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38th European Photovoltaic Solar Energy Conference and Exhibition; 330-333, Silicon heterojunction (HJ) solar cell is one of the leading technologies for single junction photovoltaic devices and it could be successfully integrated into a silicon based multijunction solar device, in conjunction with a large band gap cell. To be used as a bottom cell, the HJ device needs to adapt to the current technology for top cells, which are typically far less mature and characterized by smaller areas. However, if the HJ device is cut from a finished larger cell, the original edge passivation gets lost, causing the introduction of surface recombination paths and an overall decrease of the device performance. In this context, the development of a proper edge passivation procedure to be applied to high quality HJ solar cells becomes of major importance. We tried two different strategies of edge passivation based either on wet processes, such as the mesa etching of the device, or plasma treatment of the vertical edges. The latter consisting in the deposition of 1-10 nm of hydrogenated amorphous Si on the edges. In order to characterize the recombination losses and quantify the effect of different passivations, we did current-voltage measurements in light and dark conditions and quasi- steady-state photoconductivity measurements.

38th European Photovoltaic Solar Energy Conference and Exhibition; 501-504, Silicon heterojunctions (Si HJs) are regarded as a promising structure to develop high efficiency tandem solar cells. However, the Si HJ structure has been optimized for use as single junction device and variations might be made in order to adapt it to tandem applications. In this work we analyse by optical simulations the improvements that could be achieved in the short circuit current of Si HJs used as bottom cell in tandem devices, if the front TCO layer is made thicker. This solution allows to adapt the absorption in the Si HJ to the incident light spectrum, that is rich of high wavelength photons if a wide band gap cell is put on top of the Si HJ. The investigation has been carried out for two top cell technologies: a high efficiency InGaP solar cell and a semitransparent perovskite solar cell. The improvement in JSC is of the order of 1 mA in both cases, and is more pronounced when the edge of transparency of the top cell occurs at higher wavelength.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 66-68, In this article, we demonstrate a patch, which contains upconverting crystals of Gd2O2S:Yb3+, Er3+ mixed with a UV curable gel, integrated with a textured back-reflector that can be externally affixed to a bifacial thin-film silicon solar cell. The patch serves to provide the benefit of light trapping as well as spectrum upconversion to the solar cell fabricated on a flat substrate. Upconverting patches with different textures were studied for their effectiveness in enhancing the short-circuit current density of the solar cell and for the relationship between the current enhancement and the intensity of the incident light. The upconverting patch is shown to enhance the solar cell performance and offers a new stick-on technique to apply upconversion-based strategies to existing solar cells. The photogenerated short-circuit current densities of the solar cell show a power-law dependence on the illuminated laser light intensity when an upconverter patch is attached to the solar cell (exponent 1.5-1.6) while a linear dependence is observed for the solar cell without the upconverter patch.

In this paper, we report results on a field-effect-induced light modulation at λ = 1.55 μm in a high-index-contrast waveguide based on a multisilicon-on-insulator platform. The device is realized with the hydrogenated amorphous silicon (α -Si:H) technology, and it is suitable for monolithic integration in a CMOS IC. The device exploits the free-carrier optical absorption electrically induced in the semiconductor core waveguide. The amorphous silicon waveguiding layer contains several thin dielectric films of amorphous silicon carbonitride (α -SiCN) embedded along its thickness, thus highly enhancing the absorbing action of the modulator held in the on state. © 2006 IEEE.

28th European Photovoltaic Solar Energy Conference and Exhibition; 142-146, We present the first crystalline Si-based tandem solar cell device consisting of a monolithically interconnected c-Si wafer bottom and a Si nanocrystal (NC) top solar cell. The major part of this work has been realised within a European consortium in the frame of the FP7 project NASCEnT. Within this project we have been able to (i) develop new nanomaterials with photovoltaic compatible technologies, to (ii) improve the theoretical understanding of transport, recombination and dopant incorporation in these quantum confined absorber materials and to (iii) design devices which enable us to prove quantum confinement. In this publication we show that although solid phase crystallisation at 1100 °C for around 30 min is necessary to generate Si NCs of proper quality the wafer bottom cell as well as the c-SiC(n+) tunnel recombination junction layer can withstand such harsh conditions. Besides simulations to determine the influence of the high thermal budget on dopant profiles in wafer and c-SiC lifetime measurements on parallel processed wafers still show minority carrier lifetimes of 600 μs. The bifacial character of the tandem device enables us to clearly distinguish between bottom and top cell contributions. With a NC layer thickness of initially 3 nm Voc values as high as 978 mV could be achieved at 1 sun illumination from the front-side. Under rear-side illumination the short circuit current Jsc is with 1.0 mA/cm2 4 times higher that under front side illumination due to the better absorption and carrier lifetime of the bottom cell but still limited by the series resistance of the intrinsic NC absorber material. However, at 510 mV the Voc is much lower, indicating the difference in Voc is due to photogenerated voltage in the NC top cell which can only be created by illumination with low-wavelength light, i.e. front illumination. This very first tandem solar cell device including quantum dot absorber layers produced with photovoltaic compatible processes is the next step to successfully overcome the Shockley-Queisser limit for wafer based Si solar cells.

26th European Photovoltaic Solar Energy Conference and Exhibition; 22-27, We present an overview of the project NASCEnT and the first results we were able to achieve after one year of project duration. The overall objective of the project is to develop new nano-materials with new production technologies and to fabricate silicon quantum dot (Si QD) materials for all-silicon tandem solar cells to achieve increased efficiencies. The understanding of electrical transport and recombination mechanisms in these newly developed nano-materials will enable us to design novel solar cell structures that help to overcome the efficiency limits of conventional solar cell concepts. So far we developed simple device structures and process sequences for both material systems (Si QDs in silicon oxide and silicon carbide) and tested a pin solar cell structure. Another objective of the project is to ensure the compatibility of the newly developed technologies with high-throughput processing leading to further cost-reduction. Within the scope of this project the novel concept of band gap-tuneable quantum dot superlattices is to be exploited in combination with a wafer-based high efficiency silicon solar cell as well as with a thin-film solar cell structure. The outcome of the project will be significant progress in solar cell evolution and will lead to higher efficiencies for solar cells and to ongoing cost-reductions from both a short-term and a long-term perspective.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2121-2125, Heterojunction solar cells are fabricated on 1 Ωcm boron or phosphorus doped multicrystalline silicon wafers. As-cut material is treated by industrial isotexturing and home-made acidic texturing in order to obtain textured surfaces, as well as CP4 and modified CP6 chemical polishing, in order to obtain reference samples. A double a-Si:H/c-Si is deposited by Plasma Enhanced Chemical Vapour Deposition to sandwich the substrate, at the sunward surface to form the emitter and at the rear side to act as back surface field. High open-circuit current (625 mV) is obtained on flat p-type mc-Si, whereas commercial acidic surface treatment, commonly used in diffused junction mc-Si solar cell fabrication, requires a level of passivation which is not easily achievable using the very thin a-Si:H film needed in heterojunction technology. On n-type substrates, conversion efficiency around 12% is found, despite low VOC values. The effectiveness of a-Si:H/mc-Si technology in achieving high photovoltaic efficiency using low thermal budget manufacturing process is demonstrated.

In this paper, we report results on a field-effect-induced light modulation at lambda = 1.55 mu m in a high-index-contrast waveguide based on a multisilicon-on-insulator platform. The device is realized with the hydrogenated amorphous silicon (alpha-Si:H) technology, and it is suitable for monolithic integration in a CMOS IC. The device exploits the free-carrier optical absorption electrically induced in the semiconductor core waveguide. The amorphous silicon waveguiding layer contains several thin dielectric films of amorphous silicon carbonitride (alpha-SiCN) embedded along its thickness, thus highly enhancing the absorbing action of the modulator held in the ON state.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 662-666, We present our results on plasma deposited Si-SiC. The optical properties of the silicon rich SiC matrix as a function of stoichiometry are reported. The results are then used as starting values for the simulation of the annealed material. The optical simulation correctly predicts the initial silicon nanoparticles precipitation upon low temperature annealing. For higher annealing temperatures of 900°C and higher, we observe separate crystallization of the Si and SiC phases. The phase composition is determined by making use of optical simulation. The crystallized Si results to be well simulated using the optical function of unstructured c-Si. The excess absorption of the crystallized SiC matrix is evidenced and determined.

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.