LIGHT EMITTING DIODE DEVICE USING CHARGE ACCUMULATION AND METHOD OF MANUFACTURING THE SAME

A light emitting device using charge accumulation and a method of manufacturing the light emitting device are provided. The light emitting device includes a substrate, a first electrode formed on the substrate, a hole transport layer formed on the first electrode, an electron transport layer formed on the hole transport layer, and a second electrode formed on the electron transport layer. A thickness of the hole transport layer may be greater than 20 nm and a thickness of the electron transport layer may be greater than 40 nm. A quantum dot (QD) layer may be disposed between the hole transport layer and the electron transport layer.

DETAILED DESCRIPTION

Hereinafter, a light emitting diode (LED) device using charge accumulation and a method of manufacturing the device according to an exemplary embodiment will be described in detail. In the process, thicknesses of layers or areas illustrated in the drawings are exaggerated for clarity of the present application.

FIG. 1illustrates absorptivity and photoluminescence spectra with respect to an electron transport layer of a quantum dot-LED (QD-LED) device, which is an example of a LED device according to an exemplary embodiment.

InFIG. 1, a graph shown with a reference figure □ represents an absorption spectrum, and a graph shown with a reference figure Δ represents a photoluminescence spectrum. A material of the electron transport layer that has such properties may be, for example, F8BT.

Referring toFIG. 1, a luminescent center of the electron transport layer is green.

FIG. 2illustrates absorptivity and photoluminescence spectra with respect to a hole transport of a QD-LED device, which is an example of an LED device according to an exemplary embodiment.

InFIG. 2, a graph shown with a reference figure □ represents an absorption spectrum, and a graph shown with a reference figure Δ represents a photoluminescence spectrum. A material of the hole transport layer that has such properties may be, for example, F8BT.

Referring toFIG. 2, a luminescent center of the hole transport layer is blue.

FIG. 3illustrates an absorptivity spectrum with respect to a QD layer of a QD-LED device according to an exemplary embodiment.

Referring toFIG. 3, an absorbing area of the QD layer is evenly distributed through the visible region.

The luminescent center of the electron transport layer is green as shown inFIG. 1, and the luminescent center of the hole transport layer is blue as shown inFIG. 2, thus green light and blue light respectively emitted from the electron transport layer and the hole transport layer may be absorbed by the QD layer as the electron transport layer and the hole transport layer are each disposed on both sides of the QD layer. In this regard, a quantum efficiency of the QD layer may be improved, and as a result, a luminescence efficiency of the QD-LED device may be improved.

FIG. 4schematically illustrates the processes above. That is, blue light30L emitted from a hole transport layer30and green light50L emitted from an electron transport layer50may be delivered to the QD layer40. The green and blue light50L and30L delivered to the QD layer40may be used as energy that excites the QD layer40, and accordingly, light40L of an another region of wavelengths may be emitted from the QD layer40.

FIG. 5illustrates charge accumulation according to an energy level difference between an electron transport layer and a hole transport layer in a QD-LED device according to an embodiment of the present invention.

InFIG. 5, a reference number50E represents an energy level of the electron transport layer50. A reference number30E represents an energy level of the hole transport layer30. A reference number40E represents an energy level of the QD layer40. Thus, the reference number50E may symbolically represent an electron transport layer, and the reference numbers30E and40E may each represent a hole transport layer and a QD layer.

As shown inFIG. 5, the energy level50E of the electron transport layer50is lower than the energy level30E of the hole transport layer30. When power is supplied to the electron transport layer50and the hole transport layer30according to the energy level difference, charges (electrons/holes) may be accumulated at an interface between the electron transport layer50and the hole transport layer30.

Referring toFIG. 5, electrons50N are accumulated on the electron transport layer50, and holes30P are accumulated on the hole transport layer30.

Such accumulation of charges may increase a coupling efficiency of the electrons50N and the holes30P, thus a luminescence efficiency of the light emitting device may be increased.

In addition, as shown inFIG. 5, because there is the QD layer40between the electron transport layer50and the hole transport layer30, a quantum efficiency according to the charges50N and30P accumulated at an interface of the electron transport layer50and the hole transport layer30may be increased. Also, a quantum efficiency of the QD layer40may be increased as the light emitted from the electron transport layer50and the hole transport layer30is delivered to the QD layer40. In other words, a quantum efficiency of the QD layer40may be increased due to the charges50N and30P accumulated at the interface of the electron transport layer50and the hole transport layer30, and due to forster resonant energy transfer (FRET) between an interface between the electron transport layer50and the QD layer40and an interface between the hole transport layer30and the QD layer40.

FIG. 6illustrates an electrical field E formed inside the device due to an increase in charge density as the charges accumulate in the QD-LED device according to an exemplary embodiment.

Due to the electrical field E, a drift velocity (v) of the charges increases, and thus electron mobility increases. Due to the increase in electron mobility, an intensity of turn-on voltage is relatively reduced.

Therefore, in a QD-LED device according to an exemplary embodiment, although an overall thickness of the device increases due to an increase in a thickness of an electron transport layer and/or a hole transport layer, a turn-on voltage of the device may be relatively reduced. This may be confirmed by the results of experiments that will be described later (FIG. 9).

FIG. 7illustrates a QD-LED device as an example of the LED device according to an exemplary embodiment.

Referring toFIG. 7, the hole transport layer30is on an anode20. The anode20may be, for example, a transparent electrode such as indium tin oxide (ITO). The QD layer40is on the hole transport layer30. The electron transport layer50is on the QD layer40. The QD layer40may be, for example, a CdSe layer. A cathode60is on the electron transport layer50. The cathode60may be, for example, a Ca/Al electrode. The electron transport layer30may be a material layer having a high hole mobility and luminescent properties which may be, for example, a poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB) layer. Also, the hole transport layer30may be a QD layer or a monomolecular layer. The electron transport layer50is a material layer having a high hole mobility and luminescent properties which may be, for example, a poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT) layer. Moreover, the electron transport layer50may be a QD layer or a monomolecular layer.

The hole transport layer30has a first thickness t1. The first thickness t1may be greater than a thickness of a conventional hole transport layer. For example, if a previously known thickness of a hole transport layer is 20 nm, the first thickness of the hole transport layer30may be greater than 20 nm. A range of the thickness of the hole transport layer30may be, for example, from about 60 nm to about 300 nm. Here, a second thickness t2of the electron transport layer50may be equal to or greater than a thickness of a conventional electron transport layer.

Meanwhile, the second thickness t2of the electron transport layer50may be greater than a thickness of a conventional electron transport layer. For example, if a previously known thickness of an electron transport layer is 40 nm, the second thickness of the electron transport layer50may be greater than 40 nm. A range of the thickness of the electron transport layer30may be, for example, from about 60 nm to about 300 nm. Here, the first thickness t1of the hole transport layer30may be equal to or greater than a thickness of a conventional hole transport layer.

A thickness of the QD layer40may be in a range of, for example, about 20 nm to about 40 nm.

When the first thickness t1of the hole transport layer30and the second thickness t2of the electron transport layer50are in the corresponding ranges above, the hole transport layer30, the QD layer40, and the electron transport layer50may form a micro optical cavity70. When the cavity70is used, a particular wavelengths may be selectively emitted from the QD layer40due to the resonance characteristics of the cavity70, and thus an intensity of the light of the particular wavelengths may be increased. Also, by including the cavity70, the QD-LED device according to an exemplary embodiment may become a QD-laser. In this case, the QD layer40is formed of a monolayer and may be designed in a photonic crystal structure.

The device illustrated inFIG. 7may be manufactured by sequentially forming the anode20, the hole transport layer30, the QD layer40, the electron transport layer50, and the cathode60on a substrate (not shown). If necessary, a patterning process may be included. Each layer may be formed using a conventionally known method. Here, the electron transport layer50and the hole transport layer30may be formed with the thicknesses within the ranges stated above.

Meanwhile, as shown inFIG. 8, an LED device may include the electron transport layer50and the hole transport layer30without the QD layer40. The device ofFIG. 8may be an OLED. The ranges of the thickness of the electron transport layer50and the hole transport layer30may be same with the corresponding thicknesses ofFIG. 7.

The device ofFIG. 8may be manufactured by sequentially forming the anode20, the hole transport layer30, the electron transport layer50, and the cathode60on a substrate (not shown).

FIG. 9is a table summarizing the results of experiments showing an increase in luminescence efficiency and a relative decrease in driving voltage according to increases in thicknesses of the electron transport layer and the hole transport layer in the light emitting device ofFIGS. 7 and 8. In the experiments, a thickness of the electron transport layer50is increased from 80 nm to 125 nm, and a thickness of the hole transport layer30is increased from 40 nm to 140 nm.

Referring toFIG. 9,FIG. 9shows charge accumulation according to the thickness increase of the hole transport layer30and the electron transport layer50and that quantum dot efficiency due to the charge accumulation is increased.

Particularly, a luminescence efficiency of the light emitting device is increased from 5.5 cd/A to 35 cd/A as the thicknesses of the hole transport layer (or TFB)30and the electron transport layer (or F8BT)50are increased. Also, the luminescence efficiency is rapidly increased from 4 lm/W to 23 lm/W. Such results are caused by an increase in luminance of the device from 28,500 cd/m2 to 51,200 cd/m2 regardless of the decrease of current density due to the increase in the thicknesses of the hole transport layer30and the electron transport layer50.

InFIG. 9, the voltages in the parentheses indicate voltages applied to obtain the results of the corresponding items. When the thicknesses of the hole transport layer30and the electron transport layer50are increased, each voltage value increases but is much less value than an increased value of the predicted voltage in consideration of the increased thicknesses of the hole transport layer30and the electron transport layer50. For example, a driving voltage is predicted to be increased from 2.6 V to 8 V as the thicknesses increased as above, but the actual measured value was about 4.2 V. The results may be interpreted as it is caused by the electrical field E formed inside the device according to charge accumulation.

Thus, it has been confirmed through the results from the experiments that a driving voltage with respect to an increase of the thickness may be relatively reduced while charge accumulation may be increased and accordingly a luminescence efficiency may be increased by increasing thicknesses of the hole transport layer30and the electron transport layer50.

Next, an experiment to prove that quantum dots generated in each layer are delivered to the QD layer40as the thicknesses of the electron transport layer50and the hole transport layer30are increased will be explained hereinafter.

In the current experiment, the QD layer40was interposed between the interfaces of the electron transport layer50and the hole transport layer30. Here, a thickness of the hole transport layer50was fixed to be 75 nm, and a thickness of the QD layer40was fixed to be 20 nm.

As the results of the current experiment,FIG. 10illustrates a change in the current density according to the change in thickness of the hole transport layer30.FIG. 11illustrates a change in luminance, andFIG. 12illustrates a change in a luminescence spectrum.

InFIG. 10, a horizontal axis shows voltages, and a vertical axis shows current densities. InFIG. 10, a first graph G1represents the results of the hole transport layer30with a thickness of 25 nm, and a second graph G2represents the results of the hole transport layer30with a thickness of 50 nm. Also, third to fifth graphs G3to G5each represents the hole transport layer30with a thickness of 75 nm, 100 nm, and 140 nm, respectively.

Referring toFIG. 10, as a thickness of the hole transport layer30increases, a current density is reduced.

InFIG. 11, a horizontal axis shows voltages, and a vertical axis shows luminance.

InFIG. 11, a first graph G11represents change in luminances when the hole transport layer30has a thickness of 25 nm. Also, second to fifth graphs G12to G15each represents change in luminances when the hole transport layer30has a thickness of 50 nm, 75 nm, 100 nm, and 140 nm, respectively.

Referring toFIG. 11, as a thickness of the hole transport layer30is increased to 75 nm, an overall luminance first increased and then reduced later. Such result is interpreted as it is caused by reduction of quantum dot generation due to charges/holes density imbalance.

InFIG. 12, a first graph G21represents luminescence spectrum change when the hole transport layer30has a thickness of 25 nm. Also, second to fifth graphs G22to G25represent the results of luminescence spectrum change when the hole transport layer30has thicknesses of 50 nm, 75 nm, 100 nm, and 140 nm, respectively.

Referring toFIG. 12, as a thickness of the hole transport layer30is increased to 75 nm, a luminescence spectrum with a center of the QD layer40may appear, but when the thickness of the hole transport layer30exceeds 75 nm, green light, center light of the electron transport layer50, was observed.

Next, the results of luminescent intensity amplification experiments as the hole transport layer30, the electron transport layer50, and the QD layer40form the micro optical cavity70will now be explained.FIGS. 13 and 14illustrate the results of the experiments.

FIG. 13illustrates an electroluminescence (EL) spectrum change according to formation of the micro optical cavity when only a thickness of the electron transport layer50was changed.

InFIG. 13, a first graph G31represents the results of the electron transport layer50with a thickness of 25 nm. Also, second to fourth graphs G32to G34represent the electron transport layer50with thicknesses of 55 nm, 215 nm, and 245 nm, respectively.

Referring toFIG. 13, as a thickness of the electron transport layer50increases, an intensity of a range of wavelengths for yellow is increased.

FIG. 14illustrates an electroluminescence (EL) spectrum change according to formation of the micro optical cavity when only a thickness of the hole transport layer30is changed.

InFIG. 14, a first graph represents the results of the hole transport layer30with a thickness of 25 nm. Also, second to fourth graphs G42to G44represent the hole transport layer30with thicknesses of 75 nm, 110 nm, and 135 nm, respectively.

Referring toFIG. 14, as a thickness of the hole transport layer30increases, an intensity of a range of wavelengths for blue is increased.

As described above, according to the one or more of the above-described exemplary embodiments, it may be known that a quantum efficiency of a device may be additionally increased as an intensity of a specific wavelength may be increased with an formation of a micro optical cavity70by increasing thicknesses of a hole transport layer30and an electron transport layer50of an OLED device.