Hydrogen induced contact resistance in PERC solar cells

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LONGi Solar Energy Technology Co. Ltd. has achieved 23.83% for a commercial ptype Cz PERC cell. From a batch of over 40 000 cells, the average line efficiency achieved was 22.5%. R&D studies investigating hydrogenation and degradation show the importance of hydrogenation processes for efficiency improvements and controlling the hydrogen to prevent light-induced degradation. Such degradation is shown to appear very differently under different illumination and temperature conditions. This degradation impacts V OC , I SC , and especially fill factor. Current injection and thermal anneal can be used to recover the degradation, but the recovery may not be stable. Reducing the hydrogen content within the cell is shown to minimise degradation without sacrificing performance, provided that enough hydrogen is retained to passivate boron-oxygen defects. 1 | INTRODUCTION The advanced hydrogenation technology developed at University of New South Wales (UNSW) Sydney is focused on controlling the hydrogen in silicon solar cells to enable enhanced bulk defect passivation and minimise light-induced degradation (LID). 1 Typically studied on lab produced samples and cells, this work with LONGi Solar Energy Technology Co., Ltd. investigates the advanced hydrogenation technology on LONGi's high efficiency p-type Cz commercial cells. LONGi focuses on the R&D, production and sales of monocrystalline silicon wafers, and cells and modules, and at present (Q1, 2019), has 5.5 GW cell and 6.5 GW module capacities. LONGi has always attached great importance to R&D. The cost of R&D investment in the last 5 years has reached 2.38 billion Chinese RMB. The cell R&D department has always been committed to the research and development of high-efficiency monocrystalline solar cell technology. Among them, Cz PERC cells have repeatedly broken the world record; here, LONGi has achieved an efficiency of 23.83%. This cell performance is already almost equal to a 2017 forecasted industrial efficiency prediction of 24% 2 and on the path towards commercial implementation of the 25% efficient PERL lab cell of 1999. 3 LONGi has a long-term partnership with the UNSW Sydney and has in-depth collaborative research on advanced hydrogen passivation technologies to improve the performance and long-term stability of silicon solar cells. 1 In Cz silicon material, hydrogenation is very effective for passivating boron-oxygen(BO) defects that otherwise cause LID. 4,5 The BO defect system can be understood using a three-state model of (1) defect precursors, that under light form (2) active BO defects, and can be passivated by hydrogen to form (3) passivated BO defects. 6,7 More recently, there has been a phenomenon of a new LID caused by hydrogen, which is commonly referred to as light and elevated temperature-induced degradation (LeTID), 8,9 but perhaps more Stuart Wenham equally contributed to this work.

Section: Lid Under Varied Illuminationsupporting

confidence: 58%

23.83% efficient mono‐PERC incorporating advanced hydrogenation

Chen

Tong

Zhu

et al. 2020

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“…228,229,231,232,244,248,249 Attempts to eliminate the degradation in the bulk can result in the migration of hydrogen to the surfaces, leading to increased contact resistance. 82,94,95 There are a number of mechanisms that could lead to increased contact resistance such as counter doping of the heavily doped region, or the passivation of dopants or defects at the metal/silicon interface, in what is now becoming known as "hydrogen-induced degradation". 250 After such processes, the contact resistance can be modulated at room temperature with an electric field, consistent with a presence of a highly mobile charged species such as hydrogen.…”

Section: Advanced Hydrogenation Of Silicon Solar Cellsmentioning

confidence: 99%

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

Hallam

Hamer

Wenham

et al. 2020

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The understanding and development of advanced hydrogenation processes for silicon solar cells are presented. Hydrogen passivation is incorporated into virtually all silicon solar cells, yet the properties of hydrogen in silicon are still poorly understood. This is largely due to the complex behaviour of hydrogen in silicon and its ability to exist in many different forms in the lattice. For commercial solar cells, hydrogen is introduced into the device through the deposition of hydrogen-containing dielectric layers and the subsequent metallisation firing process. This process can readily passivate structural defects such as grain boundaries but is ineffective at passivating numerous defects in silicon solar cells such as the boron-oxygen complex, responsible for light-induced degradation in p-type Czochralski silicon. This difficulty is due to the need to first form the boron-oxygen defect and also due to atomic hydrogen naturally occupying low-mobility and low-reactivity charge states. However, these challenges can be overcome using advanced hydrogenation processes incorporating excess carrier generation from illumination or current injection that increase the concentration of the highly mobile and reactive neutral charge state. As a result, after fast firing, additional low-temperature advanced hydrogenation processes incorporating illumination can be implemented to enable the passivation of difficult defects like the boron-oxygen complex. With the implementation of such processes for industrial silicon solar cells, efficiency improvements of 1.1% absolute can be obtained.

“…Several postprocessing methods have been introduced to suppress LeTID, all of which involve annealing of the finished solar cell either under illumination, 14,15,17,20 with current injection 2,15,121 or in darkness, 41,73,88,122 demonstrating that there are likely two paths to mitigating the LeTID defect, through the injection of carriers at elevated temperatures and through a purely thermal route. Annealing under laser illumination to accelerate the degradation and regeneration process was proposed by both Payne et al and Krauß et al, in an approach similar to that proposed for rapid mitigation of the BO defect in the past.…”

Section: Mitigation Of Letidmentioning

confidence: 99%

“…41,73,88 Similar to the approach in the belt-furnace, dark annealing has been shown to result in a drop in device FF for longer durations and higher temperatures. 122 To the avoid the drop in device FF, Sen et al 127 proposed a possible alternative method whereby the anneal is performed prior to metal contact firing. This approach has shown to be effective on lifetime test structures but has yet to be demonstrated on cells, and the effect of changes in density of the SiN x :H layer after annealing on the cofiring process may need to be considered.…”

Section: Mitigation Of Letidmentioning

confidence: 99%

Progress in the understanding of light‐ and elevated temperature‐induced degradation in silicon solar cells: A review

Chen

Contreras

Ciesla

et al. 2020

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At present, the commercially dominant and rapidly expanding PV-device technology is based on the passivated emitter and rear cell (PERC) design developed at UNSW. However, this technology has been found to suffer from a carrier-induced degradation commonly referred to as 'lightand elevated temperature-induced degradation' (LeTID) and can result in up to 16% relative performance losses. LeTID was recently shown to occur in almost every type of silicon wafer, independent of the doping material. Even though the degradation mechanism is known to recover under normal operation conditions, it is a lengthy process that drastically affects the energy yield, stability and, ultimately, the levelized cost of electricity (LCOE) of installed systems.Despite the joint effort of many research groups, the root cause of the degradation is still unknown. Here, we provide an overview of the existing literature and describe key LeTID characteristics and how these have led to the development of various theories of the underlying mechanism. Further, given the continuously appearing and strong evidence of hydrogen involvement in LeTID, many mitigation methods concerning hydrogenation have been suggested. We discuss such reported methods, bearing in mind crucial consumer necessities in terms of sustained cell performance and minimised LCOE.