Light emitting polymers are an attractive material for applications ranging from domestic lighting to flat-screen displays. Such applications require devices that can emit light of each the three primary colors, red, green and blue. Steady progress has been made in the development of red and green polymer light emitting devices (PLEDs), but to date, the development of blue PLEDs has been hindered by poor emission efficiency.

Show-An Chen and colleagues1 from the Chemical Engineering Department of the National Tsing-Hua University, Taiwan, have taken a novel molecular-level approach to engineering the contacts that inject charge into PLEDs. In doing so they achieved significant improvements in the efficiencies of their deep blue and sky blue PLEDs, of around 140% and 81%, respectively.

One of the reasons for the poor efficiency of blue PLEDs is the difficulty in injecting equal numbers of positive and negative charges—hole and electrons—into blue light emitting polymers, where they recombine and emit light. This difficulty arises because although electrons can be injected into such a material, the emergence of a large potential barrier between it and most hole-transporting materials makes it difficult to inject holes.

Previous attempts to overcome this hole-injection problem have involved incorporating hole-transport elements into the molecular structure of a light emitting polymer, or including interfacial monolayers at the junction with a hole transporting contact. But these have had little success with even the best deep blue (or sky blue) PLED external light emitting efficiencies remaining below 3%.

Here, instead of trying to remove the barrier completely, the researchers minimized its impact by allowing the holes to travel over in a step-by-step fashion. This was achieved by introducing a series of graded electronic levels between the hole transporting contact and the blue light emitting polymer—each supported by a different type of molecule grown in sequence between the materials.

Fig. 1: Deep blue (left) and sky blue (right) emission from the authors polymer light emitting devices. Through molecular scale engineering of the hole-injecting contact, the authors achieve substantial improvements in the efficiency of these devices.

This enabled the production of deep blue and sky blue PLEDs (Fig. 1) with external light emitting efficiencies of up to 4.54% and 7.53%, respectively, approaching the theoretical upper limit expected to be between 8.4%–10.8%.