Article pubs.acs.org/JPCC
Role of Defects as Exciton Quenching Sites in Carbon Nanotube Photovoltaics Jialiang Wang,†,‡ Matthew J. Shea,†,‡ Jessica T. Flach,§ Thomas J. McDonough,§ Austin J. Way,† Martin T. Zanni,§ and Michael S. Arnold*,† †
Department of Materials Science and Engineering and §Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States 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/C60 planar heterojunction photovoltaic devices via both experiment and modeling. First, diazonium chemistry is used to introduce covalent sp3 sidewall defects to s-SWCNTs at various densities that are estimated using Raman and transient absorption spectroscopy. s-SWCNT/C60 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/C60 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.
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INTRODUCTION
While an internal QE of >85% is excellent, high external QE devices will need to capture more light and thus drive excitons through thicker s-SWCNT layers to the heterointerface. Experiments show that internal QE decreases with increasing s-SWCNT layer thickness because of the poor diffusion of excitons to the heterointerface.1 Although excitons have long diffusion lengths along the long axis of s-SWCNTs (typically >100 nm),22−24 most s-SWCNTs are oriented in the plane of the substrate in thin film devices, necessitating inter-SWCNT exciton transport for which the diffusion length is much shorter, typically ≤5 nm.1,17,25 Models have indicated that metallic SWCNTs and spurious small band-gap s-SWCNTs might be factors that prevent long-range inter-SWCNT exciton transport.26 However, experiments have demonstrated that the interSWCNT exciton diffusion length is short even when using sSWCNTs that are >99.9% semiconducting,1 and transient photobleaching and two-dimensional white light spectroscopy data show that most excitons in s-SWCNT films are lost to recombination even before they can become trapped on spurious small band-gap SWCNTs, suggesting that neither metallic nor small band-gap SWCNTs are primarily responsible for poor exciton diffusion to the heterointerface.27−29
Semiconducting single-walled carbon nanotubes (s-SWCNTs) are appealing materials for the light absorbing donor layer(s) of donor/acceptor heterostructure photovoltaic solar cells and photodetectors1−9 because of their strong optical absorptivity,10 fast exciton and charge transport,11 solution-phase processability,12 and excellent thermal13,14 and chemical stability.15 However, the performance of s-SWCNT-based devices has been limited thus far with a solar power conversion efficiency of less than a few percent4 and a peak external quantum efficiency (QE) of less than 50%.3 Several studies have sought to understand the fundamental behavior of excitons and charges in these heterostructures,16−18 but the dominant factors that limit external QE are still poorly understood. It is well known that photogenerated carriers in s-SWCNT layers are strongly bound as excitons with binding energies of 0.2−0.3 eV.19,20 Bindl et al. have shown that this binding energy can be overcome in bilayer s-SWCNT/C60 donor/ acceptor heterostructures with very thin (85%.1 Ihly et al. recently confirmed this electron transfer QE and showed that it can be described using Marcus theory.21 © XXXX American Chemical Society
Received: January 31, 2017 Revised: March 16, 2017 Published: March 28, 2017 A
DOI: 10.1021/acs.jpcc.7b01005 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
magnetic stirring. The dispersion is then centrifuged at 25 500 rpm (50 000g) at 4 °C for 24 h, which selectively pellets sSWCNTs, preferentially leaving free PFO in solution. The precipitated s-SWCNT pellet is then redispersed again in THF by heating and mild tip sonication (10% amplitude, < 1 min) and centrifuged for 12 h. This process is repeated for 4−5 cycles until the PFO to s-SWCNT mass ratio in the precipitate is about 1:1 as assessed by absorbance spectroscopy using methods outlined in our previous work.25 Finally, the sSWCNT powder is collected and dispersed in o-dichlorobenzene (ODCB) by using mild tip sonication (10% amplitude,
