This PDF file contains the front matter associated with SPIE Proceedings Volume 8824, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The impact of CdTe nanoscale semiconductor spectral sensitizers on the energy conversion efficiency of a poly-(hexylthiophene) (P3HT)-ZnO thin film (TF) photovoltaic (PV) cell was examined utilizing a one-dimensional computational model (Solar Cell Capacitance Simulator) (SCAPS). Output characteristics (quantum efficiency spectra, current-voltage characteristics) of TF PV cells containing the CdTe phase embedded within the n-type (ZnO) region of the junction were investigated with the modeling parameters derived from previous experimental studies of the component materials. The study focused on the influence of the spatial position of the CdTe region, relative to the P3HT-ZnO heterojunction, on the spectral characteristics of the energy conversion efficiency of the device. The contribution of this sensitizer phase to energy conversion was confirmed and the magnitude of the effect was found to increase as the semiconductor nanophase region was moved to within 20 nm of the heterojunction.

We describe a design for a photonic crystal dye-sensitized solar cell (DSSC) that can attain at least a factor of one-third enhancement in solar light absorption relative to a conventional cell. The design consists of a periodic array of modulated-diameter TiO₂ nanotubes filled with TiO₂ nanoparticles and interstitial regions filled with electrolyte. Using nanotubes filled with nanoparticles provides not only light trapping and absorption enhancement, but offers improved electrical transport through the nanotube walls. Keeping the volume of dye-coated TiO₂ nanoparticles in the cell constant, our design gives a maximum achievable photocurrent density (MAPD) of over 21mA/cm² in 2D simulations, well beyond the current record for C101-based cells. The design is shown to be feasible using current manufacturing techniques.

Triplet-triplet annihilation photon upconversion (TTA-UC) is a promising candidate for mitigating sub-band gap absorption losses in solar cells. In TTA-UC, sensitiser dyes absorb sub-band gap photons, cross to a triplet state, and transfer triplet excitons to emitter dyes. Two triplet-excited emitters can undergo TTA, raising one emitter to a higher-energy bright singlet state. The quadratic efficiency of TTA-UC at device-relevant light intensities motivates a push towards the higher chromophore densities achievable in the solid phase. We have begun this process by tethering tetrakisquinoxalino palladium porphyrin to 20nm silica nanoparticles using peptide chemistry techniques, achieving a total-volume concentration of 1.5mM. The phosphorescence kinetics of the tethered porphyrins was measured to quantify quenching by rubrene emitter. Upconverter performance was measured in a solar cell enhancement experiment.

Upconversion of low-energy photons presents a possibility to overcome the Shockley-Queisser efficiency limit for solar cells. In silicon 20% of the incident energy is lost due to transmission of these photons with energies below the band gap. Unfortunately, upconversion materials known today show pretty low absorption and quantum yields which are too low for this application. One possibility to boost the upconversion luminescence and even the quantum yield could be the embedding of the material in a suitable photonic structure environment. This influences the local irradiance onto the upconverter and the local density of states at the transition wavelengths. Thus, the radiative recombination from a specific energy level can be influenced. Hence, this approach has the potential to beneficially influence the upconversion quantum yield. For the buried grating structure shown here, a luminescence enhancement by a factor of 1.85 could be achieved, averaged over the grating.

A smart light trapping scheme is essential to tap the full potential of polycrystalline silicon (poly-Si) thin-film solar cells. Periodic nanophotonic structures are of particular interest as they allow to substantially surpass the Lambertian limit from ray optics in selected spectral ranges. We use nanoimprint-lithography for the periodic patterning of sol-gel coated glass substrates, ensuring a cost-effective, large-area production of thin-film solar cell devices. Periodic crystalline silicon nanoarchitectures are prepared on these textured substrates by high-rate silicon film evaporation, solid phase crystallization and chemical etching. Poly-Si microhole arrays in square lattice geometry with an effective thickness of about 2μm and with comparatively large pitch (2 μm) exhibit a large absorption enhancement (A900nm = 52%) compared to a planar film (A900nm ~ 7%). For the optimization of light trapping in the desired spectral region, the geometry of the nanophotonic structures with varying pitch from 600 nm to 800 nm is tailored and investigated for the cases of poly-Si nanopillar arrays of hexagonal lattice geometry, exhibiting an increase in absorption in comparison to planar film attributed to nanophotonic wave optic effects. These structures inspire the design of prospective applications such as highly-efficient nanostructured poly-Si thin-film solar cells and large-area photonic crystals.

We describe results of our investigations of the structural, optical, and electronic properties of PbS-QD films fabricated using layer-by-layer dip coating based on 1,2-ethanedithiol as an insolubilizing agent. Our investigations extend to a study of the photovoltaic properties of heterojunction thin film solar cells fabricated by sputter-deposition of a CdS ntype thin film followed by deposition of a PbS-QD thin film. Our CdS/PbS-QD solar cells exhibit open circuit voltage in excess of previously reported PbS-QD solar cells. Under standard simulated AM1.5G illumination, we observe short circuit current density as high as 12 mA cm^-2, open circuit voltage as high as 0.65 V, and a maximum efficiency of 3.3%.

In this work we introduce the main principles behind efficient light trapping in silicon nanowire structures. The ultimate performance of vertical periodic crystalline silicon nanowire arrays has been determined and optimized values have been presented as a function of the nanowire length. The further improvement of the performance has been demonstrated using dual-diameter periodic silicon nanowire arrays where the already optimized nanowire structure has been used as the starting point. The improved efficiency of this densely packed structure has been compared with the reference flat films in order to evaluate theoretical improvement of the light trapping efficiency. In the last part of our work we present the efficient light trapping inside amorphous silicon nanowire based radial junction solar cells fabricated using plasma enhanced vapor-liquid-solid process.

We show that with only one micron, equivalent bulk thickness, of crystalline silicon, sculpted into the form of a slanted conical-pore photonic crystal and placed on a silver back-reflector, it is possible to attain a maximum achievable photocurrent density (MAPD) of 35.5mA/cm2 from impinging sunlight [1]. This corresponds to absorbing roughly 85% of sunlight in the wavelength range 300-1100nm and exceeds the Lambertian limit suggested by previous “statistical ray trapping” arguments. When the silicon volume is reduced to an equivalent thickness of only 380nm, the MAPD remains as high as 32mA/cm2. This suggests the possibility of very high efficiency, ultra-thin-film silicon solar cells. Our one-micron structure consists of a photonic crystal square lattice constant of 850nm and slightly overlapping inverted cones with upper (base) radius of 500nm and 1600nm cone depth. When the solar cell is packaged with silica (each pore filled with SiO₂ and modulation on the top is added), the MAPD in the wavelength range of 400-1100nm becomes 32.6mA/cm² still higher than the Lambertian 4n² benchmark of 31.2mA/cm². Thinner structures are considered by keeping the lattice constant and cone radius fixed but by decreasing the cone depth. The MAPD dependence on the overall depth of nanopores indicates that using roughly half the amount of silicon leads to only about 5% drop in the MAPD. In the near infrared regime light is absorbed within slow group velocity modes, that propagate nearly parallel to the interface and exhibit localized high intensity vortex-like flow in the Poynting vector-field.

We explore an approach to enhance the efficiency of solar cells using photonic nanostructures for solar ther-mophotovoltaics. Our focus is on designing photonic nanostructures that can provide broadband absorption in a narrow angular range for solar thermophotovoltaic systems which do not employ sunlight concentration. We consider structures consisting of an aperiodic multilayer stack of alternating layers of silicon and silica on top of a thick tungsten layer. The layer thicknesses are optimized to maximize the angular selectivity in the absorp-tivity for both TE and TM polarizations. Using such an approach, we design structures with highly directional absorptivity for both polarizations.

Although the concept of using optical rectenna for harvesting solar energy was first introduced four decades ago, only recently has it invited a surge of interest, with dozens of laboratories around the world working on various aspects of the technology. An optical rectenna couples an ultra-high-speed diode to a submicron antenna so that the incoming radiation received by the antenna is rectified by the diode to produce a DC power output. The result is a technology that can be efficient and inexpensive, requiring only low-cost materials. Conventional classical rectification theory does not apply at optical frequencies, necessitating the application of quantum photon-assisted tunneling theory to describe the device operation. At first glance it would appear that the ultimate conversion efficiency is limited only by the Landsberg limit of 93%, but a more sober analysis that includes limitation due to the coherence of solar radiation leads to a result that coincides with the Trivich-Flinn limit of 44%. Innovative antenna designs are required to achieve high efficiency at frequencies where resistive losses in metal are substantial. The diode most often considered for rectennas make use of electron tunneling through ultra-thin insulators in metal-insulator-metal (MIM) diodes. The most severe constraint is that the impedances of the antenna and diodes must match for efficient power transfer. The consequence is an RC time constant that cannot be achieved with parallel-plate MIM diodes, leading to the need for real innovations in diode structures. Technologies under consideration include sharp-tip and traveling-wave MIM diodes, and graphene geometric diodes. We survey the technologies under consideration.

We have previously presented a method for optical rectification that has been demonstrated both theoretically and experimentally and can be used for the development of a practical rectification and energy conversion device for the electromagnetic spectrum including the visible portion. This technique for optical frequency rectification is based, not on conventional material or temperature asymmetry as used in MIM or Schottky diodes, but on a purely geometric property of the antenna tip or other sharp edges that may be incorporated on patch antennas. This “tip” or edge in conjunction with a collector anode providing connection to the external circuit constitutes a tunnel junction. Because such devices act as both the absorber of the incident radiation and the rectifier, they are referred to as “rectennas.” Using current nanofabrication techniques and the selective Atomic Layer Deposition (ALD) process, junctions of 1 nm can be fabricated, which allow for rectification of frequencies up to the blue portion of the spectrum (see Section 2).

Efficiency bounds for the rectification (AC to DC conversion) efficiency of non-coherent broadband radiation are derived, motivated by determining a basic limit for solar rectifying antennas. The limit is shown to be 2/π for a single full-wave rectifier. We also derive the increase in rectification efficiency that is possible by cascading multiple rectifiers. The approach for deriving the broadband limit follows from an analysis of sinusoidal signals of random phase. This analysis is also germane for harvesting ambient radio-frequency radiation from multiple uncorrelated sources.

We combine nanolaminate bilayer insulator tunnel barriers (Al₂O₃/HfO₂, HfO₂/Al₂O₃, Al₂O₃/ZrO₂) deposited via atomic layer deposition (ALD) with asymmetric work function metal electrodes to produce MIIM diodes with enhanced I-V asymmetry and non-linearity. We show that the improvements in MIIM devices are due to step tunneling rather than resonant tunneling. We also investigate conduction processes as a function of temperature in MIM devices with Nb₂O₅ and Ta₂O₅ high electron affinity insulators. For both Nb₂O₅ and Ta₂O₅ insulators, the dominant conduction process is established as Schottky emission at small biases and Frenkel-Poole emission at large biases. The energy depth of the traps that dominate Frenkel-Poole emission in each material are estimated.

Transparent conductors (TCs) are becoming extremely popular in many different electronic applications such as touch panels, displays, light emitting devices, light sensors and solar cells. The commonly used electrode in these applications is Indium Tin Oxide (ITO). However, the cost of ITO is increasing rapidly due to the limited supply of Indium. Other issues such as lack of flexibility and cost of the deposition process make ITO less favorable in transparent electrode applications. Graphene has been under exploration as an alternative material for TC applications in the recent years. Graphene based TCs have been shown experimentally to exhibit promising electrical and optical properties. In this paper, the prospects of graphene for transparent conductors in photovoltaics are discussed. The recent advancements in this field as well as the theoretical predictions and possible pathways for improvements are presented. In the process section, we discuss methods to synthesize few-layer graphene (FLG) with high quality in a controllable manner.

We present biomimetic antireflective AlInP nanostructures fabricated by inductively coupled plasma etching using Ag etch masks, which were easily formed by spin-coating Ag ink and subsequent sintering process on a hotplate without any lithography process and complicated equipments, for compound semiconductor based solar cell applications. This lithography-free technique is a simple, cost-effective, and high throughput method. The fabricated AlInP nanostructures demonstrated drastically reduced the hemispherical reflectance and solar-weighted reflectance (SWR) compared to that of bulk AlInP in the wavelength range of 300-870 nm. The incident angle-dependent SWR of the AlInP nanostructures remained below 4% up to an incident angle of 50°. Therefore, the biomimetic antireflective AlInP nanostructures fabricated by using the lithography-free method hold great potential for use in compound semiconductor based solar cell applications.

We investigated the upconversion and downconversion luminescence in (Tb³⁺, Yb³⁺) and (Tb³⁺, Yb³⁺) co-doped lithiumlanthanum- aluminosilicate oxyfluoride glass. Upon excitation at 980 nm, where crystalline CdTe solar cells no longer absorb, the sub-bandgap photons can be converted to the higher-energy ones via upconversion. In addition, under excitation between 470 nm and 490 nm, one blue photon might be split up to two near-infrared ones via downconversion. The downconversion luminescence matches the spectral response of crystalline Si solar cell well. We observed much more intense upconversion luminescence from (Tm3+, Yb³⁺) codoped glass than that from ( Tb³⁺, Yb³⁺) codoped glass under the same 980 nm excitation conditions. Our results indicate that the sequential energy transfer from Yb³⁺ ions to Tm3+ ions is much more efficient than the cooperative energy transfer from Yb³⁺ ions to Tb³⁺ ions.

We investigated optical properties of planar Si wafers and Si microwire (MW) arrays with and without ZnO thin films using the finite-difference time-domain (FDTD) method. Reflectance of the MW array (diameter: 4 μm and period: 12 μm) was smaller than that of the planar wafer in the wavelength range from 400 to 1100 nm, which could be originated from antireflection effects due to low optical density and guided-mode-assisted field enhancement. The reflectance of ZnO (thickness: 50 and 80 nm)-coated MW array was drastically reduced compared with the bare array but somewhat larger than that of the coated planar wafer. This could be attributed to less-confined guided modes in the wires, which was supported by the field distribution simulation results. Our results provide some insights into possible roles of transparent conducting layers on MW arrays for photovoltaic applications.

Solar energy has been sought as one of the prominent candidates among the energy harvesting methods. The energy conversion efficiency of solar cell is limited by its ability of harvesting energy from limited range in solar energy spectrum. We approach this issue by using the down-conversion effect with conventional CdSe quantum dots (QDs), increasing probability of electron-hole generation in designated solar cell. In our study, we fabricated GaAs single junction solar cells and applied QDs for down-conversion. We examine the effects of such application on the solar cell properties with various methods including TR-PL technique.

California’s aggressive energy policy goals include a Renewable Portfolio Standard, RPS, which requires 33% of energy produced must come from renewable resources by 2020. Additionally, SDGE as of the end of March 2013 has approximately 23,000 installations of rooftop photovoltaics with a nameplate capacity 171 MW that do not count towards the RPS goal. Renewable resources are intermittent power generators that introduce new challenges to planning, designing and operating a utility grid. SDGE has been developing solutions to these challenges and will discuss this topic further in the plenary talk.

There is an increased interest in exploiting photophysical phenomena to enhance the performance of photovoltaic technologies, pushing their efficiency toward, and ultimately beyond, the Shockley-Queisser limit for a single-junction solar cell. This presentation will discuss the applicability of molecular chromophores to this endeavor, focusing on two well-known phenomena: (1) photochemical upconversion and (2) singlet fission. Recent discoveries have demonstrated that these ‘scientific curiosities’ exhibit significant promise for solar photoconversion applications. I will outline the mechanisms that underlay these two photon harvesting processes; highlight scientific questions that remain to be answered; and identify strategies for, and obstacles to, their incorporation into realistic photoconversion systems.