Critical Role of the Sorting Polymer in Carbon Nanotube-Based

Nov 27, 2016 - A prerequisite for carbon nanotube-based optoelectronic devices is the ability to sort them into a pure semiconductor phase. One of the...
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Critical Role of the Sorting Polymer in Carbon Nanotube-Based Minority Carrier Devices Arun T. Mallajosyula,‡,† Wanyi Nie,‡ Gautam Gupta,‡ Jeffrey L. Blackburn,§ Stephen K. Doorn,‡ and Aditya D. Mohite*,‡ ‡

Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, United States

§

ABSTRACT: A prerequisite for carbon nanotube-based optoelectronic devices is the ability to sort them into a pure semiconductor phase. One of the most common sorting routes is enabled through using specific wrapping polymers. Here we show that subtle changes in the polymer structure can have a dramatic influence on the figures of merit of a carbon nanotube-based photovoltaic device. By comparing two commonly used polyfluorenes (PFO and PFO-BPy) for wrapping (7,5) and (6,5) chirality SWCNTs, we demonstrate that they have contrasting effects on the device efficiency. We attribute this to the differences in their ability to efficiently transfer charge. Although PFO may act as an efficient interfacial layer at the anode, PFO-BPy, having the additional pyridine side groups, forms a high resistance layer degrading the device efficiency. By comparing PFO|C60 and C60-only devices, we found that presence of a PFO layer at low optical densities resulted in the increase of all three solar cell parameters, giving nearly an order of magnitude higher efficiency over that of C60-only devices. In addition, with a relatively higher contribution to photocurrent from the PFO-C60 interface, an open circuit voltage of 0.55 V was obtained for PFO-(7,5)-C60 devices. On the other hand, PFO-BPy does not affect the open circuit voltage but drastically reduces the short circuit current density. These results indicate that the charge transport properties and energy levels of the sorting polymers have to be taken into account to fully understand their effect on carbon nanotube-based solar cells. KEYWORDS: carbon nanotube layer, single chirality, polymer wrapping, polyfluorenes, solar cell

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device with a focus on the exciton dissociation occurring at the SWCNT-C60 interface. The electron transfer at the SWCNTfullerene interface is very efficient, and it occurs in the femtosecond time scale.21 Once separated at this interface, the free carriers could have lifetimes of 100s of nanoseconds.22 Figure 1a shows a schematic of the solar cell structure. Thickness values used for each layer in the structure are also given. Polymer-nanotube and C60 layer thicknesses were chosen so that the photocurrent is maximized at the absorption peak of the corresponding nanotube.20 PEDOT:PSS is used as the hole transport layer (HTL), and bathocuproine (BCP) is used as the exciton blocking layer. Electrons transfer from C60 to the metal cathode through the intermediate states in this BCP layer.23 The energy level (eV) diagram of this device under flat-band conditions is given in Figure 1b. It can be seen that the nanotube-C60 interface is a type-II heterojunction, forming a

emiconducting single-walled carbon nanotubes (sSWCNTs) are efficient light absorbers in the visibleNIR range. Exciton dynamics, photocurrent generation, and transport of both excitons and charges have recently been intensively studied.1−6 These studies, along with the reports of high carrier mobilities in SWCNTs, have resulted in their being used for efficiency enhancement of planar as well as bulk heterojunction organic solar cells.7−12 In addition, thin films of SWCNTs have been used as transparent anodes, and sSWCNTs have been used as an active donor material for charge carrier generation and transport with efficiencies close to 3% being recently reported.13−16 Fullerenes are the typically used acceptors in these devices. Polyfluorene polymer wrappings are generally employed to isolate single chirality nanotubes for such solar cell studies.17−19 Typically, (7,5) SWCNTs are selectively wrapped using poly(9,9-dioctylfluorene-2,7-diyl) (PFO) polymer and the polymer used for separating (6,5) SWCNTs is 9,9dioctylfluorenyl-2,7-diyl and bipyridine copolymer (PFOBPy).9,10 Solar cells with thin films of such single chirality SWCNTs have reached efficiencies close to 1%.20 These reports treat the polymer-SWCNT-C60 system as a bilayer © 2016 American Chemical Society

Received: July 22, 2016 Accepted: November 27, 2016 Published: November 27, 2016 10808

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example, bulk heterojunction devices have been demonstrated where the nanotubes were wrapped with poythiophene, which acts as an active polymer generating charge carriers.24,25 In this work, we report on the influence of two wrapping polymers (PFO and PFO-BPy) on the performance of s-SWCNT-C60 solar cells.

RESULTS AND DISCUSSION Absorption and Photocurrent Spectra of PolymerSWCNT-C60 Devices. The peak absorption wavelengths of PFO and PFO-BPy thin films are 390 and 365 nm, respectively. Approximate band gap values of 2.87 and 3.07 eV can be calculated from direct linear extrapolation of the absorption spectra of these thin films. The absorption spectra are shown in Figure 2. The highest occupied molecular orbital (HOMO)

Figure 2. Normalized absorption spectra of PFO and PFO-BPy thin films spin-coated at 1000 rpm from a 1 mg/cm3 concentration solution in toluene stirred at 70 °C. The films were annealed at 120 °C.

level for PFO is 5.7−5.8 eV.26−29,35 On the other hand, various values for the lowest unoccupied molecular orbital (LUMO) level of PFO have been reported, ranging from 2.1 to 2.9 eV.26−29 Based on the band gaps mentioned above, we assume a LUMO level of ∼2.9 eV for PFO. The energy levels of the polymer PFO indicate that the PFO-C60 interface can form a type-II heterojunction. Corresponding values for PFO-BPy are unavailable in the literature, and their calculation is beyond the scope of the present work. The solution absorption and device photocurrent spectra of PFO wrapped (7,5) and PFO-BPy wrapped (6,5) nanotubes are shown in Figure 3. For the photocurrent spectra, absorption of a C60 layer having a peak near 450 nm is also included. The electronic transition energy levels are also marked in these plots. For (7,5) SWCNTs, the E11 absorption peak (optical band gap) is at 1.07 eV (1044 nm), while that for (6,5) SWCNTs is at 1.12 eV (998 nm). The contributions from phonon side bands are indicated by the peaks marked E11 + X. In addition, the absorption peaks of wrapping polymers (385 nm for PFO and 353 nm for PFO-BPy), SWCNT E22 (653 nm for (7,5) and 573 nm for (6,5)) and E33 (343 nm for (7,5) and 305 nm for (6,5)) transitions are also given in these plots. It is worth noting here that a typical C60 fullerene thin film can absorb below 500 nm as well.30 A 12 nm red shift can be observed between absorption and photocurrent E11 peaks of (7,5) tubes. A similar shift can be seen for (6,5) tubes as well. The E11 full width half-maximum (fwhm) peak widths are 16 and 31 nm for absorption and photocurrent spectra of (7,5) SWCNTs, respectively. These are expected because absorption spectra were taken for nanotubes

Figure 1. (a) Device structure of the SWCNT-C60 fullerene bilayer solar cell. Thickness values of each layer in the structure are also given. (b) Flat band energy level (in eV) diagram of the various layers in the SWCNT-C60 solar cell. (c) J−V characteristics of a PFO-BPy wrapped (6,5) SWCNTs:C60 solar cell under AM 1.5G solar spectrum. (d) Chemical structures of PFO and PFO-BPy.

bilayer organic solar cell with the nanotube layer acting as donor and C60 layer acting as acceptor. The current density− voltage (J−V) characteristics of the best device prepared using (6,5) SWCNTs is given in Figure 1c. This device had an efficiency of 1.33%, measured under standard AM 1.5G solar spectrum. The chemical structures of PFO and PFO-BPy are shown in Figure 1d. Often, the functionality of the polymer that wraps the sSWCNTs is not taken into consideration when estimating the solar cell efficiency. This becomes especially significant in the case of polymers with semiconducting properties that thus have the potential to act as active components in the device. For 10809

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Figure 3. Normalized absorption and photocurrent spectra of (a) PFO-(7,5) SWCNT solution in toluene and PFO-(7,5) SWCNT| C60 solar cell, respectively; (b) PFO-BPy-(6,5) SWCNT solution in toluene and PFO-BPy-(6,5) SWCNT|C60 solar cell, respectively. E11, E22, and E33 peaks correspond to the first, second, and third optical transitions of the semiconducting SWCNTs. E11+X corresponds to the contribution from phonon sidebands. The spectra are normalized with respect to the corresponding E11 peaks.

Figure 4. (a) Photocurrent spectrum of bichiral [(6,5) + (7,5)]|C60 solar cell with nearly equal E11 contributions from both chiralities. The relative values of various peaks in the spectrum depend on the ratio of (6,5) to (7,5) tubes in the solution used for drop casting the SWCNT film. (b) J−V characteristics of a drop-cast device under AM 1.5G solar spectrum.

contrast, JSC for PFO|C60 devices shows an initial increase at low OD values. At ODs higher than 2 (which are extrapolated), JSC decreases, ultimately approaching the value found for the reference C60-only device. The device efficiency values follow a similar trend. The efficiency of the PFO|C60 bilayer device can be an order of magnitude higher than that of the C60 reference device, as shown in Figure 5c. This behavior may be understood noting the p-type or hole-transporting nature of polyfluorenes.31−35 The higher JSC could be due to two factors: (a) light absorption by PFO produces excitons that subsequently dissociate at the PFO|C60 interface via electron transfer to C60; and (b) PFO acts as an efficient interfacial layer that enhances dissociation of excitons that are primarily generated only in the C60 layer. If the possibility (a) were to be true, then the PFO|C60 bilayer will be a type-II heterojunction, with hole transport occurring through the PFO layer and electron transport through the C60. At much higher PFO OD values, the JSC decrease will be due to the increased layer thickness and series resistance. This likely arises due to the low hole mobility of PFO (∼10−4 cm2 V−1 s−1).35 As a consequence, any holes generated at the PFO|C60 interface will form a space charge inside the thick PFO layer, which will ultimately reduce the current flow. Thus, one would expect that as the thickness of PFO is increased, the device photo current will reduce. To verify this, the external quantum efficiency of PFO|C60 devices is plotted in Figure 6a, where it can be seen that the external quantum efficiency is lower for lower spin speed, i.e., higher PFO OD. A comparison of the PFO|C60 device with the C60 only device shows that the former has relatively higher

in solution form, where individual tubes are well separated, while the photocurrent is from nanotubes in thin film form, where the individual tubes are closely packed. In addition to the E11 peak, significant contributions to photocurrent originate from C60 and also from higher electronic transitions of the SWCNTs. Combining both (7,5) and (6,5) SWCNTs will result in contributions from both of them. This can be seen from the photocurrent spectrum given in Figure 4a, where the two E11 peaks from (7,5) and (6,5) SWCNTs can be distinctly seen. By varying the ratio of (7,5) to (6,5) SWCNTs in the initial solution, the peak intensities can be well controlled. Typical J−V characteristics of a drop-cast device with 1:1 ratio of (6,5) and (7,5) SWCNTs are shown in Figure 4b. Varying the ratio can increase the efficiency further. An important point to note here, however, is the increase in the open circuit voltage compared to the solar cell with (6,5)-SWCNTs. This aspect will be explored further. Dependence of JSC of Polymer-C60 Bilayer Devices on the Polymer OD. Bilayer devices were fabricated without any SWCNTs to understand the potential roles of polymers in photocurrent generation. Photocurrent response from polymer|C60 devices (under AM 1.5G illumination) was obtained as a function of relative polymer layer thickness, as inferred by the optical density (OD) of the polymer solution used to spin-coat the film. The variation of short circuit current density (JSC) and the device fill factor with polymer OD are shown in Figure 5a,b, respectively. The values for a reference C60-only device are also shown. JSC is found to decrease monotonically with PFO-BPy OD for PFO-BPy|C60 devices. In 10810

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Figure 6. (a) External quantum efficiency of thick PFO|C60 devices at different spin speeds. (b) Normalized photocurrent spectra of PFO|C60 and C60-only devices along with normalized PFO absorption spectrum and standard AM 1.5G photon flux.

PEDOT:PSS can cause exciton quenching owing to its quasimetallic nature,36 a thin layer of PFO between PEDOT:PSS and C60, with its higher HOMO level and hole-transporting nature, could be acting as an efficient exciton blocking layer that improves the JSC of PFO|C60 devices relative to C60 only devices (at low PFO OD). The behavior of the PFO-BPy|C60 device is distinctly different. The monotonic decrease in JSC for PFO-BPy indicates that there is effectively no charge generation (i.e., no dissociation of excitons at the PFO-BPy|C60 interface). PFOBPy instead solely adds to the series resistance of the device and impedes any charge transfer from C60 to the anode. Thus, both PFO and PFO-BPy can act to limit charge transport within these devices, albeit at different concentration levels. In the case of PFO-BPy, the limitation is likely because of the polymer structure itself. The presence of the bipyridine side group appears to limit polymer interaction with C60, inhibiting charge transfer as a result. This observation is also in line with the report by Glanzmann et al. where, using time-dependent density functional theory, it has been shown that PFO-BPy polymer does not have π-conjugation between the PFO and the pyridine side group.37 In such a case, the intrachain charge transport will be severely restricted. In addition to this, the side group may also affect the band alignment between the PFO portion and the nanotube. In the case where only PFO is present without the side group, however, its poor charge mobility will become the limiting factor for transport at higher thicknesses. Use of PFO at low ODs, however, could be beneficial for the device efficiency. Dependence of VOC of Polymer-C60 and PolymerSWCNT-C60 Devices on the Polymer OD. The open circuit voltage VOC of organic solar cells originates from the splitting of

Figure 5. Variation of (a) short circuit current density, JSC, (b) power conversion efficiency, and (c) fill factor of polymer|C60 devices with the OD of the polymer. The OD values were taken from the peak absorption wavelengths of the polymer-only solutions using a 3 mm path length, and are used as an indicator of relative film thickness for the spin-coated films. OD values above OD 2 are extrapolated. These efficiency values are obtained from J−V characteristics under 1 sun intensity using a standard AM 1.5G solar spectrum.

contribution in the wavelength range below 400 nm. Incidentally, this is the region where PFO absorption is maximum. Figure 6b demonstrates that the additional photocurrent in the PFO|C60 device, relative to the C60 device, is in line with what is expected from absorption in PFO. However, an overlap with the AM 1.5G solar spectrum shows that in this range, the available photon flux is low. Thus, even though there is a possibility that PFO could be contributing to the photocurrent in in a PFO-(7,5)-C60 device, its magnitude would be very low, especially since the PFO OD is much smaller in the PFO-(7,5)-C60 device than it is in the PFO|C60 device. It is likely that at low PFO ODs, any contribution to the increase in JSC could be because of the possibility (b). Since 10811

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ACS Nano electron and hole quasi-Fermi levels in their respective charge extraction layers. The maximum open circuit voltage, VOC,max, that is thermodynamically possible for such a solar cell is given by the difference between the LUMO level of the acceptor and the HOMO level of the donor, assuming no interface energy barriers at the corresponding transport layers.38,39 In practice, the VOC values obtained will be lower than VOC,max because of losses due to exciton and/or polaron binding energy as well as recombination at the interface. For fullerene-based solar cells, these losses typically amount to ∼0.3 eV. Considering these in conjunction with the energy level diagram, we can see that a VOC,max in the range of 0.1−0.5 V can be obtained from the nanotube-C60 interface. Indeed, to date, the VOC values for devices reported with this monochiral SWCNT|Fullerene configuration fall in this range.7,13,20 The VOC values of C60|SWCNT solar cells obtained, using (6,5)-only SWCNTs, (7,5)-only SWCNTs, and (6,5) + (7,5) tubes with equal concentrations, are given in Table 1. These

Figure 7. Variation of open circuit voltage (VOC) of polymer|C60 devices with the OD of the polymer. The OD values were taken from the peak absorption wavelengths of the polymer-only solutions using a 3 mm path length.

[HOMO|PFO − LUMO|C60] difference is as high as 1.3 eV, indicating that the VOC from the PFO|C60 interface can be nearly 1 V. This is indeed the case, as can be seen from Figure 7 where at high PFO ODs, a VOC of 0.85 V is obtained. Also, the VOC values do not saturate, even at the limit of PFO OD used in our devices, implying that its final value could be higher than 0.85 V. There have been reports where PFO derivatives have been used as interfacial layers both for regular device structures on the cathode side as well as for inverted structures.40,41 PFO is an archetype polymer used in blue light-emitting diodes and, as noted earlier, has a bulk hole mobility of the order of 10−4 cm2 V−1 s−1. For perspective, the bulk electron mobilities of C60 are reported to be in the range of 10−2 to 1.5 cm2 V−1 s−1.42−44 Thus, there is a high mobility imbalance between these two materials, which is one of the main reasons that can be attributed to a maximum efficiency of only 0.6% in our case. For higher ODs, this efficiency value reduces, as expected, due to higher recombination and higher series resistance of the thick polymer. Nevertheless, in the present context, the efficiency values obtained for PFO|C60 devices are of high significance because it is comparable to those of the PFOSWCNT|C60 devices reported by various groups. Light Intensity Dependence of JSC and VOC. The variation of JSC and VOC on the intensity of incident light intensity (I) are shown in Figure 8. From Figure 8a, the slope of the linear fit to JSC−I curve in logarithm scale for the PFO| C60 device is lower than unity, while that of the C60-only device is almost equal to unity. This shows that only monomolecular recombination exists in C60-only single-layer devices. On the other hand, polymer|C60 bilayer devices show a sublinear slope, indicating the presence of a bimolecular recombination component as well.45,46 Also, from Figure 8b, it can be seen that the VOC of the C60-only device increases at 70 mV/dec. Following the metal−insulator−metal picture, the VOC device can reach the built-in potential of the device. On the other hand, the VOC values for both PFO-BPy|C60 and PFO|C60 devices are nearly independent of I, indicating that they are saturated to maximum possible values. Further, the same saturation behavior in VOC−ln(I) plot is exhibited for different ODs of PFO as shown in Figure 9. In Figure 9a, the VOC variation of PFO|C60 devices was plotted for PFO ODs of 0.1, 1, and 10. The VOC saturated close to 0.55, 0.65, 0.8 V, respectively. Thus, we can increase the VOC of PFOSWCNT|C60 devices by increasing the PFO OD. For

Table 1. Open Circuit Voltages (VOC) of C60|SWCNT Bilayer Solar Cells with (6,5), (7,5), and [(6,5) + (7,5)] Chirality Nanotubesa SWCNT type

VOC (mV) median ± σsd

VOC (mV) best value

(6,5) only (6,5):(7,5) (1:1) (7,5) only

419 ± 37 471 ± 44 483 ± 38

475 530 542

a

The median and standard deviation values are from 30 devices. The efficiency values obtained for all these devices are in the range of 1.05− 1.33%.

statistical values are calculated using data from 30 devices for each of the three configurations mentioned. It can be seen that the VOC values for (7,5)-only and (6,5) + (7,5) SWCNT devices are nearly equal, with the latter being lower by about 12 mV. On the other hand, maximum and median VOC values for (6,5) SWCNT devices are lower by 64 and 52 mV, respectively, than that for the other two configurations. The E11 energy gap for (7,5) SWCNTs is lower than that of (6,5) SWCNTs by nearly 50 meV. Thus, [HOMO|(7,5) − LUMO|C60] difference will be lower than [HOMO|(6,5) − LUMO|C60]. Consequently, one would expect a lower VOC from (7,5)-only devices than that from (6,5)-only devices. However, the results from Table 1 indicate that this simple relation is not followed in these devices, and in contrast, (7,5)-only devices gave higher VOC. These observations indicate that the VOC is not solely determined by HOMO|(SWCNT) energy levels. To understand this aspect further, dependence of VOC on the content of wrapping polymer is investigated without the presence of SWCNTs. The variation of VOC with polymer OD, as shown in Figure 7, presents contrasting results for PFO|C60 and PFO-BPy|C60 devices. The VOC values for PFO-BPy|C60 are nearly the same as that of the C60 reference device (i.e., zero polymer OD). These observations reiterate that the PFO-BPy|C60 interface does not provide any significant charge carrier generation. Consequently, in any SWCNT device, where the SWCNTs are wrapped by PFO-BPy, the OD of PFO-BPy should be kept to the minimum value that is practically possible. Multiple highspeed centrifugation steps are generally employed for this purpose. In contrast to the case of PFO-BPy, the VOC values for PFO|C60 devices provide a different picture. Considering the energy band diagram given in Figure 1b, it can be seen that the 10812

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Figure 9. Variation of VOC with incident light intensity (natural log scale) for spin-coated (a) PFO|C60 bilayer at 1500 rpm. The black, red, and blue curves correspond to PFO OD values of 0.125, 1.21, and ~75 respectively. (b) PFO-(7,5)|C60 devices at 1500 and 1000 rpm with PFO ODs of 0.10 and 0.25, respectively. Both types of devices show similar saturation behavior of VOC indicating that the maximum VOC values for a particular polymer thickness are insensitive to the incident light intensity.

Figure 8. (a) Variation of short circuit current density (JSC) of polymer|C60 bilayer and C60-only single devices with incident light intensity. The plot is in log−log scale. Lower slopes with polymers show that charge extraction is less efficient in their presence. (b) Variation of VOC of polymer|C60 bilayer and C60-only devices with incident light intensity. The x-axis of plot (b) is in natural log scale. C60-only single device gives a slope of 70 mV/dec, while PFO and PFO-BPy devices have voltage saturation.

comparing spin-coated devices, a decrease in the spin speed translates to an increase in PFO OD as the film thickness will increase accordingly. This is indeed the case, as shown in Figure 9b. It can be seen that the VOC is higher for lower spin speed and also that it saturates with light intensity at 0.45 and 0.56 V. In addition to this, spin-coated devices with different PFO to SWCNT ratios can be obtained during the PFO removal step. Typically, the PFO-(7,5) films are soaked in toluene before deposition of C60. Here, though, the aim is to remove PFO only. Along with it, (7,5) tubes will also be removed, i.e., the PFO:(7,5) ratio does not remain 1:1 after the toluene soak step. Because of the loss in (7,5) tubes, the JSC will be reduced. This is indeed the case as can be seen from Figure 10a, where the J−V characteristics are plotted for the devices at 1 sun intensity. The device parameters are listed in Table 2. In the limiting case where all PFO:(7,5) is removed, the JSC and VOC are reduced to that of a single layer of C60, i.e., 0.67 mA cm−2 and 0.32 V, respectively. One would expect the VOC also to be reduced or, at best, remain nearly constant when JSC is reduced due to a decrease in the amount of (7,5) tubes in the PFO: (7,5) film. However, here, the decrease in JSC is accompanied by an increase in VOC. Comparing the contributions of (7,5) tubes to the photocurrent from devices having films with and without a toluene soak, one can quantify the effective amount by which (7,5) tubes were lowered. The photocurrents spectra of these devices are plotted in Figure 10b, and it can be seen that E11 and E22 peaks from (7,5) tubes are reduced after the toluene soak. It can be deduced that relatively, the contribution to JSC from the other constituent (namely the PFO-C60

interface) has increased. For example, the ratio of peaks at 460 and 1050 nm reduces from 0.41 to 0.19 for the device with a lower (7,5) content. It is, thus, likely that the relative amount of PFO has a direct influence on the maximum VOC that can be achieved in these devices. A fine control of this ratio is, hence, important in order to balance the JSC and VOC values in spincoated PFO-(7,5)-C60 devices. These results clearly show that, in addition to SWCNT and C60, the VOC of polymer-SWCNT| C60 devices is influenced by the polymer as well.

CONCLUSIONS In summary, we have shown that the wrapping polymer has a significant role in determining the open circuit voltage of polymer wrapped SWCNT solar cells. Due to the presence of pyridine side groups with poor π-conjugation, PFO-BPy inhibits charge transfer, and hence, its removal should be near complete for efficient (6,5)-SWCNT-based solar cells. On the other hand, PFO can act as an efficient interfacial layer on the anode side, which increases the open circuit voltage of the devices. Instead of complete removal, its absolute mass should be controlled in order to balance the short circuit current density and, thus, increase the efficiency of (7,5)-SWCNTbased solar cells. METHODS Materials and Device Fabrication. Ten mg of CoMoCAT SWCNTs (Southwest Nanotechnologies SG65i) was dispersed in 10 mL of 25 mg/mL solution of polymer (American Dye Source) in toluene. PFO and PFO-BPy were used for separating (7,5) and (6,5) 10813

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respectively. A 100 nm Al electrode was then evaporated at 1.0 Å s−1 using a shadow mask to complete the device with an area of 3.2 mm2. Finally the devices were encapsulated using epoxy by UV-curing inside an argon-filled glovebox. Device Characterization. The current−voltage characteristics of the encapsulated devices were taken using a Keithley 2400 source meter. For the characteristics under standard AM 1.5G spectrum, a solar simulator (Oriel LCS-100) was used. A shadow mask (same as the one used for Al deposition) was used during light measurements to avoid edge effects. To vary the intensity of light incident on the device, the distance between solar simulator light outlet source and device was varied. At each distance, the intensity was calibrated using an NIST calibrated silicon solar cell (Newport 532, ISO1599).The photocurrent spectra were measured using calibrated monochromator (QEX10) at a frequency of 151 Hz coupled with a xenon lamp.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Stephen K. Doorn: 0000-0002-9535-2062 Aditya D. Mohite: 0000-0001-8865-409X Present Address †

Department of Electronics and Electrical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam 781039, India.

Figure 10. (a) J−V characteristics of two PFO-(7,5)-C60 devices with and without toluene soak, giving different nanotube and PFO contents. The characteristics were taken at an illumination intensity of 1 sun. (b) External photocurrent quantum efficiency of these devices.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported in part by the LANL LDRD program and performed in part at the Center for Integrated Nanotechnologies, a DOE Office of Science user facility. Jeff Blackburn was supported by the Solar Photochemistry Program of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract No. DE-AC36-08GO28308 to NREL.

Table 2. Solar Cell Parameters of Two PFO-(7,5)-C60 Devicesa

a

PFO-C60:(7,5) E11 photocurrent peak ratio

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

0.41 0.19

2.45 1.89

0.48 0.53

56.2 56.0

0.68 0.56

Light intensity used was 1 sun with standard AM 1.5G spectrum.

REFERENCES

SWCNTs, respectively. The dispersions were sonicated for 35 min using a probe type sonicator (Sonics Vibra-Cell). These were then centrifuged at 13,000 rpm for 12 min at room temperature to remove heavier impurities. The supernatant is centrifuged at 24,100 rpm for 20 h at 0 °C to remove excess polymer. This step is repeated until the ratio of absorption peaks of SWCNTs to polymer is 1:4. For the PFO|C60 and PFO-BPy|C60 devices, PFO and PFO-BPy solutions were diluted with varying amounts of toluene to achieve polymer optical densities (ODs) in the range of 0.1 to 75. The OD values for the polymers in solution are used as an indicator of relative film thickness for the spin-coated films that are derived from each solution. OD values above OD 2 are extrapolated. The ODs up to 2 were measured from absorption spectra (Cary 6000i) of the solutions recorded using quartz cells of 3 mm path length. Device fabrication was performed on ITO-coated glass substrates. The substrates were cleaned using isopropanol and oxygen plasma. PEDOT:PSS (Sigma-Aldrich) was coated on these substrates at 5000 rpm to make a 40 nm thin layer. The substrate was subsequently dried at 120 °C for 40 min. For spin-coated devices, the polymer-SWCNT layer was spin-coated at 1000 rpm, and the substrate was dried at 70 °C for 30 min. For drop-cast devices, 120 μL of solution was used for an 1 × 1 in. area substrate. For spray-coated devices, the SWCNT solution was sprayed at 250 μL min−1 with a nitrogen flow carrying dispersion maintained at 6.8 SLPM. C60 and BCP were thermally deposited on the polymer/SWCNT layer at 0.5 Å s−1 and 0.1 Å s−1,

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DOI: 10.1021/acsnano.6b04885 ACS Nano 2016, 10, 10808−10815