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A resist-free metallization of copper-plated contacts is attractive to replace screen-printed silver contacts and is demonstrated on large-area silicon heterojunction (SHJ) solar cells. In our approach, a self-passivated Al layer is used as a mask during the plating process. In this study, Al/AlOx or Al2O3 plating masks are further functionalized by a self-assembled monolayer (SAM) of octadecyl phosphonic acid (ODPA). The ODPA adsorption is characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy in attenuated total reflectance (FTIR-ATR) (in situ), and contact angle measurements to link the surface chemical composition to wetting properties. The SAM leads to homogeneous hydrophobic surfaces on large-area textured solar cells and planar flexible printed circuit boards (PCBs), which allows reproducible patterning of narrow lines by inkjet printing of an etchant. Selective copper plating is then performed to complete the metallization process and produce Cu contacts in the patterned areas. Silicon heterojunction (SHJ) solar cells metallized by the complete sequence reached up to 22.4% efficiency on a large area.

Plating is an emergent cost-effective metallization technology for depositing solar cell metal contacts. Plated contacts, however, typically plate in-homogeneously where fingers plate higher than busbars and solar cell edges plate particularly higher. The impact of these systematic inhomogeneities on solar cell efficiency and total plated mass is not yet well understood. To generate plating distributions, this work proposes a purely resistive plating simulation approach, employing the solar cell simulator Quokka3, which is validated by reproducing a variety of experimentally observed inhomogeneities. A simulation pipeline is then used to show the impact of initial grid and tool design on plated height distributions, as well as their impact on solar cell efficiency and total plated mass. As such, simulation shows that large-scale inhomogeneities, i.e., increased plating toward cell edges, can be avoided by adjusting the design of the anode and bath and/or reducing the cell-to-cell distance in an inline plating tool. Furthermore, results suggest that finger height inhomogeneity does not significantly reduce the maximum achievable cell efficiency, whereas it can significantly lower the plated mass required to reach that efficiency. This implies that systematic finger height inhomogeneities typical for plating could be acceptable if not beneficial for the dollar-per-watt tradeoff between cell efficiency and total deposited mass.

A resist-free metallization of copper-plated contacts is attractive to replace screen-printed silver contacts and is demonstrated on large-area silicon heterojunction (SHJ) solar cells. In our approach, a self-passivated Al layer is used as a mask during the plating process. In this study, Al/AlO

The metallization of bifacial tunneling oxide and passivating contacts (TOPCon) solar cells without initial metal seed layer by electroplating of Ni/Cu/Ag is demonstrated. The presented approach allows a lead-free metallization with narrow contact geometries and low contact resistivity. A metal plate provides electrical contact to the silicon via micrometer size laser contact openings and allows electroplating of bifacial TOPCon solar cells using industrial type inline plating tools without the need of a previously applied seed layer. Challenges such as direct contacting of silicon and dissolution of contacts are identified, and potential solutions are discussed. An optimized process sequence is developed and with this approach a solar cell efficiency of 22.5% is demonstrated on industrial bifacial TOPCon solar cells reaching the same level as the screen-printed reference solar cells.

Our novel metallization approach for silicon heterojunction (SHJ) solar cells takes advantage of a PVD metal-stack covered with a structured self-passivated Al layer as mask for electroplating a Cu grid. This plating metallization route enables a very low cost of ownership (COO) for SHJ solar cell back-end processing for busbars interconnection. Encouraging efficiencies of 22.1% are reached with FF > 81% and R s down to 0.39 Ω·cm2 on large area. A variation of the contact layer to ITO shows that of the investigated materials, TiW reached a sufficiently low contact resistivity down to 0.7 ± 0.3 mΩ·cm2 while for pure Ti it was too low to be measured (0.1 ± 0.3 mΩ-cm2). High busbar peel-off forces above 3 N/mm (Ti) are demonstrated.. Furthermore, a carefully adjusted etch-back procedure proves that AZO can be combined with our plated metallization to manufacture Ag- and In-free SHJ solar cells.

As the efficiency of silicon solar cells increases continuously, recombination at the metal contacts becomes more and more limiting, unless the contacts are passivated. Passivating contact layers on the front side, however, usually lead to parasitic absorption. To prevent this we develop a new self-aligned deposition process to implement a passivating layer only under the metal contacts. As passivating layer for the contacts on an optically highly transparent boron emitter we use PEDOT:PSS, which can be electro-polymerized at an anodic surface in an aqueous electrolyte. This electro-deposition technique is used to deposit PEDOT:PSS in the openings of the patterned dielectric passivation layers on the boron emitter of an n-type silicon solar cell. We demonstrate that the electrical properties of the electro-deposited PEDOT:PSS layer are suitable for a passivating contact layer and that the Fermi-level is de-pinned at the interface with silicon.

Confocal micro-Raman spectroscopy allows for spatially resolved measurements of the phonon energy in silicon, which is correlated to mechanical stress. Mechanical stress is a tensorial quantity. For the confocal measurement geometry and certain crystal orientations approximations have been derived in the past which correlate the shift of the Raman frequency to a scalar stress value. For optimization of mono-crystalline solar cell manufacturing steps the determination of induced mechanical stress from the top view perspective is desirable. However, this method is so far restricted to planar wafers or cross sections. we find that the anti-reflection surface texture strongly affects the measurement result. To enable quantitative stress determination of alkaline textured silicon a suited measurement procedure is investigated using 3-point bended solar cell segments. Preceding elasticity test revealed Young’s module and allowed a prediction of maximum stress values for any bending radius. We propose a measurement procedure which yields an observable shift of the Raman frequency proportional to the induced stress. We state the requirements for determination of a calibration factor to quantify stress on any alkaline textured solar cell. Adapting the conversion factors allows calibrated measurements for textures with different average pyramid heights. By varying the numerical aperture and the focus setting even changes of the induced stress within the texture pyramids are resolved.

We introduce a new plated metallization process for Silicon Heterojunction (SHJ) solar cells by selective plating of copper onto a positively masking seed layer. This process tackles the issues of high silver consumption and low grid conductivity of screen printed contacts on SHJ solar cells. Compared to other metallization approaches of plating into a negative mask, our process does not require expensive process steps or consumables to plate and then remove the mask. The process involves the deposition of a full area PVD layer of a valve metal like Al or Ti onto the ITO surface of a SHJ solar cell. The valve metal layer is self-passivating in nature and provides the ideal mix of conductivity for plating current distribution and selectivity for copper plating. As the next step, the grid is locally defined by printing or laser transferring the seed layer. Now a pulsed Cu plating step is applied to plate on the seed layer selectively while the valve metal layer is partly oxidized and parasitic Cu deposition on the valve metal surface is dissolved during reverse pulses. The valve metal layer is finally etched off in a subsequent etch bath.

Light-induced plating of a Ni-Cu-Ag stack allows for high efficient and potentially low cost solar cell metallization. The definition of the contact structure is performed by local laser ablation of the solar cells surface passivation layer. By using a femtosecond pulsed UV laser the crystal damage due to ablation is minimized in respect to any other approach of laser-structuring. Within the study presented plated contacts on femtosecond structured silicon are evaluated regarding electrical and mechanical properties. The results show that contact resistance is determined by a highly inhomogeneous interface oxide layer that is formed while femtosecond pulsed laser ablation on textured solar cell surfaces. Silicidation of the silicon/nickel contact interface is assumed to be essential for adhesion and low contact resistances. However, the results presented indicate that mechanical adhesion of contacts plated on femtosecond laser structured surfaces is ensured by the laser-induced microstructure and that low contact resistance and high fill-factors are reached when interface oxides are fully dissolved. Therefore, controlling the surface properties before metal-deposition is essential for plated contacts on femtosecond pulse laser structured silicon while silicide formation may be superfluous.

Electroplated copper contacts on small-area single-junction perovskite solar cells (PSCs) using an atomic layer deposited (ALD) Al2O3 masking layer on ITO are demonstrated for the first time. The photoconversion efficiency of ≈11% after manufacturing the Cu contacts confirms that PSCs can survive the wet-chemical plating process. From the successful realization of plated contact fingers, the creation of an electrical contact between the Cu electrode and the ITO on the FA0.75Cs0.25Pb(I0.8Br0.2)3 perovskite absorber is inferred. Furthermore, scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) analysis shows the formation of a compact interface between ITO and plated Cu. An additional plating approach, using self-passivated aluminum as mask, allows to produce well-defined 30 mm wide Cu contacts on the PSC. Such a plating process allows for plating a low-resistive Cu grid simultaneously on both sides of a perovskite silicon heterojunction tandem solar cell with TCO, independent of substrate size.

In this work we present SiOx films deposited in cost-effective laboratory scale three-dimensional printed atmospheric pressure chemical vapor deposition setup. As SiOx films are deposited at room temperature without complex vacuum systems, they can be a good candidate for the use in commercial c-Si solar cell production lines. The quality of the deposited films was investigated as to their integrity, conformity with various surfaces, and post-treatment resilience such as stability against etchants and annealing. Several applications of the SiOx film prepared with the atmospheric pressure chemical vapor deposition (APCVD) were discussed. In one application, the APCVD SiOx was utilized to effectively promote single-side texturing of Float Zone and Czochralski Si wafers by coating only one side with SiOx and subsequently annealing prior to texturing in an alkaline aqueous solution. Another application was to exploit the APCVD SiOx as a plating mask for silicon heterojunction solar cells. Two processing options prior to the oxide-film deposition were investigated: i) application of an Ag seed-layer, which promotes subsequent electroplating, and ii) printing of an organic grid, which, after stripping, creates openings in the SiOx that facilitate electroplating of the solar cell's electrode on the underlying transparent conducting oxide. In a different application, the APCVD SiOx films acted as protection against parasitic plating on the front side of passivated emitter and rear solar cells. The deposited films were characterized by ellipsometry, hemispherical reflectance measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy.

Light-induced plating on laser-structured passivation layer openings is an alternative method to silver screen printing for fabrication of the front-side metal grid on silicon solar cells. Fundamental contact properties, such as adhesion and contact resistance, are determined at the highly inhomogeneous interface of laser-structured silicon and plated nickel. Usage of laser structuring and plating holds the risk that an interfacial oxide layer is incorporated to the contact stack. It is shown within this article that both native and laser-induced oxide formation influence contact properties. Although, a native oxide layer does not impede metal deposition during plating, the dynamics of the layer formation is shown to be changed. It is known that interface oxides harm the contact resistance, and it is reported that postplating annealing decreases the contact resistance. In this article, the microstructural basis of the temperature-induced changes is examined. By transmission electron microscopy and energy dispersive X-ray spectroscopy, no temperature-induced modification of the interface microstructure, such as silicide formation or metal-induced crystallization of the amorphous surface, is observed. Results suggest that the decrease is attributed to a temperature-induced change of the electrical properties of the interface oxide. Considering reports that partially closed interface oxide layers are an inherent feature of laser-structured plated contacts, the results presented motivate reconsidering the objective of postplating annealing. The results indicate that the beneficial impact of silicide formation is overrated for the case of laser-structured plated contacts, whereas the temperature-induced modifications on residual interface oxide layers are neglected in the scientific discussions on the given contact stack.

The metallization of silicon heterojunction (SHJ) solar cells by selective Cu electroplating onto a structured PVD Cu-Al seed and mask layer stack is established by using the native oxide of the Al as insulating barrier. The NOBLE metallization (native oxide barrier layer for selective electroplating) allows reaching a first promising efficiency of 20.0% on full area SHJ solar cell with low contact resistivity on ITO. The approach features several advantages: low temperature processing, high metal conductivity of plated copper, no organic making and low material costs (almost Ag-free).

The metallization of silicon heterojunction (SHJ) solar cells by electroplating of highly conductive copper onto a multifunctional patterned metal layer stack is demonstrated. The approach features several advantages: low temperature processing, high metal conductivity of plated copper, no organic making, and low material costs (almost Ag‐free). A PVD layer stack of copper and aluminum is deposited onto the cell subsequently to TCO deposition. The aluminum layer is patterned with a printed etchant and its native oxide on the remaining areas inhibits plating. The full area aluminum layer while electroplating supports plating current distribution and allows homogeneous plating height distributions over the cell. The NOBLE (native oxide barrier layer for selective electroplating) approach allows reaching a first encouraging SHJ solar cell efficiency of 20.2% with low contact resistivity.

We present a detailed study on aluminum-boron codoping of silicon by alloying from screen-printed aluminum pastes containing boron additives (Al–B pastes). We derive an analytical model for the formation of the Al–B acceptor profiles by quantitatively describing (i) the composition of the Al–B–Si melt and (ii) the incorporation of Al and B acceptor atoms into the recrystallizing Si lattice. We show that measured Al–B dopant profiles can be excellently described by this model, which therefore offers a straightforward method for the comprehensive investigation of alloying from Al–B pastes. The formation of a characteristic kink in the Al–B dopant profile curve can thus be ascribed to the exhaustion of the B additive dissolution during alloying. By intentionally adding elemental B powder to an Al paste, we demonstrate that only a low percentage of the B powder actually dissolves into the melt. We show that this incomplete dissolution of the B additive strongly affects the recombination characteristics of Al–B–p+ regions and, thus, is an important element of alloying from Al–B pastes. This study therefore provides improved understanding of aluminum-boron codoping of silicon. Copyright © 2015 John Wiley & Sons, Ltd.

Plated Ni/Cu/Ag contacts are an industrially feasible metallization approach for high efficiency c-Si solar cells with low surface doping concentrations (10 18 cm -3 N D 20 cm -3 ). The 2d-simulations of this work define the minimum requirements on the contact resistivity of metal contacts in a high efficiency solar cell design. The following experimental study of the contact resistivity of plated Ni/Cu/Ag contacts on lowly doped phosphorus emitter demonstrates low contact resistivities in the mΩcm 2 regime, which enable solar cells with high fill factors. Furthermore, the paper analyzes the influence of the thermal silicidation process on pseudo-fill factor losses and on the mechanical contact adhesion. The contact adhesion is also studied with respect to the laser contact opening process. The results of this work demonstrate that the right choice of back-end processes enable plated Ni/Cu/Ag contacts with low contact resistivities in combination with high contact adhesions above 1 N/mm.

Laser-induced selective Si doping and simultaneous ablation of a dielectric passivation layer is a promising technology for the creation of efficient and cost-effective solar cells. In this paper, the electrical quality of emitters produced with a 532-nm continuous-wave laser will be discussed using elaborate analysis of quasi-steady-state photoconductance (QSSPC) measurements. It will be shown that these emitters cause good charge carrier shielding, which leads to emitter saturation current densities as low as 240 fA/cm $^2$ for unpassivated surfaces. If an SiN $_{\it x}$ layer is present during laser doping, the emitter recombination increases by a factor of three. This detrimental effect is put down to the formation of microcavities within the recrystallized Si. A model of the ablation mechanism and cavity formation for long laser pulses is proposed, with the experimental data in this study serving as a limiting case for long irradiation lengths.

In this paper, the corrosion mechanism behind damp heat-induced degradation of screen-printed silver front-side contacts of silicon solar cells due to the presence of acetic acid, which is known to be a decomposition product of the most common module encapsulation material ethylene vinyl acetate, is investigated. Scanning electron microscope (SEM) investigations and solder and peel-tests show that the interaction between screen-printed contacts and acetic acid results in a significant adhesion loss of the front-side metallization due to the corrosion of the glass layer inside the contacts. The application of a reductive potential to the front contact accelerates this process considerably and enables it at lower acetic acid concentrations. The analysis of the dissolved corrosion products of the contact (lead, silver) by chemical trace analysis (inductively coupled plasma optical emission spectrometry) and the generation of Pourbaix diagrams of lead in acetic acid enable the elaboration of a detailed model, which suggests a cycle of dissolution and subsequent reduction of the dissolved species as the mechanism behind the corrosion of the contacts in the model experiment and most probably in module application. Cyclic voltammetry measurements with acetic acid dipping solutions support this conclusion.

This work deals with a possibility to simplify the processing of back-contact back-junction solar cells. A novel metallization process without using any additional mask is presented. The main focus is set on the contact separation resulting in the interdigitated metal pattern. In case of using evaporated aluminum as contacting material to silicon, aluminum anodizing is a convenient process to convert electrically conductive aluminum to electrically isolating aluminum oxide. In the established processes in which anodizing of aluminum is used for aluminum structuring, masks are used to achieve local anodized areas. In order to make the contact separation process by anodizing more economic, several in-situ anodizing processes either using structured processing units or printing techniques are developed and tested.

In this study, we present detailed theoretical and experimental investigations on full-area alloying from screen-printed aluminum pastes on silicon surfaces for solar cell applications. We introduce a simple analytical model for the description of the alloying process derived from existing models for evaporated Al layers, which we adapt to printed Al pastes. Thereby, we particularly account for the recrystallization of Si within the paste particles, which we refer to as parasitic Si recrystallization. Applying our model, we demonstrate good accordance of calculated with measured eutectic layer thicknesses. We show that the model can be versatilely used to investigate screen-printed Al-alloyed contacts in detail: We demonstrate that the latent heat of the Al paste significantly influences the alloying process. Thus, the effective peak temperature of the alloying process can be several 100 °C below the set peak temperature of the firing furnace. By combining calculations of the effective peak temperature with measurements of the Al doping concentration, we determine a parameterization of the solid solubility of Al in Si down to the eutectic temperature of 577 °C. Our investigations therefore provide improved understanding of alloying from printed Al pastes and enable the specific optimization of Al-alloyed contacts.

The proof of concept of a novel metallization route for bifacial silicon heterojunction (SHJ) solar cells by selective plating – i.e. organic mask-free, is demonstrated by a first lab scale solar cell (η=15.5%). A patterned metal-seed is inkjet-printed or deposited by PVD on a PVD-Al layer covering the ITO layer. The copper electroplating process is investigated considering different electrolytes and plating settings in order to selectively cover the metal-seed and not the PVD Al layer. As well a processing chemistry to etch the PVD-Al layer without affecting the ITO and the electroplated contact is presented.

Reduction in production costs is essential for newsolar cell concepts to be competitive. A large cost factor in interdigitated back-contact solar cell manufacturing is the metallization. Several process steps are needed to form an interdigitated metal pattern out of a full faced metal layer to realize n- and p-metal contacts. Generating electrically separated conductor tracks is not only a challenge in PV but also in the circuit board sector. To drive down the costs for metal structuring, an innovative approach called electrochemical screen printing (ESP), which combines screen printing and electrochemical etching is developed in this paper. It permits localized etching of aluminum layers with very short process durations. The process speed yields 100 mm/s and is comparable with the conventional screen printing speed in solar cell manufacturing. Hence, it is faster than conventional structuring processes using masks and chemical etchants. By using a self-made water-based NaNO3 paste, narrow etched grooves (

Continuous wave laser processes used to create solar cells with selective emitter and plated Ni–Cu front contacts are a widely discussed topic. Particularly low adhesion of the front metal contact has been identified as a serious challenge. In this work a detailed surface characterization of laser doped and patterned front sides of solar cells shows that formation of silicon oxynitride hinders nickel silicide formation and reduces contact adhesion of Ni–Cu plated contacts. In order to overcome the observed tradeoff between metal contact adhesion and penetration of the pn-junction, this paper presents a novel process sequence based on the formation of a deep selective emitter using a green continuous waver laser and subsequent patterning of the dielectric using a nanosecond-pulsed laser.

Solar cells showing shunts due to potential-induced degradation (PID-s) were investigated by combining electron-beam induced current (EBIC), microscopic light beam induced current (μLBIC) and microscopic Raman spectroscopy (μRS) at the same positions with high local resolution. A direct correlation of compressive stress measured by μRS and local shunting as observed by μLBIC was found. Comparing μRS mappings before and after degradation proves that stress is induced locally by PID-s which can be explained by the formation of sodium decorated stacking faults.

Investigating modifications of the silicon/metal interface during the manufacturing process and the influence on properties of laser-structured Ni-Cu-plated contacts enabled optimizing the metallization process. The micro pattern induced by ultrashort pulsed lasers ensures adhesion even without nickel silicide formation. Minimizing growth of laser-induced and native oxide at the surface of the laser patterned contact openings before plating enables high fill-factors without HF -treatment before metal deposition. Skipping the wet-chemical treatment eliminates parasitic plating on precursors with defective ARC. Compared to the state-of-the-art plating sequence, the approach presented features less process steps and enables improved solar cell aesthetics.

Aerosol jet printing is a technology for contactless printing of narrow line shaped features down to a width of about 20 μm. Until now, process optimization was only done empirically because there was no manageable model to describe it. The process was only modeled numerically regarding Stokes and Saffman force on the aerosol particles. In this article, we introduce a simple analytical model to predict the printed line width. Experimentally, we show that the model is a valid approach for a wide range of parameters. A close look on the quality of the printed lines reveals that depending on the process parameters the droplets are distributed more or less inhomogeneously within the printed lines. We attribute this to the Saffman force, which should be taken into account for a more accurate description, especially when only little sheath gas is applied. However, the printed line width generally agrees with our model, making our model a valuable tool for process control.Copyright 2014 American Association for ...

We have studied surface passivation layers for the application on n -type p+ nn+ bifacial silicon solar cells. Thereby, we have examined their optimal composition and thickness with regards to passivation quality, optical properties, and especially the contact formation during a co-firing step. These parameters were addressed in separate investigations: 1) simulation of the optical properties of a bifacial silicon solar cell, 2) measurement of the passivation quality on lifetime samples, 3) measurement of contact resistance (of aerosol printed fingers) to analyze the contact formation during the co-firing process, and 4) differential scanning calorimetry measurement were conducted to fundamentally understand reactions during contact formation in a fast firing furnace. The passivation layers tested were silicon nitride (SiNx), titanium oxide (TiO 2), and silicon oxide (SiO2) on lowly phosphorus-doped silicon n+-layers, whereas aluminum oxide (Al2O3) stacks, capped with SiNx and TiO2, were studied on lowly boron-doped silicon p+-layers. The results show that a dielectric stack, consisting of 10-nm-thick Al2O3 and 60-nm-thick SiNx layers on the boron-diffused silicon front side and a single 50-nm SiNx layer on the phosphorus-diffused silicon rear side, provides low emitter saturation current density (J0e), high optical absorption current density, and low contact resistance for printed and co-fired contacts.

The adhesion of Ni–Cu-plated contacts requires annealing, which can lead to an electrical degradation of the solar cell. A combination of optical imaging methods and electron microscopical cross-section analysis was used to investigate the annealing-induced shunts on monocrystalline solar cells. The results show that Ni spikes cause the lowering of the pseudo fill factor and reveal that these nonlinear shunts show the same behavior as recombination active breakdown sites on multicrystalline silicon solar cells, including radiation of visible light while breakdown. Using reverse biased electroluminescence set-up the shunts could be localized in top view with µm size lateral resolution. Subsequently, a cross-section was prepared on a radiative breakdown spot. Advanced electron microscopic investigations reveal defect structures featuring nickel precipitates on the position of the light source. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An evolutionary approach to increase the cell efficiency of the industrially widely used Al-BSF Si solar cell design is the implementation of a lightly doped emitter and plated Ni–Ag front side contacts. This work shows that this approach can give a significant advantage to fully screen printed solar cells with cell efficiencies up to 19%. However, the adhesion of the plated contacts strongly depends on the tempering induced silicidation of the Ni layer. The application of macroscopic and microscopic characterization techniques indicates that the tempering induces local non-ohmic shunts due to a silicide spiking in the space charge region. The application of an additional selective emitter doping in this experiment could decrease the shunting probability but could not prevent it completely due to inhomogeneous doping depths on the textured surface.

In this work, aluminum as contact material with low transfer resistance to p- and n-silicon is combined with the zincate process which enables plating on aluminum. In this way the excellent contact quality of aluminum and the high conductivity of plated layers are combined. Examples of application are given on two solar cell concepts. One is the both side aluminum contacted concept suitable for p-type and n-type base material. The double sided metallization approach is called DACAPO (Double sided Aluminum Contacted And Plated Overcoating) process. On p-type wafers, satisfactory results are shown for the metallization in terms of contact geometry (contact width down to 50 μm). Challenges of the approach such as laser and inkjet alignment and resist stability in aluminum etching solution, are discussed. Calculations of VOC and RC in dependence of the contact area show the potential of the DACAPO process for high efficiency solar cells. The second solar cell concept tested with zincate and plating is the IBC (interdigitated back contact) concept. Different possibilities of plating processes on both polarities are discussed in this context. Thickening the aluminum/silicon layer by plating results in an increased FF by 1.3% and consequently in higher cell efficiency up to 20.4%, even on small sized solar cells. The constructed metal stack is characterized by SEM-EDX analyses.

In this work the development and evaluation of a copper solar cell front side metallization based on a screen printed silver seed layer, a plated nickel diffusion barrier, a plated copper conductive layer and a plated silver capping is presented. Due to a slight adjustment of the standard screen printing process, and subsequent plating of Ni, Cu and Ag, silver consumption was reduced to < 16 mg per cell front side on standard BSF 15.6 cm x 15.6cm mono crystalline wafers, produced only with inline capable techniques. Contact adhesion forces of 1.5 N/mm and, efficiencies of 18.4 % were achieved. For the long term stability analysis on cell and module level, cells with different nickel diffusion barrier masses (10-40mg/cell) were treated on hotplates at 200, 225 and 250°C while detecting degradation due to copper diffusion by measuring the pFF. Sufficient cell life times are predicted for 10 mg Ni by the resulting Arrhenius plot. After 750h damp heat test of one cell modules no visible degradation was determined. This means that accelerated degradation on hotplates show the degradation due to copper diffusion more strongly than 750hours damp heat test. In an additional SEM analysis the plated nickel diffusion barrier showed a closed layer on the fingers and an incomplete layer on the busbars, indicating busbars as possible weak points for copper diffusion due to inhomogeneous Ni plating

To allow plating on aluminum, zincate processes are introduced in metallization of silicon solar cells. Different zincate solutions are investigated regarding their adhesion on typical solar cell surface morphologies. Sufficient adhesion (>1.5 N/plus 45 degree rule}mm) on PVD aluminum layers is demonstrated by zincate processes with subsequent plating of nickel, copper and silver on both random pyramid and damage etched surfaces. Small-size (4 cm2) back-contact back-junction solar cells with an aluminum or aluminum-silicon seed layer are successfully processed in this way. On screen printed aluminum, adhesion has been found to be below 1 N/plus 45 degree rule}mm. Microstructural investigation of the failure mode reveals that the aluminum peels off the silicon surface in this case.

For low cost high efficiency fully plated NiCu contacts on silicon solar cells, a thermal formation of nickel silicide is commonly performed for adhesion promotion. Since the results of this strategy have been found to be ambiguous, and as the interface situation is not understood, a microstructural analysis of the metal–silicon interface after thermal annealing of plated nickel layers on silicon solar cells has been carried out. The point of failure upon performing solder and peel testing on sintered contacts has been identified to be at the silicide–nickel interface. Microstructural analysis shows that void formation can be observed at this interface. The mechanisms of silicide and void formation from plated layers on solar cells are studied taking into account different electrolytes and pretreatments. The cavities can be removed by selective nickel etching and replating nickel onto the formed silicide after silicide pretreatment. Excellent adhesion of up to 2.5 N/mm has been measured, which represents the force of silicon wafer breakage rather than true metal adhesive force.

Plated silicon solar cells with nonoptimized antireflective coatings (ARCs) may feature parasitic plating (PP), i.e., unwanted metal depositions in ARC defects/pinholes. The present work introduces the easy plating sequence that takes advantage of native oxide growth to avoid plating in unwanted ARC pinholes or defects due to the electrical insulation of these defects. This prevents PP for plated solar cells. Independent of the ARC pinhole density, an aesthetic immaculate appearance can be achieved using easy plating. It is shown that it is possible to gain up to 0.5 mA/cm2 in short-circuit current density and 6 mV in open-circuit voltage compared to the reference plating sequence at Fraunhofer ISE. The importance to avoid drawbacks in terms of contact resistance and contact adhesion due to laser-induced or native oxide at the Si–Ni interface in the easy plating sequence is discussed in detail and important influencing factors in the process chain are lined out. At this stage of development, low contact resistances are possible in some cases but further research is necessary to fully understand the impact of solar cell design and process-related influencing factors. Easy plating can be an option to avoid PP independent of prior ARC pinhole density.

We present metallization approaches for silicon heterojunction solar cells by plating onto a structured seed layer. Our approaches do not require expensive processing steps or consumables. The process starts with a full area deposition of a dielectric or alternatively a blanket metal layer such as Al onto indium tin oxide (ITO). Next, the grid-shaped seed layer is applied via printing or laser-transfer. An efficiency of 22.2% was obtained using a dielectric layer and laser-transfer, outperforming screen-printing by 0.4%. Applying Al instead of a dielectric has the advantage that the plating current spreads more homogeneously on the solar cell surface, which is of particular importance for plating on bifacial solar cells. For using an Al blanket layer on top of ITO and a grid-shaped seed layer, we developed a plating process that selectively plates on the seed layer and not on the Al layer.

We present a detailed study on aluminum-boron doping profiles formed in silicon by alloying from screen-printed aluminum pastes containing boron additives. We show that an increase in the effective peak temperature Tpeak,eff (determined from phase diagram calculations) of the alloying process leads to higher concentrations of both Al and B atoms within the alloyed p+ region, resulting in (i) higher potential barriers for electrons, but also (ii) increased densities of recombination-active Al defects. While the improved potential barrier predominates for Tpeak,eff ≲ 770 °C, the increased defect density prevails for Tpeak,eff ≲ 800 °C, thus defining an optimal effective peak temperature. Furthermore, we show that, by increasing the amount of elemental B added to the paste, the acceptor concentration can be increased without affecting the defect density. Therefore, the optimal printing and firing conditions comprise high B amounts and low, adapted effective peak temperatures. For a B content of 0.9 wt% and Tpeak,eff = 765 °C, we have achieved a saturation current density of 253 fA/cm2, corresponding to an implied open-circuit voltage of 665 mV, which demonstrates the high potential of B additives within Al pastes to further improve the efficiency of Si solar cells with Al-alloyed p+ rear.

In this paper, we introduce a predictive, physics-based model, i.e., the so-called tilted-mirror model (tm-model), for optical modeling of rough rear surfaces on silicon solar cells. An enhanced method of using transfer matrices at the rear-side interface of solar cells is developed and combined with Monte Carlo ray tracing. As a result, a physically consistent and precise simulation of the spectral reflectance is achieved, thus leading to a predictive quality of the simulations that could previously not be reached for solar cells with a remaining irregular rear-surface roughness. This advance in optical simulation enables the researcher to directly analyze the effects of varying rear-side passivation materials and thicknesses, as well as the impact of different surface morphologies on the gained charge-carrier generation rate of a solar cell. A comparison with the Phong model shows that the tm-model is able to simulate the generated photocurrent Jph more accurately, as it is shown that the Phong model tends to overestimate this value due to imprecise calculation of charge-carrier generation. In an application of the tm-model to passivated emitter and rear cells, it is shown that a strong planarization of the rear surface leads to an improvement in photogenerated current up to 0.13 mA/cm2 compared with a weak planarization.

We investigate the application of nickel-plated front contacts to front and rear emitter silicon solar cells. We compare identically processed p - and n -type Si solar cells featuring (i) a homogeneous phosphorus-diffused n + front, acting as emitter or front surface field and (ii) a full-area aluminum-alloyed p + rear, acting as back surface field or rear emitter. This results in an n + pp + front emitter and an n + np + rear emitter solar cell structure, respectively. We show that the contact annealing temperature for the thermal formation of the nickel silicide (NiSi x ) after the electroless Ni plating has a significant influence on the NiSi x layer thickness. Increasing the temperature from 300 to 450 °C shifts the average thickness to higher values and augments deep NiSi x spikes. The intensified penetration of the n + front by the NiSi x spikes leads to a strong degradation of the front emitter p -type Si solar cell performances due to (i) decreased shunt resistances and (ii) the contamination of the emitter space charge region. We demonstrate that the rear emitter n -type Si solar cells, in contrast, are explicitly more stable. Therefore, by applying n - instead of p -type Si as base material, an improved stability against the thermal Ni contact formation can be easily realized without significant changes in the solar cell fabrication process.

The etching mechanism of silver pastes with glass frit through the SiN x anti reflection coating of a standard silicon solar cell during the fast firing step is well understood. In this paper we investigate the firing through behavior for different alternative dielectric layers or stacks that are of great interest for new high efficiency solar cell concepts. On n-doped surface SiO 2 and TiO 2 were compared with SiNx. On p-doped surface TiO 2 and SiN x as single layers and as capping layers on a thin Al 2 O 3 layer were compared. The contact formation was analyzed using contact resistance measurement and scanning electron microscopy. Generally, it turns out that SiN x is the most stable passivation layer during a firing process when compared with all other tested materials. However, stacked on Al 2 O 3 the contact resistance is decreased. Contact resistances as low as ρ c = 3 mΩcm 2 were obtained on lowly doped boron surface with a surface concentration of NA = 2.3×10 19 cm -3

This paper presents the development of a fine line seed layer screen printing paste with low silver lay down ∼ 30 mg per cell for front side metallization. The contacts of these seed layers are subsequently thickened with light induced plating. Standard screen printing pastes contain between 2%-5% glass frit while aerosol seed inks contain > 15% glass frit. The amount of glass needed for best contact formation in fine-line seed layer screen printing paste is unknown. In this study, an empirical approach varying the glass content in the seed layer pastes has been chosen. To adjust the glass amount to the low metal content, the glass content was varied at 4%, 8% and 12% for the printed seed layer. 156 x 156 mm2 p-type silicon solar cells with 90 O/sq. emitter were used. Contact fingers < 60 mm wide and 2 mm high were achieved. It was found that an increase in glass content improves the ability to achieve low contact resistance of about 5 mOcm2 even under moderate firing conditions. Open circuit voltages of Voc = 632 mV, fill factors of = 77% and efficiencies up to i =18% have been achieved. Additionally, an increase in glass content from 4% to 8% improved the maximum contact adhesion after nickel and copper plating from 0.5 N/mm to 1.1 N/mm.

The present work deals with the attack of different chemicals used in nickel and copper plating electrolytes, on screen printed silver front side contacts, which lowers the adhesion and hinders industrial implementation. Dipping tests with four different self-made pastes in isolated electrolyte compounds show the dissolution of silver for pastes with glass content and acidic exposure conditions in chemical trace analysis (ICP-OES) of the dip solution. These results can be correlated to the macroscopic contact adhesion, measured with quantitative peel force tests. The results show that there is a link between the amount of silver that is found in the dipping solutions after the exposure of the contacts, the measured contact adhesion, and the presence of glass in the contact. The reaction between glass and silver during the firing process forms an oxidation layer between silver and glass which provides good adhesion. This interface seems to be dissolved by acidic plating chemicals. An additional SEM investigation shows the failure mechanism to take place exactly between the glass that covers the wafer after the firing process and the silver bulk, indicating a chemical attack at this interface.

We report on a concept for organic solar cells where the layer sequence is inverted compared to the conventional setup. In such a configuration a conducting polymer layer is used as the transparent anode which is able to transport the photocurrent laterally to a metal grid. For the anode a low sheet resistance and a work function matching approximately the chemical potential of the holes of the illuminated photoactive layer is required. We showed that the poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) fits to these requirements. In our setup an aluminium cathode was used. It turned out that for the inverted setup the interface between aluminium and the photoactive layer has to be protected against oxidation. Our investigations show that with a thin layer (20 nm) of electron-beam deposited titanium between aluminium and the photoactive layer the electrical contact is drastically improved. A solar cell efficiency of (1.4 ± 0.3)% was reached in this case. © 2005 Elsevier B.V. All rights reserved.

One key problem in optimizing organic solar cells is to maximize the absorption of incident light and to keep the charge carrier transport paths as short as possible in order to minimize transport losses. The large versatility of organic semiconductors and compositions requires specific optimization of each system. We investigate two model systems, the MDMO-PPV:PCBM blend and the P3HT:PCBM blend. Due to the small thickness of the functional layers in the order of several ten nanometers, coherent optics has to be considered and therefore interference effects play a dominant role. The influence of the thickness of the photoactive layer on the light absorption is investigated and compared with experimental data. The potential of an optical spacer which is introduced between the aluminium electrode and the photoactive layer to enhance the light harvesting is evaluated by optical modelling. Optical modelling becomes more complex for novel solar cell architectures based on nanostructured substrates. Exemplary optical simulations are presented for a nanoelectrode solar cell architecture.

We investigated organic bulk-heterojunction solar cells based on an absorber blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) by electrical impedance spectroscopy (EIS). A strong neck in the modulus plot of the EIS-spectra indicates that the absorber is divided into two regions of different conductivities. A similar behaviour was observed for pure P3HT-diodes. Hence, it can be concluded that the PCBM:P3HT absorber is p-doped by impurities of P3HT, so that a Schottky-like contact with aluminium is formed. It is known from literature, that annealing of PCBM:P3HT solar cells leads to drastic improvement of the photovoltaic performance. We compared the current-voltage characteristics and impedance spectra before and after consecutive annealing steps. After the annealing an expansion of the depletion region was observed, indicating that volatile dopants were evaporated out of the absorber. This contributes to an improved photovoltaic performance as the separation of the generated charges in the depletion region is more efficient than in the non-depleted region. Also an improved rectification behaviour might be caused by a lower doping level.

An approved measurement method for evaluating space charge region recombination (SCR-recombination) induced by the screen printed front side metallization is applied for industrial solar cells with aluminum back surface field (Al-BSF) on monocrystalline Czochralski silicon (Cz-Si). The H-patterned base grid of all processed solar cells is screen printed with a commercially available front side silver paste, ensuring sufficiently good contacting and therefore good electrical performance of the solar cells. Additionally, irregular contact fingers are realized between the regular contact fingers of the base grid by applying different screen printing layouts with commercially available front side pastes to increase the front side metallization fraction linearly as well as by applying various test pastes for the layout with the highest metallization fraction. Well-known components of glass frits as lead oxide, zinc oxide, bismuth oxide and boron oxide can affect SCR-recombination. Therefore we fabricated test pastes with these specific paste/glass frit components. By applying this test structure, a comparable qualitative evaluation of SCR-recombination with respect to the specific paste component is demonstrated. Quantitative evaluations regarding the electrical performance of the solar cells as well as related microstructural investigations are presented.

The measurement of current-voltage (J-V) characteristics is one of the most straightforward methods for the characterization of solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. The internal series resistance is one limiting parameter of the fill factor and the efficiency of these devices. A second limiting parameter is the p-n junction space charge region recombination. In this paper, we present a method to determine the lumped series resistance by combining the J-V characteristics in the dark and under 1-sun illumination. As a first approximation, the lumped series resistance under illuminated conditions is used for the dark J-V characteristic at small currents. Based on this, we present a method to quantify resistive losses and space charge region recombination only from the dark and illuminated J-V curves so that a simple separation of both losses becomes possible with all inline cell testers.

Pastes and inks to contact silicon solar cell emitters have been further developed since decades and considerable improvements have been achieved. According to different R&D roadmaps for silicon solar cells emitter sheet resistances will further increase and the surface dopant concentration will decrease as narrower contact lines support such a development. This leads to new requirements of contact pastes. In this paper an improved contact ink based on silver will be presented which is able to contact phosphorous emitters with low surface dopant concentrations of N D = 8a10 18 cm -3 .

Double side collecting silicon solar cells such as the emitter wrap through and the both sides contacted and collecting concept are one possibility to maintain high efficiencies even on low lifetime material. To investigate the intrinsic advantages and limitations of this concept we use the solution of the continuity equation under low level injection conditions for a double side collecting (n + pn + ) and a single side collecting (n + pp + ) solar cell. In general, compared to the single side collecting structure, the double side collecting structure yields the highest advantages concerning the roduct of the short circuit current density and the open circuit voltage Jsc·Voc on highly doped material with diffusion lengths in the range of half the cell thickness. A realistic calculation of the fill factor of such devices is done using two dimensional numerical simulations. It is shown that a major reduction of the fill factor is caused by laterally varying voltage induced non-generation losses and ohmic losses, both caused by the majority carrier flow in the base. Electro- and photoluminescence images verify and illustrate the lateral voltage distribution under different illumination levels and terminal voltages.

Measuring the lifetime of excess charge carriers gives the opportunity to access the electric quality of the material. However, on as-cut wafers before production this quantity is strongly limited by the surface of the material. On a batch of solar cells we show that the open circuit voltage of the finished cells only scales with the lifetime, measured on as-cut wafers, if the material quality is very low. The difference between moderate and high material quality cannot be resolved. Using photoluminescence imaging the lifetime can be acquired with high spatial resolution. We show that by analyzing crystallization-related features in the images, certain defects can be identified: Such features are defects of crystal growth (e.g. dislocations) and areas of reduced lifetime that form at the edges of the crystallization crucible or near the top or bottom of a brick. Those features can be detected before production and we show their influence on cell parameters. By recognizing and rating these features, a more accurate quality control for wafers can be introduced.