FCA-FTIR
6 Diode breakdown behavior in multicrystalline silicon solar cells
6.5 Breakdown type II: Soft breakdown at recombination ac- ac-tive defects
6.5.6 Influence of the surface morphology
Besides inducing hard breakdown at deep etch pits (see section 6.6), the morphology of the wafer surface has also a significant influence on the onset voltage of the soft break-down.
In order to investigate the dependence of the onset of soft breakdown light emission, the following experiments were carried out in collaboration with Jan Nievendick (details on the process including the texturization recipes and on further results can for example be found in reference [203])54:
A set of parallel wafers was taken from the center ingots of three standard multicrystal-line blocks (“Ref mc 3-5”), which had been crystallized by different companies. While the impurity concentration and the base doping were comparable, the distribution of crystal defects (dislocations and grain boundaries) and thus the crystalline quality varied.
For the following industrial solar cell process (based on screen-printing metallization), each set of wafers was divided into three groups. After the initial wet chemical wafer cleaning, different wafer surface treatments were applied: while one wafer batch was only damage-etched, two different wet chemical texturization recipes were applied to the two remaining groups.
As a result of the damage etch, surface inhomogeneities were leveled out to a high de-gree; only along the boundaries between the grains, the crystal orientation-dependent etching rate led to very shallow steps. By contrast, the application of both wet chemical texturization recipes entailed selective etching of crystal defects, in particular deep etch pits at dislocations. The difference between both texturizations lies in their aggressive-ness: While one recipe produces very steep etch pits and rough wafer surfaces (“aggres-sive recipe”), the other generates a more homogeneous morphology with smaller etch pits (“mild recipe”).
After the wafer surface treatment, each group of wafers was further processed according to its requirements (i.e. the anti-reflection coating, the firing of the contacts etc. were slightly modified). However, these adjustments are not expected to have an influence on the breakdown characteristics.
Hence, the comparison of the breakdown voltages of the differently texturized parallel wafers, possessing the same impurity concentration, base doping and crystal quality,
54 Solar cells and AFM measurements provided by J. Nievendick.
rectly reveals the influence of the surface morphology. As an example, in Figure 6.29 (b)-(d), the breakdown voltage maps of the solar cells processed from parallel wafers of ma-terial “Ref mc 3” are shown. Distributed over the surface of the damage-etched sample (b) many early breakdown sites can be seen (white and yellow spots), but hardly any soft breakdown sites appear up to a reverse voltage of approx. -13.5 V (compare to for-ward EL image (a) ). In the mildly etched sample (c), the earliest light emission from soft breakdown sites can be detected around -10.0 V, while the deeply etched wafer (d) shows soft diode breakdown already around -9.25 V in the same wafer area marked by the green circles. Taking the damage-etched sample as a reference, the decrease due to the influence of the surface structure thus amounts to 25-30%, which is in the same or-der of magnitude as the impurity impact and the base doping influence in the relevant parameter range.
Figure 6.29: Comparison of the breakdown voltage maps of three differently texturized, but parallel solar cells (“Ref mc 3”). (a) Forward EL image (+600 mV); the green circle marks a wafer region with a high concentration of recombination active crystal defects at which the Atomic Force Microscope image depicted in Figure 6.30 was obtained; (b) breakdown voltage map of the damage-etched wafer; (c) breakdown voltage map of a neighboring wafer, textured with the “mild” recipe; (d) breakdown voltage map of a neighboring wafer, textured with the “aggressive” recipe. Note that the same scaling was applied to images (b)-(d).
To quantify this assessment, the height profile of the textured solar cells made from the three standard materials “Ref mc 3-5” in selected regions was measured with the help of Atomic Force Microscopy (AFM)54; for example, the highly recombination active region denoted by the green circles in Figure 6.29 (a)-(d) was chosen. In these areas, the wet chemical texturization etches deep pits where dislocations reach the wafer surface. This is exemplarily imaged in the left part of Figure 6.30. Often, the dislocations form lines (small angle grain boundaries) and clusters (“bursts”), in some cases leading to very deep trenches, which can be seen for example in the lower part of the AFM measure-ment.
The etch pits are often triangularly shaped. In the right graph in Figure 6.30, the extrac-tion of the surface morphology parameters which are used in the following, namely the etch pit depth and the depth / width ratio, is exemplified by the blue lines. The depth was determined by measuring the distance between the bottom of the etch pit and the verti-cal line defined by the lowest point at the rim of the etch pit. This parameter is independ-ent of the pit shape and does not distinguish between differindepend-ent curvatures at the bottom of the etch pit. The diameter at the height of the lowest border identifies the width of the etch pit, from which the depth / width ratio is calculated. This parameter gives an indica-tion for the curvature at the bottom of the etch pit: The larger the ratio, the larger the average curvature.
Both parameters were obtained from several AFM scans (side length of the scans 50 x 50 µm2) situated at recombination active regions in all three materials in order to increase the statistical significance. After averaging the values from each scan, the etch pit depth and the depth-to-width ratio were correlated to the local soft breakdown volt-age measured in the respective region.
Figure 6.30: Left image: Atomic Force Microscopy (AFM) measurement of the lateral sur-face height distribution, obtained in the highly recombination active crystal defect region marked by the green circle in Figure 6.29 on the aggressively texturized sample (d).
Right graph: Exemplary surface profile extracted along the red dashed line in the AFM image. The etch pit depth is determined by measuring the distance between the bottom of the etch pit and the vertical line defined by the lowest point bordering the etch pit as shown by the blue lines. The diameter of the etch pit at the height of the lowest border identifies the width.
The results are plotted in Figure 6.31. The soft breakdown voltage shows a significant correlation with both parameters, suggesting a linear dependence in this parameter range in both cases. However, by extrapolation, on a very smooth surface one would ex-pect a soft breakdown voltage around -11.5 V, which is significantly lower than the real value detected on the damage-etched solar cell (-13.5 V). It seems therefore that the linearity is restricted to etch pits with depth ≥500 nm.
For the occurrence of soft breakdown, high electric fields are necessary (see also section 6.5.8). The influence of the surface on the breakdown may thus be explained by the con-centration of the electric field lines due to the curvature in the space charge region at the bottom of the etch pits, which is very similar to the mechanism of the third, hard break-down type described in section 6.6. However, soft breakbreak-down sets in at a lower reverse voltage than the hard breakdown and shows a different reverse I-V characteristic, sug-gesting that here, another factor plays a role, which is very likely the presence of metallic precipitates.
Figure 6.31: Plots of the depth of the etch pits (top) and the ratio between the depth and the width as a measure for the curvature at the tip of the etch pits (bottom) versus the local soft breakdown voltage. The red dashed lines signify linear fits to the experi-mental data.