Carrier single layer devices

Single-carrier-dominated transport, including a detailed treatment which includes space-charge effects that are prominent in single layer devices, have been developed to provide a satisfactory explanation of the I(V) characteristics in... 

In addition, the success of multilayer studies has caused the reexamination of single-layer devices. Although the combinations and contents of well-organized carrier transport and emitter materials resulted in the enhancement of EL efficiency, a higher efficiency than the multilayer structures has never obtained.59,60 Thus, we anticipate that organic/organic interfaces will especially provide efficient carrier recombination sites. 

Following these recent successful developments, trilayer hybrid NC/polymer devices were reported, constituting a film of CdSe/ZnS NCs a few monolayers thick sandwiched between films of PVK (hole transporter) and butyl-PBD (electron transporter) [23]. All the layers were deposited by a spin-coating technique from dissimilar solvents (either organics or water). These devices showed 20 times the external quantum efficiency (0.2 %) and less than half the threshold voltage of a single-layer device based on the PVK/NC/PBD blend. These improvements upon going from a blend to a trilayer structure were attributed to more balanced carrier conduction to and enhanced recombination in the NC layer. 

Schematic energy diagram of the NPB single-layer devices, and energy transfer mechanism of perylene-doped device includes 1, carrier injection 2, charge transport in pristine NPB 3, electrons and holes are trapped and recombine and 4, light emission.

Single-layer devices have the simplest device structure, and the fabrication process is simpler than that of the multilayer type. In these devices, both electrons and holes should be injected to the emitter layer. To this end, we have used dye-dispersed polymer systems in which polymers are molecularly dispersed with carrier-transporting... 

In single layer devices the low efficiency frequently is due to the imbalance of electrons and holes. Inserting a hole-transport (electron-blocking) or electron transport (hole-blocking) layer provides a means to enhance minority carriers and block the majority carriers and confine them to the emitter layer, which increases the probability of recombination. Poly(N-vinylcarbazole) (PVK) has been used as a hole transport layer and occasionally in blends with the emitter polymer. PVK is a well-studied photo-conductive polymer which often forms exciplexes with organic molecules, e.g., dimethyl terephthalate. An exciplex is a transient donor-acceptor complex between the excited state of the donor and the ground state of the acceptor. ... 

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