Spectroscopic Characterization of Charged Defects in Polycrystalline Pentacene by Time‐ and Wavelength‐Resolved Electric Force Microscopy

Organic semiconductors are attractive materials for microelectronic and photovoltaic applications because their energy levels, optical properties, and solubility can be independently adjusted. While signifi cant progress towards the commercialization of organic semiconductor devices has been made, the long-term reliability of most organic semiconductors remains a concern. Charge trapping, for example, causes a wide array of functional problems in organic semiconductor devices including a reduction in mobility, an increase in off-current, and an increase in operating voltage. Yet trap formation in these materials remains poorly understood, even in pentacene, the most widely studied organic semiconductor. Proposed trapping mechanisms in pentacene include the immobilization of charge at grain boundary defects[1,2] and dielectric interfaces,[2–4] formation of immobile bipolarons,[5] and chemical reactions.[6–9] Many transistor studies indicate a degradation of device performance following exposure to air, moisture, and/or light,[10,11,7,12–15,9] but whether this degradation arises from physisorbtion[10,11,13,14] or a chemical reaction[6–8,15,9] remains an open question. Although optical absorption[7] and recent X-ray photoelectron spectroscopy[16,15] studies have provided more defi nitive evidence of a chemical transformation in aged transistor fi lms, the relationship between the observed reaction products and trapped species remains unclear. Here, we introduce a new spectroscopic method for microscopically probing the electronic states of long-lived trapped charged species in a π -conjugated fi lm,and apply the method to record local trap-clearing action spectra in pentacene for the fi rst time. Pentacene’s small size allows us to carry out quantum chemical calculations, which are compared to experiment. Traps in pentacene and a wide-variety of organic semiconductors exhibit a similar 0.6 eV activation energy and, it has been suggested, may therefore share a common trapping mechanism. [ 13 ] Our results provide spectroscopic evidence for