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31st European Photovoltaic Solar Energy Conference and Exhibition; 429-432, In this paper we present results about excellent p-type emitters using a rapid vapour-phase direct doping (RVD) process. The main differences between the RVD process and standard BBr3 tube furnace emitters like single sided processing, short process durations and higher degree of freedom in doping profile design are discussed in detail. Lifetime samples featuring a simple RVD emitter on one side have been processed and resulted in an emitter saturation current of less than 16 fA/cm². Solar cells with the high efficiency TOPCon structure implementing this RVD emitter have been fabricated. They show open-circuit voltages of up to 687 mV, short circuit currents of up to 41.8 mA/cm², fill factors of up to 81% and a maximum efficiency of 23.3 %.

31st European Photovoltaic Solar Energy Conference and Exhibition; 288-291, In this publication we present free standing p- and n-type Si layers with thicknesses between 40 μm (EpiFoils) and 150 μm (EpiWafers) produced with epitaxy on electrochemically etched porous Si layers followed by mechanical lift-off. We show that reorganization and epitaxy in our quasi-inline tool leads to Si layers with very good crystal quality (etch pit density around 1000 cm-2) and low contamination levels (interstitial Fe concentration around 1010 cm-3). Thin p-type foils with just 40 μm thickness show minority carrier lifetimes of up to 80 μs mainly limited by a high dopant concentration of 5x1016 cm-3. The n-type epitaxial wafers with 150 μm thickness reach average minority carrier lifetimes of more than 1.5 ms. Appearing inhomogeneity of minority carrier lifetimes over the sample area are found to be due to surface cleaning process but not bulk quality related issues. The excellent lifetimes together with the very constant dopant concentration of our Si layers is sufficient for high efficiency solar cell architectures and a conversion efficiency of 20 % can be exceeded [1].

29th European Photovoltaic Solar Energy Conference and Exhibition; 1673-1677, This paper presents a summary of the EU project “roll to module processed crystalline silicon thinfilms” (R2M-Si). The aim of this project was to lift off c-Si-thin-films in a cost effective manner, and to process them to integrated interconnected solar cells. One major goal was therefore to provide Si foil material for solar cells with a high efficiency potential. Within the R2M-Si project both the theoretical understanding and the actual process development of porosifying and detaching layers from flat wafers as well as from the circumference of a silicon rod have been investigated. Studies on crystal orientation dependence of the porosification along with tool development towards a continuous formation of a separation layer were accompanied by in depth understanding and thus optimization of the reorganization and subsequent epitaxial growth process. Finally the introduction of a special attachment process and an integrated solar cell concept could be achieved.

29th European Photovoltaic Solar Energy Conference and Exhibition; 747-752, Cost reduction is still a major goal in photovoltaic development. Silicon foils offer the possibility to reduce silicon consumption and with this production costs while maintaining a high efficiency potential. In this work silicon foils are fabricated by implementation of a release layer in a silicon substrate, reorganization of this layer and epitaxial thickening prior to detachment. For the formation of the release layer two approaches are investigated, hydrogen implantation and electrochemical etching of porous silicon. Hydrogen implanted layers reorganize during heat treatment to layers with low amounts of small pores or with blisters depending on the implantation dose. The overall amount of pores can be increased by applying several implantation processes. With the more often applied electrochemically etched porous silicon as release layer the defect density in the foil deposited in a lab-type CVD reactor could be reduced to (6 ± 2) x 103 cm-2 by optimization of the reorganization process. Effective carrier lifetimes up to 56 μs are measured. Additionally, 5 x 5 cm2 foils deposited in a continuous CVD reactor are presented.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2657-2661, Silicon CVD is required to realise the upcoming “kerfless” c-Si thin-film solar cell concepts, like EpiCell, Lift-off and recrystallized wafer equivalent solar cells. To realise these solar cell concepts, a high throughput reactor with the possibility of continuous deposition is needed. For this reason, the ConCVD is the first epitaxy tool with a continuous wafer transport through the entire system. The requirements for such a machine are very sophisticated. Despite the open construction, the flammable and toxic reactor atmosphere must be separated from the surrounding atmosphere and the other way around, impurities must not get inside the reactor. This work reports on process improvement and solar cell results on layers deposited in the ConCVD at Fraunhofer ISE. The ConCVD (Continuous CVD) is a link between lab type reactors and a production reactor (ProConCVD), also developed at the Fraunhofer ISE. Setup improvements could enhance deposition conditions by providing a particle free environment. Furthermore, processes optimisations in this new setup lead to a solar cell result of 13.8%.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2630-2633, In this work the realization of a concept for integrated interconnected crystalline silicon thin film solar cells in a laboratory process is described. This module concept, referred to as IntegRex (Integrated interconnection of recrystallized silicon layers), is developed for recrystallized or lift-off crystalline silicon layers. Following the classical thin film approach the crystalline silicon thin film is divided into individual cell strips and interconnected monolithically on a substrate. To investigate the individual processing steps silicon on insulator (SOI) wafers with a buried oxide layer and epitaxially deposited back surface field (BSF) and base are used instead of recrystallized or lift-off crystalline silicon films. A first batch of IntegRex mini-modules on SOI wafers was processed. Despite a not yet optimized metallization scheme, a VOC of over 3 V could be reached for a mini-module consisting of five cells. Problems that occurred in the metallization process are presented here and a possible solution is discussed.

28th European Photovoltaic Solar Energy Conference and Exhibition; 827-830, Reducing the total costs of modules by increasing the efficiency of solar cells is one of the major challenges in today’s photovoltaic research. The emitter epitaxy by atmospheric pressure chemical vapour deposition (APCVD) offers a cost-efficient and faster alternative to the standard furnace diffusion process. The epitaxial emitter formation at 1050 °C only takes 1-2 min whereas the diffusion process using POCl3 takes up to 60 min. The purpose of this work is to show the potential of epitaxial grown emitters compared to diffused emitters. PC1D simulations show an increase in voltage of VOC = + 10 mV and a reduction in saturation current J0e of 30% for the epitaxial emitter. These advantages are due to lower surface recombination velocity and reduction of Auger recombination of the optimised emitter profile. The lifetime experiments including an epitaxial emitter show a diffusion length Leff of 750 μm and an emitter saturation current of J0e = 46 fA/cm² on a planar 10 cm p-type FZ wafer. Another important aim of this work is to determine the limitations of epitaxial emitters due to thermal degradation of the base material, interface recombination and the change of reflective properties on textured wafers due the deposition process. In a first batch, solar cell efficiencies up to 18.4 % underline that emitter epitaxy by APCVD is a competitive process for the emitter formation.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2662-2665, Low-cost high-throughput epitaxy is one of the keystones for reducing the PV module cost to far below 50 €ct/Wp by new “kerfless” cell concepts. At Fraunhofer ISE the ProConCVD (Production Continuous Chemical Vapour Deposition) was developed to demonstrate feasibility of such a process. First long term experiments could proof the reliability and the reproducibility of the tool. 1300 multicrystaline silicon (mc-Si) wafers of 156 mm x 156 mm size were coated during 24 experiments in 4 weeks. With SiCl4 as a precursor it was possible to deposit 9 μm thick epitaxial layers within 45 minutes. Thickness homogeneity higher than 90% was achieved over nearly 270 mm deposition width. Since at the edges of the reaction chamber a reduction of the deposited Si film can be observed, we investigated in Computational Fluid Dynamic (CFD) to identify the underlying reasons. The purpose of this work is to report the potential and the limitations of the current models. It could be shown that the gas inlet setup of the ProConCVD already uses 80 % of its theoretical potential in reference to the growth rate and the layer thickness homogeneity in transport direction is higher than 87 %.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2453-2458, A concept for integrated interconnected crystalline silicon thin film solar cells is presented. Following the classical thin film approach the crystalline silicon thin film is divided into individual cell stripes and interconnected monolithically, directly on the substrate. Taking advantage of the crystalline quality of the material, the interconnection is realized using screen printing pastes. In this paper we focus our experimental investigations on the separation of cell stripes and the interconnection via screen printing. The cell separation is either done by oxide masking and KOH etching or by laser ablation and a subsequent etch step. For the interconnection not only metallization pastes are required, but also an isolating material to protect the edge of the cell against shunting. Different isolating screen printing pastes were tested for their isolation properties against metallization pastes. In further experiments the influence of different firing parameters was investigated. The cell stripe width strongly affects the series resistance of the module, since it is a single side contacted concept where only the back surface field (BSF) is responsible for the electrical conductivity on the rear. Therefore the series resistance is estimated in dependence on the cell stripe width.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2386-2392, The rapid thermal direct deposition of micro-crystalline silicon (μc-Si) layers by atmospheric pressure chemical vapour deposition (APCVD) can be done on different intermediate layers and on various substrates. The deposition is done at temperatures between 850 °C and 1150 °C. A deposition rate of 1.6 μm/min has been achieved using standard process conditions. The microcrystalline structure changes depending on the deposition temperature and the layer thickness. Electron backscatter diffraction measurements show grain sizes of 0.5-10.0 μm with a columnar structure. The doping concentration depending on cell concept and layer thickness can be adjusted from 1 x 1016 cm-3 to 1 x 1020 cm-3 using diborane and from 1 x 1017 cm-3 to 1 x 1020 cm-3 using phosphine. Based on these process parameters and material properties the μc-Si layers have been simulated and optimised for two different solar cell concepts featuring different metallisation schemes and a comparison between epitaxial μc-Si emitter, diffused POCl3 emitter, and plasma enhanced chemical vapour deposition (PECVD) deposited a-Si heterojunction solar cells.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2466-2470, The ongoing challenge for cost reduction of PV modules to below 0.5 €/Wp demands for solutions combining significantly reduced material usage and consumables cost with high module efficiency. Crystalline silicon thin-film (CSiTF) solar cells can play a major role in achieving these cost targets. The most crucial process for the respective concepts, namely the cost-effective silicon deposition and epitaxy, is still not available in an industrial environment. We have developed the ProConCVD, a high-throughput prototype chemical vapour deposition (CVD) reactor for silicon epitaxy capable of a throughput more than 1000 wafers/h. In a high-volume production, this reactor will be able to process silicon layers for less than 10 €ct/wafer, by this enabling breakthrough of the most competitive CSiTF solar cell concepts like Lift-off, EpiCell or Recrystallized Wafer Equivalent approaches. At present, the ProConCVD has been fully set up. This paper reports the first CFD (Computational Fluid Dynamic) simulations, as well as process test and deposition results.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2459-2462, This paper presents the investigations on two major process steps for recrystallised wafer equivalent solar cells: First the properties of the intermediate layer with respect to its optical performance in the material and in the final device were evaluated. Second, the performance of different doping concentrations in back surface fields and the resulting effects on passivation and lateral conductivity in a laser fired access concept have been investigated. Due to these process optimisations efficiencies of 13.2 % have been obtained. So far a Jsc of 33.3 mA/cm², Voc of 614 mV and FF of 78 % have been achieved as champion values however these values were measured on different samples. This paper presents the comprehensive analysis of the shortcomings, the underlying effects and presents ways to remedy those.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2202-2205, ThinSi is a medium-scale focused research project founded by the European Commission, which aims to develop a solar cell processing chain for high throughput, cost-effective manufacturing of thin film silicon based solar cells on low-cost silicon supporting substrates. The substrates are made from the low-cost Si powder on the basis of innovative powder-to-substrate concepts using (i) spark plasma sintering, (ii) thermal spay, (iii) any other alternative method. Moreover, casting of a multi-Si ingot followed by sawing of wafers is used as a reference approach to prepare wafer equivalent based solar cells. Essential part of the ThinSi activity was concentrated so far on fabrication of Si powder with the desired purity, crystallinity and particle size. Experience from the partners using the Si powder for thermal spraying and tape casting followed by a spark plasma sintering (SPS), showed that a special refinement and adjustment of the Si powder quality was needed for each Si wafer sintering method, both with regard to conductivity and particle size distribution. It was established that a spark plasma sintering (SPS) of silicon powder with and without doping elements is a promising method to produce Si wafers with the desirable properties (dense structure and conductivity < 0.01 Ωcm) with diameters up to 200 mm2. Free standing ~300-500 μm thick 50x65 mm2 as well as 156x156 mm2 Si wafers have been produced from a low-cost and solar grade Si powders by the thermal spray method. These wafers exhibit all necessary properties (mechanical stability, ~0.01 Ωcm conductivity), Cost effective processes for the formation of a thin film silicon base on top of the low-cost supporting substrate based on: (i) high-temperature CVD, (ii) high temperature magnetron sputtering, (iii) thermal spray are under the development in this project. An advanced approach for the optical confinement based on incorporation of a porous Si Bragg reflector in the thin Si solar cell structure is applied for the solar cell processing. First solar cells on Si powder based substrates prepared via wafering of an ingot made of re-melted B-doped low purity silicon powder have been processed. The conversion efficiency for the best solar cell processed so far reached 11.9%.