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24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 191-195, In this paper the preparation of Si nanocrystals within a SiC matrix using two different methods, namely a single layer and a multi layer approach is investigated. For the preparation of the single and multi layers PECVD was used. Subsequently after deposition a thermal annealing at various temperatures up to 1100°C was performed. The influence of the thermal annealing on the optical and structural properties of the single and multi layers is analysed in detail. For both layer systems the crystallization of Si and SiC was observed by GIXRD and diffraction contrast bright field and dark-field TEM imaging. As a general trend, Si crystallizes at lower temperatures than SiC. When the annealing temperature is raised the Si- C bond density increases drastically. This is shown by FTIR spectroscopy. The SiC single layer annealed at 1100°C showed a strong PL signal centred at 1.8 eV. After a remote H passivation process the PL peak is absent.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 395-397, We report on the growth and characterization of copper indium disulfide (CIS) nanorods by electrodeposition using anodized aluminum oxide (AAO) as the growth mask. The technique facilitates effective control of both the stoichiometry and the diameters of the nanorods. The AAO nano-mask was prepared using a two-step anodization process. The pore size in the anodized aluminum foil can be controlled by varying the anodization potential. Anhydrous salts of CuCl2 and InCl3 were dissolved in an aqeous solution during the electrodeposition process. Using this technique we have successfully grown CIS nanorods with diameters varying between 50 nm to 300 nm. The lengths of the nanorods were between 5 μm to 20 μm. The stoichiometry of the nanorods can be effectively controlled by varying the electrodeposition potential. The nanorods were separated from the substrates by ultrasound and were subsequently placed on a glass plate. Metallic interdigitated structures were fabricated using standard photolithographic technique to facilitate electrical contacts with the nanorods. Detailed characterizations of the structural and the optoelectronic properties of the nanorods were performed by X-ray diffraction, transmission electron microscopy and I-V measurements on the materials.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 196-199, We investigated the effect of electric field on the carrier collection process in 10 layer-stacked InAs/GaNAs quantum dot solar cells (QDSCs). The quantum efficiency (QE) measurements were done at various applied external bias voltage (Vappl) between - 1.6 V (reverse) to + 0.4 V (forward). We found that the applied bias strongly affects the QE spectral shape for QDSCs. Further, a strong electric field is required to extract the photocarriers generated in p-GaAs emitter layer across QD region. As a result, a higher ISC = 16.7 mA/cm2 was achieved for QDSC compared to ISC = 14.6 mA/cm2 for a homojunction GaAs SC, but under a sufficiently strong electric field of > 20 kV/cm.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 218-221, Target of the Japanese Innovative Photovoltaics R&D Program started from fiscal year 2008 is to develop high efficiency solar cells with conversion efficiency of more than 40% and low electricity cost of less than 7 JPY/kWh until 2050. In this program, the concentrator III-V compound multi-junction (MJ) tandem solar cell is the most important research theme for high efficiencies of over 50% and low cost of less than 30JPY/W. InGaAsN material is good candidate for 4-6 junction solar cells. In our preliminary stage of our project, 2% efficiency has been obtained with GaAsN single-junction solar cells. In order to increase efficiency of single and 4-junction solar cells, it is very important to understand and control nitrogen incorporation, and to understand and reduce defects in the grown films. In this paper, outline of the Japanese Innovative Photovoltaics R&D Program is presented, and epitaxial grown of GaAsN films by using CBE (Chemical Beam Epitaxy) and characterization of the films by Hall effect and DLTS (Deep Level Transient Spectroscopy) measurements are also reported.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 366-368, We report on the the possible utilization of photonic crystal layers to enhance the efficiency of solar cells in various configuration, both for crystalline silicon solar cells, thin solar cells or to enhance luminescent up- or down conversion. We use a relatively simple spin-coating technique to fabricate colloidal photonic crystals with good crystalline quality with high troughput and applicable for large areas up to wafer-scale sizes, compatible to standard silicon technology.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 431-433, Hot carrier solar cell (HCSC) may be a solution to overtake the Schockley-Queisser limit for solar cell efficiency. Models for hot carrier solar cell efficiency simulation have been proposed for HCSC where carrier thermalization is neglected, giving efficiency up to 86%. Partial carrier thermalization is included here with results showing efficiencies lower than in the ideal case. This new model as input the thermalization rates that are extracted from continuous wave photoluminescence experimentsas a function of pump power. These characteristics are used to evaluate the potential efficiency of realistic cells, showing predicted maximal efficiency reaching 60% for our best sample.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 295-298, Chemical bath deposition of semiconductor thin films is by now recognized as a versatile low-cost method to produce thin films of I-VI, II-VI, III-V, IV-VI and V-VI compounds. This technique is the most cost saving, convenient and simple technique, which is highly reproducible. It makes use of the fact that films can be deposited on substrates whether metallic or not by dipping them into suitable solution baths containing metal salts without applying any external field. CdS/PbS and CdS/TlS thin films were synthesized on the glass substrate by this technique within the pores of polyvinyl alcohol (PVA) at room temperature. The chemical bath for the deposition of CdS is made up of solution of cadmium chloride (CdCl2), ammonia (NH3), thiourea ((NH2)2CS) and PVA solution. The chemical bath for the deposition of PbS thin film on clean microscope glass slide was achieved by using lead nitrate (Pb(NO3)2), sodium hydroxide (NaOH), thiourea ((NH2)2CS) and PVA solution. Thallium sulphide (TlS) thin films were deposited on glass substrate using a bath that contains thallium nitrate, sodium citrate, sodium hydroxide, thiourea and PVA solution. A chemical synthesis process for the fabrication of CdS/PbS and CdS/TlS thin films are presented herein. In this present work, these films were annealed in air at 373K and characterized for the structural, morphological and optical properties. These properties were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical spectrophotometer for their potential application in solar energy devices. The optical properties revealed the presence of direct band gaps with energies 1.20eV for PbS/CdS and 1.40eV for TlS/CdS thin films. The films show poor transmittance in the visible and near infrared region of the solar spectrum.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 285-288, We concentrate our attention on the research for p-conducting materials with high bandgap for improved integration in p-i-n solar cells. We show alternative techniques for synthesis of p-conducting materials with a wide band gap and our progress in improving these materials concerning doping and structure. We present results of the research on ptype delafossites, proved by Seebeck measurements, synthesized by sol-gel technique, in particular CuCrO2 und CuAl1-xCrxO2:Mg, and also by sputtering techniques, in particular CuAlO2 and CuCrO2. Furthermore, we could considerably lower the relative high synthesis temperature of the delafossite by systematic doping and annealing in oxygen-deficient atmosphere. Synthesis temperatures below 700 °C were reached for the first time by the sol gel route.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 207-212, In this paper different concepts to increase the efficiency of fluorescent concentrator systems are investigated. First, the combination of different fluorescent concentrator materials with spectrally matched solar cells is investigated. This approach leads to very high efficiencies up to 6.9%. Second, different photonic structures are investigated that reduce the escape cone losses of the fluorescent concentrators. These include multi-layer systems and three dimensional photonic structures.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 350-353, A near-infrared to visible upconversion phosphor (-NaYF4:Yb3+(18%), Er3+(2%)) has been applied at the back of a thin film hydrogenated amorphous silicon (a-Si:H) solar cell in combination with a white back reflector to investigate its response to subbandgap infrared irradiation. Current-voltage measurements and spectral response measurements were performed on experimental solar cells. Spectral response measurements showed a current increase of 6% for a silver back reflector and 10% for a diffuse white back reflector with respect to a cell with just a single-layer ZnO:Al back contact. An enhancement of 6.2 μA was measured under illumination with a 980 nm diode laser, at 28 mW. A part of the current was due to subbandgap absorption. Further, a close match between the non-linear behavior of the upconversion material and the EQE was found by varying the intensity of the laser light.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 434-438, Silicon carbide thin films that incorporate silicon nanocrystals are investigated. The optical band gap of the films is controlled by the deposition and annealing conditions. The band gap of as-deposited films was varied between 2.1 eV and 3 eV. Si0.5C0.5 films were successfully doped in-situ with B2H6 and conductivities of 5·10-5 S/cm to 2·10-4 S/cm were obtained. Temperature dependent conductivity shows that the conductivity follows an Arrhenius relation for certain temperature regimes. However, the temperature dependence over a larger temperature range is not Arrhenius-like, which is in agreement with previous results.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 241-242, One of the last manufacturing steps in the encapsulation of PV modules is that the glass and the foil are laminated. To be sure that the quality will be good for at least 20 or 30 years Bystronic glass offers machinery for butyl sealing. At first there is a back end system for the production of thin film modules without any foil for the encapsulation. Furthermore Bystronic glass has developed a system to apply butyl on the border of a thin film substrate before the lamination process with a lamination foil. In the end there is a new invention for wafer based modules, where butyl application is one of the key points of the project.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 258-261, For the past decade TNO has been involved in the research and development of atmospheric pressure CVD and plasma enhanced CVD processes for deposition of transparent conductive oxides, such as tin oxide and zinc oxide. The use of atmospheric deposition processes allows for large scale roll to roll manufacturing, and is therefore expected to provide a breakthrough for lowering the price of thin film PV modules.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 391-394, The dimension reduction of materials to a nanometer scale gives rise to several advantages in terms of device applications, due to size and quantum confinement effects. Crystalline nanostructures such as nanowires have received increasing attention due to potential applications in solar cells, nanoscale electronic, optical and sensing devices, etc. This work reports the synthesis of nanostructured ZnO by the method of electrochemical deposition on glass substrates covered with a thin film of SnO2 doped with F (SnO2:F). The influence of the deposition parameters on the structural properties of the obtained ZnO films was studied by SEM and Raman spectroscopy. ZnO nanostructured films were deposited by an electrochemical process using a three-electrode potentiostatic system with saturated calomel electrodes as reference electrodes, from aqueous solution containing ZnCl2, KCl, pH = 4.00 and flowing air or/and additional supplement of H2O2. The SEM micrographs demonstrate that the ZnO films consist of nanowires. The Raman spectra of the obtained ZnO films with nanowires demonstrate the typical for ZnO vibration bands. They most pronounced for the sample deposited with the highest H2O2 concentration. In order to increase the conductivity experiments for doping of the obtained nanostructured ZnO films during the electrochemical deposition are in progress.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 336-339, Apart from costs, one of the keys to the market introduction of the fluorescent solar concentrator (FSC) is its outdoor lifetime, which up to now, is limited a few years by the lifetime of the fluorescent dye or quantum dot. This study shows that omitting the dye or quantum dot still can give power conversion efficiencies (PCE), which are comparable to, or even larger than that of the FSC. This can be done by using a special type of bottom mirror. It is shown that a PCE of 4.5% can be achieved

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 222-228, It has been proposed that slowed phonon decay and correspondingly long “hot” carrier lifetimes could be obtained through the careful engineering of quantum dot (QD) superlattices (SLs). These QD superlattices would have applications to Hot Carrier Solar Cells (HCSCs). Here, two QD superlattice geometries, FCC and HCP, are investigated numerically to see if these geometries yield the desired properties for long phonon lifetimes and slowed carrier cooling. The result is that the HCP system yields quite large phonon bandgaps for features of a reasonable size. Typical phonon lifetimes in HCP superlattices are on the order of 22 ps for a 1.8 nm diameter QD. The FCC system, for extremely small feature sizes and precise spacing, appears to yield the required phonon bandgap to inhibit first order phonon decay processes almost entirely. For the FCC SL geometry, high energy phonon lifetimes should be greatly enhanced.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 421-424, The authors describe the method of obtaining of nanoheterostructures with quantum dots by liquid phase epitaxy with pulse cooling of a substrate. The structures with Ge, InAs, QD grown inside of GaP p-n junction have been prepared. Spectral dependencies of photoluminescence, and photocurrent as well as current-voltage characteristics were applied for characterization.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 247-250, Polycrystalline germanium thin films on silicon nitride coated glass had been investigated with the aim to develop a material for the bottom cells of low cost monolithic tandem solar cells in our previous work. In this paper, the structural and electrical properties of germanium thin films are explored with respect to deposition temperature and the technique of in situ hydrogenation. The films were deposited by radio-frequency magnetron sputtering in a mixture of 15 sccm Ar (and 10 sccm H2 if in situ hydrogenation is required) at a variety of substrate temperatures (Ts) ≤ 450 °C. Raman, X-ray diffraction, and UV-vis reflectance spectra reveal the structural evolution from amorphous to crystalline phase with increasing Ts. It is found that the introduction of hydrogen does not apparently affect the structural properties of the film sputtered at 450°C although the onset crystallization temperature is ~90°C higher than that sputtered without H2. On the other hand, the introduction of H2 significantly improves the dark conductivity of the films sputtered at various temperatures.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 203-206, Quantum wells enable solar cell bandgaps to be tailored to the theoretical optimum, under the assumption of unity absorption. In practice however, quantum well regions are thinner and hence absorb less strongly than bulk material. We predict detailed balance efficiency limits for varying levels of absorptivity in single- and double-junction quantum-well solar cells. These calculations allow us to find the absorption levels required for quantum wells to increase cell performance. It is found that the ideal band edge under concentration for a conventional single-junction cell for all bulk absorptivities lies near 1100nm, whilst for quantum well solar cells it is pinned at approximately 925nm for quantum well absorptivities up to ~90%. In the case of tandem cells our model indicates that bottom cell QWs should be advantageous for low absorptivities whilst additional top cell QWs improve performance for absorptivities above 50%. The effect of increased bottom cell photocurrent due to radiative losses in the top cell is also quantified.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 282-284, The front grid of screen printed solar cells strongly limits the efficiency in terms of shading and resistive losses. Main focus of this work was to provide an alternative two-step front contact process consisting of fine printing followed by an adapted plating stack of Ni/Cu/Sn. Results on improved fine line printing as well as a new plating technique will be presented for the first time. Optimized parameters for printing with newly developed techniques as well as adapted plating techniques will be shown. Solar cells using the new developed front metallization have been characterized and compared with standard screen printed solar cells. Efficiency gain of more than 0.4% has been achieved so far.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 322-325, In this paper we describe innovative nanostructured substrates which are proposed as supports for thin film PV devices with the aim of achieving plasmon enhanced photon harvesting. We report on a novel approach for the physical synthesis of laterally ordered arrays of noble metal nanowires supported on transparent substrates (glass slides) which provides a viable alternative to time consuming serial nanopatterning approaches. The formation of periodic metal nanostructures is induced by self-organized sputtering processes of glass substrates followed by glancing angle metal deposition and agglomeration into the troughs. The nanowire arrays exhibit a tunable broadband plasmonic response which matches the solar spectrum for optimal PV conversion.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 305-309, Silicon nanowire (SiNW) ensembles with vertical architecture have been successfully realized by a wet chemical etching process of a mc-p+nn+-Si layer on glass. The highest obtained open-circuit voltage under AM 1.5 illumination was 500 mV. The highest power conversion efficiency of a SiNW based solar cell on glass was around 5%. Strong absorption (>90%) and low reflection (

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 326-331, Rare-earth oxide materials have been investigated for use as spectral conversion elements for monolithic integration into single crystal silicon solar cells. Gd2O3:Er and Gd2O3:Tb thin solid layers were deposited on thin-film c-Si solar cells, for use as upconverting and downshifting layers, respectively. Photoluminescence from Gd2O3:Er shows upconversion of 1480nm light to 980nm, and concentration quenching of the luminescence for Er doping above 10%. The direct deposition of the rare-earth oxide layer allows for a modulated Er doping profile, which has been used to suppress concentration quenching. Thin layers of Er doped Gd2O3 were grown within undoped Gd2O3 layers to confine resonant energy transfer to two dimensions, and thus improve luminescence efficiency. Taking into account the optical standing wave in the layers, when the active Gd2O3:Er layer is less that 130nm thick, we see an improvement in the photoluminescence per Er ion. With these layers integrated into silicon solar cell devices, a similar increase in short circuit current is measured for thin doped layers when excited with 1480nm radiation. Gd2O3:Tb,Yb layers were produced as candidates for quantum cutting material. Downshifting from 488nm was observed, however no emission from the 980nm Yb level was observed. All rare-earth oxide layers have been shown to be stable through solar cell processing steps.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 299-304, Silicon is an excellent platform for multi-junction solar cells. High performance materials can be grown on the Si platform using liquid phase epitaxy (LPE) which is a versatile growth technique for fabricating multi-junction solar cells. A range of high band gap solar cells can be grown by first growing GaP on silicon and then growing the lower band gap solar cells on this GaP layer, such as GaAsP, InGaP and GaP. Si:Ge low band gap alloy solar cells can be grown as well as high performance silicon solar cells by LPE. In this paper, a first principle calculation method has been used to calculate the theoretical efficiency of a 5-junction solar cell system based on a Si platform. We have demonstrated that it is possible to grow various junctions of this 5-junction system by LPE. These include a GaP/Si hetero structure, a stand alone GaP(2.26eV) solar cell which can be grown on the previous GaP/Si template in the future, a high quality Si solar cell, and a Si:Ge solar cell on Si substrates. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to characterize the surface morphology and quality of our growth layers. I-V measurements are used to test the performance of these solar cell devices.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 317-321, The optical properties of silicon nanodots embedded in a dielectric matrix are studied. A critical review of the existing literature is presented, with focus on the specificity of the compositional and structural characteristics of the involved material. The results are applied to the simulation of spectrophotometric measurements taken on thermally treated silicon rich oxide (SRO) grown by Plasma Enhanced Chemical Vapor Deposition. Our results include the determination of the optical properties of the SRO host matrix. Simulation of the R&T data indicate the precipitation of amorphous silicon clusters for annealing temperatures below 1000°C, and subsequent crystallization for higher temperatures. The dielectric function of silicon nanoparticles that form after annealing is observed to closely approach the literature data obtained for samples fabricated by thermal precipitation from a silicon implanted SiO2 matrix.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 313-316, A solution-based, anti-reflective coating, SOLARCTM coating with improved performance is developed and tested for durability in our laboratory. The SOLARCTM coating formulation is prepared by synthesizing a transparent polymer in a solvent mixture, which is then deposited on glass by using a non-vacuum coating technique such as spray, roller, dip, curtain, slot-die and spin-on. A 3.5-4.2% transmission gain over 350-800 nm and a 2-2.5% transmission gain over 800-2500 nm are obtained by coating commercial PV-grade sodalime glass with SOLARCTM on one side of the glass. A 7.2- 8.5% transmission gain over the visible range and 4.0-6.0% gain over the IR region are obtained by coating both sides of the glass. Transmittance gain is maximized by targetting refractive index of the coating at 1.23 and thickness of the coating at one-quarter of a wavelength of incident light. The coating is applied onto the glass surface at room temperature followed by a thermal bake. The coating showed excellent performance on industry standard durability tests.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 385-387, We present a simple set-up with a fluorescent collector on top of an a-Si solar cell, which achieves 7 % more current than the solar cell alone. With a photonic structure on top, the current increases by 95 %. The fluorescent collector decreases the induced current directly above the solar cell by 43 %, but increases the photovoltaic active area. The solar cell alone has an active area of 1.96 cm2. On the measured area of 20.25 cm2 around the solar cell, the fluorescent concentrator provides photons to induce current in the solar cell. The application of the photonic structure increases the active area even further.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 229-231, Er-doped ZBLAN glasses emit intense upconverted fluorescence in the visible and near infrared spectral range when illuminated with a 1540 nm laser diode. These emissions can be excited by a 2-photon upconversion process (near infrared) and a 3-photon upconversion process (visible). For low excitation powers of a few mW the main upconverted fluorescence can be attributed to the 2-photon upconversion emitting at 980 nm. For increasing excitation power the contribution of the 2-photon upconversion to the overall emission decreases, while the contribution of the 3-photon upconversion increases. It was found that the power dependence of the integrated intensity, normalized to the excitation power, shows a maximum for all samples at different Er-doping levels. This maximum shifts to smaller excitation power for higher Er-doping levels.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 369-372, Silane-free polymeric PECVD silicon carbon nitride (SiCxNy) film was evaluated and characterized for its use as an alternative to passivating dielectric and antireflection coating (ARC) being used for silicon solar cells. Our previous work evaluated SiCxNy film for 45 Ω/ phosphorus emitter passivation and AR [1,2], where ammonia (NH3) flow rate and plasma power were varied to control the quality of SiCxNy film. Optimized SiCxNy coating on 45 Ω/ emitter produced solar cells with identical efficiency to the conventional SiNx coated solar cells. In this paper, we tailored SiCxNy film for 60 Ω/ phosphorus doped emitter. The polymer gas, NH3 flow rate and deposition temperature, at constant power, were varied to fine tune the film quality. We fabricated 60 Ω/ phosphorus emitter screen-printed solar cells on 149 cm2 textured 2.5 Ω-cm p-type Czochralski (CZ) wafers to evaluate the dielectric properties. SiNx coated solar cells were also fabricated for comparison. We found that the best efficiencies for SiNx and SiCxNy coated cells were 17.8% and 17.5%, respectively. The finished solar cells were analyzed by spectral response, Suns Voc, dark I-V, and PC1D modeling. Efficiency loss mechanism was also studied. We found that a lower passivation quality on front surface contributed to a decrease in voltage and current, and higher reflection caused current loss.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 270-275, Highly developed and sophisticated production techniques of modern solar cells with functional optical elements show the need for a quick, precise, cost effective and operator-friendly characterisation technique. Variable Angle Spectroscopic Ellipsometry (VASE) can fulfil this task. Even though well established for a range of known materials, the application to new materials isn’t always straightforward. Since VASE is based on an iterative modelling process by optimisation of some figures of merit, there is a lot of experimental work to do to prove assumptions about the sample which must be made beforehand. In this work, we correlate PECVD deposition parameters with VASE model parameters and certain features of hydrogenated amorphous silicon carbide (a-SixC1-x:H). a-SixC1-x:H is a very versatile material that can be adapted to a very wide range of concepts to enhance the efficiency of silicon solar cells. To prove this, we show the production of a Bragg filter consisting of ten alternating layers of this material.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 187-190, Several complex compounds that result from inserting atoms of certain transition metals in known semiconductors are proposed as intermediate band materials that, used as light absorbers, would allow to increase substantially photovoltaic efficiency. In particular, these are sulphides containing indium in octahedral coordination as the spinel CdIn2S4 or scandium (Sc2S3, with a defect NaCl structure) with the trivalent element partially substituted by Ti or V, and the silicon clathrate of type II (having a fcc Bravais lattice with 34 Si atoms per primitive cell) in which V substitutes partially Si or Ag is occluded in the intracrystalline cavities. DFT calculations show that in several of these structures the added metal gives rise to a partially occupied, relatively narrow band located between the conduction and valence bands. Some of these structures have been synthesized experimentally, and we show UV-Vis-NIR spectra agreeing with the DFT predictions.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 236-238, We apply the technique of extended x-ray absorption fine structure (EXAFS) spectroscopy to measure the chemical state of selenium atoms in highly doped silicon. Coating the surface of crystalline silicon with metallic selenium and irradiating it with a femtosecond-laser results in high concentrations of selenium (~1020 atoms cm-3) in the near-surface layer. After laser irradiation, this material demonstrates near unity optical absorption for all measurable wavelengths (400-2500 nm) including below the fundamental bandgap of elemental Si. Further annealing results in a decrease of absorption in the infrared portion of the spectrum. We report on the apparent evolution of the chemical state of the Se atoms as the material is annealed. Comparisons with absorption measurements suggest a direct correlation of the optical properties and chemical state of Se in highly doped Si, suggesting that the absorption enhancement is related to the local environment of the dopant.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 251-254, Polycrystalline sulfides of the PbXSn1-XS (0 ≤ X ≤ 0.25) alloys were synthesized by the direct fusion technique. The thin films of these materials were prepared by a hot wall deposition method on glass substrates at Tsub. = 240 - 300 oC and vacuum of about 10-5 Torr. The microstructures of the films were characterized by XRD, SEM, EDX, AES and AFM. The thin film powder patterns matched the powder target patterns satisfactorily and indicated the absence of binary phases like PbS and SnS2. The SEM and AFM analyses show that the films are densely packed with layered crystallites which have good mechanical adhesion to each other. The as-grown PbXSn1-XS films show p-type conductivity. The electrical conductivity decreases with increasing Pb/Sn ratio of the thin film composition. As the Pb/Sn ratio increases, the activation energy increases from 0.121 eV for SnS to 0.264 eV for Pb0.25Sn0.75S.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 340-345, Excitonic solar cells and solar cloths were fabricated by a commercially viable nanofabrication technique, known as electrospinning. The electrospun continuous metal oxides were either used as such or broken into high aspect ratio (10:1) nanowires for fabrication of excitonic solar cells. Transient photocurrent spectroscopic measurements have shown an order of magnitude increase in the diffusion coefficient of TiO2 nanowires in the presence of the iodide/triiodide electrolyte compared to the random continuous nanofibers. For solar cloths, the electrospun nanowires duly anchored to a dye were dispersed in a polymeric hole conducting medium and the resultant solution was spun into a cloth of non-woven fibers. The solar cloths showed relatively high open circuit voltage. Attempts are currently underway to optimize the performance of solar cloths.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 398-403, It was found that the dark conductivity of a stoichiometric a-SiC thin film, which is used as barrier layers in a Si quantum dots superlattice (Si-QDSL), increased drastically after thermal annealing above 800 oC. This is due to the crystallization of an a-SiC phase in the films. To resolve this problem, N2 gas was introduced during the deposition of stoichiometric a-SiC thin films. As a result, the dark conductivity of the films annealed at either 900 or 1000 oC was reduced below 10-9 S/cm, since the introduction of nitrogen atoms into the films prevented the a-SiC phase from crystallizing during the annealing. N-containing Si-QDSLs were prepared and the dark conductivity was measured. The dark conductivity of the N-containing Si-QDSLs was in the range of 10-6 to 10-8 S/cm, suggesting that the Si-QDSLs got close to intrinsic. The solar cell structure using a N-containing Si-QDSL as a light absorption layer was fabricated. The open circuit voltage and short circuit current were improved compared with the Si-QDSL without the introduction of nitrogen atoms. The quantum efficiency data indicated that the absorption edge was shifted to higher energy as the diameter of Si-QDs decreases, suggesting the quantum size effect.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 289-294, In 2008, Japanese KYOSEMI Corporation and Spanish Technology Corporation TECNALIA signed a Collaboration Agreement for the development of a BIPV curtain wall based on KYOSEMI’s Sphelar® technology and TECNALIA’s developed concept of distributed architecture. In this paper, the main aspects of the product design and the validation plan, as well as the first tests results carried out by TECNALIA are presented. They show that the proposed BIPV solution presents higher efficiency and lower cell-occupated area than other BIPV products already available in the market, for the same power. Additionally, new features are achieved: unique aesthetical design and indoor lighting, possibility of different patterns and drawings without compromising efficiency, variable ranges of cell occupation, continuous monitoring of PV module performance, more efficient light capture by means of proper selection of glass and selective glass coatings, etc. First results show that the proposed combination is a really attractive option to be taken into account for building applications as solar power-generating glazing systems. The first prototypes will be concluded in October and ready to be installed in KUBIK®, the experimental building that TECNALIA has designed and built to serve as a testing laboratory on the performance of energy efficient products in buildings.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 255-257, A simple and efficient method is described for integrating a silicon nanowire array into a solar cell. After nanowire growth by chemical vapour deposition, the nanowire array is planarized by using spin-on glass and chemical-mechanical polishing. This planarization allows to deposit a continuous and uniform conductive layer on top of the nanowire array, and thus to form a high-quality front electrical contact. This process leads to solar cells with a parasitic series resistance as low as 5 .cm², which is a clear improvement compared to the recent literature.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 266-269, With the development of flexible, weather-resistant high barrier materials for the encapsulation of flexible solar cells an innovative, resource-friendly production process for photovoltaic modules can be achieved. The market of flexible thin film and organic solar cells, such as amorphous silicon (a-Si), microcrystalline silicon (μc-Si), Copper-Indium-Gallium-Selenide (CIGS) and Copper-Indium-Diselenide (CIS) cells is growing very fast requiring new techonolgies in terms of cell production and encapsulation. Today state of the art is encapsulating flexible modules with glass – glass or glass – backsheet set-ups. The used backsheets provide an incorporated moisture barrier such as an aluminum foil or a metal oxid like AlOx or SiOx. Such encapsulated modules are limited in flexibility. Consequently the development of flexible, rollable high barrier material is crucial for thin film and organic solar cells. However requirements for flexible encapsulation material in terms of transparency, weatherability and permeability of gases are very challenging. Transparency of at least 85 % and permeation values as low as 10-5 g/cm2d for water vapor transition (WVTR) and respectively 10-5 cm3/m2datm for oxygen transition (OTR) are needed to protect flexible cell systems. Beside material development processability of the high barrier material in roll to roll manner is very important. The principle for establishing a flexible high barrier material is a polymer substrate delivering sufficient mechanical strength and thermal resistance. The polymer substrate is combined with inorganic layers, such as MeOx (SiOx, AlOx) layers and functional barrier coatings of tailor made hybrid polymers concerning adhesion, UV stability or weatherability. With the use of weather-resistant high barrier materials flexible solar cells can be enccasulated in large quantities in a roll to roll process. An innovative, resource-friendly production process for photovoltaic modules can be achieved.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 408-410, In this work, large-area arrays of Si nanowires (Si-NW) have been fabricated by means of the chemical electroless etching of a Si substrate. This technique implies low cost and large-scale production of robust Si-NW arrays with properties inherited from the bulk substrate. By using different techniques, such as SEM (Secondary Electron Microscopy), Optical Absorption and Spectroscopic Ellipsometry, the effect of the etching conditions in the structure and optical properties of the as grown Si-NW arrays have been systematically investigated.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 174-177, GaSb based structures were developed and fabricated by the Zn-diffusion technique for solar and thermophotovoltaic applications. The effect of Zn diffused emitter etching by means of anodic oxidation on the behaviour of characteristics of cells fabricated from GaSb (100) wafers was investigated. An optimum Zn diffused profile and penetration depth of a p-n junction have been found, which allowed obtaining the maximum efficiency of GaSb cells at a high generated photocurrent density up to 10 A/cm2.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 381-384, Luminescent solar concentrators suffer from a high loss of emitted photons through the surfaces. Previous measurements on filled waveguides showed 40-50% of all the absorbed energy and 50-60% of all absorbed photons were lost through the top and bottom of the waveguide. Aligning the dye molecules in a preferred direction can be beneficial to the light output, since fluorescent dyes tend to be dichroic in absorption and emission. Planar alignment of the dye molecules leads to an increase in surface losses, while the total edge output of these planar arrays is similar to the total output of an isotropic dye array. Homeotropic aligned waveguides show a decrease in surface-loss compared to an isotropic waveguide. Emitted photons are directed more towards the edges in this alignment and so surface losses are greatly reduced. However, edge output does not increase, likely a result of increased re-absorption of an emitted photon by other dye molecules. These internal losses are related to the small Stokes-shift of the dye molecules, non-unity quantum efficiency and trapping of photons in the thin dye layer.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 373-376, To reduce the surface losses in the luminescent solar concentrator (LSC) we have applied an organic wavelength-selective mirror, a chiral nematic (cholesteric) liquid crystal, onto a fluorescent waveguide with a small air gap in between. We compare the light outputs of the waveguide with absorbing, scattering, and mirrored bottoms. The highest light output is achieved using a scattering background and a cholesteric mirror with a reflection band significantly red-shifted with respect to the emission spectrum of the fluorescent dye. The use of an air gap results in refraction and light bending away from the normal and, consequently, a red shift of the cholesteric mirrors is required which also negatively affects its performance. A 15% increase in edge power output is demonstrated at low dye concentrations, suggesting light that is emitted to the surface of the waveguide is re-directed by the mirrors to the edge with an efficiency of about 35%. At high fluorescent dye concentrations, the chiral-nematic mirrors have reduced impact on dye emission.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 332-335, Several works have appeared in the literature as a result of the joint activity of Imperial College London with the European Solar Test Installation and other European research centres on luminescent solar concentrators: some have been specifically related to indoor and outdoor characterisation. Within the characterisation processes, the external quantum efficiency is particularly important: by means of external quantum efficiency measurements, spectral mismatch correction can be performed, enabling performance predictions for any given irradiance conditions. In recent works an increase in module efficiency due to a diffusive reflector at the backside of the concentrator has been shown. In this work we present recent results on the impact of a non-index-matched backside diffusive reflector on the external quantum efficiency of a luminescent solar concentrator and therefore on the short-circuit current of the device: the result is particularly relevant in view of using luminescent solar concentrators in building integration of photovoltaics.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 262-265, Polycrystalline SnS thin films have been prepared using a two-step process consisting of annealing of Sn precursors in S ambient. The effect of the growth conditions on the structural, optical and electrical properties was investigated using XRD, SEM, AES and optical absorption measurements. X-ray diffraction studies have shown that sulfurization temperature (Ts) affects crystalline structure of the deposited material as well as its optoelectronic properties. Single phase SnS films of orthorhombic structure (OR) with [040] preferred orientation were formed under temperatures between 250 and 350 °C. SnS films were of p-type conductivity and had electrical resistivity of 4.101 – 2.102 .cm and thermopower value of 240 – 950 μv.K–1. The films were highly absorbing with an optical absorbing coefficient over 104 cm–1 near the fundamental absorption edge. The calculated direct optical band gap value (Eg) for single phase SnS films varied in the range Eg = 1.15 – 1.24 eV.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 363-365, We have shown, that photoactive, highly (001)-textured tungsten disulfide (WS2) films can be prepared both on insulating as well as metallic substrates. The WS2 films are crystallized from X-ray-amorphous WS3+x films by a rapid annealing process in an H2S atmosphere with the assistance of metal-sulfide droplets, which are formed, when the eutectic temperature in the system metal-sulfur is exceeded. As promoters Ni, Co and Pd can be used, where the eutectic temperatures with sulfur are in the range from 620 to 680 °C. From ex situ- and in situ-analysis a modified model for the formation of the WS2 nanosheets was derived, which resembles the well-known VLS (vaporliquid- solid)-model for the formation of nanorods (or whiskers), except that in our systems the precursor is also an (amorphous) solid, and instead of 1-dimensional rods, 2-dimensional nanosheets are grown. Especially, the preparation on metallic layers (W, TiN), which was not successful before, offers now the opportunity, to prepare thin film solar cells from these WS2 films. First experiments to prepare thin film solar cells show, that short circuits occur, most probably at grain boundaries, which have to be avoided or passivated.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 346-349, Lanthanide (Ln) complexes consisting of rare earth ions such as Europium (Eu3+) or Ytterbium (Yb3+) with highly absorbing organic ligands attached, offer advantages over the fluorescent dyes and quantum dots that are more often considered for use in luminescent solar concentrators (LSCs). The large Stokes shift of such complexes (

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 418-420, Silicon nanowire thin film solar cells on glass were investigated. On a borosilicate glass substrate a multicrystalline boron doped silicon base layer was prepared by laser crystallization. The surface of this film was silanized and covered by gold colloids. By VLS growth phosphorus doped silicon nanowires were grown with diameters in the 30 to 150 nm range. The nanowire carpet was embedded into PMMA and contacted by Al. If the base layer has a doping profile so that the surface was only weakly boron doped, the system shows rectifying and photovoltaic behavior. Obviously an axial pn-junction between the nanowires and the base layer formed.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 354-358, Photovoltaic modules with glass as cover or as superstrate show a poor blue and near UV response due to absorption in the glass and in the doped layer. This poor response of all glass covered solar cells promises an enhancement by spectral conversion of the short wavelength part of the solar spectrum. Thin transparent dielectric films containing Tb3+ are developed for this application. These spectral converters with absorption bands between 200 and 400 nm shall be deposited on top of the glass. Their emission bands in the region 400 - 600 nm are well suited for the spectral response of thin film silicon solar cells. Here, the results on the deposition by RF magnetron co-sputtering and characterization of thin SiO2 and Al2O3 films containing Tb3+ ions are presented. The highest photoluminescence intensity was found at about 1 at.% Tb in both host materials for as-deposited films. The measured lifetime of the 5D4 - 7F5 (~ 546 nm) transition was determined to about 1 ms. Annealing studies lead to an enhancement of the photoluminescence intensity by a factor of 2.5 - 3 times.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 276-278, A new emerging generation of photovoltaic devices needs the development of new high-efficient electrodes. One common feature is that the side exposed to the sun must have a current collector electrode based on transparent conducting oxides (TCOs). One possibility could be a TCO based on a double-layered antimony-doped tin oxide (ATO)/ titanium oxide (TiO2) film developed for dye-sensitized solar cells by using DC reactive magnetron sputtering. It has been found that doping SnO2 with Sb improves both, transmittance and conductivity because the density of free charge carriers increases. ATO thin films with adequate properties in terms of light transparency and conductivity were obtained using room temperature processes by oxygen-reactive DC magnetron sputtering. Effects of the O2/(Ar+O2) ratio in the sputter flow gas has been studied. Results regarding to optical and electrical properties, microstructure and composition measurements, and the influence on the film properties by the application of sputtered films of TiO2 onto SnO2:Sb coated glass substrates were investigated.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 232-235, Ag nanoparticle arrays, located on the front or rear surface of a Si solar cell, can provide effective lighttrapping via the excitation of localised surface plasmons. We identify key parameters in engineering random Ag nanoparticle arrays for optimum plasmonic light-trapping. We demonstrate that there is an asymmetry in scattering behaviour between particles located on the front and rear of a Si substrate, which we attribute to differences in the driving field at the position of the nanoparticles. Applying the design considerations presented, we report a relative photocurrent increase of 27% over the light trapping spectral region for thin c-Si solar cells incorporating a rearlocated self-assembled Ag nanoparticle array, constituting the largest reported enhancement due to plasmonic light trapping for such cells. Inclusion of a detached rear reflector behind these particles increases the photocurrent enhancement to 33%