Incorporating interdigitated back-contact electrodes into organic-inorganic halide perovskite solar cells overcomes the optical losses and low architectural defect tolerance present in conventional "sandwich" cell configurations. However, other factors limit device performance in back-contact architectures, such as the short charge-carrier diffusion length within the perovskite film relative to the electrode spacing. As charge-carrier diffusion length is crystal-size related, in order to understand the effect of perovskite morphology on the performance of back-contact perovskite solar cells (bc-PSCs), perovskite films with four different grain cluster sizes, i.e. large, medium, small and extra small, were fabricated via a solvent annealing approach. Crystallization of the perovskite is found to be closely related to the surface chemistry and topography of the substrate. The bc-PSC photovoltaic performance correlates positively with the perovskite grain cluster size. Through a detailed analysis of transient photovoltage decay measurements, time-resolved photoluminescence, and space charge-limited current measurements, the effect of defect densities associated with grain cluster boundaries was elucidated.