The subject of this thesis is the dynamics of charge carriers and optical excitations in semiconducting chalcogenides, specifically compounds that contain sulfur or selenium. Chalcogenides are fascinating, as they display a wide array of properties that are attractive for both solar energy conversion and optoelectronics. With band gaps in the visible and near infrared, they strongly absorb light in the visible range, resulting in the excitation of charge carriers with long lifetimes and high mobility. Some of these materials exhibit room temperature ferroelectricity and ferroelasticity, pronounced nonlinear optical effects, and topologically protected surface states. Their properties can be further tailored by their structure and dimensionality, from bulk, three-dimensional (3D) materials to nanocrystals and inherently two-dimensional (2D) van der Waals materials. 2D chalcogenides are particularly appealing, as they feature enhanced light-matter interactions due to reduced dielectric screening, confinements of charge carriers in individual layers, and no dangling bonds at their surfaces. They are flexible and can be deposited on a variety of substrates.

This thesis discusses photoexcited carrier dynamics in a number of 2D and 3D chalcogenides: GeS, GeSe, SnSe, SnS2, PbS, Bi2S3, and (Bi1-xInx) 2Se3. Of them, 2D GeSe and GeS have strongly anisotropic electronic and optical properties, owing to robust room temperature ferroelectric polarization in the layers. 2D (Bi1-xInx) 2Se3 undergoes a transition from a topological to trivial band insulator behavior with the indium content increasing from zero to a few percent, and exhibits pronounced changes in its carrier density, mobility, and response to photoexcitation. Another 2D chalcogenide, SnS2, was investigated as a possible photoanode material due to its high optical absorption and mobility. Additionally, we discovered that it also exhibits intriguing nonlinear effects upon photoexcitation that result in the emission of terahertz (THz) radiation. Finally, the polycrystalline (quasi-3D) chalcogenides PbS and Bi2S3 were investigated as photovoltaic and photodetector materials. Polycrystalline films