With the Power of the Sun

Electricity prices are rising, and environmental awareness is increasing. Many consumers are looking to gain more independence from the big energy suppliers – with renewable energy sources, of course. Upon closer look, setting up a wind turbine in the backyard or a private biomass power plant is an unrealistic endeavor. That leaves solar energy. The federal and state governments are promoting this initiative. At the same time, research and technology are working on more efficient and cost-effective ways to harness the power of the sun. Laser technology plays a decisive role in implementation. Will we soon be able to turn our houses into small solar power plants without having to cover them with “ugly” mirror surfaces?

Will Transparent Solar Cells Soon Be Available?

Laser Innovation Brings Photovoltaic Production Back to Europe

Solar cells have been used to generate electricity since the 1950s – initially, mainly in places where no other power sources were available (e.g., to power satellites in space). With its increase in energy yield, photovoltaics (PV) also became interesting for business and politics. It is now considered a cornerstone of a sustainable energy supply. In August 2021, the Green Party submitted a motion in parliament for a Solar Energy System Expansion Acceleration Act. According to this act, a photovoltaic system would be mandatory for every new building.¹ In some federal states, corresponding provisions are already being implemented.

Principally speaking, a solar cell is any technology that uses the photoelectric effect to generate electricity from light. Typically, silicon and other semiconductors are used. Silicon exists abundantly in the form of chemical compounds (e.g., as silicon dioxide in sand). However, for solar cell technology a purity of 99.99% is required because any contamination has a negative effect on the service life of the solar cells. The production of this so-called solar silicon is a complex, energy-intensive process with numerous intermediate steps. Congruently, the production of PV modules is expensive.

Industry and research have, therefore, long been searching for alternatives to the classic solar cell. Two of them are presented in this article: In the first case, the aim is to increase the efficiency of Si cells and make production more efficient by using laser technology. In the second example, an alternative material is used. Here, too, lasers play a crucial role in production.

Heterojunction – Higher Efficiency

Heterojunction technology (HJT) is when two different semiconductor materials come together. In the case of solar cells, this involves silicon in two different crystal structures: crystalline and amorphous silicon. HJT cells, therefore, absorb more solar energy than conventional cells. At the same time, the resistance in the module decreases, causing the efficiency to increase to up to 25 percent. Thus, HJT cells can still deliver electricity even, for example, when the sky is cloudy. In addition, their performance does not diminish even at high temperatures. They are said to have a low temperature coefficient.

The Swiss manufacturer Meyer Burger Technology Ltd has further developed this technology, which originated in Japan, and is currently starting production in Germany. The fact that production was not outsourced to a low-wage Asian country, as is usually the case, is also due to a novel laser cleaving process developed by Innolas Solutions. Instead of scratching the wafer and then breaking it mechanically, the machine does both in one step. The local, induced voltage of the laser beam allows the silicon wafer to be split along an almost freely selected cell edge. This is not only faster than the conventional method, but the particle-free method also results in fewer microcracks. This would otherwise impair the quality of the solar cells. Since the wafers are not broken mechanically, the cell breakage rate is also considerably lower.

Organic Materials – The Future of Photovoltaics

The future could belong to organic photovoltaics (OPV), which, in contrast to classic Si designs, rely on materials from organic chemistry – primarily synthetic carbon compounds. Most of these solutions are still in the developmental stage. However, there are some research projects running that are already testing their industrial production.

Organic solar cells offer many advantages. First, they eliminate the need for time- and energy-consuming silicon preparation. The only metallic components of the cells are electrodes, through which the generated current is channeled. In most cases, copper, which is abundant in nature, is used for this purpose. The plastics used in the generation of electricity can also be used sparingly. Three grams of active material are enough for an area of ten square meters. The plastic layers are suitably thin; therefore, they can be “printed” onto almost any type of substrate using the roll-to-roll process. This also makes it possible to produce flexible modules and transparent versions. All these advantages open numerous new application possibilities. Organic solar cells could be integrated into buildings, facades, and glass surfaces, for example, and could capture solar energy directly where it is consumed.

The biggest disadvantage of this technology is currently still its low efficiency. The highest value achieved in the laboratory was around 12 percent. On average, however, experts currently expect around 7 percent. This means that considerably larger areas would be needed to achieve the same effect as with conventional cells.²

Precise Cuts in the Femtosecond Region

The roll-to-roll process allows large areas of solar cells to be produced quickly. However, this also poses a challenge because the larger the area, the higher the amperage of the generated current. To transport the current, cables with a large cross-section are needed, which would impair the flexibility of the cells. But there is a trick: if the total area is divided into numerous small cells, the energy yield remains almost the same and the current intensity drops to an acceptable level.

“The challenge is to remove the plastic layers, which are only nanometers thick, in such a way that the underlying layers are not damaged or short-circuited,” says Ludwig Pongratz of the Fraunhofer Institute for Laser Technology (ILT) in Aachen. “Only a laser can do that.” To scratch off the layers (in technical jargon, it is referred to as “scribing”), researchers use a femtosecond laser. For an extremely short period of one quadrillionth of a second, a beam of such high intensity is generated there that the material removed is converted directly into plasma without leaving any residue, while the substrate does not heat up to any notable temperature. The individual pulses are repeated at a repetition rate of 200 kHz. This results in very precise cuts. At ILT, diffractive optical elements divide the beam into eleven partial beams and direct them onto the material, creating a module with twelve parallel rows of cells.³

In addition to scribing, the research project also uses lasers to carry out other operational steps (e.g., a highly efficient drying process and the encapsulation of the photovoltaic cells). Thanks to the latest laser technology, nothing should stand in the way of industrial mass production and the use of organic solar cells.