Atomic-Layer Approaches Towards 'Extremely Thin' Chalcogenide-Based Photovoltaics—A unique Combinatino of Advantages

Atomic layer deposition (ALD) and variants are techniques ideally suited to the generation of ‘extremely thin absorber’ (ETA) solar cells, in which three distinct semiconductors are combined as electron transport (TiO₂, ZnO), light absorption (Sb₂S₃, Sb₂Se₃), and hole transport (polythiophenes, V₂O₅) layers.<br/>(1) ALD can deliver a material quality significantly improved with respect to solution processing, with crystals of 10 µm lateral size obtained in planar films of 50 nm thickness.<br/>(2) It enables one to vary and optimize the thickness of each layer systematically.<br/>(3) It offers the opportunity to engineer interfaces with the use of interfacial layers with extreme thickness sensitivity, on the scale of approximately 1 nm.<br/>(4) It enables conformal coatings of non-planar substrates. Parallel arrays of cylindrical, coaxial heterojunctions decouple the paths for light absorption and charge separation. Ordered monolayers of nanospheres scatter near-bandgap light, enhancing its conversion.<br/>(5) Our extension of ALD to the use of precursors dissolved in the liquid phase (‘solution ALD’ or sALD) expands the range of materials accessible by ALD. It also provides additional experimental tools and an inexpensive access to ALD.<br/>(6) The recent invention of ‘atomic-layer additive manufacturing’ (ALAM) circumvents the limitations associated with traditional blanket layering methods, opening the door to rapid prototyping approaches.