A low-cost non-doped heterojunction crystalline silicon solar cell and a method for manufacturing the same

Abstract

The application discloses a low-cost non-doped heterojunction crystalline silicon solar cell and a preparation method thereof. A growth device used in the application comprises a mixing chamber and a reaction cavity. A growth source solution is evaporated by an evaporator and enters the mixing chamber in a hydrogen carrying mode, and then enters the reaction cavity. Two metal grid electrodes are arranged in the reaction cavity. A radio frequency voltage is applied between the two metal grid electrodes. The cell has the structure of Ag / ITO / Mn z1 MoO y2 / MoO y1 / SiO p / p‑c‑Si / SiO p / TiO x1 / Mn z2 TiO x2 / Ca / Al. The application can improve the passivation effect of a titanium oxide or molybdenum oxide film on crystalline silicon, and greatly reduces the bombardment of plasma on the film during the preparation process, and improves the passivation performance of the film.

Description

Technical Field

[0001] This invention belongs to the field of crystalline silicon solar cells and relates to a low-cost undoped heterocrystalline silicon solar cell and its preparation method. Background Technology

[0002] To improve the efficiency of crystalline silicon (c-Si) solar cells, it is essential to reduce losses such as surface carrier recombination, contact resistance, and parasitic absorption. In current mainstream emitter-passivated (PERC) cells, the direct contact between the metal electrode and the silicon wafer leads to severe recombination losses of photogenerated electrons and holes. Although heavy doping below the contact electrode can significantly reduce carrier recombination, it increases Auger recombination and free carrier absorption, thereby degrading cell performance. Therefore, to eliminate the direct contact between the metal and silicon, full-surface passivation and selective carrier contact technologies have been proposed to further improve the efficiency of crystalline silicon solar cells.

[0003] Titanium oxide (TiO) x ) and molybdenum oxide (MoO) y It has a large band gap energy (E) g The band gap of TiO2 (>3.0 eV) significantly reduces parasitic absorption compared to the 1.7 eV band gap of amorphous silicon. Furthermore, due to the low work function (3.8–4.2 eV) of titanium oxide, TiO2... x The / c-Si interface exhibits a small conduction band shift and a large valence band shift, making it well-suited as an electron-selective contact layer for crystalline silicon heterojunction solar cells. Molybdenum oxide possesses a high work function (5.4–6.1 eV), while TiO2... x The / c-Si interface has a large conduction band offset and a small valence band offset, making it very suitable as a hole-selective contact layer for crystalline silicon heterojunction solar cells.

[0004] Compared to heterocrystalline silicon (HIT) cells, which consist of an intrinsic hydrogenated amorphous silicon film deposited on the front and back surfaces of crystalline silicon, and a doped hydrogenated amorphous silicon film with the opposite doping type, current TiO2 cells... x / c-Si / MoO y The efficiency of undoped heterojunction silicon solar cells is still relatively low, mainly due to the TiO₂ structure. x and MoO yThe high resistivity of thin films necessitates a thickness of less than 10 nm for use as passivation contact layers; otherwise, it leads to significant contact resistance. However, excessively thin passivation layers result in poor passivation performance and increased surface recombination current. Furthermore, as the deposition temperature or subsequent annealing temperature of titanium oxide or other oxide films increases, the film transforms from an amorphous to a crystalline state, deteriorating its passivation effect on crystalline silicon surfaces. Currently, both titanium oxide and molybdenum oxide films are prepared using thermal evaporation or atomic layer deposition (ALD) methods. Thermal evaporation methods struggle to precisely control film thickness and oxygen composition, while ALD methods suffer from slow deposition rates and expensive equipment. Summary of the Invention

[0005] The purpose of this invention is to provide a low-cost, undoped heterocrystalline silicon solar cell and its preparation method.

[0006] The applicant's research found that during the growth of titanium dioxide and molybdenum oxide thin films, when the radius of the doped manganese ions differs significantly from that of titanium or molybdenum ions, the substitution of titanium or molybdenum ions by manganese ions causes lattice distortion. This distortion-induced stress hinders grain boundary movement and inhibits grain growth. Furthermore, the doped manganese ions, replacing titanium or molybdenum ions, are randomly distributed at the positions of titanium or molybdenum ions in the lattice, forming titanium dioxide-manganese oxide or molybdenum oxide-manganese oxide, which also inhibits further grain growth. Therefore, incorporating manganese ions into titanium dioxide or molybdenum oxide can suppress the transformation of titanium dioxide or molybdenum oxide thin films from an amorphous phase to a crystalline phase, or in other words, increase the phase transformation temperature, thereby improving the passivation effect of titanium dioxide or molybdenum oxide thin films on crystalline silicon.

[0007] Plasma-enhanced chemical vapor deposition (PECVD) is a novel fabrication technique that activates gas molecules in reactants into active ions, causing a chemical reaction in the gaseous substances containing the thin film composition, thereby achieving thin film growth. The deposited films exhibit excellent uniformity and are highly suitable for large-area industrial fabrication. Using PECVD to deposit titanium oxide and molybdenum oxide films on the front and back surfaces of silicon, if hydrogen and chlorine gases are introduced into the reaction atmosphere, hydrogen atoms diffuse to the film and silicon surface during deposition and subsequent annealing, passivating dangling bonds on the silicon surface. Simultaneously, chlorine atoms also diffuse to the film and silicon surface, combining with silicon near the silicon interface to form chlorine-silicon bonds, reducing oxygen vacancies and silicon dangling bonds, thus reducing the fixed charge and interface state density of the film and improving the passivation effect on the silicon surface. Furthermore, during the subsequent film annealing process, oxygen atoms in the film diffuse to the interface and combine with silicon atoms to form an ultrathin silicon oxide layer on the silicon surface, providing excellent passivation for the silicon surface.

[0008] Based on the above research, the technical solution adopted to achieve the first inventive objective is as follows: a low-cost, undoped heterocrystalline silicon solar cell, characterized by having the following structure: Ag / ITO / Mn z1 MoO y2 / MoO y1 / SiO p / pc-Si / SiO p / TiO x1 / Mn z2 TiO x2 / Ca / Al, where 1 <x1<1.8,1<x2<1.8,1<y1<2.5,1<y2<2.5,0<z1<0.15,0<z2<0.15,1<p<2,SiO p The layer thickness is less than 1.5 nm.

[0009] To achieve the second objective of the invention, the technical solution adopted is as follows:

[0010] A low-cost method for fabricating undoped heterocrystalline silicon solar cells, characterized in that: the growth equipment used includes a mixing chamber and a reaction chamber; the growth source solution is evaporated through an evaporator and carried into the mixing chamber by hydrogen gas, and then enters the reaction chamber; the reaction chamber is provided with upper and lower metal mesh electrodes, and a radio frequency voltage is applied between the upper and lower metal mesh electrodes; the fabrication method includes the following steps:

[0011] 1) Silicon wafer cleaning;

[0012] 2) Molybdenum oxide thin film growth: After cleaning, the silicon wafer is fixed on a rotatable stage in the reaction chamber. The power supply to the MoCl5 evaporator is turned on, and oxygen and MoCl5 carried by hydrogen are introduced. The radio frequency power supply is turned on, and plasma is generated under the excitation of the radio frequency voltage. Then, a reaction occurs, forming the reactant MoO. y1 Deposited onto the surface of a silicon wafer;

[0013] 3) Growth of molybdenum manganese oxide thin films: MoO y1 After the thin film growth is complete, the power to the MnCl2 evaporator is turned on. MnCl2, carried by hydrogen gas, enters the reaction chamber to participate in the reaction, forming the reactant Mn. z1 MoO y2 Deposition to MoO y1 above;

[0014] 4) Titanium oxide thin film growth: A robotic arm is used to flip the silicon wafer, the power to the MoCl5 and MnCl2 evaporators is turned off, and the power to the TiCl4 evaporator is turned on. After the MoCl5 carried by hydrogen is mixed with oxygen, plasma is generated under the excitation of radio frequency power, forming TiO2 on the back side of the silicon wafer. x1film;

[0015] 5) Titanium oxide manganese thin film growth: TiO x1 After the thin film growth is complete, the power to the MnCl2 evaporator is turned on. MnCl2, carried by hydrogen gas, enters the reaction chamber to participate in the reaction, forming the reactant Mn. z2 TiO x2 Deposition to TiO x1 above;

[0016] 6) In-situ annealing to form ultrathin silicon oxide: After the growth of molybdenum oxide, molybdenum manganese oxide, titanium oxide, and titanium manganese oxide films is completed, all gas valves and radio frequency power supply are closed, the chamber is kept under vacuum with a vacuum degree better than 0.01 Pa, and the sample is annealed in situ for 5 to 10 minutes at an annealing temperature of 300 to 350 °C to form ultrathin silicon oxide.

[0017] 7) Ca / Al back electrode fabrication: Mn is deposited on the back surface of the silicon wafer using a vapor deposition method. z2 TiO x2 First, a 10-20 nm calcium film is grown on the thin film, and then a 300-500 nm aluminum film is grown as the back electrode of the battery.

[0018] 8) Growth of ITO and silver electrodes on the front surface: Mn electrode growth on the front surface of the silicon wafer was achieved using magnetron sputtering. z1 MoO y2 A 70-80 nm ITO conductive film and a 300-500 nm interdigitated silver electrode are sequentially grown on the thin film.

[0019] In this invention, MoO y The growth source for the thin film is a molybdenum pentachloride (MoCl5) solution. Hydrogen gas (H2) is used as the carrier gas to bring MoCl5 into the PECVD reaction chamber, where it reacts with oxygen (O2) to generate MoO. y The reaction equation is as follows:

[0020] MoCl5 (gaseous) + H2 (gaseous) + O2 (gaseous) → MoO y (Solid) + HCl (Gaseous)

[0021] Similarly, Mn z MoO y The thin film is formed by carrying MoCl6 and manganese dichloride (MnCl2) into the cavity with hydrogen gas to react with oxygen. The reaction equation is as follows:

[0022] MoCl5 (gaseous) + MnCl2 (gaseous) + H2 (gaseous) + O2 (gaseous) → Mn z MoO y (Solid) + HCl (Gaseous)

[0023] TiO x and Mn z TiO x The thin film was grown from titanium tetrachloride (TiCl4) solution and MnCl2 solution, with hydrogen gas as the carrier gas. The hydrogen gas reacted with oxygen to form the thin film. The reaction equation was similar to that described above.

[0024] Ultrathin SiO p The layer is formed during the annealing process after thin film growth, where oxygen atoms in the film diffuse to the interface between the film and silicon and combine with silicon atoms. In Mn... z TiO x Inserting a layer of low work function metallic calcium (Ca) between the aluminum (Al) electrode and the aluminum electrode is to reduce the Mn content. z TiO x The potential barrier between Al and Al can reduce contact resistance.

[0025] During the deposition of PECVD thin films, the bombardment of the film by plasma increases the defect density within the film, leading to a decrease in the passivation performance of the film on silicon. To address this, the present invention designs a metal mesh electrode scheme, which significantly reduces the bombardment of the film by plasma and improves the passivation performance of the film. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the solar cell of the present invention.

[0027] Figure 2 This is a schematic diagram of the PECVD growth equipment used in this invention.

[0028] Figure 2 The reference numerals in the attached figures are as follows: 1-vacuum pump, 2-rotatable base, 3-heatable stage, 4-substrate, 5-metal grid, 6-reaction chamber, 7-RF transmitter and matching device, 8-gas mixing chamber, 9-MoCl5 solution, 10-TiCl4 solution, 11-evaporator, 12-MnCl2 solution, T1~T7-gas valves. Detailed Implementation

[0029] Example 1: Preparation Process

[0030] 1. PECVD growth equipment

[0031] The upper and lower metal mesh electrodes are made of 60-100 mesh stainless steel, such as... Figure 2 As shown, the number of mesh openings per square inch is called the mesh count. The mesh diameters for 60-mesh and 100-mesh are 0.25 mm and 0.15 mm, respectively. An RF voltage (frequency from 50 kHz to 13.56 MHz) is applied between the upper and lower metal mesh electrodes, with the lower electrode grounded.

[0032] Since the growth source designed in this invention is a solution, the solution is evaporated by an evaporator and enters the mixing chamber by being carried by hydrogen gas, and then enters the PECVD reaction chamber. All pipe connections and the mixing chamber need to be heated to avoid condensation.

[0033] 2. Heating temperature of growth source solution and evaporator

[0034] The evaporator heating temperature for TiCl4 solution is 70–80℃. MnCl2 solution is formed by heating hydrated manganese chloride in the evaporator to 70–80℃. MoCl5 solution is formed by heating molybdenum pentachloride in the evaporator to 210–230℃.

[0035] 3. Silicon wafer cleaning

[0036] The sample substrate was a double-sided polished single-crystal silicon wafer (p-type, resistivity 0.5–5.0 Ω·cm, thickness 0.2–0.3 mm). The substrate was cleaned using the standard RCA (first created by Kern and Puotinen et al. in the RCA laboratory at NJ Princeton) cleaning process, then rinsed repeatedly with deionized water and dried with nitrogen gas before use.

[0037] 4. Molybdenum oxide (MoO) y1 Thin film growth

[0038] After cleaning, the silicon wafer is fixed on a rotatable stage within the PECVD chamber. The stage temperature is maintained at 100–150°C, and the chamber's base vacuum is better than 0.01 Pa. The power to the MoCl5 evaporator is turned on, and gas valves T5, T6, and T7 are opened to introduce oxygen and MoCl5 carried by hydrogen. The radio frequency power supply is then turned on at a power of 50–100 W. Under the excitation of the radio frequency voltage, plasma is generated, and then a reaction occurs, forming the reactant MoO. y1 The film is deposited onto the surface of a silicon wafer with a thickness of 20–30 nm, and the film thickness is monitored by a quartz crystal thickness gauge.

[0039] 5. Molybdenum manganese oxide (Mn) z1 MoO y2 Thin film growth

[0040] MoO y1 After the thin film growth is complete, the power to the MnCl2 evaporator is turned on, and the T3 and T4 gas valves are opened. MnCl2, carried by hydrogen gas, enters the PECVD chamber to participate in the reaction, forming the reactant Mn. z1 MoO y2 Deposition to MoO y1 The thickness on the top is 10-20 nm.

[0041] 6. Titanium oxide (TiO2)x1 Thin film growth

[0042] Using a robotic arm, the silicon wafer is flipped, gas valves T3, T4, T6, and T7 are closed, the power to the MoCl5 and MnCl2 evaporators is turned off, the power to the TiCl4 evaporator is turned on, and gas valves T1 and T2 are opened. After the MoCl5 carried by hydrogen mixes with oxygen, plasma is generated under the excitation of the radio frequency power supply, forming TiO2 on the back side of the silicon wafer. x1 Thin film with a thickness of 20–30 nm.

[0043] 7. Titanium manganese oxide (Mn) z2 TiO x2 Thin film growth

[0044] TiO x1 After the thin film growth is complete, the power to the MnCl2 evaporator is turned on, and the T3 and T4 gas valves are opened. MnCl2, carried by hydrogen gas, enters the PECVD chamber to participate in the reaction, forming the reactant Mn. z2 TiO x2 Deposition to TiO x1 The thickness on the top is 10-20 nm.

[0045] 8. In-situ annealing to form ultrathin silicon oxide (SiO2) p )

[0046] After the growth of molybdenum oxide, molybdenum manganese oxide, titanium oxide, and titanium manganese oxide thin films is completed, all gas valves and RF power supplies are turned off, and the chamber is kept under vacuum with a vacuum level better than 0.01 Pa. The samples are annealed in situ for 5–10 minutes at an annealing temperature of 300–350 °C. Oxygen atoms in the thin film diffuse to the silicon interface, forming ultrathin silicon oxide.

[0047] 9. Ca / Al back electrode preparation

[0048] Mn deposited on the back surface of a silicon wafer using a vapor deposition method z2 TiO x2 First, a 10-20 nm calcium (Ca) film is grown on the thin film, and then a 300-500 nm aluminum (Al) film is grown as the back electrode of the battery.

[0049] 10. Growth of ITO and silver electrodes on the front surface

[0050] Mn was deposited on the front surface of a silicon wafer using magnetron sputtering. z1 MoO y2A 70–80 nm ITO conductive film and a 300–500 nm interdigitated silver electrode are sequentially grown on the film. The sputtering targets are both ITO and silver, with a purity greater than 99.99%, and the sputtering working gas is argon. First, the ITO film is sputtered, with a base vacuum in the sputtering chamber better than 0.001 Pa, a sputtering pressure of 0.1–0.3 Pa, and a deposition temperature of room temperature. Second, the interdigitated silver electrode is sputtered using a grid line mask to form a silver grid line electrode.

[0051] Example 2: Obtaining the product

[0052] The product obtained by using the preparation method of Example 1 is as follows: Figure 1 As shown. The top-to-bottom structure is Ag / ITO / Mn. z1 MoO y2 / MoO y1 / SiO p / pc-Si / SiO p / TiO x1 / Mn z2 TiO x2 / Ca / Al, the thickness of each layer is shown in the figure.

Claims

1. A low-cost method for fabricating undoped heterocrystalline silicon solar cells, characterized in that: The solar cell described has the following structure: Ag / ITO / Mn z1 MoO y2 / MoO y1 / SiO p / pc-Si / SiO p / TiO x1 / Mn z2 TiO x2 / Ca / Al, where 1 <x1<1.8, 1<x2<1.8,1<y1<2.5,1<y2<2.5,0<z1<0.15, 0<z2<0.15,1<p<2,SiO p The layer thickness is less than 1.5 nm; The growth equipment used includes a mixing chamber and a reaction chamber. The growth source solution is evaporated through an evaporator and carried into the mixing chamber by hydrogen gas, and then enters the reaction chamber. The reaction chamber is equipped with two metal mesh electrodes, one above the other, and a radio frequency voltage is applied between the two metal mesh electrodes. The preparation method includes the following steps: Silicon wafer cleaning; Molybdenum oxide thin film growth: After cleaning, the silicon wafer is fixed on a rotatable stage within the reaction chamber. The power supply to the MoCl5 evaporator is turned on, and oxygen and MoCl5 carried by hydrogen are introduced. The radio frequency power supply is then turned on, and plasma is generated under the excitation of the radio frequency voltage. A reaction then occurs, forming the reactant MoO. y1 Deposition onto silicon wafer surface; Molybdenum manganese oxide thin film growth: MoO y1 After the thin film growth is complete, the power to the MnCl2 evaporator is turned on. MnCl2, carried by hydrogen gas, enters the reaction chamber to participate in the reaction, forming the reactant Mn. z1 MoO y2 Deposition to MoO y1 above; Titanium oxide thin film growth: A robotic arm is used to flip the silicon wafer, the power to the MoCl5 and MnCl2 evaporators is turned off, and the power to the TiCl4 evaporator is turned on. After MoCl5 carried by hydrogen is mixed with oxygen, plasma is generated under radio frequency power excitation, forming TiO2 on the back side of the silicon wafer. x1 film; Titanium oxide manganese thin film growth: TiO x1 After the thin film growth is complete, the power to the MnCl2 evaporator is turned on. MnCl2, carried by hydrogen gas, enters the reaction chamber to participate in the reaction, forming the reactant Mn. z2 TiO x2 Deposition to TiO x1 above; In-situ annealing to form ultrathin silicon oxide: After the growth of molybdenum oxide, molybdenum manganese oxide, titanium oxide, and titanium manganese oxide films is completed, all gas valves and radio frequency power supply are closed, the chamber is kept under vacuum with a vacuum degree better than 0.01 Pa, and the sample is annealed in-situ for 5~10 minutes at an annealing temperature of 300~350 ℃ to form ultrathin silicon oxide. Ca / Al back electrode fabrication: Mn on the back surface of the silicon wafer was deposited using a vapor deposition method. z2 TiO x2 First, a 10-20 nm calcium film is grown on the thin film, and then a 300-500 nm aluminum film is grown as the back electrode of the battery. Front surface ITO and silver electrode growth: Mn was deposited on the front surface of the silicon wafer using magnetron sputtering. z1 MoO y2 A 70-80 nm ITO conductive film and a 300-500 nm interdigitated silver electrode are sequentially grown on the thin film.

Images