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35th European Photovoltaic Solar Energy Conference and Exhibition; 431-433, Silicon heterojunction (SHJ) solar cells can be manufactured through relatively simple process steps. However, optoelectrical losses in doped a-Si:H and complex optimization of the doping process makes the cell fabrication difficult. Therefore, an electron contact without n-a-Si:H would be interesting and the i-a-Si:H/TiOx/low work function metal (ATOM) contact could be one of the alternatives. In this study, TiOx grown by ALD and Yb (: 2.5-2.6 eV) are used in the ATOM contact. Specific contact resistivities (c) and dark saturation current density at metalized area (J0,metal) are investigated as a function of a TiOx thickness. 1 nm thick TiOx with Yb/Ag stack on 8 nm thick i-a-Si:H achieves the minimum c of 48 mΩ·cm2 and a J0,metal below 15 fA/cm2.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 740-744, The i2-module concept is one of the approaches for processing very thin (< 50 μm) silicon foils into highly-efficient back-contacted devices. In this work, we process kerfless epitaxial foils, that are lifted-off and layertransferred onto glass, into heterojunction interdigitated back-contacted (HJ-IBC) devices. Ethylene vinyl acetate (EVA) is used as the bonding agent. As reference, thick and thin float zone wafers are used. With a carefully-selected and optimized process sequence that is compatible with EVA-bonded silicon, we obtain extraordinary VOC values of 738 mV and 751 mV for the thick and thin FZ cells, respectively, which demonstrates the high quality rear-side passivation achieved with our process. However, epifoil cells have a much lower VOC compared to the FZ cells and this is attributed to a lower bulk lifetime (30 μs). The best efficiency for an epifoil cell is 16.1% and that for a thick FZ cell bonded using EVA is 21.1%. Improving the epifoil lifetime to a level comparable to that of FZ material could increase the epifoil efficiency to above 20%.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 262-265, In a previous contribution, we presented a multi-wire approach for interconnecting back-contact cells. While a proof-of-concept was clearly demonstrated, some remaining fill factor (FF) losses were not fully explained and initiated the need for a more in-depth analysis. The current abstract therefore reports on this analysis. As a first step, we explain the reasoning towards an optimized theoretical cell-module co-design for multibusbar interdigitated back-contact cells. Indicating the opportunities, we also present the measured results of the fabricated demo modules. Then, looking closely at the remaining FF losses, we determine a hypothesis for this unforeseen pitfall and confirm it through basic circuit simulations. Relying on this hypothesis, the solution is very straightforward and could be implemented at either cell- or interconnect-level. After also experimentally verifying this phenomenon and its solution, we finally present 1-cell laminates to indicate the potential of the technology in terms of Cell-to-Module-performance and compare with previous modeling results on expected FF loss.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 765-767, We demonstrate solar cell fabrication on large area (125mm 125mm) epitaxial wafers of 50μm thickness. During handling and transport no mechanical support was used for the wafers, and the process was identical to the process that was used for 50μm and 160μm Czochralski wafers. Due to the wafer preparation procedure, thin epitaxial wafers were found to be mechanically weaker than Czochralski wafers. This resulted in wafer breakage especially during wet processing. The efficiency of epitaxial cells reached 17%, which was lower than the 19% and 20% efficiencies obtained with 50μm and 160μm Czochralski wafers. The cell results were thus found to be a good metric for substrate quality.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 328-330, In imec’s i2-module concept, silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells are fabricated on monocrystalline foils bonded to glass. The proposed technology allows for cell processing on thin wafers mechanically supported by the glass, increasing the yield of processing such thin wafers. A process sequence for SHJ-IBC cell fabrication that can be applied to bonded thin foils is described. We investigated and optimized individual process steps on thick wafers. Then the developed steps were integrated into a process flow to fabricate solar cells on wafers with different thicknesses and bonding agents. On wafers with a thickness of 190 μm, functional cells with efficiencies of 22.6% and 21.7% were made on freestanding and silicone bonded wafers, respectively. On thin wafers of 57 μm, our best SHJ-IBC cell on an EVA bonded wafer exhibits excellent Voc of 740 mV and efficiency of 20.0%, which demonstrates the high potential of the i2-module concept.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1-4, This paper reports on a novel and highly innovative technology to interconnect back-contact cells. First a background section explains the currently developed interconnection technologies. Then our novel concept is introduced: interconnection of the cells is achieved with a woven fabric, combining both conductive wires, needed for the interconnection, and insulating wires, crucial to avoid shunting between both polarities at the backside of the cell. Weaving also allows out-of-plane stress relief features. In the third section, we explain that our technology combines the advantages of multi-wire interconnection such as low-stress and reduced resistive losses with known and low-cost materials of traditional module assembly. We demonstrate that the technology can be tuned broadly in terms of costs and resistive losses using an optimized interconnection pattern. These loss simulations are also validated with the measured additional resistance in IBC test samples. Finally, first functional proof-of-concept with MWT cells as well as IBC cells have been fabricated.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 915-921, Standardizing illuminated IV measurements of bifacial solar cells and modules is a central objective for the introduction of bifacial products into the market. In this paper, the application of monofacial IV measurement conditions to bifacial solar cells is evaluated in an inter-laboratory comparison among seven research institutes in Europe. Bifacial silicon solar cells manufactured at five different sites with five different fabrication technologies were used in this investigation. We demonstrate that several characteristics of the measurement setups which are of minor importance for the measurement of monofacial solar cells can significantly affect bifacial solar cell measurements: (i) the reflectance of the measurement chuck and (ii) the electrical conductance of the chuck implying specific contacting schemes. When dividing the measurement results of this round robin into two groups according to chuck reflectance and conductance, the deviations in the IV parameters among the different partners are mostly within the uncertainty limits commonly reported for monofacial solar cell measurements. For standardization of bifacial solar cell measurements, it is therefore important to define admissible ranges for the chuck reflectance and to specify the contacting scheme in the standard.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 313-316, Layer transfer process based on a porous silicon template is one of the promising replacements for the conventional wafer sawing technologies. For the process to be cost-effective, one challenge is the multiple recycling of the Si substrate used as a template for epitaxial growth. The Si substrate after detachment has different types of defects. Therefore, reusing the substrate without surface reconditioning induces low detachment yields and generates low quality epitaxial Si-foils. The current work presents the first steps taken into the realization of successful substrate re-use. Firstly, possible defects and undesirable features on the surface after the detachment process are identified. Secondly, a wet-chemical reconditioning process is developed. Finally, the effectiveness of this reconditioning process is evaluated by monitoring the evolution of the foil quality through different generations. Until now, 5 generations of epitaxial foils have been demonstrated at imec.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 350-353, Today, the silicon solar cells with the highest efficiency are interdigitated back contact solar cells. At imec, the cell concept for 15.6x15.6 cm2 IBC cells relies on a rear design with multiple busbars and a Cu plated rear metallization. Previously, we reported efficiencies up to 21.9% by introducing Cu plated contacts. Further developments in the rear design to overcome FF and Jsc losses were implemented and reported as well, showing IBC cells with 22.5% efficiency. In a next optimization round, the passivation quality at the rear of these IBC cells needed to be improved. To do so, we demonstrate in this paper the application of a rear Al2O3 based passivation stack on both emitter and BSF. A 10 nm ALD Al2O3 - PECVD SiOx stack was implemented and a thermal activation step (firing) post deposition was applied and optimized to guarantee a good passivation on both the emitter and the BSF. Best cells with this process showed an efficiency of 22.8% and Voc up to 688mV.

31st European Photovoltaic Solar Energy Conference and Exhibition; 377-380, IBC process development at imec has focused on upscaling from 2x2 cm2 to full area 15.6x15.6 cm2 cells, while making use of industrially viable processing techniques. First results were published recently, with a best cell efficiency of 21.3%, limited by a FF of 77.4% due to high series resistance. The process flow to obtain this result involved a 3 m thick sputtered aluminium metal layer, which is not straight forward from an industrial perspective and not readily viable for module interconnection. We therefore developed a Cu-plated metallization for large area IBC cells. This metallization is the topic of this work. We developed and studied a suitable thin sputtered seed layer for the plating process, which serves as a barrier against Cu and which has good contact properties to Si. Sputtering of various materials could cause damage to the underlying passivation layer and Si, at cell level leading to lower Voc and pFF. The damage caused by the different layers is investigated in detail and the developed process is implemented on 15.6x15.6 cm2 IBC cells which yields efficiencies up to 21.9% over the full wafer area of 239 cm2.

31st European Photovoltaic Solar Energy Conference and Exhibition; 466-469, We propose a new process flow for interdigitated back contact (IBC) cells, based on plasma enhanced chemical vapor deposition (PECVD) SiOx and silicon epitaxy. These technologies contribute to a simplification of the flow by reducing the number of high temperature processing steps. We show that PECVD SiOx followed by thermal annealing exhibits a passivation quality equivalent to thermal oxide. Moreover we demonstrate that both the p-type emitter and the n-type BSF can be realized by epitaxial growth of silicon. Solar cells of 20mm×20mm have been fabricated, both with a homogeneous BSF and with a local BSF architecture. Best cells achieved an efficiency of 22.9% and 23.2% for homogeneous BSF and local BSF respectively.

31st European Photovoltaic Solar Energy Conference and Exhibition; 1997-2000, In this paper we would demonstrate how we developed a model to determine cell-to-module performance starting from available literature. After fabricating 60-cell modules with 2 different types of cells and 2 different stringing configurations, measurements of these modules then allowed us to validate the developed model. In an attempt to demonstrate potential applications for this model, we then show how potential improvements can be quantified and pinpoint which losses are affected. This way, we indicate a potential for improving the fabricated MWT modules with an increase in absolute efficiency up to ~1.7% for the module. On top of that we demonstrate that an additional increase of ~0.1% in absolute efficiency can be obtained by dividing the cells, e.g. into half or third cells.

29th European Photovoltaic Solar Energy Conference and Exhibition; 523-527, For cost reduction as well as reduction of environmental footprint of solar cells, it is important to reduce the use of silver (Ag) in metallisation. In this work we report on large reduction of Ag consumption for industrial high-efficiency bifacial n-type cells with front and rear Ag grid, by modest process changes. Both frontand- rear contacted, as well as metal-wrap-through cells, are considered. The approaches explored are, for the front, seeding with a small amount of Ag fire-through ink or paste, and plate the thus formed contact grid with copper (Cu). For the rear we use the same approach, or replace the Ag grid by a Al layer deposted by physical vapour deposition (PVD). Front Ag consumption of 10mg per wafer and rear Ag consumption of 20mg per wafer were demonstrated for inkjet seeding. The front finger width after inkjet seed&plate is about 55m. With screen print or stencil print seeding a significant Ag reduction can be obtained too, but not to the same level as with inkjet printing. The replacement of a rear Ag grid by an Al contact layer deposited by PVD requires optimisation of the rear dielectric in order to obtain good rear internal reflection. We model and experimentally demonstrate that a small modification of the rear dielectric already yields an acceptable reflection. In all experiments we find approximately equal or better efficiency than in the reference process; and also the cost of ownership estimates are favorable. This shows that a better environmental footprint can be well combined with improved production cost.

29th European Photovoltaic Solar Energy Conference and Exhibition; 497-502, This work presents an overview of the opportunities in bulk crystalline silicon photovoltaics that have been explored using silicon epitaxy as doping technology. Epitaxy demonstrates to be an elegant and versatile technology which brings a lot of new opportunities to further simplify and improve the design and performance of bulk solar cells. Advantages are the doping profile flexibility, the reduced thermal budget, the absence of additional steps to remove glassy layers or activate dopants, the simplified integration of local doping by means of selective epitaxy, and the possibility of single-side deposition. The results presented herein demonstrate the potential of epitaxy by applying the process in three cell structures to grow a boron-doped layer. First, epitaxy is used to grow blanket doped layers as emitters on the full rear side of n-type PERT cells. Second, selective epitaxy is applied to locally grow the interdigitated emitter in n-type IBC cells. Third, selective epitaxy is applied to form the local BSF in p-type PERL cells. For each of these cell concepts, silicon epitaxy helped to simplify the reference BBr3 diffusion-based process, while keeping high efficiencies: 20.5 % for n-type PERT (226 cm2 cell), 22.8 % for IBC (4 cm2 cell) and at least +0.5 mA/cm2 and +10 % escape reflectance for p-type PERL cells compared to the standard PERC.

29th European Photovoltaic Solar Energy Conference and Exhibition; 834-837, We demonstrate a numerically optimized double layer antireflective coating (DLARC) composed of dielectric materials, the application of which led to the imec best IBC solar cell of 23.4 [%]. Synthesis, optimization procedures and technological aspects were investigated in the context of application of the DLARC to the existing IBC cell architecture at imec. The numerical simulation results are demonstrated on both planar and textured surfaces. Good agreement with experimental data is obtained. Subsequently, the influence of deposition error posed by industrial PECVD deposition on the spectral characteristics of the front reflectance is investigated numerically, and confirmed experimentally. The constraints for stable, repetitive deposition of such multilayer coating are extracted. Finally, the design was tested on the cell level yielding 0.26 [%] average, absolute efficiency gain. The cell were 4 [cm2] in size, manufactured on Czochralski-grown silicon.

Porous silicon plays an important role in the concept of wafer-equivalent epitaxial thin-film solar cells. Although porous silicon is beneficial in terms of long-wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron-doped (1016/cm3) epitaxial layers grown on reorganised porous silicon as well as on pristine mono-crystalline, Czochralski, p+ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100-115s) than that of layers on top of bare p+ substrate (32-50s). It is believed that proper surfa ce closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p+ substrate interface (S103cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface.

28th European Photovoltaic Solar Energy Conference and Exhibition; 941-946, The present R&D activities in crystalline silicon photovoltaics include the exploration of doping technologies alternative to the mainstream diffusion process. The goal is to identify those technologies with potential to increase the solar cell efficiency and reduce the cost per watt peak. In that respect, this work presents the selective epitaxial growth (SEG) of silicon as a candidate for boron doping; showing the results of the evaluation of borondoped silicon epitaxial emitters on slurry and diamond-coated wire sliced Czochralski material, their integration in IBC solar cells, and the development of a novel process sequence to create the interdigitated rear junction of these devices using SEG. Boron-doped silicon epitaxy is demonstrated to perform in the high efficiency range (>22 %) and the use of SEG is proposed as an alternative route to simplify the number of steps in the IBC flow.

28th European Photovoltaic Solar Energy Conference and Exhibition; 956-960, A baseline process for small area (4 cm2) interdigitated back contact (IBC) silicon solar cells at imec is presented, based on n-type 156x156 mm2 CZ silicon wafers. This process has been stabilised, and best obtained (calibrated) conversion efficiencies of 23.1% (average 22.8%) have been achieved. Recent developments have focused on process simplification, and are the subject of this report. The key findings include the benefits of introducing a wet oxidation step as the boron activation/emitter-passivation step (higher sheet resistance, lower J0, and consequently higher efficiency). The surface cleaning routines have also been revised, resulting in slight increases in efficiency for simplified cleaning process (with consequent reduced process time/chemical consumption). Finally, results on contact definition by laser ablation are presented, showing a strong impact of the contact grid density on the BSF and emitter regions, on cell performance. The maximum achieved efficiency of 22.9% shows the potential for lithography replacement with an industrially viable patterning technique. These steps are driven by the goal of achieving a simplified and cost effective IBC process flow reaching high cell efficiencies.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2471-2474, The creation of foils by lifting off a thin, high quality layer of a silicon substrate is one of the promising substitutes for wafer sawing to create substrates thinner than 100 μm. The porous silicon-based layer transfer process is a well known method to obtain high quality foils. Despite a number of convincing lab-based solar cell show-cases, there is no breakthrough of this technology at (semi)-industrial level, because of the poor yield of processing free standing foils. This paper presents a method to fabricate back contacted solar cells based on epitaxial foils avoiding processes on free-standing foils. First, a porous silicon layer is electrochemically etched, acting as a weak sacrificial layer to detach the foil that is epitaxially grown on top of the porous silicon layer. An effective lifetime around 100 μs is obtained for those epitaxial foils, indicating sufficiently good quality for solar cell fabrication. Front-side processing is done while the epitaxial foil is still attached to its parent substrate. Crack initiation is obtained by laser scribing the foil. A UV nanosecond pulse laser is used to induce the weakening by scribing trenches. Afterwards, the front-side of the foil is permanently bonded to a quartz carrier by silicone adhesives and detached from its parent substrate. This lift-off process is studied in detail. A yield of around 80% is obtained for samples that went through the process chain up to the detachment step. The rear side of the solar cell is processed while bonded to glass. So far, the rear-side processing sequence was performed on Float-zone reference wafers as a proof of concept and still needs to be applied on epitaxial foils.

27th European Photovoltaic Solar Energy Conference and Exhibition; 571-575, At imec a small area (4 cm2) baseline process for interdigitated back contact (IBC) silicon solar cells based on n-type float-zone (FZ) and Czochralski (CZ) wafers has been developed. We obtained best cell conversion efficiencies of 23.3 % and 22.8 % on n-type 100 mm FZ and 125 mm CZ silicon substrates, respectively. Performing an identical process on one particular set of 156 mm n-type CZ substrates resulted in a large distribution of cell efficiencies within one wafer, likely to be related to the formation of oxygen precipitates during the high temperature process steps leading to degradation of the bulk lifetime. Further investigation will be done to make the process more tolerant to be applied on a variety of CZ substrates with different oxygen concentration. This IBC baseline serves as a test vehicle to innovate the integration sequence and is a starting point to upscale the IBC process to large-area IBC cells and explore new finger grid designs on 156 mm CZ silicon wafers. The final goal is to realize a simplified and cost effective IBC process flow reaching cell efficiencies above 22%.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1165-1170, The handling of thin wafers in today’s production lines demands high standards of the automation as well as complex investigations with a closer look on the actual and future needs for an economic and competitive production of solar cells. This paper describes the analysis and evaluation methods developed by the authors for ultrathin wafers (

27th European Photovoltaic Solar Energy Conference and Exhibition; 2207-2211, There are several reasons why key actors in the PV industry target thinner Si cells. It is an unresolved question whether stand-alone wafer processing will ever become a yielding production process on wafers thinner than ~80 μm. The approach proposed in this paper, termed i2-module, is targeted to back contact cells with a thickness down to 40 μm. The i2- module concept, aims at interconnecting very thin (~ 40 μm) back-contact cells. This is a disruptive approach starting from (epitaxial) Si foils which are silicone-glued to the module. Subsequent process steps like patterning, BSF formation and metallization, which would be critical to execute on stand-alone wafers of this thickness, are executed on bonded tiles of wafers, at the “module level”. We report here experimental results both related to cell processing and to thin wafer supply. The process development has been initiated using 170 μm thick FZ Si wafers as test platform. Front side of the wafers were processed according to imec’s i2BC baseline including a back side emitter, afterwards they were subsequently silicone- ’glued’ to the module. Further rear-side processing is done while the substrates are bonded to the module glass and should therefore have to be compatible with the glass and silicone glue. Epitaxial foils are investigated as a “supply” of very thin Si wafers. Processes have been developed for growing these 40 μm thick foils on top of a weak porous silicon layer. Those foils are transferred to the module glass after front side processing is finished, using a silicone-based glue. Afterwards the parent substrate can be removed and the rear-side processes developed at this moment for reference FZ bulk wafers will be applied on these bonded foils.

26th European Photovoltaic Solar Energy Conference and Exhibition; 1418-1422, It has already been proven that Al2O3 can provide a high level of surface passivation for p and n-type crystalline silicon (c-Si) [1-4]. The passivation mechanism is considered to be related to two components: low interfacial trap density (Dit) and high amount of negative fixed charges (Qf), which induce a built-in electric field. In order to get understanding on the electrical and process parameters which govern the passivation quality, a Corona charging tool was employed. This technique enables to bring together spatially resolved information on charges and carrier lifetime. Furthermore, the injection level can be tuned, which allows decoupling the bulk and surface contribution in the carrier lifetime measurement. Two main process parameters: the Al2O3 thickness and the post deposition anneal were varied to evaluate the sensitivity of the technique. The carrier lifetime information extracted from the “Quantox” shows to be consistent with the one extracted by QSSPC-PL measurements. As a result, the “Quantox” tool appears to be an appropriate tool for solar applications in order to study the quality of any passivation layers and Al 2O3 in particular.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 381-384, The classification of solar cells and the comparison of cells from different origins is typically based on the cell efficiencies. As a consequence, the efficiency and the related parameters of short circuit current, open-circuit voltage and fill factor need to be measured precisely. This requires an IV setup with a small bias error and a good stability over time. In this study, an IV setup is gradually improved by the stepwise implementation of several adaptations. The effect of these improvements on the measured solar cell parameters is tracked and after completion of the improvement process, the measured parameters are compared to the values measured at a calibration lab to quantify the bias error of the final setup.