Electron Trap Dynamics in Polymer Light‐Emitting Diodes

Abstract

Semiconducting polymers are being studied intensively for optoelectronic device applications, including solution-processed light-emitting diodes (PLEDs). Charge traps in polymers limit the charge transport and thus the PLED efficiency. It is firmly established that electron transport is hindered by the presence of the universal electron trap density, whereas hole trap formation governs the long-term degradation of PLEDs. Here, the response of PLEDs to electrical driving and breaks covering the timescale from microseconds to (a few) hours is studied, thus focusing on electron traps. As reference polymer, a phenyl-substituted poly(para-phenylene vinylene) (PPV) copolymer termed super yellow (SY) is used. Three different traps with depths between ≈0.4 and 0.7 eV, and a total trap site density of ≈2 × 10 17 cm −3 are identified. Surprisingly, filling of deep traps takes minutes to hours, at odds with the common notion that charge trapping is complete after a few hundred microseconds. The slow trap filling feature for PLEDs is confirmed using poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and poly(3-hexylthiophene) (P3HT) as active materials. This unusual phenomenon is explained with trap deactivation upon detrapping and slow trap reactivation. The results provide useful insight to pinpoint the chemical nature of the universal electron traps in semiconducting polymers.

Traps for Electrons and Holes Limit the Efficiency and Durability of Polymer Light‐Emitting Electrochemical Cells

“…This overshoot is the result of electron de‐trapping at rest. [ 22 ] At switch‐on, the light temporarily increases because non‐radiative SRH recombination between trapped electrons and free holes is small. Trap filling during operation increases SRH recombination and the light recovers to the trend line before the break.…”

“…[21] Recently, we examined the dynamics of electron traps in PLEDs. [22] We fabricated devices using a phenyl-substituted poly(para-phenylenvinylene) copolymer termed super yellow (SY) as emitting material (Figure S1, Supporting Information) [23][24][25][26][27][28] and studied the device response to electrical driving and breaks. In an operated PLED traps are filled and after turning-off, deep electron traps de-trap slowly via thermal emission over many minutes.…”

“…Therefore, electrical and optical transients in PLECs have to be analyzed as a superposition of effects due to a temporary imbalance in charge injection and electron trap filling. With the known electron trap density from PLEDs, [15,22] we can quantify the experimental optical transients of the PLEC very well. Based on the identified similarity between PLEDs and PLECs, we then argue that during long-term operation also hole traps form in a PLEC.…”

“…Such a remarkable appearance is consistent with a slow diffusion process between precursor trap species that form a trap only when meeting, with the hydrated oxygen complex as a possible candidate. [ 22 ] Because the electron trap density is constant, the light emission and voltage response to a break does not change over time, i.e., the same rest period at different moments leads to similar initial performance improvement and recovery trends.…”

“…Details on the PLED and PLEC fabrication are summarized in references. [11,22] Electroluminescence spectra were measured with an integrating sphere on an Ocean Optics spectrometer QE Pro, calibrated before each measurement. Angular dependent EL measurements were performed with the Phelos measurement system (Fluxim AG, Switzerland).…”