CN110648728A - Method for predicting passivation effect of organic molecules on silicon surface - Google Patents

Abstract

The invention provides a method for predicting the passivation effect of organic molecules on silicon surface, which adopts the first-principles method based on density functional function to predict, including: establishing an adsorption model between organic molecules and silicon surface, calculating adsorption energy, determining Stable passivation structure when organic molecules passivate the silicon surface; calculate the electronic structure properties, charge transfer and bond formation of the interface of the stable passivation structure; judge the passivation ratio of silicon surface atoms and the passivation strength of silicon surface atoms, predict Passivation effect. The prediction method of the invention can quickly judge the passivation structure, can also be used to find and design suitable organic passivation materials, guide the synthesis of experiments, save time and cost, reduce the interference of human factors and environmental factors in the experimental process, and is reliable High performance and good reproducibility.

Description

A prediction method for the passivation effect of organic molecules on silicon surface

technical field

The invention relates to material passivation, in particular to a method for predicting the passivation effect of organic molecules on silicon surfaces.

Background technique

Solar energy is the source of all energy on the earth. Today, when other non-renewable energy sources on the earth are increasingly exhausted, how to better apply solar energy, a clean and green renewable energy source, has become a topic of concern to the country and scientists. The process of converting solar energy into electricity is called solar photovoltaic power generation, and the core tool to achieve this process is the solar cell. In 1883, the first solar cell was successfully prepared by Charles Fritts, which was a semiconductor metal junction formed by covering a very thin layer of gold on a selenium semiconductor, but the device was only 1% efficient.

With the development of technology, people have also developed solar cells of various materials. Among them, silicon solar cells are favored by people due to their suitable band gap, no pollution to the environment, easy industrial production, and high photoelectric conversion efficiency. material in solar cells dominates. Theoretically, the highest conversion efficiency of silicon solar cells is 29%. In the energy of incident light, most of the energy loss comes from transmission loss and quantum loss, and about 10% is carrier recombination, surface reflection loss and series resistance loss, etc. . However, the defects and impurities on the silicon surface will greatly affect the performance of the device. In order to reduce the cost and improve the conversion efficiency of solar cells, various silicon surface passivation technologies have been developed. Because surface passivation can neutralize surface electrons, Reducing defects and surface-induced electron recombination centers is critical to increasing the carrier lifetime of crystalline silicon and plays a key role in solar cells. Existing passivation methods mainly include chemical passivation, field effect passivation, electrochemical passivation, and the like.

Chemical passivation adopts the method of forming chemical bonds between the atoms in the passivation material and the surface atoms of crystalline silicon, so as to achieve the purpose of inhibiting the surface recombination and eliminate the dangling bonds from the root. The most typical materials for chemical passivation are _SiO2 and hydrogenated amorphous silicon (a-Si:H). SiO ₂ uses Si-O bonds to fill the dangling bonds on the silicon surface, but SiO ₂ passivation technology requires a high temperature of 900 ° C in application. In order to overcome this difficulty, people have developed wet oxidation technology, such as Fraunhofer, Germany. Fraunhofer ISE uses this technology to develop a back-contact tunnel oxide passivation contact (TopCon) cell with a conversion efficiency of 25%. The a-Si:H thin film passivation uses H-Si-Si and Si-H bonds to fill the Si dangling bonds, but the PECVD equipment used in this method is relatively expensive, and it is difficult to control the process of producing ultra-thin amorphous silicon films.

Field-effect passivation is the introduction of passivation materials (such as SiNx, Al ₂ O ₃ ) with internal fixed charges, which repel the carriers through the Coulomb electrostatic field of the charges and block them from combining with surface defects, thereby achieving the purpose of inhibiting surface recombination. . SiNx film is the most commonly used passivation anti-reflection film in the production of crystalline silicon solar cells. SiNx film has a fixed positive charge and has a good passivation effect on the n-type silicon surface, but the passivation effect on the highly doped p+ surface not good. Another typical application of the field-effect passivation mechanism is Al ₂ O ₃ thin film passivation. Contrary to the SiNx film, the Al ₂ O ₃ film has a high fixed negative charge on the contact surface with silicon, which can shield the electrons on the p-type silicon surface, so the Al ₂ O ₃ film has a better field passivation effect on the p-type silicon surface. , which is currently mainly used in the backside passivation of p-type cells. The PERC cell is based on the traditional p-type cell, adding Al ₂ O ₃ back passivation, and the conversion efficiency is significantly improved.

In addition to chemical passivation and field effect passivation, electrochemical passivation mechanisms are also applied and studied. Electrochemical passivation is still essentially chemical passivation. The principle is to use atoms in the electrolyte to combine with silicon surface atoms, which is characterized by the fact that this combination can be controlled by charge transfer.

Although traditional passivation technologies such as silicon oxide, aluminum oxide, and amorphous silicon thin films are very mature, their preparation usually requires harsh conditions such as high-temperature processes or high-vacuum equipment, which seriously hinders the reduction of device costs. The use of organic molecules to passivate the silicon surface has the advantages of low cost and mild reaction conditions. The passivation effect of organic small molecule TFSI on the surface of C-Si devices has been confirmed. When this superacid is used for passivation of silicon wafers, it can produce millisecond-level minority carrier lifetimes. However, it is necessary to screen among many possible passivation materials, and it is possible to obtain a small number of test results that meet the requirements after a lot of time-consuming experiments.

To sum up, on the basis of the existing technology, how to study the passivation mechanism and passivation nature of organic molecules on silicon wafers, and how to predict the passivation effect of organic molecules on the surface of silicon wafers, is the current need to solve. one of the technical problems.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for predicting the passivation effect of organic molecules on silicon surface.

The object of the present invention is achieved in this way:

A method for predicting the passivation effect of organic molecules on a silicon surface, which uses a density functional-based first-principles method to predict, including the following steps:

(a) Establish the adsorption model between organic molecules and the silicon surface, calculate the adsorption energy, and determine the stable passivation structure when organic molecules passivate the silicon surface;

(b) Calculate the electronic structure properties, charge transfer and bond formation at the interface of the stable passivation structure;

(c) Judging the passivation ratio of the silicon surface atoms according to the result of step (a), judging the passivation intensity of the silicon surface atoms according to the result of step (b), and predicting the passivation effect.

In step (a), the adsorption energy E _ads is defined as the difference between the total energy after adsorption and the energy of the organic molecule and the surface of pure silicon, and the average value of the number of bonded silicon atoms is calculated as:

E _ads =(E _total – E _{organic molecules} -E')/n

Among them, E _total is the total energy of organic molecules adsorbed on the surface of the pure semiconductor material, E _{organic molecules} and E' represent the energy of organic molecules and the energy of pure silicon surface, respectively, and n is the number of atoms on the passivated silicon surface.

In step (a), the determination of the stable passivation structure is determined by comparing the adsorption energies of various structures. According to the principle of minimum energy, the lower the adsorption energy, the more stable the structure.

Specifically, the organic molecule is bis-trifluoromethanesulfonimide (TFSI), and the silicon surface is a Si(001) surface.

In step (b), first-principles software is used to calculate the Bader charge and ELF (electron local function) of the stable passivation structure (the model with the lowest energy), and the Bader charge is used to judge the charge transfer of the silicon surface atoms. ELF is used to judge the bonding strength of silicon surface atoms and organic molecules.

In step (c), the result of step (a) is analyzed, that is, the passivation structure of organic molecules on the silicon surface, and the ratio of the number of bonds between silicon surface atoms and organic molecules and the ratio of the number of unbonded atoms is calculated as the basis for judging the passivation effect of the silicon surface. The greater the number of bonds, the more obvious the passivation effect.

According to the result of step (b), namely the electronic structure, the electronic structure properties after passivation of organic molecules on the silicon surface can be obtained, and the gain and loss of the silicon surface charge can be obtained by analyzing the electronic structure properties, which is another basis for judging the passivation effect of the silicon surface. The stronger the bond, the more charge transfer and the higher the degree of passivation.

The invention uses the first-principles method to establish the adsorption model of organic molecules on the surface of silicon, and by calculating the adsorption energy, Bader charge, ELF (electron local function) of organic molecules on the surface of silicon materials, etc., to study the passivation of organic molecules on the surface of silicon materials The microscopic mechanism of passivation can be predicted to predict the passivation effect. This theoretical prediction method can be used to predict the passivation effect of existing materials on silicon surface, effectively avoid the blindness of experiments and a lot of waste of resources, quickly judge the passivation structure, and then guide the synthesis of experiments, saving time and cost ; It can also be used to find and design suitable organic passivation materials, and on the basis of the predicted results, find or efficiently design passivation materials with good passivation effect, and improve the efficiency of scientific research. The prediction method of the invention reduces the interference of human factors and environmental factors in the experimental process, and has high reliability and good reproducibility.

Description of drawings

Figure 1 is a schematic diagram of the prediction flow of the Si surface effect of TFSI molecular passivation.

Figure 2 is a structural model of the TFSI ^- ionic group.

Figure 3 shows the structural unit of the Si(001) plane.

Figure 4 shows the adsorption model of Si(001) adsorbed by TFSI ^- ion groups in the OA direction.

Figure 5 is an adsorption model for Si(001) adsorbed by TFSI ^- ion groups in the OB direction.

Figure 6 is a diagram showing the gain and loss of Bader charge before and after the bonding between the TFSI ^- group and the Si surface layer. "+" indicates electron gain, and "-" indicates electron loss.

Figure 7 (a) and (b) are the ELF images on the left and right sides of the TFSI ^- group after bonding with the Si surface, respectively.

Detailed ways

The present invention will be further described below in conjunction with the examples, and the following examples are only for illustration and do not limit the protection scope of the present invention in any way.

The processes and methods not described in detail in the following examples are conventional methods well known in the art. The reagents used in the examples are all analytically pure or chemically pure, and can be purchased commercially or prepared by methods well known to those of ordinary skill in the art. The following embodiments all achieve the purpose of the present invention.

The calculation tool used in the present invention is the VASP software package based on the framework of density functional theory (DFT), and the pseudopotential adopts potpaw-PBE, namely GGA-PBE. When the plane wave cutoff energy is set to 500eV, the convergence criterion of the self-consistent field is set to be within EDIFF=1×10 ^-6 eV/atom. In the calculation, the (001) surface of Si is selected for adsorption, and the Brillouin zone integration adopts the special k-point method in the form of Monkhrst-Pack. When the Si (001) surface substrate extends in the A and B directions, its structural characteristics are slightly different: the dangling bonds on the Si surface in the A direction are arranged in parallel with the same distance; the dangling bonds in the B direction are parallel to each other , the distance between each pair and the adjacent dangling key is slightly farther than the A direction. The special k points of the Monkhrst-Pack network of the structure extending in the A direction are taken as 2 × 4 × 1, and the k points of the structure extending in the B direction are taken as 4 × 2 × 1.

In the embodiment of the present invention, organic small molecule TFSI and Si are used as adsorption materials, wherein the Si (001) surface is the adsorption surface, and the passivation effect of organic small molecule TFSI on the silicon surface is explored. The schematic flowchart of the prediction method is shown in Figure 1. .

First, the adsorption structure of the silicon surface passivated by organic molecules is calculated, and the adsorption energy is calculated to determine the adsorption mode and adsorption structure; then, the electronic structure properties, charge transfer, and bonding of the interface when the organic molecules passivate the silicon surface are calculated. Finally, the passivation ratio of the silicon surface atoms was judged according to the adsorption structure, and the passivation strength of the silicon surface atoms was judged according to the electronic structure properties, and the passivation effect was predicted.

First, the adsorption model of Si(001) surface passivated by TFSI molecule is established, the adsorption energy of TFSI molecule passivated Si(001) surface is calculated, and the stable passivation structure when TFSI molecule passivated Si(001) surface is determined.

TFSI (Triflouromethanesulfonimide), Chinese name bis-trifluoromethanesulfonimide, molecular formula C ₂ HF ₆ NO ₄ S ₂ , molecular weight 281.15; At room temperature, bis-trifluoromethanesulfonimide is a colorless needle-like crystal; TFSI It is easy to absorb water, and is easily soluble in water, alcohols, ethers, insoluble in n-hexane, benzene, etc. It will emit smoke when exposed to the air, easy to sublime, and has strong acidity and certain toxicity.

Since the TFSI molecule is a strong acid, H ⁺ is easily ionized in the solution to form a TFSI ^- ionic group, so the adsorption model adopts the ionic form of TFSI "TFSI ^- ionic group" The schematic diagram is shown in Figure 2. The schematic diagram of the structural unit of Si(001) plane is shown in Fig. 3. Due to the different distribution characteristics of the dangling bonds on the Si surface in different directions, the passivation of TFSI ^- ion groups on the Si surface has two directions: OA and OB, that is, there are two possible structures F1 and F2. The schematic diagrams are shown in Fig. 4 and 5 are shown. Based on the established adsorption model, the calculation results are summarized in Table 1.

Table 1 Adsorption energy of TFSI for Si adsorption

Remarks: The adsorption energy can be calculated by the formula E _ads =( E _total - E _{organic molecules} - E' )/n

From the results in Table 1, it can be seen that the adsorption energy of TFSI molecules adsorbed on the Si surface is negative, which means that TFSI can achieve passivation effect on Si through adsorption. Among the two adsorption models, F1 has the lowest energy, and this structure is the most stable.

Secondly, the corresponding model of F1 (Fig. 4) is further calculated, and its Bader charge (Fig. 6) and ELF (electron local function) (Fig. 7) are calculated respectively. From the perspective of Bader charge transfer, the charge is transferred from Si atoms to O atoms and S atoms, thus weakening the ability to combine with carriers. From the point of view of the bonding properties of atoms before and after the reaction, the O atom and the S atom in the TFSI molecule before the reaction form a covalent bond with strong locality; after the reaction, the locality of the bond at the corresponding position is weakened, forming a covalent bond. The strength of the bond is weakened. The locality of bonding between Si atoms and O atoms is centered, but the change in the local distribution characteristics of electrons can prove that the binding of TFSI molecules to Si weakens the surface activity of Si.

Finally, based on the above calculation results, the passivation effect of TFSI ^- groups on the Si(001) surface originates from the combination of O atoms and Si atoms, which neutralizes the dangling bonds that easily become recombination centers on the Si surface. In this model, the number of bonds between silicon surface atoms and organic molecules is 4, the number of unbonded atoms is 0, and the passivation ratio is 100%. From the point of view of the gain and loss of electrons before and after passivation, the process of charge transfer from Si atom to O atom is the process of bonding, and the four bonded Si atoms have undergone a large number of charge transfer, thus weakening the activity of Si. , to achieve a passivation effect.

Claims (6)

1. an organic molecule is to the prediction method of silicon surface passivation effect, it is characterized in that, adopt the first-principles method based on density functional to predict, comprise the steps: (a) Establish the adsorption model between organic molecules and the silicon surface, calculate the adsorption energy, and determine the stable passivation structure when organic molecules passivate the silicon surface; (b) Calculate the electronic structure properties, charge transfer and bond formation at the interface of the stable passivation structure; (c) Determine the passivation ratio of the silicon surface atoms according to the result of step (a), determine the passivation intensity of the silicon surface atoms according to the result of step (b), and then predict the passivation effect. 2. The method for predicting the passivation effect of organic molecules on silicon surface according to claim 1, wherein in step (a), the adsorption energy E _ads is the total energy after adsorption and the energy of organic molecules and pure silicon surface The difference between the two is the average of the number of bonded silicon atoms, which is expressed by the formula: E _ads =(E _total – E _{organic molecules} -E')/n, where E _total is the adsorption of organic molecules on pure semiconductors The total energy of the material surface, E _{organic molecules} and E' represent the energy of organic molecules and the energy of pure silicon surface respectively, n is the number of atoms on the passivated silicon surface. 3 . The method for predicting the passivation effect of organic molecules on silicon surface according to claim 1 , wherein in step (a), the determination of the stable passivation structure is determined by comparing the adsorption energy of various passivation structures. 4 . The lower the adsorption energy, the more stable the structure. 4 . The method for predicting the passivation effect of organic molecules on silicon surface according to claim 1 , wherein in step (b), first-principles software is used to calculate Bader charge and electron localization of the stable passivation structure, respectively. The Bader charge is used to judge the charge transfer of the silicon surface atoms, and the electron local function is used to judge the bonding strength of the silicon surface atoms and organic molecules. 5 . The method for predicting the passivation effect of organic molecules on silicon surface according to claim 1 , wherein in step (c), the result of step (a) is analyzed to calculate the number of bonds between silicon surface atoms and organic molecules. 6 . The ratio of the number of unbonded and unbonded is used as the basis for judging the passivation effect of the silicon surface. The more the number of bonds, the more obvious the passivation effect. 6 . The method for predicting the passivation effect of organic molecules on silicon surface according to claim 1 , wherein in step (c), the electronic structure properties after passivation of organic molecules on silicon surface are obtained according to the result of step (b) , and analyze the electronic structure properties to obtain the gain and loss of the silicon surface charge. The stronger the bond, the more charge transfer, and the higher the passivation degree.

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