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+.