Method for precise measurement of passivated contact resistance of crystalline silicon solar cells

Abstract

The application discloses a method for precisely measuring passivated contact resistance of a crystalline silicon solar cell, which comprises the following steps: opening two microgrooves on the surface of a sample by laser etching, wherein the width of the microgrooves is equal to the width of the sample, and the depth of the microgrooves is greater than the thickness of a functional medium film; then filling conductive silver paste into the microgrooves; finally, measuring the resistance between the microgrooves and the silver electrode and the resistance between the microgrooves, and calculating the contact resistivity of the passivated contact structure. The application can conveniently and accurately measure the contact resistivity of the passivated contact structure.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of crystalline silicon solar cells, and relates to a method for accurately measuring the passivated contact resistance of a crystalline silicon solar cell. BACKGROUND

[0002] The surface or interface characteristics of a high-efficiency crystalline silicon solar cell have a great influence on the performance of the device. For example, in the current mainstream passivated emitter and rear contact (PERC) cell, the local direct contact between the rear metal electrode and the silicon produces significant carrier recombination loss. The passivated contact technology completely eliminates the direct contact between the metal and the silicon by inserting an ultra-thin functional dielectric layer material between the metal electrode and the silicon wafer, greatly reducing the carrier recombination loss on the silicon surface. Existing passivated contact technologies can be divided into three types: amorphous silicon heterojunction (HIT) technology, silicon oxide / polysilicon passivated contact (TOPCon) technology, and non-doped heterojunction (DFH) technology. The HIT and TOPCon technologies have been relatively mature, while the DFH technology is still under exploration. The DFH technology uses functional dielectric layer materials to achieve surface passivation and selective collection of carriers. These materials are generally divided into two categories: high work function hole-selective contact materials such as transition metal oxides such as molybdenum oxide, tungsten oxide, and vanadium oxide, and low work function electron-selective contact materials such as lithium fluoride, magnesium oxide, and titanium oxide. Due to the advantages of these functional dielectric layer materials, such as small optical parasitic absorption, a wide variety of materials, and simple preparation methods, they have the potential to obtain low-cost high-performance solar cells, making the DFH technology one of the mainstream technologies for high-efficiency crystalline silicon solar cells in the future.

[0003] Generally speaking, in order to achieve a high-performance DFH solar cell, both good passivation and good contact must be achieved. The specific contact resistance (p c ) is one of the most important factors for characterizing the DFH passivated contact structure, because the value of p c is closely related to the transport barrier of the majority carriers inside the DFH solar cell, and ultimately affects the fill factor (FF) of the cell. In essence, p c quantitatively characterizes the ability of carrier-selective contact to allow current to pass through, therefore, how to accurately measure the contact resistance of a non-doped heterojunction solar cell is of great significance for designing high-efficiency DFH solar cells. The Cox-Strack method (CSM) and the transmission length method (TLM) have been widely used to extract the p c of ohmic contact through current-voltage characteristics. However, in the structure of a DFH solar cell, the contact between the carrier-selective contact material and the crystalline silicon substrate is not an ohmic contact, but a Schottky heterojunction contact, making it difficult for the traditional CSM and TLM methods to accurately extract the p c .

[0004] For a DFH passivated contact electrode, such as silver / molybdenum oxide / n-type monocrystalline silicon (Ag / MoO x / n-Si), the molybdenum oxide as a hole-selective material induces a reverse layer on the silicon surface, the thickness and carrier concentration of which are closely related to the stoichiometric ratio of the molybdenum oxide, and thus the sheet resistance (R sh , the resistance of a square thin layer along its opposite edge plane, in units of Ω / □) of the thin silicon layer in contact with the molybdenum oxide is determined by the carrier concentration of the reverse layer and the silicon substrate. If the carrier concentration of the reverse layer is much greater than that of the silicon substrate, the sheet resistance of the thin silicon layer is determined by the reverse layer, and the sheet resistance of the reverse layer is unknown. Therefore, it is difficult to accurately obtain the contact resistivity of the Ag / MoO x / n-Si passivated contact structure according to the conventional TLM measurement method. SUMMARY

[0005] The purpose of the present application is to provide an accurate measurement method for the passivated contact resistance of a crystalline silicon solar cell.

[0006] A typical DFH passivated contact structure is silver / molybdenum oxide / n-type monocrystalline silicon (Ag / MoO x / n-Si), which is defined as the current transmission length L T to characterize the characteristic distance of the current flowing laterally from the semiconductor thin layer (MoO x ) into the contact point, the sheet resistance of the thin silicon layer with the functional medium film (such as molybdenum oxide) is R sh , and the contact resistivity between the functional medium film and the silicon is p c , then

[0007]

[0008] The unit of p c is Ω·cm 2 , and the corresponding contact resistance R c can be written as

[0009]

[0010] where W is the width of the metal electrode.

[0011] According to equations (1) and (2), p c can be expressed as

[0012]

[0013] As can be seen from equation (3), if R sh and R c can be accurately measured, p c can be obtained.

[0014] Based on the above idea, the present application provides the technical scheme as follows:

[0015] The present application provides a method for precisely measuring the passivated contact resistance of a crystalline silicon solar cell. The measuring sample comprises a single crystal silicon layer, a functional medium film grown on the single crystal silicon layer, and a strip-shaped silver electrode evaporated on the functional medium film. The width of the strip-shaped electrode is W. The method comprises the following steps: two microgrooves are opened on the surface of the sample by laser etching. The distance between the two microgrooves and the distance between the microgrooves and the center of the silver electrode are equal. The width of the microgrooves is equal to the width of the sample. The depth of the microgrooves is greater than the thickness of the functional medium film. Then, conductive silver paste is filled in the microgrooves. Finally, the current-voltage characteristics between the microgrooves and the silver electrode and between the microgrooves are measured, so as to obtain the resistance R1 between the microgrooves and the silver electrode and the resistance R2 between the microgrooves. The contact resistivity p of the passivated contact structure is calculated by the following formula (4). c :

[0016]

[0017] Since the resistance of the silver electrode and the conductive silver paste is very small and can be ignored, R1=R sh +R c , R2=R sh , the aforementioned formula (3) can be rewritten as formula (4). Therefore, the present application can measure R sh and R c by experiments, so as to obtain p c .

[0018] It can be seen that the present application can conveniently and accurately measure the contact resistivity of the passivated contact structure. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic diagram of the present embodiment. DETAILED DESCRIPTION

[0020] 1. Silicon wafer cleaning

[0021] The sample substrate is a single crystal silicon wafer (n-type, resistivity 0.1-1.0 Ω.cm, thickness 0.3 mm). The substrate is cleaned by a standard RCA cleaning process. The RCA process is first created by Kern and Puotinen et al. in RCA laboratory of N.J. Princeton in 1965. Then, the substrate is repeatedly washed by deionized water for more than three times and dried by nitrogen.

[0022] 2. Ultra-thin molybdenum oxide growth

[0023] A 2-20 nm thick MoO2 film is grown on a silicon wafer by evaporation using MoO2 powder with a purity of 99.99%. The silicon wafer, which has been cleaned according to Step 1, is immersed in dilute hydrofluoric acid (1% HF) for 10-20 s before being placed in the evaporation chamber to remove the oxide layer formed on the surface of the silicon wafer. The vacuum degree of the chamber is better than 1 x 10 -4 Pa, the heating current is adjusted so that the evaporation rate of the MoO2 is 0.05 nm / s, and the shutter is opened to start the evaporation after the rate is stable. When the thickness reaches 2-20 nm, the shutter is closed and the heating current is turned off. The thickness of the MoO2 film during the growth process is monitored by a quartz crystal resonator and further confirmed by an ellipsometer.

[0024] 3. Growth of silver electrode

[0025] A 500 nm thick silver electrode in the form of a strip is grown on the MoO2 film by evaporation using a mask. The width W of the electrode is equal to the width of the sample, and the length L of the electrode is 2-10 mm.

[0026] 4. Laser grooving and filling with conductive silver paste

[0027] First, two strip-shaped grooves are etched on the surface of the silicon wafer on which the MoO2 film is grown using a laser scribe machine. The distance between the two grooves and the distance from the center of the silver electrode to the grooves are equal, the depth h of the grooves is much greater than the thickness of the MoO2 film and can be 1.0-3.0 μm, the width of the grooves is equal to the width of the sample, and the length of the grooves is 0.5-1.0 mm. It should be noted that Figure 1 The drawing method commonly used in the semiconductor industry is used, and the drawing is not to scale. Then, low-temperature conductive silver paste (bulk resistivity less than 1 x 10 -5 Ω·cm) is filled in the two grooves, and then annealed at 120°C for 30 min.

[0028] 5. Current-voltage (I-V) measurement

[0029] The current-voltage characteristics between the grooves and the silver electrode and between the grooves are measured using a current source meter, i.e., A1\ V1\ A2\ V2, so as to obtain the resistance R1 between the grooves and the silver electrode and the resistance R2 between the grooves. According to equation (4), the contact resistivity p of the Ag / MoO x / n-Si passivated contact structure can be accurately obtained. c ​

Claims

1. A method for precise measurement of passivated contact resistance of a crystalline silicon solar cell, said measurement sample comprising a single crystalline silicon layer, a functional dielectric thin film grown on the single crystalline silicon layer, and a strip-shaped metal silver electrode evaporated on the functional dielectric thin film, the width of the strip-shaped electrode being characterized in that: Two microgrooves are opened on the sample surface by laser etching, the distance between the two microgrooves and the distance between the microgrooves and the center of the silver electrode are equal, the width of the microgrooves is equal to the width of the sample and the width of the strip electrode , and the depth of the microgrooves is greater than the thickness of the functional medium film Subsequently, the microgrooves are filled with conductive silver paste, and finally the current-voltage characteristics between the microgrooves and the silver electrode and between the microgrooves and the microgrooves are measured, so as to obtain the resistance R1 between the microgrooves and the silver electrode and the resistance R2 between the microgrooves and the microgrooves; the contact resistivity of the passivated contact structure is calculated by using the following formula (4) : （4）。

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