This thesis focuses on the design, synthesis and characterisation of small molecule, non- fullerene acceptors. Initially, C3-symmetric truxenone derivatives were developed, which demonstrated broad absorption and the ability to carefully tune the frontier energy levels of the molecule. However, it appeared that the poor electron mobility, as well as an unfavourable morphology due to large scale crystallisation of the acceptor, limited device performance. The second part of this thesis explores linear small molecules with rhodanine end groups, which also demonstrated an excellent ability to tune the frontier energy levels through changes to the chemical structure. Compared with the truxenones, these acceptors were relatively amorphous and formed a more favourable, intermixed morphology with the donor poly(3-hexylthiophene) (P3HT). Device efficiencies of 4.1% were achieved with this blend, however performance was again limited by microstructure, which in this case was slightly too intermixed, leading to recombination losses. In addition, the lack of complementary absorption of the donor and acceptor reduced the amount of photocurrent that could be generated. The third section of this thesis describes how the molecular structure of this acceptor was modified to overcome both of these issues, by the replacement of an 9,9’-dioctylfluorene core unit with indacenodithiophene, leading to a more planar molecular structure. The increased crystallinity and red-shifted absorption of this acceptor resulted in an improved efficiency of 6.4%, which at the time of writing is the highest efficiency for non-fullerene devices with P3HT. In addition to high efficiency, these devices also had improved air stability compared to P3HT:fullerene devices as well as devices with high-performance donor polymers, demonstrating the real potential application for these materials in commercialisable OPV technology.