Energy Alignment of Frontier Orbitals and Suppression of Charge Recombinations in P3HT/SWNT

Abstract

Blends of poly­(3-hexylthiophene) (P3HT) and single-walled carbon nanotubes (SWNTs) have been studied as promising materials for organic photovoltaic devices because of the excellent electronic properties of SWNT and a broad interfacial area guaranteed by the helical structure of P3HT at the interface. However, the P3HT/SWNT blends show a low energy conversion efficiency, and thus, a deeper understanding of the charge separation process in the P3HT/SWNT blends is required to achieve improved efficiency. In this paper, the electronic structures of P3HT at the interface and in the bulk phase were studied to elucidate the charge separation process in the P3HT/SWNT blends. We show the existence and origin of the difference between the HOMO levels at the interface and those in the bulk phase. This explains observations in a previous experiment where long-lived charge carriers were only observed in blends containing excess P3HT. In the course of the investigation on the electronic structures, the role of the side chains on the polythiophene (PT) that form the helical structure at the interface was also investigated.

Carbon Photodetectors: The Versatility of Carbon Allotropes

“…Furthermore, the polymer aggregation causes a shift of the highest occupied molecular orbital, leading to an energetic driving force away from the interface into the aggregates. [149] Theoretical calculations estimated an energetic offset of 0.5 V between the sheet-like P3HT at the nanowire interface and the aggregated P3HT in the bulk phase. [149] These considerations explain the findings of Stranks et al who observed the free carrier formation only in the presence of pristine and aggregated P3HT domains next to the P3HT-wrapped SWCNTs.…”

“…[149] Theoretical calculations estimated an energetic offset of 0.5 V between the sheet-like P3HT at the nanowire interface and the aggregated P3HT in the bulk phase. [149] These considerations explain the findings of Stranks et al who observed the free carrier formation only in the presence of pristine and aggregated P3HT domains next to the P3HT-wrapped SWCNTs. [146] To avoid metallic carbon nanotubes that can severely limit the performance of SWCNT photodetectors, highly selective dispersion of carbon nanotubes using aromatic polymers like PFO were used to obtain nanotubes of a single chirality in high purity.…”

Thermal effect on the morphology and performance of organic photovoltaics

“…14 Meanwhile, various theoretical studies on OPVs have been presented to obtain a detailed understanding of the mechanism of photocurrent generation. 15 Quantum chemical calculations estimate properties such as energy levels and distributions of molecular orbitals, absorbance, [16][17][18][19] and rate of an elementary process. 20 However, such molecular-level investigations alone are not sufficient to provide guiding principles for OPV optimization because photocurrent generation is a complicated macroscopic process.…”