Recent developments in the photophysics of single-walled carbon nanotubes for their use as active and passive material elements in thin film photovoltaics

Abstract

The search for environmentally clean energy sources has spawned a wave of research into the use of carbon nanomaterials for photovoltaic applications. In particular, research using semiconducting single-walled carbon nanotubes has undergone dramatic transformations due to the availability of high quality samples through colloidal separation techniques. This has led to breakthrough discoveries on how energy and charge transport occurs in these materials and points to applications in energy harvesting. We present a review of the relevant photophysics of carbon nanotubes that dictate processes important for integration as active and passive material elements in thin film photovoltaics. Fundamental processes ranging from light absorption and internal conversion to exciton transport and dissociation are discussed in detail from both a spectroscopic and a device perspective. We also give a perspective on the future of these fascinating materials to be used as active and passive material elements in photovoltaics.

Origin and Perspectives of the Photochemical Activity of Nanoporous Carbons

“…For instance, in single‐wall carbon nanotubes' excitons may have both Wannier–Mott and Frenkel characters depending on the chirality and the different screening of Coulomb interactions in 1D systems 67. The dielectric function of the nanotubes is large enough to allow for the spatial extent of the wavefunction to extend over a few to several nanometers along the tube axis, while poor screening in the vacuum or dielectric environment outside of the nanotube allows for large (0.4–1.0 eV) binding energies 65. On the other hand, Gutierrez and Lopez described Frenkel‐type excitons in amorphous carbon films, accounting for the core exciton in sp 3 ‐hybridized carbon atoms 68.…”

Energy transfer pathways in semiconducting carbon nanotubes revealed using two-dimensional white-light spectroscopy

“…It is composed primarily of four different diameter nanotubes, referred to by their (n,m) chiral indices (7,5), (7,6), (8,6) and (8,7). Compared with the absorption spectrum of these nanotubes in solution, the bands are broadened and slightly shifted (o5 nm) 5,15 , indicating that the nanotube bandgap and photophysics are perturbed when the nanotubes are in close contact. A standard technique to study photophysics of CNTs and other materials is photoluminescence excitation (PLE) spectroscopy, and the PLE spectra of this film, which is shown in Fig.…”

“…These films can be used in photovoltaics and photodetectors, in which the semiconducting tubes are photoabsorbers, analogous to electron donor polymers in polymer solar cells 8 . Like polymers, CNTs have large and tunable optical absorption and are solution processable, but nanotubes exhibit ultrafast charge and energy transport over longer distances and are more air stable 5 . While the photophysics of individual nanotubes is an established topic of study 9,10 , the properties of these new mesoscale films are largely unknown.…”

“…1a is the absorption spectrum for the CNT film that is the focus of this paper and has been used in previously reported prototype photovoltaic devices 5,16 . It is composed primarily of four different diameter nanotubes, referred to by their (n,m) chiral indices (7,5), (7,6), (8,6) and (8,7). Compared with the absorption spectrum of these nanotubes in solution, the bands are broadened and slightly shifted (o5 nm) 5,15 , indicating that the nanotube bandgap and photophysics are perturbed when the nanotubes are in close contact.…”

“…Semiconducting carbon nanotubes (CNTs) are one candidate for next-generation semiconductor electronics 3 , near-infrared optoelectronics 4 and light absorbers or transparent conductors for solar cells 5 . Semiconducting CNTs, with controlled bandgap, can now be purified from their metallic counterparts and fabricated into mesoscale networks of densely interwoven bundles 6,7 .…”