Following Chemical Charge Trapping in Pentacene Thin Films by Selective Impurity Doping and Wavelength‐Resolved Electric Force Microscopy

Abstract

Charge trapping is one of several factors that limit the performance of organic electronic materials, yet even in pentacene, a prototypical small‐molecule semiconductor, the precise chemical nature of charge trapping remains poorly understood. Here the effects of three chemical trap‐precursor candidates are examined by layering thin‐film pentacene transistors with different pentacene defect species. The resulting charge trapping is studied in each device via scanning‐probe electric force microscopy coupled with variable‐wavelength sample illumination. Firstly, it is found that layering with pentacen‐6(13H)‐one (PHO) readily produces uniform charge trapping everywhere in the transistor channel, as expected for an active blanket‐deposited trap‐precursor. However, layering with 6,13‐dihydropentacene (DHP) produces fewer, more‐isolated traps, closely resembling the surface potential distribution in pristine pentacene thin films. Secondly, the rates of trap‐clearing versus illuminating wavelength (trap‐clearing spectra) are measured, revealing enhanced trap‐clearing rates at wavelengths assigned to the absorption of either pentacene or the charged trap species. The trap‐clearing spectrum for the PHO‐layered sample closely resembles the spectrum obtained from pentacene aged in a working transistor, while the trap‐clearing spectrum for the DHP‐layered sample resembles the spectrum observed in pristine pentacene. We conclude that PHO competently creates traps in pentacene that match the expected trap‐clearing spectrum for degraded pentacene, while DHP does not, and that the chemical trap species in aged pentacene is very likely PHO+.

Organic Optoelectronic Materials: Mechanisms and Applications

“…Such reactions were not restricted to grain boundaries, and the traps could be cleared by illumination at wavelengths within the absorption spectrum of the film. 793 In contrast, in TES-ADT TFTs, traps were associated with grain boundaries, a similar finding to those in thin Pn films studied by KPFM. 780,794 Ginger and co-workers demonstrated feedback-based timeresolved EFM operation with a 100-μs time resolution, 782 which enabled time-resolved measurements of accumulation of photogenerated charges in OPVs.…”

“…The experiments revealed that traps in Pn and TIPS-Pn films were carrier concentration-dependent, and were created slowly by reactions of holes with a localized structural defect or impurity. Such reactions were not restricted to grain boundaries, and the traps could be cleared by illumination at wavelengths within the absorption spectrum of the film . In contrast, in TES-ADT TFTs, traps were associated with grain boundaries, a similar finding to those in thin Pn films studied by KPFM. , …”

Exploring the Origin of High Dechlorination Activity in Polar Materials M2B5O9Cl (M = Ca, Sr, Ba, Pb) with Built-In Electric Field

“…These results illustrate that the materials with built-in electric fields can generate spontaneous polarization that works as a driving force to direct the separation of photogenerated electrons and holes and highly reduces the charge recombination. Due to the screening effects, the photogenerated electrons will move along to the polarization direction and lead to the reduction product formation, while the photogenerated holes will move opposite to the polarization direction and lead to the oxidation product formation; during this process, the electron–hole pairs can be highly separated. − Furthermore, Dai et al also reported that the built-in electric field combined with the electron effective mass and their synergistic effect on the charge separation and transport in such materials are responsible for the high photocatalytic efficiency . Therefore, it can be seen that photogenerated e–h pairs with high separation efficiency have an effect on the transfer of the charges from the bulk to the surface active site, which is a critical process for a photocatalytic reaction, and thus takes on a significant role for the photocatalytic activity .…”

Hydrogen- and oxygen-related effects in phthalocyanine crystals: formation of carrier traps and a change in the magnetic state

“…The latter may trap charge carriers, thus limiting carrier mobility in electronic transistors, or acting as recombination centers in solar cells. These effects seem to be ubiquitous and have been detailed through experimental and theoretical studies on several different organic semiconductors, including acenes, [4][5][6][7][8][9] fullerenes, [10][11][12][13][14] and carbon-based nano-materials. [15][16][17] Metal phthalocyanines (MPcs) belong to a special class of organic materials.…”