Phillip Hamer

Manipulation of Hydrogen Charge States for Passivation of P-Type Wafers in Photovoltaics

Passivation of defects in silicon solar cells using hydrogen has long been an area of significant interest to the photovoltaic community. In this paper, we explore the importance of the charge states of hydrogen for passivation of defects in p-type silicon and how these charge states might be manipulated using illumination. We show that by using illumination during hydrogenation processes at temperatures between 475 and 625 K, the lifetime of wafers containing high concentrations of hydrogen can be strongly increased. The magnitude of the increase depends on temperature, with the most significant increase occurring at 545 K with samples under illumination showing an average effective lifetime of 167 μs, while samples without illumination had an average lifetime of 67 μs. This increase in the lifetime with illumination is explained in terms of how the electron quasi-Fermi energy and, hence, the relative concentrations of the hydrogen charge states respond to illumination at these temperatures. We show a correlation between the predicted charge states of interstitial hydrogen and the effective lifetimes of the wafers.

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.

Modelling of hydrogen transport in silicon solar cell structures under equilibrium conditions

This paper presents a model for the introduction and redistribution of hydrogen in silicon solar cells at temperatures between 300 and 700 °C based on a second order backwards difference formula evaluated using a single Newton-Raphson iteration. It includes the transport of hydrogen and interactions with impurities such as ionised dopants. The simulations lead to three primary conclusions: (1) hydrogen transport across an n-type emitter is heavily temperature dependent; (2) under equilibrium conditions, hydrogen is largely driven by its charged species, with the switch from a dominance of negatively charged hydrogen (H−) to positively charged hydrogen (H+) within the emitter region critical to significant transport across the junction; and (3) hydrogen transport across n-type emitters is critically dependent upon the doping profile within the emitter, and, in particular, the peak doping concentration. It is also observed that during thermal processes after an initial high temperature step, hydrogen prefer...

Ultrathin Silicon Solar Cell Loss Analysis

A detailed loss analysis is presented for a 15.9% large area ultrathin silicon (UTSi) solar cell. The loss analysis is based on a comprehensive study of the electrical and optical parameters of the champion solar cell. The results indicate that the UTSi solar cell has an efficiency potential of 19.9% using currently available technologies and is capable of achieving 22.2% efficiency in the long run.

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.