RU2392694C2 - Method of making photovoltaic cell - Google Patents

Abstract

FIELD: physics, semiconductors.

SUBSTANCE: invention relates to optoelectronics and can be used as a silicon photocell for converting radiation energy to electric energy. The method of making a photovoltaic cell in which the working area of the photodiode and an anti-refraction coating are made through formation of a Schottky barrier by depositing a nickel film on n-type silicon with subsequent annealing at temperature ranging from 350 to 450°C for 30-60 minutes. Contacts to the working area of the photodiode are made either before annealing by depositing an array of electrodes from a noble metal (gold, platinum) onto a nickel surface or after annealing by depositing an array of nickel electrodes via a lithographic process with local NiO etching.

EFFECT: method of making a photovoltaic cell enables reduction of the number of process steps, that way reducing the cost of making solar module, while maintaining key characteristics such as efficiency and fill factor at the level of modern silicon photovoltaic cells.

3 cl, 1 ex

Description

Technical field

The invention relates to the field of optoelectronics and can be used as a silicon photocell for converting radiation energy into electrical energy.

State of the art

Photovoltaic cells are a semiconductor diode. In semiconductor materials, the absorption of incident photons leads to the generation of electron-hole pairs. In this process, the decisive parameter is the band gap of the semiconductor. The optimal value of the band gap, at which about half of the solar energy is spent on the formation of electron-hole pairs, is ~ 1.1 eV [1]. At the second stage, the separation of charges occurs due to the internal electric field created by the diode structure. The equivalent circuit of a photovoltaic cell device is a photovoltage source and a parallel-connected resistor that displays the recombination losses and shunting of the cell, a diode with a potential barrier, as well as a series-connected internal resistance of the cell. The current-voltage characteristics (CVC) of a photovoltaic cell when exposed to light irradiation repeat the dark CVC with a current offset by an amount equal to the short-circuit photocurrent. The ratio of the maximum output power at the point of maximum efficiency to the product of the short circuit current J _sc by the open circuit voltage V _oc is called the fill factor FF. J _sc , V _oc and FF are the three key electrical characteristics of solar cells. The maximum value of J _sc is determined by the photocurrent density of the solar cell J _ph .

The following requirements are met for photovoltaic cells.

1. High efficiency - a fraction of the energy of light irradiation converted into electrical energy.

2. High values of short circuit currents and open circuit voltage.

3. High fill factor - the ratio of maximum output power to the product of short circuit current and open circuit voltage.

4. The simplicity of the preparation of the element, cheap and efficient production technology for mass production.

Currently, the main method for the production of silicon solar cells includes the following steps:

- the formation of the pn junction (Schottky barrier) on the front side of the n-Si substrate by injection of a dopant,

- the formation of an ohmic contact with p-Si using nickel silicide obtained by metallization of silicon with nickel, followed by sintering (annealing) in an inert atmosphere or vacuum,

- applying an antirefractive layer to reduce reflection-related losses.

These manufacturing steps can be equally applied to both mono- and polycrystalline silicon. From the point of view of production of solar cells, the latter are more advantageous. However, due to the large number of disadvantages of polycrystalline silicon, such as losses on minority carriers at grain boundaries, dislocations, etc., the efficiency of solar cells on polycrystalline silicon is generally worse than for a single crystal. This circumstance can be improved by introducing monovalent elements, such as hydrogen, into the structure in order to reduce the phenomena associated with structural defects, thereby minimizing the recombination losses on minority carriers. After hydrogen passivation, silicon is heat-treated in vacuum, which returns the amount of hydrogen introduced to the level prior to passivation (see US patent 4321283, 26 10.1979).

In the patent US 4321283, 10.26.1979, a method for electrolytic metallization of silicon wafers with nickel is proposed. In particular, a method has been proposed for the production of solar cells with this nickel deposition method, followed by annealing in a hydrogen or nitrogen atmosphere and the formation of nickel silicide as an ohmic contact to the p-Si layer. The main component for the electrolytic metallization of silicon was an aqueous solution of nickel chloride. The formation of layers of nickel silicide forms an ohmic contact with p-type silicon. In addition, temperature ranges have been proposed for the formation of nickel silicide layers of various stoichiometry (Ni ₂ Si, NiSi and NiSi ₂ ).

In the patent US 4989059 06/30/1989 describes a method for the production of silicon solar cells, which proposes the alloying of silicon tape from both surfaces to form a pn junction. Phosphorus doping is performed to form the n-Si layer, and aluminum doping to produce the p-Si layer. It is proposed to use silicon nitride and nickel silicide as ohmic contacts to various types of silicon surfaces.

The closest analogue to this object of research is a photovoltaic cell, proposed in patent US 4609565, 12/13/1984. This patent describes a method for creating a silicon solar cell by forming a pn junction on silicon with a dopant (Al). Nickel silicide is used to create ohmic contact with the p-type silicon layer. The sprayed nickel layer is annealed in an inert gas or nitrogen atmosphere to form nickel silicide at the interface with the silicon substrate. As an antirefractive coating, sprayed TiO ₂ or SiN is used.

The manufacturing procedure of a photovoltaic cell is as follows:

(1) doping with aluminum n-Si silicon substrate to create a potential barrier,

(2) the formation of a thin grid of Nickel electrodes,

(3) hydrogen passivation of the p-junction,

(4) sintering the Si / Ni boundary to form nickel silicide,

(5) deposition of additional metal on nickel electrodes,

(6) anti-refractive coating.

Thus, this method of creating a photovoltaic cell includes a large number of separate technological steps for the formation of the final structure, which in turn is one of the factors of the high cost of production of solar cell modules.

Disclosure of invention

The objective of the invention is to create a silicon solar cell based on the structure of Si-NiO, having the main indicators at the level of modern silicon solar cells (efficiency> 10%, FF> 40%) and having a relatively cheap technology for production.

The objective of the invention is solved by creating an element based on a Schottky diode, where the barrier is provided by the n-Si / NiSi boundary, MO serves as an antirefractive layer, and both NiSi and NiO layers are produced in one technological process. In addition, the proposed invention is a thin-film structure, with a NiO layer thickness on the silicon substrate of not more than 100 nm.

The physical principle of the functioning of this research object as a photovoltaic cell is as follows. When thin nickel oxide films are formed by thermal annealing in the atmosphere, an intermediate layer of nickel silicide is formed between the oxide and the silicon substrate. Obtaining an ohmic contact to the intermediate layer can be carried out both using a channel of metallic Ni obtained as a result of breakdown of a NiO film, and using standard lithography methods. The internal photoelectric effect in the structure under study is associated with the formation of a Schottky barrier between the intermediate NiSi layer and the n-Si substrate. In this case, the generation of electron-hole pairs when exposed to radiation in the visible range occurs in the intermediate layer of NiSi, the space charge region and silicon.

The technical result of the invention in comparison with the prototype US 4609565, 12/13/1984, is that NiSi creating a Schottky barrier with n-Si, as well as an anti-refractive NiO coating, which serves to reduce reflection-related losses, are carried out at one technological stage. Thus, in comparison with the prototype, the stage of alloying with aluminum to obtain a p-Si layer is excluded, as well as a separate step of coating the front surface of the element with an anti-refractive coating.

The structure is annealed at temperatures from 350 to 450 ° С; the annealing time is 30-60 min. This temperature regime is justified by the fact that a temperature of at least 350 ° C is necessary for the formation of nickel silicide. At temperatures above 450 ° C, the NiSi layer is oxidized, which leads to the destruction of the structure and the disappearance of the photovoltaic effect. The annealing time is determined by the oxidation time of the NiO film to the silicon substrate, with an annealing time of less than 30 min, a layer of underoxidized nickel remains, which prevents light from entering the working area of the photovoltaic cell. An increase in the annealing time of more than 60 min is not technologically advanced.

The implementation of the invention

Metallization of n-Si is carried out by deposition of Ni by vacuum spraying methods. The next step is thermal annealing in air under normal conditions. The temperature regime during the annealing is from 350 to 450 ° C, which allows nickel oxidation to NiO, as well as sintering at the nickel-silicon interface with the formation of NiSi. The annealing time is 30-60 minutes, after which the resulting structure cools to room temperature. The creation of ohmic contacts to the working area of a photovoltaic cell can be carried out both by spraying a grid of electrodes of a noble metal (gold, platinum) on the surface of nickel before annealing (which prevents nickel oxidation under the electrode), and after annealing, by spraying a grid of nickel electrodes with using the lithographic process, with local etching of NiO.

An example implementation:

~ 60 nm thick Ni metal film is sprayed using DC magnetron sputtering in an industrial unit VUP-5M. The formation of ohmic contacts is carried out by spraying a grid of electrodes of a noble metal (gold, platinum) on the surface of nickel before annealing. The samples were then placed into a furnace and thermally oxidized in air at a temperature T _ox = 400 ° C for 60 minutes, then cool to room temperature. The thickness of the final NiO film is about 80 nm, and the specific resistance is ρ ~ 3 · 10 ⁴ Ohm · cm. After that, the photocurrent is recorded in the structure, when it is illuminated. Moreover, the short circuit current density J _sc = 4.28 mA / cm ² , the open circuit voltage U _oc = 95.3 mV, the efficiency η = 11.2% and the filling factor FF = 40.3%.

Thus, this method of producing a photovoltaic cell can reduce the number of technological steps, thereby making it possible to reduce the cost of production of solar modules. At the same time, the main indicators, such as η and FF, remain at the level of modern silicon photovoltaic cells.

Literature

1. A.Shah, P. Tascharner, N. Wyrsch, H. Keppner // ISNN 0036-8075 692, 285, (1999).

Claims (3)

1. A method of producing a photovoltaic cell, including the creation of a photodiode working area, ohmic contacts to it, as well as an anti-refractive coating, characterized in that the working area of the photodiode and anti-refractive coating are created by forming a Schottky barrier by sputtering a nickel metal film on n-type silicon, followed by annealing at a temperature of 350 to 450 ° C for 30-60 minutes. 2. The method of producing a photovoltaic cell according to claim 1, characterized in that the contacts to the working area of the photodiode are created before annealing by spraying a grid of electrodes of a noble metal (gold, platinum) on the surface of nickel. 3. The method of producing a photovoltaic cell according to claim 1, characterized in that the contacts to the working area of the photodiode are created after annealing by spraying a grid of nickel electrodes using a lithographic process with local etching of NiO.