Carrier induced recombination

Light-induced degradation (LID), also known as Carrier-Induced Degradation (CID), is a major problem in the photovoltaics industry and is intensively researched around the world. Light-induced degradation has been shown to causes losses of up to 10% relative in solar efficiency (~1-2% absolute) on Czochralski-grown [1] and up to 16% relative in multicrystalline silicon solar cells [2]. The underlying cause for CID in monocrystalline silicon is relatively well understood, while there is still a lot of work ongoing to identify the cause and mitigate the cause for CID in multicrystalline silicon.

CID in monocrystalline silicon

Boron Oxygen Defect formation

In boron-doped p-type Czochralski (Cz) grown silicon wafers, the metastable boron-oxygen (BO) defect formation is the primary source of carrier induced degradation known widely throughout the industry. Interstitial oxygen, unintentionally introduced into the silicon in the Cz process, react with the substitutional boron atoms which are introduced in the Cz process to obtain p-type silicon. The formation of the BO complex can occur at very low illumination intensities of < 0.02 Suns and can cause severe degradation in solar cells within 48 hours of illumination at room temperature. This formation has also been demonstrated using injection of current in the dark, thus it is more correct to refer to it as carrier-induced degradation (CID). The defect is known to be metastable due to this ability to dissociate when annealed at elevated temperatures, allowing the process to be reversible.

Due to the detrimental impacts of the BO defect on cell efficiency, various methods of defect suppression, elimination and passivation have been developed and some of them are currently used by the leading solar cell manufacturers.

Mittigating CID in monocrystalline silicon

Magnetic Czochralski

One method of eliminating BO defect formation is by significantly reducing the concentrations of oxygen which is an appealing approach as the recombination strength of the BO defects scales with the square of the oxygen concentration and only linear with the boron concentration. The magnetic Czochralski (MCz) is a technique used to increase the dopant homogeneity within the ingot by modifying the fluid motion of the melt. This technique has also been shown to help reduce incorporated interstitial oxygen concentrations in the melt to as low as 0.54 ppm in contrast to 13.14 ppm in standard Cz. By altering the fluid motion of the silicon melt using magnetic fields, we can selectively increase the proportion of oxygen transport via convection to the melt-gas interface as opposed to the melt-crystal interface where the oxygen can be incorporated into the crystal. At the melt-gas interface, oxygen evaporates as silicon dioxide (SiO2) gas and is blown away by the inert argon gas. Such reduction in [O2i] is demonstrated to suppress the formation of B-O related degradation [3,4].

Gallium Doping

Another method of preventing boron-oxygen formation is to change the dopant from boron to another Group III elements such as gallium (Ga). By using Ga doped p-type wafers, we can eliminate B-O defect entirely [3]. The use of such method for preventing CID in silicon, however, is rarely used due to the disadvantages of Ga-doping in practice. Ga has a relatively low segregation coefficient (0.008) in Si which implies that Ga concentration in the melt is significantly higher than in the solid during the crystallisation process. As a result, a significant gradient in resistivity is present along the length of the ingot, much of which cannot be used, thus leading to reduced yield.

Conventional Hydrogen Passivation of Boron Oxygen defects

A more popular method is the hydrogen passivation of BO defects. Hydrogen is commonly found in modern silicon solar cells after metallization co-firing as a result of hydrogen release from the antireflective silicon nitride (a-SiN:H) dielectric layers. A conventional method using low-intensity illumination (approx. 0.1 W/cm2) at elevated temperatures (between 100 – 250 °C ) have been demonstrated [5,6] in what is known as regeneration or permanent deactivation process.

Advanced Hydrogenation

The Advanced hydrogenation of BO defects was developed at UNSW Sydney [7]. It is well known that hydrogen can occupy three different charge states defined by the number of electrons within its outer shell; these being a positive (H+), a neutral (H0) and a negative (H) charge state. These charge states play an important role in determining the diffusivity and reactivity of hydrogen. It has been shown that the fractional concentration of hydrogen in each charge state is determined by the position of the Fermi level. Consequently, hydrogen predominantly exists primarily as H+ in p-type silicon at low temperatures. Likewise, in n-type silicon hydrogen exists in the H form and in both cases the H0 fraction will be very small. The diffusivity of both H+ and H is significantly lower than H0 (approximately five orders of magnitude) as due to electrostatic interactions with the charged impurities and the silicon lattice. [8] In addition, in p-type silicon the existence of H+ concentrations is also less favourable for the passivation of positively-charged defects such as Fei+, Cr+ and especially, B-O. T

The Advanced Hydrogenation process addresses both the challenges regarding the hydrogen diffusivity as well as the reactivity. This is done using high-intensity illumination at elevated temperatures which increases the concentration of minority carriers (i.e. electrons in p-type materials) via photo generation. This excess concentration in electrons increases the density of hydrogen in the H state as well as in the H0 state:

By transitioning through the H0 charge state, hydrogen is allowed to diffuse deep into the silicon lattice while in the Hstate, it is capable of bonding to and passivating positively charged defects. The patented Advanced Hydrogenation process has been demonstrated permanently deactivate BO defects within 8 s and has since been implemented into commercially available tools [9–11].

CID in Multicrystalline Silicon

CID in multicrystalline silicon (mc-Si) wafers was first observed by Ramspeck et al. in 2012 [12] in the mc-Si PERC development by Schott Solar, and it has been a hot topic in the photovoltaic community in recent years. Experiments conducted on Ga doped mc-Si wafers demonstrated that this CID could not be attributed to the BO defect. It was found that the multi CID is activated by processing, more precisely by a high-temperature firing step in combination with a hydrogen-containing dielectric coating (e.g. SiNx:H and AlOx:H). Unlike the Boron-Oxygen defect, which can be formed at room temperature in only 48 hours, the defect formation time-scale at room temperature for the multi-defect has been found to be over a thousand hours. Therefore, various studies have used elevated temperatures of approximately 75 °C to study this defect. This degradation mechanism was consequently dubbed “Light and Elevated Temperature Induced Degradation (LeTID)” by Hanwha Q Cells [2,13–15]. Even at elevated temperatures, the defect formation can take more than a 100 hours with subsequent recovery taking another 700 hours. Recovery in the field might take up to ten years, making it crucial that an industrially applicable mitigation approach is developed as soon as possible. Although there have been many attempts to identify the defect or develop a method of elimination and suppression, the cause of the defect is still yet to be identified.

 

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