High Performance Solar Cells Exceeding 17 ... - ISC Konstanz

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Average solar cell efficiency [%] Lifetime [s] texturisation all samples were processed by POCl 3 diffusion, PECVD SiN x deposition, full Al BSF screen printing, Ag finger grid screen printing and firing through. Measurements of the solar cell parameters were carried out and the reverse current was determined at -14.5V to see the influence of the texturisation on both results. 3 RESULTS 3.1 Results of diffusion optimization Minority carrier lifetime measurements were performed to screen the feedstock quality of the investigated ingots. Therefore the saw damage of wafers from the center bricks of each ingot was removed and the samples were passivated by a PECVD SiN x . The lifetimes of those samples were determined using QSSPC measurements and are shown in figure 1. 60 50 40 30 20 10 0 ESS 1 ESS 2 ESS 3 Poly 0 10 20 30 40 50 60 70 80 90 100 Ingot Height [%] Figure 1: Minority carrier lifetime of wafers from different ingot positions, measured by QSSPC. Ingots ESS 1 and 2 were purified by IMCC and thus are not made from the standard ESS feedstock. Lifetimes between 10µs and 60µs have been measured on both ESS and poly-Si wafers. Figure 1 shows that the minority carrier lifetime of the ESS based wafers is at least on the same level or even higher compared to the poly-Si reference wafers. The bottom wafers of both ingots ESS 3 and poly show reduced lifetimes most likely due to some crucible induced impurities. For the highest wafer positions the lifetime of all four ingots is decreasing. In case of the poly-Si ingot it is likely that the higher boron doping at the top, because of the segregation of impurities is a reason for the decrease in lifetime. For the ESS based samples dislocations, found at the higher ingot positions, might limit the carrier lifetime, because they act as active recombination centers. and are thus not made from standard ESS feedstock Ingot Brick V oc [mv] J sc [mA/cm²] FF [%] η [%] ESS 1 Corner 621.9 33.7 79.5 16.7 ESS 1 Center 618.4 33.5 79.7 16.5 ESS 2 Corner 619.1 33.9 79.3 16.6 ESS 2 Center 616.8 33.8 79.1 16.5 ESS 3 Corner 613.6 33.5 79.1 16.3 ESS 3 Center 618.7 33.9 79.3 16.6 poly Corner 615.1 33.7 79.0 16.4 poly Center 617.1 33.9 79.2 16.6 Table II: Average solar cell parameters over at least 18 cells per brick, which were evenly distributed over the complete ingot height, processed using an optimized diffusion regarding more efficient gettering. Ingots ESS 1 and ESS 2 have been grown using IMCC technology as a previous step of purification and are thus not made from standard ESS feedstock. Ingot Brick V oc [mv] J sc [mA/cm²] FF [%] η [%] ESS 1 Corner 623.8 33.8 79.5 16.7 ESS 1 Center 620.2 33.6 79.6 16.6 ESS 2 Corner 620.5 33.9 79.2 16.7 ESS 2 Center 618.4 33.9 79.0 16.6 ESS 3 Corner 614.5 33.5 78.9 16.3 ESS 3 Center 619.8 34.0 79.2 16.7 poly Corner 616.2 33.8 78.9 16.4 poly Center 618.5 34.0 79.2 16.6 The ingots ESS 1 and ESS 2, which were purified using IMCC technology, show the highest achieved solar cell efficiencies, due to high FF, because of the low resistivity and thus a low series resistance, high V oc caused by the Fermi level shift as a result of the higher boron concentration and respective higher doping levels [6], and J sc similar to the reference wafers. Ingot ESS 3 also shows a good performance at the centre brick, whereas the corner brick shows the lowest average efficiency of all investigated bricks. 17,0 16,8 16,6 16,4 16,2 Optimized Standard IV measurements show average efficiencies, over at least 18 solar cells from each brick, between 16.3% and 16.7% for the ESS based solar cells and between 16.4% and 16.6% for the poly-Si based solar cell reference (see table I and II). Table I: Average solar cell parameters over at least 18 cells per brick, which were evenly distributed over the complete ingot height, processed using the standard diffusion. Ingots ESS 1 and ESS 2 have been grown using IMCC technology as a previous step of purification 16,0 ESS 1 Center ESS 1 Corner ESS 2 Center ESS 2 Corner ESS 3 Center ESS 3 Corner Poly Center Poly Corner Figure 2: Comparison of average solar cell efficiency of solar cells diffused by either an optimized or a standard POCl 3 diffusion. The optimized diffusion was improved towards a more efficient gettering. As it can be seen in table I and II and figure 2, the optimized diffusion leads to an absolute gain of 0.1% in
Interstitial oxygen [ppma] Reverse current @ -12V [A] Relative loss in cell efficiency [%] Relative loss in cell efficiency [%] cell efficiency for most of the groups, due to enhanced V oc and J sc values, which we claim to be an effect of a more efficient gettering. The optimization of the diffusion leads to solar cell efficiencies exceeding 17%, with best solar cells even reaching 17.1% efficiency. The results of the best cells of each ingot and brick are shown in table III. Table III: Solar cell parameter of the best solar cells of each group. All of the best solar cells were processed using the optimized diffusion. Ingot Brick V oc [mv] J sc [mA/cm²] FF [%] η [%] ESS 1 Corner 625.2 33.9 79.5 16.8 ESS 1 Center 628.5 34.1 79.7 17.1 ESS 2 Corner 625.0 34.2 79.3 16.9 ESS 2 Center 628.1 34.3 79.4 17.1 ESS 3 Corner 615.8 33.8 79.2 16.5 ESS 3 Center 622.1 34.1 79.2 16.8 poly Corner 618.7 34.1 79.3 16.7 poly Center 619.4 34.1 79.4 16.8 Another indication for a more efficient gettering resulting in lower impurity concentrations on cell level is the improved reverse current-voltage characteristics. The reverse breakdown is often caused by metallic impurities [7]. As shown in figure 3 the solar cells with the optimized diffusion show an improved reverse current 2 Optimized Standard for the SoG-Si based solar cells for most ingot positions, even though the boron concentrations are higher compared to the poly-Si. The ingots ESS 1 and ESS 2 show LID mostly below 1%, whereas ingot ESS 3 and the poly reference show LID up to 1.7% at the bottom wafers. 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0 10 20 30 40 50 60 70 80 90 100 Figure 4: Relative losses in solar cell efficiencies of solar cells from the center bricks, depending on the ingot height. 1,8 1,6 1,4 1,2 1,0 Ingot Height [%] ESS 1 ESS 2 ESS 3 Poly ESS 1 ESS 2 ESS 3 Poly 0,8 0,6 1 0 ESS 1 Center ESS 1 Corner ESS 2 Center ESS 2 Corner ESS 3 Center ESS 3 Corner Poly Center Poly Corner Figure 3: Comparison of reverse current, as an indicator of more efficient gettering, of solar cells diffused by efficient POCl 3 gettering compared to the standard POCl 3 diffusion. 0,4 0,2 0,0 0 10 20 30 40 50 60 70 80 90 100 Ingot Height [%] Figure 5: Relative losses in solar cell efficiencies of solar cells from the corner bricks, depending on the ingot height. All of the ingots show a decreasing tendency of LID towards higher ingot positions, what is in good agreement with FTIR measurements of interstitial oxygen, which can be seen in figure 6. LID behavior of solar cells, originating from different ingot heights ranging from bottom to top from both center and corner bricks, was investigated. Therefore the solar cells were exposed to 1 sun illumination at elevated temperature using a Semilab PV2000 tool, to reach a completely degraded state due to the formation of BO 2i complexes. The exact measurement procedure is explained in [9]. The solar cell parameters were measured before and after this procedure and the relative loss in cell efficiency was calculated out of both measurement results. The results of the relative loss in cell efficiency for solar cells from the center bricks are shown in figure 4, the results for the solar cells from the corner bricks in figure 5. The measurement results show lower LID losses 7,00E+017 6,00E+017 5,00E+017 4,00E+017 3,00E+017 2,00E+017 1,00E+017 0 10 20 30 40 50 60 70 80 90 100 Ingot Height [%] ESS 1 ESS 2 ESS 3 Poly
Page 1: HIGH PERFORMANCE SOLAR CELLS EXCEED
Page 5: to be comparable to poly-Si based s

High Performance Solar Cells Exceeding 17 ... - ISC Konstanz

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