**4.1. Passivated emitter with rear totally diffused (PERT) cells**

Even though *p*‐type silicon solar cells dominate the PV market today, in recent years, several academic groups and companies have started to investigate front junction *n*‐type crystalline silicon solar cells. The schematic of front junction *n*‐type PERT 'passivated emitter rear totally diffused' [52, 53] cell is shown in **Figure 10**. PERT cell structure featuring a bifacial architecture is shown in **Figure 10(A)**, which can collect radiation from the rear side of the solar cell, hence has the potential to achieve an increased energy yield (5∼20%) in certain module configura‐ tions. The PERT cell structure in **Figure 10(B)** shows the rear point contact that can reduce the rear metal‐induced recombination due to lower metal coverage, as well as reduce the lateral resistance due to smaller contact pitch [22].

**Figure 10.** (A) Schematic of front junction *n*‐type PERT cell with bifacial feature [52, 53]. (B) Schematic of front junction *n*‐type PERT cell with rear point contacts [22].

**Table 1** shows that 21.9% cell efficiency has been reported on thermal SiO2‐passivated boron emitter formed by BBr3 diffusion [53] while 22.7% cell efficiency has been achieved via Al2O3‐ passivated boron emitter formed by ion implantation and photolithography on small area *n*‐ type float zone (Fz) substrates [54] with rear point contacts. To make manufacturable *n*‐type PERT cells, screen‐printing metallization has been applied on large area *n*‐type Czochralski (Cz) silicon substrate with excellent cell efficiencies approaching 21% featuring bifacial architecture [17, 55], as listed in **Table 1**.


**Table 1.** Light *I‐V* results of high‐efficiency front junction *n*‐type PERT silicon solar cells.

doping in the *n+*

106 Nanostructured Solar Cells

silicon layer, resulting in very weak field‐effect passivation. In addition, to

has

facilitate the solid‐phase crystallization and activate the phosphorus dopants in the as‐ deposited *n+* amorphous silicon layer, a proper polysilicon anneal temperature is necessary as shown in **Figure 9(B)**. However, a strong degradation in the interface passivation quality is observed if the anneal temperature is too high (≥900°C), again due to more dopant diffusion into the silicon base causing high Auger recombination and possible local disruption of tunnel oxide due to polycrystalline silicon grain growth and more interface defects [45, 46]. It has also been demonstrated that both the tunnel oxide growth temperature in nitric acid and the high temperature firing process can affect the passivation quality. A very low *J*0*b*' of ≤5 fA/cm2

been achieved by optimizing the TOPCON fabrication processes [50]. Similar passivation performance has also been achieved by depositing intrinsic amorphous silicon layer on top of the tunnel oxide layer followed by ion implantation of phosphorus and thermal annealing [51].

All front junction *n*‐type crystalline silicon solar cell structures fabricated to date feature some degree of surface passivation. In the following section, we discuss several types of advanced high‐efficiency front junction solar cells that have been developed on *n*‐type silicon substrates.

Even though *p*‐type silicon solar cells dominate the PV market today, in recent years, several academic groups and companies have started to investigate front junction *n*‐type crystalline silicon solar cells. The schematic of front junction *n*‐type PERT 'passivated emitter rear totally diffused' [52, 53] cell is shown in **Figure 10**. PERT cell structure featuring a bifacial architecture is shown in **Figure 10(A)**, which can collect radiation from the rear side of the solar cell, hence has the potential to achieve an increased energy yield (5∼20%) in certain module configura‐ tions. The PERT cell structure in **Figure 10(B)** shows the rear point contact that can reduce the rear metal‐induced recombination due to lower metal coverage, as well as reduce the lateral

**Figure 10.** (A) Schematic of front junction *n*‐type PERT cell with bifacial feature [52, 53]. (B) Schematic of front junction

**4. High‐efficiency front junction** *n***‐type crystalline silicon solar cells**

**4.1. Passivated emitter with rear totally diffused (PERT) cells**

resistance due to smaller contact pitch [22].

*n*‐type PERT cell with rear point contacts [22].

The detailed characterization and analysis show that the 22.7% efficient PERT cell is largely limited by the rear side recombination (*J*0*<sup>b</sup>*') due to the totally diffused BSF layer (*J*0*b,*pass), with *J*0*<sup>b</sup>*' ( 0, pass <sup>+</sup> 0, metal = 29 + 4) = 33 fA/cm2 , *J*0*<sup>e</sup>* ( 0, pass <sup>+</sup> 0, metal = 12 + 16) = 28 fA/cm2 and *J*0*b,*bulk = 10 fA/cm2 [54]. Therefore, to further improve the cell performance, the recombi‐ nation in this heavily doped full BSF layer needs to be reduced. This can be accomplished by locally diffused BSF (PERL) structure on the rear side.
