This PDF file contains the front matter associated with SPIE Proceedings Volume 9562, including the Title Page, Copyright information, Table of Contents, Authors, and Conference Committee listing.

Photochemical upconversion has been put forward as a candidate technology to improve the light-harvesting capabilities of thin-film photovoltaic cells, by harvesting transmitted sub-bandgap light and re-radiating the absorbed energy at a usable wavelength. Efficiencies of 10% have been observed under solar-level irradiation, and up to 86% (quantum yield of 43%) has been observed under strong irradiance. In this proceeding, we explain the triplet-triplet annihilation mechanism underlying photochemical upconversion and delve into the chemical kinetics to extract strategies to improve device performance. We suggest that one of these strategies, concentrating the sensitizer species, may be flawed without proper consideration of the sensitizer identity, due to enhanced emitter triplet decay caused by the external heavy atom effect.

We suggest a new paradigm for solar cells that uses a nanostructured crystalline collector (silicon) in an amorphous absorber matrix (hydrogenated amorphous silicon). Previously amorphous absorbers have received no serious consideration because of their low carrier mobilities. Specifically, we demonstrate that carriers generated in the amorphous region are transported out of this region before losing their energy to heat. This result establishes the possibility of using a wide range of nanostructured amorphous matrices to dramatically increase the efficiencies of solar cells. The use of an amorphous absorber provides a highly desirable and flexible approach to producing low-cost, hot carrier solar cells. Since amorphous materials can be grown over a much wider composition space than crystalline materials, this surprising result greatly broadens the absorbing materials that can be used to dramatically increase the efficiencies of solar cells.

A study of the photovoltaic properties of the GaAs-based solar cells with InGaAs quantum wire had been conducted. The research included the investigation of the photovoltage rise and decay transients, spectral photovoltage dependences at different temperatures. The objects investigated were GaAs-based solar cells with InGaAs quantum wire (QWR) embedded into space-charge-region of p-i-n junction. Samples with different In content and size of InGaAs nanoobjects had been created using molecular beam epitaxy. Unlike the reference cell, the ones containing the InGaAs QWR had shown higher sensitivity in the energy range 1.2 - 1.38 eV. This is caused by the spatial separation of electron-hole (e-h) pairs excited in the QWR due to band-to-band transition. Under selective excitation of the e-h pairs only in the InGaAs quantum wire the photovoltage rise transient is slower compared to the e-h generation in GaAs. This effect is explained by charge carriers release from the InGaAs quantum well into delocalized states of the surrounding GaAs. It was determined that the InGaAs quantum wires increase the recombination rate of the non-equilibrium carriers in the temperature range 80 to 290 K, which means that the quantum wires are the additional recombination centers.

One potential way to increase photovoltaic efficiency is to take advantage of hot-carriers. Nanocrystal based solar cells aim to take advantage of hot-carrier capture to boost device performance. The crucial parameter for gauging a given nanocrystal material for this application is the electron-phonon coupling. The electron-phonon coupling will dictate the thermalization time of hot-carriers. In this study we demonstrate a method of quantifying the electron-phonon coupling in semiconductor nanocrystals. By employing ultrafast transient absorption spectroscopy with temporal pulse shaping, we manipulate coherent phonons in CdTe_{1-x}Se_{x} nanocrystals to quantify the efficiency of the electron-phonon coupling. The Raman active longitudinal optical phonon (LO) modes were excited and probed as a function of time. Using a temporal pulse shaper, we were able to control pump pulse pairs to coherently excite and cancel coherent phonons in the CdTe_{1-x}Se_{x} nanocrystals, and estimate the relative amount of optical energy that is coupled to the coherent CdSe LO mode which is the dominant thermalization pathway for the hot-electrons in this system.

The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalize to the band edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 65% under 1 sun and 86% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the required slowing of carrier cooling. A number of materials and structures are being investigated with these properties and test structures are being fabricated. Initial measurements indicate slowed carrier cooling in III-Vs with large phonon band gaps and in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the way for their development to fully functioning high efficiency solar cells.

Colloidal quantum dot heterojunction thin film solar cells (CQD-TFSC) utilize facile thin film deposition methods and promise high photon conversion efficiencies (PCE) to cost ratio which is highly desired for commercialization. So far, surface passivated PbS CQD-TFSCs show the highest PCE results, reaching 9.2% with good stability. Among other potential candidates, CuInSe2 CQDs stand out as a non-toxic material with high potential for performance, judging on bulk Cu(Ga,In)(S,Se)2 TFSCs reaching 20% PCE, with high stability. CuInSe2 CQDs has advantage over bulk films, mainly the much less expensive manufacturing cost of uniform deposition on large areas. Ga is known to cause phase separation in the bulk CIGS system. In a CQD form, CuInSe2 band gap can be tuned between 1 to 1.6 eV by quantum confinement without need for Ga and this eliminates the phase separation issue. Within our best knowledge, there are no reports on surface trap passivated CuInSe2 CQD-TFSCs. However Cu(In,Ga)(S,Se)2 colloidal particles were cast in thin film form and fused to form bulk-like crystals by various annealing conditions for solar cell devices. In this work, we investigated well-passivated CuInSe2 CQDs on n-type TiO2 and ZnO layers to form depleted heterojunction structure. We prepared luminescent CuInSe2 CQDs by synthetic wet chemistry methods and passivated the surface with 3-mercaptopropionic acid or tetrabutylammonium iodide using solid-state ligand exchange. X-ray photoelectron spectroscopy was used to confirm the ligand boding and surface coverage of the quantum dots. We will present the effect of synthesis and thin film preparation conditions on the solar cell device performance

The luminescent properties of InGaAs/GaAs heterostructures with InGaAs nanoscale objects were investigated. Multilayer heterostructures were grown using molecular beam epitaxy technique. The shapes of the photoluminescence spectra were studied in the temperature range from 10 K to 290 K. The electronic spectrum of heterosystems as well as the energy of interband transitions for InGaAs nano-objects were calculated for different sizes and InGaAs component composition. It is shown that the shape of the photoluminescence spectra is determined by the Gaussian distribution of the energy of band-to-band optical transitions between the ground states of the conduction band and valence band of nanoscale objects. The physical reason for the observed energy dispertion is the variation of sizes, heterogeneity of component composition and strain relief in the ensemble of InGaAs nano-objects. Non-monotonous temperature dependence of the width of the photoluminescence spectra indicates the existence of temperature-dependent redistribution of photoexcited charge carriers between neighbouring nanoislands having different energy of the ground states.

Solid-state energy upconversion has many potential applications, from nonlinear photonics and biophotonics to expanding the spectrum available for solar energy harvest. In organic molecular systems, upconversion is frequently done in solution to mitigate aggregation-induced photoluminescence quenching or to facilitate the diffusion of triplet donors in Triplet-Triplet Annihilation (TTA) systems. Here we demonstrate an organic thin film upconversion system utilizing two-photon absorption (TPA) properties to improve upconversion efficiency. In blend films of Stilbene-420 and Rhodamine 6G we observe a tenfold increase in up-converted fluorescence compared to the fluorescence yield of TPA in pristine stilbene films. While TPA normally has quadratic dependence on excitation intensity, these blend films exhibit sub-quadratic intensity dependence, indicating a combination of linear and quadratic upconversion processes and dramatically improving upconversion efficiency at lower excitation intensities. This improvement in intensity dependence allows for relatively efficient upconversion upon excitation by a nanosecond laser pulse, in contrast to the more expensive femtosecond lasers generally required for excitation in TPA microscopy and similar systems. Time-resolved photoluminescence decay measurements reveal that all excited states involved in this upconversion process are singlets, which indicates the potential for reduced energy losses when compared to TTA upconversion systems and their inherent intersystem-crossing energy losses. We observe emission from both the Rhodamine 6G donor molecules and Stilbene-420 acceptor molecules, indicating the presence of prompt fluorescence from the donor as well as upconversion to the acceptor, and FRET losses from acceptor back to donor. By fitting to a kinetic model we extract rates for these competing processes.

We have theoretically and experimentally investigated the effects of Ag-grating electrode on the performance of polymer:fullerene based bulk heterojunction organic solar cells. First, an integrated numerical model has been developed, which is capable of describing both the optical and the electrical properties simultaneously. The Ag-grating patterned back electrode was then designed to enhance the absorption in sub-bandgap region of P3HT:PCBM binary devices. Laser interference lithography and metal lift-off technique were adopted to realize highly-uniform and large-area nanograting patterns. We measured almost 5 times enhancement of the external quantum efficiency at the surface plasmon resonance wavelength. However, the overall improvement in power conversion efficiency was not significant due to the low intrinsic absorption of active layer in this sub-bandgap region. We, then, investigated about the effect of surface plasmon on the ternary device of P3HT:Si-PCPDTBT:ICBA. It was demonstrated that the infrared absorption by the Si-PCPDTBT sensitizer can be substantially enhanced by matching the surface plasmon resonance to the sensitizer absorption band. Besides, we also observed an additional enhancement in the visible range which is due to the scattering effect of the gratings. An overall short-circuit current enhancement of up to 40% was predicted numerically. We have then fabricated the device by the lamination technique and observed a 30% increase in the short circuit current. Plasmon enhancement of sensitized organic solar cell presents a promising pathway to high-efficiency, broadband-absorbing polymer:fullerene bulk heterojunction organic solar cells.

This work will discuss our recent advances focused on integrating high power energy storage directly into the native materials of both conventional photovoltaics (PV) and dye-sensitized solar cells (DSSCs). In the first case (PV), we demonstrate the ability to etch high surface-area porous silicon charge storage interfaces directly into the backside of a conventional polycrystalline silicon photovoltaic device exhibiting over 14% efficiency. These high surface area materials are then coupled with solid-state ionic liquid-polymer electrolytes to produce solid-state fully integrated devices where the PV device can directly inject charge into an on-board supercapacitor that can be separately discharged under dark conditions with a Coulombic efficiency of 84%. In a similar manner, we further demonstrate that surface engineered silicon materials can be utilized to replace Pt counterelectrodes in conventional DSSC energy conversion devices. As the silicon counterelectrodes rely strictly on surface Faradaic chemical reactions with the electrolyte on one side of the wafer electrode, we demonstrate double-sided processing of electrodes that enables dual-function of the material for simultaneous energy storage and conversion, each on opposing sides. In both of these devices, we demonstrate the ability to produce an all-silicon coupled energy conversion and storage system through the common ability to convert unused silicon in solar cells into high power silicon-based supercapacitors. Beyond the proof-of-concept design and performance of this integrated solar-storage system, this talk will conclude with a brief discussion of the hurdles and challenges that we envision for this emerging area both from a fundamental and technological viewpoint.

Due to large variation of the solar energy availability in a day, energy storage is required in many applications when solar cells are used. However, application of external energy storage devices, such as batteries and supercapacitors, increases the cost of solar energy systems and requires additional charging circuitry. This combination is bulky and relatively expensive, which is not ideal for many applications. In this work, a novel idea is presented for making electrochemical devices with dual properties of solar energy harvesting and internal charge storage. The device is essentially a supercapacitor with a photoactive electrode. Energy harvesting occurs through light absorption at one of the electrodes made of a composite of a conducting polymer (i.e. PEDOT:PSS) and a Porphyrin dye. The energy storage takes place in the both photoactive and counter electrode (CE). We have studied the effect of the CE material on the device characteristics. Using Y-Carbon (a commercial available electrode), an open circuit voltage of 0.49 V was achieved in light across the cell with ~1 mF capacitance. The other two choices for CE were activated carbon and carbon nanotube based electrodes. The cyclic voltammetry and impedance spectroscopy demonstrated that the Y Carbon electrode was a better match.

We designed and studied a radial junction composed by a photovoltaic and thermoelectric array based on ZnO and CdTe nanowires surrounded by an absorbing organic self assembled in order to efficiently convert UV-visible and IR energy into electricity. The hot anode of n-type ZnO nanowires was fabricated using a thermal process on pre-seeded layer and results to be crystalline with a transmittance up to 92 % and a bandgap of ~ 3.32 eV. Conductivity measurements reveal diode-like behavior for the ZnO nanowires. The organic layer was deposited between the anode and cathode at room temperature The organic layer is composed of oppositely charged porphyrin metal (Zn(II) and Sn(IV)(OH)2) derivatives that are separately water soluble, but when combined form a virtually insoluble solid. The electron donor/acceptor properties (energy levels, band gaps) of the solid can be controlled by the choice of metals and the nature of the peripheral substituent groups of the porphyrin ring. A defect free sub nanometer deposition was achieved using a layer-by-layer deposition onto both ZnO and Bi2Te3 nanowires. The highly thermoelectric structure, which acts as a cold cathode, is composed of p-type Bi2Te3 nanowires with a thermoelectric efficiency (ZT) between ~0.7 to 1, values that are twice that expected for bulk Bi2Te3. Optoelectronic and structural properties shows that with 6 nm of organic layer it is possible to form a 3% efficient solar device with an enhanced thermo electric effected with a temperature gradient of 300 C.

Recently, lead halide perovskites which are organic-inorganic hybrid structures, have been discovered to be highly efficient as light absorbers. Herein, we show the investigation of the excited state dynamics and emission properties of non-stoichiometric precursor formed lead halide perovskites grown by interdiffusion method using steady-state and time-resolved spectroscopic measurements. The influence of the different ratios of the non-stoichiometric precursor solution was examined. The observed photoluminescence properties were correlated with the femtosecond transient absorption measurements.

GaSb thermophotovoltaic (TPV) devices were fabricated using a Molecular Beam Epitaxy (MBE) technique. Different emitter thicknesses (de) were studied to maximize the TPV cell’s short circuit current density. In this regard, the fabricated TPV device’s emitter was incrementally wet-etched and characterized to find the optimal thickness value. Simulations were performed using the Crosslight APSYS® platform over the full-spectrum range in order to predict device performance for different designs, while maximizing the photocurrent generation and enhancing the emitter sheet resistance. TPV devices were characterized electrically and optically. These experimental data showed that the etched emitter has minimal impact on the measured short circuit current density (J_sc) while simulated results demonstrated an optimal d_e of 200 nm.

Clean-burning binder systems used in paste formulations for front side solar cell applications offer advantages of reduced residual carbon, improved conductive feature density, and overall performance and reliability. This paper presents the technical advantages of employing polyalkylene carbonates (QPAC®) as the principle binder for paste formulations used in front side solar screen printing applications. Thermal and rheological characteristics are presented and compared with standard or conventional pastes currently employed in production lines producing solar cell front side geometry. Microstructural comparisons of conductive features of the front side geometry are examined and related to aspects of adhesion performance and resistive losses.

Optical properties of silicon nanocrystals dispersed in SiO₂ matrix were investigated in terms of photoluminescence quantum yield at room temperature. Two multilayer samples, prepared from substoichiometric silicon oxide layers by annealing at 1150°C were used to investigate the influence of Si concentration. Significant reduction of photoluminescence quantum yield and a very specific change of its excitation energy dependence upon variation of silicon excess are concluded from the experimental data. Possible mechanisms leading to these changes are discussed.

Recently thermo-electrical nanoantennas, also known as Seebeck nanoantennas, have been proposed as an alternative for solar energy harvesting applications. In this work we present the optical and thermal analysis of metallic nanoantennas operating at infrared wavelengths, this study is performed by numerical simulations using COMSOL Multiphysics. Several different nanoantenna designs were analyzed including dipoles, bowties and square spiral antennas. Results show that metallic nanoantennas can be tuned to absorb electromagnetic energy at infrared wavelengths, and that numerical simulation can be useful in optimizing the performance of these types of nanoantennas at optical and infrared wavelengths.