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35th European Photovoltaic Solar Energy Conference and Exhibition; 1060-1064, Solar energy has become increasingly economical, especially in recent years. Defects in solar systems can endanger this economic efficiency. In solar modules electric currents generated by electrical or light induced injection flow distributed over the module area. Each flowing current generates a magnetic field depending on the strength and direction of the electric current. Defects limiting the conversion efficiency of solar modules are changing the quite specific current distribution of a working solar module and hence also the magnetic field distribution. Therefore, by analyzing the magnetic field distribution one can trace back to electrical defects within the solar module. In this work, a novel approach to analyze the direction and strength of the electrical currents by analyzing the magnetic fields is shown by using a new sensor capable of measuring all three spatial directions (x, y and z). The magnetic field is recorded as a function of the position by stroking over the solar module. The method was applied to different electrical defects such as broken cross and inter connectors, defective junction boxes, cracks, solder flux variations and to investigate single solder joints. The influence of the lamination process and thermal cycling on broken connectors is analyzed.

35th European Photovoltaic Solar Energy Conference and Exhibition; 557-560, The Black-Si method is a promising alternative technique to reduce the reflection of c-Si solar cells. However, drawbacks of this technique are (I) the ion bombardment of the silicon surface, (II) the plasma induced impurities and (III) the increase of the silicon roughness. The Black-SiN method bypasses these drawbacks by a maskless plasma texturing of a SiN-layer. This approach separates the optical properties of the SiN-layer and the electrical properties of the silicon bulk and p-n junction. The Black-SiN texture is etched by a capacitively coupled power supply with a SF6/O2 gas mixture in a direct glow discharge. First results show a parabolic like Black-SiN texture in a scale of approx. 100 nm. A simulation of the Black-SiN optical properties revealed a global reflection minimum of 7.40 % between 300-1200 nm wavelength with respect to the air mass 1.5 spectrum on a plane silicon surface. Hence, the Black-SiN method can contribute to an improved solar cell efficiency.

35th European Photovoltaic Solar Energy Conference and Exhibition; 686-689, The effect of the film structure of SiNx on the hydrogen evolution at elevated temperatures was investigated by an effusion measurement system at constant heat rate. Control of hydrogen diffusion-limited effusion has been achieved by providing three different structures of SiNx: (i) Si-rich (ii) N-rich and (iii) layer with identical content of N and Si. The hydrogen evolution peak shifts to higher temperature up to 900 °C by increasing the N content suggesting that N-rich SiNx is a perfect compact layer for limiting hydrogen effusion due to firing. On the other hand, the Si-rich layer shows good surface passivation with large amount of hydrogen, however the hydrogen was almost effused out at temperatures lower than 800 °C indicating a void-rich structure. The desorption energy of hydrogen has been calculated out of the measurements at high temperature hydrogen evolution peaks and reveals a proportional relation to the N content.

35th European Photovoltaic Solar Energy Conference and Exhibition; 561-563, Plasma etching can be applied to silicon and its compounds used in solar technology. Plasma texturing of Si surface allows getting a moth-eye surface texture which has a light trapping effect. This will increase efficiency of solar modules, due to a decrease of the reflectance. In this work, silicon samples were etched using fluorine-based plasma (CHF3/H2). First, screening experiments were conducted, aimed at identifying the main factors that affect the resulting surface structure. Next, the experiment is planned to obtain a black surface, which is a nanostructure of columns and cones.

Black silicon (bSi) is promising for integration into silicon solar cell fabrication flow due to its excellent light trapping and low reflectance, and a continuously improving passivation. However, intensive ion bombardment during the reactive ion etching used to fabricate bSi induces surface damage that causes significant recombination. Here, we present a process optimization strategy for bSi, where surface damage is reduced and surface passivation is improved while excellent light trapping and low reflectance are maintained. We demonstrate that reduction of the capacitively coupled plasma power, during reactive ion etching at non-cryogenic temperature (-20 deg C), preserves the reflectivity below 1% and improves the effective minority carrier lifetime due to reduced ion energy. We investigate the effect of the etching process on the surface morphology, light trapping, reflectance, transmittance, and effective lifetime of bSi. Additional surface passivation using atomic layer deposition of Al2O3 significantly improves the effective lifetime. For n-Type wafers, the lifetime reaches 12 ms for polished and 7.5 ms for bSi surfaces. For p-Type wafers, the lifetime reaches 800 mu s for both polished and bSi surfaces.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 978-982, Light-induced degradation (LID) is a well-known problem of industrial silicon solar cells and can lead to severe electrical performance loss through the formation of recombination-active defects during the excess carrier injection by illumination or forward biasing. Thereby an effective LID inspection for industry is essential. Unfortunately, a standard LID test procedure based on the solar cell level is currently missing. Moreover, the maintenance of the traditional degradation source – light is challenging. In order to solve these concerns, a series of experiments are carried out for the development of a reliable and rapid LID test procedure upon a customer friendly LID test machine. Consequently, the commercially available machine, which utilizes current bias instead of light as the degradation source, was developed. Furthermore, four practical experiments which are of great interests to the solar cell suppliers are demonstrated. The high flexibility of operation of this machine was validated

32nd European Photovoltaic Solar Energy Conference and Exhibition; 722-725, In this contribution, the wet chemical saw damage removal (SDR) and texturing of crystalline silicon wafers are replaced by maskless inductively coupled plasma (ICP) processes. For the SDR, an isotropic ICP process is applied with SF6 as etch gas leading to semicircle surface structures with few microns width. Subsequently, an anisotropic ICP texturing is performed with SF6/O2 forming parabolic like surface structures with lateral extent of approx. 500 nm. A superposition of nanotexture on the SDR microtexture is performed. Optical measurements reveal an average reflection of 3.71 %, transmission of 1.31 % and absorption of 94.98 % integrated from 200-1120 nm under a normal incidence of light after PECVD SiNx passivation. SEM images show a homogenous 70 nm SiNx deposition on the ICP textures. A sufficient passivation is achieved by ICP SF6 SDR with approx. 100 μs effective minority carrier lifetime at 1015 cm-3 minority carrier density (MCD). In combination with the ICP SF6/O2 texture, the samples have approx. 70 μs effective minority carrier lifetime at 1015 cm-3 MCD.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 354-357, Black silicon (b-Si) is of particular interest for the PV industry since it can significantly reduce reflection losses. A promising preparation method of b-Si surface textures is maskless plasma etching. However, the detailed mechanism of maskless plasma texturing in an SF6/O2 atmosphere is still unclear. In particular, the self-masking process is not well understood. For that reason, the change of the elemental composition of the Si surface caused by the plasma process is investigated by X-ray photoelectron spectroscopy (XPS) to discuss the formation of the self-masking layer. Evidence of a fluorine containing oxide layer was found on the surface of b-Si for the first time. Furthermore, it is shown that fluorine desorbs quickly out of the top layer under ambient air, which makes it challenging to characterize the self-masking layer. These results are an important contribution towards the understanding of the formation of plasma etched b-Si and the resulting oxide layer.

Potential-induced degradation (PID) can cause massive power losses in photovoltaic power plants with serially connected modules when high electrical potential between solar cells and the module front glass surface leads to shunting of Si solar cells. The shunting is reversible to a large extend through elevated temperatures and/or by application of reversed voltages. It was previously shown that PID-related shunting (PID-s) of the p-n junction occurs due to Na-decorated stacking faults in {111} planes. Defects at former PID-shunt positions that have been thermally 'healed' (recovered) do not show a Na decoration anymore. In this contribution we investigate stacking faults with Na decoration and with vanished Na decoration after thermal recovery. HAADF STEM imaging is used for principal investigation of the atomic structure of PID-s defects. The Na decorated stacking fault appears as a dark region in the HAADF STEM image. By contrast, the atomic structure of the recovered stacking fault can be clearly imaged. The orientation of Si dumbbells indicates an extrinsic stacking fault that remains after recovery by Na out-diffusion.

29th European Photovoltaic Solar Energy Conference and Exhibition; 764-768, During the production of silicon wafers and solar cells, a wafer’s surface is altered by different process steps, such as wire sawing or texturing. However, the surface not only influences a wafer’s mechanical and optical properties, but also that of the resulting solar cell. In these investigations, mechanical strength, reflectivity and surface roughness of four different types of wafers was measured and correlated to each other. Therefore, wafers processed by F600 and F1000 grain sized slurry based wire sawing were compared, whereas one half underwent a texturing step and the other half was left as-cut. Further, all parameters were analysed depending on their location between entry and exit of the sawing wire. Results show a clear dependence of fracture stress and roughness between different surface types, as well as between wire entry and exit of the wafer. Mechanical strength is significantly higher for F1000 sawn wafers (smaller SiC grain size) and increases after texturing step for both F600 and F1000 wafers. The mechanical strength differs locally on the wafer, increasing from sawing wire entry to exit. This effect can be observed for F600 and F1000 as well as for as-cut and texturized wafers. A correlation of decreasing roughness to increasing fracture strength can be made, which, however, only applies within a single type of surface. Reflectivity of wafers strongly depends on initial surface topology. Whereas etched F600 wafers show decreased reflectivity, reflectivity increases after F1000 wafer were etched, which needs to be considered for solar cell production.

29th European Photovoltaic Solar Energy Conference and Exhibition; 567-572, In this contribution the method to determine the crystal orientation with the example of chemical treated silicon wafers by means of optical microscopy recently introduced will be reviewed. The procedure represents an easy method to obtain all relevant parameters to describe the crystal structure of the investigated material i.e. the crystal grain orientation and the grain boundary character. The chemical treatment is a standard mono-texture for solar cells well known in the solar industry. In general, this concept can also be applied to other crystalline materials, i.e. GaAs, SiC etc., the only thing that needs to be adjusted is the texturing method to reveal specific crystal planes and the calculation model. An application of this method will be shown with the example of the defect classification of recombination active defects in mc-Si solar cell. The method combined with the shown application demonstrates a simple and quick opportunity to improve the crystallization process and the quality of electronic devices by means of an optical microscope and a chemical treatment of the material.

Through metal-assisted chemical etching (MaCE), superior purification of dirty Si is observed, from 99.74 to 99.9884% for metallurgical Si and from 99.999772 to 99.999899% for upgraded metallurgical Si. In addition, large area of silicon nanowires (SiNW) are fabricated. The purification effect induces a about 35% increase in photocurrent for SiNW based photoelectrochemical cell.

Multicrystalline solar cells break down strongly at reverse voltages well below the theoretical limit. Previous explanations were based on assuming a constant depth of the junction below the surface. In this work, preferred phosphorous diffusion at special line defects in grain boundaries is shown to lead to spikes in the p-n junctions even below flat surfaces. The curvature radii of the spherical p-n junction bending are measured by electron beam-induced current to be in the range of 300-500nm, leading to the observed type III avalanche breakdown voltages.

It had been shown already earlier by X-ray microanalysis that, in positions of defect-induced junction breakdown in industrial multicrystalline (mc) silicon solar cells, iron-containing precipitates may exist. However, the nature of these precipitates was unknown so far. Here, in such positions, scanning transmission electron microscopy was performed after defect-controlled focused ion beam preparation. First of all, the defect site was localized by microscopic reverse-bias electroluminescence imaging. The high accuracy of following FIB target preparation (

Mono- and multicrystalline solar cells have been stressed by potential-induced degradation (PID). Cell pieces with PID-shunts are imaged by SEM using the EBIC technique in plan view as well as after FIB cross-section preparation. A linear shaped signature is found in plan-view EBIC images at every potential-induced shunt position on both mono- and multicrystalline solar cells. Cross-sectional SEM and TEM images reveal stacking faults in a {111} plane. Combined TEM/EDX measurements show that the stacking faults are strongly decorated with sodium. Thus, the electric conductivity of stacking faults is assumed to arise under the influence of sodium ion movement through a high electric field across the SiNx anti-reflective layer, resulting in PID.

27th European Photovoltaic Solar Energy Conference and Exhibition; 723-728, In this contribution two models explaining the recombination behavior of the different defect types introduced in a previous publication are discussed. For this we have applied detailed microscopic investigations, temperature-dependent EBIC measurements and root cause analysis on the nm-scale. We could separate the large amount of recombination active defects into four different classes: type-AP, type-AG, type-B and type-C. It was shown that iron precipitates are responsible for the recombination behavior at type-AP sites leading to an interface recombination. At type-AG regions, most likely decorated with iron impurities, deep defect states EM are responsible for the recombination behavior and can be described by the model of Kveder et al. The recombination processes at Type-B defect structures are either due to a low impurity level or a shallower position of the defect level EM both explaining the observed high intensity of defect luminescence. It is concluded that the recombination process at type-C defects is similar to type-AG defect structures. The temperature dependency of the recombination behavior is in good agreement with the model described by Kveder et al. for all defect types.

27th European Photovoltaic Solar Energy Conference and Exhibition; 857-860, The root causes of breakdown of multicrystalline silicon solar cell well below the theoretical limit are investigated in this contribution. The main focus of this paper is on avalanche, or so called type-3, breakdown sites in alkaline texturized solar cells. However, the similarities and differences of the same breakdown type in isotextured solar cells are described as well. Using EBIC charge collecting contrast of p-n junctions at FIB prepared cross sections it is found that possible enhanced phosphorous diffusion at certain line defects in the material leads to spikes of the p-n junction despite of a flat surface of alkaline textured solar cells. Avalanche breakdown occurs at these spikes similar to the avalanche breakdown at etch pits in isotextured solar cells, but at slightly higher absolute reverse voltages as compared to the latter ones. It is assumed that the differences of the breakdown onset voltages are due to the slightly different p-n junction curvature shapes in alkaline and isotextured solar cells.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1246-1250, Investigations of electrical breakdown in multicrystalline silicon solar cells in the last couple of years revealed different breakdown mechanisms. One breakdown type is found to occur dominantly at highly recombinative defect structures and is called type-2 breakdown. It was reported that metal precipitates, containing mainly iron, play a major role for the formation of such breakdown sites. In this contribution type-2 breakdown sites were localized with high precision using micro reverse-bias electroluminescence, focused ion beam preparation, and different electron microscopy methods. It can be shown that type-2 breakdown sites found here are caused by rod-like a-FeSi2 precipitates. These precipitates are located at large angle grain boundaries and can be assigned to dislocation lines. Taking into account the quasi metallic behavior of a-FeSi2 rods, their position directly at the p-n junction, and their size, a model of the breakdown mechanism based on a Schottky junction between the p-doped solar cell’s base and the metal precipitate is proposed.

Two easy and efficient methods are demonstrated to detect SiC and Si3N4 filaments and precipitates in multi-crystalline silicon (mc-Si) wafers and solar cells by Optical Light (OLM) Microscopy and Scanning Electron Microscopy (SEM). In the case of OLM the image is created in the dark field mode which clearly visualizes the grain boundaries decorated by filaments in acidic etched silicon solar cells. In the case of SEM the image is created by a combination of Secondary Electrons (SE) imaging and Back Scattered Electrons (BSE) imaging. By using this method a clear discrimination of precipitates with different chemical compositions is possible. Note that both methods make it possible to investigate and analyze relatively large sample areas in a comparatively short period of time.

26th European Photovoltaic Solar Energy Conference and Exhibition; 1839-1842, Correlation and prediction of cell parameters from PL images of as-cut wafers is a topic of main interest. The prediction of the open circuit voltage (VOC) for different material qualities by using PL images combined with a software routine with equal assumptions can lead to significant deviations in predicted and measured VOC, thus adapting the software routine for each material is required [4,5]. In this contribution we will discuss one effect that can explain such deviations in predicting solar cell parameters and analyze this effect in terms of recombination activity and prebreakdown behavior by means of PL imaging and reverse biased electroluminescence (ReBEL).

26th European Photovoltaic Solar Energy Conference and Exhibition; 2784-2787, Non-destructive and quantitative measurements of the electron beam induced current (EBIC) as a function of the electron beam energy were performed on thin film solar cells from different technologies in order to determine the depth and width of the p-n junction by using an iterative deconvolution algorithm. Additionally, the exact position of the p-n junctions of the thin film solar cell is confirmed independently by EBIC investigations at cross sections. We show that voltage dependent EBIC investigations on the native surface and EBIC measurements at cross sections are in good agreement.

In this paper a method for studying p-n junctions is described. Different electron and ion beam charactersiation methods are introduced to determine the p-n-junction position using two different examples from Crystalline Silicon on Glass (CSG) thin film technology. In a first example lateral and cross section electron beam induced current (EBIC) measurements revealed that oxygen rich columnar growth at textured substrates disturbs strongly the p-n junction. In a second example diffusion from glass specimen is identified by TOF-SIMS to influencing the electrical and structural characteristics of the thin Si layer are responsible for the modified p-n junction. A model describing the formation of both defect structures is introduced.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1254-1258, The main aims of the photovoltaic industry are an enhancement of solar cell efficiency, the reduction of production costs and also to improve the production yield. The presence of various kinds of precipitates in block-cast multicrystalline Si for wafer-based solar cell production has a significant impact on the subsequent process technology. In this work the microstructural properties of SiC and Si3N4 precipitates as well as SiC filaments on grain boundaries are investigated with NIR microscopy, SEM and TEM studies. The elemental composition is analyzed with the help of ToF-SIMS and the electrical properties are investigated by LIT, BS-EBIC and 4-point probing. Our results contribute to new insights in solar cell shunting processes.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2053-2055, Strong correlation between the pre-breakdown sites visible in dark lock-in thermography due to local heating and the intensity of spatially resolved electroluminescence of reverse-biased solar cells was observed. By comparing differently texturised structurally similar solar cells we could show that the positions of the observed prebreakdown sites are identical for acidic and alkaline texturised solar cells and therefore are not correlated with the surface morphology. The pre-breakdown sites are directly related to bulk defects in the wafer. Nevertheless, the breakdown voltage is lower for acidic texturised solar cells. The quantitative parameters of breakdown are influenced by the texture. Also pre-breakdown sites at bulk defects are observed in areas without specific surface features for alkaline texturised solar cells.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2100-2102, The authors compare electrical properties of defects contained in standard p-type multi-crystalline silicon (mc- Si) and upgraded metallurgical silicon (umg-Si) material and in solar cells processed from these wafers by a commercial production line. For that thermal admittance spectroscopy allowing the determination of a defects capture cross-section and thermal activation energy, both being fingerprints of defects, was used. Further, the density of defect levels can be estimated. The authors found, that umg-Si contains more defect levels compared to mc-Si, however, the processed solar cells have similar defect composition. For mc-Si wafers and mc-Si solar cells the authors studied the impact of the dislocation density on the existence and density of electrically active defect states.