Lifetime Testers & Suns-Voc

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

What lifetimes can be measured?

0.1 to 20,000 microseconds on passivated wafers.
0.1 to 10,000 microseconds on unpassivated silicon ingots.

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.

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.

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.

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.

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.

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.

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.

Module & Cell Testers

Module Flash Tester

Is the Suns-Voc included in the analysis?

Yes, every module test solution we offer includes the familiar Suns-Voc analysis. This technique was developed & commercialized by Sinton Instruments and has been included in our instruments since 1995.

Does the module tester have a class A spectrum?

In its standard configuration, the module tester has a class C spectrum. There are options available to filter the light source to meet class A spectrum requirements, although Sinton Instruments recommends the unfiltered option for testing Si modules.

Why would I want to use a class C light source?

Spectral class does not predict measurement accuracy, so there is no guarantee of good data quality simply by using a specific class of light source (for example, see “Advanced Intercomparison Testing of Testing of PV Modules in European Test Laboratories” by Herrmann et al. in Proc. 22nd EU PVSEC, 2007). Our unfiltered class C light source requires less power, has lower cost of ownership, and is overall a more environmentally friendly & more compact solution that does not compromise measurement accuracy.

What intensities can be reached using a Sinton flash tester?

The Sinton light source decays from 1.2 to 0.2 suns and we can construct I(V) curves for any intensity in this range; however, the user should be aware that the spectrum red shifts through the decay of the pulse and the spectrum has been optimized for 1 sun operation.

What is the throughput of the instrument?

In a standard configuration the FMT-500 is capable of flashing once every second. In high throughput applications it is suggested the tester be used in a five- or eight-flash configuration to accurately measure Voc, Jsc, and Vmp, with a total test time of less than 10 seconds.

Does the FMT-500 include the ability to mount my module?

No. Fixtures for mounting modules are not included with the tester; the user is responsible for mounting the module. Sinton Instruments can help with discussions on good module mounting configurations.

What is the maximum power the tester is capable of measuring?

The standard tester is capable of measuring modules with powers of 500W, maximum short circuit current of 15A and open circuit voltage of 120V are standard, however customized ranges are available.

What is included with a standard module tester?

Sinton Instruments sells module test instruments as complete systems including: light source & power supply, electronic load, computer and data acquisition, and analysis software.

Are these testers used in production?

Yes. Over 7 GW of modules have been tested by multiple module manufacturers and in some cases the entire production is tested by Sinton Instrument’s module testers.

Cell Flash Testers

Is the Suns-Voc included in the analysis?

Yes, every cell test solution we offer includes the familiar Suns-Voc analysis developed & commercialized by Sinton Instruments and included in our instruments since 1995.

Does the cell tester have a class A spectrum?

Xenon flashes are well-suited to match the solar spectrum although they are somewhat IR rich. The Sinton light comes standard with in-line optical filtering options which meet the class A requirements.

What intensities can be reached using a Sinton flash tester?

The intensity can vary greatly with the size of the device, 0.2 to 1.2 suns for standard cells, 2 to 20 suns for large area medium concentration cells and over 1000 suns for small area high concentration devices. The Sinton tester uses a multiple flash technique rather than a flat top pulse. This allows for high peak intensities to be obtained with relative ease.

How are I(V) curves at multiple irradiances obtained for a single I(V) scan?

Due to the nature of the decay of the flash, the Sinton tester is capable of acquiring current and voltage profiles as a function of light intensity for each flash. The Sinton tester effectively scans intensity at a level of constant, steady-state charge in the solar cell; this is opposed to conventional testers in which the voltage is scanned during a single light pulse at constant intensity. All of this data, from the peak intensity down through lower intensities, is available from each individual flash in order to construct I(V) curves at multiple intensities from a specified sequence of flashes.

What is the throughput of the instrument?

The FCT-450 tester is best suited for R&D applications, and in a standard configuration is capable of measuring the solar cell I(V) characteristic, Suns-Voc curve, substrate doping, and dark shunt resistance in less than 10 seconds. The FCT-750 is our production tester, which provides the same measurement results, at a throughput of 3600 units per hour.

Is the tester capable of fixturing my cell?

Yes. The standard fixture for front contact cells is fully adjustable and customizable and has been used to measure cells ranging from dual busbar 60mm x 60mm cells to five busbar 156mm x 156mm cells. We have a lot of experience designing custom contacting chucks for any cell design including back contact and bifacial cells.

What current is the tester capable of measuring?

The standard one sun cell tester is capable of measuring up to 15 A. Custom options to measure higher currents are possible please contact Sinton Instruments for details..

What is included with any of the I(V) cell testers I purchase?

Sinton Instruments sells cell test instruments as complete systems including: light source & power supply, electronic load, computer and data acquisition, and analysis software. At this time we do not sell the fixtures and electronic loads of the cell test instruments independently.

Do the chucks have temperature control?

Yes. Our standard cell testers include cooling and heating elements that will maintain a constant chuck temperature or take the chuck to approximately 10°C above or below ambient. There is very little temperature excursion during testing due to the very low duty cycle of the flash lamp and our temperatures are therefore very stable. Higher temperature options to increase the range of the tester are available.

Are these testers used in production?

Yes, the FCT-750 is used in production; it is especially suited for accurate, high-throughput testing of high efficiency solar cell designs (PERC, HJT, etc…). Sinton Instruments is open to discussions about inline integration of our testers; please contact us if you are interested in this application.