Phonons spreading from laser-heated gold nanoparticle array accelerate diffusion of excitons in an underlying polythiophene thin film

Abstract

Localized surface plasmon (LSP) photophysical phenomena occurring in metal nanostructures are often presented as a method to effectively couple light into photovoltaic devices of sub-wavelength-scale thickness. However, the excitation of LSP is also associated with rapid energy dissipation leading to local heating, which affects the excitation energy pathway. We studied a system consisting of a planar gold nanoparticle (AuNP) array deposited at the surface of a semiconducting polymer thin film (P3HT). We observed heat transfer from laser pulse excited AuNPs into the P3HT, which was evidenced as a long-living thermochromic effect on transient optical absorption. By modeling of the ultrafast kinetics of exciton population evolution, we determined that their decay was caused by their mutual annihilation. The decay rate was controlled by a phonon-assisted one-dimensional diffusion mechanism with a diffusion constant of 2.2 nm ps. The transferred heat resulted in an increase of the diffusion constant by a factor of almost 2, compared to the control system of P3HT without AuNPs. These results are of practical use for the design of plasmon-enhanced optoelectronic devices.

Kinetics of the Photoexcited States in Thin Films of Metallo-Supramolecular Polymers With Ditopic Thiophene-Bridged Terpyridine Ligands

“…On the other hand, for low initial excitation population , we obtained very high initial value of the diffusion coefficient of singlets . This is in contradiction to typical values of a few found in polymers ( Rais et al, 2017a ). Realizing that for times t up to ca 10 ps , we can estimate the mean diffusion distance at early times.…”

Evolution of Diffusion Coefficient of Photoexcited Species in Excimer Forming Organic Thin Films

“…41 For high excitation densities with the mean distance between two excitations of ca. 1 nm and with typical values of the excitation diffusion coefficient D 1d ∼ 1 (nm) 2 • ps −1 , found in the literature, 11 we can expect t edge to be of the order of picoseconds, while for low initial excitation population densities it may be prolonged even up to tens of picoseconds. Whenever experimental data of the population decay are controlled by the bimolecular decay process according eq 1, the time evolution of the population should provide quasi-steplike behavior with changeover at the time t edge .…”

“…Recently, the kinetics of photoexcited species in thin solid films of bis­(terpyridine-40-yl)­terthiophene (T) were studied using transient absorption spectroscopy . The spectral evolution showed an excimer (EM) formation on the time scale of tens of picoseconds, and the rate of the excited state decay increased with the intensity of the pump pulse, indicating thus its bimolecular collision nature of the process. ,− The formation of the excimer states was confirmed simultaneously by the analysis of the luminescence spectrum that was strongly red-shifted with respect to the optical absorption. A mutual exciton annihilation via direct Förster resonant energy transfer over long distances can be excluded since the photoluminescence quantum yield was low.…”

“…First, the time dependences of charge carriers mobility, − electron–hole collision rate, electron–hole separation distance, or exciton–exciton collision rate have been observed in many organic and hybrid systems. Second, in many cases − the time-resolved experimental data of the decay of the photoexcited species population, which is assumed to be controlled by the bimolecular mechanism, satisfy the power-law asymptote t – x with x < 0.5. This is in a contradiction with the prediction of x ≥ 0.5 for the processes with the time-invariant diffusion coefficient .…”

Growth of Boron-Doped Diamond Films on Gold-Coated Substrates with and without Gold Nanoparticle Formation

“…The formation of AuNPs by annealing thin films of gold has been of significant recent interest , owing to the use of AuNPs as catalysts for growing nanowires and for plasmonic phenomena. − The use of photonic nanoparticles can aid in the performance of photocatalytic chemical production, and diamond thin films with embedded silver nanoparticles have been shown to enhance optical absorption and photocatalytic activity for the reduction of N 2 to NH 3 . Arrays of AuNPs could also be used to maximize the absorption in the active layer of photovoltaic cells, where the particle size and shape are critical for precisely tuning the light scattering and trapping to maximize the performance …”