Strong, conductive carbon nanotube fibers as efficient hole collectors
© Jia et al; licensee Springer. 2012
Received: 17 October 2011
Accepted: 17 February 2012
Published: 17 February 2012
We present the photovoltaic properties of heterojunctions made from single-walled carbon nanotube (SWNT) fibers and n-type silicon wafers. The use of the opaque SWNT fiber allows photo-generated holes to transport along the axis direction of the fiber. The heterojunction solar cells show conversion efficiencies of up to 3.1% (actual) and 10.6% (nominal) at AM1.5 condition. In addition, the use of strong, environmentally benign carbon nanotube fibers provides excellent structural stability of the photovoltaic devices.
Keywordscarbon nanotubes fibers heterojunction solar cells
As a symbolic nanomaterial, carbon nanotube (CNT) with unique properties like high strength, high electrical conductivity, and chemical inertness has found important applications in optoelectronics , being an ideal candidate for various components in photovoltaic devices . CNT bundles can be organized into two typical macrostructures: fibers (1D) and films (2D). The fabrication of homogeneous CNT films with a controllable thickness has been an important basis for the research on CNT-involved devices where CNTs mainly function as transparent electrodes . Our recent work on CNT/Si heterojunction solar cells [4, 5] have stimulated a series of studies on the photovoltaic properties of various heterostructures, including CNT/Si [6–16], CNT/CdTe , and graphene/Si Schottky junctions [18, 19]. Among these devices, the CNT film serves multiple functions as a hole collector, charge transport path, and transparent electrode. However, the CNT film composed of CNT networks has a lot of inter-bundle voids, which should be fairly controlled to achieve high transparency while maintaining sufficient lateral conductivity of the film. The junction resistances between tubes/bundles also yield a limiting value for the conductivities for CNT films .
The CNT fiber is yet another macroscopic assembly of CNT bundles in a densified manner. CNT fibers have attracted intensive experimental and theoretical interests and are of increasing practical importance because of their unique 1D structure inherited from individual CNTs . Early research efforts mainly focused on organizing discontinuous nanotubes into ribbon/fiber-like materials. We first reported that long single-walled CNT (SWNT) strands consisting of aligned SWNTs could be synthesized directly with a vertical floating chemical vapor deposition (CVD) method . Many approaches have been developed since then for the assembly of CNTs into continuous fibers through direct spinning [23–26] and post-synthesis spinning [27–30]. Compared to the CNT film, the 1D CNT fiber composed of densely aligned CNT bundles has higher conductance. When forming a heterojunction with silicon, though the fiber itself (generally microns thick) is essentially opaque, the photo-generated charge holes excited from the exposed underlying silicon wafer will transport to it.
The purposes of this work are to introduce the design of the heterojunction solar cells using SWNT fibers as upper electrodes and n-type silicon wafers (n-Si) as photoactive electrodes and to investigate experimentally the photovoltaic properties of the SWNT fiber/Si heterojunctions, verifying the role of SWNTs as hole collectors.
Results and discussion
As illustrated in the bottom panel of Figure 5a, the fiber acted as a hole collector to extract the photo-excited holes generated within the rectangle region (marked with a dashed line) defined by the minority diffusion length (Lp) (approximately 20 μm for n-Si at 2 × 1015 cm-3 doping level) of the silicon and the fiber length. Figure 5b shows a SEM image of the SWNT fiber/n-Si junction.
where (JmVm) is the maximum power point of the J-V characteristic of the solar cell.
Photovoltaic performance of the three SWNT fiber/n-Si solar cells.
As shown in Figure 5c, the Voc and FF of the SWNT fiber/Si device are 0.445 V and 49.1%, respectively, which are comparable to the values for CNT film/Si cells . The overall ηn of the fiber device (approximately 10.6%) is about 43% higher than that of the film device (approximately 7.4%). This disparity arose mainly from the different definition of the junction area for these two devices. In this fiber device, the ηa is 3.17% when the entire effective area is used instead of only the fiber projection area. It is worth mentioning that the size of the inter-bundle voids within a CNT film is < 5 μm , which is substantially smaller than the Lp (20 μm). This implies that the SWNT bundles with an inter-spacing of 2 Lp will give the optimal charge collection. The cell efficiencies are expected to be further improved by acid doping .
Consistent with the characteristics of the 1D/2D junction, we note that the device only shows a moderate rectification ratio which is approximately 1,680 at ± 0.8 V, and a typical reverse current at -1.0 V is 250 nA. As shown in Figure 5d, at low forward voltages, the current follows an exponential dependence with ideality factor (n) equal to 1.38. At higher voltages, the current follows an exponential dependence with an ideality factor of 2.9. This variation corresponds to a transition between two regimes : (1) the current is dominated by diffusion and generation-recombination outside the space charge region (n = 1), and (2) the high-injection regime, where the density of the minority carrier is comparable with that of the majority (n = 2). A dV/d(lnI)-I plot (Figure 5d, inset) is used to analyze the current-voltage characteristics when the series resistance (Rs) begins to dominate, yielding a Rs of approximately 62 Ω.
The 1D nature of the SWNT fiber offers a tremendous opportunity for exciton dissociation. SWNTs in the devices are involved in multiple processes including hole collecting and transporting. Despite its opaque feature and the relatively small interfacial area for charge separation, the SWNT fiber provides many 1D paths, forming a conducting channel for charge transport.
The devices present a great potential for use as photovoltaic solar cells and light sensors. In addition to enhancing photovoltaic conversion efficiency, the incorporation of the robust SWNT fibers can potentially improve the mechanical and environmental stability of the devices.
To conclude, we have demonstrated the photovoltaic properties of the SWNT fiber/Si heterojunction and revealed that SWNTs can be used as efficient hole collectors. The SWNT fiber/n-Si solar cell studied here represents an addition to the CNT film/n-Si counterparts reported by us previously. The photovoltaic devices also show excellent structural stability due to the use of strong, environmentally benign CNT fibers.
This work was supported by the National Science Foundation of China (50972067) and the Research Fund for Doctoral Program of Education Ministry of China (20090002120019 and 20090002120030).
- Avouris P, Freitag M, Perebeinos V: Carbon-nanotube photonics and optoelectronics. Nat Photon 2008, 2: 341–350. 10.1038/nphoton.2008.94View Article
- Zhu HW, Wei JQ, Wang KL, Wu DH: Applications of carbon materials in photovoltaic solar cells. Sol Energy Mater Sol Cells 2009, 93: 1461–1470. 10.1016/j.solmat.2009.04.006View Article
- Zhu HW, Wei BQ: Assembly and applications of carbon nanotube thin films. J Mater Sci Tech 2008, 24: 447–456.View Article
- Wei JQ, Jia Y, Shu QK, Gu ZY, Wang KL, Zhuang DM, Zhang G, Wang ZC, Luo JB, Cao AY, Wu DH: Double-walled carbon nanotube solar cells. Nano Lett 2007, 7: 2317–2321. 10.1021/nl070961cView Article
- Jia Y, Wei JQ, Wang KL, Cao AY, Shu QK, Gui XC, Zhu YQ, Zhuang DM, Zhang G, Ma BB, Wang LD, Liu WJ, Wang ZC, Luo JB, Wu DH: Nanotube-silicon heterojunction solar cells. Adv Mater 2008, 20: 4594–4598. 10.1002/adma.200801810View Article
- Zhou H, Colli A, Ahnood A, Yang Y, Rupesinghe N, Butler T, Haneef I, Hiralal P, Nathan A, Amaratunga GAJ: Arrays of parallel connected coaxial multiwall carbon nanotube amorphous silicon solar cells. Adv Mater 2009, 21: 3919–3923. 10.1002/adma.200901094View Article
- Arena A, Donato N, Saitta G, Galvagno S, Milone C, Pistone A: Photovoltaic properties of multi-walled carbon nanotubes deposited on n-doped silicon. Microelectronics J 2008, 39: 1659–1662. 10.1016/j.mejo.2008.02.012View Article
- Li ZR, Kunets VP, Saini V, Xu Y, Dervishi E, Salamo GJ, Biris AR, Biris AS: SOCl2enhanced photovoltaic conversion of single wall carbon nanotube/n-silicon heterojunctions. Appl Phys Lett 2008, 93: 243117. 10.1063/1.3050465View Article
- Li ZR, Kunets VP, Saini V, Xu Y, Dervishi E, Salamo GJ, Biris AS: Light-harvesting using high density p-type single wall carbon nanotube/n-type silicon heterojunctions. ACS Nano 2009, 3: 1407–1414. 10.1021/nn900197hView Article
- Ong PL, Euler WB, Levitsky IA: Hybrid solar cells based on single-walled carbon nanotubes/Si heterojunctions. Nanotechnol 2010, 21: 105203. 10.1088/0957-4484/21/10/105203View Article
- Li CY, Li Z, Zhu HW, Wang KL, Wei JQ, Li X, Sun PZ, Zhang H, Wu DH: Graphene nano-"patches" on carbon nanotube network for highly transparent/conductive thin film applications. J Phys Chem C 2010, 114: 14008–14012. 10.1021/jp1041487View Article
- Jia Y, Li PX, Wei JQ, Cao AY, Wang KL, Li CL, Zhuang DM, Zhu HW, Wu DH: Carbon nanotube films by filtration for nanotube-silicon heterojunction solar cells. Mater Res Bull 2010, 45: 1401–1405. 10.1016/j.materresbull.2010.06.045View Article
- Shu QK, Wei JQ, Wang KL, Zhu HW, Li Z, Jia Y, Gui XC, Guo N, Li XM, Ma CR, Wu DH: Hybrid heterojunction and photoelectrochemistry solar cell based on silicon nanowires and double-walled carbon nanotubes. Nano Lett 2009, 9: 4338–4342. 10.1021/nl902581kView Article
- Shu QK, Wei JQ, Wang KL, Song S, Guo N, Jia Y, Li Z, Xu Y, Cao AY, Zhu HW, Wu DH: Efficient energy conversion of nanotube/nanowire-based solar cells. Chem Commun 2010, 46: 5533–5535. 10.1039/c0cc00512fView Article
- Jia Y, Cao AY, Bai X, Li Z, Zhang LH, Guo N, Wei JQ, Wang KL, Zhu HW, Wu DH: Achieving high efficiency silicon-carbon nanotube heterojunction solar cells by acid doping. Nano Lett 2011, 11: 1901–1905. 10.1021/nl2002632View Article
- Jia Y, Cao AY, Li PX, Gui XC, Zhang LH, Wei JQ, Wang KL, Zhu HW, Xu Y, Wu DH: Encapsulated carbon nanotube-oxide-silicon solar cells with stable 10% efficiency. Appl Phys Lett 2011, 98: 133115. 10.1063/1.3573829View Article
- Zhang LH, Jia Y, Wang SS, Li Z, Ji CY, Wei JQ, Zhu HW, Wang KL, Wu DH, Shi EZ, Fang Y, Cao AY: Carbon nanotube and CdSe nanobelt Schottky junction solar cells. Nano Lett 2010, 10: 3583–3589. 10.1021/nl101888yView Article
- Li XM, Zhu HW, Wang KL, Cao AY, Wei JQ, Li CY, Jia Y, Li Z, Li X, Wu DH: Graphene-on-silicon Schottky junction solar cells. Adv Mater 2010, 22: 2743–2748. 10.1002/adma.200904383View Article
- Li X, Li CY, Zhu HW, Wang KL, Wei JQ, Li XM, Xu EY, Li Z, Luo S, Lei Y, Wu DH: Hybrid thin films of graphene nanowhiskers and amorphous carbon as transparent conductors. Chem Commun 2010, 46: 3502–3504. 10.1039/c002092cView Article
- Pereira LFC, Rocha CG, Latgé A, Coleman JN, Ferreira MS: Upper bound for the conductivity of nanotube networks. Appl Phys Lett 2009, 95: 123106. 10.1063/1.3236534View Article
- Behabtu N, Green MJ, Pasqualia M: Carbon nanotube-based neat fibers. Nanotoday 2008, 3: 24–34.View Article
- Zhu HW, Xu CL, Wu DH, Wei BQ, Vajtai R, Ajayan PM: Direct synthesis of long single-walled carbon nanotube strands. Science 2002, 296: 884–886. 10.1126/science.1066996View Article
- Li YL, Kinloch IA, Windle AH: Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis. Science 2004, 304: 276–278. 10.1126/science.1094982View Article
- Motta M, Moisala A, Kinloch IA, Windle AH: High performance fibres from 'dog bone' carbon nanotubes. Adv Mater 2007, 19: 3721–3726. 10.1002/adma.200700516View Article
- Koziol K, Vilatela J, Moisala A, Motta M, Cunniff P, Sennett M, Windle A: High-performance carbon nanotube fiber. Science 2007, 318: 1892–1895. 10.1126/science.1147635View Article
- Vilatela JJ, Windle AH: Yarn-like carbon nanotube fibers. Adv Mater, in press. doi: 10.1002/adma.201002131 doi: 10.1002/adma.201002131
- Zhang M, Atkinson KR, Baughman RH: Multifunctional carbon nanotube yarns by downsizing an ancient technology. Science 2004, 306: 1358–1361. 10.1126/science.1104276View Article
- Ericson LM, Fan H, Peng H, Davis VA, Zhou W, Sulpizio J, Wang YH, Booker R, Vavro J, Guthy C, Parra-Vasquez ANG, Kim MJ, Ramesh S, Saini R, Kittrell C, Lavin G, Schimdt H, Adams WW, Billups WE, Pasquali M, Hwang WH, Hauge RH, Fischer JE, Smalley RE: Macroscopic, neat, single-walled carbon nanotube fibers. Science 2004, 305: 1447–1450. 10.1126/science.1101398View Article
- Zhang XF, Li QW, Tu Y, Li Y, Coulter JY, Zheng LX, Zhao YH, Jia QX, Peterson DE, Zhu YT: Strong carbon-nanotube fibers spun from long carbon-nanotube arrays. Small 2007, 3: 244–248. 10.1002/smll.200600368View Article
- Zhang XF, Li QW, Holesinger TG, Arendt PN, Huang JY, Kirven PD, Clapp TG, DePaula RF, Liao XZ, Zhao YH, Zheng LX, Peterson DE, Zhu YT: Ultrastrong, stiff, and lightweight carbon-nanotube fibers. Adv Mater 2007, 19: 4198–4201. 10.1002/adma.200700776View Article
- Li X, Li CY, Li XM, Zhu HW, Wei JQ, Wang KL, Wu DH: Force- and light-controlled electrical transport characteristics of carbon nanotube bulk junctions. Chem Phys Lett 2009, 481: 224–228. 10.1016/j.cplett.2009.09.097View Article
- Li Z, Jia Y, Wei JQ, Wang KL, Shu QK, Gui XC, Zhu HW, Cao AY, Wu DH: Large area, highly transparent carbon nanotube spiderwebs for energy harvesting. J Mater Chem 2010, 20: 7236–7240. 10.1039/c0jm01361gView Article
- Sze SM, Ng KK: The Physics of Semiconductor Devices. 3rd edition. New York: Wiley Interscience; 2007.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.