Fourier-transform infrared spectroscopy of two prototypical high-mobility polymer organic semiconductors (OSCs), regioregular poly(3-hexylthiophene) (rr-P3HT) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT), reveals photoinduced doping that involves both oxygen and water dissolved in the polymer matrix when exposed to light. The equilibrium concentration of water at room temperature and 60% relative humidity in these films is $2 Â 10 19 cm
À3, and exists primarily as monomers, with a small population of dimers and trimers. Photo-excitation in room light ultimately generates a polaron density of the order of a few 10 17 cm
À3, which is sufficient to degrade the saturation and 'on-off' characteristics of organic field-effect transistors, and the dark current of organic photovoltaics. The dopant anion has been identified primarily to be hydroxide ion species. This process occurs to a smaller extent in wet nitrogen, but even less in dry oxygen, which points to a key role of the dissolved water. The relative stability of PBTTT over rr-P3HT is found to be largely kinetic in origin, attributed to its higher crystallinity (X-ray diffraction crystallinity 27% vs 21% in rr-P3HT), and shorter pÁÁÁp stacking distance (3.64 Å vs 3.78 Å in rr-P3HT), which gives better moisture exclusion from its thiophene backbone.Understanding the degradation mechanisms of polymer organic semiconductor (OSC) devices is an essential step to develop more robust OSC systems and their devices, whether in light-emitting diodes (LEDs), field-effect transistors (FETs), or photovoltaics (PVs). Despite decades of research, detailed spectroscopic studies have seldom been reported, [1,2] primarily because of the considerable challenges to identify chemical transformations that occur on sub-1-mol % of repeat units in the thin films. Yet changes at these levels can be electronically significant. The typical carrier density for LED operation [3] is $1 Â 10 18 cm À3 , which is approximately 0.05 mol % of repeat units, assuming a unit molecular weight of 300 g mol À1 and density of 1.1 g cm
À3. For organic FETs, doping at this level can open a parallel source-drain conduction path in the bulk that degrades the shut 'off' and saturation characteristics. [4,5] A simple performance figure-of-merit is the 'on-off' ratio, which for long-channel FETs with a small drain voltage V d and an off-state defined at a gate voltage (V g ) of 0 V is given by i on / i off ¼ mC ox (V g À V th )/(sd), where m is the carrier mobility, C ox is the gate-dielectric capacitance, (V g À V th ) is the effective gate voltage, s is the shunt conductance, and d is the film thickness. For m $ 0.1 cm 2 V À1 s À1 , d $ 30 nm, and typical values for the other parameters, we require s < 10 À8 S cm À1 to give an i on /i off ratio > 10 6