SOLACYLIN has developed preparative methods for making solar cells out of alternative (inexpensive, abundant, non-toxic) materials in a novel geometry consisting of parallel nanocylinders in ordered arrays, each of which combines three semiconductors arranged coaxially.
One piece of work has focused on the nanoporous matrices to be used as templates for the cylinders. We use "anodization", an electrochemical procedure used industrially for the surface treatment of some metals. In SOLACYLIN, we have explored simple methods for generating anodized arrays of pores that display a high degree of order and a homogeneous geometry from the very beginning. This contrasts with existing methods, in which either the pores start out disordered and only become ordered later, or expensive manufacturing techniques (lithography) are used to define the order preliminarily. In particular, we have established how to generate arrays of short (0.1 to 3 µm) pores directly on a transparent conductive substrate in a manner such that the materials deposited inside them are in direct electrical contact to the underlying substrate.
In another line of research, we have developed methods allowing for the homogeneous, conformal coating of such elongated pores based on surface chemical reactions, a strategy called "atomic layer deposition" (ALD). We have optimized methods for depositing thin silica films by ALD, into which we mix in small amounts of either aluminum or antimony. Upon thermal reduction using lithium vapor, the material is converted to amorphous silicon with either p or n doping as needed in photovoltaics. Further ALD work has been pursued towards controlling the surface reactions from 'precursors' dissolved in liquids, instead of bringing them from the gas phase in a vacuum chamber. This evolution has rendered novel types of ALD chemistry possible, and thereby, allowed for the ALD coatings of materials which to date have not been accessible. The 'solution ALD' (sALD) approach has enabled us to deposit materials inaccessible by traditional ALD, such as a solid hydride, a polymer, a metal-organic framework, and ionic solids, some of them with outstanding purity and crystallinity at room temperature. In particular, we have been able to generate hybrid perovskites of the type that is relevant to photovoltaics, both lead-based and antimony-based.
We have prepared functional perovskite solar cell prototypes in various geometries -- planar films, colloidal nanoparticles, and coaxial nanocylinders. We have proposed a material that is directly accessible by sALD and which could replace the lead-based perovskites used currently, thereby circumventing the two major impediments of lead-based perovskites, namely their toxicity and their instability in air. In parallel to this, we have also developed an "extremely thin absorber" (ETA) solar cell materials system based on antimony sulfide as the light absorber. The deposition of individual layers by ALD has enabled us to find by a systematic approach that the optimal thickness of antimony sulfide is 60 nm in a planar configuration. We have identified ZnSas an interfacial layer which provides proper adhesion and antirecombination barrier properties, with an optimized thickness of 0.6 nm. We have transferred this interface engineering to coaxial nanoyclindrical geometry, and varied the length of the cylinders as well as the thickness of each layer. We have explored a number of materials as alternatives to the classical semiconductors, in particular MoS2, HfS2, SnO2, and V2O5.
