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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.

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.

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.

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.

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).

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 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.

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.

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.

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.

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

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.

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.

After completion of the solar cell manufacturing process the current–density versus voltage curves (J(U) curves) are measured to determine the solar cell's efficiency and the mechanisms limiting the efficiency. An accurate and robust analysis of the measured curves is essential. In this work it is shown that fitting the two-diode model is inappropriate to quantify recombination in the space charge region and ohmic losses due to series resistance. Three fill factors, namely the fill factor of the illuminated J(U) curve, the pseudo fill factor of the sunsVoc curve and the ideal fill factor of the single diode model, are the base of a quick loss analysis that is evaluated in the present paper. It is shown that for an accurate analysis the distributed character of the series resistance and the network character of the solar cell cannot be neglected. An advanced current–voltage curve analysis including fill factors and fit is presented. Copyright © 2010 John Wiley & Sons, Ltd.

In this paper we present a high throughput testing setup for organic solar cells that is necessary for an efficient analysis of their behaviour. The setup comprises process parameter logging, automated measurement data acquisition and subsequent data management and analysis. Utilising this setup the reproducibitity of solar cells and the effect of production parameter variations has been tested with a set of 360 solar cells based on thepoly-3- hexylthiophene:1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 bulk heterojunction. Variations in power conversion efficiency between 1 and 3% were observed on varying production parameters hardly mentioned in literature. The conditions during the vacuum deposition of the aluminium cathode turned out to have a significant effect. The key solar cell parameter affecting the performance was the fill factor (FF).As such the work exemplifies the necessity for a combined approach to analyse the complex behaviour of organic solar cells. The developed high throughput testing setup provides a basis for an efficient testing of production parameter variations and materials and additionally opens the door for statistical analysis. Copyright © 2008 John Wiley and Sons, Ltd.

We report on the development of two types of organic solar cell modules one for energy autonomous systems and one for large area energy harvesting. The first requires a specific tailoring of the solar cell geometry and cell interconnection in order to power an energy autonomous system under its specific operating conditions. We present an organic solar cell module with 22 interconnected solar cells. A power conversion efficiency of 2% under solar illumination has been reached on the active area of 46.2 cm 2 . A voltage of 4 V at the maximum power point has been obtained under indoor illumination conditions. Micro contact printing of a self assembling monolayer was employed for the patterning of the polymer anode. Large area photovoltaic modules have to meet the requirements on efficiency, lifetime and costs simultaneously. To minimize the production costs, a suitable concept for efficient reel-to-reel production of large area modules is needed. A major contribution to reduce the costs is the substitution of the commonly used indium tin oxide electrode by a cheap material. We present the state of the art of the anode wrap through concept as a reel-to-reel suited module concept and show comparative calculations of the module interconnection of the wrap through concept and the standard ITO-based cell architecture. As a result, the calculated overall module efficiency of the anode wrap through module exceeds the overall efficiency of modules based on ITO on glass (sheet resistance 15 Ω/square) and on foils (sheet resistance 60 Ω/square).

Organic solar cells have the potential to make cheap photovoltaic devices feasible. In order to achieve this, material and production costs have to be minimised by using device architectures, which are suited to tap the full potential of reel-to-reel production.The inversion of the layer sequence in organic bulk-heterojunction solar cells is motivated by the possibility to omit the commonly used expensive indium tin oxide electrode utilising the so-called wrap through concept. In this concept, the hole contact is formed by a highly conductive formulation of poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate), which is led through via holes in the solar cell to the backside of the substrate in a regular pattern, where it is contacted with a metal layer with low sheet resistance. In this way, a scalable parallel connexion is realised. If higher voltage is desired, one can also connect several such cell segments in series monolithically. We will show that the inversion of the layer sequence is possible without loss of device performance. Using the results of small area inverted devices, we calculate the optimal dimensions of the wrap through solar cell module.First devices with active areas of 2–4 cm2 with parallel and serial wrap through connexion will be shown as proof of concept. © 2006 Elsevier B.V. All rights reserved.

We report herein large area (>10 cm 2 ), interconnected organic solar cell modules both on glass substrates as well as on flexible ultra-high barrier foils, reaching 1.5% and 0.5% overall power conversion efficiency under AM1.5 conditions. Series connection is described, as these modules consist of up to three cells. Using our flexible barrier material, a shelf lifetime of polythiophene-based solar cells of 6000 h could be realized. Furthermore, we compare the photovoltaic performance of efficient conjugated polymer:fullerene solar cell modules with established technologies. Under typical indoor-office lighting, our modules are competitive with these systems.

A novel cell architecture for organic solar cells is presented which is based on a functional microprism substrate. In contrast to the most widely used planar cell architecture with a transparent indium tin oxide (ITO) electrode, the microstructure results in a folded solar cell. Of benefit are the light trapping effect and the substitution of the ITO-electrode by a highly conductive polymer layer with a supporting metal grid. Optical simulations were performed and reveal a gain in absorption in the photoactive layer due to the inclined incidence of radiation and due to the second reflection. Optimal dimensions of the microstructure were calculated by taking the sheet resistivity of the polymer anode and the shading effect of the metal grid into account. The metal grid with a low effective sheet resistance below 1 Ω/□ was realised by evaporation techniques using the microstructure as a self aligning mask. Investigations on the thin film formation are presented. First microprism solar cells with solar efficiencies comparable to planar ITO-reference solar cells were realised.

One limiting factor to the efficiency of the bulk heterojunction solar cell is the weak absorbance of the thin photoactive layer. The thickness is restricted by the small charge carrier mobility. Two cell concepts—light trapping with diffraction gratings and buried nano-electrodes—were investigated. Optical simulations based on rigorous coupled wave analysis were performed with supporting experiments in order to evaluate the concept of an embossed diffraction grating in the active polymer film. An increased absorptance in the active layer is calculated for transversal electric polarisation. High losses in the corrugated aluminium electrode in transversal magnetic polarisation would require a displacement of the corrugated boundary between two dielectrics. The second approach is based on vertical nano-electrodes. The planar semi-transparent electrode is substituted by comb-like array of electrodes embedded in the photoactive polymer blend. The potential of this approach is discussed and initial experimental results are presented.

This work deals with requirements regarding the solar cell process that allow or facilitate the introduction of fabrication processes for front side metallization. By taking experience with plating on solar cells both from the literature and from practical lab work, design rules for the solar cell and the plating process have been derived. Regarding the surface texture, small features (e.g., random pyramids < 5μm) have been found beneficial for plating processes both on printed seed layers and directly on silicon. A dense, pinhole-free anti-reflective coating suppresses so-called ghost plating as well as careful handling in production. Shunting, which is possible for laser edge isolation can be prevented by applying wet chemical edge isolation. An adjusted soldering process which brings in high energy within a short time is recommended. Infrared or inductive soldering are ideal for the connection of plated surfaces with ribbon. For plating on paste it is recommended that using plating electrolytes with pH>2 result in a good adhesion of the seed layer on the silicon surface. Further the glass content in the silver paste and the size of the glass particles have a big influence on the adhesion. A high glass content and small particles are beneficial for a following plating process. For direct plating on silicon an ablation of the ARC with a 355nm ps-laser is the best option for adhesion.

We present a laser‐based method for the metallization of silicon heterojunction solar cells by Cu‐plating. It consists of first applying a dielectric layer on the transparent conductive oxide (TCO) as a plating mask. Then, a NiV seed is transferred by laser induced forward transfer (LIFT) from a plastic carrier foil onto the wafer. In the second laser step, the NiV layer is fired through the dielectric layer to form a contact to the TCO. After the laser process, Cu‐fingers are produced by plating. The dielectric plating mask remains on the cell. Parasitic plating is prevented by using an advanced reverse pulse plating process. With the first solar cells we reach a maximum efficiency of 22.2% and an efficiency gain of 0.5%abs compared to low‐temperature screen printing reference cells, due to a higher short circuit current and fill factor. The 30 mm wide fingers reach specific contact resistances down to 0.6 mO cm2 and also, pass a tape adhesion test. We further demonstrate that the laser process does not cause any measurable open circuit voltage loss and that a precise alignment of the two laser steps is not necessary.

In the context of nickel silicide formation from plated nickel layers for solar cell metallization, there are several open questions regarding contact adhesion and electrical properties. Nanoscale characterization by transmission electron microscopy has been employed to support these investigations. Interfacial oxides and silicide phases were investigated on differently prepared samples by different analytical methods associated with transmission electron microscopy analysis. Processing variations included the pre-treatment of samples before nickel plating, the used plating solution and the thermal budget for the nickel–silicon solid-state reaction. It was shown that interface oxides of only few nm thickness on both silicon and nickel silicide are present on the samples, depending on the chosen process sequence, which have been shown to play an important role in adhesion of nickel on silicide in an earlier publication. From sample pretreatment variations, conclusions about the role of an interfacial oxide in silicide formation and its influence on phase formation were drawn. Such an oxide layer hinders silicide formation except for pinhole sites. This reduces the availability of Ni and causes a silicide with low Ni content to form. Without an interfacial oxide a continuous nickel silicide of greater depth, polycrystalline modification and expected phase according to thermal budget is formed. Information about the nature of silicide growth on typical solar cell surfaces could be obtained from silicide phase and geometric observations, which were supported by FIB tomography. The theory of isotropic NiSi growth and orientation dependent NiSi2 growth was derived. By this, a very well performing low-cost metallization for silicon solar cells has been brought an important step closer to industrial introduction.

In this work we present the impact of Watts-type nickel electrolytes on the contact microstructure of the screen-printed silver contacts of silicon solar cells. Both an SEM and TEM analysis were used to investigate the reasons for poor contact adhesion following nickel plating. A failing interface was identified and located exactly between the glass frit and the bulk silver inside the contact. A clearly visible gap occurs at this interface due to a chemical dissolution of the boundary layer between the glass and the silver. In TEM measurements, no distinctive features in the elemental composition were identified at the failing interface that might be responsible for the site-specific dissolution. This suggests that the dissolution starts at the very thin oxide layer between the glass and the silver. Furthermore it became clear that the electrolyte reaches the identified failing interface via pores and cavities in the screen-printed silver layer. Closing these cavities prior to nickel electrolyte exposure thus lead to significantly better contact adhesion values.

The sintering behavior of screen printed front side silver contacts on silicon solar cells was studied by adding different lead containing additives to the silver paste. We added lead in the form of an element, three different oxides and in the form of glass. The pastes were studied with in-situ optical microscopy during the sintering process, crystallite formation and densification using scanning electron microcopy, contact resistance measurements to evaluate the metal-semiconductor contact of solar cell structures and finally with simultaneous thermal analysis. Results show that the sintering density is dependent upon the lead additive and that the oxide is necessary to etch through the anti-reflection coating.

Progress in metallization techniques and emitter formation technologies needs fast and reliable tools for the optimization of the metallization pattern of crystalline silicon solar cells. We present and compare three models and discuss their validity and accuracy for different emitter sheet, finger and contact resistances, and finger widths. All models yield a similar optimal number of metallization fingers for a 156 mm × 156 mm large solar cell and broad plateaus concerning the dependence of the conversion efficiency on the finger spacing, but they differ concerning the absolute value of the fill factor and the dependence of the open-circuit voltage on the number of fingers.

Selective emitters will soon become a standard feature of industrial silicon solar cells. RENA offers a laser chemical process (LCP) to locally ablate the antireflection coating and form a highly doped emitter at the opened area. The front side metallization is then applied by light induced plating (LIP) of nickel and silver. However, it is still a challenge to optimize this process sequence for precursors coming from different production lines. For this purpose we use photoluminescence imaging (PL) as a valuable tool to reduce the impact of the quality spreading of the precursors on the evaluation of our experiments. With PL it is possible to calculate an implied open circuit voltage or dark saturation current as well as a pseudo fill factor before metallization. Instead of comparing different groups of an experiment only by their final cell parameters, we compare the quality parameters obtained with PL after each process step. Like this we can exactly quantify the impact of each production step on the final solar cell performance. In this paper we will demonstrate how using PL improves the significance of our experiments and allows fast process optimization with a small amount of wafers. In general this procedure for process control can also be applied for other process steps.

We use photoluminescence imaging (PL) for quality control of the crystallization process at Fraunhofer ISE in order to find defects which will later limit the efficiencies of solar cells. Bricks of multicrystalline feedstock from electronic grade (EG) and upgraded metallurgical (UMG) silicon are subsequently wafered and solar cells are manufactured using a standard industrial solar cell process. PL is employed on bricks, as-cut wafers and finished cells and supplemented by additional measurement techniques such as microwave photo conductance decay (MW-PCD) or I–V curve measurements. In UMG material, the major problem is the presence of background dopants, which lead to compensation effects such as a pn-type changeover. As the cell efficiency of wafers within or beyond the type changeover drops significantly in the standard process, these wafers have to be detected reliably in the incoming test to be separated and introduced to an adapted solar cell process or discarded. To do such a separation, the position of the wafer from the brick needs to be known.

The working principle of organic bulk-heterojunction solar cells is a widely discussed topic. For thin film solar cells it is commonly supposed that the built-in potential Vbi is the driving force for charge separation and determines the open-circuit voltage Voc. In former works, Vbi was estimated by measuring Voc in the saturation regime. To check the validity of this model, the direct measurement of the built-in potential is desirable. We have investigated the origin of the open-circuit voltage of organic bulk-heterojunction solar cells by means of electroabsorption spectroscopy. This technique allows measurement of the built-in potential directly and therefore permits an independent measurement of Vbi and Voc. In our experiments on indium tin oxide/poly(3,4-ethylendioxythiophene) : poly(styrene-sulfonate)/poly(2 -methoxy-5-(3′,7′-dimethyloktyloxy)-p-phenylene-vinylene) : 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61/metal bulk-heterojunction solar cells no significant correlation between the open-circuit voltage and the built-in potential was observed. For certain electrode materials, i.e. gold and copper, Voc exceeded Vbi which is revealing of semipermeable membranes. © 2005 Elsevier B.V. All rights reserved.

Along with efficiency and lifetime, costs are one of the most important aspects for the commercialization of organic solar cells. Thinking of large scale production of organic solar cells by an efficient reel-to-reel process, the materials are expected to determine the costs of the final product. Our approach is to develop functional substrates for organic solar cells which have the potential for cost effective production. The functionality is obtained by combining periodically microstructured substrates with lamellar electrode structures. Such structured substrates were fabricated by cost effective replication from masterstructures that were generated by large area interference lithography. Two cell architectures were investigated - holographic microprisms and interdigital buried nanoelectrodes. A structure period of 20 μm in combination with a 2 μm wide metal grid was chosen for the microprism cells based on the results of electrical calculations. Current-voltage curves with reasonable fill factors were measured for these devices. A significant light trapping effect was predicted from optical simulations. Interdigital buried nanoelectrodes are embedded in the photoactive layer of the solar cell. Separated interdigital metal electrodes with a sufficiently high parallel resistance were manufactured despite a small electrode distance below 400 nm. Experimental results on first photovoltaic devices will be presented. We observe an insufficient rectification of the photovoltaic device which we attribute to partial electron injection into the gold anode.

We measured the electrical impedance spectra of organic bulk-heterojunction solar cells based on an absorber blend of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester. Comparing the spectra of the non-treated device and after two consecutive treatments with applied forward bias voltage at 110 °C, we observed a region in the semiconductor with a low conductivity, which was expanding after the treatments. We concluded that this region is a depletion region at the aluminium contact. This was confirmed by the bias dependence of the impedance spectra. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a novel method to determine spatially resolved the dark saturation current of standard silicon solar cells. For this two electroluminescence images are taken at two different voltages. From these two images, first the spatial voltage distribution can be calculated. Second by applying the Laplacian to the voltage image from Ohm's law and the continuity equation, the current through the device at a certain position can be determined. Knowing the local current through the device, the local voltage, and the emitter sheet resistance allows to determine the local dark saturation current. The clue of this method is to cope with the noise by using an appropriate noise reduction algorithm. By simulating electroluminescence images with realistic noise and known dark saturation current we demonstrate the applicability of the method with our noise reduction algorithm. Experimentally we compare our method with spectral response light beam induced current on multicrystalline solar cell.

This work describes a method to measure the metallized area on the front side of silicon wafer solar cells. The method is especially applicable to detect and quantify background plating, which can occur in the production of solar cells with plated front-side metallization. The metallized area of plated solar cells is determined by an image processing algorithm (“MetDetect”) using images of the solar cell front side obtained by a simple commercially available flatbed scanner. The algorithm is verified by the comparison of scanned images from test samples with microscope images. Furthermore a correlation of the metallized area with the measured short-circuit current density of the samples justifies the proposed method. With “MetDetect” a precise quantification of the background plating on the plated solar cell front side can be realized. It is suitable for inline inspection in solar cell production or quantification of pinholes and cracks in the surface dielectric. In this work “MetDetect” is applied to corroborate the observation that background plating of solar cells with a nickel copper front side metallization can surprisingly be removed during plating by a thin Sn-capping.