With power conversion efficiencies (PCEs) of <13% and plagued by stability issues, organic photovoltaics (OPVs) still lack wide adoption, despite significant recent advances. Currently, the most progress in OPV device performance is achieved by "trial-and-error" preparation procedures that lead to complex and largely unknown-despite tremendous analytical efforts-morphologies. Here, we demonstrate a proof-of-principle, chemical imaging methodology that combines experimental high spatial sensitivity and chemical selectivity with theoretical modeling, capable of analyzing the three-dimensional composition and morphology of virtually any device. Allowing the precise measurement of composition and direct visualization of film morphology with depth, our approach reveals the intricate buried donor/acceptor (D/A) interface of a model polymer/fullerene system, poly(3-hexylthiphene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM). In particular, our technique is able to identify and quantify the D/A interface length, that is, the extent of molecular mixing at the D/A interface, a parameter crucial for device performance, yet never measured. Extracting this parameter allows demonstrating that, contrary to the general understanding, when starting with a fully mixed D/A phase in our model system, thermal annealing, which is known to substantially (however limited) increase the device performance by phase segregation, does not create but small amounts of pure phases, leaving the device mostly mixed, which limits the performance improvement.