CN109212402B - Method for evaluating quantum efficiency in light-emitting diode - Google Patents
Method for evaluating quantum efficiency in light-emitting diode Download PDFInfo
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- CN109212402B CN109212402B CN201811156093.1A CN201811156093A CN109212402B CN 109212402 B CN109212402 B CN 109212402B CN 201811156093 A CN201811156093 A CN 201811156093A CN 109212402 B CN109212402 B CN 109212402B
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Abstract
The invention discloses a method for measuring the quantum efficiency in a light-emitting diode, which comprises the steps of measuring a volt-ampere characteristic curve of the light-emitting diode in a low-current region; determining a corresponding model and a carrier transport rule thereof according to the material type of the light-emitting diode by utilizing a volt-ampere characteristic curve of the light-emitting diode in the small current region and the light-emitting characteristic of the light-emitting diode, and carrying out curve fitting on the volt-ampere characteristic curve to obtain a parameter of radiation recombination and non-radiation recombination so as to obtain the internal quantum efficiency of the light-emitting diode in the small current region; measuring the external quantum efficiency of the full current interval of the light-emitting diode, and determining the ratio of the external quantum efficiency to the internal quantum efficiency according to the external quantum efficiency and the internal quantum efficiency of a low current region; and dividing the external quantum efficiency of the full current interval by the light extraction efficiency to obtain the internal quantum efficiency of the full current interval. The light extraction efficiency measured by the small current region of the invention can be expanded in the full current interval, and the internal quantum efficiency in the full current interval can be easily obtained.
Description
Technical Field
The invention belongs to the field of semiconductor optoelectronic devices, and further relates to a method for evaluating quantum efficiency in an LED.
Background
The internal quantum efficiency of an LED, which represents the ratio of the number of photons generated from an active region per unit time to the number of electrons injected into the LED, is an important index for evaluating the performance level of the LED. However, only the external quantum efficiency of the LED can be accurately measured in the current experiment, and no known measurement method for the internal quantum efficiency is available. Several of the methods commonly used face some of the following problems:
one is a temperature-variable electroluminescence method, which measures the external quantum efficiency of the LED at different injection currents at room temperature and low temperature, respectively, and assumes that the corresponding internal quantum efficiency is 100% when the external quantum efficiency of the LED reaches the peak value at low temperature. And dividing the external quantum efficiency at room temperature by the peak value of the external quantum efficiency at low temperature to finally obtain the change of the internal quantum efficiency of the LED along with the injection current. However, this method faces problems: it is assumed that when the LED external quantum efficiency peaks at low temperature, it is not based on the corresponding internal quantum efficiency being 100%. Since the internal quantum efficiency is determined by the radiative recombination efficiency of the active region and the injection efficiency of the carriers. As temperature decreases, radiative recombination efficiency increases and injection efficiency decreases, so the product of the two does not typically change monotonically. For most commercial LEDs, the external quantum efficiency is found to be at a maximum in the 100-200K temperature range, and to decrease at low temperatures. Therefore, when the external quantum efficiency of the LED peaks at low temperatures, the internal quantum efficiency is not necessarily 100%.
Yet another is a theoretical simulated light extraction efficiency method, the external quantum efficiency being the product of the internal quantum efficiency and the light extraction efficiency. The light extraction efficiency can be obtained through theoretical simulation by modeling LED chips and packaging theories, assuming parameters such as refractive index, absorption coefficient and the like and tracking by utilizing optical simulation software. The internal quantum efficiency of the LED is obtained by dividing the external quantum efficiency measured by the experiment by the light extraction efficiency. However, this method has problems in that: the assumed parameters such as refractive index and absorption coefficient greatly affect the result, so that the reliability of obtaining the internal quantum efficiency is not high, and the modeling needs to be performed specifically for different devices, which is very complicated.
Disclosure of Invention
Technical problem to be solved
In view of the above, the present invention provides a method for evaluating quantum efficiency in an LED to solve at least some of the above problems.
(II) technical scheme
In order to achieve the above object, the present invention provides a method for measuring quantum efficiency in a light emitting diode, comprising:
measuring a voltage-current characteristic curve of the light-emitting diode in a small current region;
determining a corresponding model and a carrier transport rule thereof according to the material type of the light-emitting diode by utilizing a volt-ampere characteristic curve of the light-emitting diode in the small current region and the light-emitting characteristic of the light-emitting diode, and carrying out curve fitting on the volt-ampere characteristic curve to obtain a parameter of radiation recombination and non-radiation recombination so as to obtain the internal quantum efficiency of the light-emitting diode in the small current region;
measuring the external quantum efficiency of the full current interval of the light-emitting diode, and determining the ratio of the external quantum efficiency to the internal quantum efficiency according to the external quantum efficiency and the internal quantum efficiency of a low current region, wherein the ratio is the light extraction efficiency of the full current interval;
and dividing the light extraction efficiency by the external quantum efficiency of the full current interval to obtain the internal quantum efficiency of the full current interval.
In a further embodiment, the low current region is selected as follows: according to the relation graph of the current and the voltage of the light-emitting diode, the logarithm of the current is taken, so that a relation curve of the current and the voltage under a logarithmic coordinate is obtained; finding the point with the maximum slope, namely the point with the minimum ideal factor, in the relation curve, wherein the point is called as the starting point of the large injection; and selecting a current region corresponding to 0.99-1 times of the initial point voltage of the large injection from the volt-ampere characteristic curve, and selecting the current region as a small current region.
In a further embodiment, the low current region is 1 × 10-6A～1×10-5A。
In a further embodiment, the low current region is 10-6A。
In a further embodiment, the determining a corresponding model for the material type of the light emitting diode specifically includes: for red and blue LEDs, the current-voltage relationship of the model is as follows:
wherein IRRAnd ISRHRespectively a radiation combined current and a Shockley-Reed-Harr combined current, Is1And Is2The respective intensities of the radiation complex current and the schorly-reed-harl complex current are represented as fitting parameters corresponding to the current, k is the boltzmann constant, and I and V are the experimentally measured current and voltage, respectively.
In a further embodiment, the determining a corresponding model for the material type of the light emitting diode specifically includes: for the green led, considering the effect of tunneling current, the current-voltage relationship of the model is:
wherein IRRAnd ISRHRespectively a radiation combined current and a Shockley-Reed-Harr combined current, Is1And Is2The respective intensities of the radiative recombination current and the Shockley-Reed-Harr recombination current are represented as fitting parameters for the respective currents, ITUNFor tunneling current, GTThe strength of the tunneling current is represented as a parameter that requires fitting.
In a further embodiment, the internal quantum efficiency IQE of the light emitting diode on the low current region is calculated as follows:
In a further embodiment, the relative value of the external quantum efficiency of the light emitting diode in the full current interval is measured by using a photomultiplier tube, and the actual value of the external quantum efficiency in the full current interval is integrated, so that the external quantum efficiency in the full current interval is obtained.
In a further embodiment, the light extraction efficiency is calculated as follows:
calculating the light extraction efficiency through the internal quantum efficiency IQE and the external quantum efficiency EQE on the small current area: LEE is equal to EQE/IQE and is considered a constant.
In a further embodiment, the measuring of the external quantum efficiency of the full current interval of the led is performed at room temperature.
(III) advantageous effects
(1) By measuring the light extraction efficiency in a low current region, the influence caused by carrier leakage, Auger recombination and series resistance can be ignored;
(2) the light extraction efficiency measured by the small current region can be expanded in a full current region, and the internal quantum efficiency of the full current region can be easily solved according to the external quantum efficiency;
(3) the measurement needed in the invention is only the volt-ampere characteristic curve of the light-emitting diode at room temperature and the external quantum efficiency curve, which are easy to obtain.
Drawings
Fig. 1 is a flowchart of a method for measuring quantum efficiency in a light emitting diode according to an embodiment of the present invention.
FIG. 2 is a graph of current versus voltage for blue, green, and red LEDs, respectively, according to an embodiment of the present invention.
Fig. 3 is a schematic diagram of ideality factors for the various diodes of fig. 2.
FIG. 4 is a diagram of a model of a red or blue LED and corresponding internal and external quantum efficiency curves.
FIG. 5 is a diagram of a green diode model and corresponding internal and external quantum efficiency curves.
Detailed Description
In order that the objects, technical solutions and advantages of the present invention will become more apparent, the present invention will be further described in detail with reference to the accompanying drawings in conjunction with the following specific embodiments. Furthermore, in the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the disclosure. It may be evident, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in schematic form in order to simplify the drawing.
According to the basic concept of the invention, a method for measuring the internal quantum efficiency of an LED is provided. Fitting to obtain the internal quantum efficiency of the LED by utilizing a volt-ampere characteristic curve of the LED under low current and a carrier transport rule thereof. And calculating the internal quantum efficiency of the LED by taking the external quantum efficiency curve as a reference and combining the external quantum efficiency curve obtained by testing.
Fig. 1 is a flowchart of a method for measuring quantum efficiency in a light emitting diode according to an embodiment of the present invention. The method for measuring the quantum efficiency in the light-emitting diode according to the embodiment of the invention comprises the following steps:
s1: measuring a voltage-current characteristic curve of the light-emitting diode in a small current region;
s2: determining a corresponding model and a carrier transport rule thereof according to the material type of the light-emitting diode by utilizing a volt-ampere characteristic curve of the light-emitting diode in the small current region and the light-emitting characteristic of the light-emitting diode, and carrying out curve fitting on the volt-ampere characteristic curve to obtain a parameter of radiation recombination and non-radiation recombination so as to obtain the internal quantum efficiency of the light-emitting diode in the small current region;
s3: measuring the external quantum efficiency of the full current interval of the light-emitting diode, and determining the ratio of the external quantum efficiency to the internal quantum efficiency according to the external quantum efficiency and the internal quantum efficiency of a low current region, wherein the ratio is the light extraction efficiency of the full current interval;
s4: and dividing the light extraction efficiency by the external quantum efficiency of the full current interval to obtain the internal quantum efficiency of the full current interval.
For step S1, the LED is measured at a small current (10)-6Magnitude a) of the voltage current characteristic. The effect of carrier leakage, auger recombination and series resistance is negligible because the current is chosen to be small. Preferably, the measurement accuracy of the volt-ampere characteristic curve of the light-emitting diode can be improved by reducing the intervals between the sampling points, so that errors caused by subsequent curve fitting are reduced.
In some embodiments, the low current region is selected as follows: according to the relation graph of the current and the voltage of the light-emitting diode, the logarithm of the current is taken, so that a relation curve of the current and the voltage under a logarithmic coordinate is obtained; finding the point with the maximum slope, namely the point with the minimum ideal factor, in the relation curve, wherein the point is called as the starting point of the large injection; and selecting a current region corresponding to 0.99-1 times of the initial point voltage of the large injection from the volt-ampere characteristic curve, and selecting the current region as a small current region. The current of a small current area (the voltage is 0.99-1 times of the voltage of a large injection starting point) selected from the volt-ampere characteristic curve is generally 1E-6-1E-5A, so that the current meets the following three conditions: without the influence of large injection, the luminescence can be measured, and the interval is small enough that the fitting parameters can be considered as constants.
FIG. 2 is a graph of current versus voltage for blue, green, and red LEDs, respectively, according to an embodiment of the present invention. According to the formula, the method comprises the following steps of,
the ideality factor (n) in FIG. 3 can be obtainedideal) Curve line. And, a point corresponding to the minimum value of the ideality factor in the curve can be found, and a corresponding current and voltage can be found from the volt-ampere characteristic curve. And then finding a required small current region (voltage range is 0.99-1 times of the voltage at the point) from the volt-ampere characteristic curve. From fig. 3, the point corresponding to the minimum of the ideality factor in the curve can be found, and the corresponding current and voltage can be found from the current-voltage characteristic curve. And then finding a required small current region (voltage range is 0.99-1 times of the voltage at the point) from the volt-ampere characteristic curve.
For step S2, a curve fit is then performed using the parallel model described above based on the current-voltage characteristic curves to determine the current flowing through each portion of the model: a radiation recombination section whose current exponentially rises with voltage by an ideality factor of 1; the current of the Shockley-Reed-Harr composite part rises exponentially with the voltage by taking an ideal factor as 2; for green light, the tunneling current rises proportional to the voltage. And further calculating the proportion of the radiation composite current in the total current, namely the internal quantum efficiency. Therefore, the change of quantum efficiency in the LED along with the current in a small current interval is obtained.
Aiming at the respective characteristics of the red and blue light-emitting diodes and the green light-emitting diodes, the embodiment of the invention designs a double-diode model and a model corrected on the basis of the double-diode model. (the respective models are in fig. 4 and 5).
For red and blue light, the current flowing through the LED chip at this time can be divided into two types: radiative recombination currents that emit light and non-radiative recombination currents that do not emit light. According to the knowledge of semiconductor physics, the ideality factor of radiative recombination current is 1, while the mode of non-radiative recombination is mainly the schorly-reed-harl recombination, whose ideality factor is 2. The entire LED can then be equated with two diodes in parallel with idealistic factors of 1 and 2 respectively. For red and blue LEDs, the current-voltage relationship of the model is,
wherein IRR，IsRHFor radiating complex currents and for Shockley-Reed-Harr complex currents, Is1And Is2The respective intensities of these two currents are represented as parameters that need to be fitted. k is the boltzmann constant, and I and V are the experimentally measured current and voltage, respectively.
For the green LED with larger tunneling current, correction is still needed on the basis, and a parallel term of resistance is added. For the green light emitting diode, the influence of the tunneling current is considered, and the current-voltage relationship of the model at this time is,
wherein ITUNFor tunneling current, GTThe strength of the tunneling current is represented as a parameter that requires fitting.
Further, after fitting is performed on a given small current interval, fitting parameters are obtained, so that the internal quantum efficiency, namely the proportion of the radiation composite current in the total current, can be calculated in the interval:
for step S3, the external quantum efficiency of the full current interval of the led is measured, and the ratio of the external quantum efficiency to the internal quantum efficiency is determined according to the external quantum efficiency and the internal quantum efficiency of the low current region, which is the light extraction efficiency of the full current interval. The relative value of the external quantum efficiency of the light-emitting diode in the full current interval can be measured by utilizing the photomultiplier, and the real value of the external quantum efficiency in the full current interval is measured by integration, so that the external quantum efficiency in the full current interval is obtained.
Finally, for step S4, the light extraction efficiency is considered to be constant over the full current interval, so the experimentally measured external quantum efficiency is proportional to the actual internal quantum efficiency. And calculating the ratio of the accurate internal quantum efficiency obtained in the small current region and the external quantum efficiency measured in the same interval to obtain the light extraction efficiency in the whole interval. Therefore, according to the external quantum efficiency of the full current interval, the internal quantum efficiency of the full current interval can be obtained by dividing the external quantum efficiency by the light extraction efficiency.
Fig. 4 and 5 show the results of laser measurements with different luminescence. In fig. 4 and 5, the mark points of the inverted triangle represent the external quantum efficiency values measured by the integrating sphere, and the mark points of the regular triangle represent the values obtained by the photomultiplier tube after the relative external quantum efficiency is adjusted by the integrating sphere result, i.e., the External Quantum Efficiency (EQE) in the full current interval. When the integrating sphere is used for measuring the external quantum efficiency, the number of averaging times of multiple measurements is increased, and the precision of the part can be improved. The points marked by boxes represent the fitted internal quantum efficiency values (IQE) in the small current region we have selected, and the light extraction efficiency can be calculated from IQE and EQE over this interval:
LEE is equal to EQE/IQE and is considered a constant.
After the LEE is obtained, according to the external quantum efficiency over the full current interval, IQE ═ EQE/LEE, the internal quantum efficiency over the full interval, that is, each dot on the graph, can be obtained. Therefore, the curve of the internal quantum efficiency is obtained through the volt-ampere characteristic curve and the measured curve of the external quantum efficiency.
Preferably, the temperature of the light emitting diode should be maintained during the experiment to avoid errors caused by temperature rise due to long-time operation.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. A method for measuring the quantum efficiency in a light-emitting diode is characterized by comprising the following steps:
measuring the current-voltage characteristic curve of the light-emitting diode in a small current region, wherein the small current region is 1 multiplied by 10-6A～1×10- 5A；
Determining a corresponding model and a carrier transport rule thereof according to the material type of the light-emitting diode by utilizing a volt-ampere characteristic curve of the light-emitting diode in the small current region and the light-emitting characteristic of the light-emitting diode, and carrying out curve fitting on the volt-ampere characteristic curve to obtain a parameter of radiation recombination and non-radiation recombination so as to obtain the internal quantum efficiency of the light-emitting diode in the small current region;
measuring the external quantum efficiency of the full current interval of the light-emitting diode, and determining the ratio of the external quantum efficiency to the internal quantum efficiency according to the external quantum efficiency and the internal quantum efficiency of a low current region, wherein the ratio is the light extraction efficiency of the full current interval;
dividing the light extraction efficiency by the external quantum efficiency of the full current interval to obtain the internal quantum efficiency of the full current interval;
wherein the low current region is selected as follows:
according to the relation graph of the current and the voltage of the light-emitting diode, the logarithm of the current is taken, so that a relation curve of the current and the voltage under a logarithmic coordinate is obtained;
finding the point with the maximum slope, namely the point with the minimum ideal factor, in the relation curve, wherein the point is called as the starting point of the large injection;
and selecting a current region corresponding to 0.99-1 times of the initial point voltage of the large injection from the volt-ampere characteristic curve, and selecting the current region as a small current region.
2. The method according to claim 1, wherein the determining of the respective model for the material type of the light emitting diode comprises:
for red and blue LEDs, the current-voltage relationship of the model is as follows:
wherein IRRAnd ISRHRespectively a radiation combined current and a Shockley-Reed-Harr combined current, Is1And Is2The respective intensities of the radiation complex current and the schorly-reed-harl complex current are represented as fitting parameters corresponding to the current, k is the boltzmann constant, and I and V are the experimentally measured current and voltage, respectively.
3. The method according to claim 1, wherein the determining of the respective model for the material type of the light emitting diode comprises:
for the green led, considering the effect of tunneling current, the current-voltage relationship of the model is:
wherein IRRAnd ISRHRespectively a radiation combined current and a Shockley-Reed-Harr combined current, Is1And Is2The respective intensities of the radiative recombination current and the Shockley-Reed-Harr recombination current are represented as fitting parameters for the corresponding currents, ITUNFor tunneling current, GTThe intensity of the tunneling current is represented for the parameters that need to be fitted, I being the total current.
4. A method according to claim 2 or 3, characterized in that the internal quantum efficiency IQE of the light emitting diode on the small current area is calculated as follows:
5. The method according to claim 1, wherein the measuring the external quantum efficiency of the full current interval of the led specifically comprises:
and measuring the relative numerical value of the external quantum efficiency of the light-emitting diode in the full current interval by using a photomultiplier, and integrating the actual numerical value of the external quantum efficiency in the full current interval, thereby obtaining the external quantum efficiency in the full current interval.
6. The method of claim 4, wherein the light extraction efficiency is calculated as follows:
calculating the light extraction efficiency through the internal quantum efficiency IQE and the external quantum efficiency EQE on the small current area:
LEE is equal to EQE/IQE and is considered a constant.
7. The method of claim 1, wherein the measuring of the external quantum efficiency of the full current interval of the led is performed at room temperature.
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