pv-tools - Model LOANA - Solar Cell Analysis System

External quantum efficiency & reflectance
The LOANA system includes all features of the IQE-SCAN.
When combined with other methods in the LOANA system you have various benefits:

• Automatic spectral mismatch correction of subsequent ISC-measurements.
• Calibration of LBIC measurements to global values.
• Correct sample thickness and grid shadowing as input for the automated IQE analysis.

Quantum efficiency mapping
This method uses the focussed light spot from the monochromator to create maps of EQE and/or REFL at any wavelength between 280 nm and 1200 nm with a spatial resolution of approximately 1 mm. The EQE map at 1000 nm of a 156 x 156 mm² solar cell on the right was taken in 5 minutes.

Short circuit current
Instead of tracing the entire IV-curve during the short illumination with a Xenon flash we maintain the solar cell in short circuit during the decay to measure the short ciruit current only.

If an EQE is available the spectral mismatch is calculated automatically and the result used as a setpoint to regulate the intensity of the LED-array. Subsequently, the IV-trace is recorded without compromises in accuracy because plenty of time is available. The spectrum of the Xe-flash is filtered to match class A according to IEC 60904‑9.

Current-Voltage
The IV characteristics is measured under the illumination with an infrared LED array to determine Voc, FF, and efficiency at arbitrary light intensities. The Isc‑Voc characteristics is also measured and fitted to the 2-diode model. The global series resistance is found both from comparing Isc‑Voc with light-IV and from the double light method.

The noise limit in the finest current range is 0.3 µA and low enough to measure the dark characteristics on small solar cells of only a few cm².

Busbar-to-busbar
To separate the total series resistance into its contributions from the fingers, the base, the emitter, and  the metal-silicon interfaces it is necessary to know the line resistance of the grid fingers. The resistance between the busbars measured with a test current of several amps contains this information.

Capacitance-Voltage

The base doping can be determined from the plot of 1/C² versus voltage. The width of the space charge region of a silicon solar cell, Wj, depends on the base doping concentration and can be varied with an applied reverse voltage. The solar cell's capacitance is inversely proportional to Wj and can be measured from the phase and magnitude of the AC current that results when applying a sine modulated reverse voltage.

The base doping concentration must be known for:

• the calculation of base contribution to saturation current J0,base
• the calculation of base contribution to series resistance
• estimating the current limit for small local shunts due to spreading resistance in the base.

Spectrally resolved LBIC

For the features and capabilities please follow this link to our stand-alone LBIC.
The advantages of having LBIC integrated into the LOANA system are:

• Automatic combined evaluation with other methods, for example with EL or with the global EQE & reflectance in order to rescale the data to absolute values.
• Availability of the sample loading system.

Electroluminescence imaging
These images show the near-bandgap light that a solar cell emits when it is driven in forward bias. The intensity depends mainly on the local minority carrier concentration, so a number of effects that are affecting it become visible: inhomogeneous recombination properties in the base volume and at the rear surface, voltage drops due to series resistance, and cracks. The acquisition of a high-current image takes only a few seconds and should generally accompany IV measurements to reveal e.g. poor contacting quality which may affect the fill factor measurement. Images of series resistance and saturation current are calculated from images under different bias conditions.

The camera's height and focus settings are motorized and automatically adjusted to the sample size. A zoom mode uses the minimum possible object distance for high resolution close-ups. The images are scaled to mm.

Lock-in thermography
LIT remains the ultimate method for localizing shunts, but also offers other information about recombination, series resistance, etc. The lock-in principle makes the resolution a matter of modulation frequency and time.

The IR camera is motorized in height and focus and automatically adjusts to the sample size. A zoom mode uses the minimum possible object distance for high resolution close-ups. The images are scaled to mm.

Microscope camera
A camera with manually adustable magnification is used to measure the contact finger width automatically, which needs to be known in order to calculate the grid shadowing for EQE and reflectance measurements. Feature sizes can be measured because the images are scaled to milimeters. An alternative and precise method to measure dimensions is to utilize the chuck positioning and a crosshair in the image. The microscope camera is also useful to read laser labels.

Thickness measurement
Similar to a dial gauge the sample thickness is measured wth a contacting rod near the egde of the solar cell. The reproducibility is +/- 3 µm.

After a certain part of the measured thickess is subtracted it is used as the thickness of the silicon in the models for the electro-optical evaluation.

The handling system complements the thickness measurement with the weight measurement.

Calibration unit
The calibration unit is integrated in the sample stage and holds up to 4 standards for external quantum efficiency and reflectance as well as a geometrical light trap. The system therefore needs to be re-calibrated only once a year using standards that have recently been calibrated at the PTB in Braunschweig.

The EQE standards also serve as an internal reference solar cell for IV measurements to which the calibration of your external reference solar cells can be transferred. The latter thus need to be re-measured every few months.

Optics:

Change-over between EQE and REFL
The integrating sphere is motorized to be inserted in the optical path such that both reflectance and EQE are measured with exactly the same monochromatic light beam.

Monochromatic light spot size
The light spot size can be changed from a 20 x 20 mm² square to a slit of 1 x 10 mm² or to a point of 1 x 3 mm² by software.

Motorized contact setting
The currrents during EQE and LBIC measurements are below 200 mA and flat contact needles are used which allow a close distance to the LBIC reflectance sensor.

For higher currents of several amps a contact frame with narrow pin arrays is used to make the voltage drop along the busbars negligible.

The quality of the contacting is ensured by measuring the resistance between voltage and current pin of each polarity.

Measurement batches and sequences
In a measurement sequence the methods are applied on a solar cell in a fixed order. Most of the methods offer a batch mode in order to take a series of measurements under different conditions.

You may choose to shut down the machine automatically at the end of a sequence, or to receive an SMS notification or an email with the results overview.

Sample loading system
With the box-to-box sample loading system a stack of up to 100 solar cells (125 mm or 156 mm) can be measured without further action by the operator. Sensitive solar cell surfaces may be protected with paper interleaves. Two cameras are used to check the integrity of the solar cell, the oriention of the gridlines, the number of busbars, and whether an interleaving paper needs to be removed. A third camera takes a photo of labels written onto the backsides of the solar cells. Also part of the sample loading system is a weight scale with an accuracy of 0.02 g.

Custom chucks
We build customized chucks for your latest solar cell designs. The peltier cooling and vacuum connections are part of the table so that changing the chucks is a matter of one minute. In the software the data specific to the chuck such as PID control parameters and location of the stop pins are stored in the software.

Hand panel
This tool is for manual positioning of the table, e.g. to place the light spot onto a certain grain or between fingers. It can also be used to control camera heights and focus, the chuck height and more.

In combination with the microscope camera you can make precise measurements of sample dimensions and feature sizes.

Mini-module kit
This kit consists of a frame with fixtures to securely place mini-modules, a set of cables with crocodile clamps, and a cover to keep the extra cables down to avoid their entanglement while the chuck is moving.