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8th World Conference on Photovoltaic Energy Conversion; 708-711, A new approach is presented to determine the angular response of PV modules outdoor, using a dual axis solar tracker and a reference module with known angular response. The method is based on measuring Isc data for both sample and reference module, rotating both modules away from the sun to vary the angle of incidence from 0-90 °. From these data, the Incident Angle Modifier (IAM) curve is calculated. Thanks to the quick rotation (< 1 min), no temperature sensors are needed. For outdoor calibration of reference (mini-)modules, a 1.2 m long tube is used, in which the reference module is rotated while the tube is tracking the sun. In this way, the IAM curve can be determined without diffuse light disturbance. Using such a reference module, IAM curves were determined for several modules. In particular, a mini-module previously measured indoor at PI-Berlin was measured, the maximum difference between the two curves was 1.1%. Compared to the outdoor IAM measurement of PV modules in IEC 61853-2, the method above requires less equipment/sensors and is faster. The method is also accurate, the use of a reference module and the calculation method ensure correction for the varying diffuse irradiation across the sky.

8th World Conference on Photovoltaic Energy Conversion; 678-681, Current more novel interconnection methods for Si solar cells such as shingling, multi-wire, conductive backsheet, and multi-ribbon approaches, are structurally more complex than conventional 2-5 busbar stringing. The most common techniques used for the development of these concepts are electrical characterization (current-voltage measurements and electroluminescence imaging) in combination with a cross-section analysis using SEM and EDX. Other characterization methods, such as scanning acoustic microscopy (SAM) are implemented, especially to search for voids and delamination within the module. Using micro-computed tomography (μCT) is reported as a viable method to inspect new module concepts in the past, however, very few effective results are published. Current state-of-the-art μCT devices are capable to achieve a lower voxel size and higher resolution on larger samples than ever before. In this work, the multi-ribbon interconnection for back-contact cells under development at imec is being used as a case study to investigate the potential of these new μCT devices for PV research. A 20 x 20 cm2 one-cell module is electrically characterized before, during and after reliability tests in the form of thermal cycling and humidity-freeze cycling. The results show an unstable shunt being present in the module. Before and after the reliability tests, a TESCAN UniTOM XL scanner is used to scan the one-cell module using μCT. Due to a large amount of metal in the module, artifacts such as photon scattering and shadowing in the region around the ribbons were observed. The use of binning and different metal filters was tested as a way to decrease these artifacts. Using a 1 mm tin filter in combination with binning seems to achieve the best results when scanning (part of) a PV module.

8th World Conference on Photovoltaic Energy Conversion; 1099-1104, In this work we give an overview of the module monitoring results achieved within the H2020 funded project called HighLite. A dedicated online monitoring platform has been developed for the project to group and treat the data from all partners with homogeneous procedures. Monitoring of advanced prototype modules developed in the project and based on various cells and interconnection approaches, like silicon-heterojunction (SHJ)-shingle, TopCon-shingle, cut interdigital back-contact cells (cut-IBC) are reported. We describe key learnings from monitoring modules for various applications including building-applied PV (BAPV) and building-integrated PV (BIPV). Annual energy yield up to 1900 kWh/kWp is expected in Cadarache (France) based on HighLite PV technology. Temperature corrected performance ratios are used for a ranking of the new technologies against the reference PERC modules.

38th European Photovoltaic Solar Energy Conference and Exhibition; 670-673, Since huge numbers of PV modules are expected to be discarded over the next decades, it is important to think about end-of-life management for those modules, like recycling and re-use. However, the re-use of decommissioned modules is a quite complex subject since there are concerns from technical, economic and legislative point of view. An evaluation of possible applications for second-hand modules shows that currently, the use of these modules in high-income countries is only interesting to repair PV systems (usually still receiving feed-in tariff) and to replace old modules to extend PV system lifetime. For developing regions in Africa and Asia, second-hand modules are interesting to build new small to medium size PV systems (often off-grid). The typical decommissioned module is a crystalline silicon glass-backsheet module from a utility power plant. Most modules originate from plants that have been partly damaged by severe weather or from repowered plants. Currently, technical requirements to qualify PV modules for re-use are lacking. In the legislation also a clear criterion is needed for a module to be considered as a functional product instead of waste, since it is not an easy yes/no situation like for a typical electronic device. In this paper, it is proposed to set a clear performance threshold at 70% of the original power. Also guidelines for a low-cost quality inspection and cost-effective module repair are given. With this, we aim to open the dialogue on a commonly accepted protocol and criteria. Currently, the worldwide re-use market size is estimated to be around 1 GWp/year, of which 0.3 GWp/year originating from Europe (mainly Germany, with Italy rapidly coming up). Many second-hand modules are shipped to developing countries without recycling facilities which might be a risk for disposal on the longer term. To create a healthy and sustainable market for re-use of modules it will be important that standards for re-use module evaluation become available and that the existing electronic waste legislation will be adapted for energy producing products like PV modules.

38th European Photovoltaic Solar Energy Conference and Exhibition; 24-27, In this work we report on our approach on integrating c-Si PV into lightweight structures, in particular towards vehicle integration. To this end we want to get rid of the (bulk weight of the) glass but seek a suitable replacement in terms of mechanical stability. First we elaborate on the most basic standards and norms that VIPV products should relate to in terms of (thermo-)mechanical testing. Then, for the experimental part, 2 concepts are investigated. In a first approach, we reinforced the encapsulant with glass fibre material, while in a second one we applied a dedicated glass-fibre-reinforced sheet as a replacement of the backsheet. In both cases we stay as close as possible to using commercially available material. Extending our previously reported work, we then elaborate for each approach the testing that has been carried out so far, and what is still ongoing and being planned in the short-term future (before the conference): indicating the promising results so far on thermal cycling and vibration testing, the hail impact testing is a challenge to be tackled still

38th European Photovoltaic Solar Energy Conference and Exhibition; 533-537, The EU crystalline silicon (c-Si) PV manufacturing industry has faced strong foreign competition in the last decade. To strive in this competitive environment, the EU c-Si PV manufacturing industry needs to focus on highly performing c-Si PV technologies and on reducing environmental impact (lower CO2 footprint, improved sustainability, recyclability, etc.). In this work, we give an overview of the latest results achieved within the H2020 funded project called HighLite which addresses the work programme LC-SC3-RES-15-2019: Increase the competitiveness of the EU PV manufacturing industry. We report the progress made when implementing novel layers and processes in existing pilot-lines producing high-efficiency n-type silicon heterojunction (SHJ) cells with a shingle layout and interdigitated back-contact (IBC) cells with passivated contacts. In particular, we present the work done together with several industrial partners towards innovative production equipment for assembly of cut-cells into PV modules. Finally, we explain key learnings made so far when tailoring c-Si PV modules for various applications including building-applied PV (BAPV), building-integrated PV (BIPV) and vehicle-integrated PV (VIPV).

38th European Photovoltaic Solar Energy Conference and Exhibition; 728-731, Back-contact solar cells are gaining terrain in the PV industry, although the lack of a standardized and low-cost interconnection method inhibits their market growth. To overcome this challenge, the three-dimensional multi-ribbon interconnection is proposed by imec as a novel interconnection concept for highly efficient back-contact cells. It is based on an interconnection fabric of encapsulant with incorporated metal ribbons that form a three-dimensional network. Module fabrication consists of a layup phase and a lamination step. In the layup step, a stack is established with the frontsheet, a front encapsulant, the cells, the interconnection fabric a glass or polymer backsheet. During the lamination step, soldering and encapsulation happen simultaneously. In previous research, the concept was tested in two four-cell modules with interdigitated back-contact (IBC) cells, both passing three times the IEC 61215 standard for thermal cycling reliability testing. In this work, these modules were tested using damp-heat and humidity-freeze tests. No significant sign of degradation was observed based on electrical characterization and electroluminescence imaging after consecutive 1000 hours of damp-heat and 50 cycles of humidity-freeze exposure.

38th European Photovoltaic Solar Energy Conference and Exhibition; 181-185, The aim of this work is to demonstrate the maturity of the TOPCon technology by conducting a round-robin on symmetrically processed lifetime samples in the leading European PV institutes EPFL, ISC, CEA-INES, ISFH, IMEC and Fraunhofer ISE within the H2020 funded project called HighLite. For all layers, dark saturation current-densities ranging between 2-10 fA/cm² can be reported. Simultaneously, no metal induced recombination for the two lower sintering temperatures have been observed pointing towards a true passivated contact. Furthermore, contact resistivities below 10 mcm² have been achieved. It seems that the industrial passivating contact matured to a fully passivated and conducting contact enabling full efficiency potential. The fact that this can be realized using either PECVD or LPCVD from various manufacturer is expected to drive costs down and contribute to the increased adoption of the TOPCon technology.

In this work we propose an approach for integrating c-Si PV into lightweight structures targeting applications away from more standard PV installations as BAPV and ground-mounted PV power plants. Allowing also to adapt to curved surfaces, this approach would be very suitable for vehicle integration. The technology is based on reinforcing the encapsulant with glass fibres, and in first instance is being developed with commercially available material. After motivating the work in an introduction, some considerations are outlined in terms of performance, cost, weight, curvature, aesthetics and reliability. These will need to be translated into requirements in the future. Then we elaborate on the initial tests that have been carried out. Using either standard tabbing-stringing and shingled interconnection, stacks with different material combinations were prepared and laminated. After testing the IV-performance, the laminates have been submitted to thermal cycling and are being monitored. The initial results that are available are reported.

36th European Photovoltaic Solar Energy Conference and Exhibition; 157-160, This work reports the latest results at imec of rear emitter bifacial Passivated Emitter, Rear Totally diffused (PERT) cells, processed on large area (243.3 cm2), 170 µm thick n-type Cz substrates. The cells feature a selective front side surface field (FSF) formed by laser doping using phosphorous silicate glass (PSG) as dopant source from POCl3 diffusion and a homogeneous rear Boron emitter with local p+ back surface field (BSF) formed during contact firing of Aluminium paste fingers. The FSF and rear emitter are passivated with thermal oxide (SiO2) + PECVD Silicon Nitride (SiNx), and with atomic layer deposited (ALD) Al2O3 + a stack of PECVD SiOx/SiNx respectively. The cells are fully screen-printed with 12 busbars and solderable Ag pads on the rear. We achieve an average cell efficiency of 23.10.1% with the best cell efficiency of 23.2% (both independently confirmed) with a reverse current (Irev) below 0.5A at -12V.

35th European Photovoltaic Solar Energy Conference and Exhibition; 1201-1204, This contribution handles about BIPV modules made with plated bifacial n-PERT cells from imec. For the cell interconnection the advanced smart wire connection technology (SWCT) was used that can reduce silver consumption at cell level considerably. These modules were made for installation in a facade installation of the KU Leuven, with monitoring of module performance, weather conditions and module temperatures. The modules are all glass-glass, both transparent and white. The modules were inspected with EL, and I-V measurements at standard test conditions were performed (also from the back side for the transparent modules). The modules were installed in September 2017, monitoring started one month later. The most important finding was that the bifacial gain when using a white surface behind the module was only considerable when the modules were ventilated. The probable reason is that without ventilation, the extra heat generated by double-sided illumination results in a module temperature that is so high, that the decrease in Voc and FF almost completely compensates for the gain in Isc. The monitoring data still have to be analysed in more detail, including a comparison with energy yield calculations with the imec energy yield software that has been adapted for bifacial modules.

35th European Photovoltaic Solar Energy Conference and Exhibition; 354-358, In this paper, we present a simple processing sequence to fabricate large area (244.3 cm2) n-type Czochralski-grown (Cz) monofacial screen-printed rear-emitter Passivated Emitter, Rear Totally diffused (nPERT) cells that can be implemented in production as an upgrade to conventional p-type Cz Passivated Emitter Rear Cell (p- Cz PERC) manufacturing. We use conventional POCl3 and BBr3 tube diffusions, selective n++ laser doping from phosphorous silicate glass (PSG), atomic layer deposited (ALD) Al2O3 emitter passivation, and front and rear screenprinting metallization using commercially available pastes and conventional calendered mesh screens. In a first experiment optimizing rear emitter contact pitch, we achieve average cell energy conversion efficiencies of 22.6±0.1% (internal measurement). In a second experiment implementing fine line double printing (FLDP) and double antireflective coating (DARC), we improve average conversion efficiencies to 22.8±0.1% (internal measurement) with the champion cell measured at 23.03% (externally confirmed). Finally, we identify main loss mechanisms using Free Energy Loss Analysis (FELA) and perform Quokka2D simulations to define a feasible roadmap towards >24% efficiencies.

35th European Photovoltaic Solar Energy Conference and Exhibition; 584-586, We report on the progress of wet chemical process development for silicon surface texturing of n-type passivated emitter with rear totally diffused (n-PERT) solar cells. A KOH-based texturing process sequence has been developed which provides potentially both improved conversion efficiency and reduction in cost of ownership (CoO). The improvement in conversion efficiency is attributed to decreased saturation current density after diffusion and laser processing, whereas, reduction of CoO is reached by reduced material consumption and process time. We have determined the saturation current density after subsequent process steps like Boron emitter diffusion, Phosphorous back surface field diffusion, passivation, laser ablation and selective laser doping for small pyramid (pyramid base length

35th European Photovoltaic Solar Energy Conference and Exhibition; 439-443, In this work, we report on progress made when using busbarless (0BB) bifacial Ni/Ag plated n-type Czochralski-grown (Cz) Passivated Emitter, Rear Totally diffused (nPERT) cells and Smart Wire Contacting Technology (SWCT) for interconnection. We show that Ni/Ag glass-glass (GG) SWCT laminates with an adapted laser pattern display a limited average power loss of 1.7±0.0 %rel. after 600 thermal cycles (TC) between -40 °C to +85 °C. Using this adapted laser pattern, we made a 60-cell GG SWCT module from a new batch of 0BB Ni/Ag nPERT cells using automated stringing equipment at Meyer Burger facilities. For this module, we measured a maximum power of 296.9 Wp under standard test conditions with a good cell-to-module ratio of 1.01. In addition, we also measured BiFi100 and BiFi200 values of 327.6 Wp and 356.7 Wp respectively. These results show the high potential of SWCT GG modules with 0BB Ni/Ag nPERT cells.

35th European Photovoltaic Solar Energy Conference and Exhibition; 580-583, We report detailed studies on laser doping from phosphorus silicate glass (PSG) to form the selective front surface field (FSF) for screen printed n-type passivated emitter, totally diffused (nPERT) solar cells. All relevant parameters for the laser doping process have been characterized at different laser conditions. These include measurements of sheet resistance, specific contact resistivity, recombination losses at the passivated laser-treated and metallized surface (measured by J0n++pass and J0n++metal respectively), optical and scanning electron microscopy SEM investigations. State of the art and very low dark saturation current densities have been achieved on passivated n+, passivated laser doped (n++) and metalized surfaces while maintaining low specific contact resistivity. The developed process for selective front surface field (FSF) with chosen doping parameters has been implemented in the monofacial nPERT devices with screen printed contacts resulting in independently confirmed efficiencies up to 23%.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 212-217, This paper describes a n & p surface Ni/Ag co-plating process for the metallization of bifacial cells suitable for multi-wire module integration. The process, based on electroless Ni and immersion Ag plating, mitigates typical problems of achieving simultaneous electroless Ni plating onto n & p surfaces by the use of a novel silicon selective activation step. Key attractive features of this metallization process include low cost from low Ag usage (typically < 8 mg/cell) and low stress batch processing with no cell contacting using low Capex high throughput tools. The plating process is applied to metallize bifacial nPERT+ cells achieving average cell efficiencies of 22.4% with typical cell bifacialities above 95%. Limited 1 cell laminate reliability data is given with IEC61215 thermal cycling and damp heat test criteria being met. Finally we present cost-of-ownership estimates at the cell level ($/cell), module level ($/Wp) and system level LCOE ($/kWh) for multi-wire modules made from nPERT+ cells using the co-plating process and compare the results to conventional modules with screen printed pPERC+ cells. Reductions in the LCOE of ≥ 5% are estimated for co-plated nPERT+ cell multi-wire modules for albedo values ≥10%.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 560-564, This work reports on the fabrication of advanced nPERT (passivated Emitter, Rear Totally diffused) cells by imec on epitaxial wafers from Crystal Solar obtained by direct gas to wafer technology. Crystal Solar has developed a process to grow full-epitaxial monocrystalline wafers of ~180-200μm thick with a limited stacking fault defect density of

32nd European Photovoltaic Solar Energy Conference and Exhibition; 439-442, Large area n-type rear-junction Passivated Emitter Rear Totally-diffused (PERT) solar cells with efficiency of 22.5% were recently reported by imec using rear sputtered Al. To develop a more industrially relevant process, screenprinting of Al is also currently investigated as the rear metallization. In this case, an Al p+ emitter is locally formed by Al- Si alloying during a firing step. The quality of this p+ emitter is crucial to the performance of the final devices. However, this is a challenging and less controlled process in terms of thickness of the doping layer and possible formation of voids. In this work, the contact geometry and the firing are optimized and their impact on the void and Al p+ layer formation is studied. Using the optimized processing parameters, large area n-PERT solar cells with screen-printed Al with efficiency up to 21.7% are fabricated. These cells demonstrate high values for the open circuit voltage (Voc) up to 690mV. This proves the feasibility of using screen-printed Al for high performance n-PERT cells.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 420-425, In this paper we introduce bifacial n-type PERT cells (Bi-PERT) with ultra-thin (< 3 m) and narrow (< 20 m) Ni/Ag plated fingers on both the n+ and p+ side of the cell. This cell architecture is designed for multi-wire interconnection and has the potential to deliver low Cost of Ownership at module level (< 0.40 $/Wp), as it can profit from bifacial gain, a low cost metallization sequence and the high efficiency potential of multi-wire interconnection. We describe the Bi-PERT cell features and show that the metallization sequence here proposed is well suited for integration with the multi-wire technology as it features finger section well below 60 m2 and no highly conductive Cu inter-layer. We also demonstrate a self-aligned fully plated metallization sequence able to form Ni/Ag electrodes on both sides of the cell with a specific contact resistance (c) below 1 m∙cm2 for both the n+ and p+ side. We benchmark this novel technique to a seed-and-plate sequence where the plating seed is obtained by physical vapor deposition of Ni (PVD-and-plate). Finally, we present light I-V results for rear-junction and front-junction Bi-PERT cells and report efficiency up to 21.0% for cells featuring the fully plated Ni/Ag metallization sequence and 22.6% for cells fabricated with the PVD-and-plate sequence.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 718-721, We report on the development of industrial wet chemical cleaning processes for n-type passivated emitter, rear totally diffused (n-PERT) solar cells. PERT silicon solar cells with 22% efficiency have been manufactured comparing established silicon surface cleaning processes like sulfuric ozone mixture (SOM) with novel wet chemical processes based on DI water/ozone (DIW/O3) and industrial cleaning processes like diluted hydrochloric acid/hydrofluoric acid mixture (HCl/HF) as used in solar cell mass production. We have tested the different wet chemical cleaning methods on prominent metal impurities like Fe and Cu. These metals typically occur on the silicon wafer surface after alkaline processing like saw damage etch and texturing. It has been shown that the impurity removal efficiency of the different cleaning methods is varying strongly for different metals. This information is used for optimizing wet chemical cleaning processes in terms of bath lifetime and chemical consumption reduction. In this paper the results of the metal removal efficiency study is applied to a n-PERT solar cell process flow, but it could be applied on other high efficiency cells concepts like IBC or HIT as well.

31st European Photovoltaic Solar Energy Conference and Exhibition; 502-506, In this work, we demonstrate the formation and characterization of selective emitter structures on n-type substrates. A selective p+ emitter is formed by a combination of boron diffusion and laser doping through a stack of Al2O3 and SiOx respectively deposited by atomic layer deposition (ALD) and plasma enhanced chemical vapor deposition (PECVD) while simultaneously ablating the dielectric stack to define the contact openings. The selective emitter consists therefore of boron doping under the passivated area, and boron and aluminum doping under the contact area. ALD Al2O3 acts as both the passivation layer of the p+ diffused emitter as well as a doping source of aluminum (Al). First, the influence of laser power and speed, as well as dielectric stack thickness, are studied with the aim to obtain consistent dielectric ablation and doping. Then, effective lifetime measurements are conducted as a function of contact fraction (c.f.) to extract the saturation current (J0e, laser) at the laser process surface. Finally, contact resistance measurements are carried out to assess the influence of different capping layers. The developed selective emitter structure is used as proof of concept for large area, n-type Cz-Si, rear junction solar cells achieving efficiencies as high as 20.9%.

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; 433-439, This work focuses on the (local) doping capability of selective epitaxy, and the application of the technology in n-type PERT solar cells. A deposition process was developed to grow ultra-shallow and thick 1-step profiles (uniform box-type profile) as well as 2-step profiles in a broad doping range. An evaluation of the recombination losses in those doped regions resulted in J0,pass values below 1 fA/cm2 and iVoc values up to 754 mV for an ultra-shallow and highly doped layer (50 nm, 4∙1019 cm-3) where the field-effect passivation is the dominant mechanism. J0,pass and iVoc in the range of 4-10 fA/cm2 and 725-745 mV, respectively, were measured for several 1- step and 2-step profiles of around 500 nm. The final goal of this work was the integration of the most promising newly developed profiles as the rear emitter of n-type PERT solar cells. In this direction, the integration of 500 nm epitaxial emitter with a uniform doping of 9∙1018 cm-3 (215 ohm/sq) and J0,pass=4.71.0 fA/cm2 (ALD Al2O3/PECVD SiOx passivation) proved efficiencies up to 22.5 % (ISE CalLab) in 238.9 cm2 n-type PERT solar cells fabricated on CZ silicon.

31st European Photovoltaic Solar Energy Conference and Exhibition; 365-368, We describe the process for fabricating interdigitated back-contacted cells compatible with thin (40μm) monocrystalline silicon foils. Throughout the process, the foils are supported by either the silicon parent substrate, or by a host (glass) superstrate using adhesive bonding. For a number of process steps, we describe possible issues, boundary conditions and performance. Using the integrated process which is fully compatible with bonded foils, cells were fabricated on a freestanding, non-textured wafer. An efficiency of 18.0% was achieved, the main limitations being the absence of surface texturing and a non-optimized amorphous silicon emitter.

31st European Photovoltaic Solar Energy Conference and Exhibition; 410-413, We report on the progress of imec’s n-PERT (Passivated Emitter, Rear Totally diffused) rear junction solar cells. Selective laser doping (LD) has been introduced in the flow replacing the laser ablation step (LA) to improve the front surface field and reduce the recombination current in the contact area. Simplifications have been introduced towards a more industrial annealing sequence, by replacing all expensive forming gas steps with a belt furnace annealing (BFA). The passivation quality of this process has been evaluated on test structures with doped surfaces. Applying this process to our rear junction design, 22.5% efficient cells have been measured on 6”- commercial Czochralski (Cz)-Si. This result has been independently confirmed by ISE CalLab. Smart Wire Connection Technology (SWCT) has been applied to our cells showing a power output over 5 W in a 1-cell laminate, which could be translated into a 60-cell module power over 300 W.

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; 1024-1027, In this paper, the electrical and optical characteristics of the Indium-Doped Tin Oxide (ITO) are tuned in order to design an ITO-based spacer between the a-Si:H and the metal contact suited for Amorphous / Crystalline Heterojunction interdigitated Back Contact Solar Cells (a-Si:H / c-Si HJ i-BC). ITO carrier concentration, mobility, resistivity, extinction coefficient, and work function, as well as the ITO / a-Si:H specific contact resistances are evaluated as a function of O2 relative flow rate during ITO deposition. An ITO having carrier concentration of 4.5x1019 cm-3, mobility of 20.2 cm2/(V.s) and resistivity of 6.24 mΩ.cm is found to be an optimum leading to ITO / a-Si:H(p/i) and ITO / a-Si:H(n/i) specific contact resistances values of 0.6 Ω.cm2 and 0.1 Ω.cm2 respectively and a RS reflectance of 58% at 1200nm when integrated in Silver / ITO / a-Si:H / c-Si(textured) test structures.

28th European Photovoltaic Solar Energy Conference and Exhibition; 757-760, This work reports on the feasibility to implement a cost effective dry oxidation step in a Passivated Emitter and Rear Cell (PERC) structure on p-type Cz Silicon substrates. PERC solar cells have been extensively investigated in the recent years for industrial implementation. One of the main challenges is to combine the integration of cost effective process steps while maintaining high cell efficiency. In our previous work, we introduced at imec the use of a thermal oxide below the front ARC and below the rear dielectric PECVD SiOx/SiNx stack for enhanced surface passivation. In this study, we demonstrate that a low thermal budget of 800oC coupled with a drastic reduction of oxidation time down to 15 min is sufficient for the growth of a high quality thermal oxide without adversely affecting the emitter properties. We show that the level of Dit at the silicon interface after oxidation is not determinant for the level of surface passivation but rather the hydrogenation provided by the SiNx after firing. We also show that the resulting SIMS emitter Phosphorous profiles after the low temperature oxidation maintain a surface concentration above 1×1020/cm3, which is fully compatible with screen printing technology. Assisted by the latest developments on commercial silver paste, this enables to contact homogeneous emitters up to 90 Ω/ with measured FF above 79% on the cell level after firing optimisation. All devices were fabricated on substrates area of 156 mm × 156 mm with a base resistivity of 2 Ohm.cm and a thickness of 170 μm. The PERC cells have front Ag screen printed contacts with a 3- busbar pattern, a thin thermal oxide on both sides with a rear PECVD SiOx/SiNx stack. Average conversion efficiency above 19.7% are achieved on such large area substrates. Several optimisation cycles have led to best cell efficiency of 20.1%.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1144-1147, In this work, the activation of an implanted boron (B) junction by an Excimer Laser Annealing (ELA) process is investigated. The objective is to substitute the standard Aluminium Back Surface Field (Al-BSF) normally used at the rear side of PERC cells by a Boron-BSF, taking advantage of the higher solubility of B in Si compared to Al. The long annealing step used to re-crystallize the amorphized region after implantation is usually performed in an oven at high temperature. In this study, we replaced it by the ELA based on a system from Excico. The laser has a 150 ns pulse duration, a wavelength of 308 nm, a top-hat beam profile with excellent energy uniformity (±2%) and an annealing area up to 400 mm2. The samples are analysed by sheet resistance, SIMS profiles and lifetime/J0e for different laser energy densities and implantation conditions (dose and energy), suggesting process conditions for a BBSF implementation. The B-BSF formed by implantation and subsequent ELA shows sheet resistance values as low as 20 Ohm/sq, J0e values down to 200 fA/cm2 and dopant profiles with surface concentration higher than 1020 at/cm3.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1008-1012, Despite cost and technological advantages, the commercialization of nickel/copper (Ni/Cu) plated contacts has so far been limited. Reasons for this include increased process complexity, availability of suitable lowcost production tools, and doubts over long-term reliability. We previously reported on a simple Cu based metallization sequence consisting of i) defining the front contact pattern by ps-UV laser ablation ii) self-aligned plating of the contacts using Ni/Cu/Ag and finally iii) sintering in N2 for nickel silicidation. We also reported that 10 cells strings modules featuring these Ni/Cu/Ag contacts met extended (1.5x) IEC61215 criteria for thermal cycling and damp heat reliability. In this work, this process sequence is transferred to 156x156 mmm2 substrates using pilotline processing equipment available at imec that include inline plating and sintering tools. The process sequence is further simplified to yield an industrial 0.5μm deep emitter without compromising efficiency results nor ribbon pull force values (average >3N/mm). This 0.5μm emitter also proves to be more robust to changes in sheet resistance induced during processing than the previously used high-efficiency 1μm deep homogeneous emitter. Applying the 0.5μm deep emitter and optimizing both front grid design and plating thickness we demonstrate efficiencies above 20% on 109 cells (average of 20.5%), with the best cell achieving 20.7% (externally confirmed by FhG-ISE CalLab).

28th European Photovoltaic Solar Energy Conference and Exhibition; 976-981, In this paper we present an overview of the integration of amorphous aluminum oxide (Al2O3) as rear side passivation layer for large area industrial PERC solar cells (i-PERC). The technique used for the deposition of the Al2O3 layers is spatial Atomic Layer Deposition (ALD), for its industrial relevance due to the high deposition speed and low Trimethyl-Aluminum (TMA) consumption. After analyzing the surface passivation properties, we will describe how Al2O3 can be integrated in an i-PERC process flow in a simple and cost-effective way, while maintaining high levels of solar cell efficiency. In particular, a special effort is devoted in finding solutions for process simplification and in the reduction of the cost per wafer (€/W) of the spatial ALD process step. Top efficiencies of 20.1 % and 20.6 % have been reached (Cz-Si, 6 inch cells) when coupling a spatial ALD Al2O3 rear passivation to a standard i-PERC integration flow with front Ag-screen printed or Ni/Cu/Ag plated contacts, respectively. Besides passivation, ALD Al2O3 can also be used as doping source of aluminum to form localized p+ back surface field (BSF) regions at the rear contact by means of laser processing. In this way, dielectric opening and BSF are formed simultaneously and firing step is not needed anymore, leading both to a suppression of Al2O3 blistering and optical enhancement. This process is a suitable complement of front contact formation based on plating schemes. Ni/Cu/Ag-plated solar cells with rear laser doping from ALD Al2O3 layers have been fabricated with efficiencies topping 20.4 % (FF = 79.7 %) and outperforming the control group featuring local BSF formed via a firing step.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1409-1412, In this work, we used a finite element model to simulate a novel nickel silicide formation process based on Excimer Laser Annealing (ELA). We evaluate the robustness of this new metallization process scheme that is selfaligned and compatible with shallow emitters. Study structures have been irradiated by ELA at several densities of energy, and also modelled into the simulation program. The results were compared and we suggest an explanation for the nickel silicidation process. The simulation results show a strong dependence of the energy density threshold to melt silicon with the thickness of the studied capping layer: Ni or SiNx. This is directly related to the reflectance of the stacks at the wavelength of the UV laser, but also to the thermal conductivity K and heat capacity cp of the materials. Phosphorus profiles after ELA can be predicted but the model still needs a calibration to match perfectly with the experience. A process path for self-aligned NiSi alloying with Excico laser system through full sheet ELA on real solar cells has been found. This has been confirmed with the realization of PERC cells with conversion efficiency up to 20%.

27th European Photovoltaic Solar Energy Conference and Exhibition; 561-566, In this work we study the impact of chemical polishing on the performance of mono-crystalline PERC cells. Starting from random pyramid textured surfaces, the rear side polishing step is performed by means of an inline single-side wet-etch tool. The influence of the pyramid smoothening on light trapping, surface recombination and contact formation is here studied separately. We conclude from our analysis that a moderate smoothening, corresponding to around 5-6 μm Si etch, is required in order to maximize the cell performance and that, on the other hand, a complete planarization of the rear surface can be detrimental, besides being cost un-effective. While the surface recombination generally benefits from a flat rear surface, light absorption and contact formation are more effective when roughness Ra is in the range between 300-500 nm. Contact formation is also influenced by the roughness because mass exchange between Al and Si takes place all over the rear side surface. On large area industrial PERC cells with Ag screen printed contacts we achieve a best cell efficiency of 20.1% with 6 μm of Si polishing removal and 19.9% with only 3 μm of Si removal. In the latter case the total Si consumption, including saw damage removal and texturing, is lower than 20 μm per cell.

27th European Photovoltaic Solar Energy Conference and Exhibition; 780-788, New process architectures are entering into industrial c-Si solar cell production. The device architectures and respective process flows lead therefore to new requirements with respect to the Si material. This contribution aims at improving understanding for requirements in advanced industrially applicable PERL-type solar cell processing and adequate material specifications. Analysis and understanding of CZ-Si defect patterns are addressed. The focus is on advanced industrially applicable CZ-Si solar cells applying new emitters, improved contacts and passivation schemes. Depending on the type of process sequence and parameters that are applied major reductions in solar cell performance are noticed and require to adapt Cz-Si material specification and processing. It is essential to influence base doping and defects like interstitial oxygen, vacancies, Si self interstitials as well as metal impurities already during crystal growth. The thermal history of the CZ-Si material (from crystal growth) when starting solar cell processing is often not known or communicated to solar cell manufacturers but has strong influence on the final solar cell results. Roads to industrial p-type CZ-Si i-PERL processing with stable efficiencies exceeding 21% are indicated and resulted so far in average efficiencies of 20.4% over a complete ingot length and a best efficiency of 20.6%.

27th European Photovoltaic Solar Energy Conference and Exhibition; 691-695, In this work, we report on progress in nickel silicide formation by excimer laser annealing (ELA) using single large area (10x10 mm2) pulses. The impact of both Ni and SiNx:H thicknesses were evaluated in order to precisely determine the ELA process window on shallow homogeneous emitters (~85 Ω/sq). Large area PERC type solar cells were fabricated using the ELA process, consisting in a step-and-repeat approach, to form self-aligned nickel silicides contacts that were subsequently copper plated. Cells processed with the ELA process showed a 0.5% absolute increase in efficiency compared to reference SP-Ag contacts thanks to reduced shading and contact resistance losses. In this work, the best performing cell with the ELA process reached a 20.0% energy conversion efficiency with Jsc=39.3 mA/cm2, Voc=650mV, and FF=78.3%. Finally, metal contact adhesion was evaluated by measuring soldered ribbon adhesion at a 45° angle with encouraging average values of 1.5 N/mm.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1602-1606, An industrially applicable process flow for the formation of rear side passivated solar cells has been suggested by IMEC in 2005. Since then considerable improvements in solar cell efficiencies have been obtained with that type of process. In parallel simplifications and cost reductions were needed to bring it to production. IMEC’s Industrial Affiliate Programme (IIAP) offers industrial partners the possibility to actively join development efforts with IMEC and other partners sending residents to work at IMEC as part of a research team. Selected development results of this IIAP are presented here. On large area homogeneous emitter CZ-Si solar cells FhG-SE Cal-Lab confirmed solar cell efficiencies of 20.0 and 20.3% for screen printed Ag and reliable plated Cu front contacts, respectively. Fill factors of 80% have been achieved for fully screen printed (front and rear) solar cells with low cost rear side dielectric stack layers. Cu plated contacts show reliable module data in IEC test conditions. Latest results with average solar cell efficiencies of 20.4% over the length of a CZ-Si ingot with top efficiencies of 20.6% are just an intermediate step. IMEC sees a road to >21% efficient i-PERL solar cells in production.

27th European Photovoltaic Solar Energy Conference and Exhibition; 538-543, This work reports on using copper (Cu) as the main conductor as an alternative to screen printed silver (Ag) front contacts for homogeneous emitter silicon solar cells. The work is not only motivated by the limitations that Ag screen printed contacts have regarding solar cell efficiency (high contact shading, limited line conductivity, and poor contact resistance to moderately doped emitters) but also the PV industries desire to reduce Ag usage for cost reasons. A relatively simple process sequence is investigated consisting of i) defining the front contact pattern by ps-UV laser ablation ii) self-aligned plating of the contacts using Ni/Cu/Ag and finally iii) sintering in N2 for nickel silicidation. The process sequence is applied to 12.5x12.5 cm2 p-type CZ-Si PERC type solar cells with 1μm deep 120Ω/ homogeneous emitters. Solar cell efficiencies of up to =20.3% and average pull tab adhesion results >2N/mm are achieved. Thermal cycling and damp heat reliability data for single cell laminates and small modules are reported meeting extended (1.5x) IEC61215 criteria.

Atomic layer deposition (ALD) of thin Al2O3 (=10¿nm) films is used to improve the rear surface passivation of large-area screen-printed p-type Si passivated emitter and rear cells (PERC). A blister-free stack of Al2O3/SiOx/SiNx is developed, leading to an improved back reflection and a rear recombination current (J0,rear) of 92¿±¿6¿fA/cm2. The Al2O3/SiOx/SiNx stack is blister-free if a 700°C anneal in N2 is performed after the Al2O3 deposition and prior to the SiOx/SiNx capping. A clear relationship between blistering density and lower open-circuit voltage (VOC) due to increased rear contacting area is shown. In case of the blister-free Al2O3/SiOx/SiNx rear surface passivation stack, an average cell efficiency of 19.0% is reached and independently confirmed by FhG-ISE CalLab. Compared with SiOx/SiNx-passivated PERC, there is an obvious gain in VOC and short-circuit current (JSC) of 5¿mV and 0.2¿mA/cm2, respectively, thanks to improved rear surface passivation and rear internal reflection.

26th European Photovoltaic Solar Energy Conference and Exhibition; 1195-1198, In this work, a first reliability assessment of cells in which the standard Ag screen printed front side metallization is replaced by electroplated Cu, is made. The thermal stability of Ti/Cu plated cells is investigated in thermal ageing tests for a temperature range between 175 and 300oC. The time at which the threshold value of 5% cell efficiency degradation occurs, follows a linear relationship in the Arrhenius plot. This indicates a similar failure mechanism by thermal diffusion throughout this temperature range. The Cu cells are also used for preparing 1-cell modules in order to test their resistance to oxidation in 85/85 test conditions. First tests show a promising result of 4 out of 5 modules surviving 2000h of testing for unprotected Cu.

26th European Photovoltaic Solar Energy Conference and Exhibition; 1413-1417, An alternative approach to the standard screen printing of silver for the front side metallization of solar cells is the implementation of nickel silicide contacts thickened by Cu plating. Properties of the metallization like contact resistance, junction shunting, adhesion, and line resistance need to be optimized. In this work, we use epitaxially grown phosphorous emitters to evaluate contact resistance of nickel silicide contacts both using TLM structures and finished solar cell devices. Additionally we report solar cells with Ni2Si/Cu contacts and epitaxially grown phosphorous emitters on random pyramid textured Cz with Jsc=36.6 mA/cm2, Voc=627 mV, and FF=78.6% leading to energy conversion efficiencies of 18% on full Al BSF large area solar cells.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2276-2279, In this paper we report large area 125x125 mm2 solar cells fabricated on random textured p-type Cz-Si with resistivity of 1-3 Ω.cm. A combination of optimized front and back dielectrics, local point contacts on the rear to form PERC type solar cells, and Cu plated contacts on the front lead to the independently confirmed efficiency of 19.6% with Voc up to 654mV and short circuit current density Jsc up to 38.9 mA/cm2. The fill factors measured were in the range of 76-77% indicating that there is considerable room for improvement. A detailed loss analysis is performed and 2D computer simulations are conducted to provide the guidelines for achieving energy conversion efficiencies above 20%.