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n-Type Rear Junction Solar Cells with Locally Contacted Al-Alloyed Emitter

n-Type Rear Junction Solar Cells with Locally Contacted Al-Alloyed Emitter

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RAABE, Johannes Nils, 2012. n-Type Rear Junction Solar Cells with Locally Contacted Al-Alloyed Emitter [Master thesis]

@mastersthesis{Raabe2012nType-22918, title={n-Type Rear Junction Solar Cells with Locally Contacted Al-Alloyed Emitter}, year={2012}, author={Raabe, Johannes Nils} }

Raabe, Johannes Nils 2013-10-30T08:18:54Z Raabe, Johannes Nils 2012 2013-10-30T08:18:54Z eng terms-of-use n-Type Rear Junction Solar Cells with Locally Contacted Al-Alloyed Emitter This diploma thesis was focused on enhancing the rear side performance of the improved PhosTop solar cell concept by means of dielectric rear side passivation and reduction of the highly doped emitter area. Stack systems and passivation layers were applied on lowly doped n-type silicon bulk and highly doped aluminium p-type emitters in order to reduce the effective rear surface recombination velocity and hence improve the open-circuit voltage. Furthermore, using a dielectric rear passivation leads to an improved internal reflection on the rear side, which results in a short-circuit current density gain. Three different solar cell concepts were realized.<br /><br />Besides the improved PhosTop solar cell, which is representing the reference, the Al-LARE (Aluminium - Locally Alloyed Rear Emitter) solar cell featuring a passivated n-type bulk and locally alloyed emitter is presented. Furthermore, the FALCON (Full Area Locally CONtacted emitter) solar cell is realized, which exhibits a full area alloyed and etched back emitter that is passivated and locally contacted.<br /><br />n-type silicon substrates were passivated using different layers and stacks of which Al<sub>2</sub>O<sub>3</sub>/SiNA-SiN<sub>x</sub>, SiNA-SiN<sub>x</sub> and CT-SiN<sub>x</sub> passivation performed best after a firing step and featured effective lifetimes of up to 9.5 ms. In the contrary to highly doped n-type silicon, the SiO<sub>2</sub>/SiNA-SiN<sub>x</sub> passivation on n-type substrates showed a severe firing instability for temperatures above 800 celsius.<br />The characterization of the emitter formed by three different aluminium pastes revealed very low emitter saturation current densities in the range of 150 to 180 fA/cm<sup>2</sup>, showing further no influence on the set-firing peak temperature. The passivation of the etched back emitter was not found to be on a satisfactory level on samples that featured an etched back emitter, which is thicker than 1.5 µm, being below the passivation due to the field-effect of the non-etched back emitter. Compared to the fully alloyed and metallized rear side of the PhosTop solar cell, no<br />improvement of the rear could be made by using different passivation layers and stacks.<br /><br />Al-LARE solar cells were simulated using PC2D, a novel two-dimensional simulation tool. The simulation predicted a possible short-circuit current density gain of 0.5 mA/cm<sup>2</sup> for an emitter and contact width of 100 µm and emitter spacing of 200 to 300 µm. This is in good agreement with a followed emitter width and spacing variation that was carried out on 5x5 cm<sup>2</sup> Al-LARE solar cells. The emitter width and spacing resulted in the conclusion that the highest values are obtained for a minimum of 100 µm emitter width and an emitter spacing of 300 - 400 µm (depending on the emitter width).<br /><br />Large-area Al-LARE solar cells featuring an emitter width of 100 µm and an emitter spacing of 300 µm were further fabricated and analysed. The best performing Al-LARE solar cell that was passivated by SiO<sub>2</sub>/SiNA-SiN<sub>x</sub>, reached an efficiency of 17 %. Furthermore, a maximum short-circuit current density gain of 0.45 mA/cm<sup>2</sup> compared to jointly fabricated PhosTop solar cells was found for a Al-LARE solar cell passivated by a Al<sub>2</sub>O<sub>3</sub>/SiNA-SiN<sub>x</sub> on the rear side. This solar cell concept was found to be basically limited by extremely high values of j<sub>02</sub> in combination with an in some cases slightly increased series resistance due to contact formation problems at the rear side. Furthermore, the passivation quality of the Al<sub>2</sub>O<sub>3</sub>/SiNA-SiN<sub>x</sub> passivated rear was the only passivation able to compensate diffusion losses to the emitter and hence to sustain a comparable IQE plateau to the PhosTop solar cell.<br /><br /><br /><br />Finally, FALCON solar cells were fabricated that feature an etched back 2 µm deep, screenprinted full area aluminium alloyed passivated emitter. Two different process sequences were carried out, allowing the rear side of one experiment to be passivated by a SiO<sub>2</sub>/SiNA-SiN<sub>x</sub> stack. An overall short-circuit current density gain, similar to the Al-LARE solar cells, of 0.5 mA/cm<sup>2</sup> was found for the best performing SiO<sub>2</sub>/SiNA-SiN<sub>x</sub> FALCON solar cell. This is only half of the short-circuit current density gain that was expected from the simulation. This is probably caused by a discrepancy of the assumed reflection difference between the unpassivated and passivated rear side for the simulation and the difference for real solar cells. Furthermore, a strong discrepancy of in some cases almost 20 mV was found between the simulated and actually measured V<sub>oc</sub>. This discrepancy can be attributed to a lower passivation quality and hence the rear SRV in the fabricated experiments compared to the simulation and increased j<sub>02</sub>.<br /><br />The best performing FALCON solar cell was achieved by passivating the rear using a SiO<sub>2</sub>/SiNA-SiN<sub>x</sub> stack that resulted in an efficiency of 18.9 %. Especially for low performing FALCON solar cells, a high reduction in FF and hence in efficiency was found to be due to an increased series resistance reaching approx. 1 Ohm cm<sup>2</sup> and in some cases, an extremely high j<sub>02</sub>. j<sub>02</sub> in combination with a not improved or even increased j<sub>01</sub> compared to the non-passivated rear of the PhosTop solar cell, resulted in a moderate to strongly reduced V<sub>oc</sub>, as well.<br /><br />The main advantage of the FALCON compared to the Al-LARE solar cell is the full area etched-back and passivated emitter that leads to a constant plateau in the IQE in the visible light range and hence allows a higher j<sub>sc</sub> compared to the decreased plateau of the Al-LARE solar cell. Furthermore, this full area emitter can lead to a much lower j<sub>02</sub> that is found for FALCON solar cells compared to the Al-LARE solar cells.<br /><br />In conclusion, since Al-LARE solar cells are mainly limited due to an extremely high j<sub>02</sub>, this diploma thesis suggests that unless improvements can be made, the increased fabrication effort is not justified, since a maximum obtained efficiency of 17 % is much lower than the 19.4 % of the improved PhosTop solar cell, while the latter is much easier to fabricate.<br /><br />The FALCON solar cell concept has a higher potential, since it is mainly limited due to process parameters such as unfilled line contacts on the rear that result in an increased series resistance.<br /><br />Furthermore, the passivation on the etched-back emitter needs to be further increased. For an industrial implementation of this solar cell concept the needed processing steps for fabrication have to be reduced.

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