industrial large area n-type solar cells with ... - ISC Konstanz

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3. RESULTS Solar cell process optimisation In order to optimise the solar cell performance we have varied individual cell processing steps, especially the rear side morphology. Figure 2 shows the experimental flow. The alkaline texture and chemical edge isolation was optimised already in previous experiments. Here we have chosen e.g. a textured versus a polished surface . group G1 G2 G3 G4 number of wafers 16 16 5 5 base type n n p p alkaline texture single side polish FSF 65 Ohm/sq wet chemical edge isolation PECVD SiNx screen printing fast Firing Figure 2: Experimental process flow chart on n- and p- type solar cells. Table 1 depicts the average solar cell parameters for all 4 groups. Table 1: Solar cell results from different groups explained in figure 2. group G1 G2 G2 G4 η [%] 16.9 17.0 17.4 17.4 V oc [mV] 627.6 627.7 627.9 628.5 J sc [mA/cm 2 ] 34.5 34.6 36.0 36.3 FF [&] 78.1 78.3 76.9 76.3 Average cell efficiency was taken (5 cells for p-type and 16 cells for n-type) from each group. It is visible that the p- type solar cells are better performing because the n-type concept is very sensitive to the material quality. In addition it can be seen that polishing of the rear side does not have any additional advantage as the formed Al emitter is completely dissolving and re-crystallising 7-10µm of the rear surface. The average efficiencies for the better n-tyoe cell group are 17.0% with a best cell of 17.2%. PC1D simulations show that further optimisation of front side properties like emitter with higher sheet resistance FSF with the combination of improved SiN x passivation will further increase the efficiency to values close to 18%. This will be explained in the following. Figure 3: PC1D simulations of efficiency as a function of front surface recombination velocity for Al-rear emitter solar cells. When reducing the SRV by a factor of 10 (improved FSF and passivation) efficiencies of 17.8% are reachable and by a factor of 100 even 18% are possible. We have already tested this enhancement on p-type cells using selective P-emitter and therefore we are optimistic to transfer it to our n-type structure as well. An additional improvement is foreseen when the shadowing is reduced or even completely eliminated (IBC Al or B-rear junction). Then efficiencies of around 19.5% are possible with screen printed contacts. Concepts for solar cell interconnection One of our solution of implementing rear tabbings without causing tremendous shunts is that the AgAl paste is printed on the emitter which can be done in several ways. We propose an easy process which is demonstrated in Figure 4. After the fully printed Al rear contact is fired and emitter formed the stripes for AgAl pads are mechanically removed, filled with AgAl paste by screen printing and fired again. We have shown that this mechanical abrasion is not causing any surface damage in our last publication [6]. Further potential of improvement Figure 3 shows a PC1D simulation of the reachable efficiency as a function of surface recombination velocity assuming a FF of 78.5% and a metallization shadowing of 8%. Currently we have reached average efficiencies of 17.0% with a SRV 5x10 5 cm/s. Figure 4: Schematic drawing of the mechanical abrasion of the stripe areas for AgAl pads.
Other possibility would be to print or contact directly on the Al paste which actually today also works with several techniques such as ultrasonic soldering or bonding. The attached contact might stick perfectly to the Al, however the problem is that the Al does not hold on the Si and the contact can be easily pealed off as already showed by us at the Hawaii conference in 2006 [2]. Figure 5: Schematic drawing of the rear side with different interconnection options: a) standard p-type AgAl pad geometry, b) selective opened Al-areas after fully sided printed fired rear side c) contacting of selective opening and d) direct contacting of Al. Figure 5 shows different possibilities of the rear side of an n-type rear Al-emitter solar cell. Picture 5 a) is the classical implementation the AgAl pads. However for our n-type cells this is not possible as we would contact the base directly with the emitter contacts. Therefore we have proposed to fully print the Al rear side in the past which results in a full Al emitter layer. After abrasion of the rear stripes behind the front bus-bars (picture 5 b)) a low firing AgAl paste printed and fired as depicted in picture 5c). The last picture 5d) shows a AgAl paste directly printed on the Al which gives a good adhesion but gets pealed off on the interface SiAl eutectic-Al contact. Our method works perfectly but includes one more firing step and an additional abrasion step. Therefore we have looked for other possibilities. Together with Hitachi Chemical we have developed an interconnection concept based on their existing product conductive film CF and our selective abrasion concept. Product CF was developed for the electronic industry and can be used for bonding of tabbings on different surfaces such as Ag, AgAl, Al and Si. Figure 6 shows such tabbing experiments on a) AgAl-paste, b) SiAl eutectic, c) Al with no Si edge and c) Al with Si edge. The adhesion of all samples is excellent, however for sample c) the tabbing is pealing off because of the already explained effect. However if, as in standard cells, a shallow Si-edge is left, the tabbing is attached to that and avoids the peal-off. This brought us to the idea co combine the use of CF with our abrasion method in a selective way as depicted in figure 7. Figure 7: Schematic drawing of a ) fully metallised rear Al-emitter solar cell b) selectively abrased rear side and c) tabbed rear side on the selective openings. The rear side process will then be extremely simple resulting in three steps. Fully screen printing of Al rear side and firing; selective abrasion of the Al rear contact; bonding of tabbings. The opened regions guarantee a good adhesion to the emitter and the Al regions a good electric contact to the entire fully metallised rear side. Now we even have several advantages compared to the standard p-type side such as: - elimination of one printing step - saving of AgAl paste - low temperature bonging process (lower stress) Figure 6: Schematic cross sections (left) and top view pictures (right) of different interconnection geometries of the tabbing. This method can be of course used for p-type rear sides as well leading most likely even to slightly higher Vocs because of the fully developed BSF. This method is currently under intensive testing and the results will be presented at the EU PVSEC 2010 in Valencia. At hat time mechanical as well as electrical properties will be evaluated in detail. In addition degradation tests as well as cost of owner ship calculations will be conducted in order to see the full potential of such contacting technology.
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industrial large area n-type solar cells with ... - ISC Konstanz

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