The invention relates to a photoactive device with organic layers, especially a solar cell, with a layer arrangement having an electrode and a counterelectrode as well as a sequence of organic layers arranged between the electrode and the counterelectrode, wherein two layers bordering on one another are formed in a photoactive region encompassed by the sequence of organic layers, namely, an exciton-harvesting layer (EHL) and an exciton-separating layer (ESL); in which the exciton-harvesting layer (EHL) is a mixed layer containing an organic material (A) and at least one further organic material (B), in which (i) a lowest singlet excitation state for excitons (S1A) of the organic material (A) is energetically higher than a lowest singlet excitation state for excitons (S1B) of the further organic material (B), (ii) the further organic material (B) is chosen such that it transforms singlet excitons into triplet excitons with a quantum yield of at least approximately 20%, preferably of at least approximately 50% by an ISC mechanism (ISC-Inter-System-Crossing), and (iii) a lowest triplet excitation state for excitons (T1B) of the further organic material (B) is energetically higher than a lowest triplet excitation state for excitons (T1A) of the organic material (A); and wherein a donor-acceptor heterojunction is formed between the exciton-harvesting layer (EHL) and the exciton-separating layer (ESL) converting triplet excitons of the organic material (A) into free charge carrier pairs in the vicinity of the interface.