Self-Assembly and Its Impact on Interfacial Charge Transfer in Carbon Nanotube/P3HT Solar Cells

Abstract

Charge transfer at the interface of conjugated polymer and nanoscale inorganic acceptors is pivotal in determining the efficiency of excitonic solar cells. Despite intense efforts, carbon nanotube/polymer solar cells have resulted in disappointing efficiencies (<2%) due in large part to poor charge transfer at the interface. While the interfacial energy level alignment is clearly important, the self-assembly and the interface structure also play a major role in facilitating this charge transfer. To understand and control this effect to our advantage, we study the interface of commonly used conductive polymer poly-3-hexylthiophene (P3HT) and single-walled carbon nanotubes (SWNTs) with a combination of molecular dynamics simulations, absorption spectra experiments, and an analysis of charge transfer effects. Classical molecular dynamics simulations show that the P3HT wraps around the SWNTs in a number of different conformations, including helices, bundles, and more elongated conformations that maximize planar π-π stacking, in agreement with recent experimental observations. Snapshots from the MD simulations reveal that the carbon nanotubes play an important templating role of increasing the π-conjugation in the system, an effect deriving from the π-π stacking interaction at the interface and the 1-dimensional (1D) nature of the SWNTs, and independent of the SWNT chirality. We show how this increase in the system conjugation could largely improve the charge transfer in P3HT-SWNT type II heterojunctions and support our results with absorption spectra measurements of mixtures of carbon nanotubes and P3HT. These findings open possibilities for improved preparation of polymeric solar cells based on carbon nanotubes and on 1D nanomaterials in general.

Microscopic and Spectroscopic Investigation of Poly(3-hexylthiophene) Interaction with Carbon Nanotubes

“…Referring to our previous results, we can speculate that the non-constant coiling angle could be a result of the mismatch between the underlying carbon nanotube's chirality and the energy minima of the P3HT self-assembled structure. Recent theoretical studies based on classical molecular dynamics simulations [36] show that the P3HT wraps around the single-wall carbon nanotubes (SWNTs) in a number of different conformations, including helices, bundles, and more elongated conformations that maximize planar stacking, matching perfectly the above experimental observations. The simulations reveal that the 1-dimensional 1D nature of the carbon nanotubes plays an important templating role in the polymer attachment.…”

Self-Assembly and Visualization of Poly(3-hexyl-thiophene) Chain Alignment along Boron Nitride Nanotubes

“…The evolution of this band intensity is also shown in Figure 2c. The relative intensity of the 603 nm band has been used to evaluate the degree of crystallinity of rr-P3HT/MWCNT composites 8 and P3HT nanowhiskers. 16 The intensity ratio for rra-P3HT/BNNT as a function of increasing P3HT to BNNT weight ratio is included in Figure 2c.…”

Hybrid Carbon Nanotube Networks as Efficient Hole Extraction Layers for Organic Photovoltaics

“…In terms of the geometry of rr-P3HT in the presence of SWNTs, previous reports on nanohybrids prepared using techniques similar to that described in this work have indicated experimentally (through scanning probe microscopic techniques) as well as computationally, a natural tendency for the rr-P3HT to wrap around SWNTs due to π-π type super-molecular interactions. 10,[21][22][23][24] To ascertain this fact further and to obtain a better understanding of the non-covalent functionalization process, optical absorption, valence and core level X-ray spectroscopy (XPS) measurements were carried out on the rr-P3HT/s-SWNT nanohybrid system. A representative UV-Vis absorbance spectrum for rr-P3HT/s-SWNT nanohybrids in 1,2-dichlorobenzene is shown in Fig.…”