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)

Abstract

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.

The Importance of Microstructure in Determining Polaron Generation Yield in Poly(9,9-dioctylfluorene)

“…Experimental determinations of the relative positions of the β-phase HOMO and LUMO levels are, however, inconclusivespectroelectrochemical hole doping of β-phase PFO samples has shown that the β-phase oxidation potential is lower than the glassyphase by ΔE ≈ 100-150 meV, 63 indicative of a shallower HOMO level; and photocurrent and impedance spectroscopies suggest that the β-phase HOMO is also shallower by ΔE ≈ 300 meV. 76 Meanwhile, trap-filling thermally stimulated current measurements exhibit no evidence of a difference in glassy and β-phase HOMO levels, but find the LUMO of the βphase to be deeper than the glassy-phase by ΔE ≈ 120 meV. 32 Given this uncertainty, in combination with the local variation in HOMO and LUMO energies of glassy-phase chains due to conformational variation, it is reasonable to speculate that within a PFO film there may be a (small) fraction of glassy/β-phase interfaces which form type II rather than type I heterojunctions, where offsets between HOMO or LUMO levels of neighbouring glassy and β-phase chain segments favour charge separation over energy transfer.…”

Relationship between Fill Factor and Light Intensity in Solar Cells Based on Organic Disordered Semiconductors: The Role of Tail States

“…We note that gpvdm has been validated against experimental data in the past. [33,63,64] Despite that for extreme low values of !,#$% FF goes down with !,#$% , within commonly observed FF values in the range from 50% to 70%, we find that FF increases with ! for both direct and trap-mediated recombination.…”

The Influence of Photo‐Induced Space Charge and Energetic Disorder on the Indoor and Outdoor Performance of Organic Solar Cells

“…[ 4–8 ] This development was mostly fueled by the use of novel, small‐molecule acceptor materials that replaced the previously used fullerene‐based acceptors and led to improved absorption and reduced nonradiative recombination losses. [ 9–15 ] Despite the substantial amount of structural and conformational disorder present in these molecular semiconductor blends, [ 16–18 ] efficiencies of organic solar cells under standardized test conditions have been improved to >18%, [ 7,8 ] that is, substantially higher than that of highly disordered inorganic solar cells such as those based on amorphous Si absorber layers. [ 3 ] In addition, applications of organic photovoltaics beyond the traditional large‐scale power generation market have become more important, both from a scientific and economic point of view.…”