The mechanisms that govern the transformation and dissipation of energy in semiconductors
must be understood to advance optoelectronic technologies to their fullest extent. This thesis
aims to unravel the evolution and energy losses of electronically excited states in novel solution-processed
materials for photovoltaics. This is achieved by implementing ultrafast laser spectroscopies devised to target efficiency-limiting processes in conditions relevant to device operation. Part of the research herein examines the deleterious relaxation of high-energy ("hot") charge carriers in metal-halide perovskites and their nanocrystal analogues using an all-optical "pump-push-probe" approach. The results show that carrier "cooling" in these materials is dictated by the lattice composition, and unlike other low-dimensional semiconductors, insensitive to the crystal size or surface properties. The multi-pulse methodology also reveals that hot carriers can lose their energy to cold carriers, which highlights a previously unaddressed competition between carrier-carrier and carrier-lattice interactions in these materials. The remainder of the thesis deals with charge separation at the interface between electron-donating and accepting organic semiconductors. Spectroscopies with optical, emission and photocurrent detection convey slower charge separation when the donor and acceptor are more closely aligned in energy. In spite of the slower dynamics and small "driving energy" of these systems, efficient long-range delocalisation of charges occurs in a manner that does not entail inexorable non-radiative recombination. Moreover, the electric field inside working solar cells is discovered to directly aid the preliminary step to this process, contrary to conventional wisdom. Both research strands of the thesis bring unique insight into the photophysics of "soft" semiconductors, and also establish frameworks for the design of high-performance solar cells based on molecular and/or nanoscale materials. These guidelines and the device-oriented approaches developed herein could be instrumental in unlocking the functionality of other up-and-coming materials systems for energy, photonic and nanoelectronic applications.