Article pubs.acs.org/JPCC
Efficient Exciton Relaxation and Charge Generation in Nearly Monochiral (7,5) Carbon Nanotube/C60 Thin-Film Photovoltaics Dominick J. Bindl and Michael S. Arnold* Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, 53706, United States S Supporting Information *
ABSTRACT: We report on photovoltaic diodes based on bilayer heterojunctions between nearly monochiral, polymer wrapped (7,5) semiconducting carbon nanotube photoabsorbing films and C60. The internal quantum efficiencies (IQEs) for exciton dissociation and subsequent charge collection at the nanotubes’ visible E22 and near-infrared E11 and E11 + X resonances are 84% ± 7%, 85% ± 5%, and 84% ± 14%, respectively. The high IQE at each transition shows that recombination losses during relaxation and/or direct dissociation of “hot” E11 + X and E22 excitons are negligible. A peak external quantum efficiency (EQE) of 34% is achieved at the E11 transition. Zero-bias photoresponsivity is invariant up to short-circuit current densities of at least 23 mA cm−2, indicating negligible losses via trion, charge-exciton, and charge−charge recombination relaxation pathways. An open circuit voltage of 0.49 V and power conversion efficiency of 7.1% are achieved in response to monochromatic excitation of the diodes at the E11 transition. The high IQE across multiple spectral windows, invariant photoresponsivity, and attractive open circuit voltage relative to the 1.18 eV optical bandgap demonstrate the future promise of using monochiral and multichiral semiconducting carbon nanotube films for broadband solar photovoltaic applications.
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1.18 eV). This chiral distribution is nonoptimal, however, because the small-bandgap (8,7) and (9,7) s-SWCNTs have insufficient energetic offsets with C60,4 can potentially trap both excitons and free carriers,1,11 and reduce the optical density of the other larger-bandgap species in the s-SWCNT films, limiting performance. In addition to decreasing performance, the s-SWCNT polydispersity present in these devices and the resulting spectral congestion make it difficult to analyze the efficiency of photocurrent generation for above-bandgap ‘hot’ sSWCNT absorption. The optical absorption spectrum of a semiconducting nanotube is defined by strong absorption at not only its E11 bandgap transition, but at higher order, “hot” interband transitions (e.g., E22 and E33) and at transitions arising from phonon−exciton coupling. The successful exploitation of s-SWCNTs in broadband single and multijunction devices will require an efficient means for harvesting energy, and charges from these “hot” excitons. However, the mechanisms by which “hot” excitons relax and the efficiency of photocurrent collection before and/or after relaxation are both poorly understood. To overcome these challenges, we have fabricated nearly monochiral (7,5) s-SWCNT/C60 bilayer photovoltaic devices. We show that employing nearly monochiral s-SWCNTs increases efficiency by minimizing the spurious, large-
emiconducting single-walled carbon nanotubes (sSWCNTs) have recently attracted significant research interest as the light absorbing components of solar cells and photodetectors.1−7 This application is motivated by the strong optical absorptivity of s-SWCNTs and their near-infrared bandgaps, easily tuned from 0.9 to 1.5 eV (ca. 850 to 1400 nm) with the potential to be tailored well beyond this range.8 Additionally, s-SWCNTs demonstrate excellent charge transport characteristics and are solution-processable via noncovalent interactions with dispersants. For these reasons, sSWCNTs are exceptionally well-suited for light harvesting applications. Toward this end, we have recently demonstrated that excitons photogenerated at the near-infrared optical bandgap (E11 transition) of s-SWCNTs can be effectively dissociated and separated into free charge carriers at semiconductor heterojunction interfaces with sufficient energetic offsets.9 Along with other groups, we have used the interface between small diameter (8−10 Å) s-SWCNT donors and C60 acceptors, in particular, to demonstrate proof of concept photovoltaic devices,4,10 extend our knowledge of inter- and intrananotube exciton transport in s-SWCNT thin films,1,11 and quantify the s-SWCNT diametric and bandgap dependence for exciton dissociation.4 To date, the highest reported external quantum efficiency (EQE) for an s-SWCNT photoabsorber in a photovoltaic device has been 22% at 1205 nm using a five chirality mixture of the (7,5), (7,6), (8,6), (8,7), and (9,7) s-SWCNTs with E11 bandgap absorption ranging from 1050 to 1330 nm (0.93 to © 2013 American Chemical Society
Received: November 6, 2012 Revised: January 4, 2013 Published: January 16, 2013 2390
dx.doi.org/10.1021/jp310983y | J. Phys. Chem. C 2013, 117, 2390−2395
The Journal of Physical Chemistry C
Article
Figure 1. (A) Normalized absorbance of (7,5)-enriched s-SWCNT solutions (top, green) and thin films on quartz (bottom, violet) cast from the above solutions. Solution absorbance spectra have been offset. (B) Characteristic EQE of photocurrent generation from ITO/active layer/10 nm BCP/100 nm Ag devices. Active layers displayed are (7,5)/50 nm C60 (solid, blue), (7,5)/90 nm C60 (solid, red), 50 nm C60 (dashed, blue), and 90 nm C60 (dashed, red).
thermally evaporating C60 films of various thickness on (7,5)/ ITO substrates, followed by a cathode of 10 nm bathocuprione and 100 nm silver evaporated through 1 mm diameter circular shadowmasks. All evaporations were conducted at a background pressure of 5%. Additionally, insignificant absorbance was observed from metallic carbon nanotube M11 transitions, which would appear as sharp peaks from 400−600 nm for carbon nanotubes in this diameter range.19 Excess PFO has largely been removed from the solutions, as inferred from comparison of s-SWCNT spectral features to PFO absorbance at 390 nm. Given the high monochirality of these solutions, it is possible to attribute several distinct absorbance features to the (7, 5) chirality, including both E11 and E22 absorbance at 1050 and 655 nm, respectively, and also a feature at 900 nm referred to as the E11 + X sideband20 and attributed to a superposition of phonon sidebands of the bright singlet (directly excited via E11 absorbance) and K-momentum dark singlet excitonic states.21−23 Despite noticeable broadening, these pronounced
diameter/small-bandgap s-SWCNTs. The use of nearly monochiral (7,5) s-SWCNTs also reduces spectral congestion, thereby making it possible, for the first time, to quantify the efficiency of photocurrent generation at both bandgap and “hot” transitions. The devices demonstrate a peak EQE of 34% at 1055 nm. The high EQE allows us to drive the diodes to relatively high current densities at photon fluxes comparable to terrestrial solar applications. This, furthermore, provides a new opportunity to gauge trion, charge-exciton, and charge−charge recombination relaxation pathways in s-SWCNT-based photovoltaic devices. Trions are charged excitons that result on sSWCNTs from the capture of a free charge carrier by an exciton, resulting in a bound, three-charge state.12−16 Positively charged trions would likely result in s-SWCNT photovoltaics at high irradiance and would problematically drift in response to the built-in field away from the active heterointerface where bound charges dissociate, thereby decreasing efficiency. Nearly monochiral (7,5) s-SWCNTs were prepared for this study as described elsewhere17 and briefly below. Suspensions of 1 mg mL−1 SG65 CoMoCAT single-walled carbon nanotubes (SWCNTs) and 2 mg mL−1 poly(9,9-dioctylfluoreneyl-2,7-diyl) (PFO, American Dye Source) in 100 mL toluene were ultrasonicated for 1 h at 40% amplitude with a horn-type sonic dismembrator (Fisher Scientific, 400W). The ultrasonicated suspensions were immediately centrifuged at 50 000g for 15 min. The collected supernatant was filtered with 5 μm Millipore Millex-SV syringe filters and concentrated via vacuum distillation. PFO-wrapped s-SWCNTs were then removed from the supernatant by centrifuging at 4 °C for 24 h, at 50 000g. This rinsing was repeated 3 times by redispersing the s-SWCNT pellet into tetrahydrofuran (THF), heating the solution for 5 min on a hot plate set to 90 °C and repelleting. The final, low-PFO-content s-SWCNT pellet was redispersed into ortho-dichlorobenzene (o-DCB). Immediately before casting films, persistent aggregates were removed by centrifuging the o-DCB solution for 10 min at 30 000g and extracting the supernatant for use. (7,5)-enriched films were deposited from o-DCB solutions by doctor-blade casting in a nitrogen glovebox, on a hot plate set to 140 °C. Films were cast on quartz substrates for optical characterization and on indium tin oxide (ITO) coated glass (Prazions Glas & Optik, < 20 Ω/□) for device fabrication. Prior to s-SWCNT film deposition, all substrates were cleaned in acetone, trichloroethylene, and isopropanol, then UV/ozone exposed for 20 min at 100 °C. Devices were completed by 2391
dx.doi.org/10.1021/jp310983y | J. Phys. Chem. C 2013, 117, 2390−2395
The Journal of Physical Chemistry C
Article
Figure 2. (A) Measured 1 − Reflectance (i.e., ηA) for device stacks of ITO/active layer/10 nm BCP/100 nm Ag devices. Active layers displayed are (7,5)/50 nm C60 (violet), and 50 nm C60 (green). (B−D) Display ΔηA (solid green); fits to ΔηA (dashed blue); and measured EQE (solid red) for three devices. (E) Extracted IQE for optical excitation at E11, E11 + X, and E22 transitions, following the treatment outlined in the Supporting Information.
spectral features persist upon film deposition (Figure 1A). Using the PFO and s-SWCNT optical cross sections, as we describe in detail elsewhere,11 we estimate from the solution absorbance spectrum that the resulting films contained 61% and 39% PFO and s-SWCNTs, respectively. The measured EQE spectra of device stacks fabricated using 50 and 90 nm of C60, with and without s-SWCNTs, are compared in Figure 1B. The E11, E11 + X, and E22 transitions strongly manifest in the EQE spectra of the s-SWCNT/C60 device stacks (solid lines). The relative amplitude of each transition is modulated by optical interference effects, largely determined by the overall C60 thickness. A constructive interference node is spatially commensurate with the sSWCNT film in the visible spectrum when the C60 thickness is 50 nm, maximizing the E22 EQE. This constructive interference node shifts to the near-infrared when the C60 thickness increases to 90 nm, maximizing the E11 and E11 + X EQE, whereas a destructive interference node simultaneously minimizes the E22 EQE. The peak E11 EQE for an ∼7 nm sSWCNT/90 nm C60 device stack is 34%, whereas the peak E22 EQE for an ∼7 nm s-SWCNT/50 nm C60 device stack is 17%. Peak E22 EQE is lass than the peak E11 EQE because of the smaller E22 absorption cross-section. By contrast, photocurrent generation in control device stacks without s-SWCNTs is limited to the C60 response
