Engineering Efficient Type-Sorted Carbon Nanotube Photovoltaics
Abstract
Single-walled carbon nanotubes (SWCNTs) are an intriguing photoabsorbing material for photodetectors and photovoltaics due to their strong optical absorptivity, fast charge transport, solution processability, and chemical stability. However, isolating electronically pure semiconducing (s-) SWCNTs from as-synthesized mixtures of metallic (m-) and s-SWCNTs remains a challenge. Over the last decade, a class of polymers known as polyfluorenes has been shown to selectively wrap s-SWCNTs in organic solvents like toluene, allowing them to be isolated in solution at purities exceeding 99.98%. This simple, scalable, and efficient one-step separation has led to breakthroughs in both fundamental physics and the performance of s-SWCNT photovoltaics (PVs). s-SWCNTs are excitonic absorbers, and photoexcited excitons must be dissociated at a type-II heterojunction for efficient energy conversion. While exciton energy transfer along the axis of the nanotube is very fast, diffusion in the radial direction is orders of magnitude slower. Due to the poor out-of-plane exciton diffusion and the strength of their absorption bands, s-SWCNT films for PVs are typically very thin, often less than 10 nm. In this regime, optical interference plays an important role in determining device efficiency. Through modeling, I identify the optical spacing necessary to optimize device efficiency, and fabricate devices with > 1% power conversion efficiency for AM1.5G broadband illumination, and > 7% for near-infrared laser illumination. By studying s-SWCNT PV device characteristics at low temperature, I obtain information about the recombination in s-SWCNT/C60 PVs, and measure the offset energy between the donor s-SWCNT highest occupied molecular orbital and the fullerene-C60 acceptor lowest unoccupied molecular orbital, which sets the maximum open circuit voltage in these devices. The outstanding challenge of poor power conversion efficiency in all s-SWCNT PVs to date is attributed to poor exciton energy transfer within s-SWCNT films. I study the role of defects and length on limiting device efficiency, and pairing with modeling, suggest nanotube length and defect concentration regimes for improved device performance. Finally, using less harsh separation techniques, I isolate s-SWCNTs with lower defect density to fabricate PVs with improved photovoltaic efficiency.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2017
- Bibcode:
- 2017PhDT.......395S
- Keywords:
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- Materials science;Nanoscience;Energy