Effect of molecular weight on the efficiency of poly(N-vinylcarbazole)-based polymer light-emitting diodes

Polymer light-emitting diodes based on poly(N-vinylcarbazole) (PVK) with molecular weights M_W of 1.1 × 10⁶ and ∼7.5 × 10⁴ are compared. For devices without an electron transport layer (ETL), the high M_W PVK yields higher external quantum efficiency (0.67% vs 0.18%), but for devices with an ETL, the low M_W PVK yields higher efficiency (1.13% vs 0.83%). This intriguing difference is believed to result from higher energetic disorder in the high M_W polymer and different recombination zone-quenching metal electrode distances, in agreement with Konezny et al. [Appl. Phys. Lett. 97, 143305 (2010)].