Precision printing and optical modeling of ultrathin SWCNT/C₆₀ heterojunction solar cells

Abstract

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising candidates as the active layer in photovoltaics (PV), particularly for niche applications where high infrared absorbance and/or semi-transparent solar cells are desirable. Most current fabrication strategies for SWCNT PV devices suffer from relatively high surface roughness and lack nanometer-scale deposition precision, both of which may hamper the reproducible production of ultrathin devices. Additionally, detailed optical models of SWCNT PV devices are lacking, due in part to a lack of well-defined optical constants for high-purity s-SWCNT thin films. Here, we present an optical model that accurately reconstructs the shape and magnitude of spectrally resolved external quantum efficiencies for ultrathin (7,5) s-SWCNT/C60 solar cells that are deposited by ultrasonic spraying. The ultrasonic spraying technique enables thickness tuning of the s-SWCNT layer with nanometer-scale precision, and consistently produces devices with low s-SWCNT film average surface roughness (Rq of <5 nm). Our optical model, based entirely on measured optical constants of each layer within the device stack, enables quantitative predictions of thickness-dependent relative photocurrent contributions of SWCNTs and C60 and enables estimates of the exciton diffusion lengths within each layer. These results establish routes towards rational performance improvements and scalable fabrication processes for ultra-thin SWCNT-based solar cells.

Trap-limited carrier recombination in single-walled carbon nanotube heterojunctions with fullerene acceptor layers

“…[22]. The same correlation was found in recent device studies [7,26], and indicates that similar exciton densities are able to diffuse to the interface and be dissociated, regardless of the state from which they originate. shown for S 11 (yellow-filled diamonds; λ ex = 1048 nm) excitation of the neat film, and both S 11 (greenfilled circles; λ ex = 1048 nm) and X 11 (blue-filled squares; λ ex = 886 nm) excitation of the bilayer film.…”

Probing Exciton Diffusion and Dissociation in Single-Walled Carbon Nanotube–C60 Heterojunctions

“…Following this initial rapid decay, the S 11 bleach rises very slowly (over ca. 150 ps) for the bilayer, deviating dramatically from the transient observed for the neat (6,5) film. This deviation reflects the contribution of a new species to the SWCNT−C 60 bleach dynamics, namely, holes created by diffusion-limited interfacial exciton dissociation where the slow rise reflects exciton diffusion through the relatively thick C 60 layer.…”

“…35 In this case, the exciton diffusion length reported here would be a lower limit, and slightly longer L D may be realized at solar fluences. 5,14,15 The low values found for L D , in both SWCNT and C 60 thin films, suggest that SWCNT/ fullerene device efficiencies will likely benefit from bulk heterojunction geometries, as demonstrated in recent studies. 9,10 However, the low fill factor of these initial demonstrations of bulk heterojunctions 9,10 suggests that the source of significant recombination losses should be explored and mitigated.…”

“…26 We focus on the results for the (6,5) bilayers here, but as shown in the Supporting Information, the results are similar for (7,5) bilayers (Figures S1−S3). A neat (6,5) SWCNT film exhibits strong absorption at the first excitonic transition (S 11 ) at 1000 nm, and other notable features include the X 1 phonon sideband at 800 nm 27 and second excitonic transition (S 22 ) at (6,5) SWCNT and SWCNT−C 60 bilayer films at long delay times (∼300 ps) after excitation at 450 nm. Photoinduced hole transfer from C 60 to the SWCNT layer results in a long-lived S 11 bleach and an induced absorption at 1174 nm corresponding to the trion transition, X + .…”

“…To avoid any spectral congestion caused by polychiral SWCNT samples, we focused on monochiral (6,5) and (7,5) SWCNT dispersions to determine the diffusion length of photogenerated C 60 excitons. As we previously demonstrated for an interface between semiconducting SWCNTs and a perovskite absorber layer, 19 the sharp optical transitions of monochiral s-SWCNT layers can serve as sensitive reporters for exciton diffusion in this model system.…”