Correlation of pre‐breakdown sites and bulk defects in multicrystalline silicon solar cells

Abstract

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 solar cells we could show that the pre‐breakdown sites are not correlated to the surface morphology, e.g. etch pits resulting in local field enhancement. The positions of the pre‐breakdown sites are identical for acidic and alkaline texturised solar cells and therefore are directly related to bulk defects in the wafer. Nevertheless, the breakdown voltage is lower for acidic texturised solar cells; the parameters of breakdown are influenced by the texture in contrast to the position. Also pre‐breakdown sites are observed in areas without specific surface features for alkaline texturised solar cells. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Understanding junction breakdown in multicrystalline solar cells

“…It can be seen that all breakdown sites visible in DLIT are also visible in ReBEL (with one exception, see arrow), that the two signal heights are well correlating (hence the ReBEL signal is reflecting the magnitude of the breakdown current), and that all breakdown sites are lying on dark lines visible in forward bias EL (b). Similar results have been found by Usami et al, 27 Wagner et al, 28 and, with even better spatial resolution, by Lausch et al 13 Small deviations in the position may be explained by grain boundaries lying inclined to the surface. The exception (arrow) is an ohmic shunt.…”

“…32 Note that in these cells the onset voltages are higher than in the acid-etched cells shown until now, see Ref. 13. As Fig.…”

“…Maybe there are also other precipitate types involved in this mechanism (by TEM, besides Fe-also Cu-, Sn-, and Ca-containing precipitates have been found), and also a tip effect at the small precipitates at the base side should play a role for reducing the breakdown voltage. This explains why the onset voltage of type-2 breakdown sites spreads over an extended reverse bias range from about À9 to À13 V. Note that this breakdown type also exists on flat surfaces, where it shows somewhat higher breakdown voltages, 13 see Fig. 11.…”

“…Small-angle GBs are sometimes also called intragrain defects since they do not significantly alter the grain orientation. EL under reverse bias (called in the following ReBEL), 13 on the other hand, relies on acceleration with subsequent scattering or recombination of carriers in high electric fields. The light emission has been attributed to hot carrier interband recombination 14 or relaxation 15 and shows a wide-band spectrum including contributions in the visible range.…”

Electric properties and carrier multiplication in breakdown sites in multi-crystalline silicon solar cells

“…Therefore, intense research into the reasons and the physical understanding of breakdown were undertaken, especially for (multi-) crystalline silicon solar cells. [1][2][3][4][5][6][7][8] Like in other silicon-based electronic devices, breakdown causes the emission of visible light, the reverse biased electroluminescence (ReBEL). 9,10 It is therefore possible to investigate and locate the occurring high currents not only with (Lock-in) thermography 2 (LIT) but also with CCD cameras offering higher spatial resolution.…”