Role of Defects as Exciton Quenching Sites in Carbon Nanotube Photovoltaics

Abstract

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted significant attention as a photoactive component in thin film photovoltaic solar cells and photodetectors due to their strong optical absorptivity and high charge transport mobility. However, the external quantum efficiency (QE) of s-SWCNT/acceptor heterojunction solar cells has been limited by poor exciton harvesting efficiency. Exciton trapping and quenching at defects are a suspected source of loss. Here, we study the influence of defects on bilayer s-SWCNT/C 60 planar heterojunction photovoltaic devices via both experiment and modeling. First, diazonium chemistry is used to introduce covalent sp 3 sidewall defects to s-SWCNTs at various densities that are estimated using Raman and transient absorption spectroscopy. s-SWCNT/C 60 heterojunction photovoltaic cells are then fabricated that show a significant decrease in peak external QE (e.g., from 40% to 8%) with increasing defect density. Second, a diffusion-limited contact quenching Monte Carlo model is developed to assess the contributions of exciton quenching defects on exciton migration in bilayer s-SWCNT/C 60 heterojunction devices. The model indicates that current state-of-the-art s-SWCNT-based devices are defect limited and suggests that significant gains in exciton harvesting efficiency can be realized if more pristine, longer s-SWCNTs are utilized.

Carbon Nanotubes for Photovoltaics: From Lab to Industry

“…By preparing solar cells from PFO‐BPy wrapped (6,5) exposed to different degrees of harshness in their dispersion; extended and brief ultrasonication and shear force mixing, they were able to show an improvement in FF and V oc for the SFM sample, Figure 2e. [ 68b ] EQE from the CNTs also depends on the defect density [ 76 ] and exciton lifetime, and the SFM sample achieved 49% (4.2 ps) compared to 38% (3.0 ps) and 28% (2.1 ps) for the brief or extended sonication samples, respectively. [ 68b ]…”

“…[ 13a,60b,65,70 ] Although this is much thicker than the exciton diffusion length in C 60 , it is an approach used to smooth out inhomogeneities in the SWCNT film, and more importantly to optimize the electric field intensity within the layer stack such that it is matched to the physical position of the SWCNTs within the device and their optical transitions using transfer matrix calculations. [ 13a,70,76 ]…”

Luminescent Defects in Single‐Walled Carbon Nanotubes for Applications

“…First demonstrations of the direct functionalization of polymer-wrapped nanotubes with the goal of a controlled luminescent defect density used either dip-doping of pre-deposited nanotubes with very reactive diazonium salts [70] or in situ generation of aryl diazonium from 4-nitroaniline, which required elevated temperatures and inert conditions. [99,100] A scalable, simple and very controllable method to facilitate the reaction of aryl diazonium salts with polymer-wrapped nanotubes is the addition of a phase-transfer agent (a crown ether) and a polar co-solvent (acetonitrile) to the dispersion. Berger et al demonstrated that monochiral (6,5) nanotubes that were sorted and wrapped with a polyfluorene-bipyridine copolymer in toluene could be functionalized this way with different aryl-substituents at room temperature, showing E 11 * emission and enhanced PL yield after work-up.…”

Effect of benzophenone on the physicochemical properties of N-CNTs synthesized from 1-ferrocenylmethyl (2-methylimidazole) catalyst

“…The two prominent peaks observed at ∼1330 and ∼1573 cm −1 are assigned to the D-and G-bands, respectively. The intensity ratios of the D-and G-bands (I D /I G ) shows the defect level of graphitic carbon materials [56,57,58]. The I D /I G ratio of N-CNTs-0% and N-CNT-Fe is 0.74 and 0.66, respectively (Table 4).…”