**4.2. Passivated emitter with rear locally diffused (PERL) cells**

The concept of 'passivated emitter rear locally diffused (PERL)' structure was introduced and developed in 1990s [56], in order to reduce the recombination from the rear side. The PERL schematic is shown in **Figure 11(A)**, which can diminish the heavy doping effect by using locally phosphorus‐diffused area and decrease the metal‐induced recombination si‐ multaneously via heavy doping (*n++*) underneath the contact metal. By upgrading from PERT to PERL structure, the *J*0*<sup>b</sup>*' can be significantly reduced (from 33 to 15 fA/cm2 ), which can lead to an increase in *V*oc from 691 to 704 mV [10]. However, because the rear metal contact of PERL cell is restricted to a small fraction (<1%) of the rear surface, the carrier flow towards contact is constricted, which is beneficial for minority carriers (holes) but detrimental for majority carriers (electrons). On one hand, for minority carriers, such constriction is equivalent to reducing the conductance in the direction of the rear contact, which facilitates the build‐up of their concentration inside the solar cell resulting in high‐ er *V*oc. However, for majority carriers, a lower conductance causes higher lateral resistive losses resulting in lower *FF* [57]. Hence, a trade‐off between *V*oc and *FF* needs to be con‐ sidered. By optimizing the cell fabrication process to reduce contact resistance, a 23.9% efficient front junction PERL cell on small area *n*‐type Fz substrate with photolithography contacts was reported [58] with *V*oc of 705 mV and *FF* of 82.5%. The detailed characteriza‐ 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 ( 0, pass = 10, 0, metal = 20), *J*0*b,*bulk = 10 fA/cm2 and *J*0*<sup>b</sup>*' = 15 fA/cm2 [54]. To further reduce 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/ cm2 ) resulting in *J*0*e,*metal reduction from 20 to 2 fA/cm2 on the contacted region assuming ∼1.1% metal coverage [10, 59].

**Figure 11.** Schematic of front junction *n*‐type PERL cell (A). (B) Front junction *n*‐type PERL cell with selective emitter [56, 58].

#### **4.3. Tunnel oxide passivated contact cells**

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 the PERT and PERL cells.

**Figure 12.** (A) Schematic of front junction *n*‐type TOPCON cell with selective emitter [59, 61]. (B) Schematic of large area front junction *n*‐type TOPCON cell with screen‐printed and fired front contact [45].

In addition, because both the heavy doping effect and the metal‐induced recombination are minimized in TOPCON structure, *J*0*b*' is dramatically reduced from ≥15 fA/cm2 in a PERL cell to ≤8 fA/cm2 in TOPCON cell [50, 54, 59]. A small area (4 cm2 ) 24.9% efficient TOPCON cell has been reported [59] with selective boron emitter and photolithography contacts on the front in conjunction with TOPCON back (**Figure 12(A)**). This cell had a *V*oc of 719 mV, *J*sc of 41.5 mA/cm2 and *FF* of 83.4% [59]. More recently, an efficiency of 25.1% is reported by moving the busbars outside the cell with *J*sc of 42.1 mA/cm2 [61].

**Figure 13.** Simulated *J*<sup>0</sup> decomposition with each technology for large area front junction *n*‐type PERT, PERL and TOP‐ CON cells [62, 63].

To implement TOPCON on a more manufacturable cell structure, large area front junction n‐ type Cz silicon solar cells have been developed with ion‐implanted boron emitter, SiNx anti‐ reflection coating and screen‐printed front contact, as shown in **Figure 12(B)**, and cell efficiency of 21.4% with evaporated rear contact has been reported [45]. **Figure 13** shows the simulated road map to achieve ≥23% efficient large area front junction *n*‐type silicon solar cells, including the reduction of all the *J*0 components (*J*0*<sup>e</sup>*, *J*0*b,* bulk and *J*0*<sup>b</sup>*') in the cell [62, 63]. It is clearly shown that selectively doping is one of the most manufacturable and elegant way to reduce *J*0 of the metallized and diffused regions simultaneously. This is because heavy diffusion underneath grid reduces metal *J*0 and light diffusion in between the gridlines reduces *J*<sup>0</sup> of the diffused region. The 2D modelling shows that the cell efficiency can be increased from 21.0 to ∼22.5% (bifacial architecture) by applying selectively doped emitter and BSF on large area *n*‐type Cz wafers. Moreover, implementing TOPCON structure on the rear side can raise the cell efficiency over 23% with screen‐printed contact on the front in combination with improved bulk material (10 Ω‐cm, 3 ms lifetime).
