No abstract

Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic speci®cations for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Achieving balanced electron-hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (<100 A) charge-injection interfacial layers in polymer LEDs. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15-20 per cent) at a luminance of 1,600 candelas per m2 at 5 V.

Organic semiconductors fabricated as thin-film light-emitting diodes, LEDs, now provide a promising new display technology.[1] Solution-processed semiconductor polymers make possible direct printing (using ink-jet deposition) and allow high-resolution full-color displays to be conveniently manufactured. [2] Multiple-layer deposition, used in vacuum-sublimed molecular semiconductor LEDs, is difficult to achieve by solution processing. We have instead fabricated distributed heterojunction' structures that are formed by de-mixing of two polymers co-deposited from common solution. We have used hole-accepting and electron-accepting derivatives of polyfluorene, and have optimized these structures to achieve high-efficiency diodes (above 19 lm W ±1 for green emission) that operate at very low voltages (100 cd m ±2 at 2.1 V for green emission). This very low voltage operation is achieved because electron±hole capture across the heterojunction is arranged to be a barrier-free process to form an interface state (termed an exciplex) that has significant charge-transfer character and is lower in energy than the charge-separated state. With respect to the bulk exciton, the exciplex is red-shifted (here between 140 and 360 meV) and its radiative lifetime is strongly increased (between 68 and 118 ns at low temperatures). The barrier for thermal excitation of the exciplex to allow it to move away from the heterojunction is small (100± 250 meV), and this process can give efficient bulk exciton emission at room temperature. The heterojunction formed between dissimilar organic semiconductors is generally found to be remarkably free of gap-states and other defects that would otherwise compromise semiconductor device operation. Heterojunction LEDs are designed so that the offsets between conduction and between valence band edges are type II' and electrons and holes accumulate on opposite sides of the heterojunction (Fig. 1). In a non-interacting electron scheme, type II heterojunctions would destabilize an exciton present in either semiconductor, since the exciton state would be higher in energy than the charge-separated state. However, organic semiconductors are low dielectric constant materials (typically having values less than 4) so that the coulomb interaction between electron and hole gives a substantial exciton binding energy (of order 0.5 eV). When this binding energy is larger than the band-edge offsets, excitons are stable at the interface. By selecting semiconductors with larger band-edge offsets, charge separation at the heterojunction can be readily achieved, giving efficient photovoltaic behavior. [3] LEDs made using molecular semiconductors are generally fabricated as multiple-layer heterojunction structures by successive vacuum sublimation steps.[1] However, with solutionprocessed polymers it is possible to make distributed heterojunction' diodes by de-mixing of two polymers spin-coated from common solution. [4] This is an obviously desirable structure for photovoltaic diodes, because it allows excitons photogenerated...