Could you clarify this for me.... Both emitting layers are (necessarily) still between the anode & cathode. Doesn't that mean that current is free to flow throughout the entirety of the stack without an additional charging layer?
Every time I try to dig up an answer to that fundamental question, I end up mired in the differences between singlet and triplet states.
For light to be emitted the electrons and holes must recombine in the emitter layers. In a stacked device this means there are two layers where this must occur. Without a generating layer there would only be one recombination site.