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Silicon nanowires for solar photovoltaic applications

The majority of solar photovoltaic modules sold are siliconbased, but in recent years increased demand for silicon solar cells has inflated the price of raw silicon materials. The shortage of high-quality silicon has lead to research to find novel ways to design photovoltaic cells using inexpensive, low-quality silicon alternatives. Photovoltaic cells based on silicon nanowire arrays have emerged as a promising candidate for solar energy harvesting.1, 2 Silicon nanowire solar cells consist of arrays of radial p-n junction nanowires (see Figure 1) where the darker outer shell is composed of n-type silicon, to which the electron acceptor phosphorous has been added, and the lighter inner core from p-type silicon, to which the electron donor boron has been added. Each individual nanowire in the array has a p-n junction and acts as a tiny photovoltaic cell. Silicon solar cells based on nanowires have much shorter p-n junctions that thin film solar cells. In the nanowire structure, photo-excited electrons and holes (carriers) travel very short distances before being collected by the electrodes. This results in a higher carrier-collection efficiency in the core-shell nanowire structure, and this advantage leads to a higher tolerance for material defects and allows the use of a lower-quality silicon. The core-shell nanowire structure addresses the carrier-collection issue, one of the key factors that determine the overall efficiency of a solar cell. However, the efficiency of photon capture in the nanowire structures, another very important factor, has not yet been determined. Nanowire arrays are expected to possess significantly different optical properties from their bulk-length counterparts because they are smaller than the wavelength of visible light. In our recent work,3 we performed numerical simulations to study optical absorption in silicon nanowire structures with a diameter between 50 and 80nm. Wave effects are taken into account by numerically solving the full-wave Maxwell’s equations. Our Figure 1. Diagram of the silicon nanowire solar cell. Each individual nanowire is a tiny p-n junction. The darker outer shell is n-type silicon. The lighter inner core is p-type silicon.