The efficiency of solar energy conversion to electricity is one of the basic parameters of photovoltaic cells. The first values for silicon cells were recorded in the early 1950s in connection with the installation of batteries in satellites. Regular performance publications for all types of photovoltaic cells have been presented annually by the NREL laboratory of the U.S. Department of Energy since 1976 [1]. These data presented in Fig. 1, Fig. 2 illustrate the dynamics of changes in energy conversion efficiency for the different types of photovoltaic sources produced. These data are based on laboratory work. Nevertheless, they show where individual technologies stand and how far they are from the physical limits of conversion efficiency. The first data from 1976 were small. For amorphous silicon \( 1\% \), for monocrystalline silicon \( 14\% \), for CIGS thin film cells \( 6\% \), and CdTe \( 9\% \). Only the GaAs cells showed a higher conversion of \( 21\% \). Efficiency gains occurred as a result of continually ongoing research efforts in both corporations and research units. Some innovations have led to jumps in cell efficiency. It should also be noted that some research has led to the introduction of completely new designs, such as multi-junction cells, or new materials or even entire classes of materials, such as polymers or perovskites, into photovoltaics. Three classes of materials deserve special attention either because of the performance values that cells constructed with these materials achieve or because of the significantly higher dynamics of change compared to other materials. The first category is that of multi-junction cells especially those built on gallium arsenide GaAs. The introduction of two-, three-, and later four-junction cells started in the second half of the 1980s, the second half of the 1990s, and the last ones already in this century after 2005, respectively. These cells already in the early stages of research had high efficiencies above \( 28% \). Intensive work on their development has led to 2019 performance ranges lying in the previously unattainable range from \( 30\% \) to \( 47.1\% \).

Figure 1: Cell efficiency growth for all cell types from 1976-2019. compiled. National Renewable Energy Laboratory (NREL) graph, CC0 license, source: Wikipedia.

Figure 2: Description of the symbols used in the performance graph. Elaborated from a National Renewable Energy Laboratory (NREL) chart, CC0 license, source: Wikipedia.

The second group of materials are polymers. Their main advantage is undoubtedly their low price. The history of polymers in photovoltaics began only at the beginning of our century, but despite such a short period of time they have already managed to achieve efficiencies of \( 17\% \). A second class of materials with an even higher rate of change is perovskites. Produced on mineral-based materials, recent designs have reached \( 21.5\% \) in less than a decade, bringing them close to the theoretical conversion limit for these materials of about \( 31\% \). Unfortunately, both classes of materials lack a feature very important for photovoltaic materials, i.e., long-term stability. Photovoltaic power plants operate for 25 years and more, so work on improving this parameter seems crucial for the widespread use of these cheap and already efficient photovoltaic sources. Constructions using achievements of modern physics, i.e., photovoltaic cells made of quantum dots, are developing equally dynamically. These cells require advanced technology, but in less than 10 years since their discovery they have already achieved \( 16.6\% \) efficiency. CIGS and CdTe thin-film cells are developing without such spectacular results, however, they have already reached efficiencies of \( 23.4\% \) and \( 22.1\% \) respectively, which not long ago was the limit for silicon cells. An unquestionable advantage of these cells is minimal material consumption and possibility of their application on flexible substrates, which significantly extends their installation possibilities. And finally, silicon cells. Started in the 1950s with an efficiency of a few percent in niche applications such as powering satellites, they are now the main material in most of the photovoltaic power plants being built worldwide with efficiencies achieved in laboratory conditions of \( 27.6\% \). We currently have two areas of activity related to photovoltaic cell materials. One is the search for materials, especially for multi-component semiconductors with high yields. The second is the area related to the development of methods of production of materials with obtained high efficiencies on an industrial scale. The increasing efficiency of cells and the nearly million-fold increase in production has resulted in a roughly thousand-fold decrease in the price per 1 Wp from 1975 to today. These changes are shown in Fig. 3 (based on data from [2], [3]). Such significant reductions in selling prices have resulted in increased research efforts into low-cost photovoltaic materials such as perovskites and polymers. As a result, in recent years we have seen a dramatic increase in the conversion efficiency ( Fig. 3 ) achieved for cells made from these materials.