(26c) Controlling Morphology of Photovoltaic Thin Films By Phase Transformation of Metastable Colloidal Nanocrystals

Colloidal nanocrystals are promising solution processible precursors to thin films of complex semiconductors, including CuZnSnS₄ and CuInSe₂. Detrimental impurity phases and spatial fluctuations of the composition can be minimized by controlling stoichiometry during colloidal synthesis. However, the preparation of large-grained, electronic quality thin films from such nanocrystals has been challenging. We synthesize nanocrystals of these materials in a metastable hexagonal crystal phase and leverage their thermally driven conversion to the stable phase to engineer grain size and morphology in the resulting polycrystalline films. The activation energy for the phase transformation, determined by variable heating rate in situ X-ray diffraction, is found to depend strongly on nanocrystal shape. Using nanorods around 40 nm in length, we apply classical nucleation and growth principles to tune the thermal processing conditions to achieve large, micron scale grains in a matter of minutes. To exclude carbon impurities that can negatively impact optoelectronic properties, the same nanocrystals can be processed using inorganic chalcogenidometallates (ChaMs) as ligands. Upon heating, the ChaMs form a glassy matrix around each nanocrystal that inhibits nucleation of the stable phase (observed as a higher activation energy for nucleation), yet facilitates grain growth by coarsening. Interdiffusion ultimately results in alloyed thin films with grains that span their full thickness. Optical and electronic characterization of the resulting thin films shows promise for their application in thin film photovoltaic devices.