**5. Summary and outlook**

tion and analysis shows that this 23.9% efficient PERL cell is limited by metal recombina‐ tion on the front (*J*0*e,*metal) and the rear side recombination *J*0*<sup>b</sup>*', with total *J*0*<sup>e</sup>* of 30 fA/cm2

recombination losses, selective emitter with heavier and deeper boron doping profile un‐ derneath front contact metal is typically implemented, as shown in **Figure 11(B)**, which can dramatically decrease the front metal‐induced recombination (from 1800 to 200 fA/

) resulting in *J*0*e,*metal reduction from 20 to 2 fA/cm2 on the contacted region assuming

**Figure 11.** Schematic of front junction *n*‐type PERL cell (A). (B) Front junction *n*‐type PERL cell with selective emitter

The implementation of polysilicon tunnel junction as an alternative to either totally or locally diffused junction to reduce the recombination at the contact of silicon solar cells has been reported in the 1980s [60]. Because of its excellent surface passivation and carrier selectivity, a full area TOPCON shown in **Figure 12(A)** was applied, which also enables one‐dimensional (1D) carrier transport on the rear side to eliminate *FF* losses due to 2D/3D carrier transport in

**Figure 12.** (A) Schematic of front junction *n*‐type TOPCON cell with selective emitter [59, 61]. (B) Schematic of large

In addition, because both the heavy doping effect and the metal‐induced recombination are

in a PERL cell

area front junction *n*‐type TOPCON cell with screen‐printed and fired front contact [45].

minimized in TOPCON structure, *J*0*b*' is dramatically reduced from ≥15 fA/cm2

0, metal = 20), *J*0*b,*bulk = 10 fA/cm2 and *J*0*<sup>b</sup>*' = 15 fA/cm2 [54]. To further reduce

(

cm2

[56, 58].

0, pass = 10,

108 Nanostructured Solar Cells

∼1.1% metal coverage [10, 59].

**4.3. Tunnel oxide passivated contact cells**

the PERT and PERL cells.

In this chapter, the physics and operation of front junction *n*‐type silicon solar cells is described, including detailed cell parameters, *pn*‐junction formation, metallization approaches and fundamental power loss mechanisms. To reduce surface recombination velocity for achieving high cell efficiency, Al2O3 has been approved to be very efficient on passivating heavily boron‐ doped front emitter and thermal SiO2 is very effective to passivate phosphorous‐diffused regions. In addition, to minimize the metal‐induced recombination, implementation of selective boron emitter on the front side and locally‐diffused back surface field on the rear side are preferred. Furthermore, TOPCON passivation scheme shows even better performance by simultaneously reducing the metal‐induced recombination and the heavy doping effect, and allowing for 1D carrier transport. However, it is only suitable for being applied on the rear side of solar cell because the heavily doped polysilicon layer can absorb significant amount of incoming photons (hence low *J*sc), if it is located on the front side.

High‐efficiency front junction *n*‐type silicon solar cells, including PERT, PERL and TOPCON cells, are reviewed and analysed. In combination with low‐cost screen‐printed metallization technology, PERT bifacial cell structure with homogeneous doping profiles on both front and back sides is a promising candidate of next‐generation solar cells for industrial application in terms of process simplicity and energy yield while the recently developed cell structure with TOPCON on the rear and selective boron emitter on the front demonstrates the promise of this technology option for higher cell efficiency.
