Lightsoaking (LS) of n-type silicon heterojunction (SHJ) solar cells is a topic that raised increasing attention of the PV industry. The treatment of n-type SHJ with high light intensity and high temperature in parallel leads to a boost in efficiency (eta) that is driven by improved passivation at open-circuit and MPP (V-oc, pFF) and reduced series resistance (RS), both together leading to a higher FF of the cell. Within this paper we investigate the effects of LS and show that LS can be an effective means to modify and improve layer and interface properties. However, to fully take advantage and maximize eta the post processing by LS and the individual cell / module processing steps must be well aligned. Here we show that transparent conductive oxide (TCO) deposition conditions can have a significant influence for a given LS treatment. Furthermore, we combine the LS treatment with cell separation and edge passivation technology, the so-called post processing of the cell to overcome losses resulting from separation of half-cut (HC) SHJ cells. Further, we demonstrate that SHJ cells were successfully separated with low. loss applying a slim post processing sequence. From these cells a monofacial and a bifacial module were built and characterized. Finally, the topic reliability of the LS gains is shortly addressed.

In this work, we present results on various low-temperature approaches for the metallization and interconnection of high-efficiency solar cells as silicon heterojunction (SHJ) or perovskite silicon tandems. By using fine line screen printing for the cell metallization and Ag-free or-reduced interconnection technologies, we demonstrate the potential of these approaches both for SHJ and perovskite silicon tandem cells. Furthermore, low temperature (LT,-200 degrees C) or ultra-low-temperature (ULT,-150 degrees C) processes are utilized for metallization and interconnection to treat these temperature-sensitive solar cells with a reduced energy consumption. We compare LT soldering of SHJ cells with Pb-free alloys to state-of-the-art soldering processes and interconnection with electrically conductive adhesives (ECAs). For successful module integration of perovskite silicon tandem solar cells, these findings provide the basis to build full-size tandem modules with different interconnection technologies.

The current work focuses on the question if shingling can be a suitable interconnection method for perovskitesilicon tandem (PVST) cells. Cell-to-module (CTM) analysis was conducted to investigate the effect of the number of the metallization fingers and cut size (1/4, 1/5, 1/6 and 1/7 of the original wafer) on the I-V characteristics of PVST shingle cells, defining an optimum number of fingers for each of the cut sizes. The simulation was based on M6 wafers with an edge length of 166 mm, showing that smaller cut size with more fingers exhibit lower current, but higher fill factor and efficiency. Furthermore, power gains and losses from cell to module depending on the shingle size were analyzed. The simulation was based on a module design with 10 strings in parallel and shingles of 1/4, 1/5 and 1/6 cut sizes. CTM analysis revealed that smaller shingle cut sizes result in higher module efficiencies due to their higher initial efficiency and optical gains from overlap. According to the performed simulations, assuming an initial cell efficiency of 25.0 % (1/6 cut with 95 fingers), a module efficiency of 23.4 % can be reached. In order to demonstrate the feasibility of shingle interconnection and full-format module integration of PVST cells, low-temperature silver metallization with a screen-printing approach was utilized on M6 precursors provided by Oxford PV. After metallization, cells were cut (166 mm x 33.2 mm) and interconnected into shingle strings with an automatic process. They were then integrated in glass-glass (GG) solar modules. The prototype modules exhibited no visual defects and demonstrated performance in line with the initial cell efficiency. The highest achieved module efficiency was 22.8 % based on the aperture area of 1.5 m2 and exhibited a power PMPP of 336.5 W. Throughout the whole prototype fabrication process, only commercially available materials and industrial manufacturing equipment was used. To conclude, shingling has been demonstrated as a viable interconnection approach for PVST cells. Simulation and prototype production confirmed the potential of shingle modules to achieve high efficiencies.

To unlock the full potential of perovskite-silicon tandem solar cells with >30% efficiency at presumably low cost, the transparent conductive oxides (TCOs) and metal grid at the front side need to be adapted compared to classical silicon heterojunction (SHJ) solar cells. By means of optical and electrical modelling, we consider the main aspects to optimize the front electrode for the tandem case, where in contrast to silicon single junction devices, there are (i) different optical properties including a lower refractive index of the perovskite absorber (ii) about half the current, thus quarter the resistive power losses for the same series resistance contribution (iii) lateral transport at the front needs to be provided solely by the front TCO layer and (iv) lower thermal stability of the perovskite, which affects TCO deposition conditions and results in a less efficient sintering of the silver screen printing pastes. This study concludes that compared to silicon heterojunction cells, the thickness of the front TCO should be reduced from 75 nm to around 20 nm, resulting in less parasitic absorption and a potential cost reduction of 1.46 euroct/cell for ITO. We investigate the impact of different front metallization including plating and silver screen printing and showcase that for multi-wire interconnection concepts, the number of wires can be reduced from 18 wires to 9 or even less depending on the front metallization. Finally, we give an outlook on the silver consumption and levelized cost of electricity.

Within this work, we present industrially feasible and well-applicable methods to effectively lower the costs of silicon heterojunction (SHJ) solar cell processing by reducing cycle time of the screen printing and the subsequent thermal curing step using industrial process equipment. For six out of nine tested low-temperature silver pastes, process velocity in screen printing could be doubled compared to typical printing velocity, equaling a reduction of cycle time in this process step by 1 sec. Using a convection curing process supported by infrared radiation (IR) and an adapted process temperature, curing dwell time of SHJ solar cells can be substantially reduced from 10 min to 0.5 min, allowing for higher throughput and/or smaller machine dimensioning (footprint). Furthermore, an improved solar cell conversion efficiency by up to +0.1 %abs is demonstrated which is likely to be caused by the so-called light soaking effect. When applying an additional process step with IR at high illumination density, a further increase of cell efficiency by up to +0.3 %abs can be achieved. With regard to the soldered interconnection, the combination of a short and hot curing process and their diametrical effects on adhesion properties resulted in similar peel force values as for the reference process. Also, treatment of the SHJ solar cells with a laser-based IR light soaking process does not exhibit a significant effect on peel forces. In summary, our work shows that process times of the metallization for SHJ solar cells can be strongly reduced while maintaining cell performance and applicability for soldered interconnection.

The use of electrically conductive adhesives (ECAs) and ribbons is a cost-efficient solution for the interconnection of silicon heterojunction (SHJ) solar cells already implemented in fully automated stringing equipment. A better understanding of the materials, the interconnection process and the reliability of the modules is still required. In this paper we present results of this interconnection approach focusing on material level, string production and performance analysis in outdoor operation. Firstly, the curing of ECAs is investigated. Furthermore, the volume and contact resistivity initially and after aging is characterized. ECAs in combination with Ag-coated and bare Cu ribbons are processed on an industrial glue stringer TT1600ECA from teamtechnik GmbH to manufacture SHJ solar cell strings. The uniformity of the cured bond line thickness is analyzed with optical microscopy. The bond line thickness is 32 to 38 μm on the sunny side. The peel force is characterized and the strings are further encapsulated in modules to test reliability and outdoor performance. The peel force ranges from 0.5 N mm−1 to 1.0 N mm−1 for certain ECAs. Adequate material combinations for PV modules showed degradation lower than 5% even in extended thermal cycle and damp heat testing. The outdoor test of SHJ modules over one year showed a 2% to 3.5% higher specific energy yield compared to a PERC reference module due to the absence of p-type related light induced degradation and a lower temperature coefficient of SHJ cells. With this work we contribute to the industrialization of ECA technology for SHJ cells by addressing relevant aspects of mass production.

Silicon Heterojunction (SHJ) solar cells represent a key technology to approach very high conversion efficiencies close to the theoretical limit of silicon solar cells. The main advantages of SHJ solar cells are a lean production chain with low temperature processes and carrier selective hetero-structure for excellent passivation. This paper focuses on the implementation of partially new, alternative, cost effective and production-feasible process steps as ozone-based cleaning, inline PECVD and screen printing in the Silicon Heterojunction process chain enabling efficiencies up to 22.7%. (C) 2017 The Authors. Published by Elsevier Ltd.

Interconnecting silicon heterojunction (SHJ) solar cells by low-temperature ribbon soldering allows the use of standard stringing equipment and might therefore be the cheapest and most straightforward implementation in existing fabrication lines. However, solder joints on low-temperature metallization pastes of SHJ cells are known for a weak adhesion to the cell surface. This work is dedicated to a better understanding of the interaction between solder and low-temperature metallization on SHJ solar cells. We evaluate soldering during a lamination process with standard copper ribbons featuring six different low-temperature solder alloys. The mechanical adhesion of the solder joints and microstructural changes in the metallization pastes during the soldering process are analyzed. Our study includes three metallization pastes, two of them show poor adhesion on the wafer surface after soldering (up to 0.03 N/mm) and one paste performs slightly better (up to 0.28 N/mm). We find this difference to be caused by an interaction of the pastes and the liquid solder, neither depending on the composition of the solder alloy nor on the soldering time. A fine grain structure of the Ag-particles of such pastes correlates with a higher penetration of solder components (Sn and Bi) resulting in dissolution of the metallization and detachment from the cell surface.

Within this study, different metallization approaches were investigated and compared with respect to their applicability for silicon heterojunction (SHJ) solar cells. A printed line width of 34 µm with an aspect ratio of 0.59 was reached by double fine-line screen printing. In parallel, ink jet printing and its applicability regarding in-situ curing was evaluated. It was found that spreading of the ink is substantially reduced by choosing the optimum substrate temperature, enabling for a line width of 32 µm. Successful dispensing of a low-temperature silver paste was achieved by using a ten nozzle print head. By means of screen printed contact grids on both sides, solar cells with a conversion efficiency of 21.9 % were produced with industrial precursor wafers.

Catalyst layers (CLs) with varying ionomer contents are produced using a stencil coating and screen printing technique. The optimum ionomer content of 31-34 wt% confirms the findings of other groups and performance is found to be independent of production technique. A new CL impedance transition line model is developed and fitted to in-situ data. The results indicate that the protonic contact resistance between CL and membrane is an important factor for the used transfer-decal process, especially for CLs with low ionomer loading.When subjected to potentials higher than 1.2 V, an increased performance is observed for low ionomer loading CLs. It is found that by applying the high potential to the electrode a significantly increased proton conductivity is counteracting and superimposing the loss of electrochemical surface area (ECSA) due to carbon corrosion. After aging, the performance of the 15 w CL is at the same level as the 31-34 wt% ionomer content CLs at the beginning of life, even though the ECSA is reduced due to carbon corrosion or platinum dissolution. The findings indicate that for the optimization of the ionomer loading, either the changing wetting properties or the redistribution of ionomer during lifetime have to be taken into account. (C) 2016 Elsevier B.V. All rights reserved.

Flexographic printing is a high-throughput technology which is capable of fine-line printing. The use of a soft and flexible printing plate keeps mechanical stress to silicon wafers during printing low. It is therefore very interesting for the industrial-scale production of seed layers for front-side contact grids on solar cells. Within this work, flexographic printing is applied to silicon solar cells for the first time. We investigate the effect of printing parameters and printing press components on finger width. An average finger width after contact firing of about 44μm was achieved on wafers of the format 22x60 mm2. Due to reduced shading losses compared to screen printed cells the best flexographically printed cells reached an efficiency gain of 0.7%abs. The highest efficiency was 18.1% and was observed on Cz silicon.

We present a standard p+pn+ solar cell device exhibiting a full-area aluminum back surface field (BSF) and a conversion efficiency of 20.1%. The front side features a shallow emitter which has been exposed to a short oxidation step and reduces the emitter dark saturation current density j0e to 160 fA/cm2 on a textured surface. The front contact is formed by light-induced nickel and silver plating. Also, devices featuring screen-printed front contacts have been realized that reach a conversion efficiency of 19.8%. PC1D simulations are presented in order to extract the electronic parameters of the BSF. Therefore, external quantum efficiency and reflectance have been determined for modeling the internal quantum efficiency by adapting surface recombination and lifetime of the PC1D-simulated silicon device. As a result, a recombination velocity of SBSF = 283 cm/s and a dark saturation current density of jBSF = 274 fA/cm2 in the Al BSF are determined. This results in an effective diffusion length Leff = 1150 μm .

We present metal wrap through (MWT) silicon solar cells with passivated surfaces based on a simplified device structure. This so-called HIP-MWT structure (high-performance metal wrap through) does not exhibit an emitter on the rear side and therefore simplifies processing. The confirmed peak efficiency of the fabricated solar cells with an edge length of 125 mm, screen printed contacts and solder pads is 20.2%. To our knowledge, this is the highest value reported for large-area p-type silicon solar cells to date. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

On the way to higher efficiencies, back contact solar cells seem to be very promising. Especially, the metal wrap through (MWT) solar cell concept, with only three additional process steps when compared to conventionally processed cells, is appropriate for a fast transfer to industry. Hence, a pilot-line process based on a modified via-metallization step was set up. Therefore, a newly developed short suction step directly after the screen-printing process was established and characterized. Using this new via-metallization technique, cell efficiencies over 16% are reached on a multi-crystalline silicon (mc-Si) material. Compared with conventionally processed cells, an efficiency gain of 0.5% absolute is observed.

Fully functional large area screen-printed multi-crystalline silicon solar cells with Laser Chemical Processing (LCP) selective emitters were obtained for the first time. Solar cell efficiencies between 14.4 and 15.2 % were achieved. The absolute efficiency gain of 0.3 %abs was obtained by the application of the LCP process on the lowly doped (120 Ω/sq) emitter in comparison to reference solar cell without the LCP process. The application of the LCP selective emitter increased the fill factor of the solar cells in comparison to the reference solar cells. The efficiency gain of these first solar cells proves the concept of applying the LCP to the standard solar cell structure. However, the overall efficiency was strongly limited by the large width of the metal fingers and the resulting shading and contact recombination losses. Also the non-optimized metal finger distance is limiting the FF of the solar cells with high emitter sheet resistance. Thus the efficiency of the solar cells with lowly doped emitter was lower than in the case of highly doped emitter. A detailed characterization of the processed solar ells is presented.

9 Printing technology has been applied in the c-Si photovoltaic industry for several decades now. Being a classic reproduction technique it is well established as a structuring technology which allows for cost-efficient mass production at high throughput rates. Through further research and development in this field, printing technologies take part in the recent progress of renewable energy to become capable of competing with fossil and nuclear energy production.