Browsing by Subject "Silicon Nanocrystals"

Now showing 1 - 3 of 3
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    Effects of water adsorption and surface oxidation on the electrical conductivity of silicon nanocrystal films
    (2012-12) Rastgar, Neema
    Electrical conductivities of thin films of silicon nanocrystals (4-6 nm) exhibit high sensitivity to water vapor. Specifically, water adsorption on the surface of silicon nanocrystal (NC) films increases their electrical conductivity by a factor of four at room temperature and an order of magnitude at 175 K. The increase in conductivity is reversible and can manifest as peaks or hysteresis loops in temperature-dependent conductivity measurements even when the measurements are conducted under vacuum at 10-5 Torr and in the presence of only residual amounts of water vapor. Hydrogen-terminated silicon nanocrystals are easily oxidized to form submonolayer to monolayer of chemically bound oxygen on their surfaces when annealed at 300°C in a glovebox with 0.1 ppm of water vapor. Annealing under vacuum at 300°C retains H-passivation without oxidation. The electrical conductivity of films made from hydrogen-terminated silicon nanocrystals is 200 times higher than the electrical conductivity of films made from silicon nanocrystals with a monolayer of chemically bound oxygen. However, the conductivities of both types of films increase upon adsorption of water on the nanocrystal surfaces. These findings underscore the importance of controlling silicon nanocrystal surfaces in determining the electrical properties of their thin films.
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    Hybrid solar cells from polymers and silicon nanocrystals.
    (2009-12) Liu, Chin-Yi
    This thesis is concerned with the application of silicon nanocrystals (Si NCs) in photovoltaic devices. Two types of novel solar cells, hybrid solar cells and Si NCs-only thin-film photovoltaic devices, have been developed. Hybrid solar cells are made from polymers and Si NCs. The first hybrid solar cells were fabricated by using poly- 3(hexylthiophene) (P3HT) which has a good hole mobility and matches the energy band alignment of Si NCs. The solar cell performance of Si NCs/P3HT devices was studied as a function of the weight ratio of Si NCs/P3HT and Si NC size. Three groups of Si NCs were used in this study: Si NCs 3-5 nm in diameter, 5-9 nm in diameter, and 10-20 nm in diameter. The open-circuit voltage and short-circuit current increased by using the smallest size NCs due to the high surface-area-to-volume ratio and quantum confinement effect. Those results indicate that Si NCs are a good candidate as an electron acceptor in hybrid solar cell application. To improve the efficiency of Si NCs/P3HT hybrid solar cells, we started to optimize the fabrication conditions by modification of the polymer concentration, usage of postproduction heat treatment, and application of different metal electrodes. After optimization, a hybrid solar cell from 50wt% (weight ratio) Si NCs/P3HT annealed at 150 °C for 2 hours with aluminum (Al) electrodes had a power conversion efficiency of 1.47% with a fill factor of 0.47, short-circuit current of 3.8 mA/cm2, and open-circuit voltage of 0.8 V under air mass 1.5 direct (AM 1.5D) one sun illumination. To understand the hole mobility of P3HT before and after post-production heat treatment, a hole-only device was fabricated by depositing gold (Au) electrodes, which block electron injection from the electrodes to Si NCs. The results suggest that the hole mobility of 50wt% Si NCs/P3HT film increases one order of magnitude after heat treatment, due to improved crystallinity in the P3HT, which can enhance hybrid solar cell efficiency. Literature has reported that the compatibility of polymers and nanocrystals plays an important role in hybrid solar cell efficiency. Although P3HT is a good hole conductor and light absorber in solar cell applications, other polymers should be tested to find the best compatibility for Si NCs. Knowing this, P3HT was replaced by poly [2-methoxy-5- (3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in 3-5 nm Si NCs/MDMO-PPV hybrid solar cells. Although Si NCs/MDMO-PPV devices have a higher open-circuit voltage than Si NCs/P3HT devices, the power conversion efficiency of Si NCs/MDMO-PPV devices is not as high as that of Si NCs/P3HT devices. To understand the reasons for the low efficiency from Si NCs/MDMO-PPV devices, the hole mobility of MDMO-PPV, energy band alignment between MDMO-PPV and Si NCs, and absorption spectrum of MDMO-PPV were studied and compared to those of P3HT. To measure the hole mobility of MDMO-PPV, Au electrodes were again utilized to block electron injection into the Si NCs. The results show that the hole mobility of MDMOPPV is lower than that of P3HT. The absorption spectrum of MDMO-PPV (400-600 nm) is narrower than that of P3HT (400-650 nm) so that exciton generation in P3HT is more efficient than in MDMO-PPV under AM 1.5 conditions. Additionally, MDMO-PPV has a lower highest occupied molecular orbital level than P3HT so the efficiency of hole injection from Si NCs into MDMO-PPV may not be as efficient as for P3HT. These reasons explain why the efficiency of Si NCs/MDMO-PPV devices is not as good as Si NCs/P3HT devices. From Si NC solution processing, we found that 10-20 nm bare Si NCs without any surface modification can form a stable cloudy colloid with 1,2-dichlorobenzene. This colloid can be spin-cast onto an ITO substrate to form a continuous and dense thin film. A Schottky photovoltaic device consisting of a single layer of intrinsic Si NCs was fabricated in a glove box to verify that films can be cast from colloid Si NCs. This photovoltaic device has a sandwich structure with a 250 nm Si NC layer between ITO and Al electrodes. Under AM 1.5D one sun illumination, the Si NC Schottky device showed a significant photovoltaic response with a power conversion efficiency of 0.02%, a fill factor of 0.26, short circuit-current density of 0.148 mA/cm2, and open-circuit voltage of 0.51 V. This result suggests that the solution processing of bare Si NCs can be a new way to manufacture low-cost and high-quality silicon-based thin films.
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    Supporting data for "3D Printed Silicon Nanocrystal Light Emitting Diodes"
    (2020-05-20) Su, Ruitao; Park, Sung Hyun; Li, Zhaohan; McAlpine, Michael C; mcalpine@umn.edu; McAlpine, Michael C; McAlpine Research Group
    The application of 3-D printing to the fabrication of light emitting diode (LED) requires the ability to integrate materials with distinct properties into one functional device by tuning the printability of materials and precisely confining the cured patterns within the predesigned 3-D structure. To meet this goal, material properties, e.g., viscosity, surface tension and degree of crosslinking are optimized to improve the compatibility with the 3-D printing technique. Particularly, silicon nano crystal (SiNC), the nontoxic active material for the printed LED, is investigated in terms of controllable dispensing of the solution-based material as well as surface roughness and uniformity of the printed layer. With the successful red-IR light emission from the printed SiNC-LED, 3-D printing displays the potential to fabricate optoelectronic devices that are flexible, biocompatible and conforming to the surface shape of the target object in a freeform manner.