20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

Abstract

In this work, we report on ion-implanted, high-efficiency n-type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laserpatterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiN x , followed by screen-printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO 2 /SiN x passivation stack on the back to form the pattern for metal-Si contact. The laser pulse energy had to be optimized to fully open the SiO 2 /SiN x passivation layers, without inducing appreciable damage or defects on the surface of the n + back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n-type passivated emitter, rear totally diffused cell.

Process development and comparison of various boron emitter technologies for high-efficiency (~21%) n-type silicon solar cells

“…This was accomplished by laser opening 300‐µm‐apart 40‐µm dots. This geometry reduced the metal coverage from ~4.5 to ~1.4% .…”

“…This was accomplished by laser opening 300-μm-apart 40-μm dots. This geometry reduced the metal coverage from~4.5 to~1.4% [18]. Figure 5 shows the comparison of process flow for the rear contact formation by screen-printing of Ag dots and PVD Al metallization.…”

Solar‐grade boron emitters by BF3 plasma doping and role of the co‐implanted fluorine

“…It is established that boron (B) emitters need high thermal activation annealing to reach low J 0e value and may require additional chemical surface treatments to remove the boron‐rich layer formed at the surface. Efficiencies around 20.5% have been obtained on large‐area n‐type homojunction cells , but to our knowledge, such performances have not been demonstrated with plasma doping (PIII) yet. As a non mass‐selective technique, PIII does not implant pure boron ions but composite ion species depending on the gas precursor we choose (BF x<3 + , B + , F + ions with BF 3 , B 2 H x<6 + , B + , BH x + with B 2 H 6 ).…”

“…Such a treatment is not necessary when we implant pure B + atoms (with a beamline). Another method could be to willingly grow a thick sacrificial oxide during the thermal annealing in order to largely consume the B‐rich emitter surface and to remove it chemically afterwards . A disadvantage, however, is that post‐annealing chemical step prevents a direct passivation of the surface by growing a high‐quality silicon oxide during the thermal activation (such an in situ passivation is very practical in a solar cell‐manufacturing process).…”

Modeling the potential of screen printed front junction CZ silicon solar cell with tunnel oxide passivated back contact

“…The unit cell cross section of this screen printed Cz TOPCon solar cell with homogenous emitter is shown in Figure . All the required input parameters for the base, emitter and screen printed front contact regions are measured or extracted from a 21.0% n‐type screen printed PERT cell fabricated in our lab (Table , column 2). Next, the full area BSF and local contacts to the n + back of this PERT cell were replaced by the TOPCon structure ( J orear = 8 fA/cm 2 and v p = 650 cm/s with n + diffusion) for modeling.…”