A scanning electron microscope (SEM) image (Figure ) of the SWNT film reveals uniformity of the film across the entire area. Upon twisting, the SWNT fiber became stronger and tougher thanks to the closer contact and improved load transfer between nanotubes due to the enhanced van der Waals forces and friction, which is consistent with previously reported results [27
]. Figure illustrates the strength of a twisted SWNT fiber which sustains a 200-g weight. As further revealed by Figure , the SWNT fiber upon twisting became much denser and possessed substantial alignment of the nanotubes along the twisting direction. The fiber diameter was reduced by approximately 35% from 17 to 11 μm. The twist angle, defined as the angle between the longitudinal direction of the SWNT bundles and the axis of the fiber, is about 26°, which is large enough to yield a strong fiber [29
]. The result shows that this simple process allowed one-step formation of continuous nanotube fibers.
SEM images. (a) The single-walled CNT (SWNT) film and (b, c, d) a densified and twisted SWNT fiber.
Before solar cell assembly, the mechanical properties of the SWNT fibers are tested. Figure shows typical stress-strain curves for three SWNT fibers before fracture. All the SWNT fibers fractured at the highest load. The tensile strength and Young's modulus of our SWNT fibers were measured in the range 0.8 to 1.0 GPa and 8 to 10 GPa, respectively. During loading to failure, the fibers, and hence the SWNT bundles, experienced two different strains, elastic strain and plastic strain, owing to slippage between aligned bundles and plastic deformation of individual nanotubes. Three different fracture morphologies were observed: (1) brittle fracture due to strong inter-bundle coupling (Figure ), (2) fan-shaped fracture surface due to fiber unwinding (Figure ), and (3) sliding of bundles due to weak inter-bundle coupling and small twist angle (approximately 11°) (Figure ).
Mechanical properties of the single-walled CNT (SWNT) fibers. (a) Tensile stress-strain curves of three SWNT fibers. (b, c, d) SEM images of fractured SWNT fibers.
The high tensile strengths of the SWNT fibers are consistent with their electrical conducting performance. Owing to the higher density, the conducting properties of the twisted fibers are superior to the original fibers. Figure shows the current density versus voltage curves of a typical SWNT fiber (approximately 1 cm long) before and after twisting. The current density is defined as the current per unit cross-sectional area of the SWNT fiber. The conductivity was enhanced featured with the resistivity reduced by approximately 40% from 9.7 × 10-4
to 5.5 × 10-4
. Raman spectra at an excitation of 633 nm show high G-band intensity (IG
) and very low D-band intensity (ID
) of as-produced CNT network (black) and CNT fiber (red) in Figure . The ratios of IG
are about 30, indicating high crystallization of CNT and negligible amorphous carbon. The two peak positions remain unchanged (D-band at 1,322 cm-1
and G-band at 1,589 cm-1
), revealing an absence of optical absorption change during the fiber twisting process.
Conducting properties of the single-walled CNT (SWNT) fibers. (a)Current density-voltage curves of a SWNT fiber before and after twisting. (b) Raman spectra of as-produced CNT network and CNT fiber.
Because the SWNT fibers were of macroscopic lengths and provided 1D electrical conducting channels, photovoltaic tests have been performed on the heterojunction solar cells made from the fibers and n-Si. The SWNT fiber/n-Si heterojunction was fabricated as illustrated in Figure . An n-type Si (100) wafer (doping density, 2 × 1015 cm-3) with a 300-nm SiO2 layer was patterned by photolithography and wet-etching to make a square window of 9 mm2. A back electrode of a Ti/Pd/Ag layer was used to ensure high-quality Ohm contact with the silicon. A SWNT fiber was then transferred to the top of the patterned silicon wafer and naturally dried. To introduce a strong adhesion between the fiber and the wafer, a piece of transparent tape was coated on the fiber. Forward bias was defined as positive voltage applied to the SWNT fiber. The current-voltage data were recorded using a Keithley 2601 SourceMeter (Keithley Instruments, Inc., Cleveland, OH, USA). The solar devices were tested with a Newport solar simulator (Newport, Beijing, China) under AM1.5 condition.
Figure 5 The single-walled CNT (SWNT) fiber/n-Si solar cell. (a) Device schematics of the SWNT fiber/n-Si solar cell. (b) SEM image of the SWNT fiber/n-Si junction. (c) Dark and light (AM1.5) J-V curves of the SWNT fiber/n-Si solar cell. (d) lnI-V plot and (inset) (more ...)
As illustrated in the bottom panel of Figure , 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 shows a SEM image of the SWNT fiber/n-Si junction.
Figure shows the measured current density-voltage (J-V) characteristics for a typical SWNT fiber/Si cell. Based on the J-V characteristics, the energy conversion efficiency (η) of the solar cell was estimated. The efficiency is defined by
where Jsc is the short-circuit current density (Jsc = Isc /S). Here, the nominal current density is defined as the current per unit projectional area (Sn = length × diameter) of the SWNT fiber; the actual current density is defined as the current per unit area when the minority diffusion in silicon is considered (Sa = Sn + 2Lp × length). Correspondingly, the actual efficiency (ηa) and nominal efficiency (ηn) will be obtained. Voc is the open-circuit voltage, Pin is the incident power density (100 mW/cm2), and FF is the fill factor, which is defined by the relation
where (JmVm) is the maximum power point of the J-V characteristic of the solar cell.
Along with the other two tested cells, the photovoltaic performance of the three cells is summarized in Table . Initial tests have shown ηa of 2% to approximately 3% and ηn of 6% to approximately 10% at AM1.5, proving that SWNT fiber-on-Si is a potentially suitable configuration for making solar cells. Comparing sample #1 and sample #2 with different diameters in Table the smaller diameter results in a smaller projectional area (Sn) and entire effective area (Sa), leading to a higher cell efficiency.
Photovoltaic performance of the three SWNT fiber/n-Si solar cells.
As shown in Figure , 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 [32
]. 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 [32
], 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 [16
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 , 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 [33
]: (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
plot (Figure , 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.