DE102007011749A1 - Solar cell manufacturing method, involves providing semiconductor substrate with pn-junction formed in it and removing dielectric layer in local areas by ultra short pulse laser with laser pulse duration of less than hundred picoseconds - Google Patents

Abstract

The method involves providing a semiconductor substrate with a pn-junction formed in it. The dielectric layer is removed in local areas by an ultra short pulse laser with a laser pulse duration of less than 100 picoseconds, particularly less than 50 picoseconds and stronger prefers less than 20 picoseconds.

Description

The Invention relates to a process for the production of solar cells, one at a surface a solar cell located dielectric layer by means of a laser is removed locally.

solar cells serve to convert light into electrical energy. For this purpose points a semiconductor cell forming semiconductor regions of different polarity. For example For example, an emitter region having a first polarity may be formed of an n-type semiconductor whereas a base region having a second polarity is one p-type semiconductor may be formed. By at the interface between the both areas formed pn junction can Charge carrier pairs, the light incident upon absorption is separated become. To the so separated charge carriers an external circuit respectively to be able to are the base and emitter regions through base and emitter contacts electrically contacted.

Many solar cells have an optically transparent dielectric layer on at least one of their surfaces. For example, a layer of silicon oxide (usually SiO ₂ ), silicon nitride (eg Si ₃ N ₄ ) or another transparent dielectric or mixtures or layer sequences thereof may be deposited on the front and / or back of a solar cell. As a method for depositing such dielectric layers, for example vapor deposition, thermal growth, PECVD (Plasma Enhanced Chemical Vapor Deposition), LPCVD (Liquid Phase CVD) can be used on an industrial scale.

The Dielectric layers can serve different purposes. Often They are used as a surface passivation layer used to the surface a semiconductor substrate forming the actual solar cell passivate to there a recombination of carrier pairs to reduce and so increase the efficiency of the solar cell. The Dielectric layer can also serve as an antireflection layer, if your layer thickness is adjusted accordingly, e.g. reflections incident light on the sun-facing side to avoid and so also to increase the efficiency of the solar cell. dielectric layers can also while of the manufacturing process of a solar cell, e.g. around certain areas during to protect a process step. For example can Areas of a semiconductor substrate during a diffusion step, at the e.g. in a p-type substrate superficial Dopants are diffused to a thin n-type layer as an emitter layer to form, completely or partially protected against ingress of dopants, by covering these areas with a dielectric layer.

at the diverse ones Applications of a dielectric layer, it is often necessary to apply a flat To open the dielectric layer locally, i.e. wholly or partially locally to remove an underlying Expose layer or underlying substrate. For example must in a typical solar cell application, when the back a surface of a solar cell with a dielectric layer is to open the dielectric layer locally to the open To be able to apply fine metal contacts that under the dielectric layer Contact directly lying semiconductor directly electrically.

The Structure sizes of to open Areas can being very small, often in the range of 10μm up to a few hundred microns, so removing the dielectric layer with conventional hardly possible by mechanical means is.

Become conventional Dielectric layers often with the help of lithography process locally open. there a photosensitive lacquer is applied to the dielectric layer, exposed locally and then The exposed areas of the photosensitive resist become chemically away. Subsequently, the dielectric layer in the areas that no longer protected by the photoresist, etched away and thus open locally become. That way you can Today structures of well below a micrometer in dielectric layers already etched become. However, lithographic processes are expensive and laborious.

Alternatively, in the last few years, in the manufacture of solar cells, lasers are also used to locally remove dielectric layers. In this case, a high-energy, coherent laser beam is directed onto the region of the dielectric layer to be removed. The use of this so-called laser ablation in solar cell production is already successfully used to locally open both silicon nitride layers and silicon oxide layers. Frequently, solid-state lasers with a wavelength of, for example, λ = 1064 nm, λ = 532 nm or λ = 355 nm but also excimer lasers (gas lasers) with a wavelength of, for example, λ = 248 nm or λ = 193 nm are used. The laser beam sources are operated in pulse mode so that high pulse peak powers are available. The pulse durations are usually in the nanosecond range (eg τ _pulse = 20 ≅ 40ns).

If the energy of the photons in the laser beam is greater than the energy band gap of the dielectric layer forming material, ie if the wavelength of the laser light is short enough, individual photons may excite the material of the dielectric layer and thus be absorbed in the dielectric layer. The dielectric layer is heated to a great extent and melts and finally evaporates locally in the areas irradiated with the laser beam. With lasers with wavelengths in the UV range, a defined ablation of silicon nitride layers can be achieved, since they absorb well in this wavelength range and thus a direct removal is possible.

at However, silicon dioxide layers are found using conventional Lasers up to the present in industrial use technically achievable minimum wavelengths no appreciable absorption of 193nm, because silica a very big one Energy band gap of 9.3eV (corresponds to λ = 133nm). A silicon dioxide layer can be with the above usual Laser beam sources only by melting the under the layer ablate lying silicon substrate and subsequent solidification. The expansion of Siliciumschmeize causes a flaking of the overlying brittle Silicon dioxide layer. This process, however, can only be very insufficient check. Furthermore For example, the silicon substrate may be damaged by the surface melting. It can Recombination centers are formed, which reduces the efficiency of the finally produced solar cell lowers. To such a reduction To avoid the efficiency, the laser would have to thermally resulting damage after lasering by etching the melted during laser and subsequently be removed again solidified layer. This is not possible for all production sequences, e.g. already generated Emitter layers or a metallization could be attacked, and would on definitely one extra Step mean.

It It is an object of the present invention to provide a method of manufacturing a solar cell having a dielectric layer formed on a surface in which the dielectric layer is removed locally in particular without the problems described above occur. In particular, a method for producing a Solar cell with a method for a low-damage, Controllable and direct removal of optically transparent Dielectric layers are proposed.

According to one Main aspect of the present invention is a method for manufacturing a solar cell according to claim 1 proposed. It includes the following steps: Deploy a semiconductor substrate having a pn junction formed therein; Forming a substantially optically transparent dielectric layer on a surface the semiconductor substrate; and removing the dielectric layer at least in local areas by means of an ultrashort pulse laser with a laser pulse duration of less than 100ps. Laser pulse durations less than 50ps, stronger preferably less than 20ps, even more preferably less than 10ps and more preferably less than 1ps are advantageous.

characteristics and preferred embodiments The present invention will be described below in detail.

The Semiconductor substrate may be any substrate that consists of a semiconducting material is formed. For the production of solar cells on an industrial scale Silicon substrates are preferably used. These can be in Shape of a wafer or as a thin-film substrate be used. The inventive method can also with be applied to any other semiconductor substrates.

Of the formed in the semiconductor substrate pn junction can be arbitrary Been created in the way. A common in industrial manufacturing Method is to place in a silicon wafer with a basic doping, e.g. a boron doped p-type wafer, a dopant such as e.g. Diffuse phosphorus superficially and so an emitter layer of the opposite conductivity type, e.g. a n-type emitter layer to produce. At the boundary layer of the emitter layer to the rest of the wafer substrate forms a pn junction. The pn junction but can also be generated in any other known manner, such as e.g. by depositing an additional Emitter layer on a base substrate, inducing an emitter inversion layer in a base substrate, etc.

The on a surface The solar cell formed dielectric layer may be a layer be made of any dielectric material. The term "dielectric" is used here for electrical used insulating materials. As more detailed below explained is to be, the inventive method is particularly advantageous if for the Dielectric layer of silicon dioxide or another dielectric with a big one Energy band gap used becomes. The dielectric layer can be formed by any method be applied, e.g. by vapor deposition, thermal growth, PECVD, LPCVD, APCVD (Atmospheric Pressure CVD), sputtering, imprinting dielectric layers e.g. by screen printing, etc.

The Dielectric layer should be substantially optically transparent. That is, ambient light having wavelengths in the visible range, ie with wavelengths between about 400 and 800nm, should be approximate unimpeded transmitted through the dielectric layer can be without there being any appreciable absorption.

The dielectric layer can be removed locally with the aid of an ultrashort pulse laser. An ultrashort pulse laser is to be understood in this context as a laser in which laser light is emitted in very short pulses of less than 100 ps (100 × 10 ^-12 s). Such lasers are also referred to as picosecond lasers. At energies of up to a few millijoules per laser pulse, picosecond laser powers of up to 10MW and more can be achieved. With so-called femtosecond lasers, where the pulse durations range between about 100 fs and 1 ps (10 ^-13 to 10 ^-12 s), even powers of up to 20GW can be achieved.

In the individual laser pulses of an ultrashort pulse laser, there is an extremely high average energy density of, for example, more than 100 mJ / cm ² and, accordingly, a very high average light intensity of, for example, more than 10 GW / cm ² . The photon density is extraordinarily high. If such laser pulses are irradiated onto the dielectric layer of a solar cell to be processed, so-called multiphoton processes occur in the material of the dielectric layer. These are often referred to as non-linear optical effects. Several photons quasi-simultaneously energize an electron across the band gap of the material, resulting in the absorption of these photons. It is noteworthy that the energy of each individual photon emitted by the short-pulse laser can be smaller than the energy band gap of the material of the dielectric layer. In this way, materials can also be processed with an ultrashort pulse laser which under normal conditions (ie photon densities in which there are hardly any multiphoton effects) would not absorb light at the wavelength emitted by the ultrashort pulse laser at all or only to a very low degree.

One Another advantage of using an ultrashort pulse laser for local Removing a dielectric layer is adjacent to the high laser power and the resulting high photon density in the brevity of Laser pulses justified. The high power of the laser is achieved by the one described above Multiphoton effects caused absorption on the electrons of the Dielectric layer forming atoms transferred. It also can the bonds of the atoms with neighboring atoms are broken up and the respective atom is thus broken out of the bond with the neighboring atoms become. If the duration of a laser pulse is very short, can the duration of the interaction between the laser radiation and the Matter of the dielectric layer to be too short as the essential Quantities of energy in the form of heat from transferred the excited atom to the neighboring atomic network could become. It comes to the so-called "cold Ablation ", at the single atoms torn out of the atomic network by a laser pulse without causing any significant heating of the remaining Atomic Network would come.

The Applicant has succeeded in locally ablating a silicon dioxide layer applied to a solar cell (band gap of SiO ₂ : 9.3 eV, corresponding to a wavelength of 133 nm) with the aid of a neodymium-vanadate ultrashort pulse laser. The Nd-VAN laser (also referred to as Nd: YVO ₄ picosecond laser) had a pulse duration of about 10ps and an average pulse power of about 28 to 68mW. The original wavelength of the laser of 1064nm was halved by frequency doubling to 532nm. The pulse energy was between about 280 and 680nJ. The mean energy density was between about 360 and 870 mJ / cm ² and the mean light intensity was between about 36 and 87 GW / cm ² . The spot size of the focused laser beam was about 10μm, which allowed ablation of structures of the same order of magnitude. The laser had a repetition rate of about 100kHz. With an ultrashort pulse laser with the foregoing parameters, a silicon dioxide layer, typically used on the back side of back contact solar cells, could be ablated locally to a thickness of the order of 100 nm with a single laser pulse. The laser can therefore be scanned in a simple, known technique on the solar cell surface. High processing speeds are possible.

alternative can also be used in the laboratory, a titanium sapphire laser, the Pulse durations in the range of a few hundred femtoseconds allows. However, lets the difficult handling of a titanium sapphire laser hitherto for the industrial Use inappropriate appear.

With The method described above can be achieved by the use of an ultrashort pulse laser to be avoided when locally removing a dielectric layer from the surface a solar cell adjacent areas of the dielectric layer or Overheat or even melt underlying areas of a substrate. Ultimately, in particular, the efficiency of a solar cell mitigating damage the substrate surface be prevented.

With the aid of the method according to the invention, solar cells can be produced in which a silicon dioxide layer grown on the rear side of the solar cell was locally opened by means of an ultrashort pulse laser. In this case, photolithographic masking as well as subsequent wet chemistry, for example, in order to etch away crystal damage that has arisen during laser cutting, is dispensed with become.

Claims (7)

A method of manufacturing a solar cell comprising the following steps: Providing a semiconductor substrate with a pn junction formed therein; Form an optically transparent dielectric layer on a surface of the Semiconductor substrate; marked by Removing the Dielectric layer at least in local areas by means of a Ultra short pulse laser with a laser pulse duration of less than 100 Picoseconds, preferably less than 50 picoseconds, and more preferably less than 20 picoseconds. The method of claim 1, wherein the energy of a individual photons emitted by the ultrashort pulse laser become smaller is as the energy band gap the material of the dielectric layer. The method of claim 1 or 2, wherein a laser pulse of the ultrashort pulse laser has a power of at least about 1 MW. The method of claim 1, 2 or 3, wherein the average intensity of the ultrashort pulse laser is at least about 10GW / cm ² , preferably at least about 40GW / cm ² . Method according to one of claims 1 to 4, wherein the average energy density of the ultrashort pulse laser is at least about 100mJ / cm ² , preferably at least about 400mJ / cm ² . Method according to one of claims 1 to 5, wherein the dielectric layer has a silicon dioxide layer. Method according to one of claims 1 to 6, wherein the short-pulse laser an Nd: VAN laser is.