Details

Autor(en) / Beteiligte

• Shi, Xingyuan
• Nádaždy, Vojtech
• Perevedentsev, Aleksandr
• Frost, Jarvist M.
• Wang, Xuhua
• von Hauff, Elizabeth
• MacKenzie, Roderick C. I.
• Nelson, Jenny

Titel

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)

Ist Teil von

• Physical review. X, 2019-05, Vol.9 (2), p.021038, Article 021038

Ort / Verlag

College Park: American Physical Society

Erscheinungsjahr

2019

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Charge transport inπ-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(9,9-dioctylfluorene) (PFO). We show that whileβ-phase introduces a striking drop of about a hundredfold in time-of-flight (TOF) hole mobility (μh) at room temperature, it reduces the steady-stateμhmeasured 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 theβ-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 drift-diffusion 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.