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

Single-walled carbon nanotube (SWCNT)-fullerene (${\mathrm{C}}_{60}$) bilayers represent an attractive “donor-acceptor” binary system for solar photoconversion, where the kinetics of photoinduced processes depend critically on the properties of the interface between the two materials. Using photoconductivity measurements we identify the kinetic scheme that describes the free carrier kinetics in such bilayers where the dominant SWCNT species is the (7,5) semiconducting nanotube. Following charge separation, the carrier kinetics, covering up to four orders of magnitude in volumetric hole density, are described by a recombination process that is limited by capture and emission at traps or states at the SWCNT-${\mathrm{C}}_{60}$ interface. The high-frequency mobility of holes in the (7,5) SWCNT phase is lower than in multichiral films, potentially due to differences in SWCNT defect density for nanotubes that have been purified more aggressively. The results obtained here provide fundamental insights into the transport and recombination of both charges and excitons within SWCNT thin films and bilayers, and point to several potential ways to improve SWCNT-${\mathrm{C}}_{60}$ photovoltaic devices.

Figure 1

(a) Absorptance spectrum of the neat ∼9-nm-thick (7,5) s-SWCNT film (without ${\mathrm{C}}_{60}$). Peaks corresponding to the first and second excitonic transitions of the (7,5) SWCNT are labeled as $S_{11}$ and $S_{22}$ whereas the corresponding vibronic transitions are labeled $X_{11}$ and $X_{22}$. (b) Redshift of the absorbance peak associated with the $S_{11}$ exciton induced by the ${\mathrm{C}}_{60}$ layer as a function of SWCNT film thickness. Inset: Cartoon schematic of the $\mathrm{SWCNT}/{\mathrm{C}}_{60}$ bilayer studied by TRMC. (7,5) s-SWCNT films with varying thicknesses are deposited on quartz, after which a 90-nm ${\mathrm{C}}_{60}$ layer is thermally evaporated. Photoexcitation pulses first pass through the quartz substrate to generate SWCNT excitons.

Figure 2

Microwave photoconductivity data for the ∼9-nm neat and bilayer (7,5) s-SWCNT films (${\lambda }_{\mathrm{ex}}=1048\mathrm{nm}$). (a) Photoconductance transients measured at an absorbed photon fluence, $I_0{F}_A\approx 1\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$. (b) Photoconductance action spectrum (black circles) overlaid with the SWCNT absorptance spectrum (solid gray line). (c) Fluence-dependent yield-mobility product $\left(\varphi \mathrm{\Sigma }\mu \right)$: data are shown for $S_{11}$ (yellow-filled diamonds; ${\lambda }_{\mathrm{ex}}=1048\mathrm{nm}$) excitation of the neat film, and both $S_{11}$ (green-filled circles; ${\lambda }_{\mathrm{ex}}=1048\mathrm{nm}$) and $X_{11}$ (blue-filled squares; ${\lambda }_{\mathrm{ex}}=886\mathrm{nm}$) excitation of the bilayer film. The solid green fit line shows extrapolation of fluence-dependent $\varphi \mathrm{\Sigma }\mu \mathrm{to}\sim 1$ sun fluence, using Eq. (1).

Figure 3

(a) Dependence of yield-mobility product $\left(\varphi \mathrm{\Sigma }\mu \right)$ on absorbed photon flux ($I_0{F}_A$) for $\mathrm{SWCNT}/{\mathrm{C}}_{60}$ bilayers with varying SWCNT thickness. (b) Normalized photoconductance $\left(\mathrm{\Delta }G\right)$ transients for five $\mathrm{SWCNT}/{\mathrm{C}}_{60}$ bilayers with varying SWCNT thickness, all taken at $I_0{F}_A\approx 4.5\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$ [corresponding to the gray shaded area in (a)]. (c) Triexponential fit (solid gray trace) to the 3-nm $\mathrm{SWCNT}/{\mathrm{C}}_{60}$ bilayer transient from (b) (solid red trace). The solid black trace shows the extrapolation of the fit to $t=0\left[{\left(\varphi \mathrm{\Sigma }\mu \right)}_{t=0}\right]$ using the sum of the prefactors of the exponential terms for each time constant obtained in the fit. The dashed lines show the magnitudes of ${\left(\varphi \mathrm{\Sigma }\mu \right)}_{\mathrm{EOP}}$ and ${\left(\varphi \mathrm{\Sigma }\mu \right)}_{t=0}$. (d) Comparison of the end-of-pulse (open circles) and $t=0$ (filled diamonds) yield-mobility products (left axis), along with the internal quantum efficiencies (IQE, gray pentagons) obtained for a large number of SWCNT-${\mathrm{C}}_{60}$ PV devices utilizing identically prepared $\mathrm{SWCNT}/{\mathrm{C}}_{60}$ bilayers.

Figure 4

(a) Photoconductance transients for the $\mathrm{SWCNT}/{\mathrm{C}}_{60}$ bilayer with ∼7-nm SWCNT layer thickness. The gray traces are photoconductance decays simulated using Eqs. (2, 3, 4). (b) Simulated decay trajectories of each individual carrier considered in the simulation of the transient at an absorbed photon flux of $36\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-2}$ in panel (a). Inset: schematic of the trap-limited recombination model used to generate the rate equations (2) and (3). (c) Photoconductance transients for the $\mathrm{SWCNT}/{\mathrm{C}}_{60}$ bilayers with varying SWCNT layer thickness at an initial volumetric carrier density of $\sim 1\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$. The gray traces are photoconductance decays simulated using Eqs. (2, 3, 4).