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

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

Measuring <i>n</i> and <i>k</i> at the Microscale in Single Crystals of CH₃NH₃PbBr₃ Perovskite

Lead-based, inorganic–organic hybrid perovskites have shown much promise in photovoltaics, and the ability to tune their band gap makes them attractive for tandem solar cells, photodetectors, light-emitting diodes, and lasers. A crucial first step toward understanding a material’s behavior in such optoelectronic devices is determining its complex refractive index, <i>n + ik</i>; however, optically smooth films of hybrid perovskites are challenging to produce, and the optical properties of films of these materials have been shown to depend on the size of their crystallites. To address these challenges, this work reports quantitative reflectance and transmittance measurements performed on individual microcrystals of CH₃NH₃PbBr₃, with thicknesses ranging from 155 to 1907 nm. The single crystals are formed by spin-coating a film of precursor solution and then stamping it with polydimethylsiloxane (PDMS) during crystallization. By measuring crystals of varying thickness, <i>n</i> and <i>k</i> values at each wavelength (405–1100 nm) have been determined, which agree with recently reported values extracted by ellipsometry on millimeter-sized single crystals. This approach can be applied to determine the optical constants of any material that presents challenges in producing smooth films over large areas, such as mixed-halide hybrid and inorganic perovskites, and to micro- or nanoplatelets

Machine learning in nanoscience: big data at small scales

Recent advances in machine learning (ML) offer new tools to extract new insights from large data sets and to acquire small data sets more effectively. Researchers in nanoscience are experimenting with these tools to tackle challenges in many fields. In addition to ML’s advancement of nanoscience, nanoscience provides the foundation for neuromorphic computing hardware to expand the implementation of ML algorithms. In this mini-review, which is not able to be comprehensive, we highlight some recent efforts to connect the ML and nanoscience communities focusing on three types of interaction: (1) using ML to analyze and extract new information from large nanoscience data sets, (2) applying ML to accelerate materials discovery, including the use of active learning to guide experimental design, and (3) the nanoscience of memristive devices to realize hardware tailored for ML. We conclude with a discussion of challenges and opportunities for future interactions between nanoscience and ML researchers

Core-Shell CdS-Cu₂S Nanorod Array Solar Cells.

As an earth-abundant p-type semiconductor, copper sulfide (Cu2S) is an attractive material for application in photovoltaic devices. However, it suffers from a minority carrier diffusion length that is less than the length required for complete light absorption. Core-shell nanowires and nanorods have the potential to alleviate this difficulty because they decouple the length scales of light absorption and charge collection. To achieve this geometry using Cu2S, cation exchange was applied to an array of CdS nanorods to produce well-defined CdS-Cu2S core-shell nanorods. Previous work has demonstrated single-nanowire photovoltaic devices from this material system, but in this work, the cation exchange chemistry has been applied to nanorod arrays to produce ensemble-level devices with microscale sizes. The core-shell nanorod array devices show power conversion efficiencies of up to 3.8%. In addition, these devices are stable when measured in air after nearly one month of storage in a desiccator. These results are a first step in the development of large-area nanostructured Cu2S-based photovoltaics that can be processed from solution

Epitaxially aligned cuprous oxide nanowires for all-oxide, single-wire solar cells.

As a p-type semiconducting oxide that can absorb visible light, cuprous oxide (Cu2O) is an attractive material for solar energy conversion. This work introduces a high-temperature, vapor-phase synthesis that produces faceted Cu2O nanowires that grow epitaxially along the surface of a lattice-matched, single-crystal MgO substrate. Individual wires were then fabricated into single-wire, all-oxide diodes and solar cells using low-temperature atomic layer deposition (ALD) of TiO2 and ZnO films to form the heterojunction. The performance of devices made from pristine Cu2O wires and chlorine-exposed Cu2O wires was investigated under one-sun and laser illumination. These faceted wires allow the fabrication of well-controlled heterojunctions that can be used to investigate the interfacial properties of all-oxide solar cells