World electricity demand is steadily increasing over years and photovoltaic conversion of solar energy into electricity stands out as a clean, sustainable and reliable way of electricity production. The dominant photovoltaic technology is based on silicon (Si) solar cells. To reduce the cost of photovoltaic electricity and to minimize the semiconductor use, the conversion efficiency of the devices should be increased.
However, after many decades of R&D, the Si technology is mature and no further efficiency improvements are expected (best conversion efficiencies are 25% in the laboratory). Significantly higher conversion efficiencies (e.g. 38.8%) have so far been reached with multi-junction solar cells based on III-V materials, but these devices are too expensive for the use in flat-plat modules for terrestrial applications.
One approach to reduce costs is to grow III-V solar cells on Si substrates, but it is still expensive and technological difficult to achieve. As an alternative, nanowires composed of III-V materials are an innovative way to significantly reduce material needs without compromising light absorption or device performance, given the small dimensions (in the order of 10-9 meters) of such structures. The overall goal of this project is therefore to develop specifically designed III-V nanowire subcells to be subsequently connected to state-of-the-art high-efficiency bottom Si solar cell. Nanowire solar cell development includes synthesis of the materials and development of specific characterization tools of single nanowire solar cells.
In conclusion, within this project a variety of nanowire solar cell structures have been developed to be ready as a building block of a multi-junction solar cell with a Si solar cell. For this to happen, still several challenges need to be addressed in the future, as well as further efficiency increase of the nanowire by, for instance, synthesizing a dual junction solar cell within a nanowire.
