Silicon Nanocrystals and Semiconducting Single-Walled Carbon

Daniel D. Tune , Frank Hennrich , Simone Dehm , Michael F. G. Klein ... 1% solar cells derived from ultrathin carbon nanotube photoabsorbing films ...
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LETTER pubs.acs.org/JPCL

Silicon Nanocrystals and Semiconducting Single-Walled Carbon Nanotubes Applied to Photovoltaic Cells V. Svrcek,*,† S. Cook,‡ S. Kazaoui,‡ and M. Kondo† †

Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, 305-8568, Japan ‡ Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8568, Japan ABSTRACT: We demonstrate the feasibility of bulk-heterojunction (BHJ) photovoltaic solar cells, which consist of surfactant-free silicon nanocrystals (size ∼3 nm) and semiconducting single-walled carbon nanotubes (diameter ∼1 nm) sandwiched between Al and ITO electrodes. In particular, we demonstrate that BHJ cells composed of p-type doped Si-ncs, in contrast with n-type, show a diode-like behavior, short-circuit current, and open-circuit voltage because the energy levels between p-type Si-ncs and semiconducting SWCNTs are adequate for exciton dissociation and carrier generation. Prototype BHJ cells made of p-type doped Si-ncs and semiconducting SWCNTs harvest sun light in broad spectral range 300 1400 nm. SECTION: Energy Conversion and Storage

ilicon nanocrystals (Si-ncs)1 4 and semiconducting singlewalled carbon nanotubes (SWCNTs)5 8 are very promising low-dimensional materials of great interest for photoconductor and photovoltaic (PV) applications. Indeed, Si-ncs in conjunction with the conjugated polymer, P3HT (poly(3-hexylthiophene)), was utilized to fabricate bulk-heterojunction PV cells. It was shown that by controlling the size of nanocrystal (in the range 3 20 nm), the optical band gap and the power conversion efficiency of the PV cells could be tuned.4 Meanwhile, semiconducting SWCNTs were also combined with fullerenes and polymers to make heterojunction and bulk-heterojunction PV cells.8,9 In particular, it was shown that the PV cells could harvest sunlight over a broad spectral range defined by the diameter of the SWCNTs. Recently, the combination of silicon and carbon nanotubes to realize PV cells has also gained interest.10 13 For example, Si nanowires were utilized in conjunction with carbon nanotubes,10 but because the latter consisted of a mixture of semiconducting and metallic SWCNTs as well as impurities (catalytic particles, amorphous carbon), the contribution of semiconducting SWCNTs was severely obscured. In a different study, SWCNTs were coated on bulk Si wafers or on Si nanowire array12,13 to fabricate heterojunction PV cells.11 13 However, unlike nanocrystal Si, bulk Si cannot exhibit the unique properties that rise from quantum confinement effects. To the best of our knowledge, the combination of Si-ncs and semiconducting SWCNTs is still unexplored despite great interest and promising results recently reported on SWCNT-Si PV cells. In this Letter, for the first time, we shall demonstrate the feasibility of bulk-heterojunction (BHJ) photovoltaic solar cells consisting of purified semiconducting SWCNTs and moieties/ surfactants-free Si-ncs. In particular, we shall discuss that BHJ cells consisting of p-type, in contrast with n-type Si-ncs, show a

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rectification behavior, short-circuit current (Isc), and open-circuit voltage (Voc) because the energy band offsets between p-type Si-ncs and the semiconducting SWCNTs are adequate for exciton dissociation and carrier generation. Figure 1a presents the optical absorption spectra of the n-type (black line) and p-type (red line) doped Si-ncs dispersed in ethanol solution. The optical absorption spectra of Si-ncs are very broad, and the onsets correspond to the transition across the band gap opened due to the quantum confinement effects.14 16 Note that the features labeled with (*) are due to the measurement system or the solvent but not due to Si-ncs. Figure 1b displays the optical absorption of the semiconducting SWCNTs, characterized by sharp peaks in the ranges of 1000 1400 and 600 900 nm, corresponding, respectively, to the first and the second excitonic transitions in (7,5), (7,6), (8,6), (8,7), and (9,7) SWCNTs.17 Note that the peak at 400 nm is due to traces of PFO, which are strongly bound to SWCNTs. Figure 2a shows the schematic structure of the PV cell that was made as follows: a mixture of Si-ncs (p- or n-type, typically ∼2 mg) and SWCNTs (typically ∼0.01 mg) in 10 mL of toluene was sonicated and was spray-coated on PEDOT:PSS/ITO/glass substrate. (PEDOT/PSS and ITO stand for poly(3,4oxyethyleneoxythiophene)-poly(styrenesulfonate) and indium tin oxide, respectively.) Then, the aluminum electrodes (100 nm thick) were deposited through a shadow mask in an ultrahigh vacuum chamber. For comparison, a reference device consisting only of SWCNTs (thickness ∼1 μm) was also made under similar conditions. Finally, Received: May 19, 2011 Accepted: June 21, 2011 Published: June 21, 2011 1646

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Figure 1. (a) Absorbance of n- and p-type doped Si-ncs dispersed in ethanol and (b) optical absorption spectra of an ensemble of semiconducting SWCNTs with traces of PFO.

the devices were loaded in a measurement cell filled with nitrogen gas. The current voltage (I V) characteristics of the devices were measured in dark and under illumination through the ITO/glass (A. M. 1.5, 100 mW/cm2, active area of a device 0.04 cm2). External quantum efficiency spectra (EQE) were measured with monochromatic light using Xe and halogen lamps and were calibrated using a standard Si cell and pyrodetector. The I V characteristics of the PV cell consisting of n-type and p-type doped Si-ncs mixed with semiconducting SWCNTs are presented in Figures 2b,c in the dark (black line) and under illumination (red line), respectively. Both p-type Si-ncs/SWCNTs and n-type Si-ncs/SWCNTs devices show a diode like behavior with nonlinear current versus voltage. However, the difference is that the n-type device has a lower Isc (∼3 orders magnitude lower) than the p-type device. From the I V curve, we see that the device shows a poor rectification and exhibits an S shape, probably due to the fact that our device is still unoptimized. In particular, the prototypical PV cell based on p-type Si-ncs and SWCNTs presents the following characteristics: Voc = 0.14 V, Isc = 0.3 mA/cm2, fill factor FF = 0.25, and power conversion efficiency η = 0.01%, but note that the ratio Sincs and SWCNTs nor the thickness of the device is optimized in this stage of the investigations. We would like to stress that we have also prepared PV cells consisting only of n-type or p-type Si-ncs, but all cells show ohmic behavior (at V = 0, Isc = 0) and show a small photocurrent response at positive/negative bias (not displayed). PV cells consisting only of semiconducting SWCNTs exhibit a very small rectification behavior, very small short-circuit current, and open circuit current (Figure 2d, to be added if necessary). In contrast, the combination of semiconducting SWCNTs and Si-ncs shows a sizable rectification behavior, short-circuit current, and open circuit current (Figure 2c), which is the sign of synergetic interactions between semiconducting SWCNTs and Si-ncs. Figure 3a presents the EQE spectra of the p-type doped Si-ncs/ SWCNTs and of the reference device. The EQE of the former is larger than that of the latter at all wavelengths. Furthermore, by normalizing the EQE spectra to the peak at 1300 nm, where only

Figure 2. (a) Schematic view of the solar cells. The I V characteristics in the dark (black line) and under A.M. 1.5 illumination (red line) of solar cells based on semiconducting SWCNTs and (b) n-type and (c) p-type doped Si-ncs, respectively.

SWCNTs contribute to the EQE (Figure 3b), we clearly observed that the EQE of p-type Si-ncs/SWCNTs device is significantly enhanced over the SWCNT reference device for wavelengths shorter than ∼1200 nm. In particular, we observed an increase in EQE peak intensity by a factor ∼2 in the range 600 800 nm, where the optical absorption from the Si-ncs strongly overlaps with the second excited states of SWCNTs. In addition, we observed a quenching of the photoluminescence (PL) emission excited at 400 nm of Si-ncs by more than a factor of two, alongside a relatively small red shift of the PL band from 610 to 613 nm (Figure 3c) when a small amount of SWCNTs (∼0.01 mg) was introduced to a solution of p-type Si-ncs (0.2 mg/ mL in ethanol). We believe that the PL quenching and photocurrent charge generation seen in devices are consistent with exciton dissociation and charge transfer processes between p-type doped Si-ncs and semiconducting SWCNTs. Note that similar processes between semiconducting SWCNTs and various types of polymers were recently discussed by Arnold and coworkers.18 It has to be noted that in our studies to enhance the electronic interactions between SWCNTs and Si-ncs the latter are moieties/surfactants-free.19 To rationalize our results, we shall refer to the energy diagram of the semiconducting SWCNTs and Si-ncs, which are reported in Figure 4. For SWCNTs, the electronic structure and the energy levels depend on the chiral index (n,m).5,6 In particular, the electron affinity (EA) and ionization potential (IP) of (7,5), 1647

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Figure 3. (a) External quantum efficiency (EQE) of the devices based on p-type doped Si-ncs and SWCNTs (red line) compared with one based only of SWCNTs (black line). (b) Normalized EQE spectra of the devices described in part a. EQE peaks are assigned to (n,m) SWCNTs. The overlap of the absorption of SWCNTs and Si-ncs is schematically shown with lines. (c) Photoluminescence spectra of p-type doped Si-ncs without (black line) and with SWCNTs (blue line) excited at the wavelength 400 nm.

(7,6), (8,6), (8,7), and (9,7) SWCNTs are in the range 3.8 to 4.1 and 5.1 to 4.8 eV, respectively, with the Fermi level (EF) almost constant 4.4 eV.20 In general, SWCNTs are ambipolar but proceeded in air; our SWCNTs are well-defined p-type in field-effect transistor configuration17 and are very weakly p-type in PV cells consisting only of semiconducting SWCNTs sandwich between Al and PEDOT:PSS/ITO/glass. For Si-ncs, because of the relatively broad size distribution, multiple energy levels are formed with the conduction band at ∼3.55 eV and the optical band gap as low as ∼1 eV;15 therefore, the valence band can be as low as 4.55 eV. In p-type doped Si-ncs device due to equalization of Fermi levels between Si-ncs and SWCNTs, the energy levels are shifted (Figure 4b), which enhances holes transfer into Si-ncs. It has been previously reported that p-type Si-ncs can serve as a hole transporting material when electronically coupled to an electron accepting such as nanocarbon material, that is, fullerenes.18

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In our devices, because Si-ncs and SWCNTs with different energy levels coexist, there is a probability that p-type doped Sincs and SWCNTs with the suitable electronic configuration can form type-II bulk-heterojunction (Figure 4b). A detailed analysis of the EQE spectra of p-type Si-ncs/SWCNTs device compared with that of the reference device consisting only SWCNTs (Figure 3a) suggests that the EQEs of the (7,5) and (7,6) SWCNTs have increased relatively more than that of (9,7), (8,7), and (8,6) SWCNTs in the range 900 1400 nm. This might originate from the fact that the alignment of the energy levels of the p-type Si-ncs are to some extent suitable with these of (7,5) and (7,6) but not with these of (9,7), (8,7), and (8,6) SWCNTs. In particular, as shown in the energy diagram in Figure 4, (7,5) SWCNTs is characterized by EA = 3.8 eV and IP = 5.1 eV, whereas (9,7) SWCNTs is characterized by EA = 4.1 eV and IP = 4.8 eV. This result suggests that the combination of Si-ncs (size ∼3 nm) and (7,5) SWCNTs (diameter ∼0.83 nm) in contrast with (9,7) SWCNTs (diameter∼1.10 nm) is electronically favorable for exciton dissociation and carrier generation. Note that (7,6), (8,6), and (8,7) SWCNTs are in an intermediate situation. We have derived the diameter dependence from the EQE spectra, but it can be confirmed by analyzing the IQE (which is the internal quantum efficiency defined as EQE/(number of absorbed photons) or approximately EQE/absorbance) to take into account the optical density of the film in the device. The optical absorption peak intensity of (7,5) and (7,6) SWCNTs (Figure 1b) is smaller than that of (9,7), (8,7), and (8,6) SWCNTs, but the EQE peak intensity is just the opposite. Therefore, the IQEs of (7,5) and (7,6) are larger than those of (8,7), (8,6), and (9,7) SWCNTs; in other words, the exciton dissociation yield of the former is larger than that of the latter. In addition, Arnold and coworkers8,18 have recently demonstrated that exciton dissociation occurs at the interface of the donor acceptor and depends on the diameter of the SWCNTs, which support our conclusions. It is important to highlight the several fold enhancement of charge generation when mixing Si-ncs with the SWCNTs and that the EQE shows more that the simple contribution of both Si-ncs and SWCNTs. In addition, according to energy levels, the formation of type-II heterojunction is possible. Our results also suggest that the photocarrier generation intrinsically involves SWCNTs and Si-ncs and is not due to extrinsic factors such as interaction with the electrodes We have also observed that n-Si-ncs/SWCNTs, in contrast with the p-Si-ncs/SWCNT device, shows very low short circuit current. One explanation is that the excess of electron in n-type Si-ncs might shift the conduction, valence, and Fermi levels, resulting in a larger energy barrier between the SWCNTs and the Si-ncs (Figure 4c). Another explanation is that in n-type Si-ncs, the excess carriers are the electrons, which might partially recombine with the photogenerated holes and eventually lower the photocarrier generation efficiency, but to provide an accurate explanation and to elucidate the origin of the high contact resistance in our devices, further experiments shall be performed in the future. We have also noticed that the optical absorption and the EQE spectra show the contribution of PFO at ∼400 nm (Figure 3a). The response of the PV cell from near-infrared to visible is essentially due to Si-ncs and SWCNTs, and the contribution of PFO is only sizable for wavelengths shorter than 400 nm. Moreover, PFO is wide band gap (∼3 eV) p-type semiconductor (Figure 4) with relatively high conduction band (∼2.2 eV) and relatively deep valence band (∼5.8 eV); therefore, PFO is not expected to make type-II bulk-heterojunction with either SWCNTs or Si-ncs. Finally, note that traces of PFO, which are 1648

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Figure 4. (a) Energy levels (in electronvolts relative to the vacuum level) of semiconducting SWCNTs with different chiral indexes (n,m), Si-ncs, PFO, Al, ITO, and PEDOT/PSS taking individually before contact. (b) Schematic representation, after contact, showing the different possible band alignment of p-type doped and (c) n-type doped Si-ncs with semiconducting SWCNTs, respectively.

adsorbed on the surface of SWCNTs, might hinder but obviously have not completely prevented the electronic interactions between SWCNTs and Si-ncs. In conclusion, in this Article, we have demonstrated the feasibility of a BHJ photovoltaic solar cell made from moieties/surfactantsfree Si-ncs combined with purified semiconducting SWCNTs. In particular, we have demonstrated that BHJ cells consisting of p-type doped Si-ncs, in contrast with n type, show a rectification behavior, Isc and Voc. Furthermore, our results suggest that the combination of p-type Si-ncs (size ∼3 nm) and semiconducting SWCNT such as (7,5) (diameter ∼0.83 nm) is electronically more favorable for exciton dissociation and carrier generation than with (9,7) SWCNTs (diameter ∼1.10 nm). Our unoptimized protypical BHJ cells harvest sunlight over a broad spectral range (300 1400 nm) with Voc = 0.14 V, Isc = 0.3 mA/cm2, fill-factor FF = 0.25, and power conversion efficiency PCE = 0.01% (at AM1.5, 100 mW/cm2). Further investigations are in progress to optimize the thickness of the different layers to engineer the optical band gap and the energy levels as well as to fund the necessary conditions to realize PV cells that take advantage of the multiple exciton and carrier generation effects expected in both Si-ncs and semiconducting SWCNTs.

’ EXPERIMENTAL DETAILS To obtain surfactant-free Si-ncs with rather same conductivity, we used a boron-doped wafer with a resistivity of ∼0.5 Ω 3 cm (p-type) and a phosphorus-doped wafer with a resistivity of ∼0.5 Ω 3 cm (n-type). Both types were produced same technique by electrochemical etching of a silicon wafers with the same crystallographic direction Æ100æ and subsequent mechanical pulverization, as previously described.22,23 Furthermore, to obtain similar average size and the size distributions of Si-ncs, the etching conditions such as current and time were varied: for the boron-doped Si-ncs, the current density was 3.2 mA 3 cm 2 and the etching time was 60 min; for

phosphorus doped Si-ncs, the current density was maintained at 1.6 mA 3 cm 2 and etching time was 90 min under illumination with an halogen lamp. Subsequently, the largest grains sizes were eliminated by sedimentation in ethanol solution for 30 min. Our previous studies showed that Si-ncs are characterized by an average size of ∼3 nm and an average energy band gap of ∼2 eV.22,23 It is worth noticing that our Si-ncs are free from organics moieties or surfactants, in sharp contrast with other reports, where Si-ncs are frequently terminated or coated to prevent their oxidation or to increase their solubility.24 Semiconducting SWCNTs were extracted from as-prepared carbon nanotubes powders (such as HiPCO, Carbon Nanotechnologies) using PFO (poly-9,9-di-n-octyl-fluorenyl-2,7-diyl, SigmaAldrich) as an extracting agent in Toluene solution assisted by ultrasonication and ultracentrifugation techniques, as previously described.17,25 In this study, the semiconducting SWCNT sample consists of the following chiral indexes (7,5), (7,6), (8,6), (8,7), and (9,7).16 The diameter and optical band gap of these semiconducting SWCNTs are in the range 0.8 to 1.1 nm and 0.8 to 1.1 eV, respectively.25

’ AUTHOR INFORMATION Corresponding Author

*Address: New Generation Device Team Research Center for Photovoltaic Technologies National Institute of Advanced Industrial Science and Technology (AIST) Central 2, Umezono 1-1-1, Tsukuba 305-8568 Japan. Tel: +81-29-861-5429. Fax: +81-29-861-3367. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the NEDO grant on Innovative Solar Cells. 1649

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’ REFERENCES (1) Magarshak, Y.; Kozyrev, S.; Ashok, K. Silicon Versus Carbon, Fundamental Nanoprocesses, Nanobiotechnology and Risks Assessment. In Proceedings of the NATO Advanced Research Workshop on Environmental and Biological Risks of Hybrid Organic-Silicon Nanodevices; St. Petersburg, 2008. (2) Koshida, N. Device Applications of Silicon Nanocrystals and Nanostructures, Nanostructure Science and Technology; Nanostructure Science and Technology XII; Springer: New York, 2009; pp 348 361. (3) Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. J. Multiple Exciton Generation in Colloidal Silicon Nanocrystals. Nano Lett. 2007, 8, 2506–2512. (4) Liu, C.-Y.; Holman, Z.; Kortshagen, U. Hybrid Solar Cells from P3HT and Silicon Nanocrystals. Nano Lett. 2009, 9, 449–452. (5) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: Singapore, 1998. (6) Avouris, P.; Freitag, M.; Perebeinos, V. Carbon Nanotube Optoelectronics. Top. Appl. Phys. 2008, 111, 423–453. (7) Gabor, N. M.; Zhong, Z. H.; Bosnick, K.; Park, J.; McEuen, P. L. Extremely Efficient Multiple Electron-Hole Pair Generation in Carbon Nanotube Photodiodes. Science 2009, 325, 1367–1371. (8) Bindl, D. J.; Wu, M.-Y.; Prehn, F. C.; Arnold, M. S. Efficiently Harvesting Excitons from Electronic Type-Controlled Semiconducting Carbon Nanotube Films. Nano Lett. 2011, 11, 455–460. (9) Kazaoui, S.; Minami, N.; Nalini, B.; Kim, Y.; Hara, Y. NearInfrared Photoconductive and Photovoltaic Devices Using Single-Wall Carbon Nanotubes in Conductive Polymer Films. J. Appl. Phys. 2005, 98, 084314–084320. (10) Shu, Q.; Wei, J.; Wang, K.; Song, S.; Guo, N.; Jia, Y.; Li, Z.; Xu, Y.; Cao, A.; Zhu, H.; et al. Efficient Energy Conversion of Nanotube/ Nanowire-Based Solar Cells. Chem. Commun. 2010, 46, 5533–5535. (11) Li, Z.; Kunets, V.; Saini, V.; Xu, Y.; Dervishi, E.; Salamo, G.; Biris, R.; Biris, A. Light-Harvesting Using High Density p-type Single Wall Carbon Nanotube/n-type Silicon Heterojunctions. ACS Nano 2009, 3, 1407–1414. (12) Wadhwa, P; Liu, B.; McCarthy, M. A.; Wu, Z.; Rinzler, A. G Electronic Junction Control in a Nanotube Semiconductor Schottky Junction Solar Cell. Nano Lett. 2010, 10, 5001–5005. (13) Jia, Y.; Cao, A.; Bai, X.; Li, Z.; Zhang, L.; Guo, N.; Wei, J.; Wang, K.; Zhu, H.; Wu, D.; et al. Achieving High Efficiency Silicon-Carbon Nanotube Heterojunction Solar Cells by Acid Doping. Nano Lett. 2011, 11, 1901–1905. (14) Kovalev, D.; Heckler, H.; Polisski, G.; Koch, F. Optical Properties of Silicon Nanocrystals. Phys. Status Solidi 1999, 251, 871–932. (15) Svrcek, V. Nanocrystaline Silicon and Carbon Nanotube Nanocomposites Prepared by Pulsed Laser Fragmentation. Pure Appl. Chem. 2008, 80, 2513–2521. (16) Holmes, J. D.; Ziegler, K. J.; Doty, R. C.; Pell, L. E.; Johnston, K. P.; Korgel, B. A. Highly Luminescent Silicon Nanocrystals with Discrete Optical Transitions. J. Am. Chem. Soc. 2001, 123, 3743–3748. (17) Izard, N.; Kazaoui, S.; Hata, K.; Okazaki, T.; Saito, T.; Iijima, S.; Minami, N. Semiconductor-Enriched Single Wall Carbon Nanotube Networks Applied to Field Effect Transistors. Appl. Phys. Lett. 2008, 92, 243112. (18) Bindl, D. J.; Safron, N. S.; Arnold, M. S. Dissociating Excitons Photogenerated in Semiconducting Carbon Nanotubes at Polymeric Photovoltaic Heterojunction Interfaces. ACS Nano 2010, 4, 5657–5664. (19) Svrcek, V.; Mariotti, D.; Shibata, Y.; Kondo, M. Hybrid Heterojunction Based on Fullerenes and Surfactant-Free, Self-Assembled, Closely-Packed, Silicon Nanocrystals. J. Phys. D: Appl. Phys. 2010, 43, 415402–415411. (20) Tanaka, Y.; Hirana, Y; Niidome, Y.; Kato, Y; Saito, S.; Nakashima, N. Experimentally Determined Redox Potentials of Individual (n,m) SingleWalled Carbon Nanotubes. Angew. Chem., Int. Ed. 2009, 48, 7655–7659. (21) Svrcek, V; Mariotti, D.; Kondo, M. Microplasma-Induced Surface Engineering of Silicon Nanocrystals in Colloidal Dispersion. Appl. Phys. Lett. 2010, 97, 161502.

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(22) Svrcek, V; Slaoui, V.; Muller, J. C. Ex Situ Prepared Si Nanocrystals Embedded in Silica Glass: Formation and Characterization. J. Appl. Phys. 2004, 95, 3158–3162. (23) Svrcek, V.; Fujiwara, H.; Kondo, M. Top-down Silicon Nanocrystals and a Conjugated Polymer-Based Bulk Heterojunction: Optoelectronic and Photovoltaic Applications. Acta Mater. 2009, 57, 5986–5995. (24) Sato, S.; Swihart, M. T. Propionic-Acid-Terminated Silicon Nanoparticles: Synthesis and Optical Characterization. Chem. Mater. 2006, 18, 4083–4088. (25) Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J. Highly Selective Dispersion of Single-Walled Carbon Nanotubes Using Aromatic Polymers. Nat. Nanotechnol. 2007, 2, 640–646.

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