This PDF file contains the front matter associated with SPIE Proceedings Volume 7047, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Photovoltaics (PV) technology is currently enjoying substantial growth and investment, owing to worldwide sensitivity to energy security and the importance of renewable energy as a means to mitigate carbon emissions. There are many options in photovoltaic cell design and fabrication, but the key performance metric is the cost per Watt of PV-generated electricity. While solar cells are semiconductor devices like integrated circuits, the processing cost/area must be several orders of magnitude less expensive than for microelectronic integrated circuit chip processing. Thus while most current solar cell manufacturing is done with crystalline silicon wafers, the future of photovoltaics could see the large-scale development of inexpensive thin film and nano-structured devices and processes. To achieve substantial market growth to the point where photovoltaics is able to seriously contribute to the overall energy supply, high solar conversion efficiency will be critical, as will be the use of earth-abundant materials in thin film cells. This paper surveys several promising new approaches to ultrahigh efficiency thin film multi-junction solar cells, options for earth-abundant materials, semiconductor nanowire-based solar cells and plasmonic structures for enhanced light absorption that open up new design approaches to very thin photovoltaic devices.

We describe experimental and theoretical analysis of coupling of light scattered by metal or dielectric nanoparticles into waveguide modes of InP/InGaAsP quantum-well solar cells. The integration of metal or dielectric nanoparticles above the quantum-well solar cell device is shown to couple normally incident light into lateral optical propagation paths, with optical confinement provided by the refractive index contrast between the quantum-well layers and surrounding material. Photocurrent response spectra yield clear evidence of scattering of photons into the multiple-quantum-well waveguide structure, and consequently increased photocurrent generation, at wavelengths between the band gaps of the barrier and quantum-well layers. With minimal optimization, a short-circuit current density increase of 12.9% and 7.3% and power conversion efficiency increases of 17% and 1% are observed for silica and Au nanoparticles, respectively. A theoretical approach for calculating the optical coupling is described, and the resulting analysis suggests that extremely high coupling efficiency can be attained in appropriately designed structures.

For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest as much as possible of the available sunlight. Typically, one uses randomly textured front TCOs to scatter the light diffusively in pin-cells on glass. Here, we investigate methods to texture the back contact with both random and periodic textures for use in nip-cells on opaque foil. We applied an electrically insulating SiOx-polymer coating on a stainless steel substrate, and textured this barrier layer by embossing. On this barrier layer the back contact is deposited for further use in the solar cell stack. Replication of stamps with various random and periodic patterns was investigated, and, using scanning electron microscopy, replicas were found to compare well with the originals. Masters with U-grooves of various submicrometer widths have been used to investigate the optimum dimensions of regular patterns for light trapping in the silicon layers. Angular reflection distributions were measured to evaluate the light scattering properties of both periodic and random patterns. Diffraction gratings show promising results in scattering the light to specific angles, enhancing the total internal reflection in the solar cell.

We report fabrication of size-controlled plasmonic nanoparticle arrays by which optically thin GaAs single junction solar cells are decorated. Ordered Ag and Al nanoparticles with average diameters of 60-150 nm and interparticle spacings of 100-300 nm were templated onto the window layers of the GaAs solar cells using nanoporous anodic aluminum oxide membrane templates. Near the surface plasmon resonances, 60nm-diameter Ag and Al nanoparticles serve as light-absorbers so that non-radiative surface plasmon resonances reduce the photocurrent of the cells, which is improved by increasing the nanoparticle size. Photocurrent enhancements are seen at wavelengths longer than surface plasmon resonance which is maximized near the band gap edge of GaAs. These enhancements can be attributed to the increased optical path in the photovoltaic layers resulting from multi-angle scattering by the nanoparticles, while high scattering efficiency nanoparticles in turn increase the back scattering light out of the cell reducing the photocurrent.

Intermediate-band (IB) solar cells were predicted to have the photovoltaic (PV) efficiency exceeding the Shockley-Queisser limit for a single junction cell. A possible practical implementation of the IB solar cells can be based on the quantum dot superlattices (QDS). The parameters of such QDS structure have to be carefully tuned in order to achieve the desired charge carrier dispersion required for the IB solar cell operation. We used the first-principle theoretical models for calculating the carrier states and light absorption in QDS. This approach allowed us to determine the actual dimensions of the quantum dots and the inter-dot spacing for inducing the carrier miniband in the band-gap region where the miniband can play the role of the IB. Using the Shockley-Queisser detailed balance theory we determined that the upper-bound PV efficiency of such IB solar cells can be as high as ~ 51%. The required QDS dimensions for the IB implementation on the basis of InAsN/GaAsSb are technologically challenging but feasible: ~ 2 - 6 nm. The proposed computational design approach may help with implementation of other solar cell concepts for advanced light-to-energy conversion enabled by nanostructures.

Copper indium selenide (CIS) or its derivatives (such as gallium doped CIS and sulfur substituted CIS) are considered the best optical absorber material used in polycrystalline thin film photovoltaic solar cells due to their favorable electrical and optical properties, and long term stability. To develop a low cost yet high throughput thin film deposition process with both composition and film uniformity control, precursor ink has been formulated using nanoparticle metal oxide of copper and indium in an organic solvent system dissolved with selenium or sulfur. Smooth thin film of precursor oxide mixture has been demonstrated by wet printing process. Upon heat treatment of the precursor thin film under atmosphere of selenium and/or sulfur, copper-indium selenide and/or sulfide (CIS) was formed. Several approaches of nanoparticle ink coating processes have been investigated through spin-coating, screen-printing and contact printing. For using glass substrate, contact printing demonstrated superior uniformity and composition control. By using a post-thermal treatment process on the nanoparticle-coated film, good morphology thin film with composition control was achieved. Both the chemical composition and physical morphology has been investigated using ICP-OES and XRD measurements. Based on molybdenum glass substrate, all-printed solar cells have been demonstrated.

We demonstrate the method of transferring aligned single crystal silicon nanowires (SiNWs) to transparent substrate. The alignment of the transferred nanowires is almost identical to the original one. The density of the transferred SiNWs can achieve 3×10⁷ nanowires/mm². The low temperature fabrication processes are compatible for a wide range of substrates. The transmission coefficient below 10 % at a wide bandwidth, 400-1100 nm, was found in the transferred SiNWs. The high dense aligned SiNWs are promising for future photovoltaic applications.

We have fabricated bulk heterojunction photovoltaic (PV) cells using a perfluoropolyether (PFPE) elastomeric stamp to control the morphology of the donor-acceptor interface within devices. Devices were fabricated using the Pattern Replication In Non-wetting Templates (PRINT) process to have nanoscale control over the bulk heterojunction device architecture. The low-surface energy, chemically resistant, variable modulus, fluoropolymer based molds used in PRINT provide a route to patterning, with nanometer resolution, general polymeric donor materials such as polythiophene and polyphenylenevinylene derivatives and 'hard' inorganic oxide structures typically used as acceptor materials in hybrid organic solar cells such as TiO₂, ZnO, and CdSe. This "top-down" approach allows for patterning over large areas and for the functionalization of the donor/acceptor interface. Specifically, nanostructured anatase titania with post-like features ranging from 30-100 nm in diameter and 30-65 nm in height was fabricated to form the ordered bulk heterojunction of a titania-poly(3-hexylthiophene) (P3HT) PV-cell. Nanostructured devices showed a two-fold improvement in both short-circuit current (J_sc) and power conversion efficiency (PCE) relative to reference bilayer cells. Additionally, we will discuss devices fabricated with other organic and inorganic materials in order to investigate the effect on cell performance of controlling the nanoscale architecture of the bulk heterojunction via patterning.

Polythiophene films can be electrodeposited on modified ITO substrates, textured to increase their active surface area, doped to enhance charge transport, and then interfaced with C₆₀ thin films to create "planar heterojunction" photovoltaic devices with power conversion efficiencies up to 1%. Preliminary results indicate that these electrodeposited films (e-P3HT) modified with appropriate ligands can serve as hosts for semi-conducting nanoparticles (CdSe NPs). These NPs may ultimately extend the device spectral sensitivity into the red and near-IR spectral regions.

We report on the realization of high-efficiency bulk heterojunction PV devices based on P3HT/PCBM on transparent plastic substrates, from one elementary cell to large area modules, and we compare with results obtained on glass. The first target consists in the optimisation of the processing parameters in order to obtain the highest possible Power Conversion Efficiency (PCE) values for individual cells. We have reached PCE close to 4% with small dispersion on plastic substrates for cells of 0.28 cm² active area, compared to 5% on glass. Modules of multiple cells are then elaborated on 5x5 cm substrates with a design aimed to minimize ohmic losses, and interconnection resistances. For glass module, with 12 individual cells on a 5x5 cm² substrate we obtain PCE of 3.26 % (12.4 cm² active surface). Larger modules with active area up to 35 cm² exhibiting PCE of 2.8 % and open circuit voltage higher than 6V are also demonstrated for glass, approaching the requirements for commercial electronic applications. On PET, record efficiency of 2.85 % is obtained for a 8.8 cm² module and PCE of 2.52 % is demonstrated for a large area module with 53 cm² active surface. The influence of the geometric parameters of the individual cells and their type of connection (parallel or series) on the module characteristics is also discussed.

The use of semiconductor nanowires for photovoltaic applications is advantageous for several reasons: 1) it permits interpenetrating networks of materials for semiconductor heterojunctions at the nanoscale, allowing efficient carrier extraction following light absorption, 2) long absorption paths are possible while maintaining short distances for carrier collection, even in imperfect materials, 3) single crystal materials can be grown in relatively thin films with little material, 4) strong light trapping is possible due to the geometry of the nanowires, and 5) manipulation of materials properties is possible by varying the size of the nanostructures. These advantages must be traded off against the difficulties of fabricating devices (particularly planarization of structures), issues with recombination centers at interfaces, and the requirement of making ohmic contacts with relatively low temperature processes. The optical and electronic properties of semiconductor nanowires, nanowire arrays, and heterojunction interfaces are discussed. Recent results for photovoltaic cells based on semiconductor nanorods and nanowires are summarized, and opportunities for improvement of device characteristics are presented.

To achieve efficient carrier extraction from nanocrystal quantum dots, we introduce a novel tandem cell device using PbSe nanocrystal quantum dots and a P3HT/PCBM bulk hetero junction cell. The device is fabricated using an all solution based process. The device consists of a hydrazine treated PbSe nanocrystal photoconductive film coupled to the P3HT/PCBM bulk hetero junction cell. In this work, the photocurrent from the PbSe nanocrystal photoconductive layer, and the role of carrier multiplication at UV wavelengths, is elucidated. By using light biased spectral response measurements, we successfully demonstrate an enhancement of quantum efficiency at photon energies greater than three times the bandgap of the PbSe nanocrystals. Additionally, this tandem structured device shows a self-passivation property that provides protection from UV irradiation to the underlying polymer cell.

We present experimental and theoretical studies of a nanopatterned photonic crystal formed between the bulk heterojunction blend, poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and nanocrystalline zinc oxide (nc-ZnO). The nanopattern is fabricated using the Pattern Replication in Non-wetting Templates (PRINT) technique. We summarize the fabrication method and show how it can be used to make a highly ordered hexagonal array of photovoltaic P3HT:PCBM posts. We also discuss theoretical studies of optical absorption for the nanopattern design that result in a 22% enhancement over a conventional planar cell. Spectroscopic ellipsometry is also used to determine the optical constants of solar cell materials that are used in the optical model. Finally, we calculate the local exciton creation profile within the photoactive nanopattern to relate the nanostructured geometry to electrical performance.

The Pattern Replication In Non-wetting Templates (PRINT) technique has been extended to patterning of isolated features as well as embossed films of sub-500 nm "hard" inorganic oxides and nanocrystalline semiconductors and "soft" semiconducting polymers including TiO₂, SnO₂, ZnO, ITO, BaTiO₃, CdSe, poly(3-hexylthiophene) (P3HT), Poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), and other polythiophene derivatives. The low surface energy, chemically resistant, air permeable elastomeric perflouropolyether (PFPE) based molds allow for numerous materials to be patterned on a variety of substrates including glass, transparent conductive oxides, and thin films of conducting polymer for a wide range of electronic and optical applications. Additionally, PRINT has been employed to pattern features with aspect ratios greater than 1, deposit a second layer of features on top of an initial layer without pattern destruction, and replicate sub-100 nm sized features for photovoltaics applications. Materials and patterns generated in this work were characterized using a variety of techniques including: Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD).

In this work preparation methods for spin-cast uniform layers of cadmium selenide (CdSe) quantum dots (QDs) with specific thickness and subsequent film treatment methodologies are presented. Dimensional and lattice structures as well as the homogeneity of the nanocrystals distribution over the film thickness are studied through high resolution transmission electron microscopy (HRTEM). Ultra violet (UV) spectrometric and spectrofluorometric measurements are performed to obtain absorption-emission spectrum of the provided films. The results show that the absorption is sensitive to size of the nanocrystals and the refractive index of the medium in which they are embedded. Refractive index of the thin films is extracted using spectroscopic Ellipsometery. Results are presented on the incorporation of the CdSe in glass matrices and a graded index structure is optimized for embedment of nanocrystals. The photon conversion ability of the fabricated layer has been verified. Effect of size and glass matrix contraction on the Raman shifts of CdSe quantum dots has been also investigated. The results from such characterization methods are vital in knowing the properties of the nanocrystals, as well as in optimizing a converter layer for solar cell applications.