Model and parameter extraction method of semiconductor heterojunction iv output kink effect

Abstract

The application discloses a model and a parameter extraction method of a semiconductor heterojunction IV output twist effect. The method is based on a circuit model used for describing the influence of a window layer conduction band barrier on a heterojunction IV characteristic. The circuit model comprises three diodes, i.e. two photo diodes corresponding to a P region and an N region of a PN junction and a bypass diode caused by a window layer conduction band barrier of a heterojunction interface. The application can quickly classify the causes of the twist effect of a battery by using a characteristic index, and then locate the causes of the twist effect by using a corresponding relationship between the characteristic index and material parameters, so as to guide the improvement of a production or a research and development process.

Description

Technical Field

[0001] The invention belongs to the field of heterojunction photoelectric effect analysis, and in particular relates to a model for describing an IV output curve of a semiconductor heterojunction and a parameter extraction method. Background Art

[0002] Solar cells are a green and energy-saving way of generating electricity. The development of solar cells mainly focuses on significantly reducing costs and improving the conversion efficiency of solar cells. Heterojunction thin-film cells are a typical representative of the second generation of solar cells. The advantage of heterojunction is that the high bandgap window layer can reduce the ineffective absorption of light and reduce the recombination current. However, continuous research has found that the selection of a window layer with a bandgap that is too high to reduce ineffective light absorption or the thickening of the window layer to reduce the recombination current will lead to an S-shaped distortion of the current-voltage (IV) curve (Solar Energy, (2018) 165: 27-34). Figure 1 As shown in Figure 3, this effect limits the fill factor and output power of the battery. Its physical essence is that the conduction band barrier of the window layer hinders the transmission of photogenerated charges.

[0003] The S-shaped distortion of the current-voltage (IV) characteristic is a common phenomenon in the development of second-generation heterojunction solar cell device structures. Using different work function materials or reducing the thickness of the window layer can alleviate the S-shaped distortion effect. For example, doping Ga in the window layer material MgZnO (Solar Energy Materials and Solar Cells, (2021) 232: 6.) can effectively eliminate the distortion in the JV curve. In the actual research and development process, although engineers can use simulation software such as SCAPS to simulate this effect, the semiconductor material parameters involved are numerous and very inconvenient. Therefore, for this important effect that exists in most heterojunction thin-film batteries, a method is needed that can quickly and easily infer the battery characteristic indicators in actual research and development and production. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, the present invention provides a model and parameter extraction method for the IV output distortion effect of semiconductor heterojunction, which can quickly classify the causes of the distortion effect of the battery using characteristic indicators, and then locate the cause of the distortion effect using the correspondence between the characteristic indicators and material parameters.

[0005] The technical solution adopted by the present invention to solve its technical problem is:

[0006] A model and parameter extraction method for the IV output distortion effect of a semiconductor heterojunction is disclosed. The method is based on a circuit model used to describe the influence of the conduction band barrier of the window layer on the IV characteristics of the heterojunction. The circuit model includes three diodes: two photodiodes corresponding to the depletion regions of the P and N regions in the PN junction, and a bypass diode caused by the conduction band barrier of the window layer at the heterojunction interface. The two photodiodes corresponding to the n-type and p-type regions are connected in series in the same direction, and the bypass diode and the photodiode corresponding to the n-type region are connected in parallel. In the bypass diode, when the conduction band of the heterojunction n-type material is higher than that of the p-type material, the bypass diode and the photodiode are in opposite directions. When the conduction band of the heterojunction n-type material is lower than that of the p-type material, the bypass diode and the photodiode are in the same direction.

[0007] The parameters of the photodiode and bypass diode are obtained by fitting the bright IV curve of the heterojunction device, and the saturation current J0 and ideality factor n of the bypass diode are calculated to quantitatively compare the potential barriers and interface recombination degrees of different heterojunction window layers.

[0008] Furthermore, the photocurrent generated by the photodiode in the n-type region is very weak, much smaller than the photocurrent generated by the photodiode in the p-type region.

[0009] In the present invention, heterojunction thin film batteries are a typical representative of the second generation of solar cells. The advantage of the heterojunction is that the high bandgap window layer can reduce the ineffective absorption of light and reduce the recombination current. However, the use of a window layer with a bandgap that is too high or a window layer that is too thick will lead to an S-shaped distortion of the current-voltage (IV) curve. This effect will limit the fill factor and output power of the battery. Since this effect exists in most heterojunction thin film batteries, there is a need for a method that can quickly and easily infer the battery characteristic indicators in actual research and development and production. Based on the industry-wide diode parallel constant current source model of homogeneous PN junction silicon batteries, this method introduces a bypass diode for the interface electric field caused by the conduction band transition of the heterojunction.

[0010] The beneficial effects of the present invention are: the causes of the distortion effect in the battery can be quickly classified by using characteristic indicators, and then the causes of the distortion effect can be located by using the correspondence between the characteristic indicators and material parameters to guide the improvement of the production or R&D process. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic diagram of the circuit model, where 1 is the p-type region, 2 is the n-type region, 3 represents photogenerated electrons, 4 is the window layer barrier, the energy band region of 5 is equivalent to a bypass diode, the energy band region of 6 is equivalent to the window layer diode J1, and the energy band region of 7 is equivalent to the absorption layer photodiode J2.

[0012] Figure 2 This is a schematic diagram of a circuit model with a bypass diode.

[0013] Figure 3 The IV curve is generated using the model of the present invention. Curve E is composed of the bypass diode curve A, the window junction curve B, and the absorber junction curve D.

[0014] Figure 4 This is a schematic diagram of the energy band and device structure of the analysis object in Example 1. (a) shows the energy band structure of the analysis object, 1 is the conductive glass layer, 2 is the Al2O3 passivation layer, and 3 is the CdS window layer. (b) shows the device structure of the analysis object, and the right side shows the equivalent circuit of the corresponding region. In the device structure, 1 is the conductive glass layer, 2 is the Al2O3 passivation layer, 3 is the CdS window layer, and 4, 5, 6, and 7 are other layers unrelated to this model. 8 is the equivalent photodiode, 9 is the equivalent window layer diode, and 10 is the equivalent bypass diode of the conduction band barrier.

[0015] Figure 5 It is the IV curve of the analysis object in Example 1, wherein (a) represents the experimental value of the IV curve of the analysis object at different aluminum oxide passivation layer thicknesses, and (b) represents the fitting result of the model of the present invention for the IV curve of 15 nm aluminum oxide thickness in (a).

[0016] Figure 6 Figure 1 is a schematic diagram of the device structure analyzed in Example 2. 1 is ordinary glass, 2 is the conductive oxide layer, 3 is the MgZnO or Ga-doped MgZnO window layer, 4 is the CdTe absorption layer, and 5, 6, and 7 are the device back contact and electrode layers unrelated to this model. Similarly, 8 is the equivalent photodiode, 9 is the equivalent window layer diode, and 10 is the equivalent bypass diode of the conduction band barrier.

[0017] Figure 7 Figures 2 and 3 are the IV curves of the device analyzed in Example 2. (a) shows the IV curve of a Ga-doped MgZnO window layer device, where A, B, and D are the bypass diode curve, the window layer diode curve, and the absorption layer photodiode curve, respectively. The dashed line is the experimental curve. (b) shows the IV curve of an undoped Ga-doped MgZnO window layer device, where A, B, and D are the bypass diode curve, the window layer diode curve, and the absorption layer photodiode curve, respectively. The dashed line is the experimental curve. DETAILED DESCRIPTION

[0018] The present invention will be further described below with reference to the accompanying drawings.

[0019] Reference Figures 1 to 7A model and parameter extraction method for the IV output distortion effect of a semiconductor heterojunction is disclosed. The method is based on a circuit model used to describe the influence of the conduction band barrier of the window layer on the IV characteristics of the heterojunction. The circuit model includes three diodes, namely, two photodiodes corresponding to the depletion regions of the P and N regions in the PN junction, and a bypass diode caused by the conduction band barrier of the window layer at the heterojunction interface. The two photodiodes corresponding to the n-type region and the p-type region are connected in series in the same direction, and the bypass diode and the photodiode corresponding to the n-type region are connected in parallel. In the bypass diode, when the conduction band of the n-type material of the heterojunction is higher than that of the p-type material, the bypass diode and the photodiode are in opposite directions. When the conduction band of the n-type material of the heterojunction is lower than that of the p-type material, the bypass diode and the photodiode are in the same direction.

[0020] The parameters of the photodiode and bypass diode are obtained by fitting the bright IV curve of the heterojunction device, and the saturation current J0 and ideality factor n of the bypass diode are calculated to quantitatively compare the potential barriers and interface recombination degrees of different heterojunction window layers.

[0021] Furthermore, the photocurrent generated by the photodiode in the n-type region is very weak, much smaller than the photocurrent generated by the photodiode in the p-type region.

[0022] Building on the industry-standard diode-parallel constant current source model for homojunction silicon cells (CN202410122908), a bypass diode is introduced to address the interfacial electric field caused by heterojunction conduction band transitions. Compared to SCAPS simulation, this method can quickly categorize the causes of cell distortion effects using characteristic indicators. The corresponding relationship between characteristic indicators and material parameters can then be used to pinpoint the cause of the distortion effect, guiding improvements to production or R&D processes.

[0023] like Figure 1 As shown, in the model of the present invention, the electric field formed within a few nanometers by the electron affinity transition between the heterojunction window layer and the absorber layer results in the formation of a bypass diode J in the opposite direction to the PN junction diode of the cell. Bypass diodes are common circuit components in silicon photovoltaic modules used to prevent module overload (CN201910394098). In the model of the present invention, bypass diode J is connected in parallel with diode J1 in the window layer, suppressing the transmission of the photocurrent generated by photodiode J2 in the absorber layer at high bias voltage.

[0024] like Figure 2As shown, in the model of the present invention, bypass diode J and window layer junction J1 are connected in parallel, and both are connected in series with absorption layer junction J2. The ideal diode expression is: I = I0(exp(qV / nkT)-1), at room temperature: kT / q = 0.026eV, I0 is the forward current, and n is the ideality factor. Of the three diodes, bypass diode J originates from an electric field within a few nanometers, and junction J1 originates from the depletion region of the window layer. The doping concentration of the window layer is several hundred times that of the absorption layer, so the thickness of the window layer's depletion region is also less than 10 nanometers. Therefore, both J and J1 have a certain degree of tunneling effect, resulting in a large forward current at the junction. In contrast, the forward current and ideality factor n of absorption layer junction J2 meet the characteristic magnitudes of a typical semiconductor PN junction.

[0025] According to the model of the present invention, the heterojunction conduction band barrier and the heterojunction interface recombination determine the forward current and ideality factor of the bypass diode J, respectively. Therefore, the bypass diode mainly reflects the characteristics of the open-circuit voltage. Due to the high doping concentration and the large number of body defects in the window junction J1, it cannot generate photocurrent. Therefore, the photocurrent of J1 is equal to zero, and it is a pure forward diode. In this model, J1 mainly reflects the characteristics of the series resistance. The absorption junction J2 is the main area for generating photocurrent. J2 mainly reflects the characteristics of the short-circuit current, open-circuit voltage and parallel resistance of the battery.

[0026] like Figure 3 As shown, curve A is the reverse bypass diode, which is reverse connected; curve B is the window junction J1 without photoelectric effect, which is forward connected; curve D is the absorption junction J2 that generates photocurrent, which is forward connected.

[0027] The fitting process is as follows:

[0028] Step 1: Use the data of curve E near 0-0.3V to determine the three characteristic parameters of curve D: short-circuit current I L2 , forward current I 02 and ideality factor n2.

[0029] Get curve D: I2=I L2 -I 02 (exp(qV / n2kT)-1)(1)

[0030] Step 2: Use the data in the area above the open circuit voltage (≈0.6V) of curve E to determine the characteristic parameters of curve B: forward current I 01 and ideality factor n1.

[0031] Curve B is obtained: I1 = -I 01 (exp(qV / n1kT)-1)(2)

[0032] Step 3: Use the data of curve E from 0.3V to open circuit voltage (≈0.6V) to determine the characteristic parameters of curve A: forward current I0 and ideality factor n.

[0033] Curve A is obtained: I = -I0(exp(-qV / nkT)-1)(3)

[0034] After combining the three curves and making fine adjustments, the fitting result is obtained.

[0035] According to the battery characteristics that can be reflected by the above three junctions, the model parameters of the present invention and the battery functions have the following corresponding rules as listed in Table 1:

[0036] parameter Battery function Deeper factors <![CDATA[I L2 ]]> Photocurrent Collection efficiency of photogenerated carriers <![CDATA[I 02 ]]> Heterojunction leakage Is the window layer complete and is there a short circuit at the edge? <![CDATA[n2]]> Absorbent layer composite Absorption layer film quality <![CDATA[I0]]> Window layer conduction band barrier height or width Window layer material selection and preparation thickness n Heterojunction interface recombination Preparation environment, film quality <![CDATA[I 01 ]]> Series resistance Window layer film quality, doping concentration, electrode quality <![CDATA[n1]]> Series resistance Window layer film quality, doping concentration, electrode quality

[0037] Table 1

[0038] Among them, the parameters of the main junction J2 and the bypass diode J have the greatest impact on battery performance.

[0039] Example 1:

[0040] The literature Solar Energy, (2018) 165: 27-34 reported a method of using a high bandgap aluminum oxide layer to passivate the front interface of the battery and reduce the battery interface recombination. Figure 4 In the case shown in (a), the heterojunction itself has almost no jump in electron affinity, so there is no reverse junction in the heterojunction itself. However, the extremely low electron affinity of aluminum oxide as a passivation layer forms a photocurrent barrier at the front interface of the cell window layer, resulting in an S-shaped distortion of the cell IV output curve. Figure 4 As shown in (b), the device is modeled using the model of the present invention, where the interface between aluminum oxide and cadmium sulfide corresponds to the bypass diode J and the junction J1, while the heterojunction composed of cadmium sulfide and cadmium telluride as a whole is a photodiode J2.

[0041] Figure 5 (a) shows the IV output curves of the cell with different aluminum oxide passivation layer thicknesses from the same paper. A high conduction band barrier of 3.43 eV is established between the aluminum oxide passivation layer and the CdS window layer, and devices with different aluminum oxide thicknesses are modeled and fitted. Figure 5 (b) is the fitting result for a typical device with a 15nm aluminum oxide passivation layer thickness. It can be seen that our model curve E is in perfect agreement with the experimental curve. Furthermore, fitting is performed for devices with 5, 15, 20, and 30nm passivation layers. The results show that the forward current J0 of the bypass diode is 10 0.33 , 10 0.2 , 10 0.05 , 10 -0.7 mA / cm 2, while the ideality factors are 3.1, 3.1, 2.8, and 2.3, respectively. This indicates that the wider the passivation barrier, the smaller the tunneling probability, the smaller the forward current J0, and the significantly reduced ideality factor. This means that passivation can reduce the interface recombination of the device but also hinder the transport of photocurrent, which is consistent with the conclusions in the literature.

[0042] Example 2:

[0043] The literature Solar Energy Materials and Solar Cells, (2021) 232:6 reported a method using a bandgap adjustable zinc-doped magnesium oxide as a window layer and an absorption layer CdSe 1-x Te x A major problem with this type of battery structure is the significant window layer barrier, which leads to an S-shaped distortion in the IV output curve. However, literature reports that doping the window layer with an appropriate amount of metallic Ga can help reduce the window barrier and alleviate the distortion in the IV curve.

[0044] Figure 6 This is the battery structure in the document, with the equivalent circuit drawn next to its heterojunction.

[0045] The IV output curves of devices with Ga-doped MgZnO as the window layer and devices with undoped Ga-doped MgZnO as the window layer were fitted using the model of the present invention. Figure 7 (a) is the IV curve and fitting curve of the MgZnO window layer device doped with Ga and (b) not doped with Ga. It can also be seen that the model of the present invention fits well regardless of whether there is distortion.

[0046] The forward current I of the window layer J1 of the Ga-doped device 01 10 1.8 mA / cm 2 , the ideal factor n1 is 1.92. The forward current I 01 10 0.35 mA / cm 2 , and the ideality factor n1 is 1.92. This indicates that Ga doping improves the carriers in the MgZnO window layer, reduces the series resistance, and does not form obvious recombination centers.

[0047] The forward current I0 of the bypass diode J of the undoped Ga device is 10 0.5 mA / cm 2 , the ideal factor n is 2.69. The forward current I0 of the bypass diode J of the Ga-doped device is 10 1.4 mA / cm 2, and the ideality factor n is 1.23. This indicates that Ga doping leads to a significant reduction in the window layer barrier and improves heterojunction interface recombination.

[0048] The embodiments of this specification are merely examples of implementations of the invention and are provided for illustrative purposes only. The scope of protection of the present invention should not be considered limited to the specific embodiments described in these embodiments. The scope of protection of the present invention also extends to equivalent technical means that can be conceived by a person of ordinary skill in the art based on the invention.

Claims

1. A model and parameter extraction method for the IV output distortion effect of a semiconductor heterojunction, characterized in that: The method is based on a circuit model used to describe the influence of the conduction band barrier of the window layer on the IV characteristics of the heterojunction. The circuit model includes three diodes, namely, two photodiodes corresponding to the depletion regions of the P-region and N-region in the PN junction, and a bypass diode caused by the conduction band barrier of the window layer at the heterojunction interface. The two photodiodes corresponding to the n-type region and the p-type region are connected in series in the same direction, and the bypass diode and the photodiode corresponding to the n-type region are connected in parallel. In the bypass diode, when the conduction band of the n-type material of the heterojunction is higher than the conduction band of the p-type material, the bypass diode and the photodiode are in opposite directions. When the conduction band of the n-type material of the heterojunction is lower than the conduction band of the p-type material, the bypass diode and the photodiode are in the same direction. The parameters of the photodiode and bypass diode are obtained by fitting the bright IV curve of the heterojunction device, and the saturation current J0 and ideality factor n of the bypass diode are calculated to quantitatively compare the potential barriers and interface recombination degrees of different heterojunction window layers.

2. A semiconductor heterojunction IV output distortion effect model and parameter extraction method according to claim 1, characterized in that: The photocurrent generated by the photodiode in the n-type region is very weak, and is much smaller than the photocurrent generated by the photodiode in the p-type region.

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