Emitter Patterning for Back-Contacted Si Heterojunction Solar Cells Using Laser Written Mask Layers for Etching and Self-Aligned Passivation (LEAP)

Abstract

A novel emitter patterning method for backcontacted Si heterojunction solar cells is presented, which combines laser processing and wet etching of a mask layer stack with self-aligned repassivation, thus reducing the process complexity, as compared with the commonly used emitter patterning methods. Lifetime samples demonstrate that with a suitable mask stack, laser scribing can be performed without inducing laser damage to the passivation. Despite nonoptimal wet etch and repassivation processes which currently limit the obtained lifetime, proof-of-concept cells on p-type wafers fabricated using this novel emitter patterning process and lithographically patterned metallization exhibit an open-circuit voltage of 694 mV and pseudo-fill-factors of 83%. With the laser written mask layers for etching and self-aligned passivation process, we have thus developed the proof-of-concept for a simple, lithography free, and contactless emitter patterning method for industrial applicationsIndex Terms-Heterojunction silicon solar cells, interdigitated back-contact (IBC), laser processing, self-alignment.

Laser Assisted Patterning of a-Si:H: Detailed Investigation of Laser Damage

“…One of the key limitations is that the back‐contact architecture results in additional fabrication complexity, mainly due to the need to form interdigitated n‐ and p‐type a‐Si:H strips. Hence, different a‐Si:H patterning approaches have been developed including photolithography, lift‐off, shadow mask deposition, screen printing, inkjet printing, and laser ablation (LA) . Several research groups focus their work on LA due to the following three advantages: LA is a fast, single‐side, and contactless process; it allows a flexible device design; and it has high process precision.…”

“…Direct LA of the a‐Si:H layer on the c‐Si substrate is infeasible due to severe laser damage introduced to the a‐Si:H/c‐Si heterocontact . To reduce such damage, an additional sacrificial a‐Si:H laser‐absorbing layer on top of a SiO x mask layer is used . Only the top a‐Si:H laser‐absorbing layer is ablated, which shifts part of the laser damage from the a‐Si:H/c‐Si heterocontact to the SiO x mask surface.…”

A novel silicon heterojunction IBC process flow using partial etching of doped a‐Si:H to switch from hole contact to electron contact in situ with efficiencies close to 23%

“…Another popular “litho‐free” alternative to patterning, which is contact‐less and also drastically reduces the number wet chemical steps, is laser ablation‐assisted patterning . In this approach, the pattern is often directly structured onto a sacrificial mask layer on top of the a‐Si:H stack to be patterned.…”

“…In this approach, the pattern is often directly structured onto a sacrificial mask layer on top of the a‐Si:H stack to be patterned. While there is a risk of laser‐induced thermal damage to the crystalline silicon and the heterocontact in the laser‐ablated areas, significant strides have been made recently in tackling this issue . In this approach, etching of the underlying a‐Si:H stack is needed, and hence, it is a subtractive route, just like in photolithography.…”

Laser-induced BSF: A new approach to simplify IBC-SHJ solar cell fabrication

“…Additionally, the use of laser ablation for SHJ-IBC patterning is being investigated by various research groups. [10][11][12][13][14] Figure 1 shows a new process flow that uses a novel laser concept referred to here as "compensation lasering".…”