Role of HF in Oxygen Removal from Carbon Nanotubes

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Role of HF in Oxygen Removal from Carbon Nanotubes: Implications for High Performance Carbon Electronics Xiaokai Li,† Jing-Shun Huang,† Siamak Nejati,† Lyndsey McMillon,† Su Huang,† Chinedum O. Osuji,† Nilay Hazari,*,‡ and André D. Taylor*,† †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States Department of Chemistry, Yale University, New Haven, Connecticut 06511, United States

‡

S Supporting Information *

ABSTRACT: Oxygen removal from SWNTs is crucial for many carbon electronic devices. This work shows that HF treatment followed by current stimulation is a very effective method for oxygen removal. Using a procedure involving HF treatment, current stimulation and spin-casting AgNWs onto a SWNT thin film, record high efficiency SWNT/p-Si solar cells have been developed.

KEYWORDS: Hydrofluoric acid, carbon nanotubes, carbon electronics, silver nanowires, photovoltaic devices films exhibit positive thermoelectric power (TEP); however, when oxygen is detached from the thin films, a negative TEP is observed. This oxygen-induced swing in the TEP is consistent with weak charge (electron) transfer from the tube wall to the adsorbed oxygen.10 In contrast, several other studies have suggested that oxygen does not induce hole doping.12,13 For example Derycke et al. suggested that the switch of SWNT FET from p-type to n-type after oxygen desorption is not due to doping effects but results from changes in the barriers at the metal−semiconductor contacts.7 It is clear that further work is required to determine if oxygen does p-type dope SWNTs. Following our previous demonstration of SWNT thin films as transparent conductive electrodes,14 another potentially important application is their use as an emitter layer in SWNT/ Si photovoltaics. In principle, these hybrid solar cells could provide the intrinsically high photovoltaic efficiency of Si in a cheap and efficient manner owing to the low-temperature solution processability inherent to the fabrication of SWNT/Si junctions. Recently, our group demonstrated high performance SWNT/n-Si and SWNT/p-Si solar cells using molecular dopants to change the properties of the SWNTs.15−17 These studies provide insight into how the doping levels of SWNT thin films affect the performance of the SWNT/Si solar cells, as well as the effect of molecular doping on SWNTs. However, a

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ingle-walled carbon nanotubes (SWNTs) are promising materials for applications in electronics because they possess high mobility (∼105 cm2/(V s)),1 large on−off current ratios (>105),2 and enormous current carrying capacity (>109 A/cm2).3 A current problem in utilizing SWNTs for practical applications is that their properties are highly dependent on both their exact structure and (the chemical nature of their) local environment. For example, it is well known that a small variation in the environment can have a considerable effect on the SWNT properties because they are composed entirely of surface atoms.4 In particular, gas molecules, such as oxygen, which naturally adsorb to SWNTs, can change various physical properties of SWNTs, such as conductance,5 thermoelectric power,6 type of field effect transistors (FET),7 and field emission.8 Therefore, an understanding of the effect of oxygen adsorption by SWNTs and the development of techniques to efficiently remove oxygen from SWNTs are crucial for many applications. At this stage, the most common oxygen removal method involves high temperature annealing of SWNTs under ultrahigh vacuum or inert gas, which is a very slow and difficult process.5−7,9,10 Furthermore, even though a significant amount of work has been performed to investigate the changes in SWNT properties caused by the interaction of SWNTs with oxygen, there are contradictory reports on the effect of oxygen adsorption. An early study suggested that the binding energy of oxygen to SWNTs is large, and thus substantial charge transfer results in the hole doping of SWNTs.11 Subsequently, Sumanasekera et al.10 showed that oxygen-loaded SWNT thin © XXXX American Chemical Society

Received: June 26, 2014 Revised: September 21, 2014

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Figure 1. (a) Schematic of SWNT/p-Si photovoltaic devices. (b) J−V characteristics of an as made SWNT/p-Si photovoltaic device under 1 sun (AM1.5) illumination and in the dark.

Figure 2. J−V characteristics of SWNT/p-Si photovoltaic devices after (a) heating in ultrahigh vacuum (black) and current stimulation (red); (b) current stimulations; (c) and (d) J−V characteristics of SWNT/p-Si photovoltaic devices after both HF treatment and current stimulation process (1 h at 40 mA) (c) under 1 sun (AM1.5) illumination and (d) in the dark; (e) SWNT/p-Si response (voltage vs time) to 20 mA constant current stimulation; (f) the change of IV curve for SWNT thin films on glass slides; (g) the response of a SWNT thin film (resistance vs time) to 20 mA constant current stimulation.

pronounced effect of oxygen on some of our SWNT/Si solar cells was observed, which was not explored.

In this work, we investigate the effects of oxygen on SWNTs and the resulting performance in SWNT/p-Si hybrid solar cells. B

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In the presence of adsorbed oxygen on the SWNT thin films, the SWNT/p-Si devices display resistor type of behavior. Here, we show that diode behavior in these same devices can be readily observed following the removal of oxygen. Oxygen removal can be achieved through heating under ultrahigh vacuum or by a new process involving treatment with HF and current stimulation. We suggest that oxygen indeed causes hole doping of SWNTs. X-ray photoelectron spectroscopy suggests that HF treatment assists in the removal of oxygen. Using a combination of HF treatment and current stimulation, along with a spin-casting process (described previously)18 in which silver nanowires (AgNWs) are deposited onto SWNT thin films, SWNT/p-Si solar cells with power conversion efficiencies (PCE) as high as 7.53%, are obtained. This study demonstrates a 63% improvement over previous devices of this type.16 High quality SWNT thin films for use in hybrid SWNT/Si solar cells were prepared utilizing a “superacid sliding coating method” we described previously,17 where detailed information about the film fabrication technology as well as film properties are given. Briefly, this process exposes the SWNTs to oxygen (from air) and involves the use of chlorosulfonic acid, which strongly p-type dopes the SWNTs due to the in situ generation of a small quantity of sulfuric acid.19 As a result, combining these SWNTs with n-Si generates efficient p-SWNT/n-Si solar cells. Previously, Sumanasekera et al.,10 suggested that the removal of oxygen can cause SWNTs to change from p-type to n-type without the use of a dopant. Here, we postulated that if this reasoning is true, we would be able to subject our SWNT thin film to an oxygen removal process (which could also remove residual sulfuric acid) and enable the demonstration of efficient SWNT/p-Si solar cells. Initially, as a control, devices with untreated SWNT thin films on p-Si were prepared (Figure 1a). As expected these devices exhibited resistor behavior with no photocurrent rectification under 1 sun illumination (Figure 1b; see the experimental methods for the SWNT/Si device fabrication method). Subsequently, we attempted to remove oxygen and sulfuric acid from the SWNT thin films by placing the SWNT/ p-Si devices under high vacuum (10−7 mbar) at 180 °C (Figure 2a). The resulting devices exhibit good rectification behavior with an ideality factor of 2.27 and a PCE of 1.9%. Unfortunately, they display a low Voc of only 0.227 V, indicating a low built-in voltage from the SWNT/p-Si junction. In an attempt to develop a low temperature oxygen removal process, we employed a current stimulation method. Recently, it has been demonstrated that running constant current through SWNTs can remove gases from the SWNT surface.18,19 We applied a constant current between the top and bottom contacts of the SWNT/p-Si devices (Figure 2b). Various intensities of current (1−100 mA) were applied for different lengths of time (20 min−6 h). In all cases, no rectification behavior was observed (Figure 2b). This is in good agreement with literature, showing that direct current stimulation, though effective in the removal of many gases, does not effectively cause desorption of oxygen.20,21 Interestingly, when devices were first placed under high vacuum at 180 °C and then subjected to current stimulation (Figure 2a), a significant increase in PCE (3.68%) was observed compared to devices, which were just subjected to high vacuum at elevated temperature (1.90%). In this case, the high vacuum and heat treatment presumably removes oxygen (and residual sulfuric acid), while the current stimulation plays an additional role, which could involve the removal of weakly bound residual

surface dioxygen molecules, or dinitrogen molecules, which were adsorbed in the glovebox. Overall, the PCE of 3.68% is impressive for a SWNT/p-Si device but still quite low compared to SWNT/n-Si devices.15,17 HF is widely used to remove silicon oxide from a Si surface in the semiconductor industry. Previously, we have demonstrated that the acid HF can increase PCE in Si solar cells, through the removal of native oxides on the Si surface.17 Here, we were interested in exploring whether HF could assist in either oxygen or residual oxide removal from SWNTs as well, for improved SWNT/p-Si devices. As made SWNT/p-Si devices were treated with HF vapor in air for 5 min (Figure 2c) and then transferred into a nitrogen filled glovebox for photovoltaic testing. Initial tests indicated that the devices did not act as rectifiers and no photocurrent was observed, similar to devices which were not treated with HF. However, continued JV tests of the HF treated devices under illumination resulted first in rectification and then gradual improvement in the Voc, Jsc, fill factor (FF), and PCE (Supporting Information Figure S1). There are two potential explanations for this improvement: (i) a light soaking effect, where efficiency increases with light soaking time, similar to what occurs in some organic solar cells,22 or (ii) a current stimulation effect.20 The light soaking effect was ruled out as the cause of the increase in efficiency because illumination without JV testing did not cause SWNT/p-Si devices to change from resistors to rectifiers. The potential role of current stimulation in improving the rectification of the devices was explored by applying constant current to a HF treated SWNT/p-Si device in the dark and the device performance was compared before and after current stimulation (Figure 2c). Devices treated with HF and then subjected to constant current showed immediate diode behavior, with no induction period, consistent with a current stimulation effect. The red curve in Figure 2c is the characteristic J−V curve of an HF treated SWNT/p-Si device after current stimulation under 1 sun illumination. Remarkably, the Voc increases to 0.475 V compared to essentially zero for a device only treated with HF; Jsc is 25.5 mA/cm2 and the FF is 0.485. As a result, a record high PCE of 5.88% was obtained for a SWNT/p-Si photovoltaic. The properties of our HF treated devices were studied further by recording ln J−V curves in the dark before and after current stimulation (Figure 2d). In the ln J−V plot, the current density displays adequate linearity over a range of three decades of J. Therefore, both the ideality factor η and the saturation current density Js can be obtained by fitting the data to the diode equation J(T,V) = Js(T)[exp(eV/ηkBT)− 1], where J(T,V) is the current density across the SWNT/p-Si interface, V is applied voltage, T is the temperature, η is the ideality factor. The prefactor, Js(T), is the saturation current density which is correlated with Voc, where Voc = kT/q × ln(Jsc/Js). It is clear from this equation that the Voc increases with decreasing Js. The dramatic increase of Voc with current stimulation can be explained by the huge decrease in Js after our current = 32,100 nA/cm2 and Jafter = 22.24 stimulation process, Jbefore s s 2 nA/cm . The improvement in the ideality factor from 1.78 to 1.52 after current stimulation is indicative of an improved SWNT/pSi diode, whereas the reduced Js implies reduced recombination in the Si bulk or at the SWNT/p-Si interface. As neither HF vapor treatment nor current stimulation affects the bulk properties of Si, the decrease in Js can reasonably be attributed C

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Figure 3. J−V characteristics of HF and current stimulation treated SWNT/p-Si photovoltaic devices before and after AgNW coating (a) under 1 sun (AM1.5) illumination and (b) in the dark. Please note the AgNW/SWNT/Si went through a second HF and current stimulation treatment. The inset in (a) is a SEM image of the AgNWs on top of SWNT network. The scale bar is 2 μm. The inset in (b) is the ln J−V curve for a SWNT/p-Si photovoltaic device before and after AgNW coating.

Figure 4. (a) Change in the J−V characteristics based on the length of time exposed to air for a HF and current stimulation process treated AgNW/ SWNT/p-Si photovoltaic device under 1 sun (AM1.5) illumination. (b) J−V characteristics of the air exposed AgNW/SWNT/p-Si photovoltaic devices after: (blue) heating in vacuum, (green) current stimulation, and (black) HF treatment followed by current stimulation. (c) Band scheme diagram of a SWNT/p-Si heterojunction. The parameters for Si were derived from the dopant concentration dependence of the band gap and of the Fermi level as described by Sze.24 The band gap of the CNTs is assumed to be 0.5 eV and the Fermi level of the SWNT was assumed to be almost equal to that of undoped SWNTs as measured by Kim et al.25

then gradually decreases, reflecting the effect of current stimulation. It should be noted that the resistance of SWNT thin films on glass slides decreases from 180 to 148 Ω, after HF vapor treatment in air (Figure 2f). In contrast, the current simulation process increases the resistance of SWNT thin films (Figure 2g). The most likely explanation for this behavior is desorption of oxygen, and Collins et al.5 have observed similar effects. In SWNT/p-Si solar cells, the SWNT layer not only forms the junction with Si for carrier dissociation, but also transports separated carriers to the respective metal electrodes. The dedoping of SWNTs due to oxygen removal, clearly improves the junction between SWNT/p-Si. However, it also reduces the density of carriers in SWNTs and increases sheet resistance, which suggests that our devices can be penalized by less effective carrier transport within the SWNT layer. This problem will become even more pronounced if the surface area of the cells is increased. Previously, it has been demonstrated that the problem of an increase in resistance can be mitigated by metal deposition.23 In the case of SWNT/p-Si devices, the metal deposition method is not applicable due to the porous nature of SWNT thin films, especially because the SWNT films are only ∼10 nm in thickness. Recently, we have reported the spin-casting of AgNWs for mitigating resistive power loss in large area pSWNT/n-Si solar cell.18 Here, we demonstrate a similar method to solve the resistive loss problem in SWNT/p-Si solar cells (Figure 3). The overall PCE increased slightly from

to reduced interfacial recombination. The built-in voltage at the SWNT/p-Si interface is related to the difference between the Fermi levels of the SWNT thin film and p-Si. Since the combination of HF and current stimulation removes the pdoping of the SWNT thin film, the Fermi level of the thin film must increase, and hence, the difference between the Fermi level of the SWNT film and p-Si must increase. This causes a larger potential drop Vbi across the depletion width and facilitates a more efficient collection of electrons and holes (vide infra). We would like to point out that the current density we apply during the current stimulation treatment is several magnitudes lower than the breakdown current density for SWNTs. We have performed Raman spectroscopy before and after current stimulation and have verified that there is no electronic breakdown of SWNTs (Supporting Information Figure S3). Currently, state of the art p-SWNT/n-Si solar cells give a PCE of 8.52%, with Jsc = 26.11 mA/cm2, Voc = 0.51 V, FF = 0.64, without optimization (i.e., antireflective coating or scattering by nanoparticles).15,17 A comparison of the performance of our best SWNT/p-Si solar cells to these values indicates that the major difference is the lower FF in SWNT/p-Si devices. A plausible explanation for the lower FF is higher series resistance in the SWNT thin films. To explore this further, the voltage change with time between the cathode and anode of our SWNT/p-Si solar cells with constant driving current was monitored (Figure 2e). The voltage first increases sharply and D

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Figure 5. (a) X-ray photoelectron spectra of a SWNT thin film on Si before and after HF treatment. The X-ray photoelectron spectra in the (b) oxygen 1s and (c) carbon 1s region for SWNT/Si before and after HF treatment.

5.51% to 5.97%, mainly due to the FF increasing from 0.427 to 0.567. The slight decrease in Jsc from 26.1 to 24.5 mA/cm2 was expected due to reduced transmittance and, thus, less light absorption in Si. Unexpectedly, there was a decrease in Voc from 0.495 to 0.417 V (Supporting Information Figure S2). This is likely caused by dissolved oxygen in the AgNW isopropanol solution, which slightly p-dopes the SWNTs. Thus, after depositing the AgNWs, the devices were again treated with HF and current stimulation. In this case, a new record PCE of 7.53% was obtained, with Jsc of 24.8 mA/cm2, Voc of 0.496 V, and FF of 0.612 (Figure 3 and Supporting Information Table S1). To further confirm that oxygen directly influences the behavior of SWNT/p-Si solar cells, our HF and current treated SWNT/p-Si devices were exposed to air (Figure 4a). The performance gradually decreases as the exposure time increases. Upon exposure to air for 5 min, the PCE decreased dramatically from 7.3% to 0.7% mainly due to a decrease in Voc from 0.470 to 0.208 V and FF from 0.615 to 0.167. The performance continued to decrease from air exposure until the SWNT/p-Si device exhibited behavior typical of a resistor (Supporting Information Table S2). Diode behavior was recovered by redesorbing the oxygen. Heating (180 °C) the device in ultrahigh vacuum (10−7 mbar) recovered the PCE to 1.26% and current stimulation (without HF) in the glovebox increases the PCE to 4.82% (Figure 4b). Further HF and current stimulation resulted in the full recovery of the PCE to 7.30%. To the best of our knowledge the reversible switching of SWNT/p-Si devices between resistors and diodes presumably based on the amount of oxygen adsorbed by the SWNT thin film, has never previously been observed in devices of this type. On the basis of the work function and band gaps of the SWNTs and p-Si, a band diagram of the SWNT/p-Si heterojunction can be drawn, following the Anderson model (Figure 4c). A large built-in voltage of around 0.52 V is expected due to the difference between the Fermi levels of SWNTs and p-Si. As oxygen adsorbs on SWNTs and injects hole carriers, the Fermi level of SWNTs decreases, resulting in a reduced built-in potential, as observed in Figure 4a. This is direct and strong evidence that oxygen p-dopes SWNTs. Our results clearly indicate that HF plays a crucial role in generating high efficiency SWNT/p-Si solar cells. Current stimulation alone does not generate diode behavior, whereas subjecting devices to high vacuum at elevated temperature

followed by current stimulation gives devices that are significantly less efficient. Although at this stage it is not clear why HF treatment needs to be combined with current stimulation, X-ray photoelectron spectroscopy (XPS) was performed to obtain information about the chemical environment of SWNT/p-Si films before and after HF treatment (Figure 5). We show there are notable differences in the spectra of films before and after HF treatment (Figure 5). The sulfur signal present in the film before treatment is not observed after HF treatment, whereas a relatively new small signal that appears at approximately 680 eV is present after HF treatment, which can be assigned to F 1s. In addition, the overall O 1s peak intensity is reduced after HF treatment. In fact, careful inspection of this signal reveals that the intensity of the signal corresponding to CO single bonds (at ∼533 eV) is more reduced than the intensity of the signal corresponding to CO double bonds (at ∼532 eV) after HF treatment. This suggests that functional groups such as ethers or hydroxyl moieties have been removed from the surface (Figure 5b). Finally, there are two features of note in the C 1s region of the spectrum: (i) After HF treatment there is a slight increase in the C 1s signal at around ∼289 eV (Figure 5c), which corresponds to the region in which either CO double bonds or C−F single bonds are observed. Given that the oxygen concentration is reduced after HF treatment, this suggests that C−F bonds have formed during HF treatment. (ii) The tail of the C 1s signal is stronger after HF treatment. This is indicative of a stronger π−π* signal, which is presumably related to a longer and better conjugated network in the SWNT thin films. Overall, the XPS studies suggest that HF plays critical roles in oxygen removal from the SWNT surface. In conclusion, we have demonstrated that oxygen dopes SWNTs with hole carriers and that the removal of oxygen leads to n-type behavior of SWNT in SWNT/p-Si devices. It has been established that HF treatment followed by current stimulation is a very effective method for oxygen removal in comparison to the commonly used procedure of heating SWNTs under vacuum. Finally, using a procedure involving HF treatment, current stimulation and spin-casting AgNWs onto a SWNT thin film, record high efficiency SWNT/p-Si solar cells have been developed. Methods. SWNT/Si Device Fabrication. A 500 nm-thermal oxide covered p-type Si (100) wafer (1−10 Ω·cm) was patterned with Au (80 nm: top contact and etch mask)/Cr E

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(5 nm: adhesion layer) by photolithography. A Si window (3 × 3 mm2) was exposed through wet-etching of the oxide. The back contacts were fabricated using Al following a buffered oxide etch (BOE) for 1 min. The SWNT thin films were floated on water and then transferred onto patterned Si wafers. HF Treatment. For the HF study, as made SWNT/p-Si devices were treated by HF using a vapor phase etcher. The backside metal was protected during the etching process. All devices were brought into glovebox for testing or processing right after HF treatment to minimize the change of surface state of Si. Current Stimulated Gas Desorption Process. We applied various currents (ranging from 20 mA to 80 mA) between the cathode and the anode of SWNT/Si devices for a period of time (1 min to 4 h), and measured the desorption of gases via the change in voltage over time. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy was performed on a Physical Electronics PHI 5000 VersaProbe with a scanning monochromatic source from an Al anode and with dual beam charge neutralization. Survey XPS spectra were acquired at 100 W with pass energy of 117 eV over the range of 0−1000 eV with 0.5 eV resolution and 50 ms dwell time, and averaged over three scans. High resolution XPS spectrum of C 1s, F 1s, and O 1s core electrons were acquired over an approximately 100 μm spot with 50 W beam power and averaged 20 times with 0.1 eV resolutions at 11.75 pass energy with 200 ms dwell time.

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ASSOCIATED CONTENT

S Supporting Information *

J−V curve after HF treatment; air exposure experiments. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

A.D.T. and N.H. thank the Yale Climate and Energy Institute for funding. The authors gratefully acknowledge the Sabotka Research Fund, Teracon Corp., and the National Science Foundation NSF-CBET-0954985 CAREER Award for partial support of this work. Facilities use was supported by YINQE and NSF MRSEC DMR 1119826 (CRISP). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Mark Schwab and Prof. Lisa Pfefferle for their help with Raman spectrscopy. Southwest Nanotechnologies are acknowledged for their kind supply of single walled carbon nanotubes.

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REFERENCES

(1) Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett. 2004, 4, 35−39. F

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