We propose silicon solar cell–integrated stress and temperature sensors as a new approach for the stress and temperature measurement in photovoltaic (PV) modules. The solar cell–integrated sensors enable a direct and continuous in situ measurement of mechanical stress and temperature of solar cells within PV modules. In this work, we present a proof of concept for stress and temperature sensors on a silicon solar cell wafer. Both sensors were tested in a conventional PV module setup. For the stress sensor, a sensitivity of (−47.41 ± 0.14)%/GPa has been reached, and for the temperature sensor, a sensitivity of (3.557 ± 0.008) × 10 −3 K −1 has been reached. These sensors can already be used in research for increased measurement accuracy of the temperature and the mechanical stress in PV modules because of the implementation at the precise location of the solar cells within a laminate stack, for process evaluation, in‐situ measurements in reliability tests, and the correlation with real exposure to climates.

The contact formation by screen printed metal pastes is widely employed in standard solar cell production. To expand the use of screen printed electrodes to n-type solar cells, both boron and phosphorus doped surfaces need to be contacted. To do so with a single material has some advantages especially for IBC solar cells. In this study we test four different screen printing pastes on different boron and phosphorus dopings and in combination with different silicon nitride thicknesses. Phosphorus doping could be contacted over a wide range of sheet resistances, nitride thicknesses and fast firing conditions, leaving much freedom to target the boron contacts. Boron dopings are successfully contacted with all materials, if no capping silicon nitride layer was present. With silicon nitride capping an AgAl and an Ag paste are found to be suitable choices. The lowest contact resistivities with 100 nm SiNX capping determined in this study are ρC = 0.5 mΩ cm2 on phosphorus (Ag) and ρC = 1.8 mΩ cm2 on boron (Ag) doping with one single paste. These results enable highly efficient homojunction IBC cells at low cost.

This study analyses thermomechanical stresses in silicon solar cells after the soldering process by finite element modeling. An experimentally validated model shows compressive and tensile stresses, longitudinal and transversal to a busbar or a pad row on the surface of a silicon solar cell. The impact of the interconnector segments at and in between two solder pads was investigated and characteristic locations of maximum stress were identified. In addition, the influence of the layout of the contact metallization on the thermomechanical stress was identified by geometry variations to reveal design guidelines that lead to reduced thermomechanical stress in a solar cell after the soldering process. The model results reveal maxima of the tensile stress located at the outermost contacts. Furthermore, a significant influence of the distance between the outermost contact areas and the solar cell edge was determined; with decreasing distance, the compressive stress maxima are higher, but in contrast the more critical tensile stress maxima decrease. For connected pad rows tensile stress maxima are larger compared to single pad connection, which shows the influence of the interconnector segments in between the solder joints of a pad row. After several stages of thermal cycling, electroluminescence measurements showed, in compliance with the model results, contact damages, mainly at the outermost contacts. Furthermore, connected pad rows revealed a steadily growing amount of damaged contacts, whereas single solder joints showed no defects up to 400 thermal cycles.

Understanding the origin of thermomechanical stress in solar cells is a key factor to extend the lifetime of photovoltaic modules. However, the methods to determine the stress are very limited. With the confocal micro Raman spectroscopy, we present a contactless method, which is able to measure through the front glass and is well-known in the field of microelectronics. One major challenge for the measurement on crystalline silicon solar cells and modules is the surface texturization of the mono crystalline solar cell, which changes the topology from a plain (100) surface to pyramids with (111) flanks and (100) valleys. We develop a procedure to cover the challenges arising from this topology, namely the inhomogeneous stress distribution on the pyramid flanks and the different crystal planes of the phonon vibrations and the photon back scattering. By studying the procedure on a reference system, we determine a factor for the conversion of a micro-Raman peak shift to stress of Sigma = -(833 +/- 49) MPa/rel. cm(-1). The presented measurements show that the factor holds for uniaxial stress, biaxial stress as well as the stress states occurring from the PV module production processes. We then apply the procedure to measure the stress from soldering 156 x 156 mm(2) solar cells and the lamination. We obtain (-21 +/- 2) MPa for the stress in the unsoldered solar cells, which arise from the cell production steps, like metallization. After soldering, we measure (-26 +/- 3) MPa and after lamination (-53 +/- 6) MPa. Additionally we perform a line scan along the cell diagonal and area scans of the quarter cell as well as the end of one busbar. All results match well with a simulation of stress induced by the soldering process and lamination using the finite element method.

In this paper we analyze the microstructure of solder coated copper ribbon and wire interconnectors for silicon solar cells from different manufacturers at different stretch levels in longitudinal and cross microsections by color etching and microscopy. The used etchant colors each grain according to its crystallographic orientation. This enables the extraction of the grain size and frequency information by image and data processing tools. Furthermore we measure the yield strength of the solar cell interconnectors for strain levels of 0.5 %, 5 % and 10 % and evaluate the impact on the microstructure of the interconnector. We find a large variation in the copper microstructure, especially for wires and observe an inverse relation between yield strength and grain sizes corresponding to the Hall-Petch relation. The lowest measured yield strength for a wire interconnector is about 82 MPa (avg. grain size: 237 μm2), which is about 20 MPa higher compared to the lowest yield strength measured for ribbons (avg. grain size: 247 μm2). The wire with the highest yield strength of 148 MPa shows fine grains (avg. grain size: 29 μm2). In the ribbon analysis we find the same overall correlation between grain size and yield strength with some exceptions. This underlines that grain size distribution is not the only attribute which affects the yield strength. The analysis of copper ribbons at different stretch levels discloses a deformation or refinement of the copper grains associated with rising yield strength. In general the results show that the approach of a color etching, optimized for solar cell interconnector cross and longitudinal sections, is a suitable, fast and cost-effective solution to quantify the grain size distribution and evaluate mechanical impacts like stretching or bending on the copper microstructure.

The idea to interconnect solar cells in a shingled manner like roof tiles dates back to the 1960’s. However, the standard interconnection technology in the PV industry nowadays is still the front to back series interconnection using solder coated copper ribbons. Shingle interconnection offers very high packing densities of solar cells and increases the module’s active area by eliminating busbars from the illuminated area. Direct interconnection of stripe-like solar cells by electrical conductive adhesives (ECA) replaces the front-to-back ribbon interconnection and therefore eliminates the interconnectors’ ohmic losses. Stripe-like solar cells additionally reduce the overall ohmic losses of the solar cell string by lower cell currents.

Cluing ribbons to silicon solar cells by using electrically conductive adhesives (ECAs) is an alternative interconnection technology for module integration to the state-of-the-art soldering process. We reveal cost reduction potentials by analyzing the influence of volume and contact resistivity, as well as the bond design of ECAs on the fill factor of photovoltaic modules. Solar cells with structured busbars are considered in the analysis. The volume resistivity is controlled by the cure temperature. We contact individual cells at different curing conditions and measure their fill factors. A volume resistivity of 1 x 10(-2) Omega . cm does not cause a significant fill factor reduction compared with an ECA with around 1 x 10(-4) Omega . cm. The contact resistivity is varied by using different ribbon coatings. Ag and So coatings achieve almost identical fill factors. A bare Cu surface reduces the fill factor. We lower the consumed ECA from 40 to 3 mg/cell by modifying the bond design. A design with 16 mg/cell achieves similar fill factors as with 40 mg/cell. A finite-element model is developed to study the combined influence of electrical properties and the bond design. We propose an optimized contact design for high fill factors and reduced material consumption.

This work describes interdigitated back-contact cells with a number of different rear side geometries, for different interconnection concepts and module integration, by means of numerical simulations. We show that a simple interconnection concept can be realized with copper wires as bus features and interrupted metal electrodes to avoid shunting, without severe losses compared to multilayer metallization concepts. Using Quokka3, which due to enhanced speed allows for very large simulation sizes, this enables principal investigations undescribed in previous literature. We use this to investigate the disconnection of a single (or multiple) solder joint(s) in terms of device performance, in the case of interrupted metal electrodes. Our findings show that disconnected emitter electrodes cause higher power losses than disconnected BSF electrodes (similar to x4), both following a linear relationship. Nevertheless, when multiple of such defects are aligned, the losses are increasing much stronger. We accordingly derive the need to balance design choices such as BSF and emitter width in an industrial implementation, with an empirically derived disconnection probability.

Foil metallized (FolMet) solar cells combine the high-efficiency PERC technology and a cost-efficient rear side metallization based on a 9 µm thin aluminum foil. Laser fired contacts are used to attach the foil to the rear side and allow for the mechanical and electrical contact to the cell. Direct soldering on aluminum is hampered by a native oxide film formed immediately when exposed to air. To realize the cell interconnection by a standard solder process the Al foil is coated by sputter deposition or roll cladding of solderable layers. This work evaluates the solderability of 200 µm thick coated Al foils with the common solder interconnection process established in photovoltaics using standard copper ribbons for module integration. Our analysis reveals for both coating approaches strong initial mechanical adhesion of > 4 N/mm after soldering and very low contact resistivities of < 1.6 µ∙cm2. The contact resistivity shows no degradation after isothermal aging within 1000 hours at 85 °C. The mechanical adhesion of the sputter coated Al foils remains at 3 N/mm under thermal aging of more than 1000 h at 85 °C whereas the adhesion for the Al foils coated by roll cladding drops strongly to values of < 1 N/mm already after 50 hours. SEM images indicate that this effect is caused by re-oxidation of the aluminum surface supported by micro cracks in the solderable top layer.

Foil metallized solar cells combine the high-efficiency passivated emitter and rear cell technology with a cost-efficient rear side metallization based on a thin aluminum foil. The cell interconnection with the soldering processes usually used in photovoltaics, is hampered by the native oxide film Al2O3. This work evaluates solder joints on Al substrates, modified with either physical vapor deposition (PVD), roll cladding or zincate treatment. Our analysis reveals low contact resistivities of ~0. 02 m Ωcm2, temperature-stable for 1000 hours aged at 85°C in ambient air. Additional tests under damp heat conditions (85°C, 85 % relative humidity) with encapsulated samples yield as well no electrical degradation for the PVD coated samples within 1000 hours aging. The mechanical characterization by a 90° peel test shows initial adhesion of 6 N/mm (PVD), 4 N/mm (roll cladding) and 0.8 N/mm (zincate). Degradation of the roll cladded samples is observed and analyzed further by EDX spectroscopy.

In this work the reliability of silicon solar cells interconnected by wires soldered directly on the contact fingers of the front side grid is analyzed in detail. The interconnection of busbarless solar cells enables significant silver reduction. We use solder coated wave-shaped wires to reduce thermomechanical stress in the solder joints, which results in minimized cell bowing. Consequently, this interconnection concept is especially suitable for the interconnection of back-contact solar cells. We analyze the mechanical and electrical properties of wave-shaped wires with different amplitudes by measuring length and electrical resistance, and by performing standard tensile tests. Furthermore, we manufacture 8 one-cell modules, 4 with full scale 156 x 156 mm(2) solar cells and 4 with half cells, as well as a module including a string of three half cells using semi-automatic infrared soldering and a conventional lamination process. Subsequently, the one-cell module samples undergo thermal cycling and are characterized by EL imaging and IV measurements to detect solder joint defects. After temperature cycling we determine additional solder joint failures, mainly in areas with initial defects and at the solar cell edges. This confirms our assumption that most defects occur at the cell edges, where the thermomechanical stress maxima are located. However, all module samples show a maximum relative power loss of < 3% after 200 temperature cycles and except one module all samples show a power loss of < 5% after 400 temperature cycles. This demonstrates the feasibility of our interconnection approach based on soldering wave-shaped wires on the finger grid or small contact pads of silicon solar cells.

In the manufacturing process of a common solar module, crystalline silicon solar cells are interconnected by soldering. Copper-based interconnectors, coated with a solder alloy are soldered on both sides of the solar cells on the contact metallization. Subsequently, the solar cell strings are embedded in between two sheets of encapsulant, commonly ethylene-vinyl acetat (EVA), which itself attaches to a front glass and a back sheet during the lamination process. Thermomechanical stress, due to the mismatch of the coefficients of thermal expansion (CTE) of copper and silicon, causes bowing of a back-contact solar cell after the soldering process. Additionally, thermomechanical stress can result in the failure of solder joints. By adapting the mechanical properties of an interconnector, thermomechanical stress can be reduced. Shaping a straight wire with a round cross section to a wave-like form reduces its effective tensile mechanical properties and leads to changes of its effective electrical conductivity. This study investigates the mechanical and electrical properties of wave-shaped wires with different amplitudes, periods and diameters. In addition, microscopic imaging reveals the geometry and potential defects due to the shaping process. The influence of the shaping process on the yield limit of commercially available copper-based wires is determined. For this reason, an algorithm is designed that allows the automatic and precise determination of the yield limit of these interconnectors. In this study, a maximum yield limit reduction of 88.5% compared to a straight wire is found. Furthermore, the influence of the shaping process on the electrical resistance is analyzed. For this purpose, the influence of wire damaging due to the shaping process, as well as the wires longitudinal length change is determined. An increase of the electrical resistance between 3.5% and 82.7% is measured. To choose an optimal interconnection concept for solar cells, the mechanical, as well as the electrical properties of the interconnectors have to be considered. This study correlates the advantageous longitudinal softness of wave-shaped wires, which potentially leads to a significant decrease of the thermomechanical stress in solar cells and of the bowing of back-contact solar cells, with the disadvantageous increase of the electrical resistance caused by the shaping process. The findings enable an optimization of a wire-based interconnection concept for back-contact solar cells.

In this work, we show the latest progress in the confocal micro-Raman spectroscopy for determination of stress within embedded solar cells. We present measurements proving that the module front glass has no influence on the stress measurement. We also present the first large area stress mapping on a quarter solar cell within a single-cell laminate. A stress map of the busbar end shows the superposition of the soldering induced tensile stress at the end of a busbar and the overall compressive stress from lamination. The stress mappings show the same trend as finite element simulations. We obtain a maximum relative compressive stress in the center of the laminated solar cell of about 50 MPa from the Raman spectroscopy and the FEM simulation.

Internal reflections within photovoltaic modules are known to contribute to power gains from cell to module. The module rear cover, usually a white backsheet, is one module component reflecting additional light onto the solar cell. A novel approach to model the effect of backsheet reflectance on the achievable coupling gain in solar modules is presented. Using a discrete ray optics approach, results can be calculated rapidly for arbitrary reflectance distributions using a partition of the emerging rays into groups. The model is fully wavelength resolved, using measured data to model optical material properties. It is therefore suitable for arbitrary material stacks in front of and behind the solar cell with a single diffusely scattering layer. We study the impact of layer thicknesses, incidence angle and distribution function on the coupling gain using the presented approach. Comparison to measurements of the coupling gain using single cell modules shows good agreement with the calculated results.

This work presents an interconnection approach for 6" back-contact back-junction (BC-BJ) solar cells by using conventional solder-coated copper ribbons with implemented wave structures for thermomechanical stress relief. We developed a process for production and advanced mechanical and electrical characterization for these interconnectors. In our study, mechanical stress is reduced up to 96.6 % (for ribbons) and up to 81 % (for wires) compared to non-structured interconnectors. In electrical terms, the relative effective resistance of the interconnector is increased by 3.1 % (for ribbons) and by 6.4 % (for wires). In general both, ribbons and wires, are suitable for the interconnection of 6" BC-BJ solar cells. A 4-cell module with 19.94 % efficiency in a standard module setup with 21.09 % -21.18 % 6" BC-BJ ZEBRA cells (CTMpower=-3.8 %) is manufactured. For interconnection 8 modified ribbons (1.5 x 0.2 mm(2)) are used on the rear side. The cells feature a multilayer metallization with a low temperature paste. The module passes TC-200 according to IEC 61215 without degradation. Our cost analysis shows that such a stress relief structure can be realized with additional material costs of lower than 0.2 for a 60 cell full size module. (C) 2017 The Authors. Published by Elsevier Ltd.

For the comparative study with focus on mechanical long-term stability, we perform a temperature cycle (TC) test-to-failure for 1250 cycles according to IEC 61215 with laminated solar cell strings. The strings were produced with Multi Busbar (MBB) solar cells interconnected by wires and with solar cells with 3 busbars (3BB) interconnected by ribbons. One module sample comprises two electrically isolated 3-cell strings, one for each technology. The 3BB string displays a power loss of more than 5 % after 850 cycles, while the MBB string only exceeds 5% loss after 1250 cycles. After TC 1250 the 3BB string shows a degradation of -8.2 % in FF and -8.0 % in power, while the MBB string shows a degradation of -6.2 % in FF and of -5.3 % in power. In the second comprehensive comparative study we use for each interconnection technology 15 cells for isothermal storage (up to 130 degrees C, 42.8h) and 10 modules for TC (up to 400 cycles). Each reliability analysis includes IV and EL characterization as well as adhesion force measurements and microsection analysis. After TC 400 the MBB interconnection shows less than 1 % degradation in efficiency compared to 4-5 % degradation for 3BB ribbon interconnection in a common module setup. The cells and modules show that most of the electrical contacts are still intact with respect to the EL image after 42.8 h isothermal storage and TC 400, although the average peel forces degraded from about 2 N/mm to less than 0.5 N/mm. Both studies indicate a superior mechanical long-term stability for Multi Busbar technology compared to ribbon interconnection. (C) 2017 The Authors. Published by Elsevier Ltd.

The microstructure and intermetallic phase growth in solder joints of photovoltaic modules are investigated because of their significance for interconnection reliability. Interconnector ribbons with Sn60Pb40, Sn62Pb36Ag2, Sn43Bi57, Sn41Bi57Ag2 and Sn91Zn9 are soldered on the front busbars of industrial crystalline silicon solar cells. Cross sections are inspected using microscopy, SEM and EDX. The interconnections are isothermally aged, whereas the intermetallic layer thickness is determined successively. The microstructural changes in the bonds are characterized. Grain coarsening, volume increase of intermetallic compounds, Sn-penetration into the metallization and growth of grain boundaries between the phases are found. The composition of the intermetallic phases within Sn91Zn9-bonds is discussed. A diffusion model is used to simulate the intermetallic layer growth after 3000h at 85°C and thermal cycling from −40°C to 85°C for 600 cycles. A prognosis of the phase growth within the photovoltaic module after 25 years at the location Freiburg in Germany is made. It is found that the Ag3Sn-phase using Sn43Bi57 extends to 6.6µm after 3000h at 85°C as compared to 2.6µm for Sn60Pb40. After 25 years in Freiburg the Ag3Sn-layer within Sn43Bi57 joints is predicted to be 2.4µm whereas within Sn60Pb40-bonds only 2.2µm.

In order to achieve a fast and reliable qualification of promising conductive adhesives for solar cells, different accelerated ageing test sequences, accompanied with analyses on material level are conducted. Damp-heat followed by damp heat at a current range of 8 A; humidity freeze and thermal cycling. The testing procedure in the herein presented work is adapted to identify advantages and also weaknesses of the materials in combination with the entire design. The conductive adhesives replace the lead-based soldering thereby allowing a lower processing temperature. Another potential reduction of material costs is the use of double redundant fingers instead of busbars. In this way, a considerably smaller amount of silver is needed. The interconnectors are glued between theses double redundant fingers with the conductive adhesive. Another advantage of the gluing is the possibility of using groove structured interconnectors that allow an increase in power of the solar module by reflecting light back to the solar cell. As major degradation indicator the performance of the modules containing conductive adhesives, redundant fingers and groove structured interconnectors is measured before and after the exposition of the solar module to different accelerated ageing tests and compared to the reference solar module with flat interconnectors soldered on busbars. After the accelerating ageing test damp heat (DH), there was already a clear difference in viability of the conductive adhesives. For two conductive adhesives, module power losses were comparable to the reference, for a third one power losses amounted to 14 %. The loss in power is caused by an increase in serial resistance. Analyses of the adhesives indicate a relation between the degradation in damp-heat and the adsorption of humidity of the material. Different subsequent aging tests (after DH) showed the feasibility of this innovative technology, but also unveiled weaknesses compared to a traditional soldered busbar technology. Two of the three conductive adhesives have fulfilled the IEC 61215 criteria in terms of power loss after damp heat, humidity freeze and thermal cycling. (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.