Towards high efficiency nanowire solar cells

Semiconductor nanowires are a class of materials recently gaining increasing interest for solar cell applications. In this article we review the development of the field with a special focus on the III-V materials due to their potential to reach high power conversion efficiencies. After introducing basic concepts of nanowire synthesis, we discuss important aspects of nanowire design for high power conversion efficiencies; first in terms of light absorption, then in terms of charge carrier separation and collection. Further, we examine methods to assess and understand the materials quality and the solar cell performance. We end the review by a discussion of strategies and challenges in achieving efficiencies above the Shockley-Queisser limit, and the potential for cost efficient production

Analyse av solcelleanlegget på Kiwi Dalgård- Sluttrapport

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Simplifying Nanowire Hall Effect Characterization by Using a Three-Probe Device Design

Electrical characterization of nanowires is a time-consuming and challenging task due to the complexity of single nanowire device fabrication and the difficulty in interpreting the measurements. We present a method to measure Hall effect in nanowires using a three-probe device that is simpler to fabricate than previous four-probe nanowire Hall devices and allows characterization of nanowires with smaller diameter. Extraction of charge carrier concentration from the three-probe measurements using an analytical model is discussed and compared to simulations. The validity of the method is experimentally verified by a comparison between results obtained with the three-probe method and results obtained using four-probe nanowire Hall measurements. In addition, a nanowire with a diameter of only 65 nm is characterized to demonstrate the capabilities of the method. The three-probe Hall effect method offers a relatively fast and simple, yet accurate way to quantify the charge carrier concentration in nanowires and has the potential to become a standard characterization technique for nanowires

InxGa1-xP Nanowire Growth Dynamics Strongly Affected by Doping Using Diethylzinc

Semiconductor nanowires are versatile building blocks for optoelectronic devices, in part because nanowires offer an increased freedom in material design due to relaxed constraints on lattice matching during the epitaxial growth. This enables the growth of ternary alloy nanowires in which the bandgap is tunable over a large energy range, desirable for optoelectronic devices. However, little is known about the effects of doping in the ternary nanowire materials, a prerequisite for applications. Here we present a study of p-doping of InxGa1-xP nanowires and show that the growth dynamics are strongly affected when diethylzinc is used as a dopant precursor. Specifically, using in situ optical reflectometry and high-resolution transmission electron microscopy we show that the doping results in a smaller nanowire diameter, a more predominant zincblende crystal structure, a more Ga-rich composition, and an increased axial growth rate. We attribute these effects to changes in seed particle wetting angle and increased TMGa pyrolysis efficiency upon introducing diethylzinc. Lastly, we demonstrate degenerate p-doping levels in InxGa1-xP nanowires by the realization of an Esaki tunnel diode. Our findings provide insights into the growth dynamics of ternary alloy nanowires during doping, thus potentially enabling the realization of such nanowires with high compositional homogeneity and controlled doping for high-performance optoelectronics devices

Machine learning assisted representative period selection as input to modelling of field degradation in photovoltaic modules

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The performance and amphibious operation potential of a new floating photovoltaic technology

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Radiation Tolerant Nanowire Array Solar Cells

Space power systems require photovoltaics that are lightweight, efficient, reliable, and capable of operating for years or decades in space environment. Current solar panels use planar multijunction, III–V based solar cells with very high efficiency, but their specific power (power to weight ratio) is limited by the added mass of radiation shielding (e.g., coverglass) required to protect the cells from the high-energy particle radiation that occurs in space. Here, we demonstrate that III–V nanowire-array solar cells have dramatically superior radiation performance relative to planar solar cell designs and show this for multiple cell geometries and materials, including GaAs and InP. Nanowire cells exhibit damage thresholds ranging from ∼10–40 times higher than planar control solar cells when subjected to irradiation by 100–350 keV protons and 1 MeV electrons. Using Monte Carlo simulations, we show that this improvement is due in part to a reduction in the displacement density within the wires arising from their nanoscale dimensions. Radiation tolerance, combined with the efficient optical absorption and the improving performance of nanowire photovoltaics, indicates that nanowire arrays could provide a pathway to realize high-specific-power, substrate-free, III–V space solar cells with substantially reduced shielding requirements. More broadly, the exceptional reduction in radiation damage suggests that nanowire architectures may be useful in improving the radiation tolerance of other electronic and optoelectronic devices

III-V Nanowire Solar Cells: Growth and Characterization

To mitigate dangerous climate change, a transition to a new and sustainable energy system is needed. In this system, solar energy will need to be a key player. Prices of electricity made from solar cells have declined rapidly over the recent decades, making solar energy competitive in more markets. However, further price reductions and new innovations are needed for solar cell technology to fulfil its potential.In this thesis, we look at a class of materials that have gained increasing interest in the recent decade; III-V semiconductor nanowires. For solar cells, the III V semiconductors hold excellent optical and electrical properties, but high material and manufacturing costs have so far prevented competitiveness with the dominating Si based technology. However, III-V material in the nanowire geometry has a number of interesting advantages when it comes to reducing cost, as well as for adding III-Vs to conventional Si in tandem solar cell architectures. This has motivated substantial research efforts during recent years, both at universities and private companies. In this thesis, we have mainly studied InP and InxGa1-xP nanowire arrays as a solar cell material. The nanowires were grown by metal organic vapor phase epitaxy (MOVPE) via gold seeded vapor-liquid-solid growth. The gold seed particles were placed in a pattern on the growth substrate by help of nanoimprint lithography. Developing strategies to preserve this pattern through the stages of nanowire growth was an important foundation for the thesis work. These strategies allowed us to reproducibly grow dense and ordered arrays of nanowires, optimized for sunlight absorption. Controllably changing the electrical properties of the semiconductor through impurity doping is important to make a good solar cell. Doping nanowires is challenging since the growth mechanism is different from established layer growth by MOVPE, and nanowire characterization is demanding. We have studied doping in our nanowires in various ways. Most importantly, we have performed some of the first systematic doping studies in ternary III-V materials, with bandgaps needed to create tandem nanowire solar cells. Knowledge from these studies allowed us to realize and study the first nanowire tunnel junction connecting two materials of appropriate bandgap to match the solar spectrum.Finally, we have developed a characterization procedure to optimize nanowire solar cell characteristics. This helped us create a better understanding of performance limiting factors in our InP nanowire solar cells. As a result, we achieved more than a sevenfold performance improvement in these cells, with the best device having a certified power conversion efficiency of 15.0%. This is the highest reported efficiency value for a bottom-up synthesized InP nanowire solar cell