Theoretical Power Conversion Efficiency Research Articles

Plasmonic Energy Collection through Hot Carrier Extraction

Conversion of light into direct current is important for applications ranging from energy conversion to photodetection, yet often challenging over broad photon frequencies. Here we show a new architecture based on surface plasmon excitation within a metal-insulator-metal device that produces power based on spatial confinement of electron excitation through plasmon absorption. Plasmons excited in the upper metal are absorbed, creating a high concentration of hot electrons which can inject above or tunnel through the thin insulating barrier, producing current. The theoretical power conversion efficiency enhancement achieved can be almost 40 times larger than that of direct illumination while utilizing a broad spectrum of IR to visible wavelengths. Here we present both theoretical estimates of the power conversion efficiency and experimental device measurements, which show clear rectification and power conversion behavior.

RF Power Conversion Efficiency of Photodiodes Driven by Mach–Zehnder Modulators

A comprehensive analysis supported by experimental results is provided for RF and microwave power conversion efficiency (PCE) of photodiodes driven by optical signals with either shaped or pure sinusoidal intensity-envelopes where the shaping is performed with a Mach-Zehnder modulator (MZM). It is shown that optical envelope-shaping provides a significantly improved maximum theoretical PCE of 67% as compared to 50% for sinusoidal intensity-envelopes. Practical PCEs of 40.5%-53.5% have been achieved as compared to previously published efficiencies in the 32%-41% range. The results demonstrate that high-current photodiodes can be used as efficient wideband high-power output stages in microwave photonic distribution systems leading to less complex and more efficient antenna array backplanes.

ChemInform Abstract: Carbazole‐Based Polymers for Organic Photovoltaic Devices

AbstractReview: 60 refs.

Carbazole-based polymers for organic photovoltaic devices

Polymers based upon 2,7-disubstituted carbazole have recently become of great interest as electron-donating materials in organic photovoltaic devices. In this tutorial review the synthesis of such polymers and their relative performances in such devices are surveyed. In particular structure-property relationships are investigated and the potential for the rational design of materials for high efficiency solar cells is discussed. In the case of the 2,7-carbazole homopolymer it has been found that electron acceptors other than fullerenes produce higher energy conversion efficiencies. To get around possible problems with the build-up of charge density at the 3- and 6-positions and to improve the solar light harvesting ability of the polymers by reducing the bandgap, ladder- and step-ladder type 2,7-carbazole polymers have been synthesised. The fully ladderised polymers gave very poor results in devices, but efficiencies of over 1% have been obtained from a step-ladder polymer with a diindenocarbazole monomer unit. Donor-acceptor copolymers containing 2,7-carbazole donors and various electron-accepting comonomer units have been prepared. An efficiency of 6% has been reported from a device using such a copolymer and by suitable choice of the acceptor comonomer, polymers can be designed with potential theoretical power conversion efficiencies of 10%. While such efficiencies remain to be obtained, the results to date certainly suggest that carbazole-based polymers and copolymers are among the most promising materials yet proposed for obtaining high efficiency organic solar cells.

Energy transfer in nanowire solar cells with photon-harvesting shells

The concept of a nanowire solar cell with photon-harvesting shells is presented. In this architecture, organic molecules which absorb strongly in the near infrared where silicon absorbs weakly are coupled to silicon nanowires (SiNWs). This enables an array of 7-μm-long nanowires with a diameter of 50 nm to absorb over 85% of the photons above the bandgap of silicon. The organic molecules are bonded to the surface of the SiNWs forming a thin shell. They absorb the low-energy photons and subsequently transfer the energy to the SiNWs via Förster resonant energy transfer, creating free electrons and holes within the SiNWs. The carriers are then separated at a radial p-n junction in a nanowire and extracted at the respective electrodes. The shortness of the nanowires is expected to lower the dark current due to the decrease in p-n junction surface area, which scales linearly with wire length. The theoretical power conversion efficiency is 15%. To demonstrate this concept, we measure a 60% increase in photocurrent from a planar silicon-on-insulator diode when a 5 nm layer of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene is applied to the surface of the silicon. This increase is in excellent agreement with theoretical predictions.

Theoretical Investigations of Polymer Based Solar Cells

AbstractWe investigate the behavior of a polymer blend (M3EH-PPV:CN-ether-PPV) bulk heterojunction solar cell using a numeric model that self-consistently solves Poisson's equation and the charge continuity equation while incorporating electric field dependent mobilities. We obtain good quantitative agreement with present experimental data for J-V curves and photocurrent action spectra. To reproduce experimental photocurrent action spectra, our model predicts 36% exciton dissociation efficiencies in the bulk of the polymer. We also study the limiting conditions of polymer solar cell development by simulating an ideal solar cell using an AM1.5 global spectrum and assuming all absorbed photons hitting a M3EH-PPV:CN-ether-PPV polymer blend (band gap ∼2.0 eV) based solar cell at normal incidence contribute to current. If such a solar cell has 100 nm length, open circuit voltage=0.6 V and 50% fill factor, then the maximum theoretical power conversion efficiency is ηp=5.6%. A similar analysis for a M3EH-PPV:PCBM bulk heterojunction cell yields, ηp=3.5%. These results further highlight the need to develop smaller band gap materials and help explain why the best polymer based solar cells have power conversion efficiencies that remain stuck at about 3%. Our model is used to investigate the important increase in power conversion efficiencies we can expect as lower band gap polymers become available.

Theoretical and Experimental Investigation of a New Type of Blazed Grating*

A rigorous electromagnetic theory of the diffraction of light by blazed lamellar gratings has been developed. It is applied to calculate the diffracted power distribution of four gratings (R1 − R4) whose grooves have the following depths and widths (b,l) in units of grating period a: (0.433, 0.750), (0.333, 0.667), (0.250, 0.500), and (0.200, 0.400). All four gratings are theoretically blazed in the minus-first order for 30° incidence and for the wavelength equal to the grating period. The blaze is, however, broad band, and the minus-first-order theoretical power-conversion efficiency (P−1) for the most interesting grating, R3, exceeds 0.8, 0.9, 0.95, and 0.98 in the wavelength bands 0.84 a−1.49 a, 0.85 a−1.47 a, 0.87 a−1.43 a, and 0.98 a−1.31 a, respectively, for linearly polarized light. These surprising performances, which far exceed those obtainable with echelette gratings insofar as the incident light is linearly polarized, have been experimentally verified with the help of a microwave interference spectrometer, and point to the potential usefulness of blazed lamellar-reflection gratings in far-infrared spectroscopy.