The cost of solar cells is still too high for them to be widely used in consumer applications, and hence succeed as a commercial technology. Many research efforts focus on reducing processing and material costs or increasing the efficiency of the photovoltaic conversion (i.e., the conversion of the optical power radiated from the sun into electrical power). Thus, although photovoltaic devices are still mostly based on silicon (Si), a multitude of other materials and device architectures have been investigated. Engineering the band gap of the absorber to convert a large portion (i.e., a broad range of wavelengths) of solar radiation in electrical power is a major area of research in the field of renewable energy.
I will illustrate how ultra-thin semiconductors offer a new avenue to band gap engineering. Direct band gap semiconductors can be isolated as large-area sheets or membranes with thickness varying between ~0.7 nanometers and few microns. These new structural elements range from monolayer transition metal dichalcogenides to sheets of single-crystalline III-V and III-N semiconductors. Freestanding membranes can be transferred and bonded to other hosts. In addition, a large variety of local and global strain fields can be established in sheets with nanoscale thickness due to their high fracture limit. I will present a few examples of broad band absorbers of solar radiation relying on these unique properties of semiconducting sheets. For instance, I will describe integration of III-Sb and Si photovoltaic devices by direct bonding to obtain a solar cell operating in the visible and in the infrared range. In another example, I will show how few-layered MoS2 can be strain-engineered to establish a spatially varying profile of the band gap and exciton confinement in a single material.