Light-emitting element and display device

Abstract

A light-emitting element (1) comprises: an anode (11) and a cathode (15); a light-emitting layer (13) positioned between the anode (11) and the cathode (15) and including quantum dots (131); and an electron transport layer (14) positioned between the light-emitting layer (13) and the cathode (15). The electron transport layer (14) includes first semiconductor particles (141) and second semiconductor particles (142) of a conduction type different from the first semiconductor particles (141).

Description

Light-emitting element and display device

[0001] The following disclosure relates to light-emitting devices that include quantum dots.

[0002] The following Patent Document 1 discloses an example of the configuration of an electron transport layer for improving the performance of a light-emitting device including quantum dots.

[0003] US Patent Application Publication No. 2019 / 0097151

[0004] An object of one embodiment of the present disclosure is to improve the performance of a light-emitting element by using a configuration different from conventional ones.

[0005] A light-emitting element according to one aspect of the present disclosure includes an anode and a cathode, a light-emitting layer located between the anode and the cathode and including quantum dots, and an electron transport layer located between the light-emitting layer and the cathode, wherein the electron transport layer includes first semiconductor particles and second semiconductor particles of a different conductivity type from the first semiconductor particles.

[0006] According to one aspect of the present disclosure, the performance of a light-emitting device can be improved by using a configuration different from conventional configurations.

[0007] 1 shows an example of the configuration of a light-emitting element according to embodiment 1; 2 shows an example of an energy band diagram for a conventional light-emitting element; 3 shows an example of the EQE characteristics of a conventional light-emitting element; 4 shows a schematic diagram of electron transport in the ETL and its periphery of a conventional light-emitting element; 5 shows an example of diode characteristics of a conventional light-emitting element; 6 shows an example of current distribution in a conventional light-emitting element; 7 shows a schematic diagram of the relationship between first semiconductor particles and second semiconductor particles in a light-emitting element according to embodiment 1; 8 shows a schematic diagram of the energy band structure formed by first semiconductor particles and second semiconductor particles in a contact state and the relationship between voltage bias; 9 shows a schematic diagram of electron transport in the ETL and its periphery of a light-emitting element according to embodiment 1; 10 shows an example of diode characteristics of the light-emitting element according to embodiment 1; 11 shows an example of EQE characteristics of the light-emitting element according to embodiment 1; 12 shows an example of the relationship between Vp2 and current allowable ratio in embodiment 2; 13 shows an example of the configuration of a display device according to embodiment 3.

[0008] [Embodiment 1] Embodiment 1 will be described below. For convenience of explanation, components having the same functions as those described in embodiment 1 will be denoted by the same reference numerals in the following embodiments, and their descriptions will not be repeated. For simplicity, descriptions of well-known technical matters will be omitted as appropriate. Unless otherwise specified, the components, materials, and numerical values ​​described in this specification are merely examples. Therefore, for example, unless otherwise specified, the positional relationship of each component is not limited to the example in each figure. Furthermore, each figure is not necessarily drawn to scale.

[0009] (Configuration example of light-emitting element 1) Fig. 1 shows a configuration example of the light-emitting element 1 in embodiment 1. Fig. 1 schematically shows the layered structure of the light-emitting element 1. In the example of Fig. 1, the light-emitting element 1 includes an anode 11, a hole transport layer 12, a light-emitting layer 13, an electron transport layer 14, and a cathode 15, in this order from the bottom.

[0010] Each part of the light-emitting element 1 is supported by a substrate (not shown). Therefore, in the example of Fig. 1, the distance between the anode 11 and the substrate is smaller than the distance between the cathode 15 and the substrate. That is, in the example of Fig. 1, the anode 11 is the lower electrode and the cathode 15 is the upper electrode. However, unlike the example of Fig. 1, the anode 11 may be the upper electrode and the cathode 15 may be the lower electrode.

[0011] The light-emitting element 1 is configured so that light emitted from the light-emitting layer 13 can be extracted to the outside of the light-emitting element 1. Therefore, in the light-emitting element 1, at least one of the anode 11 and the cathode 15 may be a light-transmitting electrode. Either the anode 11 or the cathode 15 may be a light-reflecting electrode.

[0012] The light-emitting layer 13 may be located between the anode 11 and the cathode 15. In the example of Fig. 1, the light-emitting layer 13 is located between the hole-transporting layer 12 and the electron-transporting layer 14. In this specification, the light-emitting layer may be referred to as "EML."

[0013] The light-emitting layer 13 includes quantum dots 131. The quantum dots 131 in FIG. 1 are an example of an electroluminescent (EL) material. In this specification, quantum dots may be abbreviated as "QD." The quantum dots 131 emit EL light upon recombination of holes supplied from the anode 11 and electrons transported from the cathode 15. Therefore, EL light can be generated in the light-emitting layer 13 by applying a forward voltage between the anode 11 and the cathode 15.

[0014] The electron transport layer 14 may be located between the light-emitting layer 13 and the cathode 15. In this specification, the electron transport layer may be referred to as "ETL." The ETL may contain any electron transport material.

[0015] In the example of FIG. 1, the electron transport layer 14 includes (i) first semiconductor particles 141 and (ii) second semiconductor particles 142 of a different conductivity type from the first semiconductor particles 141 .

[0016] For example, one of the first semiconductor particles 141 and the second semiconductor particles 142 may be n-type semiconductor particles, and the other may be p-type semiconductor particles. In embodiment 1, a case where the first semiconductor particles 141 are n-type semiconductor particles, and the second semiconductor particles 142 are p-type semiconductor particles is exemplified.

[0017] At least one of the first semiconductor particles 141 and the second semiconductor particles 142 may be II-VI group semiconductor compound particles. Examples of the II-VI group semiconductor compound particles include ZnO particles, MgO particles, and ZnMgO particles.

[0018] The first semiconductor particles 141 and the second semiconductor particles 142 may be II-VI group semiconductor compound particles of the same material. Therefore, in embodiment 1, a case where the first semiconductor particles 141 are n-type ZnO particles and the second semiconductor particles 142 are p-type ZnO particles is exemplified.

[0019] For example, p-type semiconductor particles according to one embodiment of the present disclosure may be II-VI compound semiconductors and may contain a Group V element. The Group V element is an example of an acceptor in the p-type semiconductor particles. Examples of Group V elements according to one embodiment of the present disclosure include N, As, P, and Sb. Preferably, the p-type semiconductor particles contain N as the Group V element (i.e., as an acceptor).

[0020] For example, the acceptor concentration in the p-type semiconductor particles according to one embodiment of the present disclosure is 10 １６ cm －３ Greater than or equal to 10 １９ cm －３ Therefore, as an example, the concentration of N in the p-type semiconductor particles may be 10 １６ cm －３ Greater than or equal to 10 １９ cm －３ The following is the result.

[0021] The hole transport layer 12 may be located between the light-emitting layer 13 and the anode 11. In this specification, the hole transport layer may be referred to as "HTL." The HTL may include any hole transport material.

[0022] (Example of Manufacturing Method of Electron Transport Layer 14) Each part of the light-emitting device 1 shown in Fig. 1 may be manufactured by a known method, except for the electron transport layer 14. The electron transport layer 14 may be manufactured (deposited) by, for example, the following procedure.

[0023] First, a colloidal solution is prepared in which n-type semiconductor particles and p-type semiconductor particles are uniformly mixed, with the mixing ratio of the p-type semiconductor particles in the colloidal solution being set to, for example, 50% by volume.

[0024] As an example, the average particle size of each of the n-type semiconductor particles and the p-type semiconductor particles may be set to about 10 nm. In this case, the deviation in the particle size of each of the n-type semiconductor particles and the p-type semiconductor particles can be kept within about 2 nm. With such average particle sizes and deviations, a colloidal solution in which the n-type semiconductor particles and the p-type semiconductor particles are uniformly mixed can be prepared.

[0025] The colloidal solution can be applied onto the light-emitting layer 13 to obtain the electron transport layer 14 located on the light-emitting layer 13. As described above, by producing the electron transport layer 14 using the colloidal solution, it is possible to obtain the electron transport layer 14 in which n-type semiconductor particles and p-type semiconductor particles are uniformly mixed.

[0026] The mixed state of the semiconductor particles (e.g., the mixed state of n-type semiconductor particles and p-type semiconductor particles) inside the electron transport layer 14 may be observed by any method. For example, the mixed state may be observed by performing TEM-EDX analysis (energy dispersive X-ray spectroscopy analysis using a transmission electron microscope) on the light-emitting element 1.

[0027] As another example, it is also possible to use a combination of a transmission electron microscope (TEM) and dynamic ion mass spectrometry (SIMS) of a cross section of the light-emitting element 1. By measuring the depth profile of the element concentration distribution from the surface of the light-emitting element 1 using dynamic SIMS, the mixed state of semiconductor particles inside the electron transport layer 14 can be evaluated.

[0028] As an example, the case where the semiconductor particles contained in the electron transport layer 14 are ZnO particles will be described. In this case, 10 ２３ cm －３ Zn and O were detected at concentrations of about 10 １６ cm －３ Greater than or equal to 10 １９ cm －３ In the region where the concentration of N is detected below 10 ２３ cm －３ Zn was detected at a concentration of about 10 ２３ cm －３ In the region where O concentrations below about 1000 nm are detected, n-type semiconductor particles are present.

[0029] Generally, the resolution in the depth direction of dynamic SIMS is on the order of 1 nm, so the distribution of n-type and p-type semiconductor particles inside the electron transport layer 14 can be determined by plotting the concentration of each atom in the thickness direction of the electron transport layer 14.

[0030] Regarding the range for observing the mixed state of semiconductor particles inside the electron transport layer 14, since the semiconductor particles are randomly distributed inside the electron transport layer 14, it is not necessary to observe the entire electron transport layer 14. The mixed state of semiconductor particles inside the electron transport layer 14 may be evaluated by observing a portion of the electron transport layer 14. Specifically, the electron transport layer 14 is divided into multiple locations and observed, and the analysis results obtained at the multiple locations are averaged to evaluate the mixed state of semiconductor particles. For example, the observation field at one location is a range in which approximately 50 semiconductor particles can be observed. In this case, observation is performed not just at one location, but at, for example, 10 locations (in this case, the total number of semiconductor particles to be observed is approximately 500).

[0031] (Regarding the conventional light-emitting element) Before describing the effects of the light-emitting element 1, a conventional light-emitting element will be described as a comparative example. In this specification, the conventional light-emitting element as a comparative example will be referred to as the light-emitting element 1r. The light-emitting element 1r is an example of a conventional QLED (Quantum Light Emitting Diode).

[0032] In this specification, the electron transport layer of the light-emitting element 1r is referred to as the electron transport layer 14r. Unlike the electron transport layer 14, the electron transport layer 14r does not contain both the first semiconductor particles 141 and the second semiconductor particles 142. In embodiment 1, a case is illustrated in which the electron transport layer 14 contains only the first semiconductor particles 141 (i.e., only n-type semiconductor particles) as semiconductor particles.

[0033] 2 shows an example of an energy band diagram of the light-emitting element 1r. As shown in FIG. 2, holes travel from the anode to the EML via the HTL, while electrons travel from the anode to the EML via the ETL.

[0034] As is known to those skilled in the art, the effective mass of electrons is much smaller than that of holes. Specifically, the effective mass of electrons is 1 / 10 or less of that of holes. This allows electrons to easily overcome the injection barrier and reach the EML compared to holes. Therefore, in the light-emitting device 1r, excessive electron injection into the QDs in the EML is likely to occur.

[0035] 3 shows an example of the EQE (External Quantum Efficiency) characteristics of the light-emitting element 1r. In the graph of FIG. 3, the horizontal axis represents current density, and the vertical axis represents normalized EQE. The normalized EQE is a quantity obtained by normalizing the EQE by the maximum value of the EQE. Therefore, the maximum value of the normalized EQE is 1.

[0036] Elements A to D in Fig. 3 are examples of conventional light-emitting element 1r. As shown in Fig. 3, in all of elements A to D, the maximum value of the normalized EQE, i.e., the peak of the EQE, is obtained at a fairly low current density. This indicates that in light-emitting element 1r, the peak of the EQE is obtained when the amount of electron injection is not very large (low injection state).

[0037] However, as shown in Figure 3, in all of the elements A to D, in the current density range higher than the current density at which the maximum normalized EQE occurs, the normalized EQE drops sharply with increasing current density. This characteristic of the light-emitting element 1r is also called roll-off. The roll-off in the example of Figure 3 indicates that the EQE of the light-emitting element 1r drops significantly in the current density range where excessive electron injection occurs.

[0038] FIG. 4 is a schematic diagram showing electron transport in the ETL (electron transport layer 14r) of the light-emitting element 1r and its periphery. In the example of FIG. 4, the anode and cathode are forward biased. In the light-emitting element 1r shown in FIG. 4, there is no component in the ETL that blocks electrons from the cathode toward the EML. Therefore, as described above, excessive electrons are likely to be injected into the QDs in the EML in the light-emitting element 1r.

[0039] As described above, excessive electron injection into the QDs in the EML results in a significant decrease in EQE. The decrease in EQE can, for example, cause overheating of the light-emitting element 1r, thereby reducing the reliability of the light-emitting element 1r. Therefore, some measures can be considered to prevent excessive electron injection into the light-emitting element 1r.

[0040] As an example, in the light-emitting element 1r, n-type semiconductor particles having a very deep conduction band minimum (CBM) can be used as the first semiconductor particles 141 in the ETL. In this case, the electron injection barrier can be increased, and excessive electron injection into the QDs can be prevented. However, this has the disadvantage of significantly increasing the driving voltage of the light-emitting element 1r.

[0041] As another example, in the light-emitting element 1r, it is possible to form electron traps by injecting heavy ions into the ETL. In this case, the electron traps can prevent excessive electron injection into the QDs. However, as described below, the formation of electron traps causes degradation of the diode characteristics of the light-emitting element 1r.

[0042] 5 illustrates the diode characteristics of the light-emitting element 1r. In the graph of FIG. 5, the horizontal axis represents the voltage V applied to the light-emitting element 1r (more specifically, the voltage between the anode and cathode). The vertical axis represents the current I flowing through the light-emitting element 1r. Ｄ (so-called diode current).

[0043] As known to those skilled in the art, the VI of a diode Ｄ The characteristics are expressed by the following formula (1): ...(1) Equation (1) is also called the diode rectification equation. In equation (1), I ０ is a constant determined by the state of the semiconductor layer in the diode, etc. q is the unit charge, k is the Boltzmann constant, and T is the absolute temperature.

[0044] In formula (1), n ​​is a constant also called an ideality coefficient. n takes a value of 1 or more, and in particular, in a light-emitting element, n is 2 or more because a recombination current flows in the light-emitting layer. Note that in a diode as an electronic device, n is 1 or more because a diffusion current occurs. n is determined by the state of the pn junction in the diode. For this reason, n can be used as an evaluation index indicating the quality of the pn junction. In an ideal diode, n = 1. On the other hand, as the quality of the pn junction decreases, n increases. For example, as the crystallinity of the pn junction interface in a diode deteriorates, n increases.

[0045] 5 shows an example of the diode characteristics of the light-emitting element 1r when no electron traps are formed. In contrast, graphs 520 and 530 show examples of the diode characteristics of the light-emitting element 1r when electron traps are formed. In the example of graph 530, more electron traps are formed than in the example of graph 520.

[0046] As can be seen from equation (1), n ​​is the slope of the diode characteristic at a certain voltage V, i.e., dI at that V Ｄ In the example of Fig. 5, the slope at V = 4 [V (Volt)] is indicated by a dotted line.

[0047] As can be seen from equation (1), dI Ｄ 5, at the point 4V, the slope of graph 520 is smaller than the slope of graph 510, and the slope of graph 530 is smaller than the slope of graph 520. This means that the more electron traps are formed, the lower n becomes.

[0048] 6 is a schematic diagram showing the distribution of current in the light-emitting element 1r. The "total current" in FIG. 6 is the current I Ｄ Reference numeral 610 in FIG. 6 corresponds to the example of graph 510 described above. As shown by reference numeral 610, I Ｄ Part of the flow goes to QD, and part of the flow goes to I Ｄ The other parts of I flow to each part other than QD. Ｄ The portion of the current flowing through the QD is called the current of interest. Ｄ The portion of the current excluding the current of interest is called the residual current.

[0049] 6 corresponds to the example of the graph 520 or the graph 530. As can be understood from the above equation (1), I Ｄ decreases as n increases, so the total current is smaller in the example 620 than in the example 610.

[0050] 5, in the light-emitting element 1r, the quality of the pn junction in the diode is reduced due to the formation of electron traps. Therefore, the ratio of the current of interest to the residual current in the example indicated by reference numeral 620 is smaller than the ratio of the current of interest to the residual current in the example indicated by reference numeral 610. In other words, in the light-emitting element 1r, when the total current is reduced, the rate of reduction of the current of interest is greater than the rate of reduction of the total current. Therefore, in the light-emitting element 1r, the above-mentioned significant reduction in EQE occurs.

[0051] (Further Description of Light-Emitting Element 1) To address the above-described problems with the light-emitting element 1r, the inventors of the present application (hereinafter abbreviated as "the inventors") created a novel light-emitting element 1 that is different from the light-emitting element 1r. As described above, the ETL (electron transport layer 14) of the light-emitting element 1 differs from the ETL (electron transport layer 14r) of the light-emitting element 1r in that it further contains p-type semiconductor particles in addition to n-type semiconductor particles.

[0052] 7 schematically shows the relationship between first semiconductor particles 141 as n-type semiconductor particles and second semiconductor particles 142 as p-type semiconductor particles in the light-emitting element 1. In the example indicated by reference numeral 710, the first semiconductor particles 141 and the second semiconductor particles 142 are spaced apart from each other. In this case, the first semiconductor particles 141 and the second semiconductor particles 142 each have an individual energy band.

[0053] In the example indicated by reference numeral 720, unlike the example indicated by reference numeral 710, the first semiconductor particles 141 and the second semiconductor particles 142 are in contact with each other. The contact between the first semiconductor particles 141 and the second semiconductor particles 142 corresponds to a pn junction in a general diode. Therefore, the first semiconductor particles 141 and the second semiconductor particles 142 in contact with each other form an energy band structure similar to that of a pn junction.

[0054] For ease of explanation, the example indicated by the reference numeral 720 shows a case where the distance between the first semiconductor particles 141 and the second semiconductor particles 142 is 0. However, it should be noted that in one aspect of the present disclosure, "the first semiconductor particles 141 and the second semiconductor particles 142 are in contact" does not necessarily mean "the distance between the first semiconductor particles 141 and the second semiconductor particles 142 is 0."

[0055] For example, even if the distance between adjacent first semiconductor particles 141 and second semiconductor particles 142 is non-zero, the first semiconductor particles 141 and second semiconductor particles 142 can be considered to be in contact as long as they form an energy band structure similar to that of the example indicated by reference numeral 720. In other words, the first semiconductor particles 141 and the second semiconductor particles 142 only need to be in contact or adjacent to each other so as to form the energy band structure. As an example, when the distance between adjacent first semiconductor particles 141 and second semiconductor particles 142 is 3 nm or less, the first semiconductor particles 141 and second semiconductor particles 142 can be considered to be in contact.

[0056] 8 schematically shows the relationship between the energy band structure formed by the first semiconductor particles 141 and the second semiconductor particles 142 that are in contact with each other and the voltage bias. In the example indicated by the reference numeral 810, a forward bias voltage is applied to the energy band structure. In this case, the electron injection barrier in the energy band structure is lowered, and electrons are more easily transported from the first semiconductor particles 141 to the second semiconductor particles 142.

[0057] On the other hand, in the example indicated by the reference numeral 820, a reverse bias voltage is applied to the energy band structure. In this case, the electron injection barrier in the energy band structure becomes high, making it difficult for electrons to be transported from the first semiconductor particles 141 to the second semiconductor particles 142.

[0058] FIG. 9 schematically shows electron transport in the ETL (electron transport layer 14) of the light-emitting element 1 and its periphery. FIG. 9 is a diagram paired with the above-mentioned FIG. 4. In the example of FIG. 9, the anode and cathode are also forward biased. In the ETL of the light-emitting element 1 in the example of FIG. 9, first semiconductor particles 141 and second semiconductor particles 142 are randomly distributed. The plurality of particles are in contact with each other.

[0059] 9, when the anode and cathode are forward biased, the energy band structure in the forward bias state (see reference numeral 810 in FIG. 8) and the energy band structure in the reverse bias state (see reference numeral 820 in FIG. 8) exist randomly. As a result, electrons traveling from the cathode to the EML can be blocked at positions in the thickness direction of the ETL where the energy band structure in the reverse bias state exists.

[0060] Thus, unlike the prior art, the light-emitting device 1 can prevent excessive electron injection into QDs without forming electron traps. The light-emitting device 1 can also prevent excessive electron injection into QDs without using n-type semiconductor particles with a very deep CBM.

[0061] FIG. 10 illustrates the diode characteristics of the light-emitting element 1. FIG. 10 is a diagram paired with the above-mentioned FIG. 5. For ease of comparison with FIG. 5, FIG. 10 again shows graph 510 from FIG. 5. Graphs 1010 and 1020 in FIG. 10 are each examples of the diode characteristics of the light-emitting element 1. In the example of graph 1020, the content of p-type semiconductor particles in the ETL is higher than in the example of graph 1010. On the other hand, in the example of graph 510, the content of p-type semiconductor particles in the ETL is 0.

[0062] In the example of Fig. 10, unlike the example of Fig. 5, no electron traps are formed. Therefore, in the example of Fig. 10, unlike the example of Fig. 5, the slopes of the diode characteristics at 4 V are almost the same in all of the graphs 510, 1010, and 1020. The example of Fig. 10 shows the decrease in I due to the increase in the content of p-type semiconductor particles in the ETL. Ｄ The decrease in does not correspond to an increase in n in the above formula (1), but to an increase in I in the formula (1). ０ This shows that the decrease corresponds to the decrease in

[0063] In this way, the inventors "keep n constant while ０ By reducing ＤThe light-emitting element 1 is created based on this novel idea.

[0064] FIG. 11 is a diagram that schematically illustrates current distribution in the light-emitting element 1. FIG. 11 is a diagram that is paired with FIG. 6. For ease of comparison with FIG. 6, FIG. 11 shows the example of reference numeral 610 in FIG. 6 again. Reference numeral 1120 in FIG. 11 corresponds to the example of the graph 1010 or the graph 1020 described above. As described above, I ０ With the decrease of I Ｄ Therefore, the total current is smaller in the example 1120 than in the example 610.

[0065] However, as described above, unlike the light-emitting element 1r, no electron traps are formed in the light-emitting element 1. Therefore, unlike the light-emitting element 1r, the quality of the pn junction in the diode does not decrease with the decrease in total current in the light-emitting element 1. Therefore, the ratio of the current of interest to the residual current in the example of reference numeral 1120 is approximately equal (ideally, completely equal) to the ratio of the current of interest to the residual current in the example of reference numeral 610.

[0066] Thus, in the light-emitting element 1, unlike the light-emitting element 1r, the rate of decrease in the current of interest when the total current is decreased is approximately the same as the rate of decrease in the total current. As described above, the light-emitting element 1 can prevent excessive injection of electrons into the QDs in the EML by a method different from that of the light-emitting element 1r. As a result, the light-emitting element 1 can prevent the significant decrease in EQE that occurred in the light-emitting element 1r.

[0067] The inventors derived the respective characteristics of the light-emitting element 1 and the conventional light-emitting element in order to demonstrate the superiority of the light-emitting element 1 over the conventional light-emitting element. FIGS. 12 and 13 respectively show examples of the characteristics of the light-emitting elements derived by the inventors. The legend "Example" in FIGS. 12 and 13 represents the light-emitting element 1, and the legend "Comparative Example" represents the conventional light-emitting element. In the comparative example in FIGS. 12 and 13, it is assumed that no electron traps are formed.

[0068] 12 shows examples of diode characteristics of the example and the comparative example. In FIG. 12, the vertical axis of the graph 1210 is represented on a logarithmic scale, while the vertical axis of the graph 1220 is represented on a linear scale.

[0069] As shown in FIG. 12, for all V, I in the example Ｄ is I in the comparative example Ｄ This indicates that the light-emitting device 1 can effectively prevent excessive injection of electrons into the QDs in the EML.

[0070] 13 shows examples of EQE characteristics of an example and a comparative example. In FIG. 13, the vertical axis of the graph 1310 indicates EQE. On the other hand, the vertical axis of the graph 1320 indicates normalized EQE. The normalized EQE at 1320 is obtained by normalizing the EQE at 1310 by the maximum value of the EQE.

[0071] As shown in the graph 1310, the light-emitting element 1 can effectively improve the EQE compared to the conventional light-emitting element. In addition, as shown in the graph 1320, the light-emitting element 1 can also improve the roll-off compared to the conventional light-emitting element.

[0072] 13, the current density at which the EQE peak is obtained in the example is almost the same as the current density at which the EQE peak is obtained in the comparative example, which indicates that in the light-emitting element 1, when the total current is reduced, the rate of decrease in the current of interest is approximately the same as the rate of decrease in the total current (see also FIG. 11 above).

[0073] As described above, according to the first embodiment, it is possible to improve the performance of the light-emitting element by using a configuration different from the conventional configuration.

[0074] (Supplementary Note) The above-mentioned Patent Document 1 discloses the idea of ​​improving the efficiency of a light-emitting device by providing an ETL containing multiple types of inorganic particles having different average particle sizes. However, Patent Document 1 does not mention the idea of ​​providing an ETL containing multiple types of semiconductor particles having different conductivity types. Thus, the light-emitting device 1 is created based on a technical concept that is completely different from the light-emitting device of Patent Document 1.

[0075] [Embodiment 2] As described in embodiment 1, as the content of p-type semiconductor particles in the ETL of the light-emitting element 1 increases, I Ｄ The inventors have further investigated this tendency.

[0076] In the second embodiment, as in the first embodiment, the first semiconductor particles 141 and the second semiconductor particles 142 are made of the same material, and the particle sizes and particle size distributions of the first semiconductor particles 141 and the second semiconductor particles 142 are equivalent. In this case, the volume density of each of the first semiconductor particles 141 and the second semiconductor particles 142 in the ETL can be considered to be the same value as the weight density of each of the first semiconductor particles 141 and the second semiconductor particles 142 in the ETL.

[0077] In this specification, the volume density of the first semiconductor particles 141 in the ETL is denoted as Vp1, and the volume density of the second semiconductor particles 142 in the ETL is denoted as Vp2. As can be understood from the above explanation, in the example of embodiment 2, Vp1 and Vp2 can be defined as follows, respectively: Vp1 = wp1 / (wp1 + wp2) ... (2) Vp2 = wp2 / (wp1 + wp2) ... (3) where wp1 represents the total weight of the first semiconductor particles 141 contained in the ETL, and wp2 represents the total weight of the second semiconductor particles 142 contained in the ETL. The right-hand side of formula (2) represents the weight density of the first semiconductor particles 141 in the ETL. The right-hand side of formula (3) represents the weight density of the second semiconductor particles 142 in the ETL.

[0078] Next, the inventors calculated the I at 4V point at a certain Vp2. Ｄ, the I at point 2V of the graph 510 described above. Ｄ The value obtained by dividing by the value of I at the 2V point in the graph 510 was determined as the current allowance rate at Vp2. Ｄ is an example of a current that is expected to cause a rise in luminance in a conventional light-emitting element.

[0079] The inventors have investigated the relationship between Vp2 and the current allowable ratio when the first semiconductor particles 141 are n-type semiconductor particles (more specifically, n-type ZnO particles) and the second semiconductor particles 142 are p-type semiconductor particles (more specifically, p-type ZnO particles), as in the first embodiment. Specifically, the inventors have derived a graph showing the relationship between Vp2 and the current allowable ratio through simulation. Fig. 14 is an example of the graph.

[0080] In Fig. 14, the horizontal axis represents Vp2, and the vertical axis represents the current allowable ratio. Specifically, Vp2 in the example of Fig. 14 represents the volume density of p-type semiconductor particles (more specifically, p-type ZnO particles) in the ETL. As shown in Fig. 14, the current allowable ratio decreases as Vp2 increases. The symbol RG in Fig. 14 schematically represents the state of lateral diffusion of p-type semiconductor particles (diffusion in a direction perpendicular to the thickness direction of the ETL).

[0081] 14, in the example of embodiment 2, the current tolerance can be reduced to 10% or less by setting Vp2 to 46% or more. In this way, by configuring the ETL so as to obtain a relatively large Vp2, excessive injection of electrons into the QDs in the EML can be more effectively prevented.

[0082] In the example of the second embodiment, Vp1 represents the volume density of n-type semiconductor particles (more specifically, n-type ZnO particles) in the ETL. To rephrase the above example regarding Vp2 for Vp1, setting Vp1 to 54% or less allows the current tolerance to be 10% or less. In this way, by configuring the ETL so that Vp1 is not excessive, excessive injection of electrons into the QDs in the EML can be more effectively prevented.

[0083] [Another Configuration Example of Embodiment 2] In the above explanations, one of the first semiconductor particles 141 and the second semiconductor particles 142 is an n-type semiconductor particle and the other is a p-type semiconductor particle. However, the conductivity type pair of the first semiconductor particles 141 and the second semiconductor particles 142 is not limited to this example.

[0084] (1) For example, one of the first semiconductor particles 141 and the second semiconductor particles 142 may be an intrinsic semiconductor particle. In this case, for example, the other may be a p-type semiconductor particle.

[0085] In one embodiment of the present disclosure, the term "intrinsic semiconductor" refers to a semiconductor to which no donors or acceptors are intentionally added and in which the effects of defects and the like can be practically ignored. Therefore, the intrinsic semiconductor in one embodiment of the present disclosure may contain a small amount of impurities. The intrinsic semiconductor may also have minor defects.

[0086] For this reason, the position of the Fermi level in the intrinsic semiconductor according to one embodiment of the present disclosure does not need to exactly coincide with the center position of the band gap of the intrinsic semiconductor, but it is sufficient that the Fermi level is located near the center of the band gap of the intrinsic semiconductor according to one embodiment of the present disclosure.

[0087] Even when intrinsic semiconductor particles and p-type semiconductor particles come into contact with each other, it is believed that an energy band structure similar to that of the example indicated by reference numeral 720 in Fig. 7 is formed. Therefore, it is believed that the same effect as that of the first embodiment can be obtained by combining intrinsic semiconductor particles and p-type semiconductor particles.

[0088] Consider, for example, a case where the first semiconductor particles 141 are intrinsic semiconductor particles and the second semiconductor particles 142 are p-type semiconductor particles. In this case, as in the example of embodiment 2, it is considered that the current allowable ratio decreases as Vp2 increases. Therefore, for example, in this example, Vp2 may be set to 46% or more.

[0089] (2) As another example, one of the first semiconductor particles 141 and the second semiconductor particles 142 may be intrinsic semiconductor particles, and the other may be n-type semiconductor particles. Even when the intrinsic semiconductor particles and the n-type semiconductor particles come into contact with each other, it is believed that an energy band structure similar to the example indicated by reference numeral 720 in Figure 7 described above will be formed. For this reason, it is believed that the same effect as in embodiment 1 can be obtained by combining the intrinsic semiconductor particles and the n-type semiconductor particles.

[0090] Consider, for example, a case where the first semiconductor particles 141 are n-type semiconductor particles and the second semiconductor particles 142 are intrinsic semiconductor particles. In this case, as in the example of embodiment 2, it is considered that the current allowable ratio decreases as Vp1 decreases. Therefore, for example, in this example, Vp1 may be set to 54% or less. In other words, in this example, Vp2 may be set to 46% or more.

[0091] (3) Various patterns can be envisioned for the combination of the first semiconductor particles and the second semiconductor particles according to one embodiment of the present disclosure, and therefore, the combination of the first semiconductor particles and the second semiconductor particles according to one embodiment of the present disclosure is not limited to the above examples.

[0092] As an example, the first semiconductor particles may be n-type semiconductor particles, and in this case, the volume density of the n-type semiconductor particles (first semiconductor particles) in the ETL may be 54% or less.

[0093] In this example (i.e., when the first semiconductor particles are n-type semiconductor particles), the second semiconductor particles may be intrinsic semiconductor particles, and in this case, the volume density of the intrinsic semiconductor particles (second semiconductor particles) in the ETL may be 46% or more.

[0094] Alternatively, in this example, the second semiconductor particles may be p-type semiconductor particles. In this case, the volume density of the p-type semiconductor particles (second semiconductor particles) in the ETL may be 46% or more.

[0095] (4) As another example, the first semiconductor particles may be intrinsic semiconductor particles. In this case, the volume density of the intrinsic semiconductor particles (first semiconductor particles) in the ETL may be 54% or less.

[0096] In this example (i.e., when the first semiconductor particles are intrinsic semiconductor particles), the second semiconductor particles may be n-type semiconductor particles, and in this case, the volume density of the n-type semiconductor particles (second semiconductor particles) in the ETL may be 46% or more.

[0097] Alternatively, in this example, the second semiconductor particles may be p-type semiconductor particles. In this case, the volume density of the p-type semiconductor particles (second semiconductor particles) in the ETL may be 46% or more.

[0098] (5) As yet another example, the first semiconductor particles may be p-type semiconductor particles. In this case, for example, the volume density of the p-type semiconductor particles (first semiconductor particles) in the ETL may be 54% or less.

[0099] In this example (i.e., when the first semiconductor particles are p-type semiconductor particles), the second semiconductor particles may be intrinsic semiconductor particles, and in this case, the volume density of the intrinsic semiconductor particles (second semiconductor particles) in the ETL may be 46% or more.

[0100] [Supplementary Note on Embodiment 2] In one aspect of the present disclosure, it is believed that electron injection into QDs can be more effectively reduced when semiconductor particles with a shallower Fermi level are located on the cathode side. Furthermore, p-type semiconductor particles have a higher hole transport property than intrinsic semiconductor particles, and n-type semiconductor particles have a higher electron transport property than intrinsic semiconductor particles.

[0101] Taking this into consideration, in one aspect of the present disclosure, it is believed that electron injection into QDs can be particularly effectively reduced when the first semiconductor particles located on the cathode side are p-type semiconductor particles and the second semiconductor particles located on the anode side are n-type semiconductor particles.

[0102] It is believed that electron injection into the QDs can be effectively reduced when the first semiconductor particles located on the cathode side are intrinsic semiconductor particles and the second semiconductor particles located on the anode side are p-type semiconductor particles, and it is also believed that electron injection into the QDs can be effectively reduced when the first semiconductor particles located on the cathode side are n-type semiconductor particles and the second semiconductor particles located on the anode side are intrinsic semiconductor particles.

[0103] As another example, it is believed that electron injection into the QDs can be reduced even when the second semiconductor particles located on the cathode side are intrinsic semiconductor particles and the first semiconductor particles located on the anode side are p-type semiconductor particles. It is also believed that electron injection into the QDs can be reduced even when the second semiconductor particles located on the cathode side are n-type semiconductor particles and the first semiconductor particles located on the anode side are intrinsic semiconductor particles. In this example, too, the electron injection barrier in the above-mentioned energy band structure becomes higher during reverse bias due to the difference between the Fermi levels of the first semiconductor particles and the second semiconductor particles. In addition, this is because the intrinsic semiconductor particles have lower carrier transport properties than n-type and p-type semiconductor particles.

[0104] 15 shows a configuration example of a display device 300 according to a third embodiment. The display device 300 may include a light-emitting element (e.g., light-emitting element 1) according to one aspect of the present disclosure. In the example of FIG. 15, the display device 300 includes, as the light-emitting element 1, a first light-emitting element 1-1, a second light-emitting element 1-2, and a third light-emitting element 1-3. As an example, the first light-emitting element 1-1 is a blue light-emitting element, the second light-emitting element 1-2 is a green light-emitting element, and the third light-emitting element 1-3 is a red light-emitting element.

[0105] 15 , the display device 300 includes a display unit DA including a plurality of subpixels SP, a first driver X1 and a second driver X2 that drive the plurality of subpixels SP, and a display control unit DC that controls the first driver X1 and the second driver X2. Each subpixel SP includes a light-emitting element 1 and a pixel circuit PC connected to the light-emitting element 1.

[0106] As an example, the pixel circuit PC is connected to a scanning signal line GL, a data signal line DL, and a light-emission control line ELL. The scanning signal line GL and the light-emission control line ELL are connected to a first driver X1. The data signal line DL is connected to a second driver X2.

[0107] [Additional Notes] One aspect of the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of one aspect of the present disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0108] REFERENCE SIGNS LIST 1 Light-emitting element 11 Anode 13 Light-emitting layer 14 Electron transport layer 15 Cathode 131 Quantum dots 141 First semiconductor particles 142 Second semiconductor particles 300 Display device

Claims

1. A light-emitting device comprising: an anode and a cathode; a light-emitting layer located between the anode and the cathode and including quantum dots; and an electron transport layer located between the light-emitting layer and the cathode, wherein the electron transport layer includes first semiconductor particles and second semiconductor particles of a different conductivity type than the first semiconductor particles.

2. The light-emitting element according to claim 1, wherein the first semiconductor particles and the second semiconductor particles are in contact with or adjacent to each other.

3. The light-emitting element according to claim 1 or 2, wherein the distance between the first semiconductor particles and the second semiconductor particles that are adjacent to each other is 3 nm or less.

4. The light-emitting device according to any one of claims 1 to 3, wherein at least one of the first semiconductor particles and the second semiconductor particles is a II-VI group semiconductor compound particle.

5. The light-emitting device according to claim 4, wherein the II-VI group semiconductor compound particles are ZnO particles, MgO particles, or ZnMgO particles.

6. The light-emitting element according to any one of claims 1 to 5, wherein the first semiconductor particles are n-type semiconductor particles, and the volume density of the n-type semiconductor particles in the electron transport layer is 54% or less.

7. The light-emitting device according to claim 6, wherein the second semiconductor particles are intrinsic semiconductor particles, and the volume density of the intrinsic semiconductor particles in the electron transport layer is 46% or more.

8. The light-emitting device according to claim 6, wherein the second semiconductor particles are p-type semiconductor particles, and the volume density of the p-type semiconductor particles in the electron transport layer is 46% or more.

9. The light-emitting element according to any one of claims 1 to 5, wherein the first semiconductor particles are intrinsic semiconductor particles, and the volume density of the intrinsic semiconductor particles in the electron transport layer is 54% or less.

10. The light-emitting device according to claim 9, wherein the second semiconductor particles are n-type semiconductor particles, and the volume density of the n-type semiconductor particles in the electron transport layer is 46% or more.

11. The light-emitting device according to claim 9, wherein the second semiconductor particles are p-type semiconductor particles, and the volume density of the p-type semiconductor particles in the electron transport layer is 46% or more.

12. The light-emitting element according to any one of claims 1 to 5, wherein the first semiconductor particles are p-type semiconductor particles, and the volume density of the p-type semiconductor particles in the electron transport layer is 54% or less.

13. The light-emitting device according to claim 12, wherein the second semiconductor particles are intrinsic semiconductor particles, and the volume density of the intrinsic semiconductor particles in the electron transport layer is 46% or more.

14. The light-emitting element according to any one of claims 1 to 9 or 11 to 13, wherein one of the first semiconductor particles and the second semiconductor particles is a p-type semiconductor particle, and the p-type semiconductor particle contains a Group V element.

15. The light-emitting device according to claim 14, wherein the p-type semiconductor particles contain N as the Group V element.

16. The concentration of N in the p-type semiconductor particles is 10 １６ cm －３ Greater than or equal to 10 １９ cm －３ 16. The light-emitting device according to claim 15, wherein:

17. A display device comprising a light-emitting element according to any one of claims 1 to 16.

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