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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

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.

    Cover page: Single-Nanowire Solar Cells
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
      Cover page: Nanowires for Photovoltaics and Artificial Photosynthesis
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2015)
      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.
        Cover page: Core–Shell CdS–Cu2S Nanorod Array Solar Cells
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2014)
        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.
          Cover page: Epitaxially Aligned Cuprous Oxide Nanowires for All-Oxide, Single-Wire Solar Cells
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2017)
          The defect tolerance of halide perovskite materials has led to efficient optoelectronic devices based on thin-film geometries with unprecedented speed. Moreover, it has motivated research on perovskite nanowires because surface recombination continues to be a major obstacle in realizing efficient nanowire devices. Recently, ordered vertical arrays of perovskite nanowires have been realized, which can benefit from nanophotonic design strategies allowing precise control over light propagation, absorption, and emission. An anodized aluminum oxide template is used to confine the crystallization process, either in the solution or in the vapor phase. This approach, however, results in an unavoidable drawback: only nanowires embedded inside the AAO are obtainable, since the AAO cannot be etched selectively. The requirement for a support matrix originates from the intrinsic difficulty of controlling precise placement, sizes, and shapes of free-standing nanostructures during crystallization, especially in solution. Here we introduce a method to fabricate free-standing solution-based vertical nanowires with arbitrary dimensions. Our scheme also utilizes AAO; however, in contrast to embedding the perovskite inside the matrix, we apply a pressure gradient to extrude the solution from the free-standing templates. The exit profile of the template is subsequently translated into the final semiconductor geometry. The free-standing nanowires are single crystalline and show a PLQY up to ∼29%. In principle, this rapid method is not limited to nanowires but can be extended to uniform and ordered high PLQY single crystalline perovskite nanostructures of different shapes and sizes by fabricating additional masking layers or using specifically shaped nanopore endings.
            Cover page: Perovskite Nanowire Extrusion.
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2016)
            Photoelectrochemistry is one of several promising approaches for the realization of efficient solar-to-fuel conversion. Recent work has shown that photoelectrodes made of semiconductor nano-/microwire arrays can have better photoelectrochemical performance than their planar counterparts because of their unique properties, such as high surface area. Although considerable research effort has focused on studying wire arrays, the inhomogeneity in the geometry, doping, defects and catalyst loading present in such arrays can obscure the link between these properties and the photoelectrochemical performance of the wires, and correlating performance with the specific properties of individual wires is difficult because of ensemble averaging. Here, we show that a single-nanowire-based photoelectrode platform can be used to reliably probe the current-voltage (I-V) characteristics of individual nanowires. We find that the photovoltage output of ensemble array samples can be limited by poorly performing individual wires, which highlights the importance of improving nanowire homogeneity within an array. Furthermore, the platform allows the flux of photogenerated electrons to be quantified as a function of the lengths and diameters of individual nanowires, and we find that the flux over the entire nanowire surface (7-30 electrons nm(-2) s(-1)) is significantly reduced as compared with that of a planar analogue (∼1,200 electrons nm(-2) s(-1)). Such characterization of the photogenerated carrier flux at the semiconductor/electrolyte interface is essential for designing nanowire photoelectrodes that match the activity of their loaded electrocatalysts.
              Cover page: Single-nanowire photoelectrochemistry
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2014)
              Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.
                Cover page: 25th Anniversary Article: Semiconductor Nanowires – Synthesis, Characterization, and Applications
                • Article
                LBL Publications (2014)
                  Cover page: Phase-Selective Cation-Exchange Chemistry in Sulfide Nanowire Systems
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