Overcoming the Challenges of Hydrogenation in Silicon Solar Cells

Abstract

The challenges of passivating defects in silicon solar cells using hydrogen atoms are discussed. Atomic hydrogen is naturally incorporated into conventional silicon solar cells through the deposition of hydrogen-containing dielectric layers and the metallisation firing process. The firing process can readily passivate certain structural defects such as grain boundaries. However, the standard hydrogenation processes are ineffective at passivating numerous defects in silicon solar cells. This difficulty can be attributed to the atomic hydrogen naturally occupying low-mobility and low-reactivity charge states, or the thermal dissociation of hydrogen–defect complexes. The concentration of the highly mobile and reactive neutral-charge state of atomic hydrogen can be enhanced using excess carriers generated by light. Additional low-temperature hydrogenation processes implemented after the conventional fast-firing hydrogenation process are shown to improve the passivation of difficult structural defects. For process-induced defects, careful attention must be paid to the process sequence to ensure that a hydrogenation process is included after the defects are introduced into the device. Defects such as oxygen precipitates that form during high-temperature diffusion and oxidation processes can be passivated during the subsequent dielectric deposition and high-temperature firing process. However, for laser-based processes performed after firing, an additional hydrogenation process should be included after the introduction of the defects. Carrier-induced defects are even more challenging to passivate, and advanced hydrogenation methods incorporating minority carrier injection must be used to induce defect formation first, and, second, provide charge state manipulation to enable passivation. Doing so can increase the performance of industrial p-type Czochralski solar cells by 1.1 % absolute when using a new commercially available laser-based advanced hydrogenation tool.

Development of advanced hydrogenation processes for silicon solar cells via an improved understanding of the behaviour of hydrogen in silicon

“…[161][162][163] Furthermore, thermal processes are also known to precipitate iron. 158,164,165 Another defect of key importance for silicon solar cells with contradictions in the literature is the boron-oxygen (B-O) complex, 60,[166][167][168] responsible for carrier-induced degradation in ptype Czochralski (Cz) silicon. 169,170 The challenge of hydrogen detection has no doubt played a role in this controversy, with estimated defect concentrations in the range of 10 11 to 10 13 /cm Subsequent work has provided further evidence for the role of hydrogen, by identifying the influence of the hydrogen concentration and thickness of dielectric layers in the passivation reaction.…”

“…212 The most recent study also identified the differences in passivation with illuminated versus dark hydrogenation processes for laser-induced defects, highlighting the impact of advanced hydrogenation processes for the heavily doped silicon regions. 60 Additional low-temperature hydrogenation processes have also proven extremely beneficial for dislocation clusters in cast-mono material, with efficiency enhancements of up to 2% absolute. 204,217,218 However, the specific benefit of illumination in these processes remains unclear.…”

“…The difficulty in passivation is complicated by many factors including the F I G U R E 2 1-sun implied open circuit voltage (iV OC ) maps of p-type multicrystalline silicon (A) before and (B) after a belt furnace firing process [Colour figure can be viewed at wileyonlinelibrary.com] defect type, whether it is structural, process-induced, or carrierinduced, and hence when the defects are introduced into the device. 60 Particularly, over the past 10 years, UNSW has spent significant research efforts focused on the development of hydrogenation processes for silicon solar cells. In this work, we discuss the properties of hydrogen in silicon and the development of new advanced hydrogenation processes that can improve the performance of silicon solar cells.…”

24.58% efficient commercial n‐type silicon solar cells with hydrogenation

“…This is also the case for PERC solar cells, where high-temperature firing allows hydrogen to move from the dielectric layers to passivate both bulk and surface defects. 21,22 In p-type PERC solar cells, it is typical to also include a secondary dedicated hydrogenation step on the cell after metallisation. This approach is often referred to as an 'advanced hydrogenation process' and involves carrier injection, either via illumination or direct current injection.…”

RACI and Academy of Science Awards 2017–18

“…Additional low-temperature hydrogenation processes implemented after the conventional fast-firing hydrogenation process are shown to improve the passivation of difficult structural defects. [3] Liam Burt (University of Tasmania) was awarded the Masson Memorial Scholarship, and in a Highlight co-written with his supervisor, Curtis Ho, on high oxidation states of organopalladium(IV) species, it is recalled that the synthetically versatile palladium-catalysed couplings usually employ palladium (0)/palladium(II) catalysis, and there is much less knowledge of palladium(II)/palladium(IV) pairs. The Highlight focuses on seminal discoveries in organopalladium(IV) chemistry and their applications in palladium catalysis, particularly C-H activation.…”