In Fig. 7.5athe solar cell efficienciesηof four wafer regions a, b, c and d are depicted versus different POCl3diffusions summarized in Tab. 7.1. Each data point is the average of two 2×2 cm2solar cells with comparable grain structure. The mc-Si material originates from bottom height of the ingot. The highest efficiencies are measured in region c compared to the other three regions independently of the applied emitter. The better cell performance and hence higher material quality of this region is in agreement with its longer distance from the crucible wall. This implies that the other three regions originating from the red zone of the ingot are limited by their low material quality. It should be mentioned that the as-grown lifetime before processing is not a good indicator for predicting the final material quality after the complete solar cell process. It is possible that even strongly contaminated material originating from the red zone of an ingot exhibits higher final material quality than material from a more central ingot position. This is shown in the detailed gettering analysis of Chapter 3.
A positive effect of the additional 90 min annealing is observed forDiff 1as well as for the Indus-700diffusion in all four sample regions a, b, c and d. The efficiency ofDiff 1in region c, for example, increases by 0.7% from 14.4% up to 15.1% due to the annealing at 700◦C. A similar increase of 0.6%abs is detected for the Indus-700 diffusion with the same unloading/annealing temperature. Annealing at 500◦C (Diff 2 + annealemitter), however, does not result in a higher cell efficiency. Note that data of the b cell withDiff 2 + annealemitter is missing due to sample breakage. Only region c cells show a slight increase by 0.2%abs due to annealing at 500◦C. In this study, the discrepancy between the efficiencies η of two cells with the same emitter is up to 0.3%. This implies that the slight improvement of c cells withDiff 2 + annealemitter is within this discrepancy and hence does not reflect actual improvement.
Fig.7.5cshows the open-circuit voltageVOC and Fig.7.6athe dark saturation current densityJ01, which is extracted from fitting the illuminated J-V curves using the double-diode model. Both quantities are determined under open-circuit conditions and therefore at the same injection level. An improved
7.3 Bottom Ingot Height 119
(a) (b)
(c) (d)
Figure 7.5: J-V parameters of lab-type solar cells versus POCl3diffusion are depicted from (a) to (d):η,FF,VOC,JSC. Four regions (a, b, c, d) of one 5×5 cm2sample originating from bottom ingot height are depicted.
open-circuit voltage is clearly accompanied by a loweredJ01, which means less recombination within emitter and base.
As already mentioned above,Le f f is determined at lower injection thanVOC, namely at short-circuit condition. This explains the same trend versus POCl3diffusion of both,Le f f andJSC. Since the effective minority carrier lifetimeτe f f, which indicates the material quality, strongly depends on the excess carrier density (injection), the different injection levels ofVOCandLe f f have to be considered. This means that differences inVOC between modified POCl3 diffusions are not necessarily accompanied by changes in Le f f. Nevertheless,VOC andLe f f exhibit the same trend in the present analysis (compare Fig. 7.5cand 7.7). This might be due to the fact that the effective diffusion lengthLe f f is in the range of 100 µm to 225 µm which is not significantly larger than the cell thickness of approximately 170 µm. Note thatLe f f, which isLe f f,Qdetermined from the IQE, might be larger than its actual value, as it was simulated by Brendel et al. [195]. Other authors claim that recombination losses in the emitter region start to dominate cell performance as soon as the diffusion length reaches twice the cell thickness which is definitely not the case here [57,58]. Another important fact is the similarity of the investigated emitter profiles. All POCl3emitters yield an emitter sheet resistance of approximately 80Ω/2.
A comparison of the Diff 1 and the Indus-700 emitter profiles measured by ECV is depicted in Fig.2.14of Sec. 2.3.3. Four emitter profiles (Diff 1/2with and without annealing) do not deviate signif-icantly. TheIndus-700emitter profile, however, exhibits a notable difference to the other four emitters.
(a) (b)
(c) (d)
Figure 7.6: Fitting the double-diode model to the J-V curves of lab-type solar cells in bottom ingot height leads to the four parameters:J01,J02,RSH,RS.
Its P+ surface concentration is higher and the profile is more shallow. This results in a higher emitter saturation current density of 175 fA/cm2 compared to the deeper emitter profiles with approximately 130 fA/cm2. Therefore, the contribution of the emitter component to the total dark saturation currentJ01 of the Indus-700 emitter is by 45 fA/cm2 higher than the ones ofDiff 1 andDiff 2. Nevertheless, the contribution of the base component still dominates the recombination losses of the cells.
The fill factorFFshown in Fig. 7.5bdecreases by approximately 1.2%abs for theIndus-700emitter compared with the Indus-700 + annealemitter. In agreement with the fill factor, the shunt resistance RSH of cells withIndus-700emitter is clearly lowered. It falls below a critical value of approximately 7×103Ωcm2 down to 2×103Ωcm2. Below this value solar cell performance is affected by shunts accompanied by a slight increase ofJ02from 60 nA/cm2to 80 nA/cm2. Note that solar cells in region b, having the lowestVOCvalues, do not show any change in shunt resistance but rather an increased series resistance from 0.25Ωcm2 to 0.3Ωcm2. J02 also increases from 89 nA/cm2 to 120 nA/cm2 and hence the fill factor is lowered by 1.3%abs. The shunts might be a result of theIndus-700emitter profile shape which might be too shallow during contact sintering. Shunts are formed within the depletion region. The enhanced series resistance of region b cells, however, cannot be easily understood. It might be due to process related difficulties during contact formation.
In summary, it can be stated that the cell results mainly reflect the different gettering efficacy of the applied POCl3 diffusions and thus the achieved material quality of the cells. The 90 min annealing at