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What is the recombination lifetime?
When light strikes silicon, photons with energy greater than a threshold energy, the bandgap of silicon, are absorbed in the silicon creating excess charge carriers. These carriers last for a characteristic time equal to their “lifetime” before falling back into their low-energy state. The process of falling back to the low-energy state is referred to as recombination. So the characteristic time it takes an excess carrier to fall back into its lower energy state is called the recombination lifetime.
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How does the solar cell efficiency depend on the lifetime?
If the lifetime of the excess carrier is longer than it takes to cross a wafer thickness, the transit time, then most of the photogenerated excess carriers can be collected as current from a solar cell. If the lifetime in the material is much longer than the transit time for the wafer, then the current can be collected at higher voltages, with each higher lifetime corresponding to a higher voltage at the same current collection fraction. With all other design elements of the solar cell equal, solar cells with higher carrier recombination lifetimes will have higher efficiencies. High-efficiency solar cell designs are critically dependent on the carrier lifetime. Standard solar cells are dependent up to a point, and then other losses tend to cause the efficiency gains to plateau with additional increases in lifetime.
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What determines the lifetime in silicon?
The lifetime in bulk silicon is determined by the chemical purity of the silicon and the details of how the crystal was grown. Single crystalline silicon has a nearly perfect structure with each atom in its optimal place in the lattice structure. This crystalline material can have very high lifetime if it is sufficiently pure. Multicrystalline silicon is less expensive to grow, but has many crystalline defects, such as grain boundaries, and dislocations. These defects result in lower lifetimes than if the same starting material had been used to grow a single crystal. Metallic impurities are especially bad for lifetime, and have been studied widely. For example, iron contamination at one part per 1010 can change the lifetime of a silicon crystal by a large amount. Unfortunately, iron is a common contaminant due to its use in the stainless steel for parts of most of the machines that grow and process silicon. For wafers, the measured lifetime is often determined by surface effects, and the quality of the surface diffusion (the emitter saturation current density) or the surface passivation. A Sinton lifetime tester can be used to monitor and optimize these aspects of the production process.
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How is lifetime measured by the Sinton Instruments tools?
Our systems use the Eddy-current method. A sensor (a coil built into the instrument stage) is placed near the silicon sample and sends electromagnetic waves into the silicon Light is then pulsed onto the sample to create the excess carriers, and the coil circuit senses the increase in conductance of the sample due to the carriers. This data is analyzed and the lifetime of the excess carriers during or after illumination is reported.
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How is the data analyzed?
We use both transient photoconductance techniques, and the Quasi-steady-state photoconductance method (QSSPC) which we developed in 1994. These measurements are considered to be the most carefully calibrated in the industry for reporting accurate values of carrier recombination lifetime. Over 1000 technical papers are available in the literature discussing data taken and analyzed by these techniques.
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Can you measure surface recombination velocity?
Yes, in the technical literature, these are the most common instruments used for reporting surface recombination velocities. The application notes that are provided with purchased instruments give the details for this analysis.
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Does the system measure emitter saturation current density?
This is another common use for the instruments that is covered in detailed application notes that are provided with the instruments.
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Can wafers be measured with no surface passivation
(“out of the box”)?
Yes, we have an application note for measuring p-type wafers with no surface passivation. Without surface passivation, the measured lifetimes are very low, since the photogenerated carriers quickly diffuse to the surface and recombine there. However, the quality of the wafer can still be determined in the range of interest for most solar cells by using the measured lifetime and the trapping characteristic that can correlate with the crystalline quality.
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Can any of these instruments do lifetime maps?
Yes, the SBS-150 does raster scanned automated X-Y mapping. The WCT-120, BCT-400, and BLS-I all can be used to do very rough (cm scale) manual maps.
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How do these measurements compare to microwave PCD?
Microwave PCD is used to generate high-resolution maps that can be used to picture spatial variations and defects in the sample. Sinton instruments are primarily used to report lifetime and surface recombination properties in calibrated units as a function of carrier density in the sample.
Sinton Instruments’ measurements are analyzed in calibrated physical units, in order that the results can be used to model solar cells and predict performance. The results can also be compared to other calibrated lifetime measurements taken by different techniques at different laboratories or companies. The microwave PCD measurements usually report a curve-fit parameter (primary-mode lifetime of the microwave reflectance signal) from the raw data from the instrument. In most cases, the microwave PCD number should be considered to be in arbitrary units. In some special cases, the results can be compared to calibrated measurements. However microwave PCD results are generally not reported in sufficient detail to compare to any other lifetime measurement method. Some sophisticated laboratories report calibrated measurements from microwave PCD, but the more frequently used techniques for microwave PCD are not calibrated.
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What lifetimes can be measured?
0.1 to 20,000 microseconds on passivated wafers.
0.1 to 10,000 microseconds on unpassivated silicon ingots.
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What is the smallest sample size?
The instruments are fully calibrated in the factory-default setup for samples that are 4-cm diameter. The user can recalibrate for small samples. We recommend that the smallest sample size be at least 1 cm square.
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How do you measure bulk lifetime on blocks or ingots?
We use infra-red light photoexcitation. This creates excess carriers deep into the silicon, with significant photogeneration in the 100 to 1000 micrometer depth range. These carriers are relatively far from the surface and are very sensitive to bulk lifetime. For the p-type material commonly used for solar cells, we have an analysis that corrects for surface recombination to report bulk lifetime. For long lifetime samples, both p- or n-type, we use the transient method that allows the surface recombination to eliminate carriers near the surface so that using data later in the decay, the true bulk lifetime is approached asymptotically.
The lifetime is reported as a function of excess carrier density in this measurement.
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At what carrier density should I report the result?
Often the entire data trace as a function of carrier density is shown in order to convey the most information. For a single-point measurement, we recommend 1015 cm-3. This has been used for the majority of reported data over the last 15 years. It is relevant to solar cell efficiencies, has good signal to noise for these instruments, exists in most data traces for a wide variety of samples, is good for Fe determination, and works quite well for measurements of emitter saturation current density.
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Can the lifetime tester be used to detect Fe contamination?
Yes! We have an application note for how to detect Fe using the data from the lifetime testers.
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How is the instrument calibrated?
The instrument is traceable to four-point-probe measurements in order to report in units of absolute conductance. This is the only calibration required for doing fully calibrated transient measurements. The light intensity sensor can be calibrated in one of several ways. One way is to compare QSSPC and transient measurements, and use this data to set the intensity calibration for the QSSPC measurement. The other is to calibrate the cell under a known light source as we do in the factory calibration.
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When should wafers be tested inline?
The in-line wafer tester can be integrated for incoming wafer test, then again after phosphorus diffusion (to monitor for doping quality and wafer contamination in the front end of the process). The nitride deposition can also be optimized and monitored using an in-line lifetime tester.
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Does the lifetime tester measure the trapping?
Yes. Within every measurement, we note the photoconductance as a function of the light intensity over a range of light intensities spanning 300X. The details of this dependence can be used to separate out trapping photoconductance from free electron-hole pair conductance by using the curve shape at low intensity to determine the trapping. Then we report both the electron-hole recombination lifetime as well as a trapping parameter.
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