Carbon-Nanotube Solar Photovoltaic Microcells with Nanowelded Contacts

Abstract

Great research interests have been shown on photovoltaic (PV) cells over the past four decades or so for their vast application in the domains of energy and communication [1–3]. To achieve efficient, low-cost PV cells, the selection of the ideal PV material, especially energy conversion material, and the optimized device structure are critical. In numerous materials, the semiconducting single-walled carbon nanotubes (SWCNTs) are attractive as the energy conversion material for PV applications due to their unique structure and excellent photoelectric properties. Almost defect-free structure and strong one-dimensional quantum local confinement effect of SWCNTs can decrease the recombination probability of photogenerated carriers and prolong its relaxation time [4, 5]. As a result, the main limitation factors of the efficiency of the traditional silicon PV device can be overcome. A wide band-gap distribution due to variation in diameter enables SWCNTs to match the whole solar spectrum and enhance the absorption of solar energy [6]. Furthermore, all semiconducting SWCNTs have direct band gaps, which make the photoexcitation process easily conducted without needing the assistance of phonons [7]. SWCNT films had been attempted to fabricate the photoelectric chemical solar cells in previous literature [8]. However, because of the lack of the efficient separation and collection of photoexcited carriers and large intertube interaction, the maximum monochromatic incident photo-to-current conversion efficiency (IPCE) acquired for the cell is only 0.15%. In this chapter, we introduce a novel SWCNT PV solar microcells. In this cell, a directed array of monolayer SWCNTs was nanowelded onto two asymmetrical metal electrodes with high and low work function, causing a strong built-in electric field in SWCNTs for efficiently separating photogenerated electron–hole pairs. The monolayer SWCNT PV cell shows a power conversion efficiency (η) of ~0:80% and 0.31% at the solar-light illumination of 8:8 W cm−2 and 100mW cm−2 respectively. Correspondingly, a high internal η of 12.6% and 5.1% was estimated for SWCNTs in the device based on the simulation of the actual absorbed incident power by SWCNTs. A promising application potential for SWCNTs in PV devices can be shown with the results.