Direct Spectroscopic Evidence for a Photodoping Mechanism in Polythiophene and Poly(bithiophene‐alt‐thienothiophene) Organic Semiconductor Thin Films Involving Oxygen and Sorbed Moisture

Abstract

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

Reversible P3HT/Oxygen Charge Transfer Complex Identification in Thin Films Exposed to Direct Contact with Water

“…This finds an explanation by considering that in water the hydration of oxygen molecules is enhanced (thus leading to more efficient doping), in full agreement with results reported by Ho and co-workers. 25 The formation of the charge transfer complex between P3HT and triplet oxygen, both in air and in water, is confirmed by DFT calculations. B97D/6-311G** BSSE corrected calculations were performed on the following systems: (a) isolated P3HT oligomer and molecular oxygen in its triplet ground state ( 3 O 2 ); (b) P3HT:n( 3 O 2 ) molecular complexes with n = 1, 2 (i.e., with one or two molecular oxygens); and (c) P3HT:H 2 O molecular complex.…”

Effect of voltage sweep direction on the performance evaluation of P3HT : PCBM solar cells

“…It is suggested that the reaction involves metastable oxygen species that are responsible for the irreversible degradation of the polymer. This formation of charge transfer complex results Effect of voltage sweep direction on P3HT:PCBM solar cells P. Kumar, A. Sharma and D. P. Singh into the p-type doping of P3HT [26,27]. The electron transfer from P3HT to oxygen molecule results in the formation of mobile holes on P3HT and immobile electrons on oxygen.…”

“…It is known, however, that P3HT forms a charge transfer complexes with oxygen, resulting in the formation of cation and anion radical species, which can be detected by electron spin resonance. Formation of these charged ions represents the doping of P3HT molecules . This doping increases with degradation of P3HT and increment in oxygen concentration.…”

Polarization effects on energy-level alignment at the interfaces of polymer organic semiconductor films

“…These histograms reveal that the step heights on the surfaces of the 2-nm-thick films comprise exclusively of the respective (100) d-spacings of the polymer lamellar crystal that are well known from x-ray scattering (P3HT, 1.65 nm; C 14 -PBTTT, 2.2 nm). 16 the thickest films, both the fundamental and its doublet were found. This shows that the films here are deposited as lamellae comprising extended polymer chains with well ordered p-conjugated backbones that are p-stacked parallel to the substrate surface.…”

“…These final values are the same as "thick-film" values previously reported. 16 This evolution is characterized by a rigid shift of the valence band away from the vacuum level, as a consequence of which, the relatively shallow E F pinning to the first ML (ca. 0.2 eV) becomes deep when referenced to the HOMO edge of the "bulk" film (0.5-0.6 eV).…”