2.  Interface passivation and defect tolerance in semiconductors

As polycrystalline thin-film PV materials are considered for their low-cost, and as single-junction bulk semiconductors improve, recombination at grain boundaries and at interfaces between device layers become increasingly important. Further, devices that require sustaining an excess carrier population in nanostructures with high surface-to-volume ratio, and virtually all solar cells, are strongly influenced by their interface properties. We are exploring the recombination characteristics of heterovalent interfaces between dissimilar materials – such as Ga(N)P and tunnel oxide layers on silicon, and ALD oxide layers such as Al₂O₃ , and amorphous group-IV layers on III-Vs and other compound semiconductors – for planar and nanostructure surface passivation, as well as for carrier-selective contact formation. The effects of doping at mixed-column (heterovalent) interfaces, atomic configuration at interfaces, and modeled interface state density and recombination activity are central to these studies.
 
Remarkably, some semiconductors have a remarkable insensitivity to the presence of grain boundaries, surfaces, and other extended defects, exhibiting low recombination rates and in spite of these disruptions to the periodic crystal lattice. Polycrystalline CuInSe₂ is an canonical example of such defect and interface tolerant (DIT) materials, which typically has better performance than single-crystals of the same composition. Lead-halide perovskites such as CH₃NH₃PbI₃ and variants containing formamidinium, cesium, and bromine also have highly recombination-benign grain boundaries (as well as point defects), resulting in their high solar cell open-circuit voltages for their bandgaps. GaInN results in highly useful transistors, light-emitting diodes, and other minority-carrier devices even when areal dislocation densities are very high in the range of 10⁶ to 10¹⁰ cm^-2. Within the family of arsenide/phosphide III-V semiconductors, InP tends to show significantly less severe Fermi-level pinning and lower recombination at bare surfaces than does GaAs. We are investigating the atomic-scale mechanisms responsible for strongly defect tolerant behavior in some semiconductors, and the highly active recombination kinetics at the interfaces and grain boundaries in other materials. Key materials systems of interest are polycrystalline GaInP and its evolution of recombination properties as Ga is added to In-rich compositions, perovskites, CdTe-related materials, and III-nitride semiconductors.