Electrodeless time-resolved microwave conductivity study of charge-carrier photogeneration in regioregular poly(3-hexylthiophene) thin films

Abstract

The electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study the photogeneration of charge carriers in spin-coated films of regioregular poly(3-hexylthiophene) ͑P3HT͒, over the photon energy range from 1.9 to 5.2 eV for incident light intensities from 10 13 to 10 16 photons/ cm 2 per ͑3 ns͒ pulse. The initial, single-photon quantum yield of photoionization, , has been estimated from the low-intensity limit to the photoconductivity based on a charge carrier mobility of 0.014 cm 2 /Vs (determined in separate pulse-radiolysis TRMC experiments on bulk P3HT). The value of is constant at ͑1.7± 0.4͒% within the range 1.9-3.0 eV, which encompasses the first electronic absorption band of P3HT. Above 3.0 eV, increases, up to a value of ͑7±2͒% at 5.2 eV. The activation energy of the photoconductivity was found to be approximately 50 meV at all photon energies. The high-intensity, sublinear dependence of the photoconductivity can be described by the occurrence of either exciton-exciton annihilation or diffusional charge recombination with rate coefficients of 2.3ϫ 10 −8 cm 3 / s and 1.1ϫ 10 −8 cm 3 /s.

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

“…30), exhibits the lowest yield among the materials presented here. It is gratifying to note that the yield we report, η = 8 ± 2 %, is comparable to values obtained by means of complementary techniques 38,39 . This last comparison supports the reasoning that donor and acceptor moieties within the polymer backbone are promoting charge separation and polaron pair formation, whereas in homopolymers, such as P3HT, charge separation is less probable and eventually driven by the energetic disorder between the polymer chromophores 40 .…”

“…A recent work by Reid et al 43 shows that the polaron yield in P3HT depends on the degree of crystallinity because the charge separation might occur at the interface between the crystalline and amorphous regions. A less pronounced carrier localization in one of the two regions can lead to large intrachain mobilities 39 and a longer polaron pair lifetime. Our results have immediate implications for organic photovoltaics.…”

Conjugated Thiophene Dendrimer with an Electron-Withdrawing Core and Electron-Rich Dendrons: How the Molecular Structure Affects the Morphology and Performance of Dendrimer:Fullerene Photovoltaic Devices

“…13 The magnitude of the TRMC signal for neat dendrimer 2 (shown in Figure 5) is very similar to that in regioregular poly(3-hexylthiophene). 25 Since transport in both systems occurs via an ordered array of oligothiophenes, a drastic change in φ would manifest itself in the magnitude of the signal, contrary to observations.…”

“…75% in blends with PCBM. 22,25 The 5-fold increase in the photoconductance of pristine 2 can predominantly be attributed to two factors: (1) the increased local order in dendrimer 2 and/or (2) the low reorganization energy for hole transfer in 2 compared to other nonsubstituted thiophene dendrimers. 16 Currently we are unable to differentiate between these two mechanisms; however, it is clear that both arise from the planar structure of dendrimer 2, while the bowl-shaped structure of dendrimer 1 does not allow packing of the molecules in an ordered film, thereby reducing the coupling of π-orbitals and lowering the hole mobility that is measured by TRMC.…”

Ultrafast Charge Separation at a Polymer−Single-Walled Carbon Nanotube Molecular Junction

“…28,36,37 Excitonexciton annihilation processes, which are significant with the fluences used here, 38 cause a fast decay of the absorption signal, which may also contain a very small transient component arising from charge absorption originating from internal hot exciton dissociation intrinsic to the polymer. [39][40][41] As expected, no long-term charge-separated state exists for the bulk P3HT sample.…”