If the EU Commission had its way, the deep sleep of photovoltaics in Europe would soon be superseded by a swift awakening. According to a paper on the EU strategy for solar energy, published in the middle of May 2022, Brussels gives this energy source high priority in order to bring the European Green Deal to a good conclusion and end the dependence on fossil fuels from Russia. An expansion of capacities is necessary – for solar systems on roofs as well as for large-scale solar plants. On the other hand, it is clear that sufficient land for the construction of solar modules is limited. The Commission therefore sees the real challenge in the technological sector: A new generation of high performance solar cells that set new standards in terms of efficiency and sustainability must be the development goal.
The classic silicon solar cell´s efficiency is limited
The solar research community is already pushing hard to raise the energy yield of solar cells and their absorption of light energy to a new level. It is quite possible that the classic silicon solar cell will fall by the wayside. With a market share of over 90 percent, this type of cell is still top dog on the market. However, its efficiency, which indicates how much electrical energy is generated by incident sunlight, is limited – common monocrystalline solar cells achieve values between 20 and 22 percent. However, Chinese solar technology company Longi achieved an efficiency of 26.81 percent in the laboratory late last year, which should also be reproducible in mass production of cells. Even though, in this case, almost three quarters of the solar energy still does not contribute to electricity generation, this result represents a veritable top performance – after all, the physical limit for the efficiency of a silicon solar cell is around 29 percent.
Solar cells process sunlight in different spectra
To a certain extent, this handicap is built into the cells themselves. They essentially consist of a silicon layer doped negatively and positively with foreign atoms, in which only a comparatively small part can be used energetically. The light quanta (photons) radiating into the so-called space charge region, release electrons from their bonds, which then generate an electric current in the conductors. Incident sunlight plays a central role in this process. The silicon layer mainly processes light waves in the near-infrared range – meaning that a large part of light energy at other frequencies is lost. This limitation can be overcome by new solar cell designs. Renowned institutes such as the Karlsruhe Institute of Technology (KIT), the Bergische University Wuppertal, the Ecole Polytechnique Fédérale de Lausanne (EPFL) and the Helmholtz-Zentrum Berlin for Materials and Energy (HZB) are already working on the next technological leap: the tandem solar cell, first presented in 2015.
The combination of different semiconductors improves luminous efficacy
The idea is to expand the capacities of a simple solar cell by combining it with another layer. The combination of silicon and perovskite is promising – the latter is a mineral that can be used with little energy input and at low cost. The interaction of these materials defines new energy limits because the two layers each operate in different regions of the light spectrum. The upper layer processes high energy light with shorter wavelengths, while the layer below processes lower energy light at longer wavelengths. This way, the available light energy can be better utilized, which in turn improves the cell’s efficiency.