Understanding a critical defect affecting silicon wafers in many solar cells

(Nanowerk News) A University of New South Wales (UNSW) solar PV research team has demonstrated for the first time that this detrimental “boron oxygen complex” in silicon wafers arises from two distinct and directly measured “traps” (H375 and H400 traps) (IEEE Journal of Photovoltaics, "Electronic Properties of the Boron–Oxygen Defect Precursor of the Light-Induced Degradation in Silicon").
“Traps” are places where an electron can be found "trapped" within a semiconductor. These traps are highly undesirable because they prevent electrons from moving around quickly so that they can be used or collected; for example in a battery.

Key Facts

– What is the boron-oxygen related degradation effect in solar cells? As a natural part of their 20+ year lifecycle spent absorbing the light and heat of the sun, solar cells’ power generation capacity incrementally degrades over time as a result of a defect that is only observable once they are exposed to sunlight. An effect that is commonly referred to as light-induced degradation, or “LID”; resulting from a combination of boron and oxygen impurities in silicon, the materials used in solar cells.
– 95% of all solar panels are affected by the boron-oxygen complex, which has been studied for decades with the hope of eliminating or largely reducing its impact. It is not yet understood what exactly is causing the degradation, but some potential causes have been debated and recorded in the literature for over 40 years:
“Modules experience power loss rates of approximately 3% within the first year of usage. Thereafter, a phenomenon known as power stabilization is said to occur, which refers to lower levels of power loss in subsequent years of usage at rates typically around 0.8%. This indicates that the rates of degradation are most prominent initially.
LID of a PV module refers to the power loss and other loss of performance of crystalline p-type boron doped silicon solar cells after the first few hours upon exposure to sunlight. LID typically varies between 1-3%. The degradation of cells by sunlight is highly dependent on the quality of the wafer manufactured and is the result of a defect known as “boron oxygen complex”. This defect occurs owing to oxygen being trapped in the silicon as part of the “Czochralski process” during manufacture.”
– LID is attributed to a combination of boron and oxygen impurities in silicon (“a boron-oxygen complex”). However, many of its properties and nature are still unknown to this day given the challenges brought by its microscopic observation and characterisation. In fact, there have not been any reports on direct chemical observation of this complex, and scientists have been relying on several indirect methods to get an understanding of the formation mechanism and calculating some rough estimates of its impact on efficiency.
These two traps were observed via a direct spectroscopy deep level transient spectroscopy (DLTS), which is a very powerful and unique technique used in the semiconductor industry for measuring the electrical activity of impurities in a given material. Since solar cells are electrical devices, making sure there are no electrically active impurities/defects in the silicon is of utmost importance.
With the combination of DLTS and other spectroscopy techniques, the researchers have also managed to directly determine the electron capture cross-section of the “boron oxygen complex” that causes LID, identifying how large the trap is where electrons can be stuck: a key property for the estimation of how harmful an impurity is for the solar cell performance.
Source: University of New South Wales
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