The Hetero-Contact

(a) The Ohmic Contact

Different coatings of silicon surfaces show different passivation qualities.

For example,aluminum oxidepassivates the cell surface in a better way than the aluminium-silicon alloy used in «standard Al-BSF solar cells». With aluminium oxide passivation layers (see Chap.5, PERC solar cells), open-circuit voltagesVoc

=660 mV can be achieved. However, since the aluminium oxide, which is applied, as a passivation layer on the back side of the PERC cell, is a very good insulator with a resistivity of 10¹² m @ 20 °C, a direct contact between the solar cell and the metal layer—through the aluminium oxide layer—must be created, so that the charge carriers find their way. Such a direct contact is usually made in the PERC cell

1Diffusion lengthL=(τ*D)^1/2.Dis the diffusion constant andτis the lifetime in seconds. Diffusion length describes the average length a carrier moves between generation and recombination.

7 Crystalline Silicon Solar Cells: Heterojunction Cells 167 with openings in the aluminium oxide through which a conductive aluminium paste is used to contact the crystalline silicon (see Chap.5). A high-temperature step in the range of 850 °C forms, as in the «standard Al-BSF solar cell», an aluminium-silicon «eutectic²» resulting in an Ohmic contact between aluminum and silicon, but this time only at the openings of the aluminum oxide layer. At these contact points not only Ohmic resistance losses appear but, like in standard Al-BSF solar cells, an inferior passivation occurs, which results in reduced efficiencyη.

The question now arises as to what an optimal contact might look like—a con-tact that has no or only very little recombination and no Ohmic losses. A reduction of the recombination losses can be achieved, for example, by making the openings even smaller; but this increases resistive losses. Very high doping of the silicon at the openings reduces resistivity losses but increases recombination. (Auger recom-bination, see Chap. 4.) Laser-Fired Contacts [3] constitute another way to reduce Ohmic losses. However, all these processes have additional process steps—a fact that renders cell production more expensive.

(b) Passivated Contacts

Another, very elegant possibility to avoid direct metallic contact to silicon is to use thin passivation layers with a thickness of only a few nanometres. Charge carriers can «tunnel³» through the passivation layer and recombination effects cannot occur since direct Ohmic contacts are not present. In addition, such passivated contacts can form selective contacts, e.g. they conduct either only electrons or only holes. This possibility is implemented in the heterojunction cell.

(c) The Amorphous Double-Layer Construction of the HJT Cell Leads to the Highest Passivation

The first silicon-based heterojunction cells used doped amorphous materialdirectly applied on the silicon and achieved satisfactory passivation properties. However, it has been shown that in this structure the charge carrier density is very high, so that increased surface recombination takes place [4]. Finally, in 1992,an intrinsic, undoped amorphous interlayer sandwiched between the silicon crystal and the doped amorphous layer was found to provide even better passivation properties [5].

The recombination rate could be reduced and the fill factorFFincreased. The cell was named «HIT», which stands for «Heterojunction withIntrinsicThin Layer^©».

Sanyo achieved 23% cell efficiency with this structure in 2009. In 2014, Panasonic obtained another breakthrough by applying passivated contacts on both sides, e.g.

on the front and on the back side. The efficiencyηincreased to 25.6%. In 2017,

2The term «eutectic» designates a homogeneous mixture of substances that melts or solidifies at a single temperature that is lower than the melting point of either of the constituents.

3The term «tunneling» comes from quantum physics: If an electron encounters a potential barrier, it can pass through the barrier with a certain probability, even if its energy is lower than that of the barrier. That would not be possible according to classical physics. This probability is called the

«residence probability»—it is given by the wave function representing the electron. The electron is described in quantum mechanics by a wave: as a wave it can also extend to the other side of the barrier. OR, in other words, it can just «tunnel» through the barrier.

168 S. Leu and D. Sontag

Fig. 7.4 Simplified cross-section of a heterojunction cell in the «back contact implementation».

On the front,nofingers shadow the cell. The electricity is taken off on the back. Thepn-junction is on the back; then-region alternates with thep-region

Kaneka even achievedη=26.6% cell efficiency on a 6×6 inch HIT cell, in which the contacts are only on the back side (back-contact cell). «HJT-IBC(Interdigital BackContact)» have the highest efficiencies. This is because the losses due to the shading of the metal fingers on the front side are completely eliminated and because there is no parasitic absorption on the front side; in fact, there is no TCO layer and no doped a-Si on the front side (see Fig.7.4). Today, HJT-IBC cells are not in mass production due to the high manufacturing costs. However, with a clever design, the costs per peak-Watt (W_p) can be kept at the same level as with «normal» HJT cells.

Figure7.4gives a schematic illustration of such an HJT-IBC structure.

The best measure for the quality of the passivation is the open-circuit voltageVoc, which, is in a HJT cell, in general, higher than 740 mV, e.g.Vocis about 15% higher for an HJT cell, than for conventional PERC cell, where we only obtain ~660 mV today.

7 Crystalline Silicon Solar Cells: Heterojunction Cells 169