinc halides. (b) The comparison among as prepared, BDA modified, and BDA/ZnI2 modified InP GQD devices. (c) The illustration shows the relationship between the passivation process and the electron/hole balance that affects EQE. (d) The voltage-dependent electroluminescent spectra of the QLED with BDA/ZnI2 modification. Inset: photographs of the operating devices.

This work therefore provides a general guide, in which direct passivation of the green InP QDs can in principle improve the device efficiency without changing the device structure, and promote the green InP QLED to play a key role in full-color display.

 

These results were recently published in Communications Materials:

https://www.nature.com/articles/s43246-021-00203-5

Pi-Tai Chou

Professor, National Taiwan University