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.
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).
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