Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K52: Nanostructured Photovoltaics |
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Sponsoring Units: GERA DPOLY Room: Hilton Baltimore Holiday Ballroom 3 |
Wednesday, March 16, 2016 8:00AM - 8:12AM |
K52.00001: Ligand engineering of lead chalcogenide nanoparticle solar cells Marton Voros, Nicholas Brawand, Giulia Galli Semiconductor nanoparticles (NP) are promising materials to build cheap and efficient solar cells. One of the key challenges in their utilization for solar energy conversion is the control of ligand-NP interfaces. Recent experiments have shown that by carefully choosing the ligands terminating the NPs, one can tailor electronic and optical absorption properties of NP assemblies, along with their transport properties.[1] By using density functional theory based methods, we investigated how the opto-electronic properties of lead chalcogenide NPs may be tuned by using diverse organic and inorganic ligands. We interpreted experiments, and we showed that an essential prerequisite to avoid detrimental trap states is to ensure charge balance at the ligand-NP interface, possibly with the help of hydrogen treatment. [1] Ryan Crisp et al., Scientific Reports 5, 9945 (2015); Carlo Giansante et al., J. Am. Chem. Soc. 137, 1875 (2015). [Preview Abstract] |
Wednesday, March 16, 2016 8:12AM - 8:24AM |
K52.00002: Verifying field-effect passivation of a SiN$_{\mathrm{x}}$ layer on a silicon nanopillar array using surface photovoltage characterization Eunah Kim, Yunae Cho, Ahrum Sohn, Dong-Wook Kim, Hyeong-Ho Park, Joondong Kim In silicon (Si) wafer based photovoltaic (PV) devices, light-trapping strategies to improve optical absorption are very important due to the indirect bandgap of Si. Surface nano-patterned Si enable omnidirectional broadband antireflection (AR) effects with the help of graded refractive index, multiple scattering, diffraction, and Mie resonance. In this work, the surface photovoltage (SPV) of periodic nanopillar (NP) arrays were investigated using Kelvin probe force microscopy (KPFM). The SPV characteristics clearly revealed that positive fixed charges in SiN$_{\mathrm{x}}$ layers induced downward band bending at the Si surface and increased SPV at the NP top surface. The similar SPV value of NPs and planar counterpart suggests that field effect passivation by the dielectric layer coating could help improve PV performance of nanostructure-based Si solar cells and that KPFM measurements are useful tool for quantitative investigation of surface electrical properties of Si nanostructures. [Preview Abstract] |
Wednesday, March 16, 2016 8:24AM - 8:36AM |
K52.00003: \textbf{Ultra-dense silicon nanowire array solar cells by nanoimprint lithography} Peng Zhang, Pei Liu, Stylianos Siontas, Alexander Zaslavsky, Domenico Pacifici, Jong-yoon Ha, Sergiy Krylyuk, Albert Davydov Nanowire (NW) solar cells have been attracting increasing interest due to their potentially superior light absorption compared to thin bulk films. In order to improve light trapping, we have used nanoimprint lithography (NIL) to fabricate high-density NW arrays with deep sub-micron pitch (P) and diameter (D). We have grown dense vertical arrays of Si axial $p-i-n$ junction NWs of D $=$ 170 nm and P $=$ 500 nm by vapor-liquid-solid epitaxy on seed arrays produced by NIL. The NWs were 9 µm length long with a 5 µm intrinsic section. The NW arrays were planarized using SU-8 photoresist, followed by reactive ion etching to expose the NW tips. Top $n$-contact was realized by sputter deposition of a transparent 200 nm IZO layer. The nanoimprinted NW array samples measured under AM 1.5 G illumination showed a peak external quantum efficiency of \textasciitilde 8{\%} and internal quantum efficiency of \textasciitilde 90{\%} in the visible spectral range. Three-dimensional finite-difference time-domain simulations of Si NW periodic arrays with varying P confirm the importance of high NW density. Specifically, due to diffractive scattering and light trapping, absorption efficiency close to 100{\%} in the 400--650 nm spectral range is predicted for a Si NW array with an even smaller P $=$ 250 nm, significantly outperforming a blanket Si film of the same thickness. Such pitch values are accessible to NIL and work on such arrays is in progress. [Preview Abstract] |
Wednesday, March 16, 2016 8:36AM - 8:48AM |
K52.00004: High Efficiency InP Solar Cells Through Nanostructuring Daniel Goldman, Joseph Murray, Jeremy Munday We describe high efficiency InP solar cells which utilize a periodic array of optically designed TiO$_{\mathrm{2}}$ nanocylinders. Optical and electronic simulations were performed to determine the spectrally resolved reflectivity and I-V characteristics of potential devices under AM1.5G illumination. The reflectivity of InP solar cells with these nanocylinders is found to have an average value of 2{\%} over the visible and near-IR spectral range, which outperforms traditional antireflection coatings. Coupling between Mie scattering resonances and thin film interference effects is found to accurately describe the optical enhancement provided by the nanocylinders. These nanostructured solar cells have power conversion efficiencies greater than 23{\%}, which is comparable to the highest quoted efficiencies for InP solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 8:48AM - 9:00AM |
K52.00005: Roles of SiN$_{\mathrm{x}}$ layers in light trapping and carrier collection of nanostructured crystalline Si solar cells Yunae Cho, Eunah Kim, Minji Gwon, Dong-Wook Kim, Hyeong-Ho Park, Joondong Kim We investigated optical properties and photovoltaic (PV) performance of nanostructured Si solar cells with and without SiN$_{\mathrm{x}}$ dielectric layers. The SiN$_{\mathrm{x}}$ layer contributed to the enhancement of internal quantum efficiency of the nanostructured cells at both short and long wavelengths. In contrast, the surface passivation of SiNx layers on flat cells helped the carrier collection mainly at short wavelength. The surface nanopatterned array enhanced the optical absorption and also concentrated incoming light near the surface in broad wavelength range, revealed by experimental data and optical simulation results. As a result, the nanostructured cells had high density of photo-generated carriers near the surface. This could lead to significant recombination loss of the cells without SiN$_{\mathrm{x}}$ layers. Our work suggested that the SiNx layer played a crucial role in the improved carrier collection of the nanostructured Si PV devices. [Preview Abstract] |
Wednesday, March 16, 2016 9:00AM - 9:12AM |
K52.00006: Gold Nanoparticles Assisted Photocurrent Enhancement in Hybrid Nanostructures Based Heterojunction Solar Cell Device Gen Long, Michael Beattie, Huizhong Xu, Mostafa Sadoqi In this presentation, we report a first hand study of plasmon enhanced photocurrent observed in hybrid nanostructures based heterojunction solar cell. The heterojunction solar cell was fabricated, using chemically synthesized narrow gap, IV-VI group semiconductor nanoparticles (PbS and PbSe), wide gap semiconductor ZnO nanowires, and gold nanoparticles, by spin-coating onto FTO glasses, in ambient conditions (25C, 1atm). The synthesized nanostructures were characterized by XRD, UV-VIS absorption, SEM, AFM, TEM, solar simulator, etc. Nanostructures of variant sizes were integrated in to the heterojunction devices to study the effects on photocurrent and solar cell performance. The architecture of film stack, i.e., the positions of Au nanoparticles and PbS, PbSe nanoparticles were also studied. We believe that introducing Au nanopartiles with proper size will lead to increase of photocurrent as well as solar cell devices. [Preview Abstract] |
Wednesday, March 16, 2016 9:12AM - 9:24AM |
K52.00007: Dielectric micro-resonator arrays for optical coupling to solar cells Dongheon Ha, Chen Gong, Marina S. Leite, Jeremy N. Munday Reflection occurs at the interface of a semiconductor and air as a result of the index of refraction contrast between the two media. In order to increase the coupling efficiency of free-space light to the modes of a solar cell, single- or double-layer dielectric thin-film anti-reflection coatings (ARC) are typically used. As an alternative approach, we introduce a new anti-reflection coating based on silicon dioxide (SiO$_{\mathrm{2}})$ nanospheres that enable high absorption and low-cost photovoltaic devices through a combination of effects including scattering, thin-film interference, and sphere-sphere coupling. From experiments and Finite Difference Time Domain (FDTD) simulations, we show that there is \textasciitilde 15-20{\%} enhancement in light absorption within the substrate (Si), which ultimately leads to increased spectral current density. We also show the enhancement of the optoelectronic properties via photo-response measurement on multi-crystalline Si solar cells with SiO$_{\mathrm{2}}$ nanosphere arrays on top. Because the layer can be made with an easy, inexpensive, and scalable process, this proposed ARC is an excellent candidate for substituting conventional ARC technologies relying on complicated, high temperature and expensive processes. [Preview Abstract] |
Wednesday, March 16, 2016 9:24AM - 9:36AM |
K52.00008: Importance of Depletion Width on Charge Transport and Interfacial Recombination in Extremely Thin Absorber Solar Cells Michael Edley, Treavor Jones, Jason Baxter The dynamics of charge carrier transport and recombination and their dependence on physical and electrochemical length scales in extremely thin absorber (ETA) solar cells is vital to cell design. We used J-V characterization, transient photocurrent / photovoltage, and electrochemical impedance spectroscopy to study electron transport and interfacial recombination in ETA cell. ETA cells were composed of ZnO nanowires coated with an ultrathin (5 nm) CdS buffer layer and CdSe absorbers with thicknesses of 10 -- 40 nm, with polysulfide electrolyte. In thinner absorbers near short circuit, the depletion region can extend radially into the nanowire, inhibiting interfacial recombination rate. However, depleting the periphery of the nanowire reduces the cross sectional area for charge transport, resulting in longer characteristic collection times. Thicker absorbers suffered more significant bias-dependent collection, and we conclude that slight radial penetration of the depletion region into the nanowires enhances charge collection. This work highlights the importance of considering the impact of depletion width on charge transport and interfacial recombination in the design of liquid junction, semiconductor-sensitized solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 9:36AM - 9:48AM |
K52.00009: MOVED ABSTRACT TO R26.011 |
Wednesday, March 16, 2016 9:48AM - 10:00AM |
K52.00010: Exciton Transfer in Carbon Nanotube Aggregates for Energy Harvesting Applications Amirhossein Davoody, Farhad Karimi, Irena Knezevic Carbon nanotubes (CNTs) are promising building blocks for organic photovoltaic devices, owing to their tunable band gap, mechanical and chemical stability. We study intertube excitonic energy transfer between pairs of CNTs with different orientations and band gaps. The optically bright and dark excitonic states in CNTs are calculated by solving the Bethe-Salpeter equation. We calculate the exciton transfer rates due to the direct and exchange Coulomb interactions, as well as the second-order phonon-assisted processes. We show the importance of phonons in calculating the transfer rates that match the measurements. In addition, we discuss the contribution of optically inactive excited states in the exciton transfer process, which is difficult to determine experimentally. Furthermore, we study the effects of sample inhomogeneity, impurities, and temperature on the exciton transfer rate. The inhomogeneity in the CNT sample dielectric function can increase the transfer rate by about a factor of two. We show that the exciton confinement by impurities has a detrimental effect on the transfer rate between pairs of similar CNTs. The exciton transfer rate increases monotonically with increasing temperature. [Preview Abstract] |
Wednesday, March 16, 2016 10:00AM - 10:12AM |
K52.00011: Ultrafast Spectroscopy Reveals Frenkel-CT Mixed Excitonic States in Copper Phthalocyanine Robert Younts, Terry McAfee, Bhoj Gautam, Daniel Dougherty, Harald Ade, Kenan Gundogdu In organic semiconducting systems, intermolecular charge transport and energy diffusion take place along the $\pi $-$\pi $ stacking direction, which is beneficial for opto-electronic devices. Therefore it is essential to study electronic state structure in the $\pi $-$\pi $ stacking direction in organic solids.~ We studied a model quasi-one-dimensional molecular crystal copper phthalocyanine, which has strong intermolecular coupling along the $\pi $-$\pi $ stack. In this work, we used polarization resolved transient absorption spectroscopy and identified the coupling~of~low-lying singlet Frenkel (intramolecular) excitons with CT (intermolecular) excitons. Our study shows an evolution between localized and delocalized excitations which can be utilized to tune charge transport properties in molecular crystals. These studies provide fundamental understanding~of~electronic state structures, which will be essential for tailoring electronic properties~of~desired applications. [Preview Abstract] |
Wednesday, March 16, 2016 10:12AM - 10:24AM |
K52.00012: Investigation of transport properties of ZnO/PbS heterojunction solar cells Yang Cheng, Michael D. C. Whitaker, Vincent R. Whiteside, Lloyd A. Bumm, Ian R. Sellers Lead sulfide (PbS) and lead selenide (PbSe) colloidal quantum dots (CQDs) are considered as a potential candidate material for solar cell applications due to their large band gap tunability range (0.5 to 1.7~eV) and cost-effective solution based processing. A series of Glass/ITO/ZnO/PbS/MoO$_{\mathrm{3}}$/Au heterojunction solar cells were processed and analyzed. A stable (reproducible) 2{\%} conversion efficiency under 1-sun is achieved based on the result of $J-V$ measurements. Absorbance and external quantum efficiency (EQE) measurements clearly show photo-generated carrier extraction from PbS active layers in the solar cell. However, a non-ideal $J-V$ behavior is observed in current-voltage measurements. This behavior may be attributed to a high density of trap states at the QD surface or defect states at the PbS/ZnO or ITO/ZnO interfaces. C-V and Impedance spectroscopy measurements are used to study this unusual behavior. These techniques could also help probe the transport properties and limitation of these heterojunction solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 10:24AM - 10:36AM |
K52.00013: Limits of Plasmonic Nanoparticle Enhancement in Solution-Processed Solar Cells Ebuka Arinze, Botong Qiu, Gabrielle Nyirjesy, Susanna Thon Solution-processed solar cells are particularly suited to benefit from plasmonic absorption enhancement due to their transport-limited film thicknesses, and the favorable compatibility of material integration. To date, experimental demonstrations of device enhancements via plasmonic nanoparticle-based strategies have achieved photocurrents that still fall below the theoretical predicted limits. We critically evaluate the prospects for plasmonic enhancements in solution-processed thin-film solar cells. We develop an effective medium model for embedded plasmonic nanostructures in photovoltaic thin films, evaluate the model in the context of previous results achieved in the field, and use the model as a framework for identifying the most promising avenues to realizing plasmonic performance enhancements in practical photovoltaic devices. Our model results indicate that further plasmonic enhancement gains may be possible in organic photovoltaic cells. For achieving maximum photocurrent potential, we identify fine-tuning of the concentration of embedded plasmonic enhancers within the absorbing medium as the crucial factor in achieving a balance between useful scattering and detrimental parasitic absorption losses. [Preview Abstract] |
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