Light harvesting for generation of electric energy is one of the most important research topics in applied sciences. First, for an efficient harvesting one needs a material with a broad light absorption window having a strong overlap with the sunlight spectrum. Second, one needs an efficient conversion of photoexcited carriers into produced current or voltage which can be used for applied purposes. The maximum light conversion coefficient in semiconductor systems is designated by so called Shockley-Queisser law, which is around 32% for an optimal bandgap value of 1,2–1,3 eV. However the efficiency may be increased using a solutions based on semiconductor nano materials such as quantum dots. Solar cells based on such a structures are included in the group of 3rd generation solar sell. 3rd generation solar cell encompasses multiple materials as a base of cell, such as: perovskite, organic, polymers and biomimetics. The most promising and in the same time most discussed are quantum dots and perovskite. Both material has a potential to revolutionize the solar cell industry due to their wide absorption range and high conversion coefficient. Nonetheless before the most common used material in photovoltaic namely silicon is replace one must overcome few major issues such as: stability and lifetime for at least 5 to 10 years or more, manufacturing process for a large surfaces and low production cost as well as recycling after the time of optimal use.
Part of the book: Solar Cells
The essence of the photovoltaic effect is the generation of electric current with the help of light. Absorption of a quantum of the energy of light (photon) generates the appearance of an electron in the conduction band and holes in the valence band. The illumination of the material, in general, is not uniform, which leads to the appearance of spatially inhomogeneous charge in the band valence and conductivity. Besides, electrons and holes generally diffuse with different velocities, which leads to the creation of a separated space charge and generation of an electric field (sometimes called the Dember field). This field inhibits further separation of cargo. The reverse processes also take place in the system, i.e. electron recombination and holes. These processes are destructive from the point of view of photovoltaics and should be minimized, which is achieved; thanks to the spatial separation of electrons and holes. The point is that electrons and holes were carried away from the area where they formed as quickly as possible, yes to prevent their spontaneous recombination. The use of semiconductor quantum dots introduced into the photoelectric material is currently a very important and effective way to increase the efficiency of photoelectric devices and photovoltaic cells. This is due to the fact that in semiconductor photoelectric materials with no quantum dots, there is always some upper limit of the wavelength λgrgr≃1,24/EgeV for absorbed light, above which the light is not absorbed.
Part of the book: Quantum Dots