摘要： Thin film solar cells equipped with polycrystalline compound semiconductors as functional layer for light absorption have continuously been improved in terms of solar energy conversion efficiency, such that they became a competitive alternative to well-established silicon-based solar cells. In 1905, Einstein published a comprehensive, physical description of the photoelectric effect [1] and thus provided the theoretical framework for upcoming research of photovoltaic technologies. The emergence of photovoltaic devices, however, only started about 50 years later, and for several decades, it persisted a niche technology mainly for aerospace applications. Among others, silicon (Si) was known to belong to the group of (extrinsic) elemental semiconductors, and due to its abundance, it was the very first absorber material to be used in solar cells. Triggered by the oil crisis in the 1970s, the research of solar energy conversion technologies finally got a tremendous stimulus. As a result, research not only of silicon-based solar cells but also of other absorber layer materials based on compound semiconductors have been much more extensively endeavored. The latter were also brought into focus in order to address some severe drawbacks of silicon-based solar cells. First of all, the high energy consumption in fabricating single crystal silicon results in a quite long energy amortization time. In addition, the requirements on crystallinity and purity are extremely high while a considerable amount of material is wasted upon slicing silicon wafers. Also, during the growth of silicon single crystals a certain concentration of dopants has to be incorporated in order to induce either extrinsic p-type or n-type conductivity. Despite the energy of the band gap of silicon fitting quite well with the optimal energy determined by the solar spectrum, silicon is an indirect semiconductor whose photonic electron transition from the valence band to the conduction band needs to be assisted by a phononic momentum transfer. This requirement of coincidence between a photon of appropriate energy being absorbed and a phonon transferring impulse to the electron leads to a reduced probability of events of photoelectric charge carrier generation. Correspondingly, the absorber thickness must be augmented in order to compensate the low absorption coefficient. These aforementioned issues, eventually, gave rise to reconsider photovoltaic technologies, being both economical and ecological reasonably applicable in a more widely spread manner. These demands have paved the way for thin film solar cell technologies using compound semiconductors. Those compound semiconductors are intrinsically conductive, and they possess a higher absorption coefficient due to direct electron band transitions (Fig. 4.1).