4.6 Edge-defined Film-fed Growth (EFG)
4.6.2 Comparison of RTP and CFP P-Al co-diffused solar cells from former
Recently, ASE Americas has managed to improve the quality of EFG wafers by modifications in the crystal growth. In the production at ASE Americas as well as in the production at its German based mother company RWE SCHOTT Solar, solar cells produced from the improved ribbons exhibited higher efficiencies compared to those of the former quality. It was shown, that the improved quality can directly be linked with a higher average carrier lifetime after crystal growth [38].
In collaboration with RWE SCHOTT Solar, laboratory solar cells with a simple n pp structure were fabricated from both EFG qualities. The goal was to compare CFP with RTP co-formation of emitter and Al-BSF for the two qualities of material. With regard to thermal budget these two processes can be considered as extremes. It is worth mentioning that we used the standard CFP process routinely applied to block-cast multicrystalline silicon materials at the Faunhofer ISE. This CFP process has been shown to yield enhanced gettering of impurities in many publications [149, 97, 150].
Experimental details
Solar cells from the new and from the old quality EFG were processed according to the sequence shown in Fig. 4.25. At first, arbitrarily chosen 10 10 cm wafers were cut into pieces of 5 5 cm which were then etched for a short time in a CP133 solution in order to avoid later wafer breakage. Afterwards they were cleaned by a modified RCA clean and a 1 to 2 m thick layer of Al was deposited by e-gun evaporation on the backside. Then P-Al co-diffusion was carried out either by CFP or by RTP. The CFP diffusion took place at 820 C for 1 h utilizing POCl3 as a P-source. In contrast, the parameters of the RTP diffusion were varied over a wide range: the diffusion temperature between 860 and 1000 C and the diffusion time between 5 and 180 s. As a P-source either of the spin-on dopants P507 and P509 was deposited on the front.
Edge isolation Forming gas anneal Front contacts (photolith.) RCA clean
AR coating Al evaporation and P spin-on
P-Al co-diffusion CP133 etch
H-passivation (RPHP) PSG removal
Fig. 4.25: Sketch of the laboratory process sequence applied to the solar cells from the former and the new EFG quality. The P-Al co-diffusion was done either by CFP or by RTP.
In all cases the emitter sheet resistance aimed at was in the 80-100 /sq range. For the profiles of the RTD emitters please refer to Fig. 3.5 (before RTO) and Fig. 3.7. The profile of the CFP emitter is shown in the right graph of Fig. 3.8. After P-glass removal in HF, the front contacts were prepared by photolithography, evaporation of Ti/Pd/Ag and plating of Ag.
Subsequent to edge isolation by laser cutting and cleaving, a FGA step at 350 C for 30 min was applied to improve the contact properties. The solar cells were treated with a standard remote plasma hydrogen passivation step (RPHP) at 350 C for 30 min. Finally, a double layer TiO2/MgF2antireflection coating was deposited.
Results and discussion for EFG of the former quality
Tab. 4.9 contains the average IV-parameters and the respective standard deviation of solar cells fabricated from EFG of the former quality at different stages of processing. Each group consists of eight cells. As no essential differences between the differently RTP diffused solar cells became evident, the solar cells´ characteristics were simply averaged.
Apparently, prior to hydrogen passivation, the RTP solar cells are superior to the CFP solar cells with respect to all average solar cell parameters: is more than 20 mV, about
Tab. 4.9:Average cell parameters of solar cells fabricated either by CFP or RTP P-Al co-diffusion from EFG-Si of the former quality. 8 cells were processed in each group. Cell size is 4.6 4.6 cm .
Process " " (
[mV] [mAcm-2] [%] [%]
CFP 501 8 17.8 0.9 72.8 1.2 6.5 0.3
+ RPHP 520 11 18.9 0.9 72.0 2.0 7.1 0.3 + ARC 536 11 28.5 1.4 74.0 1.0 11.3 0.9
best cell 553 30.3 75.5 12.6
RTP 524 5 19.8 0.8 76.3 0.7 7.9 0.3
+ RPHP 533 5 20.5 0.6 76.2 0.9 8.3 0.3 + ARC 547 4 29.6 0.7 76.5 0.6 12.4 0.4
best cell 549 30.8 76.9 13.0
2 mAcm-2 and " " more than 3 % (absolute) higher. The superiority of RTP to CFP for this kind of silicon material has not been reported before. Geiger et al. [38] have also processed solar cells from the former EFG quality using CFP diffusion similar to ours and determined the average solar cell parameters. For solar cells at a comparable stage of processing they reported 518 mV of , 18.4 mAcm-2 of , and 74.9 % of " " on average. The mean efficiency was 7.2%. These parameters are slightly better than those of our CFP solar cells, presumably because the emitter of Geiger et al. was passivated by a thin CFP oxide which accounts for the slightly higher and . Remarkably, the RTP solar cells are superior to the CFP solar cells of Geiger and co-workers as well. From FZ reference solar cells we know that the slight differences in the emitter and the BSF properties of the RTP solar cells compared to the CFP solar cells cannot account for performance differences at such a level. The results rather indicate fundamental differences in the bulk lifetime. It seems that the bulk properties of the former EFG material reacts positively to RTP due to the specific crystallographic and defect characteristics of this material. In order to substantiate this assumption, SR-LBIC measurements were per-formed before and after hydrogen passivation to measure the effective diffusion length # . The# maps of the best RTP and the best CFP solar cell are shown in Fig. 4.26 together with the distribution function. These maps are representatives of the other solar cells as all cells of a group exhibit the same trends.
The RTP and the CFP solar cells exhibit the EFG specific twin grain boundary lamellas where the diffusion length takes in general its highest values. However, only in the case of the RTP solar cells, theses areas exhibit high# 150 m already prior to H-passivation. As all RTP solar cells show this characteristic, this explains the surprisingly high efficiencies at this stage of processing compared to the CFP solar cells. The# distribution of the RTP cells is shifted significantly towards higher values compared to the CFP case. High# values up to 300 m can be measured before RPHP only in the case of RTP. This observation also holds for the distribution after H-passivation. Though the CFP solar cells improve upon RPHP, they do not reach the level of the RTP counterparts. This is also reflected by the average cell efficiency after RPHP. As shown in Table 4.9, during RPHP the CFP and the RTP cells improve. For the
RTP RTP + RPHP
CFP CFP + RPHP
0 >200
0 100 200 300
0 1 2 3
4 RTP
RTP + RPHP CFP
CFP + RPHP
Counts [x1000]
Leff [µm]
Fig. 4.26: Mapping and distribution of for the best RTP and CFP cell, respectively, made from EFG-Si of the former quality. was determined from SR-LBIC measuerements before and after hydrogen passivation by RPHP.
RTP solar cell the relative increase in efficiency is 5 % and for the CFP solar cell it is almost 10 %. The relative improvement is more pronounced in the case of the CFP cells as the initial performance was lower. However, the improvement is not as high as one might expect taking into account the frequently reported sensitivity of EFG on H-passivation. This can be attributed to the fact that the applied RPHP parameters were not optimized for EFG. For example, using an optimized remote hydrogen plasma passivation process, Geiger et al. [38] have reported a dramatic performance improvement of above 35 % for CFP solar cells of this kind of EFG material.
Finally, after deposition of the AR coating, the mean solar cell efficiency of the CFP cells is 11.3 % whereas the RTP cells reach 12.4 %.
Results and discussion for EFG of the state-of-the-art quality
The average cell parameters and the respective standard deviation of solar cells fabricated from EFG of the state-of-the-art quality are shown in Tab. 4.10 for different stages of processing.
Each group consists of eight solar cells.
Noticeably, like observed for the solar cells made of the old EFG, the RTP cells are superior to the CFP cells also for the new EFG. All average cell parameters of the RTP cells are
Tab. 4.10: Average cell paramters of solar cells fabricated from state-of-the-art EFG either by CFP or RTP P-Al co-diffusion. Per group 8 cells were processed, cell size is 4.6 4.6 cm .
Process " " (
[mV] [mAcm-2] [%] [%]
CFP 499 12 18.0 1.0 73.5 1.0 6.6 0.6
+ RPHP 515 12 18.5 1.0 74.7 0.8 7.1 0.6 + ARC 532 13 28.0 1.5 74.6 1.0 11.2 1.0
best cell 546 29.8 76.3 12.4
RTP 531 5 20.7 0.7 76.4 0.6 8.4 0.3
+ RPHP 542 5 21.3 0.7 76.9 .0.9 8.9 0.4 + ARC 554 4 30.7 0.9 77.0 0.5 13.1 0.5
best cell 558 31.8 76.4 13.6
significantly higher than those of the CFP cells after each stage of processing. For EFG of the new quality, the gap between the two processes is even more pronounced than for EFG of the old quality: is more than 30 mV and is about 2.7 mAcm-2 higher after RTP compared to CFP. The RTP cells exhibit unexpected high performance prior to H-passivation and AR deposition. For comparison, taking the same kind of EFG material, Geiger et al. [38]
have reported an average efficiency of 7.4 % for conventionally diffused solar cells including emitter passivation by oxidation. A clear hint to explain the difference between CFP and RTP diffused cells can be drawn from local SR-LBIC measurements. In Fig. 4.27 the maps and the corresponding distribution of the effective diffusion length# of the best CFP and the best RTP cell are plotted. Both cells represent well the other cells of their group.
The distribution of # is stretched towards higher values in the case of the RTP cells compared to the CFP cells. Noticeably, only the RTP cells exhibit regions of very high
# 250 m already prior to H-passivation. This explains the surprisingly high efficiencies at that stage. These good regions are where a high density of twin grain boundaries prevails.
Obviously, these regions do benefit from P-Al co-diffusion by RTP. Upon RPHP both sort of cells do improve in most areas. The strongest improvement can be observed in the already good regions and in some of the medium quality areas. Remarkably, for both cell types some of the areas with very low starting diffusion lengths 20 m hardly improve at all during hydrogen passivation. This has been observed by other researchers as well [39]. The poor reaction of these regions to hydrogen passivations seems to be of fundamental nature and independent of the passivation method and of the respective amount of atomic hydrogen provided. It is rather a consequence of the type of defects present in these areas. Currently, within the German project ASIS, a joint research program has been worked out to investigate this phenomenon further and to identify the underlying microscopic reasons [40].
As already mentioned, the RPHP process applied in this work was not optimized for EFG and as a consequence the solar cells did not improve in the way observed by other researchers.
The CFP solar cells do not reach the level of the RTP solar cells. For the RTP cells the relative increase in efficiency is 6 % and for the CFP cells it is about 8 % (relative). In contrast, Geiger
RTP RTP + RPHP
CFP CFP + RPHP
0 >200
0 100 200 300
0 1 2 3 4
RTP
RTP + RPHP CFP
CFP + RPHP
Counts [x1000]
Leff [µm]
Fig. 4.27: Mapping and distribution of for the best RTP and CFP cell, respectively, made from EFG-Si of the state-of-the-art quality. was determined from SR-LBIC measuerements carried out before and after RPHP hydrogen passivation..
and co-workers managed to increase the average efficiency of their CFP solar cells by more than 35 % by the application of an optimized H-passivation process. In our case, including the AR coating, the mean cell efficiency of the CFP solar cells is 11.1 % and 13.1 % for the RTP solar cells, respectively.
It is worth comparing the results obtained for the new EFG with those obtained for the old EFG. In the case of CFP, no significant difference in the cell parameters can be observed, although before and after RPHP the best CFP cell from the new material exhibits slightly higher
# values than its counterpart of the old EFG. Apparently, the CFP process is not capable of highlighting the slight differences in the initial material quality. In contrast, in the case of RTP, the solar cells of the new EFG show higher and especially more than 1 mAcm-2 higher than solar cells made from the old quality. The RTP P-Al co-diffusion process is capable of preserving or even increasing the difference in the initial carrier diffusion length. At the moment, we can only speculate on what happens to the initial diffusion length during CFP and RTP P-Al co-diffusion, respectively. We cannot say whether it is enhanced, preserved or even deteriorated. But certainly, the areas of high starting diffusion length even reach higher values after RTP than after CFP. Apparently, the high RTP process temperature does not harm the material. One can only speculate on the microscopic reasons. The next experiment was designed to gain further information by a direct comparison of adjacent wafers.