4. n-Type Rear Al-Emitter Solar Cell Concepts 35
4.3. Full Area Locally Contacted Emitter (FALCON) Solar Cell
the increased space charge region recombination and stays constant. On the other hand, the open-circuit voltage Voc is reduced, since it depends on the quasi-fermi levels, which are in turn affected by the carrier densities in the junction. While the carrier density is affected by recombination in the junction, Voc is reduced.
4.3. Full Area Locally Contacted Emitter (FALCON) Solar Cell
Besides locally formed emitter (Al-LARE) solar cells, also full area locally contacted rear emitter (FALCON ) solar cells were fabricated.
Figure 4.9.: Schematic of a locally contacted full rear emitter solar cell
This cell concept is depicted in figure 4.9 and is similar to the previous one, but features a full area rear emitter which is etched back to a specified emitter depth. Similarly to the locally formed emitter cell, this cell concept also features an enhanced internal rear reflection due to the dielectric rear passivation, which results in an increase in the short-circuit current. Furthermore, the rear side emitter passivation reduces the overall rear diode’s saturation current j01and hence should enable to achieve a higher Voc compared to the improved PhosTop cell. Depending on the metallization, this cell concept can be used as a bifacial solar cells, as well. If a grid is used for screen-printing aluminium paste or silver-aluminium paste on the rear, locally passivated regions remain non-metallized and hence can couple light inside the cell.
4.3.1. Contact Pitch
Similar to the locally formed and contacted emitter cells, locally contacted cells are simulated using PC2D. The pitch has been varied, while front side metallization is held constant at a 2 mm pitch. The short-circuit current jsc and the open-circuit voltage Voc have been simulated for two different saturation currents j01at the rear passivated area, 50 fA/cm2 and 150 fA/cm2. The emitter is treated as it has been etched back to roughly 2 µm, therefore having a sheet resistance of 50 Ω/sq.
While jsc is not influenced by the passivation quality between the contacts on the rear, Voc
is improving. The short-circuit current is increased with enlarged pitch due to the enhanced internal reflection by the passivated rear emitter between the contacts to the emitter.
Compared to the locally formed emitter, a full rear emitter eliminates the loss due to lateral diffusion, since in between the contacts, the holes are majority charge carriers and hence are less likely to recombine.
4.3. Full Area Locally Contacted Emitter (FALCON) Solar Cell
Figure 4.10.: Locally contacted full area rear emitter solar cell simulation for two different rear emitter saturation current densities (50 fA/cm2 and 150 fA/cm2)
4.3.2. Emitter Depth
By bringing the junction closer to the surface, recombination losses in the highly doped emitter are reduced, since the minority charge carrier lifetime is extremely low, only in the range of a few nanoseconds [63, 64]. If the junction is able to collect more generated minority carriers in the emitter, jsc is increased. On the other hand, this should lead to a higher sensitivity to the surface passivation quality. Simulations using PC1D were carried out for 10 Ωcm bulk resistivity with a bulk lifetime of 10 ms. The open-circuit voltage, the short-circuit current as well as the efficiency are presented in figure 4.11 as a function of the rear surface recombination velocity.
The basis of the simulation was a PhosTop cell, which was simulated with a measured front side reflection spectrum as well as a FSF doping profile measured using an electrochemical capacitance-voltage (ECV) measurement assembly (both have been measured in a previous experiment, which is not part of this work). Details about this simulation are shown in ap-pendix D. Since PC2D was not applicable, because no emitter or FSF profiles can be imported, one-dimensional simulation was used. It is important to note, that neither optical gain nor series resistance losses such as in the first case nor a jsc gain due to enhanced internal reflection or in the latter case a lateral resistances in the highly doped emitter are included in the simulation.
The open-circuit voltage dependence on the SRV increases the more the pn-junction gets closer to the surface. While this can improve the Voc for a low rear SRV, it deteriorates Voc for high SRV, starting roughly above 104cm/s. This is a result of a reduced field-effect passivation of the emitter due to the doping profile. Therefore, far etched back emitters are much more sensible to the actual rear SRV, if the surface is passivated compared to less etched back emitters (not Seff, which is reduced).
The short-circuit current density jscis affected by the junction depth, since the minority charge carrier lifetime and hence the effective diffusion length in the emitter is low, the distance from the junction is important. This results in a potential gain from a 8µm deep junction to 0.5µm of about 0.43 mA/cm2. Moreover, the short-circuit current density is only marginal influenced
4.3. Full Area Locally Contacted Emitter (FALCON) Solar Cell
Figure 4.11.: Simulation of jsc, Voc and η as a function of the rear SRV for different emitter depths
by the SRV.
Overall, this results in a enhanced efficiency for a low SRV and far etched back emitter, compared to the standard roughly 5µm deep PhosTop emitter. For less etched back emitters the efficiency is determined by the loss of the open-circuit voltage and hence deteriorates for increased SRV.
Interestingly, the improved PhosTop cell with an unpassivated rear (S = 107 cm/s) and an emitter depth of 5 µm shows the highest efficiency for the respective SRV, which is also only marginally affected by the SRV, too. The reason for this is the behaviour of jscand Voc. While jsc varies only marginally due to the SRV, Voc is strongly influenced.
5. Experiments and Methodology
The main goal of this thesis is to use the already well-performing front side of the PhosTop solar cell concept, which includes a s-FSF and a SiO2/CT-SiNxstack, and improve the rear side by means of a dielectric passivation.
Basically, three different solar cell concepts based on n-type mono-crystalline silicon are realized, which are prepared by preparatory experiments. These experiments are focused on evaluating the rear side passivation quality of different passivation layers and stacks on two differently doped silicon surfaces. That involves the passivation of n-type bulk substrates (10-12 Ωcm) and highly-doped p++ layers (11-13 Ω/sq). To complete, the emitter formation and quality is analysed.
Then, the Aluminium-locally alloyed rear emitter (Al-LARE) solar cell, which uses the PhosTop cell as reference, is presented. Starting with an emitter spacing analysis in order to find the opti-mum rear side geometry, results of fabricated 6”Al-LARE solar cells are given and furthermore, analysed and discussed.
Finally, FALCON solar cells are presented, which feature a full rear emitter that is locally contacted. Results of fabricated FALCON solar cells are given and further analysed.