Influence on grain boundaries

To investigate the recombination activity of the grain boundaries regarding the four different processing schemes, high resolution LBIC measurements were carried out.

24For correct interpretation of the IV data it should be added that all solar cells are neither textured nor is the ARC layer thickness at an optimum for a single anti-reflection coating (minimum of reflectance at 540 nm). From this point of view it is astonishing how high the efficiency potential even of the highly contaminated material is. Especially a Voc of 628 mV (middle of ingot 3) is unusually high even for very pure mc silicon material with Al-BSF.

Investigation of defects in the solar cell bulk Figure 5-17 (left side) shows LBIC maps of the same area on the four different solar cells from the bottom of ingot 2, processed according to process A, B, C, and D, respectively.

While the extended gettering (C and D) does not show deviations compared to the standard gettering (A and B), a clear influence of the hydrogenation via the SiNx:H is visible. Some grain boundaries which are clearly visible (highly recombination active) on the samples from processes B and D are almost completely deactivated in the hydrogenated samples (A and C). Interestingly not all grain boundaries behave the same way. This leads to the assumption that decoration and/or type of the grain boundary play a major role concerning the possibility of passivation during hydrogenation. Similar effects were observed e.g. by Zuschlag et al. on pure mc Si material (‘mc FZ’) [145].

Figure 5-17: LBIC measurement of neighbouring cells from the bottom (left) and top (right) of ingot 2 which underwent the different processing schemes (mapped area: 17x8 mm² and 8x5 mm² respectively).

Examples for grain boundaries that are positively affected by the hydrogenation are encircled in red.

Figure 5-17 (right side) shows LBIC scans from neighbouring solar cells originating from the top of ingot 2. The behaviour of the grain boundaries is similar to the observation made at the bottom of the ingot, although the grain boundaries are no longer completely passivated. The recombination activity of some grain boundaries (encircled in red) is only significantly reduced by the hydrogenation. This indicates that there is a certain concentration limit for impurities which can be passivated (after a gettering step) via high temperature hydrogenation. This limit seems to be exceeded for the examined grain boundaries in the top region (90% height) of this ingot due to the high overall impurity concentration caused by segregation and back-diffusion during ingot casting which also leads to larger precipitates.

To gain direct insight into the bulk lifetime of a readily processed solar cell, the metallization, SiNx, emitter and Al-BSF is removed from the surfaces²⁵. Afterwards the sample is wet chemically passivated (IE) and a spatially resolved lifetime map is obtained by µPCD measurement. Figure 5-18 shows the correlation of the spatially resolved bulk lifetime and the LBIC image taken from the solar cell before the surface removal. The area of low bulk lifetime clearly correlates with recombination active grain boundaries visible in the LBIC picture.

25 The metallisation is removed by agua regina, the SiNx layer in diluted HF, emitter and Al-BSF in a CP etch.

Figure 5-18: LBIC map (left) and µPCD lifetime mapping of the bulk Si (right) from one and the same 2x2 cm² solar cell. A good correlation, particularly of the areas with the lower material quality is evident.

The solar cell originates from the bottom of ingot 2.

To compare the processes A, B, C, and D on bulk lifetime level, the respective neighbouring solar cells are stripped as described above and measured together in one step to grant highest comparability of the measurements. The result of the investigation is depicted in Figure 5-19 where the lifetime differences of neighbouring solar cells originating from the bottom region of ingot 2 are shown.

Figure 5-19: Comparison of the four processes on lifetime level after the solar cell process. Hydrogenation clearly improves the bulk properties (left), and also a slight improvement is visible from the extended gettering (two lower maps). The back etched solar cells originate from the bottom of ingot 2.

The positive effect of the hydrogenation (process A and C) is clearly detectable. The measurement method also shows the positive effect of the extended gettering (process C and D). The data clarifies the unexpected IQE data obtained for the bottom region of ingot 2 (Figure 5-13), where the expected effects of gettering and hydrogenation are not clearly observable. A more detailed insight, especially into highly recombination active regions, however, is limited due to the low measurement resolution (> 250 µm).

Investigation of defects in the solar cell bulk A deeper insight into the aforementioned passivation behavior of certain grain boundaries can be obtained by using high resolution Electron Beam Induced Current (EBIC) and, after stripping the solar cell front and rear surfaces, by Electron Back Scattering Diffraction (EBSD) techniques (chapter 1.2.6). The left side of Figure 5-20 shows EBIC scans on a solar cell revealing the same recombination behavior already observed in LBIC measurements but in a much higher resolution. This allows the identification of single grain boundaries. The right side of Figure 5-20 shows an EBSD mapping of the  coincidence values for the grain boundaries in the same wafer region. From the depicted data it can be concluded, that grain boundaries exhibiting a high coincidence number ( 27, highlighted in blue) are very recombination active and are not easily passivated by hydrogen (compare Figure 5-20 upper left, hydrogenated solar cell and lower left, not hydrogenated solar cell).

Figure 5-20: EBIC scan of a hydrogenated solar cell and a neighbouring non hydrogenated solar cell below (left side). The right side depicts an EBSD graph illustrating the coincidence number of the grain boundaries in the corresponding solar cell area after stripping the surface layers of one solar cell. Different

 numbers are colour coded. The  27 configurations, highlighted or circled in blue, appear very recombination active and are not significantly affected by hydrogenation. Solar cells originate from the bottom of ingot 2.

Grain boundaries that exhibit lower  coincidence numbers ( 3,  9) by contrast show a very positive response to hydrogenation in most cases. This might be due to the less disturbed boundary area for grain boundaries exhibiting low  values where by trend less impurities are agglomerated during gettering steps [146]. These less decorated areas then of course are more easily passivated by hydrogen.

As an extended gettering step at a constant temperature of 700°C did not show significant improvements on solar cell level, in further investigations the influence of a slow cooling ramp down to 600°C during one hour (Figure 5-1, red curve) after the emitter drive-in is investigated. The chosen material originates from a standard mc Si ingot and from ingots consisting of block cast Upgraded Metallurgical Grade (UMG) mc Si.

Figure 5-21 depicts the influence of the slow cooling ramp on jsc and Voc values of neighbouring solar cells from different ingot positions of a standard block cast mc Si ingot. Although the differences are small, for all cells an improvement is detected and only for the top region the standard deviation is too high to confirm a real improvement.

bottom middle top

Figure 5-21: Comparison of jsc and Voc values for two to four neighbouring solar cells concerning the response to a slower wafer cooling (ramp) after the POCl3 diffusion. Wafers are taken from bottom, middle, and top of a standard mc Si ingot, respectively.

Interestingly, for the examined UMG materials the results are not equally clear. Although a similar trend towards higher jsc and Voc values due to the cooling ramp is observed for the majority of the ingot positions, it is not always the case. In any case, however, the observed differences are within the respective standard deviation. Spectral response measurements here also show very little difference between the two processes.

Conclusions

The conclusions drawn from the above described experiments on the intentionally contaminated and the UMG material for the process development are as follows:

The gettering efficiency of the long low temperature emitter diffusion process is already very high. For further improvements a slow cooling of the wafers after the emitter drive-in seems to have more positive effect than a post-getterdrive-ing step at a fixed temperature of 700°C. Hydrogenation during the firing of the SiNx:H is very important for all mc Si materials and might be worth further investigations concerning individual optimization of the hydrogenation conditions for mc Si materials featuring different defect compositions and concentrations. The finding, that grain boundaries featuring low  values are much easier passivated than grain boundaries featuring high  values might trigger efforts to avoid grain boundaries with high  values during crystallisation.

Investigation of defects in the solar cell bulk