This paper reviews the main research results related to PERC+ silicon solar cells. Compared to today’s industry typical passivated emitter and rear cell (PERC) silicon solar cells with full-area rear aluminum layer, PERC+ solar cells apply an aluminum finger grid on the rear side and hence are able to absorb diffuse light from the rear side in addition to the direct sunlight which is absorbed from the front side. This bifaciality increases the energy yield of silicon solar modules by up to 25%. Since its first publication in 2015, the PERC+ cell concept has been rapidly adopted by several solar cell manufacturers due to the very similar process technology of bifacial PERC+ cells and main stream monofacial PERC cells. We summarize technological challenges, published PERC+ conversion efficiencies and PERC+ module technologies. First energy yield data of PERC+ field installations demonstrate the high energy yield potential of PERC+ solar cells.

SmartWire Connection Technology (SWCT) developed by Meyer Burger uses multiple thin copper wires with alloy coating to interconnect solar cells. These thin copper wires separate a 156 mm solar cell into unit cells, which typically have ~4 mm finger length instead of 26 mm as in the case of three busbar solar cells, thereby substantially reduce the requirement of finger line conductivity. Such a low requirement of finger conductivity has enabled diverse metallization schemes to be successfully applied to many cell designs to reduce silver consumption and to be integrated into modules easily. In this paper, we present the module integration results of silicon heterojunction solar cells using various metallization schemes developed within Meyer Burger group, including fine line silver screen printing down to 80 mg per 6 inch bifacial cell, inkjet direct silver printing down to 10 mg per 6 inch monofacial cell, inkjet masking and nickel/copper plating to finger thickness as low as 1 μm on 6 inch monofacial cell. We demonstrate the high durability of these modules with extensive thermal cycling tests.

This paper reports a patterning and metallization method for silicon solar cells fabrications. Patterning was achieved by the inkjet printing of a dye-based ink as a mask to protect the photoresist from UV-light initiated crosslinking. The patterned photoresist was used to facilitate the etching of a pattern in the underlying dielectric layer and also to act as a metal plating mask. This method resulted in fine point openings in the photoresist layer with a diameter of 15 μm and line openings with a width of 30 μm. Nickel/copper plated homogeneous emitter silicon solar cells with an efficiency of 18.2% on small size Cz wafers were fabricated using this method, may find applications in the metallization of future heterojunction, rear contact and PERC cells.

This paper reports the use of injection-dependent local ideality factors, obtained from quasi-steady state photoconductance and photoluminescence measurements, to investigate the effects of various cell processing steps on recombination in solar cells, fabricated using boron-doped Czochralski silicon wafers, with an ∼ 100 Ω/□ phosphorus-doped emitter and silicon nitride passivation of both surfaces. It is shown that activation of boron-oxygen complexes in the cells by light soaking can impact the pseudo fill factor and is manifest in increased local ideality factors in the injection range between maximum power point and open circuit voltage. The introduced recombination was modeled as a single Shockley Reed Hall (SRH) recombination centre at Ec - Et = 0.41 eV and an electron:hole capture cross section ratio of ∼14. The effects of boron and phosphorus laser doping on the injection-dependent local ideality factor were also investigated. Boron laser doping was shown to introduce additional recombination in cells, as indicated by the increased local ideality factor in the injection range between maximum power point and open circuit voltage. However, in this case, the additional recombination was not well-modelled by a single SRH recombination centre, especially in the mid-injection range. Finally, it is shown that high temperature belt furnace anneals can place cells into a specific recombination state, and cells can be returned to this state by subsequent anneals even after additional recombination is introduced into cell by processes like laser doping.

In this paper we report on the fabrication of laser-doped p-type bifacial cells using self-aligned metal-plating with energy conversion efficiencies as high as 19.2%. A key fabrication step for these cells is recognising that the p-type silicon regions can be made cathodic by forward biasing the p–n junction in a process which we call here field-induced plating (FIP). Used in conjunction with light-induced plating (LIP) in the same plating apparatus, FIP can be used to form low cost nickel/copper grids on both surfaces of a cell. Furthermore, the simplicity of the FIP process means that it can potentially be performed using the same plating equipment and chemistry as used for LIP. Plating rates similar to LIP were achieved (i.e., ~10µm of copper in 10min), however there is potential to plate at much faster rates with FIP because the junction is forward-biased. This bifacial cell plating method could be adapted to metallise a range of bifacial cells including heterojunction cells.

Strong contact adhesion is an important requirement for durable, manufacturable solar cells. Advanced contacting technologies require new methods to measure adhesion. We describe a scratch test for measuring contact adhesion that involves scanning a weighted stylus across the cell while measuring the horizontal force FD required to dislodge the contacts. FD is characteristic of the adhesive bond but independent of the contact height, stylus weight and scan speed. We observe that contact peeling depends also on the tensile strength of the metal finger. The tests provide a valuable way to assess and optimize the adhesion of metal contacts.

ABSTRACT Increasing silver prices and reducing silicon wafer thicknesses provide incentives for silicon solar cell manufacturing to develop new metallisation strategies that do not rely on screen printing and preferably reduce silver usage. Recently, metal plating has re‐emerged as a metallisation process that may address these future requirements. This paper reports on the evolution of metal plating techniques, from their use in early silicon solar cells, to current light‐induced plating processes. Unlike screen‐printed metallisation, metal plating typically requires an initial patterning step to create openings in a masking layer for the subsequent self‐aligned metallisation. Consequently, relevant recently‐developed dielectric patterning methods are also reviewed because, in many cases, the plating process must be adapted to the properties of the patterning method used. The potential of new light‐induced plating processes to form cost‐effective copper metallisation is supported by the recent activity in the development of metal plating tools for commercial silicon solar cell manufacture. Copyright © 2012 John Wiley & Sons, Ltd.

A method to laminate a thin monocrystalline Si layer to a conductive and fracture-resistant carrier such as steel has been developed, resulting in a practical design for high volume production of robust ultra-thin (10-20 μm) “kerfless” Si wafers. With this technology front and rear cell features based on the world-record PERL cell design have been integrated. A confirmed efficiency of 15.1% has been achieved on a 20-micron thick one-cm2 solar cell. This 15.1% is believed to be the highest confirmed efficiency achieved with ultra-thin silicon integrated with a conducting substrate.

Light-induced plating (LIP) can be used to form self-aligned metal contacts to selective-emitter silicon solar cells. Although LIP of metal contacts is attractively simple, it is difficult to directly measure parameters, such as the average plating current density, which are required if the plating chemistry is to be optimised. In this paper we describe the use of inductively-coupled plasma (ICP) spectrometry techniques to characterise LIP of laser-doped selective-emitter silicon solar cells. The average plating current density and the effect of different anode and cathode configurations on the aluminium dissolution from the rear surfaces of the solar cells were evaluated. The quality of LIP nickel layers was also characterised using ICP measurements for various critical parameters in the cell design, such as the conductivity of the laser-doped grooves and the lateral resistance of the emitter.

A method for spatially-selective etching of dielectric layers without the use of a mask has been developed at the University of New South Wales (UNSW). This 'direct etching' method, which was first implemented using inkjet printing, is now being further developed using Optomec's Aerosol Jet Printer (AJP), in order to achieve the patterning resolution and processing throughput required for commercial photovoltaic applications. Results presented in this paper show that the use of the AJP enables etched grooves as narrow as 15-20 mu m. Grooves can be etched in similar to 75 nm layers of SiO2, SiNx, SiONx and PECVD Al2O3 dielectric layers. Furthermore, the etching process can be tailored to different applications by varying processing parameters, such as the gas flow rates, platen movement speed and number of printing passes. Finally, the accurate alignment enabled by the AJP allows etched patterns to be formed in pre-patterned surfaces, a property that may find application in a number of selective-emitter solar cell designs which use aligned screen printing for metallization.

For many years, the selective emitter approach has been well-known to yield cell efficiencies well above those achieved by conventional screen-printed cells. A simple and effective way of forming a selective emitter can be achieved by laser doping to simultaneously pattern the dielectric with openings as narrow as 8 µm, and create heavy doping beneath the metal contacts. In conjunction with laser doping, light-induced plating (LIP) is seen as an attractive approach for forming metal contacts on the laser-doped regions, without the need for aligning masks or other expensive, long laboratory processes. As laser-doping is gaining increasing interests in the PV industry, selection of the most appropriate laser and processing conditions is important to ensure high yields in a production environment. In this work, we have identified a suitable laser that enables good ohmic contacts for a wide range of laser scan speeds. Sheet resistances of laser-doped lines as low as 2 ohms/sq was achieved at a scan speeds of <1 m/s, while a sufficiently high doping (∼20 ohms/sq) is still achievable at scan speeds up to 6 m/s. Optimization of the laser parameters in this work lead to a cell efficiency of 18.5% being achieved with the laser-doped selective emitter (LDSE) structure. The cell also has an excellent pseudo fill factor (pFF) of 82.3% and a local ideality factor n nearing unity. This indicates there is minimal laser-induced damage and junction recombination as a result of the laser doping process.