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Potential-induced degradation in n-type modules is typically associated with a surface polarization effect. This paper shows that modifications at the cell level can minimize the potential-induced degradation of modules caused by a polarization effect. As is demonstrated on n-PERT cells, the potentialinduced degradation can be reduced effectively by modification of the silicon nitride antireflection coating. Potential-induced degradation tests on mini-modules confirmed the predictions by the stipulated polarization-type potential-induced degradationmodel that the potential-induced degradation is minimized when an Si-rich, conductive layer with refractive index n = 2.4 is inserted between the wafer and the outer SiNx layer with refractive index n = 2.0. In this way, the optical and passivation properties of the cell are maintained. The proposed modifications are easy to implement in the manufacturing process and are therefore cost-effective.

High quality passivating contacts can be realized by using the combination of a thin interfacial oxide (SiOx) and doped polysilicon (polySi). Recombination losses are minimized by providing very good passivation between the thin hydrogenated oxide and the cSi, a high field effect by the highly doped polySi [1-2], combined with the low level penetration of dopants in the wafer [2-3]. To realize this low level in-diffusion of dopants, several interacting options are evaluated in this work: the quality of the thin oxide layer (growth method), combined with a diffusion blocking method (nitridation), doping concentration levels in the polySi and temperature of diffusion. It is shown that for Phosphorus (P)-doped polySi, in-diffusion can be reduced by adding an i-layer in between the oxide and the highly doped polySi, lowering the overall doping level in the system slightly. For Boron (B)-doped polySi, in-diffusion can be blocked by nitridation of the SiO2 layer.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1908-1911, In this paper we report on the high stability of our n-type front junction solar cells (n-PERT) exposed to potential-induced degradation (PID) and UV-induced degradation (UVID) conditions. These intrinsically stable n- Pasha cells enable PID- and UVID-free modules even with industrially low-cost standard EVA encapsulant, independent of system grounding and system voltage. Based on intentional modifications of the Boron emitter and/or the dielectric layer in the PID-free and UVID-free n-Pasha solar cells, we are able to replicate reported degradation effects and study the mechanisms behind it. A combination of altering the boron profile and the dielectric properties together with increasing the interface defect density Dit is detrimental for the stability. Applying our standard optimal B-diffusion and passivation scheme assure that the UV radiation and system voltage have virtually no effect on our n- Pasha cell and module performance.

Two solar cell types are discussed in this thesis. Firstly, the Metal Wrap-Through cell, where the emitter-contact metallization of the front side is wrapped through holes in the wafer to the cell back. Optimization of several cell processing steps led to an increase of more than 2% absolute in cell efficiency. By integrating these optimized process steps a cell efficiency of 17.9% was reached. Using these multicrystalline cells in modules, a highest module efficiency of 17.0% on aperture area was obtained, which was the world record at the end of 2009 and the beginning of 2010. Secondly, the Interdigitated Back Contact cell, where both the emitter and base region and their contacts are positioned at the cell back. This thesis describes the fabrication of this cell based on all-screen-printed patterning and metallization. By optimizing the front surface passivation, a short circuit current of 41.5 mA/cm2 and a cell efficiency of 19.1% was obtained. To further improve cell efficiency, surface passivation by a-SiNx:H is studied. By applying a NH3 plasma prior to a-SiNx:H layer deposition, it was found that the amount of K-centers and thereby the surface passivation could be varied without changing the actual a-SiNx:H deposition process and thus the a-SiNx:H bulk layer properties. Detailed characterization showed a gradual change in the composition of a-SiNx:H in the first 2 nm. By changing the temperature of the NH3 plasma, the amount and penetration depth of N in the first few nm's in the Si substrate were altered.

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.

28th European Photovoltaic Solar Energy Conference and Exhibition; 736-740, The n-Pasha cell is a bifacial solar cell concept with average efficiencies, depending on the material quality, between 19.8% and 20%. Our process has been optimized to enable high efficiencies with narrow distribution on wafers from complete n-type ingots (2 to 10 -cm). This reduces the so-called electrical yield losses from a wafer point of view, which is important since the wafer costs make up the largest part (~40%) of the total module costs for n-Pasha modules. We found that the costs/Wp for the 20% n-Pasha cell and module process are very similar to those of a 19% p-type cell, assuming the same absolute wafer and module manufacturing costs. In the paper the successful implementation of a reduction of >60% in BBr3 consumption, and a reduction of >50% in Ag consumption are described, while keeping the n-Pasha cell efficiency at the same level. According to our calculations, the achieved reduction of the Ag and BBr3 consumption will lower the costs/Wp for n-Pasha modules below that of p-type modules. For our current n-Pasha cells majority of the efficiency losses are due to recombination in the diffused layers and below the contact regions. By tuning both the emitter and BSF profile, an efficiency of 0.4% abs. was already obtained, but, there is still room for improvements. Based on the simulations and experimental results, the path towards further optimization and efficiencies approaching 21% is discussed in more detail.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1188-1193, We investigate the impact of different dielectric layers and stacks on the passivation properties of boron doped p++-emitters and phosphorous doped n+-BSFs which are relevant for competitive n-type cell conversion efficiencies. The applied passivation schemes are associated with specific properties at c-Si/dielectric interface and functional mechanisms. In this way we aim to gain a deeper understanding of the passivation mechanism of the differently doped fields within the n-type cells and identify options to further improve the efficiency. The deposition technologies in our study comprise industrial PECVD systems and/or ALD both in industrial and lab scale configurations. In case of p++-emitters the best results were achieved by combining field effect and chemical passivation using stacks of low temperature wet chemical oxide and thin ALD-AlOx capped with PECVD-SiNx. The corresponding Implied Voc values were of about (673+2) mV and J0 of (68+2) fA/cm2. For the n+-BSF passivation the passivation scheme based on SiOx with or without additional AlOx film deposited by a lab scale temporal ALD processes and capped with PECVD-SiNx layer yielded a comparable Implied Voc of (673 + 2) mV, but then corresponding to J0 value of (80 + 15) fA/cm2. This passivation scheme is mainly based on the chemical passivation and was also suitable for p++ surface. This means that we have demonstrated that for n-Pasha cells both the emitter and BSF can be passivated with the same type of passivation that should lead to > 20% cell efficiency. This offers the possibility for transfer this passivation scheme to advanced cell architectures, such as IBC.

The effects of an interface gradient in nitrogen concentration on a number of important properties of amorphous hydrogenated silicon nitride / crystalline silicon (a-SiNx:H/c-Si) interfaces in the context of solar cell devices are investigated using molecular dynamics simulations. We simulate interfaces with a gradient of nitrogen which goes from SiN1.2 to Si over widths from 2 9 nm, in the presence of 10 at % hydrogen, to recreate the conditions present when SiNx layers are deposited onto c-Si by plasma enhanced vapour deposition. We examine how changing the width of the nitrogen gradient can affect a number of atomic level structural properties which influence the optical and electrical performance of solar cells. We examine the trajectories of our simulations to search for certain geometries which have previously been identified as being important at this interface. Silicon- silicon and silicon hydrogen bonds, which are help to determine the refractive index of the interface are shown to increase with increasing N gradient width. The fixed charge in the interface is also shown to increase with the width of the gradient. The results demonstrate how altering the width of the N layer can affect the efficiency of a-SiNx:H as both an anti-reflective coating and a passivation layer, and we suggest an optimal gradient width of, in the region of, 2 nm

In this paper we present molecular dynamics simulations of silicon nitride, both in bulk and as an interface to crystalline silicon. We investigate, in particular, the bonding structure of the silicon nitride and analyze the simulations to search for de- fective geometries which have been identified as potential charge carrier traps when silicon nitride forms an interface with silicon semiconductors. The simulations reveal how the bonding patterns in silicon nitride are dependent upon the stoichiometry of the system. Furthermore we demonstrate how having an “interphase”, where the nitrogen content in silicon gradually reduces towards pure silicon across a boundary region, as opposed to an interface where there is an abrupt drop in nitrogen con- centration at the boundary, can result in significantly different numbers of certain important carrier trap

26th European Photovoltaic Solar Energy Conference and Exhibition; 2223-2226, Interdigitated Back Contact (IBC) solar cells with >19% efficiencies have been fabricated using ntype silicon wafers and well-demonstrated high-volume solar cell process technologies alone. An optimized Front Surface Field (FSF) allows excellent current collection with Jsc values as high as 41.5 mA/cm2. High Pseudo Fill Factors (PFF) of above 81% and reduced Fill Factors (FF) of below 72%, have been obtained. Modelling of series resistance indicates a major contribution of the lateral transport of the carriers, mainly limited by low FSF and bulk conductivities. The process flow described is currently being transferred to a pilot production line and further possible process improvements are discussed.

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26th European Photovoltaic Solar Energy Conference and Exhibition; 2239-2242, Interdigitated Back Contact (IBC) cells have been fabricated at ECN laboratories with a best cell efficiency of 19.1%, using n-type Cz material and a process flow based on all-screen-printed patterning and metallization. Excellent current collection due to an optimized FSF is demonstrated by a Jsc value as high as 41.5 mA/cm2, corrected for spectral mismatch, and a Voc of 641 mV. The results show high Pseudo Fill Factors (PFF) of above 81% but lower Fill Factors (FF) of below 72%.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1417-1421, We obtained 17.9% cell efficiency on 160 micron thin cells sized 156 x 156 mm2 using ECN’s Metal- Wrap-Through (MWT) concept. Several cell processing steps were optimized and this led to an increase of more than 2% absolute in cell efficiency compared with previously reported values [1]. With these high-efficiency cells a 36- cell module was manufactured in our industry scale module pilot line. The aperture area efficiency of this module was 17.0% (as independently confirmed by JRC-ESTI): a world record in his category in 2009 and early 2010 [2]. In this module the average cell efficiency was 17.8%; this shows a small difference between cell and module efficiency.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 3258-3261, We obtained an independently confirmed module aperture area efficiency of 16.4% using 36 large and thin mc-Si metal-wrap-through (MWT) cells. This is an absolute increase of almost 1% compared to the previous world record. The module was made using a conductive rear-side foil with conductive adhesive for the interconnection. The module was constructed using a dedicated module manufacturing line that is designed to be able to work with extremely thin cells and provide a high through-put of one sixty cell module per minute.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 946-949, We obtained 17.6% cell efficiency on 160 μm and 17.1% on 120 μm thin multicrystalline silicon screen printed cells of 156 x 156 mm2. These cells are the best from two batches of cells that have been produced using improved processing on ECN’s industrial metal-wrap-through (MWT) cell concept. With these batches of cells, four 36-cells modules were made: two modules with 160 μm thin cells and two modules with 120 μm thin cells. The module manufacturing, which is based on a conducting substrate foil, succeeded with 100% yield in our module pilot line. The modules were independently measured by TÜV Rheinland and JRC-ESTI, and resulted in a new certified world record module efficiency of 16.4% on an aperture area of 8904 cm2.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1000-1005, At ECN, we are developing several different cell concepts to reduce the €/Wp costs of crystalline silicon solar cells by increasing the efficiency on thin and large solar cells. Our metallization wrap through (MWT) concept PUM reduces the shading losses from the front side by 3%, which results in a gain in Jsc. Furthermore, the cells are fully back contacted and easy to implement in a module. Another way to improve efficiencies, especially on thin wafers, is to replace the full aluminum BSF at the rear side by a more appropriate passivating layer like silicon nitride in combination with partial rear metallization. In this paper we present the development at ECN from standard H-patterned cell processing towards a combination of the two concepts mentioned above: our rear side passivated PUM cell, the ASPIRe cell. The proof of concept of the ASPIRe cell will be given. Compared to PUM reference cells, ASPIRe cells have reached higher Jsc and Voc, although the FF still remained behind. The highest efficiency obtained so far for ASPIRe cells is 16.4%, and methods to improve the FF by changing the rear side Al pattern are shown.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1708-1712, Standard in industrial in-line emitter processing is to apply an excess phosphorus source on the wafer prior to diffusion. Subsequently, a constant and high phosphorus concentration is present on the surface during the diffusion process. This paper shows that a reduction in the concentration, and thus in the emitter, can give a significant gain in Voc and Jsc as the dead P-layer is reduced. Homogeneous reduction over the complete wafer is important to obtain a maximum gain in Jsc and Voc. Different application methods are tested on homogeneity and the effect on Jsc and Voc. To optimize the contacting of these emitters, different pastes are tested on their contacting ability. It is shown that an efficiency increase of 0.1% absolute can be obtained with these emitters for industrial in-line applications, which can be easily and directly implemented in new and existing lines. No additional costs are required and no yield loss is expected, while at the same time the HF usage decreases as the glass is thinner and contains less phosphorus.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1048-1050, In the StarFire project, the partners ECN, Ferro and Solland develop several breakthrough technologies with the goal of realizing of 17% average cell efficiency on 150 mm thin wafers in combination with in-line cell processing and back-contacted cell and module technology. In this contribution the first prototype cells are presented, the status of the project is discussed and the road towards mass production is drawn.

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