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.
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.
This paper discusses the role hydrogen plays in degradation of silicon solar cells and modules. Slightly unorthodox, it presents a high level view of the latest research findings and theories of Professor Stuart Wenham before he suddenly passed away. This paper includes 6 main parts: (1) a brief introduction including Professor Wenham's prior hydrogen related work; (2) evidence for the role of excess hydrogen in Light-and elevated Temperature-Induced Degradation (LeTID) (3) discussion of Prof. Wenham's 'Bucket Theory' -how hydrogen could move and change to cause LeTID or Hydrogen-Induced Degradation (HID); (4) how hydrogen can cause recombination including Prof. Wenham's realization that hydrogen can cause recombination on its own (Hydrogen-Induced Recombination); (5) demonstration that appropriate control of hydrogenation processes can enable wafers and solar cells that are stable against LeTID/HID; and (6) implications and testing.
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