Possible evidence for quantum-size effects in self-assembled ultrathin films containing conjugated copolymers

We present photoluminescence (PL), UV absorption, electroluminescence and x-ray reflectivity studies of self-assembled multilayer films containing alternate layers of conjugated copolymers, and nonconjugated insulating polymers. We show that the PL emission properties of these organic quantum wells can be ''tuned'' by a proper choice of the conjugated copolymer and the thickness of the insulating layers. Particularly, some of the self-assembled ultrathin films containing thin (similar to 7 Angstrom) insulating polymeric layers exhibit a blue shift upon decreasing the thickness of the assembly. The PL shift is roughly proportional to 1/d(2) where d is the thickness of the assembly, as expected for confined photogenerated electron-hole pair in an infinite square potential well. In contrast, the PL emission of similar assemblies but containing thick (similar to 4 Angstrom) insulating layers is independent of the assembly thickness and exhibit emission in the blue. This may suggest a strong spatial confinement. Light emitting diodes based on self-assembled multilayer films with improved efficiency and stability and with threshold voltage as low as 2.6 V could be fabricated. (C) 1996 American Institute of Physics.