Quantum dot light-emitting devices: beyond alignment of energy levels

Multinary semiconductor nanoparticles such as CuInS2, AgInS2, and the corresponding alloys with ZnS hold promise for designing future quantum dot light-emitting devices (QLED). The QLED architectures require matching of energy levels between the different electron and hole transport layers. In addition to energy level alignment, conductivity and charge transfer interactions within these layers determine the overall efficiency of QLED. By employing CuInS2–ZnS QDs we succeeded in fabricating red-emitting QLED using two different hole-transporting materials, polyvinylcarbazole and poly(4-butylphenyldiphenylamine). Despite the similarity of the HOMO–LUMO energy levels of these two hole transport materials, the QLED devices exhibit distinctly different voltage dependence. The difference in onset voltage and excited state interactions shows the complexity involved in selecting the hole transport materials for display devices.