The work presented in this Thesis focuses on the solution-phase self-assembly behaviour of a series of di- and triblock copolymers with semicrystalline, π-conjugated segments, namely poly(3-alkylthiophene) (P3AT) and poly(3-alkylselenophene) (P3ASe) derivatives. Our group has shown that crystallisation-driven self-assembly (CDSA) of P3AT and P3ASe block copolymers (BCPs) in block-selective solvents allows access to colloidally-stable nanofibers with π-conjugated cores. In this thesis, we demonstrate that the contour length of P3ASe nanofibers can be controlled using a seeded-growth technique. Furthermore, we show that hierarchical nanostructures can be accessed from the coassembly of P3ATs and P3ASes using sequential CDSA. This "bottom-up" methodology could be employed as a facile route to nanomaterials with controlled dimensions, interesting optoelectronic properties and potential applications in polymer-based devices.