Reducing the size of a semiconductor to the natural length scale of carriers and excitons provides a unique opportunity to tailor the semiconductor’s electronic structure and optical properties through quantum and dielectric confinement effects. In this dissertation, we analyze the dynamics of free carriers and excitons in two emerging classes of semiconductors: zero-dimensional coupled quantum dot (QD) assemblies and two-dimensional organic-inorganic hybrid perovskites (2DHPs). QDs are prized for their size-dependent band gaps and can be assembled with surface ligand tunable inter-QD coupling. However, control over carrier type, concentration, mobility, and lifetime remains an important challenge to their device applications. Post-synthesis modification of QD assemblies provides a route to couple QDs as well as dope and passivate QD surfaces. In PbSe QD assemblies, we modify the Pb:Se ratio to change the dominant carrier type and doping level, and in CdSe QD assemblies, we modulate the conductivity through indium doping. To study the optoelectronic properties of these and other QD assemblies, we built a time-resolved microwave conductivity (TRMC) apparatus to contactlessly probe mobility and lifetime. We correlate changes in carrier density and conductivity from surface modification with the mobility and lifetime measured in TRMC and with field-effect transistor mobilities. We show that the carrier lifetime increases with doping as the Fermi level moves near the band edges and electronically passivates trap states. 2DHPs consist of metal-halide monolayers separated by organoammonium cations and resemble interfacial defect-free quantum well superlattices. Strong confinement of carriers in the inorganic monolayers results in a large exciton binding energy, allowing exciton and carrier dynamics to be probed optically. At temperatures <75 K, we find regularly spaced fine structure resonances in excitonic absorption and photoluminescence spectra of the 2DHP phenethylammonium lead iodide, which we hypothesize are caused by excitons coupling to phonons in the organic framework. We also observe higher energy hot photoluminescence resonances which result from competition between vibrational relaxation and radiative recombination. By synthesizing and characterizing a family of phenethylammonium lead iodide derivatives with single atom substitutions on the phenyl group, we tailor the packing of the organic framework, the energetic disorder, and the rate of photoluminescence.