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We performed an extensive study of light induced degradation experiments in PERC solar cells made of Boron doped Cz Silicon material from several experimental crystals. Strong LeTID (Light and Elevated Temperature Induced Degradation) amplitudes were found in cells originating from the top and tail part of the crystals. A statistical analysis reveals a strong correlation to the bulk concentration of Aluminium in cells originating from wafers close to the tail. Close to the top, the LeTID effect is strongly enhanced whenever Oxygen precipitation occurs in the cell process.

The efficiency record of industrial type PERC solar cells exceeded 22% at the turn of the year 2015 to 2016. Our best screen-printed PERC solar cell reached 22.04% efficiency while the best cell batch showed a very narrow efficiency distribution. A detailed electrical and optical loss analysis of those industrial type high efficiency PERC solar cells is carried out which enables further optimization and strategic improvements. A variety of characterization data allows for a recombination current density, resistance and optical loss analysis based on numerical device simulation, analytical calculations and raytracing, respectively. The main recombination losses at maximum power point (MPP) occur in the homogenous and selective diffused regions of the emitter. A series resistance loss analysis is analytically performed. The emitter contribution to the lumped series resistance dominates the series resistance losses. The optical loss analysis performed with raytracing shows main reflection and absorption losses in the rear metal layer which is partly due to the light trapping capability of the PERC cells.

With efficiencies of industrial type PERC solar cells exceeding 22% and reduced electrical and optical losses in these high efficiency cells, it becomes more and more important for manufacturers to rely on high quality silicon wafers. In this contribution, we present data of the bulk excess carrier lifetime τ bulk and a doping density independent material quality parameter at maximum power point, the material saturation current density j 0,mat , of different p-type monocrystalline silicon materials. These values are obtained by measuring the effective carrier lifetime of neighbouring wafers of different thickness and thus eliminating the surface recombination contribution. We show that j 0,mat is an adequate parameter to evaluate the bulk quality of a given material. To further validate these findings, we compare the results to PERC cell efficiencies obtained with the same material.

During the crystallization of directionally solidified silicon such as multicrystalline or cast quasi-single crystalline silicon, the diffusion of metal impurities from crucible and coating into the solid silicon ingot at high temperatures creates a severe contamination issue in the edge region of the ingot, the so called red zone. We present an effective diffusion barrier which greatly reduces this detrimental metal diffusion. Silicon wafers are coated with a layer system of silicon oxide and silicon nitride by chemical vapor deposition and are placed on the bottom of the crucible. Using this technique, the metal contamination in the red zone can be reduced by about one order of magnitude. This leads to better solar cell performance of wafers fabricated from this ingot area. Also, less ingot material has to be discarded leading to higher wafer yield per ingot.

openaire: EC/FP7/307315/EU//SOLARX Both multicrystalline and Czochralski (Cz) silicon substrates are known to suffer from various mechanisms of light-induced degradation (LID), including copper-related LID (Cu-LID). Past studies on Cu-LID have mostly been performed on unprocessed wafers, omitting the impact of the solar cell process on the copper distribution. Here, we carefully contaminate Cz-substrates of different quality with different amounts of copper and process the substrates into complete industrial Cz-Si PERC solar cells, reaching a comprehensive mapping of the impact of Cu-LID for the PV industry. The results show that both the copper contamination level and Cz crystal quality are critical factors affecting the extent of Cu-LID. Most importantly, we show that copper can result in significant concentrations in the bulk of the finished PERC cells after being exposed to only trace surface contamination. Consequently, even a small local copper contamination area (~ 3–4 cm2) is sufficient to induce strong LID in the full-sized (156 × 156mm2) cell parameters, resulting e.g. in ~7% relative efficiency loss during light soaking. The corresponding short circuit current density decreases by up to a factor of two in the contaminated areas.

openaire: EC/FP7/307315/EU//SOLARX Light and elevated-temperature induced degradation (LeTID) is currently a severe issue in passivated emitter and rear cells (PERC). In this work, we study the impact of surface texture, especially a black silicon (b-Si) nanostructure, on LeTID in industrial p-type mc-Si PERC. Our results show that during standard LeTID conditions the b-Si cells with atomic-layer-deposited aluminum oxide (AlOx) front surface passivation show no degradation despite the presence of a hydrogen-rich AlOx/SiNx passivation stack on the rear. Furthermore, b-Si solar cells passivated with silicon nitride (SiNx) on the front lose only 1.5 %rel of their initial power conversion efficiency, while the acidic-textured equivalents degrade by nearly 4 %rel under the same conditions. Correspondingly, clear degradation is visible in the IQE of the acidic-textured cells, especially in the ~850–1100 nm wavelength range confirming that the degradation occurs in the bulk, while the IQE remains nearly unaffected in the b-Si cells. The observations are supported by spatially-resolved photoluminescence (PL) maps, which show a clear contrast in the degradation behavior of b-Si and acidic-textured cells, especially in the case of SiNx front surface passivation. The PL maps also suggest that the magnitude of LeTID scales with surface area of the texture, rather than wafer thickness that was recently reported, although the b-Si cells are slightly thinner (140 vs. 165 µm). The results indicate that b-Si has a positive impact on LeTID, and hence, benefits provided by b-Si are not limited only to the excellent optical properties, as commonly understood.

In this work we investigate the regeneration (permanent deactivation of light induced degradation, LID) of industrial high efficiency PERC cells. This procedure transforms the boron oxygen related defects responsible for LID into a less active state. Recently it was found that the hydrogen content in the passivation layer of boron doped Cz silicon lifetime samples plays a crucial role during regeneration. This effect is tested here on PERC cells fabricated with silicon nitride capping layers of different refractive indices and thus different compositions. We are able to confirm the results on cell level: PERC cells with high stable efficiencies of approximately 20% are achieved with an industrially applicable short regeneration step. The possibility of regeneration can be a very important factor when choosing the bulk resistivity with respect to the highest efficiency potential for PERC cells.

In this work we investigate the effect of the bulk metal contamination level in cast silicon on the electrical performance of PERC (passivated emitter and rear cell) solar cells [1] . The bulk contamination is varied in two different ways. In the first approach, cast silicon crystallized with high purity silicon feedstock and reduced metal contamination from crucible and coating is used. Second, an extended gettering step is introduced in the PERC process flow. The interstitial iron concentration is taken as a measure of the bulk metal contamination of the investigated cells. Both the higher purity cast silicon and the gettering step significantly lower the interstitial iron concentration in the resulting PERC cells. This clear positive effect is also mirrored in solar cell parameters such as the open circuit voltage UOC. In this experiment, the impact of gettering is larger than the effect which is reached by reducing the bulk metal concentration during the crystallization process. Device simulations show that most of the difference in UOC can be explained by the different interstitial iron levels while other recombination active defects might also be present.

In this work we compare experimental results of an industrial passivated emitter and rear cell (PERC) high volume pilot line production in the SolarWorld Innovations technology center with a simulation model based on 2D Sentaurus Device Simulations The PERC solar cell design shows in a well-controlled experiment a 1.1% absolute higher median efficiency compared to the aluminium back surface field (Al-BSF) solar cell reference group. We derive a calibrated simulation model by the characterization of cells and test structures. We show that the simulation model reproduces well the measured I-V data. In consequence this enables us to estimate the solar cell performance when an applied process or silicon wafer parameters will be changed for further optimization. In the second part of this study we investigate the impact of front and rear side recombination on the solar cell parameters by simulation and sensitivity analysis. We show that the quality of the local BSF underneath the local rear contacts has an important impact on the electrical solar cell performance.

International audience; We performed magnetic force microscopy (MFM) measurements in external magnetic fields parallel to the sample plane to qualitatively study their effect on the magnetization of different kinds of MFM probes. As a test structure we used an array of rectangular ferromagnetic thin film elements aligned with the external magnetic field direction. MFM images were taken while the field was increased stepwise to monitor the onset of a tilt in the MFM probe magnetization. Three different probes were investigated: a conventional pyramidal probe coated with 40 nm of CoCrTa, a coated high aspect ratio MFM probe, and a MFM probe based on an iron filled carbon nanotube (FeCNT). The results show that the magnetization of the pyramidal probe is only stable in in-plane fields of up to ∼60 mT, in larger fields a considerable in-plane component of the tip magnetization is observed. This makes it difficult to distinguish the effect of the external field on the sample from those on the MFM probe. The coated high aspect ratio probe is at first stable up to ∼200 mT and then shows a history dependent behavior, in the second sweep the onset of the magnetization tilt is already visible at ∼100 mT. With the FeCNT probe, no tilt of the probe magnetization could be observed in in-plane fields up to the experimental limit of 230 mT. Due to the large shape anisotropy of the enclosed iron nanowire its direction of magnetization stays mainly oriented along the long axis even in moderate fields that deviate from this easy axis. Consequently, this probe is best suited for MFM measurements in external fields.

We have characterized a new Magnetic Force Microscopy (MFM) probe based on an iron filled carbon nanotube (FeCNT) using MFM imaging on permalloy (Py) disks saturated in a high magnetic field perpendicular to the disk plane. The experimental data are accurately modeled by describing the FeCNT probe as having a single magnetic monopole at its tip whose effective magnetic charge is determined by the diameter of the iron wire enclosed in the carbon nanotube and its saturation magnetization 4 \pi M_s ~ 2.2 x 10^4 G. A magnetic monopole probe enables quantitative measurements of the magnetic field gradient close to the sample surface. The lateral resolution is defined by the diameter of the iron wire ~15 nm and the probe-sample separation. As a demonstration, the magnetic field gradients close to the surface of a Py dot in domain and vortex states were imaged., Comment: 6 pages, 7 figures

In addition to boron and oxygen, copper is also known to cause light-induced degradation (LID) in silicon. We have demonstrated previously that LID can be prevented by depositing negative corona charge onto the wafer surfaces. Positively charged interstitial copper ions are proposed to diffuse to the negatively charged surface and consequently empty the bulk of copper. In this study, copper out-diffusion was confirmed by chemical analysis of the near surface region of negatively/positively charged silicon wafer. Furthermore, LID was permanently removed by etching the copper-rich surface layer after negative charge deposition. These results demonstrate that (i) copper can be effectively removed from the bulk by negative charge, (ii) under illumination copper forms a recombination active defect in the bulk of the wafer causing severe light induced degradation.

A local magnetization measurement was performed with a Magnetic Force Microscope (MFM) to determine magnetization in domains of an exchange coupled [Co/Pt]/Co/Ru multilayer with predominant perpendicular anisotropy. The quantitative MFM measurements were conducted with an iron filled carbon nanotube tip, which is shown to behave like a monopole. As a result we determined an additional in-plane magnetization component of the multilayer, which is explained by estimating the effective permeability of the sample within the \mu*-method., Comment: 3 pages, 3 figures

We report on the magnetic properties of an individual, high-quality single-crystalline iron nanowire with diameter d=26 nm. The nanowire is embedded in a carbon nanotube which provides complete shielding against oxidation. Magnetization reversal is associated with domain wall formation where domain nucleation is initiated by curling. The observed nucleation fields of up to 900 mT are much higher than reported previously and nearly reach the shape anisotropy field of iron nanowires.

The magnetization reversal and switching behavior of an individual Fe-filled carbon nanotube has been measured using vibrating cantilever magnetometry. We report measurements of the magnetic field at which the 25 nanometer diameter iron core inside the nanotube reverses. The fields at which reversal occurs, characterized by an exceptionally narrow distribution (σH≤1 G at 6.3 K), are determined by thermally activated excitation over a field dependent barrier. The high precision achievable by virtue of measuring individual nanowires allows detailed quantitative understanding of magnetization reversal.

We present results on the application of an iron filled carbon nanotube (Fe-CNT) as a probe for magnetic force microscopy (MFM) in an external magnetic field. If an external field is applied parallel to the sample surface, conventional ferromagnetically coated MFM probes often have the disadvantage that the magnetization of the coating turns towards the direction of the applied field. Then it is difficult to distinguish the effect of the external field on the sample from those on the MFM probe. The Fe-CNT MFM probe has a large shape anisotropy due to the high aspect ratio of the enclosed iron nanowire. Thanks to this the direction of the magnetization stays mainly oriented along the long nanotube axis in in-plane fields up to our experimental limit of 250 mT. Thus, the quality of the MFM images remains unchanged. Apart from this, it is shown that Fe-CNT MFM probe yields a very good magnetic resolution of about 25 nm due to the small diameter of the iron filling.

Single crystal iron carbide nanowires contained in multiwalled carbon nanotubes have been prepared by aerosol-based thermal chemical vapor deposition. Investigations by transmission electron microscopy reveal the crystallographic [010] axis of the orthorhombic Fe3C nanowires to be predominantly aligned along the nanotube axis. Despite the high aspect ratio of the Fe3C nanowires, magnetic force microscopy measurements imply single domain behavior with the easy magnetic axis of the nanowires perpendicular to the wire axis. In agreement with the structural results, these findings show that the magnetic behavior is dominated by the magnetocrystalline anisotropy contribution, causing the easy axis to be along the [001] direction.

Iron-filled carbon nanotubes (Fe-CNTs) were used to prepare probes for magnetic force microscopy (MFM) by attaching them to the tips of conventional atomic force microscopy cantilevers. An optimized chemical vapor deposition process, employing a two stage furnace and ferrocene as a precursor, supplied the homogeneously filled Fe-CNTs required for the MFM probes. These can be regarded as cylindrically shaped single-domain nanomagnets that are protected from oxidation by a carbon shell. Carbon nanotubes are known to possess both great mechanical stability and elasticity, which lead to a much longer lifetime of these probes compared to conventional magnetically coated probes. It is shown that the prepared probes are suitable for magnetic imaging and so far show no sign of deterioration. Even very long nanotubes can be used as probes, which implies that they are extraordinarily stiff. It is also shown that attached Fe-CNTs can subsequently be tailored by electron-beam induced oxidation (e.g., to remove distur...

We present a technique to measure the interstitial oxygen concentration in monocrystalline silicon directly on 180 μm industrial solar wafers using an appropriate measurement setup. This avoids the material and time consuming fabrication of special thicker samples which are usually necessary for Fourier Transform Infrared Spectroscopy (FTIR) measurements on silicon. We compare the data obtained from wafer samples to results of thicker slices and use the wafer FTIR method to evaluate the impact of high temperature steps on the interstitial oxygen content of silicon wafers.

In this work we investigate the light induced degradation (LID) of industrial high efficiency PERC cells which are fabricated from p-type Cz silicon with a base resistivity ranging from 0.8 Ω cm to 3.4 Ω cm. The change of all electrical cell parameters after illumination is investigated. The total efficiency decrease after illumination is mainly driven by the FF decrease. This is due to an increased bulk recombination contribution after degradation compared to the front and rear surface recombination and also to the injection dependent excess carrier lifetime. Additionally, the so called regeneration procedure which transforms the boron-oxygen defects responsible for LID into a less active state is employed in order to reduce the degradation. The possibility of regeneration can be a very important factor when choosing the base resistivity with the highest efficiency potential for PERC cells.

In this work we present our approach to realize an industrial process that allows cell efficiencies up to and above 21%. Based on a loss analysis we systematically investigate the feasible options to improve the efficiency with device simulations and production experiments. Subsequently we perform sensitivity analyses particularly for various silicon wafer materials to ensure stable process capability. Our best prototype process with optimized front and rear side passivation and enhanced laser contact patterning has demonstrated a maximum efficiency of 20.9% with a very high V OC of 670 mV on high-lifetime mono material. We were able to assemble 60-cell based modules with more than 305Wp.

In this work, we present our progress in the industrial p-type PERC technology [1, 2]. Based on device simulations we continuously develop an efficiency roadmap for a steady improvement of our PERC (passivated emitter and rear cell) process. Following this simulation based approach, we effectively improve the front side metallization and the emitter characteristics. Currently, our best prototype process has reached a conversion efficiency well over 21% which enables the manufacturing of a 60-cell based module with a power of 310W. Our best cell so far has a conversion efficiency of 21.5% which has been confirmed by the calibration laboratory of Fraunhofer ISE. This is to our knowledge the highest efficiency reported for industrial-size silicon solar cells with screen-printed metal front and rear contacts.