Semiconducting Carbon Nanotubes in Photovoltaic Blends: the case of PTB7:PC60BM:(6,5) SWNT

Blends of carbon nanotubes with conjugated polymer and fullerene derivatives are complex nanocomposite systems, which have recently attracted a great research interest for their photovoltaic ability. Therefore, gaining a better understanding of the excitonic dynamics in such materials can be important to boost the efficiency of excitonic solar cells. Here, we studied the photophysics of a ternary system in which the polymer PTB7 and the fullerene derivative PCBM are integrated with (6,5) SWNTs. We highlight the contribution of SWNTs in the exciton dissociation and in the charge transfer process. These findings can be useful for the exploitation of these multi-component systems for organic photovoltaic and, in general, optoelectronic applications.


Introduction
The concept of excitons in semiconducting carbon nanotubes was firstly introduced by T. Ando in a pioneering theoretical work published in 1997. 1 The idea of optical resonances due to excitons in carbon nanotubes was then reprised in 2004 in an experimental work by the Vardeny and Baughman groups, 2 and a theoretical work by the Louie group. 3 In May 2005 the group of Tony Heinz, 4 and in December 2005 the group of Christoph Lienau, 5 found the smoking gun evidence to state that "the optical resonances in carbon nanotubes arise from excitons", with an exciton binding energy of 420 meV for (7,5), (6,5) and (8,3) single walled carbon nanotubes (SWNTs). 4 Finally, in 2008 the exciton size and mobility for semiconducting carbon nanotubes with (6,5) chirality were measured. 6 Such huge research effort in understanding the fundamental photoexcitation phenomena in carbon nanotubes demonstrates that the exciton picture of carbon nanotubes is in-fact a complex scenario, involving a series of bright and dark excitons as well as higher energetic excitons lying above the bottom of the conduction band. 7,8 Such higher excitons, in particular, can serve as probes for photogenerated charge carriers in the highly enriched (6,5) SWNT (see figure 1a). [9][10][11] The employment of SWNTs as active materials in organic photovoltaic diodes (OPVs) is very promising because of their optical absorption in the near infrared region and the relatively high carrier mobility. [12][13][14][15][16] Given the excitonic character of OPVs it is necessary a proper electron donor/acceptor interface to achieve effective excitons splitting and charge generation. Therefore, in organic solar cells SWNTs are usually blended with appropriate electron donor/acceptor systems either in bilayer or bulk heterojunction configurations, i.e. with conjugated polymers and fullerene derivatives. However, the relatively high level of complexity of these blends and the strong dependency of their nanomorphology on the intrinsic structural properties of the constituent materials (i.e. crystallinity) [17][18][19] and on a number of process parameters (i.e. deposition techniques and casting solvent) [20][21][22] , hinder a complete and reliable characterization of these functional nanocomposites. In this scenario, photophysical studies, as the ones reported for SWNTs in blends with poly(3-hexylthiophene) (P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM), 23,24 can give a strong support to describe these systems, with the view to improve their photogeneration ability.
Polythieno [3,4-b]-thiophene-co-benzodithiophene (PTB7) is a promising electron-donor polymer for organic solar cells. 25 Low band-gap (around 1.6 eV) is obtained via the stabilization of a quinoidal structure from thieno- [3,4-b]thiophene, resulting in a strong absorption around 700 nm, which is the region of the maximum photon flux of the solar spectrum. In addition, it features a rigid backbone that ensures a relatively high hole mobility and environmental stability, 26 whereas the presence of side chains allows a good dispersibility in organic solvents and miscibility with organic acceptors, as fullerene derivatives. As a result of these advantageous properties, a photon-to-current efficiency as high as 7.4% has been reported in OPVs incorporating PTB7. 25,27,28 Furthermore, the employment of a solvent mix has been demonstrated to improve the interpenetration of polymers in the blend. 29,30 In the case of PTB7, the mix of 1,2-Dichlorobenzene ODCB/1,8-diiodoctane (DIO) (97%-3% by volume) has proven to increase the fill factor in the I-V curve. 27 The photophysics in PTB7:PCBM blends has been widely studied [24][25][26][27][28][29][30], [31][32][33][34][35][36][37] and it is characterised by an ultrafast cation state formation via intra-and intermolecular charge separation, and the formation of polaronic states that is facilitated by the charge transfer character of the repeated units. Differently from P3HT, optical charge separation occurs within very small (2-20 nm) domain sizes. 31 Here, we extend and nicely complement our previous studies on P3HT:SWNTs blends 24 with a novel investigation on SWNTs in close contact with PTB7 and PCBM. We observe evidence of a hole transfer process between SWNTs to polymer chains, thus demonstrating that SWNTs can participate actively in the exciton dissociation process of such photovoltaic blends. For the structure PTB7: PCBM/SWNT, a relatively brief oxygen plasma treatment (5 seconds of exposure) was performed to the active layer before deposition of the SWNTs, with the aim to improve DMF wettability on the active layer.

Optical measurements
Steady state absorption spectra have been acquired with a Perkin Elmer spectrophotometer Lambda 1050 WB. Ultrafast differential transmission measurements have been performed with a Ti:Sapphire amplified laser system (repetition rate of 1 kHz, pulse duration of about 100 fs, central wavelength at 780 nm). Excitation pulses have been obtained with second harmonic generation or by optical parametric amplification. 38 Chirp-free transient differential transmission (ΔT/T) has been collected by using an optical multichannel analyser (OMA) with a dechirping algorithm.

Results and discussion
To elucidate the role of SWNTs in the exciton dynamics of PTB7:PCBM blends (see figure 1b for the chemical structure of the polymer), we employed ultrafast pump-probe spectroscopy to study the photophysics of three sample configurations, characterised by a different interfacial arrangement between the PTB7:PCBM blends and the nanotubes, namely: 1. We highlighted the contribution of SWNTs to the overall deactivation dynamics by pumping selectively either the polymer:fullerene system at 400 nm or the (6,5) SWNTs first exciton S 1 at 870 nm, and probing both in the UV-VIS (450-750 nm) and IR (900-1200 nm) spectral regions.  When exciting the S 1 of SWNTs and probing in the visible range (Figure 3a), we observe two PB signals corresponding to the S 22 transition of the (6,5) SWNT at 580 nm and, in addition, a peak at 680 nm due to the same transition of the (7,5) chirality that is also present in the SWNTs mixture. Interestingly, we note a relatively narrower PB peak from the nanotubes in the bilayers structures, and a slightly red shifted signal for the (6,5) SWNTs, for the heterojunction blend when compared with the pure SWNTs sample. Such differences detected in the transient absorption spectra are also evident from the time-decay plots recorded at probe wavelengths of 9 590 nm, 680 nm and 690 nm (Figure 3b, c and d). In particular, if we probe at a wavelength corresponding to the S 2 transition for the (6,5)   Therefore, to acquire more details on the possible charge transfer mechanism between SWNTs and PTB7, we analysed the building up of the transient spectra within the first picosecond (figure 4a-d, excitation 870 nm). Starting from the pure SWNTs sample consisting of a mixture of (6,5) and (7,5) chiralities (fig. 4a), we see that PB signal at 675 nm due to the (7,5) SWNTs is more intense than the (6,5)

Conclusions
In conclusion, we have presented a photophysical study of the behaviour of semiconducting SWNTs placed in close contact with a polymer PTB7 and the fullerene derivative PCBM. We investigated different configurations of this film structures, with the nanotubes as a separate layer above or below PTB7:PCBM, or with the SWNTs in a ternary bulk heterojunction blends. We found evidence of ultrafast SWNTsàPBT7 hole transfer, an effect that is strongly dependent on sample configuration and nanotubes proximity to the polymer-rich regions. These measurements demonstrate an active role of the nanotube in the excitonic photodynamic of such photovoltaic blends and, hence, can be useful for the optimization of photovoltaic nanocomposites integrating SWNTs as active material.

Conflict of interest
There are no conflicts of interest to declare.