Crystalline silicon solar cells with copper‐plated contacts are fabricated, encapsulated in ethylene‐vinyl acetate (EVA), and subject to extended damp heat stress (85° C and 85% relative humidity). We source cell precursors from several different cell manufacturers and employ several different patterning methods of the silicon nitride layer and deposit a plated front contact stack of nickel, copper, and tin using light‐induced plating. Across different Cu‐plated samples, we find similar degradation that impacts both series resistance and diode quality of the cells, indicating that there is some degradation of the p‐n junction. The overall degradation is on the order of 15%–20% of maximum power (PMP), and roughly half of this degradation is attributable to degradation of the p‐n junction. Control samples with silver‐screenprinted contacts do not exhibit the same degradation, and p‐n junction degradation in copper‐plated samples is prevented by changing the encapsulant from EVA to a polyolefin. The degradation mode is hypothesized to be the diffusion of copper from the contact, followed by the transport of this copper into the silicon cell via some mechanism facilitated by the degraded EVA encapsulant.

The goal of this project is to provide a commercially viable Ag-free metallization technology that will both reduce cost and increase efficiency of standard silicon solar cells. By removing silver from the front grid metallization and replacing it with lower cost nickel, copper, and tin metal, the front grid direct materials costs will decrease. This reduction in material costs should provide a path to meeting the Sunshot 2020 goal of 1 dollar / WDC. As of today, plated contacts are not widely implemented in large scale manufacturing. For organizations that wish to implement pilot scale manufacturing, only two equipment choices exist. These equipment manufacturers do not supply plating chemistry. The main goal of this project is to provide a chemistry and equipment solution to the industry that enables reliable manufacturing of plated contacts marked by passing reliability results and higher efficiencies than silver paste front grid contacts. To date, there have been several key findings that point to plated contacts performing equal to or better than the current state of the art silver paste contacts. Poor adhesion and reliability concerns are a few of the hurdles for plated contacts, specifically plated nickel directly on silicon. A key finding of the Phase 1 budget period is that the plated contacts have the same adhesion as the silver paste controls. This is a huge win for plated contacts. With very little optimization work, state of the art electrical results for plated contacts on laser ablated lines have been demonstrated with efficiencies up to 19.1% and fill factors ~80% on grid lines 40-50 um wide. The silver paste controls with similar line widths demonstrate similar electrical results. By optimizing the emitter and grid design for the plated contacts, it is expected that the electrical performance will exceed the silver paste controls. In addition, cells plated using Technic chemistry and equipment pass reliability testing; i.e. 1000 hours damp heat and 200 thermal cycles, with results similar to silver paste control cells. 100 cells have been processed through Technic’s novel demo plating tool built and installed during budget period 2. This plating tool performed consistently from cell to cell, providing gentle handling for the solar cells. An agreement has been signed with a cell manufacturer to process their cells through our plating chemistry and equipment. Their main focus for plated contacts is to reduce the direct materials cost by utilizing nickel, copper, and tin in place of silver paste. Based on current market conditions and cost model calculations, the overall savings offered by plated contacts is only 3.5% dollar/W versus silver paste contacts; however, the direct materials savings depend on the silver market. If silver prices increase, plated contacts may find a wider adoption in the solar industry in order to keep the direct materials costs down for front grid contacts.

The reliability and degradation of copper contacts is a topic of concern as copper is considered as an alternative to screenprinted silver. In this work, we perform reliability testing of modules of crystalline silicon (c-Si) solar cells with nickel-copper (Ni-Cu) plated contacts, including both traditional full-area back surface field (BSF) and local-area passivated and rear emitter (PERC) cells. In particular, we have demonstrated two separate manners in which Cu-plated cells show greater susceptibility to degradation than analogous Ag-screenprinted cells. We expose modules to environmental stress using climate chambers, and light soak modules under natural sunlight. In both circumstances we have demonstrated separate modes in which Cu-plated cells show greater susceptibility to degradation than analogous Ag-screenprinted cells.

A novel boric acid-free nickel plating chemistry has been developed to plate nickel onto silicon solar cells. This bath enables light induced plating (LIP) of nickel without the use of external rectification. The resulting deposit is low stress and has been shown to be an effective barrier to copper diffusion. Solar cells plated using this nickel bath demonstrate electrical, adhesion and reliability results similar to silver paste controls. In addition, these plated cells have lower contact resistance and higher metal conductivity than silver paste controls using similarly diffused wafers. The advantages of a boric acid-free nickel bath will be reviewed in detail.

Copper-plated contacts for front side crystalline silicon solar cells are a topic of considerable interest, with many recent publications presenting a variety of successful methods and impressive cell results. Several of the more obvious challenges yet to be proven relate to the durability and reliability of plated contacts, especially the adhesion of plated metal to solar cells and the long-term stability of the metals that could potentially result in gradual power degradation. In this work, we have fabricated copper plated cells using several different front side patterning methods. For a resist-based process, we have optimized plated cell processing to achieve adhesion comparable to screenprinted silver paste contacts. For laser-based patterning methods, greater understanding of the metal-silicon interface and microstructure effecting adhesion is still needed.

This paper reports the results of the study comparing various patterning and plating methods for the deposition of Cu electrodes on transparent conductive oxides for silicon heterojunction solar cells. We compared direct electroplating of Cu on different metal seeds (Ag, Ni, Cr and Ti deposited on transparent conductive oxide by physical vapor deposition) to the light induced plating of Ni/Cu directly on transparent conductive oxide. Patterning was done either using photoresists (formed by spin-on, screen printing or lamination) or lift-off of the PECVD dielectric using screen printed resist. The geometry of the fingers, line resistance, contact resistance and adhesion were used as comparative parameters. We identified direct electroplating of Cu on the sputtered Ag seed to achieve the lowest contact resistance and the best adhesion. All photoresists were able to achieve less than 60 micron resolution and could produce the fingers with the sought height (some, however, having a characteristic mushroom shape). The best silicon heterojunction cell with Cu contacts directly electroplated on the sputtered Ag seed achieved 21.9% efficiency on 153 cm 2 area.

Plated copper is being considered as an alternative to screenprinted silver for the front contacts of crystalline silicon solar cells. Generally, a thin nickel layer, annealed to form nickel silicide is used to improve contact resistance and adhesion of the contact to silicon. Nickel layers can also be used to prevent the detrimental diffusion of copper into the cell. One challenge to the commercialization of plated copper contacts is the potential for this nickel barrier to fail, causing catastrophic local electrical shunts of the cell. In this work, we monitor shunting during a thermal stress test to evaluate the adequacy of nickel diffusion barrier layers and identify different mechanisms by which Ni-Cu plated cells may become shunted.

Light induced plating (LIP) of front grid contacts is an industry-scalable potential alternative to silver paste, but LIP requires an additional patterning step to create openings in the silicon nitride (SiNx) antireflection coating (ARC) layer for metallization. One approach for patterning SiNx is masking and wet chemical etching. However, nitride etch rates can vary from cell to cell depending on the SiNx PECVD deposition parameters, previous processing steps, and etching solution usage and maintenance. Under-etching results in poor contact adhesion and over-etching results in undercutting and possible emitter damage. We demonstrate in situ real-time photoluminescence imaging (PLI) as a method to determine the point when SiNx has been fully removed. This method has the potential to be integrated into a commercial processing line to improve process control, uniformity, and repeatability.

Light-induced plating (LIP) of solar-cell metal contacts is a scalable alternative to silver paste. However, LIP requires an additional patterning step to create openings in the silicon nitride (SiNx) antireflection coating (ARC) layer prior to metallization. One approach to pattern the SiNx is masking and wet chemical etching. In-situ real-time photoluminescence imaging (PLI) is demonstrated as a process-monitoring method to determine when SiNx has been fully removed during etching. We demonstrate that the change in PLI signal intensity during etching is caused by a combination of (1) decreasing light absorption from the reduction in SiNx ARC layer thickness and (2) decreasing surface lifetime as the SiNx/Si interface transitions to an etch-solution/Si. Using in-situ PLI to guide the etching process, we demonstrate a full-area plated single-crystalline silicon device. In-situ PLI has the potential to be integrated into a commercial processing line to improve process control and reliability.

Light induced plating (LIP) chemistry and tooling that are scalable to industrial solar cell processing are used to deposit layers of nickel (Ni), copper (Cu), and tin (Sn) on previously defined front grid patterns of large area solar cells. The Ni plated layer is in direct contact with the silicon surface enabling the formation of a nickel silicide (NiSi) contact after annealing. This Alternative Seed Layer (ASL) process involves many variables that influence the formation of the NiSi contact. This paper will investigate two different aspects of the contact formation: 1) the position of the annealing step in the process flow; i.e. after Ni plating or after Ni/Cu plating and 2) the resulting contact formation for monocrystalline silicon (mono-Si) versus polycrystalline silicon (poly-Si) substrates. A decrease in the series resistance (Rseries) measurement and increase in efficiency after annealing has been demonstrated for both mono- Si and poly-Si cells with Ni only and Ni/Cu annealing.

An alternative seed layer (ASL) process is proposed in order to increase the efficiency of silicon solar cells by forming a low cost, front metal contact with reduced contact resistance and increased line conductivity and aspect ratio. A nickel seed layer is deposited directly on silicon to form a low resistivity nickel silicide (NiSi) ohmic contact and this contact is thickened by light induced plating (LIP) of nickel and copper. Unlike the traditional screen printing process currently used in industry, the ARC layer must be patterned to expose the silicon surface for nickel deposition. This paper investigates the compatibility of the ASL process with two different ARC patterning methods: 1) masking & wet chemical etching, and 2) laser ablation. In addition, the ASL process is demonstrated on both mono-crystalline and polycrystalline silicon substrates with ARC layers from different sources. The nickel seed layer and resulting NiSi layer are evaluated using scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS) and focused ion beam (FIB) cross section. X-ray photoelectron spectroscopy (XPS) is used to investigate the completeness of the ARC removal step. In addition, contact resistance testing will be performed to determine the quality of the ohmic contact formed from the ASL process. The importance of chemistry optimization in the development of a robust ASL process that is compatible with mono-Si and poly-Si substrates and exposed to two different ARC patterning methods will be discussed.

This paper presents a low cost process for fabrication of high efficiency silicon-based solar cells from front side ARC patterning through contact line metallization. This process utilizes a screen printable etch resist to define the contact pattern and a wet etching solution to remove the exposed ARC layer. The metallization stack for the contact line pattern consists of a nickel silicide ohmic contact and a nickel and copper metal stack plated using a light induced plating (LIP) process. The nickel silicide contact is formed by annealing a thin nickel seed layer that is deposited on the silicon surface using two different alternative seed layer (ASL) processes, chemically activated and light assisted electroless plating. Through optimization of the ASL processes, a thin, uniform NiSi layer is achieved that is less than 200 nm. This thin NiSi layer should be compatible with shallow emitter silicon solar cells. An LIP process for nickel and copper over the NiSi contact areas is demonstrated.