Organic π-conjugated materials have been studied intensively to promote their application in opto-electronic devices, especially in the field of light-emitting diodes (OLEDs) and organic solid state lasers (OSLs). OSL research is driven by the expectation of tuneable, cheap and flexible devices as well as new application areas, in particular in optical computing, data processing and sensing applications. Despite great success in optically pumped lasers, electrically driven lasers have not yet been realised, although the prerequisite - high internal quantum efficiencies of electrically generated photons - has been demonstrated in OLEDs. Thus, electrically pumped OSLs are mainly impeded by non-radiative losses by bimolecular recombination such as singlet-singlet, singlet-triplet and singlet-polaron quenching at higher excitation densities accompanied by the low charge carrier mobilities in the organic materials. These limitations might be overcome by organic crystals and crystalline films with well-defined long range molecular order which offer superior charge transport properties compared to amorphous materials. However, several challenges have to be faced integrating single crystals as the active medium in OSL devices, which define the key tasks of the project. 1. Understanding and optimising how different electronic/excitonic interactions and processes in organic crystalline materials affect the optical gain. 2. Examining the role of different bimolecular annihilation processes such as singlet singlet, singlet-triplet, and singlet–polaron in crystalline materials at high excitation densities, which limit the performance of such devices. 3. Imprinting resonator structures on the crystalline materials to reduce resonator losses. 4. Testing the viability as active materials in electrically injected lasers and also by indirect electrical pumping recently demonstrated in the host group.