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Single-Nanowire Solar Cells


The two tasks performed by a solar cell are absorption of sunlight and collection of the photogenerated charges. In a conventional planar solar cell, these two processes are coupled because charges produced by light that is absorbed deep within the semiconductor must travel a long distance to the junction near the surface before they can be separated and collected. Core-shell nanowire arrays decouple the directions of light absorption and charge separation, allowing the collection of charges from poorly absorbing materials with relatively short minority carrier diffusion lengths. Additionally, because the dimensions of nanowires are on the same length scale as the wavelength of visible light, light-trapping effects allow a film of nanowires to absorb more light than would a thin film made from the equivalent volume of material.

In the development of nanowire array solar cells, single-nanowire solar cells provide information about the optical and electronic properties of the junction in a particular material system. They are a simplified experimental platform that can be used to screen materials for their suitability for nanowire array solar cells. This work describes the opto-electronic properties of single-wire solar cells made from silicon, CdS/Cu2S, and ZnO/Cu2O. While silicon is a model system used to investigate fundamental optical effects, the oxide and sulfide heterojunctions are attractive for photovoltaics because of their low cost and elemental abundance in the earth's crust; they are also particularly well suited for nanowire array, rather than planar, solar cells because of their sub-micrometer minority carrier diffusion lengths.

Suspended, silicon single-nanowire solar cells served as a model system with which to develop characterization techniques such as scanning photocurrent mapping (SPCM) and wavelength-dependent photocurrent measurements and to understand the optical properties of single-nanowire solar cells. These measurements and electromagnetic simulations of the wire's absorption show that the devices exhibit enhanced photocurrent at the wavelengths corresponding to the optical resonances of the nanowire. After characterization of the devices, they were used to study the interaction between a nanoscale dielectric cavity, the silicon nanowire, and a plasmonic nanocrystal, which is summarized below.

In planar solar cells, the CdS/Cu2S heterojunction is formed by a low-temperature cation-exchange reaction that creates an epitaxial interface between the two sulfides. This chemistry was adapted to single-nanowire solar cells and determined to produce high-quality junctions at the nanoscale, suitable for nanowire photovoltaics. The high carrier concentration of Cu2S or one of its neighboring Cu(2-x)S phases, however, often led to full depletion of the CdS nanowire core, prohibiting efficient collection of photogenerated charges. To address this difficulty, indium-doped CdS nanowires were synthesized, and single-nanowire solar cells were fabricated from them. I-V curves under simulated sunlight and SPCM show that the difficulty in collecting from the CdS core was resolved, and these devices yielded single-nanowire efficiencies averaging 2.5%. These indium-doped solar cells were also used as platforms to study the interaction between a single-nanowire solar cell and plasmonic nanocrystals, discussed below. Finally, the cation-exchange chemistry was applied to hydrothermally synthesized CdS nanorod arrays to produce micro-array nanorod solar cells with efficiencies reaching 0.2%.

In recent photovoltaic research, nanomaterials have offered two new approaches for trapping light within solar cells to increase their absorption: nanostructuring the absorbing semiconductor and using metallic nanostructures to couple light into the absorbing layer. These two approaches are combined in the study of silicon and In-CdS/Cu(2-x)S single-nanowire solar cells decorated with silver nanocrystals. Wavelength-dependent photocurrent measurements and finite-difference time domain (FDTD) simulations show that increases in photocurrent arise at wavelengths corresponding to the nanocrystal's surface plasmon resonances, while decreases occur at wavelengths corresponding to optical resonances of the nanowire. SPCM experimentally confirms that changes in the device's photocurrent come from the silver nanocrystal. These results demonstrate that understanding the interactions between nanoscale absorbers and plasmonic nanostructures is essential to optimizing the efficiency of nanostructured solar cells.

Because of the earth-abundance, non-toxicity, and low cost of copper and zinc, the ZnO/Cu2O heterojunction is an attractive material system for solar energy conversion. Beginning with Cu2O wires synthesized via a high-temperature, vapor-phase reaction, single-wire ZnO/Cu2O and ZnO/TiO2/Cu2O heterostructure diodes were fabricated. Devices showed photocurrent and photovoltage under laser illumination, and a few exhibited photovoltaic performance under 1-sun illumination. The general lack of photoresponse under 1-sun conditions is attributed to full depletion of the Cu2O wire's core, greatly reducing its conductivity. For the devices that did function as solar cells under 1-sun conditions, a combination of the large size of the Cu2O wires, the charge-screening ability of the TiO2 buffer layer, and possible incorporation of chlorine into the wires is likely responsible for their improved performance.

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