29-01-2013, 02:01 PM
Effect of molecular weight on the efficiency of poly(Nvinylcarbazole)-
based polymer LEDs
Abstract
Polymer light-emitting diodes (PLEDs) based on poly(N-vinylcarbazole) (PVK)
with molecular weights MW of 1.1×106 and ~7.5×104 are compared. For devices
without an electron transport layer (ETL), the high MW PVK PLEDs yield higher
external quantum efficiency (0.67% vs 0.18%), but for devices with an ETL, the low
MW PVK PLEDs are more efficient (1.13% vs 0.83%). This intriguing difference is
believed to result from higher energetic disorder in the higher MW polymer and
different distances of the recombination zone from the quenching metal electrode, in
agreement with S. J. Konezny, L. J. Rothberg, M. E. Galvin and D. L. Smith [Appl.
Phys. Lett. 97, 143305 (2010)].
Introduction
Solution-processable conjugated polymers are an important class of materials for
low-cost optoelectronic applications, such as polymer light-emitting diodes (PLEDs),
polymer solar cells, and polymer field effect transistors [1-6]. Conjugated polymers
combine the properties of conventional polymers, such as light weight, mechanical
flexibility, and processability, with semiconducting properties, such as a tunable
bandgap and conductivity. However, they usually have a distribution of conjugation
lengths, resulting from a distribution of defects and weight-average molecular weights
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MW [7]. The distribution of conjugation lengths (typically 8 – 15 repeat units per
conjugated segment) and defects commonly leads to energetic disorder [8], which
plays a crucial role in device performance [9]. In general, it is believed that higher MW
PLEDs are usually less efficient than low MW devices due to higher energetic disorder
[7].
Results and discussion
The PLED and material structures are shown in Fig. 5-1. The first type of PLEDs
that were studied were ITO/PEDOTSS/PVK (high and low MW)/CsF (1 nm)/Al
(100 nm) (Type I) (see Fig. 5-1). Fig. 5-2 shows the EL spectra, normalized transient
EL intensity vs. time t, cw brightness L and current density J vs. bias V, and external
quantum efficiency ηext vs. L of these devices. As clearly seen, the turn-on V (i.e., the
bias Von required for L = 1 Cd/m2) are both ~4.4 V. But J of the high MW PVK PLED
is much lower than that of the low MW PVK PLEDs throughout the whole bias range.
This lower J is most likely due to the strong effects of energetic disorder on charge
carrier transport. Indeed, electrochemical studies have shown that the energetic
disorder in polymers increases as MW increases [13], and this increased energetic
disorder generates additional and deeper charge carrier traps [7].