Alison Maree Wenham

Advanced Bulk Defect Passivation for Silicon Solar Cells

Through an advanced hydrogenation process that involves controlling and manipulating the hydrogen charge state, substantial increases in the bulk minority carrier lifetime are observed for standard commercial grade boron-doped Czochralski grown silicon wafers from 250-500 μs to 1.3-1.4 ms and from 8 to 550 μs on p-type Czochralski wafers grown from upgraded metallurgical grade silicon. However, the passivation is reversible, whereby the passivated defects can be reactivated during subsequent processes. With appropriate processing that involves controlling the charge state of hydrogen, the passivation can be retained on finished devices yielding independently confirmed voltages on cells fabricated using standard commercial grade boron-doped Czochralski grown silicon over 680 mV. Hence, it appears that the charge state of hydrogen plays an important role in determining the reactivity of the atomic hydrogen and, therefore, ability to passivate defects.

Rapid Stabilization of High-Performance Multicrystalline P-type Silicon PERC Cells

Light-induced or, more broadly, carrier-induced degradation (CID) in high-performance multicrystalline silicon (TIP mc-Si) solar cells remains a serious issue for many manufacturers, and the root cause of the degradation is still unknown. In this paper, the impact of firing temperature on the stability of lifetime test structures is investigated, and it is found that substantial CID can be triggered if peak temperatures exceed approximately 700 °C. We then investigate two pathways to stabilize the performance of industrially produced TIP mc-Si passivated emitter rear contact cells which have been fired at CID-activating temperatures (~740 °C-800 °C) currently required for silver contact formation. The first is a fast-firing approach, whereby it is demonstrated that an additional firing step at a reduced temperature after cell metallization can suppress the extent of Voc degradation by up to 80%. The second approach is the accelerated degradation and subsequent recovery of carrier lifetime through the use of high-intensity illumination during annealing at elevated temperatures. A 30 s process is found to suppress the maximum extent of degradation in Voc by up to 60% and up to 80% for longer processes. Ultimately, the results suggest that a combined approach of fast-firing and a high-intensity-illuminated anneal could achieve the best results in terms of Voc, stability.

Laser Enhanced Hydrogen Passivation of Silicon Wafers

The application of lasers to enable advanced hydrogenation processes with charge state control is explored. Localised hydrogenation is realised through the use of lasers to achieve localised illumination and heating of the silicon material and hence spatially control the hydrogenation process. Improvements in minority carrier lifetime are confirmed in the laser hydrogenated regions using photoluminescence (PL) imaging. However with inappropriate laser settings a localised reduction in minority carrier lifetime can result. It is observed that high illumination intensities and rapid cooling are beneficial for achieving improvements in minority carrier lifetimes through laser hydrogenation. The laser hydrogenation process is then applied to finished screen-printed solar cells fabricated on seeded-cast quasi monocrystalline silicon wafers. The passivation of dislocation clusters is observed with clear improvements in quantum efficiency, open circuit voltage, and short circuit current density, leading to an improvement in efficiency of 0.6% absolute.

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.

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.

Stability of hydrogen passivated UMG silicon with implied open circuit voltages over 700mV

Interstitial hydrogen is used to passivate defects in upgraded metallurgical grade (UMG) Czockralski silicon. It is observed that the quality of the UMG material can be improved progressively, with the passivated defects appearing to be stable to subsequent light soaking. New defects however continue to form for a prolonged period of light soaking, requiring subsequent further hydrogen passivation to restore the open circuit voltages to over 700 mV for the UMG wafers. By repeatedly applying the advanced hydrogenation process, the quality and stability of UMG wafers are improved to the point of being comparable to that for Czockralski wafers produced from semiconductor grade silicon purified by the Siemens Process.

19.1 % laser-doped selective emitter P-type multi-crystalline UMG silicon solar cell

In this paper standard screen printed solar cells and laser-doped selective emitter solar cells are fabricated on p-type multi-crystalline silicon wafers from three upgraded-metallurgical grade feedstock suppliers. A cell efficiency of UMG material is demonstrated above 17 % using standard screen printing technology and a full area aluminum back surface field. By employing a laser-doped selective emitter and self-aligned light-induced plated contacts, a 1-2 % absolute increase in cell efficiency is obtained with a peak efficiency of 19.1 %, when still employing a full area aluminum-back surface field.