Femtosecond and Attosecond Electron-Transfer Dynamics in PCPDTBT:PCBM Bulk Heterojunctions

Abstract

Charge separation efficiency is a crucial parameter for photovoltaic devicespolymers consisting of alternating electron-rich and electron-deficient parts can achieve high such efficiencies, for instance, together with a fullerene electron acceptor. This offers a viable path toward solar cells with organic bulk heterojunctions. Here, we measured the charge-transfer times in the femtosecond and attosecond regimes via the decay of sulfur 1s X-ray core-excited states (with the core-hole clock method) in blends of a low-band gap polymer {PCPDTBT [poly­[2,6-(4,4-bis­(2-ethylhexyl)-4H-cyclopenta­[2,1-b;3,4-b′]­dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]]} consisting of a cyclopentadithiophene electron-rich part and a benzothiadiazole electron-deficient part. The constituting parts of the bulk heterojunction were varied by adding the fullerene derivative PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) (weight ratio of polymer/PCBM as 1:0, 1:1, 1:2, and 1:3). For low-energy excitations, the charge-transfer time varies to the largest extent for the thiophene donor part. The charge-transfer time in the 1:2 blend is reduced by 86% compared to that of pristine PCPDTBT. At higher energy excitations, the charge-transfer time does not vary with the chemical environment, as this regime is dominated by intramolecular conduction that yields ultrafast charge-transfer times for all blends, approaching 170 as. We thus demonstrate that the core-hole clock method applied to a series with changing composition can give information about local electron dynamics (with chemical specificity) at interfaces between the constituting partsthe crucial part of a bulk heterojunction where the initial charge separation occurs.

Tailoring ultra-fast charge transfer in MoS2

“…increase in the density of states. 21 Comparing the life-times of the S 1s two things stands out. The two samples with nano-sized flakes (NP and rGO) exhibit a shorter core-hole life-time than the single crystal.…”

“…The time-scales measured using core hole clock spectroscopy are different from that of pump-probe spectroscopy, as has been shown for the PCPDTBT:PCBM system. 21 The difference arises from that while an optical excitation occurs between valence orbitals (or the valence band) to the unoccupied orbitals (conduction band) which are diffuse of delocalised in the system, an excitation with an X-ray is localised since the core hole is localised. The localisation makes core hole clock spectroscopy chemically specific and the resonant Auger pathways are very sensitive to the local energy landscape of the excited electron: for instance, the MoS 2 monolayer/graphene system the charge transfer time have been measured to be 300 attoseconds have been found, 16 whereas here we study multilayered and even bulk systems and reach charge transfer times below 100 attoseconds.…”

“…19 In this paper the charge transfer dynamics have been studied using core-hole clock spectroscopy. [20][21][22] This is a technique that combines the chemical specificity of X-ray absorption and offers a higher time resolution than typical pump-probe spectroscopy as is probes dynamics reaching into the sub-femtosecond time scale.…”

Understanding ultrafast charge transfer processes in SnS and SnS2: using the core hole clock method to measure attosecond orbital-dependent electron delocalisation in semiconducting layered materials

“…21,22 In the last decade, the CHC method has been applied to study semiconducting materials, starting with the pioneering work of Rocco and Garcia-Basabe on a series of thiophene containing semiconducting polymers, [23][24][25][26] and with a couple of other groups following with incremental work on polymers and other complex conjugated systems. [27][28][29] These studies provide maps of delocalisation times across the conduction band of these semiconductor materials that have been related to the molecular composition, 30 structure, 23,24 morphology 31 and material interactions at different substrates and heterojunctions. 27 However, as recognised in a recent 2020 publication, CHC studies of complex organic semiconductor materials are still scarce.…”

“…[27][28][29] These studies provide maps of delocalisation times across the conduction band of these semiconductor materials that have been related to the molecular composition, 30 structure, 23,24 morphology 31 and material interactions at different substrates and heterojunctions. 27 However, as recognised in a recent 2020 publication, CHC studies of complex organic semiconductor materials are still scarce. 28 Studies on inorganic semiconducting materials are even scarcer probably due to the higher complexity of their conduction band, containing a high number of orbital hybridisations, which adds another degree of complexity to the analysis and interpretation of CHC data.…”