**2.4 Interdigitated back contact solar cell**

In IBC solar cells, optical shading loss is eliminated as both polarities of the metal contact are placed on the rear surface. In addition, the resistive power loss is reduced largely as the rear surface furnishes an opportunity for best design of metal contact formation. The other key advantages of IBC cell are (a) module manufacturing cost is reduced as the interconnection between the cells is simplified and (b) higher cell packing density increases the module power. The process flow of the IBC cell is shown in **Figure 8**. The major challenges present in the metallization of IBC cell includes: (i) shunting between the two polarities of metal contacts must be prevented and (ii) the metal conductors must be thick enough to ascertain the low resistive power loss. To isolate both the contacts, different cell-based metallization techniques can be used. One such method is patterning metal seed layer [78, 79], with electroplating to reduce resistance [80]. However, this plating up process needs electrical contact to the seed metal lines, which may lead to be problem with thinner wafers.

IBC solar cells with a record high efficiency of 25.6% were obtained by Sanyo/ Panasonic [81], and the pioneer SunPower Corporation achieved 25% efficiency [82]. For IBC cells, the front surface field (FSF) reduces the surface recombination at the front as it acts as an electrical field which pushes back the minority carriers at the front surface [83]. The high expensive photolithography process is replaced with laser processing or screen printing which leads to a significant reduction in position accuracy which increases the pitch. This makes the majority carriers to travel from vertical to lateral direction. Depending on the pitch and base resistivity, series resistance over 90% contributing to the lateral majority carrier transport reduces the cell efficiency. Moreover the lateral majority carrier's current transport as well as the front surface passivation has been enhanced by FSF and finally the series resistance also significantly reduced to 0.1 and 1.3 Ω cm<sup>2</sup> for the base resistivity of 1 and 8 Ω cm, respectively, for the pitch of 3.5 mm [84].

The rear metallization of IBC cells is usually done with silver (Ag) and aluminum (Al) pastes [85], and Si/Ti/Pd/Ag or Si/Al/Ti/Pd/Ag metal stack and Al-deposited by PVD form a good ohmic contact with both n- and p-type silicon [86]. In Si/Ti/Pd/Ag or Si/Al/Ti/Pd/Ag metal stack, the Ag layer is used as a conductive layer because of its low resistivity. To avoid the reaction between Ti and Ag, the Pd layer is deposited between Ti and Ag layer. The work function of Ti or Al makes it suitable to contact with low contact resistivity [87] on both p- and n-doped

**Figure 8.** *IBC process flow.*

*Review on Metallization in Crystalline Silicon Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.84820*

#### **Figure 9.**

*Process flow of AMELI interconnection process.*

regions. As Ti is a poor reflector, the Si/Al/Ti/Pd/Ag structure is adopted owing to the higher reflectance obtained with an Al layer which increases the light trapping. Couderca et al. [20] used Si/Ti/Ag stack with a thin Ti layer which has a low contact resistivity on both n- and p- doped regions. For n-doped surfaces, the specific resistivity is very low, and hence, the resistive losses are negligible. As the p-doped surfaces are lowly doped, the specific resistivity is higher though it stays under the crucial limit of 0.01 Ω cm2 . As a low-cost approach, Chena et al. [88] applied the aluminum electrodes deposited by e-beam evaporation using Al contact for high performance IBC cells. The specific contact resistivity of the Al contact cell resulted in 0.7 and 0.05 mΩ cm2 on an n-doped and p-doped surface, respectively, and the final Al-contacted IBC cell resulted in an efficiency of 22.72%.

Recently, carrier selective contacts [TOPcon] using tunnel oxide and amorphous (a-Si) layer resulted in Voc values around 720 mV and contact resistivities less than 10 mΩ cm2 [89]. With poly-Si/SiOx approach, similar values for their passivated contact have been achieved by various researchers [90–92]. Young et al. used the similar contact for the IBC solar cell patterned with ion implantation. The metallization layer consisting of thin Ti/Pd adhesion layer with 1-μm thick Ag layer and a Pd capping layer resulted in the contact resistivities less than 0.1 mΩ cm2 [93].

Electrical shading loss plays a detrimental role as it reduces the collection efficiency of the minority carriers over the BSF regions [94], which compromises Jsc. By decreasing the width of the BSF region, this detrimental effect can be resolved. In the active cell area, by decreasing the finger pitch and BSF finger width, the electrical shading loss is minimized. Nevertheless, the base busbar still enforces electrical shading. In addition, as the majority carriers generated over the emitter busbar have to traverse over the entire wafer area, result in transport losses. The electrical shading loss and the transport loss contribute to the resistive losses and FF losses in the busbar metallization. Hence for further efficiency improvement, research group from ISFH developed a busbar less metallization which omits the busbar and eliminates the resistive losses in metallization with aluminum-based mechanical and electrical laser interconnection (AMELI) process for contacting aluminum-metalized IBC cells [95] and obtained a conversion efficiency of 22.1% with a Voc of 683 mV, Jsc of 41.4 mA/cm2 , and FF of 78.1% on a cell area of 132 cm2 . AMELI process interconnects the solar cell with highly flexible interconnection geometry performed by a laser as structuring of the metallization. In addition, this AMELI process can interconnect that are as wide as the whole cell edge with a lower electrical resistance between the cells [96, 97]. **Figure 9** depicts the AMELI interconnection scheme for busbar-free solar cells. Woehl et al. introduced a pointshaped metalized IBC cells interconnected to a printed circuit board. The presence of only point-shaped metal contacts, increases the Voc as the recombination area, is significantly reduced [98].

The main advantage of interdigitated back contact solar cells over other type of solar cells is zero shadow loss due to the absence of complete front contact. Although IBC is the high efficiency single junction cells among all other type of silicon solar cells, the carrier collection efficiency in front of the back surface field is low.
