Laser Ablation and Ni/Cu Plating Approach for Tunnel Oxide Passivated Contacts Solar Cells with Variate Polysilicon Layer Thickness: Gains and Possibilities in Comparison to Screen Printing

Abstract

Recombination of photo-generated charge carriers at metalsemiconductor junctions is a strong limiting factor in achieving high efficiencies for conventional c-Si solar cells [e.g., aluminum back scatter field, passivated emitter rear contact (PERC), etc.]. [1-3] This can be reduced by squeezing the metallized area fraction, but that results in an increase of fill-factor (FF) loses. A novel approach developed at Fraunhofer ISE, namely, tunnel oxide passivated contacts (TOPCon), attempts to counter this effect by implementing passivating contact structures to drastically reduce metal-semiconductor junction recombination. [1,2,4] Passivated contact layers, such as TOPCon, constitute an ultrathin (1-2 nm) silicon oxide (SiO x) layer grown over the bulk c-Si, which is further deposited upon with a highly doped poly-Si layer and finally capped with SiN x. Together they provide excellent passivation, enabling high implied open circuit voltage (iV oc) [2] and carrier selectivity. [2,4] Furthermore, the depletion zone field generated due to high doping of poly-Si also contributes to the passivation effect. Passivating contacts are, therefore, considered to be the next major technological upgrade in the solar cell technology. [3,4] However, forming metal contacts on TOPCon is still a challenge that requires further investigation and improvements. Some commonly reported concerns pertaining to the conventional screen-printed metallization on TOPCon are 1) higher specific contact resistivity (1.5-10 Ω cm) on thick (>150 nm) poly-Si passivated contacts [5] and 2) increase of metal-induced recombination with decreasing thickness of poly-Si due to the metal spiking through the poly-Si and coming in contact with the bulk c-Si. [6] Spiking of the metal paste through the poly-Si layer drastically increases the J 0,met (recombination current at metal-silicon junction) due to poor shielding of recombination at metal contacts, resulting in a low open circuit voltage (V oc). [6] To prevent the penetration of fire-through metal pastes all the way through to the c-Si substrate, it becomes necessary either to keep the thickness of poly-Si above 120 nm or use low temperature metallization techniques. [7-10] However, a thicker poly-Si layer has two serious drawbacks-increased parasitic absorption and higher cost of ownership (COO). Parasitic absorption inflates with increasing poly-Si thickness, resulting in the loss of short circuit current (J sc), [6,11,12] whereas the COO calculations reveal a jump of around 75% if the poly-Si thickness is increased from 50 to 200 nm. [13] Hence, to keep the parasitic absorption and the cost

Direct Contact Electroplating Sequence Without Initial Seed Layer for Bifacial TOPCon Solar Cell Metallization

“…For screen-printed metallization most of recent publications require a thickness of at least 100 nm of the TOPCon layer to ensure its functionality [4]- [6]. For UV (355 nm) picosecond (ps)-laser ablation before plating the contacts 60-70 nm are sufficient so that the tunneling oxide remains intact as the penetration depth of the laser is around 50-60 nm [3], [7]. Furthermore, contacting these laser contact openings via electroplating of Ni/Cu/Ag would allow an alternative lead-free metallization technique with narrow contact geometries (≤ 25 μm), low contact resistivity ρ c (< 1 mΩcm²) [8] with high throughputs and low cost of ownership (COO) showed by Kluska et al [9].…”

Influence of Plasma‐Enhanced Chemical Vapor Deposition Poly‐Si Layer Thickness on the Wrap‐Around and the Quantum Efficiency of Bifacial n‐TOPCon (Tunnel Oxide Passivated Contact) Solar Cells

“…[ 21 ] The results of increased recombination are in accordance with observations by Arya et al showing an increase of the contact recombination and that poly‐Si layers below 100 nm remain less sensitive to laser ablation than to screen‐printing metallization. [ 16 ] Further, the analysis of the IV results of the solar cells presented by Arya et al reveal a decrease in V oc and J sc for both metallizations whereby the plated cells feature a higher level. [ 16 ] The expected reduction of the FCA can be seen in the increase of the escape reflection for thinner poly‐Si layers between 1100 and 1200 nm.…”

“…Bifacial TOPCon solar cells were manufactured from 156.75 × 156.75 mm 2 large n‐type Cz–silicon wafers featuring a resistivity of 1 Ω cm. The exact processing of the solar cells is discussed in detail in the publication of Arya et al [ 16 ] The schematic cross‐sectional layout is shown in Figure . The produced TOPCon solar cells feature a thermal SiO x layer prepared in a tube furnace and a phosphorus‐doped amorphous silicon (a‐Si) layer deposited in a Centrotherm c.PLASMA tube PECVD with horizontal carrier.…”

“…[ 7 ] Laser ablation and plating of Ni/Cu/Ag contacts was found to be a candidate to allow for metallizing of TOPCon solar cells with thinner poly‐Si layers without inducing increased excess carrier recombination. [ 9,16 ]…”

Progress in p‐type Tunnel Oxide‐Passivated Contact Solar Cells with Screen‐Printed Contacts

“…In a second route, low‐temperature metallization pastes could be used for metallization of the rear polylayer, after laser contact opening (LCO) of the rear SiN x :H layer; however, this would require a very tight alignment of laser processing and screen printing, [ 38 ] which would put additional complexity to the process route, possibly with the benefit of reduced J 0,met . Alternatively, the combination of LCO and Ni/Cu plating has shown also very promising for especially very thin n‐doped polysilicon layers [ 39 ] and could be tested for this application.…”