Past, present and future of the thin film CdTe/CdS solar cells

•The outstanding basic features of polycrystalline CdTe thin-film solar cells are highlighted.•Materials properties and device configurations are critically discussed.•Essential steps, basic techniques and “magic” treatments in fabrication process are reviewed.•Towards 25% efficiency CdTe-based solar cells: new ideas and research challenges. Recently, the CdTe solar cell technology reached a high-tech level able to realize devices showing efficiencies close to 22%. Nowadays, this technology acquires more and more market share, becoming the most promising among the thin film technologies. These important achievements are the result of more than 40 years of studies and researches that have synergically group together the photovoltaic module manufacturing technology with semiconductor and interfaces physics, modeling, characterization and measurements on electronic devices. This paper shows the main steps of the production process of the thin film CdTe/CdS-based solar cells both from a technological and from a physical point of view. In particular, the main differences between cells fabricated in superstrate and in substrate configuration will be highlighted. For each of these two structures the fundamental layers, their main alternatives, chemical and thermal treatments, interfaces, the won challenges and the still open-problems will be presented and discussed.