Solar cell and an arrangement and a method for producing a solar cell

The present invention relates generally to solar cells, material layers within solar cells, a production method of solar cells, and a manufacturing arrangement for producing solar cells. More specifically, the present invention relates to what is disclosed in the preamble of the independent claim.

Solar cells provide an ecological way of producing energy, and they have therefore been under intensive development. Solar cells are generally made of photovoltaic cells. A photovoltaic cell has a at least one semiconducting layer, wherein photons of light are absorbed. The absorption of light causes electrons or holes to transfer to a higher, conducting energy level, and this energy can be used as electricity.

In order to conduct the formed electricity out of the solar cells there must be a conducting layer at the both sides of the semiconducting layer(s). The conducting layer at the radiated surface of the solar cell must allow light to enter the semiconductor layer. A solar cell is usually partitioned into small cells which are connected in series or parallel. In this case the conducting layers and possibly the semiconductor layer(s) are patterned to provide the required circuitry.

The radiated surface of the solar cell is further coated with one or several layers for providing antireflection surface for the solar cell, and for protecting the solar cell from mechanical, chemical and physical stresses of the environment. Such surfaces may be radiation stabilized glass or plastic products. For example, a glass layer may include self-cleaning TiO₂ coating made by sputtering, Hot-Aerosol-Layering-Operation (nHALO) and by ALD-techniques. The outermost protecting layers may be integrated in the solar cell or they can be separated from the electrical layers.

At the time of filing this patent application there are two main technologies used for producing solar cells. In the first technology silicon or some other semiconducting material is used as a substrate, and further layers are provided on the substrate. So far, this technology is most generally used. However, the production of the silicon substrates as well as forming the further layers with the present manufacturing technology is costly. Also, the weight of large solar cells becomes high, and the large solar cells are relatively sensitive to mechanical strains. These disadvantages prevent an increased use of solar cells.

Another technology for producing solar cells is based on using some other substrate and producing the semiconducting layers as well as other layers as films on the substrate. The substrate may be e.g. glass or plastic. The substrate can be used as the radiated surface of the solar cell, in which case the substrate is made transparent. The solar cells made with this technology have less weight and are not as sensitive to mechanical stresses. However, it has been a problem to achieve sufficient efficiency; usually less than 10% of the energy of light can be converted into electrical energy. One reason for this is the non-homogeneity of the produced layers. Therefore the transparency of the radiated layers may be insufficient. Also, the non-homogeneity of semiconducting layers causes loss of energy.

One problem which causes lack of efficiency is the fact that a junction of semiconducting layer has a specified electric potential threshold, and the energy of photons can only be converted into an amount of energy which corresponds to the potential threshold. The solar light has a wide spectrum of wavelengths and thus the photons have a wide range of energies. If the energy of a photon is lower than the threshold potential of the semiconducting junction, the photon is not converted into electrical energy. On the other hand, if the energy of a photon is higher than the threshold potential of the semiconducting junction, the photon is converted into energy according to the potential threshold, but the energy of the photon above the threshold is converted into heat.

The problem of converting wide spectrum radiation into electrical energy could be solved by providing several successive, transparent semiconductor layers wherein each pair of semiconducting layers provides a junction for converting light to electricity. The semiconductor junctions nearest to the radiated surface have the highest threshold potential and the threshold potential decreases when the radiation enters the following junctions. This way a photon is converted into electricity at a junction which has a threshold potential close to the photon energy, and high efficiency could be achieved. However, it is difficult manufacture several successive, transparent semiconducting layers. If the surfaces of the layers are not sufficiently smooth, light is reflected at each junction layers thus decreasing the efficiency. Also, the non-homogeneity of several semiconductor layers causes loss of electrical energy due to e.g. short circuiting spots in the layer and other uneven distribution of electrical fields.

Further, the production of protecting layers at the radiated surface of a solar cell is also difficult and costly. The processes are slow and need to be made separately from producing the electrical part of the solar cell. Handling the parts in different processes and/or assembly phases may involve a risk of contamination and thus a further loss of efficiency of the final product.

The above mentioned problems become all much more difficult when producing large solar cells as it is necessary to produce layers of large surfaces. The known technologies are suitable for producing cells of small dimensions, e.g. areas of a few cm² at the most, but the quality of surfaces and homogeneity of materials become worse if known technologies would be applied in producing solar cells with layers of large surfaces.

The applicant has investigated possibilities for using laser cold ablation in production of solar cells. In the recent years, considerable development of the laser technology has provided means to produce very high-efficiency laser systems that are based on semi-conductor fibres, thus supporting advance in so called cold ablation methods. Cold ablation is based on forming high energy laser pulses of short duration, such as within picosecond range, and directing the pulses into the surface of a target material. A plume of plasma is thus ablated from the area where the laser beam hits the target. The applications of cold ablation include e.g. coating and machining.

When employing novel cold-ablation, both qualitative and production rate related problems associated with coating, thin film production as well as cutting/grooving/carving etc. has been approached by focusing on increasing laser power and reducing the spot size of the laser beam on the target. However, most of the power increase was consumed to noise. The qualitative and production rate related problems were still remaining although some laser manufacturers resolved the laser power related problem. Representative samples for both coating/thin film as well as cutting/grooving/carving etc. could be produced only with low with repetition rates, narrow scanning widths and with long working time beyond industrial feasibility as such, highlighted especially for large bodies.

Because the energy content of a pulse, the power of the pulse increases in the decrease of the pulse duration, the problem significance increases with the decreasing laser-pulse duration. The problems occur significant even with the nano-second-pulse lasers, although they are not applied as such in cold ablation methods.

The pulse duration decrease further to femto or even to atto-second scale makes the problem almost irresolvable. For example, in a pico-second laser system with a pulse duration of 10-15 ps the pulse energy should be 5 μJ for a 10-30 μm spot, when the total power of the laser is 100 W and the repetition rate 20 MHz. Such a fibre to tolerate such a pulse is not available at the priority date of the current application according to the knowledge of the writer at the very date.

The prior art laser treatment systems most often include optical scanners which are based on vibrating mirrors. Such an optical scanner is disclosed in e.g. document DE10343080. A vibrating mirror oscillates between two determined angles relative to an axis which is parallel to the mirror. When a laser beam is directed to the mirror, it is reflected with an angle which depends on the position of the mirror at that moment. The vibrating mirror thus reflects or “scans” the laser beam into points of a line at the surface of a target material.

An example of a vibrating scanner or “galvano-scanner” is illustrated in FIG. 1a. It has two vibrating mirrors, one of which scans the beam relative to X-axis and another scans the beam relative to orthogonal y-axis.

The production rate is directly proportional to the repetition rate or repetition frequency. On one hand the known mirror-film scanners (galvano-scanners or back and worth wobbling type of scanners), which do their duty cycle in a way characterized by their back and forth movement, the stopping of the mirror at the both ends of the duty cycle is somewhat problematic as well as the accelerating and decelerating related to the turning point and the related momentary stop, which all limit the usability of the mirror as scanner, but especially also to the scanning width. The present coating methods employing galvano-scanners can produce scanning widths at most 10 cm, preferably less. If the production rate were tried to be scaled up, by increasing the repetition rate, the acceleration and deceleration cause either a narrow scanning range, or uneven distribution of the radiation and thus the plasma at the target when radiation hit the target via accelerating and/or decelerating mirror.

Conventionally galvanometric scanners are used to scan a laser beam with a typical maximum speed of about 2-3 m/s, in practice about 1 m/s. If trying to increase the coating/thin film production rate by simply increasing the pulse repetition rate, the present above mentioned known scanners direct the pulses to overlapping spot of the target area already at the low pulse repetition rates in kHz-range, in an uncontrolled way. With repetition rate of 2 MHz even 40-60 successive pulses are overlapping. The overlapping of spots 111 in such a situation are illustrated in FIG. 1b.

At worst, such an approach results in release of particles from the target material, instead of plasma but at least in particle formation into plasma. Once several successive laser pulses are directed into the same location of target surface, the cumulative effect seems to erode the target material unevenly and can lead to heating of the target material, the advantages of cold ablation being thus lost.

The same problems apply to nanosecond range lasers, the problem being naturally even more severe because of the long lasting pulse with high energy. Here, the target material heating occurs always, the target material temperature being elevated to approximately 5000 K. Thus, even one single nanosecond range pulse erodes the target material drastically, with aforesaid problems.

In the known techniques, the target may not only ware out unevenly but may also fragment easily and degrade the plasma quality. Thus, the surface to be coated with such plasma also suffers the detrimental effects of the plasma. The surface may comprise fragments, plasma may be not evenly distributed to form such a coating etc. which are problematic in accuracy demanding application, but may be not problematic, with paint or pigment for instance, provided that the defects keep below the detection limit of the very application.