Investigation of defects and interfaces in silicon wafers

Defects in Czochralski n-type wafers

Striations can lead to up to 1% absolute efficiency loss for n-type Czochralski (Cz) solar cells. As n-type solar cells are predicted to take an increased market share, the striation defects have gained interest not only from research institutes but also from industry.
In this project, we use temperature dependent photoluminescence (PL) spectra and temperature and injection dependent lifetime spectroscopy to investigate the striation defect. A sub-bandgap PL peak related to the striation defect was identified. The intensity of this PL peak is found to increase with decreasing temperature, whereas the band to band PL intensity shows the opposite temperature dependency. From a mapping of the sub-bandgap PL, it was found that the defect is present throughout the wafer with varying concentration.

(a) PL image of a n-type Cz wafer (half of 6-inch pseudo square) suffering striation defect; (b) Temperature dependent PL spectra of the wafer.

Defects in mono-cast wafers

The sub-bandgap photoluminescence PL arising from dislocations in crystalline silicon (known as “D-lines”) has been studied for over half a century. However, many properties of the D-lines such as the defect parameters and the underlying recombination mechanisms are poorly understood. In this project, we perform both temperature-dependent and injection-dependent hyperspectral mapping and apply this to a cast-mono silicon sample held at room-temperature and above. We parameterise the energy levels and defect densities of the D-lines in this sample. We demonstrate for the first time that the D1 line in silicon wafers originates from the donor-acceptor pair recombination mechanism.

(a) Normalised density plot of Ed vs Ea. The white points are the measurement coordinates. The dashed, red line is a linear fit to the data. (b) Histogram of Ea. (c) Histogram of Ed.

Boron-oxygen defect

Light induced degradation in boron doped Czochralski silicon wafers is widely speculated to be related to the formation of a recombination complex involving boron and oxygen. Although the defect has been studied for decades, disagreements on its nature, formation mechanism, and recombination activity still exist. In this project, we use injection dependent lifetime spectroscopy with temperature and doping variation to extract the electrical parameters of the defect. We demonstrated for the first time that the Boron-Oxygen defect should have more than one energy level in the bandgap. The common fitting using two independent single-level defects is incorrect and can lead to significant error in the defect parameterisation. It is also found that a wide range of doping variation is necessary for an accurate determination of the defect parameters.

The measured Boron-Oxygen defect associated lifetime in a compensated n-type silicon wafer at 303 K and 343 K plotted against the ratio of hole and electron concentration. The concave shape of the curve cannot be explained by the presence of two single-levels defects.

Traps

Traps are often defined as defects with very low recombination rates. Minority carrier traps can result in high apparent low-injection lifetime when using photoconductance-based measurements. On the other hand, majority carrier traps can lead to negative photoconductance signal.
In last few years, we developed a sensitive photoconductance decay (PCD) measurement system that allows investigation of traps. We also developed analytical methods to determine their electrical parameters.
In PCD measurements the focus is on the photoconductance signal after relatively long time from excitation, unlike lifetime measurements that use the signal immediately after (or even during) the excitation.
Recently, using PCD measurements we identified a correlation between minority carrier traps and the well-known boron-oxygen defect. We clearly demonstrated that there are traps in the annealed state that are removed after the degradation process. It seems that we detected the precursor of the boron-oxygen defect. Note that such observation is not possible with lifetime measurement, simply because the precursor of the defect does not limit the lifetime. The conclusions from this study is far beyond boron-oxygen defect. Our results suggest that defects can be investigated even before they affect the lifetime (in their non-recombination active form). We are now using this method to investigate precursors of other defects. We also improve our analytical models to consider more complex trap configurations.

(a) Lifetime measurements and (b) photoconductance decay measurements of a boron-doped Czochralski sample after dark annealing and degradation cycles. (c) Evolution of the trap and defect density after various light soaking durations under 1 sun illumination at 60 °C.

Temperature dependence of surface recombination

Corona-charged dielectric films have been shown to provide an excellent surface passivation by means of field effect for silicon solar cells. This technique has been used to investigate the electrical properties of silicon-dielectric interfaces at room temperature. However, little is known about the interface electrical properties as a function of temperature. As the increase in temperature leads to a significant variation in the silicon solar cells operating characteristics, it is vital to get insight into the temperature dependent device properties of the cells. In this project, for the first time, we investigate the electrical properties of the silicon/silicon-dioxide interface, under the influence of a large fixed-charge (provided by a corona discharge), via temperature and injection-dependent lifetime spectroscopy. The passivation quality of the corona-charged silicon dioxide is found to improve with increasing temperature for moderate temperatures. At high temperatures, a non-uniform degradation is observed across the wafers, which is ascribed to corona charge leakage. The leakage of the corona charge as a function of temperature is quantified by fitting the temperature and injection-dependent maximum surface recombination obtained from lifetime measurements.

Temperature and injection dependent effective lifetime measurements for an n-type silicon wafer passivated with 100 nm thick SiO2 layer (a) with and (b) without corona charge. Ratio photoluminescence images between different temperature-dependent measurements (c).

Post-mortem of PV modules

In this project, we are developing a “post-mortem” procedure for analysing degradation of fielded photovoltaic modules.
We investigate cell fragments cored from various fielded modules using a wide range of characterisation tools, such as current-voltage (I-V), Suns-VOC, external quantum efficiency (EQE), micro-photoluminescence (µPL), photoluminescence (PL) and electroluminescence (EL) imaging and many more.
We started the project investigating two silicon heterojunction (SHJ) modules, one operated in the field for a decade and one stored in the dark as a control. We find that both the front surface passivation and bulk have likely been degraded. Through comparisons of spectral photoluminescence emissions, we conclude that although an increase in density of the pre-existing types of radiative defects is possible, it seems that any new defect types are purely non-radiative.

Typical average spectra of four heterojunction samples, where C stands for “control” and F stands for “field”.

Defects in float-zone wafers

Float-zone silicon is usually assumed to be bulk defect-lean and stable. However, recent studies have revealed that detrimental defects can be thermally activated in float-zone silicon wafers and lead to a reduction of carrier lifetime by up to two orders of magnitude. In this project, we are using a robust methodology which combines different characterisation techniques and passivation schemes to provide new insight into the origin of degradation of float-zone silicon after annealing at 500 °C. Carrier lifetime and photoluminescence experiments are performed with temporary room temperature surface passivation which minimises lifetime changes that can occur during passivation processes involving thermal treatments. Temperature and injection dependent lifetime spectroscopy is performed with a more stable passivation scheme, with the same samples also being studied by deep level transient spectroscopy.

Calibrated PL images for float zone wafers with room temperature superacid-derived surface passivation: (a) not-annealed wafer and (b) annealed at 500 °C for 30 min.

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