Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Surface passivation and ARC formation by PECVD SiNx films

The reference point of the process development within the INDHI-project was a TiO2 coated homogeneous emitter of a multicrystalline silicon solar cell. An improvement of surface passivation and bulk lifetime can be achieved by the incorporation of a silicon nitride SiNx layer into the standard industrial silicon solar cell. The antireflection properties of SiNx are comparable to those of TiO2, if the index of reflection and the layer thickness are well adapted. The open question which had to be addressed is the stability of this SiNx layer. Stability with regard to illumination with UV-light which the solar cells have to endure in the final application in a module on the one hand and the influence of heat treatment during cell processing e. g. contact firing on the other hand, has to be evaluated considering bulk lifetime and surface passivation properties of the SiNx layer.

The bulk lifetime investigations were carried out on mc-Si acquired for the project from Deutsche Solar. The process sequence carried out includes a firing step for the front contact, whereas no screen-printing had been performed. After that the SiN layer as well as the previously formed phosphorous emitter were removed and silicon nitride was deposited on both wafer surfaces, optimised for surface passivation leading to surface recombination velocities of less than 10 cm/s. Thus it was possible to determine the bulk lifetime of the material. The lifetime mappings before and after bulk passivation are 118 µs and 184 µs respectively. That means despite of a high lifetime before processing the average lifetime could have been improved by about 50%.
Experiments with FZ-Si proved the stability of the surface passivation during the firing process in the IR belt furnace.

A joint solar cell manufacturing experiment was set up between ENEA, EniTecnologie and ISFH. A series of 24 wafers was prepared to be introduced into the solar cell production line of EniTecnologie. These solar cells with a SiNx layer from ISFH showed efficiencies up to 15.1 % (Voc: 614 mV, Jsc: 32.4 mA/sqcm, FF: 76.0 %, cell area > 100 sqcm).

A further experiment between UKON and ISFH showed the combined effect of an acidic surface texture and a SiNx layer. An efficiency of 15.9 % was achieved (Voc: 613 mV, Jsc: 34.1 mA/sqcm, FF: 76.0 %, cell area: 25 sqcm). The main contribution to the efficiency improvement originates from an enhanced short circuit current leading to an average increase in cell efficiency of 1 % absolute.

The UV stability of the cells manufactured together with EniTecnologie could have been demonstrated. The cells were exposed to UV-light with wavelengths above 360 nm at about 5 suns for up to 16 hours.

The transfer of the process for surface and bulk passivation was successfully established on on an industrially relevant system, the SiNA from Roth & Rau. The surface and bulk passivation properties of the SiN films from the SiNA system are as good as from the laboratory tool. Also the stability requirements are fulfilled as stated above.

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Am Ohrberg 1
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