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

• Published in: Advanced functional materials 2012-12, Vol.22 (24), p.5096-5106
• Main Authors: Smieska, Louisa M., Pozdin, Vladimir A., Luria, Justin L., Hennig, Richard G., Hines, Melissa A., Lewis, Chad A., Marohn, John A.
• Format: Article
• Language: English
• Subjects: charge trapping; electric force microscopy; organic field-effect transistors; thin films
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201200595

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+. Thin pentacene films are layered with putative pentacene trap‐precursors and the resulting spatial distribution of charge traps and spectroscopic trap‐clearing rates under illumination are measured with time‐ and wavelength‐resolved electric force microscopy. Pentacen‐6(13H)‐one efficiently generates charged traps with trap‐clearing spectra resembling those of aged pentacene, while 6,13‐dihydropentacene does not. It is concluded that PHO+ is the charged trap species.