Charge carrier and exciton transport is a key parameter determining the performance of optoelectronic and electronic devices, from photovoltaics to transistors. While mobility or diffusivity are important empirical factors when comparing materials for a particular application, conventional ensemble approaches for measuring these parameters average over structural heterogeneities like grain boundaries or local crystal orientation, which are ubiquitous in the disordered materials produced through solution-based fabrication approaches. Furthermore, experimental measures of mobility often fail to link transport efficiency to intrinsic material parameters. To overcome these limitations, our group has utilized transient absorption microscopies which enable local measures of exciton or free-carrier transport to be correlated to the local material morphology as well as to material parameters like effective mass and mean scattering time. In lead halide perovskites, we have correlated direct measures of the ambipolar diffusivity to refractive index changes induced by photogenerated free charge carriers on individual perovskite domains. These optical measurements provide a local probe of how perovskite chemistry systematically determines transport rates through a tightly-coupled interplay of effective mass, band structure, and disorder.
Research Site: http://www.montana.edu/grumstruplab/