Electronic structure of UV degradation defects in polysilanes studied by Energy Resolved – Electrochemical Impedance Spectroscopy

Schauer, F.; Tkáčová, Miroslava; Nádaždy, Vojtěch; Gmucová, Katarína; Ožvoldová, Miroslava; Tkáč, Lukáš; Chlpík, Juraj

doi:10.1016/j.polymdegradstab.2016.02.016

Cited by 15 publications

(13 citation statements)

References 27 publications

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“…As a complementary measurement of the effect of β-phase on the hole DOS, we use a sensitive spectroscopic method for DOS mapping in organic semiconductors, namely, energy-resolved electrochemical impedance spectroscopy (ER-EIS) [81,82]. This method has previously been applied to the determination of HOMO and LUMO positions, electronic bandgap, and characteristic energy of the HOMO band tail, as well as to the characterization of defect state evolution in the bandgap of poly(3-hexylthiophene) (P3HT) induced upon degradation in air [83] or by varying layer thickness [84], and defect states in poly (silylene) materials induced by ultraviolet-light degradation [85]. Recently, the ER-EIS method, along with structural information, was used to characterize the energetic position and density of midgap trap states of the PbS nanocrystal thin film [86].…”

Section: B Thin-film Dos Mapping Using Energy-resolved Electrochemicmentioning

confidence: 99%

Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β -phase in Poly(9,9-dioctylfluorene)

et al. 2019

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Charge transport in π-conjugated polymers is characterized by a strong degree of disorder in both the energy of conjugated segments and the electronic coupling between adjacent sites. This disorder arises from variations in the structure and conformation of molecular units, as well as the weak intermolecular binding interactions. Although disorder in molecular conformation can be expected to influence the density of states (DOS) distribution-and hence, optoelectronic properties of the material-until now, there has been no direct study of the relationship between a distinct conformational defect and the charge transport properties of a conjugated polymer. Here, we investigate the impact of introducing an extended, planarized chain geometry, known as the "β-phase," on hole transport through otherwise amorphous films of poly(9,9dioctylfluorene) (PFO). We show that while β-phase introduces a striking drop of about a hundredfold in time-of-flight (TOF) hole mobility (μ h ) at room temperature, it reduces the steady-state μ h measured from hole-only devices by a factor of less than about 5. In order to reconcile these observations, we combine high-dynamic-range TOF photocurrent spectroscopy and energy-resolved electrochemical impedance spectroscopy to extract the hole DOS of the conjugated polymer. Both methods show that the effect of the β-phase content is to introduce a sharp sub-bandgap feature into the DOS of glassy PFO lying about 0.3 eV above the highest occupied molecular orbital. The observed energy of the conformational trap is consistent with electronic structure calculations using a tight-binding approach. Using the obtained DOS with a driftdiffusion model capable of resolving charge carriers in both time and energy, we show how the seemingly contradictory transport phenomena obtained via the time-resolved, frequency-resolved, and steady-state methods are reconciled. The results highlight the significance of energetic redistribution of charge carriers in affecting transport behavior. This work demonstrates how charge-carrier mobility in organic semiconductors can be controlled via molecular conformation, and it resolves a long-standing debate over how different (equilibrium versus nonequilibrium) transport techniques reveal electronic properties of disordered solids in a unified manner.

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Section: B Thin-film Dos Mapping Using Energy-resolved Electrochemicmentioning

confidence: 99%

Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β -phase in Poly(9,9-dioctylfluorene)

et al. 2019

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“…These branching state generation rates characterize the rate of UV degradation of the band electronic structure. 29,30 It can be seen that the shorter UV wavelength stimulates a faster UV degradation regardless the previous sample treatment while the thermally annealed sample is more resistant to the UV degradation at the longer UV wavelength.…”

confidence: 99%

“…27,28 The ER-EIS method was used for elucidation of the quantum efficiency of the deep branching state formation in PMPSi after UV degradation. 29,30 The PMPSi is archetypal polymer ideally suited for such a study because its electronic properties strongly depend on the backbone conformation. A strong effect of conformation on the microphysical properties and UV degradation of PMPSi was reported.…”

confidence: 99%

“…These defect states can be attributed to the formation of the crosslinking and bridging defects after the photochemical scission of Si-Si σ bond. 28,29 After reaching the branching defect concentration of ≈5.10 15 cm -3 , the defect states in the band gap below LUMO start to grow, most probably due to the formation of conformation defects. The branching states were found to be a direct measure of the Si-Si σ bond splitting in PMPSi and may be thus used as a measure of the degradation of the electronic band structure in PMPSi.…”

confidence: 99%

“…The quantum efficiency of the branching states in PMPSi generated by the 345 nm UV irradiation was found to be φ(x) = 0.0059. 29 The dependences of the total concentration c(t) of the branching states on the UV irradiation time for both NSTA and STA samples and the UV irradiation wavelengths of 345 nm and 290 nm are shown in Fig. 2.…”

confidence: 99%

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Effect of crystallinity on UV degradability of poly[methyl(phenyl)silane] by energy-resolved electrochemical impedance spectroscopy

et al. 2017

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Low stability and degradability of polymers by ambient air, UV irradiation or charge transport are major problems of molecular electronics devices. Recent research tentatively suggests that the presence of a crystalline phase may increase polymer stability due to an intensive energy trapping in the ordered phase. Using the UV degradability, we demonstrate this effect on an archetypal model σ bonded polymer - poly[methyl(phenyl)silane] (PMPSi) - with partially crystalline and amorphous-like layers. UV degradation with 345 nm, derived from the branching state generation rate, was inversely proportional to the crystalline phase content, changing from 4.8x1011 s-1 (partially crystalline phase) to 1.8x1013 s-1 (amorphous-like phase). A model is proposed where crystallites formed by molecular packing act as effective excitation energy traps with a suppressed nonradiative recombination improving thus PMPSi film stability. The molecular packing and higher crystalline phase proportion may be a general approach for stability and degradability improvement of polymers in molecular electronics.

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Unifying Charge Generation, Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

Karki

Vollbrecht

Gillett

et al. 2020

Advanced Energy Materials

129

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Even though significant breakthroughs with over 17% power conversion efficiencies (PCEs) in polymer:non-fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends was conducted to unify and correlate the cumulative effects of i) voltage losses ii) charge generation efficiencies, iii) nongeminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics were established. An extension to advanced morphological techniques, such as solid-state NMR (for atomic level insights on the local ordering and donor:acceptor π-π interactions) and resonant soft x-ray scattering (for donor and acceptor interfacial area and domain spacings), provided detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

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Electronic structure of UV degradation defects in polysilanes studied by Energy Resolved – Electrochemical Impedance Spectroscopy

Cited by 15 publications

References 27 publications

Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β -phase in Poly(9,9-dioctylfluorene)

Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β -phase in Poly(9,9-dioctylfluorene)

Effect of crystallinity on UV degradability of poly[methyl(phenyl)silane] by energy-resolved electrochemical impedance spectroscopy

Unifying Charge Generation, Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

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