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 ‘light‐ and 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.

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.

LONGi Solar Energy Technology Co. Ltd. has achieved 23.83% for a commercial p‐type 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.

Tunnelling oxide passivated contact (TOPCon) solar cells are gaining significant commercial interest, due to the potential for high efficiency. Industrially, this passivated contact scheme is typically coupled with an n‐type Czochralski (Cz) wafer. JinkoSolar Holding Co., Ltd. is one of the leading manufacturers that are producing n‐type TOPCon solar cells (referred to as ‘HOT’ cells) on a commercial scale. In this work, the influence of a post‐cell hydrogenation step, using illumination from an LED light source, on the performance and stability of n‐type TOPCon solar cells is investigated. The incorporation of this additional hydrogenation treatment led to an average efficiency enhancement of 0.64%_abs on a batch of 50 cells made in an industrial environment. This significant improvement was caused by a 6.9 mV and 1.04%_abs increase in open‐circuit voltage (V_OC) and fill factor (FF), respectively. We also assessed the stability and found almost no light‐ and elevated temperature‐induced degradation (LeTID) in hydrogenated n‐type TOPCon cells. Testing at 70 ± 5°C under 1‐sun illumination revealed that the maximum degradation is limited to 0.06%_rel. Following further stability testing, the efficiency increased beyond the initial value, up to 0.4%_rel increase after 120 h. By incorporating this hydrogenation process into the production, an average line efficiency of 24.08% and V_OC of 707.5 mV was achieved. The champion cell from the batch displayed an efficiency of 24.58%, as certified by measurement at the Chinese Academy of Sciences.