a The donor layer is illuminated at one end, generating photoexcited carriers that diffuse to the acceptor layer at the other end. The bulk homojunction provides distinct physical paths for each class of charge carrier, enabling efficient carrier transport with minimized nonradiative recombination. b PL intensity with different thicknesses of bulk homojunction (donor) film. c Normalized acceptor PL intensity as a function of donor layer thickness (dot). Fits to data (solid line) indicate the enhancement in the carrier diffusion length (dotted line) in bulk homojunction film compared with n-type and p-type CQD films. The average (symbols) and standard deviation (error bars) are calculated from a sample of three CQD films in each case.