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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.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 628-630, We report on the progress of wet chemical process development for silicon surface cleaning of n-type passivated emitter with rear totally diffused (n-PERT) solar cells. An Ozone based wet chemical cleaning process sequence has been developed which provides both, improved effective minority carrier lifetime and reduction in cost of ownership. The proposed cleaning solution (i2PV: imec industrial PhotoVoltaic Clean), is a mixture of ozonated DI water (DIW/O3) and diluted Hydrochloride and Hydrofluoric acid (HCl/HF). The silicon surface clean is performed in a single tank and provides a hydrophobic or hydrophilic silicon surface after the clean depending on the ozone concentration. In order to quantify the cleaning efficiency of the solution we have determined surface contamination concentration of prominent transition metals before and after wet chemical cleaning. We also determined the impact on solar cell performance by measuring effective minority carrier lifetime in large area 156x156 mm2 commercial ntype cz-Silicon, 3-5 Ohm.cm, 180um thickness, after a high temperature process and passivation with thermal SiOx. We benchmarked the cleaning efficiency of the solution by determining the spatial distribution of the effective minority carrier lifetime using photoluminescence measurements. The impact of pre-treatment of the silicon surface has been investigated. The low cost of ownership is mainly determined by the low concentration of the involved chemicals (HCl and HF less than 2%), the ozone that is switched on and off before and after the cleaning process, the application at room temperature and the short processing time of only 2-5min. Despite the low chemical concentrations the solution shows a highly effective impurity removal quality. Metal removal efficiency is determined and effective minority carrier lifetime is extracted on passivated wafers. As a novel parameter a measure for the spatial distribution of the effective minority carrier lifetime is introduced on large area Cz-Si. The uniformity of the lifetime distribution has been defined by measuring the color distribution in the PL spatial image using the histogram data for red in the RGB bitmap. This provides us a scale from 50 to 75. In this case 75 represents a very good uniformity and 50 a rather poor uniformity. The minority carrier lifetime distribution uniformity is an important parameter for potential solar cell efficiency losses. A uniformity vs effective minority carrier lifetime graph is depicted resulting from a parameter sensitivity screening design of experiment (DoE). There is a clear trend towards hydrophobic pre-treatment in both, minority carrier lifetime and uniformity, respectively. The large process window, the low chemical consumption and the high metal impurity removal efficiency makes this solution an interesting choice for silicon surface cleaning in solar cell production.

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.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 604-609, This works focuses on the introduction of selective epitaxy as a contact passivation approach for bifacial n-type PERT solar cells. Because of the use of a metal grid on their rear side, bifacial cells provide a higher energy yield through rear illumination but have larger recombination losses than their monofacial counterparts with point contacts. A breakdown analysis of the recombination losses in bifacial cells fabricated with a homogeneous emitter, and for which efficiency values above 22.5 % were already achieved, reveals that there is still an important contribution of the emitter J0,metal (3000-4000 fA/cm2 for 1.2 % metal contact fraction) and that the emitter J0,pass can also be further improved to increase efficiency. This works aims at reducing the recombination losses in these devices by introducing in one single step an epitaxially grown highly doped layer at the p-type regions to be contacted. The structures here fabricated up to the step just prior to metallization (including patterning of both emitter and BSF) demonstrate the potential of this approach to significantly reduce the recombination losses compared to the reference without a selective epitaxial contact passivation scheme: (a) homogeneous emitter with SiO2 passivation: J0,total=90 fA/cm2, iVoc=681 mV, (b) homogeneous emitter with Al2O3 passivation: J0,total=70 fA/cm2, iVoc=688 mV, and (c) selective emitter via selective epitaxy with SiO2 passivation: J0,total=55 fA/cm2, iVoc=695 mV. Keeping in mind that this approach also brings the possibility to independently optimize the emitter in the passivated regions to reduce Auger recombination, further gain in passivation may still be possible, which increases the potential of this structure to achieve higher efficiency values at cell level.

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; 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.