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In this study, we introduce a temperature screening image analysis to investigate the temperature dependence of boron-oxygen-related defect regeneration achieved by using one sample. For that purpose, we induce a temperature gradient in a single sample over a broad temperature range in our laser-based rapid thermal processing furnace, while other influencing factors are kept constant. Spatially resolved measurements of the temperature during the regeneration process (thermographic images) and photoluminescence (PL) images at different boron-oxygen-related defect states are recorded. By a pixelwise assignment of the PL images to the temperature image, the effectiveness of the regeneration process in terms of regeneration completeness is evaluated for each pixel. In this experiment, we investigate the temperature dependence of a boron-oxygen-related defect regeneration in a temperature range of 100-500 °C for different treatment times of 2-30 s at an illumination intensity of 100 kW/m². Thereby, we determine the temperature regimes that allow for efficient regeneration for the respective regeneration parameter set with a single sample. The results can be used for industrial optimization of a boron-oxygen-related defect regeneration process. Furthermore, this technique can also be applied to other temperature-dependent process optimizations and even fundamental research.

We present the design, implementation and optimization of laser metal bonding (LMB), a new approach for joining thin aluminum foils and back surfaces of solar cells with a focus on low impact on the semiconductor and good adhesion. After gaining deep insights into laser-material interaction using finite difference simulations, we succeeded in establishing an expulsion free laser process that can be tuned from non-ablative bonding, which leaves the substrate largely intact, to welding, which produces highly adhesive joints. Experiments were conducted on a number of substrates, showing the correlation between damage and adhesion at different process parameters.

We study the mitigation of light- and elevated temperature-induced degradation (LeTID) with high-intensity illumination treatments, placing special emphasis on inline feasibility. After the treatments, we investigate the stability upon degradation conditions close to the recently suggested standard, which allows estimating the LeTID behavior during the operating lifetime of solar modules in the field. Subsequently, we map the stability at different treatment intensities and temperatures achievable with an air-cooled tool. We show that, when applying short treatment times, the stability improves with increasing treatment intensity and deteriorates steeply with rising temperature above an optimum region around 250 °C. However, these intensity- and temperature-dependent differences largely vanish when increasing the treatment time sufficiently. We also investigate the significance of darkness/illumination during the cooling ramp from the treatment temperature in view of the LeTID stability. We discuss our results based on suggested defect models of LeTID, and provide hypotheses of the origin of the instabilities observed at the high treatment temperatures. After identifying optimal treatments, we demonstrate that the energy yield loss due to LeTID reduces by over 60% after an inline-feasible process consisting of only 30 s of high-intensity illumination, combined with cooling of the samples from the process temperature under a lower-intensity illumination.

We present the design, implementation and optimization of laser metal bonding (LMB), a new approach for joining thin aluminum foils and back surfaces of solar cells with a focus on low impact on the semiconductor and good adhesion. We succeeded in establishing an expulsion free laser process to create highly adhesive joints on dielectric layers without penetrating them. Electrically conducting contacts have been successfully created on PERC solar cells via LMB on locally opened nitride layers, achieving efficiencies up to 20.5%.

In this work, we present a novel technological approach to form highly boron-doped selective emitters. The selective emitters are formed by using a Laser Induced Forward Transfer Doping (DLIFT) process, which allows precise adjustment of the doping profile by tuning the laser parameters and choosing an appropriate doping source. Surface dopant concentrations of up to Ns≈1×1021 cm−3 were achieved by using DLIFT. Subsequent BBr3 tube furnace diffusion was performed to form a low surface concentration (Ns≈1×1019 cm−3) homogeneous emitter. In comparison to the conventionally applied BBr3 tube diffusion, an increase in open circuit voltage of up to 6 mV is predicted based on the photoluminescence measurements performed after passivation, screen-printing and firing processes. Simulations suggest that this voltage gain is most possibly due to a lower emitter saturation current density (j0e) of the highly doped DLIFT samples compared to the BBr3 diffusion samples. Moreover, it is observed that the laser-induced defects are reduced successfully after a subsequent boron tribromide (BBr3) tube diffusion, which is used to form the homogeneous emitter after the DLIFT process. The micro Raman spectroscopy results indicated that the compressive stress of −200 MPa in the silicon crystal lattice after DLIFT process is released by the subsequent BBr3 tube diffusion. These results demonstrate that the DLIFT process in combination with the conventionally applied BBr3 tube diffusion can be an effective approach to form selective boron emitters in high efficiency n-type crystalline silicon (c-Si) solar cells.

Laser contact opening is a critical step for solar cells manufacturing and needs to be optimized to achieve high efficiencies. In this paper, laser contact opening using a picosecond laser (wavelength 355 nm, pulse duration 10 ps) has been carried out on n-PERT precursors composed of a SiOx/SiOxNy stack on the rear polished side and a SiOxNy layer on the front textured side. By varying peak fluence from 0.130 J/cm2 to 2.159 J/cm2 and spot overlapping, ninety parameters combinations have been tested to open these dielectric layers. Surface morphology characterization, before and after laser ablation, has been realized using Confocal Laser Scanning Microscopy and Scanning Electron Microscopy. Bulk and surface compositions have also been investigated by Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy analysis, respectively. Results have shown the existence of four separated laser impacted areas on the polished side and a related ablation mechanism is suggested. Also, electrical characterization using four probe measurements and calibrated lifetime photoluminescence revealed that electrical properties of the silicon underlying increased when post laser annealing was performed associated with no spot overlapping. Then, nickel electroless deposition has been performed and first characterizations indicate adherence issues and inhomogeneous metallization. Characterization of metallized samples revealed that these observations were closely linked to the non-homogenous surface morphology and composition after laser ablation.

Laser diffusion from PECVD layers can result in the incorporation of impurities like nitrogen into crystalline silicon. It is shown that the nitrogen can have a significant influence on the laser doping process. The incorporated nitrogen can affect the material properties resulting in misinterpretation of measurement results and accumulation of nitrogen at the surface can lead to negative effects such as improper contact formation. An approach reducing the amount of nitrogen content in the PECVD layer and the LBSF is presented. This new approach allows for a partial decoupling of the passivation and doping properties of the passivation layers. The doping efficiency of the laser doping process was significantly improved while keeping the recombination properties low. The higher doping efficiency was found to be of major importance for a reproducible level of LBSF/metal contact resistivity on the rear side. Using the adapted process with reduced nitrogen content, it is shown that the doping concentration is high enough to be contacted by screen printed silver pastes. Solar cells using the new approach are presented reaching efficiencies up to 20.9% on a cell area of 149 cm 2 . The influence of the higher doping efficiency reflected into the new solar cells allowing fill factors of up to 80.1%.

Light-induced degradation (LID) of multicrystalline silicon (mc-Si) solar cell performance has been reported to be surprisingly strong for conditions relevant under field operation. In paticular, solar cells with dielectrically passivated rear sides such as passivated emitter and rear cells are affected by this LID effect that can cause a loss of more than 10%rel in cell efficiency. With the root cause for the observed degradation being unknown to date, the underlying defect, however, has also been reported to be permanently deactivated under the same conditions at even longer time scales. However, a severe power loss due to the mc-Si specific LID is observed before the cells recover due to the long time scales of the regeneration under these conditions. Hence, regeneration on short time scales similar to the fast regeneration processes being reported for the boron–oxygen defect within p-type Czochralski-grown silicon is highly desirable also for p-type mc-Si, especially since mc-Si currently dominates industrial solar cell production. Within this work, partial regeneration of the defect causing the mc-Si specific LID is shown to be possible within less than 30 s, reducing the impact of LID by up to 60%, which leads to a significantly increased performance of the regenerated mc-Si solar cells.

The Laser Transfer and Firing (LTF) process can be used for the creation of seed layers for electroplated front side metallization of silicon solar cells. In this contribution, an optical method to determine the coverage and the diameter of the transferred seed layers is developed and a parameter called optical fill factor is introduced. The optical method is confirmed by EDS measurements. For the transfer, NiV layers with different layer thicknesses were used for the laser transfer and their corresponding thresholds were deduced. A correlation between layer thickness, metal coverage and diameter is observed. Thus, a better understanding of the laser transfer process is achieved.The Laser Transfer and Firing (LTF) process can be used for the creation of seed layers for electroplated front side metallization of silicon solar cells. In this contribution, an optical method to determine the coverage and the diameter of the transferred seed layers is developed and a parameter called optical fill factor is introduced. The optical method is confirmed by EDS measurements. For the transfer, NiV layers with different layer thicknesses were used for the laser transfer and their corresponding thresholds were deduced. A correlation between layer thickness, metal coverage and diameter is observed. Thus, a better understanding of the laser transfer process is achieved.

Simulation results concerning laser-doped selective emitters (LDSE) for the phosphorous doped front side of passivated emitter and rear solar cells are presented. We introduce a formula for different emitter doping profiles to study the influence on the dark saturation current density at the emitter-metal interface $j _{\mathrm{0e,met}}$. It is shown that $j _{\mathrm{0e,met}}$ can be reduced by deep and heavily-doped LDSEs. In addition, we report on how energy conversion efficiency can be predicted accounting for potential misalignment between LDSE and contact structures. As a result, we recommend avoiding the occurrence of non-laser-doped metallized areas. According to our simulation, energy conversion efficiency is increased by about 0.5% abs to 21.8% by implementing the LDSE.

The upgrade of state of the art p-type silicon solar cell production lines to passivated rear side technology (PERC) will be one of the major trends in the next years and new production processes for further cost reduction will continuously gain relevance. In 2007, we have introduced the laser based foil metallization technology “FolMet”: the rear electrode of p-type PERC devices as well as the local contact is fabricated by attaching conventional aluminum foil during the so-called laser fired contact process to the silicon wafer. This process features improved internal optical properties, a huge cost saving potential and a simplified cell production process. In this publication we focus on the acceleration of the laser process, which is together with module assembly issues a remaining challenge towards industrialization. We carried out comprehensive simulations, to better understand the correlation between different laser parameters on melting- and evaporation depth of the 8 μm thin aluminum foil. We determined lower limits for crucial laser pulse parameters to successfully attach the foil onto the substrate and validated these parameters experimentally. According to these results, we set up a system based on a pulsed high power laser featuring repetition rates Frep ≤ 2 MHz with an unique ultrafast polygon scanning system, allowing for scan-speeds vscan ≤ 1000 m/s. Thereby, we demonstrate processing times tpro < 0.8 s for industrial wafer, which corresponds to a reduction in laser process time by the factor of 20 compared to state of the art laser scanning technology.

In this study, we use pump-probe microscopy to examine the melting and solidification dynamics of silicon during and after a UV laser pulse with a duration of $$30\,\hbox {ns}$$ . Below the ablation threshold, we observe lateral melt front contraction velocities of up to $$600\,\hbox {ms}^{-1}$$ . The peak velocities spatially coincide with a ring of lower crystallinity within the formerly molten area, as we show with spatially resolved Raman spectroscopy.

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

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.

Although highly doped boron selective emitters (SEs) can be formed by using laser doping (LD) from the borosilicate glass remaining after boron tribromide tube diffusion, the dopant concentration is limited. This limitation can be overcome by using a laser-induced forward transfer (DLIFT) approach. In this work, SEs formed by DLIFT and LD are compared to a homogenous tube diffusion (Diff) emitter. We investigated the correlation of the emitter saturation current density ( j 0 e ) and the specific contact resistance $(\rho _{c})$ with the sheet resistance $(R_{{\rm{sheet}}})$ and the laser pulse energy density $(E_{p,d})$ . For passivated emitters, $j_{{0}e}$ of DLIFT and LD emitters was around ten times higher than $j_{{0}e}$ of Diff emitters. For metallized emitters, simulated $j_{{0}e}$ of DLIFT emitters was lower than $j_{{0}e}$ of LD and Diff emitters for $R_{{\rm{sheet}}}\,{\rm{ glt; \,40.0\,\Omega / sq}}$ . Moreover, we show how $j_{{0}e}$ can be further reduced by increasing the surface dopant concentration of the boron emitter and by reducing the laser-induced defects in the silicon crystal. Additionally, the metallization of DLIFT emitters with aluminum–silver paste by screen printing provided low contact resistances between metal and silicon ( ρc < 1.0 mΩ · cm2). Thus, p-type emitters can be optimized by forming an SE with the highly doped boron regions achieved by DLIFT under the screen-printed metallization.

We present 6-inch n-type Cz-Si metal wrap through (MWT) solar cells with screen-printed and fired contacts achieving energy conversion efficiencies of 20%. In order to decrease the occurrence of leakage currents under forward operation to a minimum after applying reverse bias load, the structuring of the rear side phosphorus doping in the area of the external p-type contacts is necessary. The fabrication of these so-called n-type high-performance MWT+ (n-HIP-MWT+) solar cells is considerably simplified by using laser processes to locally structure the rear side phosphorus doping. The n-HIP-MWT+ cells fabricated with laser structuring achieve the same peak energy conversion efficiency of 20% as conventionally fabricated ones, which are manufactured using an elaborate inkjet-based masking process prior to phosphorus diffusion. The loss of cell efficiency after reverse biasing is decreased to – 0.1% abs independent of the structuring method, and is three times smaller than the one observed for cells without structuring of the phosphorus doping.

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 emitter wrap through (EWT) solar cells, laser drilling is used to increase the light sensitive area by removing emitter contacts from the front side of the cell. For a cell area of 156 x 156 mm 2 , about 24000 via-holes with a diameter of 60 μm have to be drilled into silicon wafers with a thickness of 200 μm. The processing time of 10 to 20 s is determined by the number of laser pulses required for safely opening every hole on the bottom side. Therefore, the largest wafer thickness occurring in a production line defines the processing time. However, wafer thickness varies by roughly ±20 %. To reduce the processing time, a coaxial camera control system was integrated into the laser scanner. It observes the bottom breakthrough from the front side of the wafer by measuring the process emissions of every single laser pulse. To achieve the frame rates and latency times required by the repetition rate of the laser (10 kHz), a camera based on cellular neural networks (CNN) was used where the images are processed directly on the camera chip by 176 x 144 sensor–processor–elements. One image per laser pulse is processed within 36 μs corresponding to a maximum pulse rate of 25 kHz. The laser is stopped when all of the holes are open on the bottom side. The result is a quality control system in which the processing time of a production line is defined by average instead of maximum wafer thickness.

Laser microstructuring of thin dielectric layers on sensitive electronic devices, such as crystalline silicon solar cells, requires a careful design of the laser ablation process. For instance, degradation of the substrate’s crystallinity can vastly decrease minority carrier lifetime and consequently impair the efficiency of such devices. Short-pulse laser ablation seems well suited for clean and spatially confined structuring because of the small heat-affected zone in the remaining substrate material [Dube and Gonsiorawski in Conference record of the twenty first IEEE photovoltaic specialists conference, 624–628 1990]. The short-time regimes, however, generate steep temperature gradients that can lead to amorphization of the remaining silicon surface. By ‘heating’ the substrate via a non-ablative laser pulse in the nanosecond regime before the actual ablation pulse occurs we are able to prevent amorphization of the surface of the silicon solar cell substrate, while lowering the ablation thresholds of a SiNx layer on crystalline silicon wafers.

In this contribution we present different results of our investigations regarding the use of aluminum foil as rear side metallization for solar cells with dielectric passivation and laser fired contacts (LFC). We investigate the impact of different laser processes on the resistance of the contacts, the adhesion properties of the foil and the efficiency potential. By fabricating highly efficient, 20×20 mm2 sized solar cells with a conversion efficiency of 21.0 %, we demonstrate the high potential of this approach, which is equal to that of LFC-cells with common screen-printed or PVD metallization on the rear side. We investigated the optical properties of such metallized rear sides which benefit from an embedded air gap between foil and passivation layer. Finally, we present first solar cell results on industrially sized wafers (A=238 cm2) demonstrating again the equal efficiency potential compared to PVD metallization.

This work focuses on manufacturing inkjet structured Emitter Wrap-Through (EWT) silicon solar cells with a side selective emitter and an evaporated metallization. Inkjet structuring is a suitable technique for the formation of interdigitated structures used in back contacted silicon solar cells because it allows small feature sizes and has high alignment accuracy. Therefore all structuring steps in this EWT solar cell process are done with the help of inkjet masking. This includes the structuring of a silicon oxide passivation layer and the evaporated aluminum metallization. For all masking processes an acid-resistant inkjet hotmelt ink is used. An evaporated thick aluminum layer and laser-fired contacts (LFC) [1] to contact the bulk region are introduced. Cell efficiencies above 15% prior to a forming gas anneal are reached. The best cell reaches an efficiency of 15.7% after a short annealing step on a hotplate.

In this paper we present first results concerning different thermal evaporation processes for thin aluminum layers, which are carried out on a pilot system with a throughput of up to 540 wafers/h (156×156 mm2). To qualify the processes the deposited aluminum layers were evaluated with respect to homogeneity and conductivity. Additionally the effect of the different processes on the passivation quality of a thermally grown 100 nm thick SiO 2 was analyzed by means of lifetime measurements, indicating a negligible effect of the conducted process variations on the passivation quality. Finally high-efficiency silicon solar cells were prepared to determine the overall potential and to compare it with an electron beam (e-gun) evaporation process, which is used as a standard process in our laboratory. An efficiency of up to 21% was achieved by the high deposition rate technique performing at least as well as our standard high efficiency process.

Laser processes have penetrated into the crystalline silicon solar cell production market some time ago already, but are still far from reaching the status they probably will achieve one day. As the largest fraction of state-of-the-art production lines still produces conventional screen-printed aluminum back surface field (Al-BSF) cells, the applicability of lasers is currently limited mainly to the process step of laser edge isolation, while only a few other companies use lasers for groove formation (fabrication of laser grooved buried contact solar cells) or via hole drilling. Due to the contactless nature as well as the possibility to process a wide variety of materials with fine structures, lasers can be used for a vast field of production steps like ablating, melting and soldering different materials. Within this paper several applications of laser processes within the fabrication of various next-generation silicon solar cell structures are presented. These processes are for example laser via hole drilling, which is inevitable for MWT and EWT (metal and emitter wrap through) solar cells, LFC (laser-fired contacts) as a fast and easy approach for the production of passivated emitter and rear solar cells as well as laser ablation of dielectric layers and laser doping which offer the chance for industrial production of several different high efficiency solar cell structures.

For optimum performance of solar cells featuring a locally contacted rear surface, the metallization fraction as well as the size and distribution of the local contacts are crucial, since Ohmic and recombination losses have to be balanced. In this work we present a set of equations which enable to calculate this trade off without the need of numerical simulations. Our model combines established analytical and empirical equations to predict the energy conversion efficiency of a locally contacted device. For experimental verification, we fabricate devices from float zone silicon wafers of different resistivity using the laser fired contact technology for forming the local rear contacts. The detailed characterization of test structures enables the determination of important physical parameters, such as the surface recombination velocity at the contacted area and the spreading resistance of the contacts. Our analytical model reproduces the experimental results very well and correctly predicts the optimum contact spacing without the use of free fitting parameters. We use our model to estimate the optimum bulk resistivity for locally contacted devices fabricated from conventional Czochralski-grown silicon material. These calculations use literature values for the stable minority carrier lifetime to account for the bulk recombination caused by the formation of boron-oxygen complexes under carrier injection.

In this contribution the novel aluminum foil based rear metallization technology FoilMet® - Connect is introduced. With this technology, we combine the optical advantages of a foil metallized rear, known from FoilMet® - Classic and the high voltages of screen printed and furnace fired contacts usually applied in industrial PERC. The applied paste volume is reduced in the FoilMet® - Connect technology which we show to come with benefits in contacts formation during the firing process in a standard belt furnace. With this technology, we report an improvement in cell efficiency by Δη = (0.25±0.15) %abs over a mono facial screen printed PERC. Therefore it is a high efficiency upgrade for most monofacial cell concepts based on screen printed metallization.