ngjia Ji

Mass production of the innovative PLUTO solar cell technology

Following 6 years of intensive R&D at Suntech Power, the world record holding PERL cell design from The University of New South Wales (UNSW) has been successfully commercialized with product sales of the PLUTO technology to commence in mid 2009. This innovative new technology is having a major impact on production activities and plans for Suntech Power, the worlds largest silicon solar cell manufacturer, with more than 100MW production capacity already installed, increasing to 300MW by the end of 2009. GW levels of PLUTO production capacity are planned by the end of 2011. Successful commercialization has required the simplification or elimination of all the expensive and sophisticated processes and materials from the record PERL cells, while simultaneously achieving high device efficiencies, production yields and throughput. A simplified version of the PLUTO technology can be easily retrofitted onto existing screen-printing production lines, with similar costs per unit area to conventional screen printed solar cells but with a 10–15% performance advantage over the latter fabricated on juxtaposed production lines using essentially the same equipment, wafers and materials.

Development of a 16.8% Efficient 18-μm Silicon Solar Cell on Steel

Thin crystalline silicon solar cells have the potential to achieve high efficiency due to the potential for increased voltage. Thin silicon wafers are fragile; therefore, means of support must be provided. This paper reports the design, development, and analysis of an 18-μm crystalline silicon solar cell electrically integrated with a steel alloy substrate. This ultrathin silicon is epitaxially grown on porous silicon and then transferred onto the steel substrate. This method allows the independent processing of each surface. The steel substrate enables robust handling and provides a conductive back plane. Three groups of cells with planar and textured structures are compared; significant improvements in Jsc, Voc, and fill factor (FF) are achieved. The best cell shows an efficiency of 16.8% with an open-circuit voltage of 632 mV and a short-circuit current density of 34.5 mA/cm2.

15%, 20 Micron thin, silicon solar cells on steel

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.

High efficiency at module level with almost no cell metallisation: Multiple wire interconnection of reduced metal solar cells

Perpendicular multiple busbar wires have proven an effective way of interconnecting standard screen printed solar cells with high reliability and low cell to module loss. The technology is also an effective way to interconnect plated solar cells, where conventional soldered interconnects may be problematic. However, the full benefits of this interconnection technology can be fully realized on plated cell structures with drastically reduced plated metallization. This paper presents a new selective emitter plated cell structure with metallization reduced to around 1 μm thickness, interconnected using perpendicular multiple busbar wires. Metal usage on the cell is reduced by more than 90% compared to conventional plated or screen print cells and the use of Ag almost eliminated, while high efficiency at the module level is achieved along with environmental durability.

Rapid mitigation of carrier-induced degradation in commercial silicon solar cells

We report on the progress for the understanding of carrier-induced degradation (CID) in p-type mono and multi-crystalline silicon (mc-Si) solar cells, and methods of mitigation. Defect formation is a key aspect to mitigating CID. Illuminated annealing can be used for both mono and mc-Si solar cells to reduce CID. The latest results of an 8-s UNSW advanced hydrogenation process applied to industrial p-type Czochralski PERC solar cells are shown with average efficiency enhancements of 1.1% absolute from eight different solar cell manufacturers. Results from three new industrial CID mitigation tools are presented, reducing CID to 0.8–1.1% relative, compared to 4.2% relative on control cells. Similar advanced hydrogenation processes can also be applied to multi-crystalline silicon passivated emitter with rear local contact (PERC) cells, however to date, the processes take longer and are less effective. Modifications to the firing processes can also suppress CID in multi-crystalline cells during subsequent illumination. The most stable results are achieved with a multi-stage process consisting of a second firing process at a reduced firing temperature, followed by extended illuminated annealing.

Improved adhesion for plated Solar cell metallisation

Experts predict that copper plated metal contacts will eventually become the dominant metallisation for silicon wafer-based technologies once several key issues are solved. Of particular importance is the adhesion strength and hence durability of such plated contacts with many of the largest cell manufacturers currently nervous about considering such metallisation for large-scale manufacturing due to concerns in this area. A new approach for enhancing the adhesion strength for plated contacts involves establishing laser-machined anchor points in the silicon surface which when plated act to enhance both the ohmic contact and the mechanical adhesion strength for the metallisation. Cells manufactured on a production line using this innovation typically lose 0.1% in efficiency in absolute terms relative to identically manufactured cells that do not use the anchor points. BP Solars Saturn cells using a similar approach have demonstrated that such plated contacts are at least as durable if not more durable than screen-printed contacts installed at the same time 20 years ago.

The investigation on the quality of aluminum rear emitter for n-type solar cells

Since rear emitter characterization is the crucial factor for aluminum back junction n-type cells, the emphasis of this paper is focused on the quality of this Al-Si alloy layer. Diffusion profile test is used to describe the diffusion of Al in the silicon rear side. It is found that the Al-Si alloy layers thickness of about 6 μm should be better. In order to get a thicker thickness, a higher temperature or a slow belt velocity is needed in the firing process. However, it is found that a higher temperature is not suitable, which will cause a higher concentration of Al atoms in the layer. A lower firing speed almost only thickens the alloy layer. A high Voc of 650 mV and a high fill factor of 0.818 have been obtained by slowing down the firing speed.

The investigation on the front surface oxidation for aluminum rear emitter n-type solar cells

Since front surface field characterization is one of the crucial factors for aluminum back junction n-type cells, the emphasis of this paper is focused on the quality of this n+ layer. Oxidation process is wildly used for high efficiency solar cells. Higher temperature and thicker SiOx layers can provide a better passivation effect. However oxidation process can also reduce the surface concentration of phosphorus, and thicken the n+ layer. Both of a too low surface concentration and a thick n+ layer are not needed. After the optimization, the oxidation temperature and time are fixed on 900 □ and 15 min, respectively. The passivation effect is well-done; the minority lifetime of the wafers after oxidation is nearly 400 μs. And the sheet resistance of about 150 Ω/□ is better for the cells fabrication. A high open-circuit voltage of over 645 mV and a high fill factor of over 0.80 have been obtained.

LEDs for the Implementation of Advanced Hydrogenation Using Hydrogen Charge-State Control

Light-induced degradation (LID) of p-type Cz solar cells has plagued the industry for many decades. However, in recent years, new techniques for solving this LID have been developed, with hydrogen passivation of the boron-oxygen defects appearing to be an important contributor to the solution. Advanced hydrogenation approaches involving the control of the charge state for the hydrogen atoms in silicon to enhance their diffusivity and reactivity are developed and evaluated in this work for commercial application using a prototype industrial tool in conjunction with solar cells manufactured on commercial production lines. This prototype tool, unlike the previous successful laser-based laboratory approaches, is based on the use of LEDs for controlling the charge state of the hydrogen atoms. The illumination from the LEDs is also used in this work to passivate process-induced defects and contamination from the respective production lines with significant improvements in both efficiency and stability. The results indicate that the low-cost LED-based industrial tool performs as well as the laser-based laboratory tool for implementing these advanced hydrogen passivation approaches.

Copper plated contacts for large-scale manufacturing

Copper plated contacts have the potential to become the dominant front metallization technology over silver contacts if several key issues can be solved. These are addressed in this work. Through large batches of cells produced at Suntech on Plutos 0.5 GW production line, it is shown that: adhesion concerns can be over come by optimized nickel seed layers or laser formed anchor points; long-term reliability can also be solved by an optimized nickel seed layer in conjunction with deep laser doping; and an absence of large-scale high-throughput inline plating equipment is no longer an issue. Average device efficiencies are 19-20% for Al-BSF and over 20% for PERC structures with record open circuit voltages but with a small efficiency loss when anchor points are applied.