Maximizing the power conversion efficiency of organic solar cells requires the simultaneous optimization of its short-circuit current density, fill factor, and open-circuit voltage Voc. Several key parameters of the device model needed to understand these quantities have not been reliably determined, even for the prototypical poly(3-hexylthiophene):phenyl-C61-methylbutyric ester (P3HT:PCBM) photoactive layer (PAL). Detailed analysis of the loss mechanisms at contacts and their rational optimization have not been possible. Here, using crosslinked P3HT network:PCBM cells with predefined ultrafine donor–acceptor morphology and very high internal quantum efficiencies, the built-in potential Vbi is measured to decouple and reliably extract other key parameters of the cells. Using the refined device parameters, the general optimization of organic solar cells is evaluated and the following is established: i) The PAL composition of the first optical absorption optimum is displaced towards the more strongly absorbing component due to thin film effects. ii) The optimal cell configuration is the one in which the slower carrier travels on average the shorter distance to the collection contact, a consequence of the asymmetric photogeneration profile. iii) The absorption thickness optima follows a simple λp/nPAL scaling law, where λp is its absorption center wavelength and nPAL is the corresponding refractive index.