Felix Book

Minimizing the electrical losses on the front side: Development of a selective emitter process from a single diffusion

In this paper we present latest results in the development of a process for the fabrication of a selective emitter structure on mono- and multicrystalline silicon solar cells. The process is based on an approach that was first introduced by Zerga et al. [1]. We have chosen a wet chemical route for an emitter etch back where the areas of the wafer that are intended for emitter metallization are shielded from etching by a screen printable etch barrier. The etch barrier is later removed by wet chemical etching. The process has yielded a gain in open circuit voltage of more than 1% and a gain in short circuit current of more than 2%. The overall efficiency gain was more than 0.3%abs due to slightly lower fill factor of the cells.

The etchback selective emitter technology and its application to multicrystalline silicon

We have developed a simple and industrially applicable selective emitter cell process using only one diffusion step and an emitter etchback to create the high sheet resistance emitter [1, 2]. The process generates a deeper doping profile with a lower surface phosphorous concentration than a directly diffused emitter with the same sheet resistance. This results in an extremely low emitter saturation current j0E even at a moderate sheet resistance of 60–80 Ω/□. The highest independently confirmed cell efficiency on Cz-Si (146 cm2 was 18.7%. In this work the etching behavior of the acidic solution at the grain boundaries is studied by SEM imaging and high resolution LBIC measurements at 405 nm wavelength. The etchback also leads to a change in reflectivity, which is quantified by reflectance measurements. We furthermore investigate the influence of the base material quality on the gain that can be achieved by this process. Large area solar cells have been processed from solar grade and UMG mc silicon.

Co-diffused APCVD boron rear emitter with selectively etched-back FSF for industrial N-type Si solar cells

The employment of a B-doped atmospheric pressure chemical vapor deposited (inline belt APCVD) borosilicate glass is an elegant technology for industrially realizing a p emitter. By drive-in of B and a subsequent POCl3 co-diffusion, p emitter and n front surface field (FSF) are established in a single process step. APCVD-SiOx is used to prevent the p emitter from being compensated during P diffusion. Its thickness needs to be adapted in order not to affect the p profile during POCl3 diffusion while keeping it removable. For rear junction solar cells, it is crucial to ensure low recombination activity at the front. Therefore, a selectively etched-back FSF is to be established in the solar cell. An adjusted etch-back solution increases n Rsheet successively and well controllably, accompanied by a drastic j0FSF reduction while simultaneously almost completely maintaining p Rsheet. A 43 /sq APCVD-AlOx passivated p emitter achieves j0E of only 52 fA/cm. Total implied VOC of a pseudo solar cell structure attains up to 695 mV. The newly developed APCVD p emitter combined with the co-diffused and selectively etched-back FSF employed in an industrial n-type solar cell achieves 18.8% efficiency in a first experiment being still limited by a poor Ag/Al contact to the B-emitter.

Phosphorous Doping from APCVD Deposited PSG

The phosphorous diffusion from atmospheric pressure chemical vapor deposition (APCVD) deposited phosphorus silicate glass (PSG) promises reduced process costs compared to the standard POCl3 diffusion process, since no POCl3 gas flow is necessary during the diffusion process. Therefore, much smaller or no spacing between the wafers is necessary and the throughput of the diffusion process can be significantly increased. Furthermore, it allows a structuring of the doping source prior to diffusion. We investigate the effect of basic process parameters concerning the deposition of the PSG and the capping layer on sheet resistance and uniformity. On standard aluminum back surface field (Al-BSF) solar cells, cell efficiencies of up to 19.6 % were achieved. In a high temperature co-diffusion process with reduced P content, the APCVD-PSG emitter passivated with fired PECVD-SiNX features low j0E of 100 fA/cm2 at 50 Ω/sq. This results in a high cell VOC of 639 mV while leading to a jSC loss due to increased Auger recombination in the deep emitter profile. This loss can partly be compensated by a selective emitter etch-back. It would not occur, when the doping profile is located at the rear side of a bifacial or back contact solar cell as a BSF.