Showing papers by "Stefan W. Glunz published in 2011"

Four-junction spectral beam-splitting photovoltaic receiver with high optical efficiency

Abstract: A spectral beam-splitting architecture is shown to provide an excellent basis for a four junction photovoltaic receiver with a virtually ideal band gap combination. Spectrally selective beam-splitters are used to create a very efficient light trap in form of a 45° parallelepiped. The light trap distributes incident radiation onto the different solar cells with an optical efficiency of more then 90%. Highly efficient solar cells including III–V semiconductors and silicon were fabricated and mounted into the light trapping assembly. An integrated characterization of such a receiver including the measurement of quantum efficiency as well as indoor and outdoor I–V measurements is shown. Moreover, the optical loss mechanisms and the optical efficiency of the spectral beam-splitting approach are discussed. The first experimental setup of the receiver demonstrated an outdoor efficiency of more than 34% under unconcentrated sunlight. Copyright © 2010 John Wiley & Sons, Ltd.

Explanation of commonly observed shunt currents in c-Si solar cells by means of recombination statistics beyond the Shockley-Read-Hall approximation

Abstract: The current-voltage (I–V) characteristics of industrially fabricated, crystalline silicon solar cells are often influenced by non-linear shunts that originate from localized, highly disturbed regions and cause ideality factors n > 2. We show that recombination within such locations needs model descriptions that go beyond the Shockley-Read-Hall (SRH) approximation, because the density of defects is so high that recombination does not occur via isolated, but coupled defect states. We use a variant of coupled defect level (CDL) recombination, the donor-acceptor-pair (DAP) recombination, but via deep levels (as opposed to shallow levels). With this model, we quantitatively reproduce the I–V curves of solar cells that we subjected to various degrees of cleaving, laser scribing or diamond scratching to form shunt locations in a controlled manner. The suggested model explains the transition from ideality factors n 2 when going from low to high defect densities. We also explain the non-saturating reverse I–V characteristics. We show that an additional source of currents with n > 2 is SRH recombination in an inversion layer that extends from the front p-n junction to the rear contact along the cell’s edge or along a micro-crack.

Excellent silicon surface passivation with 5 Å thin ALD Al2O3 layers: Influence of different thermal post‐deposition treatments

Abstract: Thin layers of Al2O3 always require a thermal post-deposition treatment to activate the passivation on crystalline silicon surfaces. In this work, we studied the influence of different thermal post-deposition treatments for the activation of passivating ALD Al2O3 single layers and Al2O3/SiNx stacks. For the stacks, especially with less than 5 nm Al2O3, a short high temperature process at ∼800 °C results in a remarkably lower surface recombination compared to a commonly applied annealing at 425 °C. We observed that four ALD cycles of Al2O3 are sufficient to reach the full potential of surface passivation, and even with one atomic layer of Al2O3 (one ALD cycle) emitter saturation current densities as low as 45 fA/cm2 can be reached on boron-diffused emitters. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of electrical shading effects in back-contacted back-junction silicon solar cells using the two-dimensional charge collection probability and the reciprocity theorem

Abstract: In this study, short-circuit current losses in high-efficiency n-type back-contacted back-junction silicon solar cells caused by the electrical shading effect have been investigated by two-dimensional simulations of the charge carrier collection probability. Based on the reciprocity theorem, the homogenous partial differential equation describing the probability of charge carriers being collected by the p-n junction on the rear side of the solar cell has been solved numerically using the finite element method implemented in the partial differential equation solver COMSOL Multiphysics. The method has been applied to study the impact of geometrical parameters of the solar cell, such as the pitch distance, as well as the emitter and back surface field width, on the local and global internal quantum efficiency and on the short-circuit current density. The influence of the rear surface recombination velocity of an undiffused gap and the effective rear surface recombination velocity of the back surface field re...

Analysis and optimization approach for the doped amorphous layers of silicon heterojunction solar cells

Abstract: Comparison of the open-circuit voltage (external voltage Voc,ext) determined by Suns-Voc measurements with the implied voltage Voc,impl determined by transient photoconductance decay lifetime measurements can yield a quick and easy analysis of silicon heterojunction (SHJ) solar cells, especially in regard to finding the optimum doping concentration of the emitter layer [or back surface field (BSF)]. A sufficiently high doping concentration of the emitter and BSF is mandatory to extract the internal Fermi-level splitting and thus the internal voltage, at the solar cell contacts. However increasing the concentration of doping gases during the deposition of doped amorphous silicon layers results in a reduction of the interface passivation quality and Voc,impl. The best trade off is realized when the ratio of Voc,ext to Voc,impl (external/internal Voc-ratio ζ) reaches a saturation value near 1 upon increasing the doping concentration. AFORS-HET (Automat FOR Simulation of HETerostructures) simulations resulted...

Investigation of Aluminum-Alloyed Local Contacts for Rear Surface-Passivated Silicon Solar Cells

Abstract: We present a comprehensive study on the rear contact formation of rear surface-passivated silicon solar cells by full-area screen printing and alloying of aluminum pastes on the locally opened passivation layer. We show that the point contact distance has a significant influence on the local alloying process for the contact formation resulting in different structural and electrical contact properties when applying conventional Al pastes. Increasing the distance leads to 1) high contact depths resulting in an enlargement of the contact area and 2) severely reduced thicknesses of the Al-doped p+ regions in the contact points, leading to a strong increase in recombination within the contact points. This inadequate contact formation can be directly linked to the deficiently low percentage of silicon that dissolves into the Al-Si melt during alloying. We demonstrate that by intentionally adding Si to the Al paste, the contact point geometry can be significantly improved and particularly becomes independent of the contact distance. Further investigations on the internal reflectance and the specific resistivity of the rear contact suggest an upper limit for the Si content added to the Al paste. In summary, we present a simple way to significantly improve the rear contact formation of rear surface-passivated silicon solar cells.

Investigation of aluminum-alloyed local contacts for rear surface-passivated silicon solar cells

Abstract: The surface passivation of rear aluminum-alloyed p+ emitters is highly beneficial to increase the efficiency of back-junction n-type silicon solar cells, thus however demanding the application of locally defined emitter contacts. The formation of the rear contact by full-area screen-printing and alloying of Al-pastes on the locally opened contact points in the passivation layers exhibits two main problems: (i) increase of the contact depth leading to an enlargement of the contact area and (ii) low Al-p+ emitter thicknesses underneath the point contacts, both implying the danger of emitter shunts. In this study we therefore focus on controlling the formation and structural properties of contact points by systematically modifying the composition of the rear Al paste. By examining the contact geometry over a broad range of pitches, we demonstrate that the contact point depth and the Al-p+ emitter thickness in the contact region are directly linked to the percentage of Si that is dissolved into the Al-Si melt during alloying. For conventional Al pastes, the Si percentage in the melt was calculated to be far too low, so that we provided additional Si by manually adding Si powder to the Al paste. Thus, we could significantly reduce the contact depth and significantly enlarge the Al-p+ thickness in the point contacts, respectively. A first quantitative evaluation of the electrical properties was carried out, showing that the saturation current density is decreased by increasing the Si content of the paste, very likely due to the decreased contact area and improved electron shielding. In summary, we demonstrate that the local rear contact formation by alloying of full-area screen-printed Al pastes can be considerably improved by intentionally adding Si to the paste. The results of this investigation are highly interesting for applications to both n-type and p-type Si solar cells with passivated rear side.

Improved diffusion profiles in back‐contacted back‐junction Si solar cells with an overcompensated boron‐doped emitter

Abstract: The performance of n-type back-contacted back-junction silicon solar cells where the boron-doped emitter diffusion on the rear side is locally overcompensated by a phosphorus-doped base-type back surface field (BSF) diffusion has been analysed theoretically and experimentally. By overcompensating the emitter diffusion the noncollecting base-type region can be reduced significantly allowing electrical shading losses to be minimized. It has been found that for solar cells with a lowly doped BSF diffusion the local external quantum efficiency and the short-circuit current density Jsc could be improved significantly. For reference solar cells with an undiffused gap between emitter and BSF diffusion and a large noncollecting base-type region, a maximum Jsc of 40.9 mA/cm2 could be achieved and for solar cells with a locally overcompensated boron-doped emitter diffusion featuring a small noncollecting base-type region a maximum Jsc of 41.4 mA/cm2 has been measured. The reduction of Jsc losses caused by free carrier absorption (FCA) in highly doped silicon at near-infrared wavelengths is also shown. Furthermore, theoretical investigations are performed by one-dimensional device simulations and the influence of highly doped and lowly doped emitter and BSF diffusions on the open-circuit voltage Voc is presented. For solar cells with a locally overcompensated boron-doped emitter diffusion Voc could be improved from 629 to 652 mV when lowly-doped diffusions and thermally grown SiO2 and antireflection plasma enhanced chemical vapour deposited (PECVD) SiNx passivation stacks are applied. For the reference solar cells with an undiffused gap between the lowly doped emitter and BSF diffusions Voc of 693 mV could be achieved for a plasma enhanced atomic layer deposited (PEALD) Al2O3 passivation layer.

Combining laser chemical processing and aerosol jet printing: a laboratory scale feasibility study

Abstract: First results showing the viability of combining laser chemical processing (LCP) and aerosol jet printing (AJP) technologies to produce a high-efficiency front side for silicon solar cells are presented. LCP simultaneously opens the anti-reflection coating (ARC) and highly dopes the underlying silicon to create a selective emitter, while AJP is the first in a two-step fine-line contact formation procedure. The electrical properties as well as the morphology of the resulting structures are presented. Performance similar to that achieved with evaporated TiPdAg metallization is demonstrated. Copyright © 2010 John Wiley & Sons, Ltd.

Si and SiC nanocrystals in an amorphous SiC matrix: Formation and electrical properties

Abstract: Layers of Si nanocrystals in a dielectric matrix have promising properties to be implemented as the absorber layer in a top cell of a Si-based tandem solar cell. Si nanocrystals in SiC are produced by plasma deposition of Si rich a-SiC:H and subsequent solid phase crystallization by thermal annealing at temperatures between 800°C and 1000°C. The Si rich a-SiC:H films were doped with boron by addition of diluted diborane (B2H6 in H2) to the plasma process. The microstructure was investigated for different gas fluxes and annealing procedures and the films were found to consist of amorphous or polycrystalline SiC with embedded Si nanocrystals. The microstructural results are then correlated with the electrical and optoelectronic properties. By choosing appropriate deposition parameters, the films micro-structure could be modified such that the crystallization of both Si and SiC nanocrystals is favoured and the conductivity is enhanced. X-ray diffraction (XRD) patterns show the formation of both Si and SiC nanocrystals after annealing at 900 °C, if the hydrogen flux in the plasma is large enough. The formation of SiC nanocrystals is confirmed by Fourier Transformed IR (FTIR) spectra where a transition from a Gaussian shaped peak of the amorphous SiC-phase to a blueshifted Lorentzian peak of nanocrystalline SiC is observed. Dark dc IV measurements reveal the increase of conductivity with diluted diborane flux from 9.32 10–6 Ω–1cm–1 to 1.98 10–2 Ω–1cm–1 for a 1000 °C anneal. Increase of annealing temperature at constant doping also increases conductivity by about two orders of magnitude. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Seed Layer and Substrate Morphology on Solar Cell Contacts Deposited by Light-Induced Plating

Abstract: To improve two-step metallization processes for silicon solar cells, the influence of different seed layers and surface textures on the light-induced plating (LIP) deposition behaviour at different plating rates has been studied. Typical seed layers of screen- and aerosol printed silver paste have been tested on alkaline (random pyramid) and acidic texture. Silver deposition at high rate has been found to be problematic on very thin aerosol printed layers, especially for alkaline texture with high roughness. In an attempt to obtain high process speed while maintaining high conductivity, aerosol seed layers were subjected to different multi-step plating processes. Morphological features (aerosol particles, alkaline texture) were leveled by a slow pre-plating step. In a second plating step, the deposition rate was varied. The results were compared to similar experiments with untreated aerosol seed layers. Both pre-plating steps were found to effectively improve the deposition quality of the second plating step with high plating rate, and to improve the process robustness. On acidic texture (multicrystalline solar cells), high plating rates were found to give better qualities compared to random pyramids, even without pre-treatment. Both behaviours are attributed to differences in local field and current density between sharply and smoothly shaped plating bases.

Boron LCP local back surface fields for high efficiency silicon solar cells

Abstract: This work shows the possibility to use local laser chemical processing (LCP) boron dopings to create local back surface fields (LBSF) in a high efficiency p-type silicon solar cell structure. In order to create boron local back surface fields with LCP an alkaline aqueous boron solution was used. Our recent work shows the potential of this doping source to create local dopings with a surface concentration of about 1020 cm−3 and a doping depth of up to 1.3 μm. For the first time the successful application of boron LCP as LBSF for high efficiency solar cells is shown. The processed LCP-PERL (passivated emitter and rear locally diffused) solar cells show a maximum cell efficiency of η = 20.9% and an absolute cell efficiency benefit of Δη = 0.3–0.4%abs. in comparison to the reference cells with undoped local rear contacts processed with water-LCP. The presented solar cells show the best cell efficiencies ever achieved with LCP dopings. For this study different LCP-parameters were varied such as the contact geometry (line openings and openings formed with a cascade of overlapping LCP dots) and the pitch of the rear side contact structure. The results show that the LCP boron doping process allows the fabrication of industrial feasible local back surface field structures with a line contact design for high efficiency p-type silicon solar cells.

Method for producing contact grid on surface of e.g. photovoltaic solar cell for converting incident electromagnetic radiation into electrical energy, involves electrochemically metalizing contact region with metal, which is not aluminum

Abstract: The method involves applying an aluminum-containing layer (5) on a surface of a semiconductor structure in a contact region i.e. cell contact region, of a surface. Aluminum is alloyed from the aluminum-containing layer into the semiconductor structure by thermal effect for forming a p-doped region (4), which is formed as a dopant by aluminum, in the semiconductor structure at the contact region. The aluminum-containing layer in the contact region is partially removed by an acid etching agent. The contact region is electrochemically metalized with metal, which is not aluminum. The acid etching agent contains hydrochloric acid, sodium hydroxide, and sodium trichloride. An independent claim is also included for a photovoltaic solar cell.

timalen Kontaktierung von Emittern sowie Verfahren zu dessen Herstellung

Abstract: Die vorliegende Erfindung betrifft ein Halbleiterbauteil aus einem Halbleitermaterial, das eine an einer Oberflache des Halbleiterbauteils angeordnete defektreiche Halbleiterschicht mit einer Defektkonzentration von mindestens 103 Defekten pro cm2 aufweist. Ebenso betrifft die vorliegende Erfindung ein Verfahren zur Herstellung des vorgenannten Halbleiterbauteils, bei dem eine Halbleiterschicht durch physikalische oder chemische Gasphasenabscheidung auf das Halbleitermaterial aufgetragen und in einem weiteren Arbeitsschritt, z. B. wahrend der Gasphasenabscheidung oder im Anschluss daran, versintert wird. Im Anschluss daran wird die gewunschte Defektkonzentration in der aufgetragenen Halbleiterschicht eingestellt.

Metal-containing composition, useful e.g. to produce contact structure, comprises electrically conductive metal powder and/or their alloy and/or organometallic compound of the conductive metal, first oxide material, and oxidizing agent

Abstract: Metal-containing composition comprises: (a) at least one electrically conductive metal powder and/or a powder of a metal alloy and/or at least one organometallic compound of the conductive metal; (b) at least one first oxide material comprising glasses, ceramics, metal oxides having a melting point of less than 1000[deg] C or organometallic compounds derived from metals contained in the ceramics and/or metal oxides; and (c) at least one oxidizing agent that releases oxygen in atomic or molecular form at 100-900[deg] C. An independent claim is included for producing an electrical contact structure on an electronic component comprising applying the composition on the electronic component in a form reproducing the contact structure to be produced and heating the component provided with the composition in a contact firing step to a temperature of 400-900[deg] C.

3D Optical Simulation of Scattering in Thin Film Silicon Solar Cells

Abstract: Efficient light trapping is of great importance for thin-film silicon solar cells. Randomly textured TCO or glass substrates provide excellent light trapping and are frequently applied. Optical simulation of such textures is challenging due to a large variety of occurring geometrical features. In this paper we suggest an approach for the 3D optical simulation of scattering structures. This approach is based on a simulation and investigation of the constituting single structure features and subsequent implementation of the solar cell structure into the rigorous coupled wave analysis (RCWA). In this paper we present first results of this method for a microcrystalline silicon solar cell deposited on a sputtered and etched ZnO surface. The simulated absorption for the single structures is compared to quantum efficiency measurements on large areas. Good agreement between simulation and measurement is found. Comparing periodic and random structures, we find that small periodic structures can increase the absorbed photocurrent density by up to 3mA/cm. The best result was obtained for a structure with a diameter of D= 800 nm.