Solar Cell Characterisation

Solar cell characterisation is important for process control and for the development of new solar cell technologies. There are a wide range of semiconductor characterisation techniques used in the photovoltaics area. In particular, the development of new photovoltaic materials uses an enormous range of sophisticated techniques to examine material composition and structure. However, many of the more sophisticated techniques are unsuitable for production lines where fast and unabbiguous measurements are required. This chapter concentrates on techniques that are either presently used in solar cell production lines or that are used to develop new solar cell technologies on the verge of commericialisation.

The characterisation techniques described in this chapter focus on those relavant to silicon solar cells.

As of 2007 this chapter is still being edited and does not have the technical polish of the other chapters.

Characterisation Types

There are three basic types of characterisation in use:

1. Full Area Measurement
   Here the entire solar cell or large fraction wafer is measured giving one data point per wafer. The measurements are typically fast enough for inline characterisation on every wafer going through a produciton line and the data produced is suitable for statistical process control (SPC). an example of full area measurement is an IED test at the end of solar cell processing, the measurement of a parameter such as reflectivity on part of a wafer, or a lifetime test using a coil based sensor. The disadvantage of full area measurements is that it does not reveal area related problems such as poor printing on part of a wafer.
2. Mapping
   mapping techniques rely on point by point measurements to scan across a wafer surface. They may provide a great deal of accuracy about each point on the wafer and reveal defects such as grain boundaries or other imperfections. The drawback of mapping techniques is the time taken to cover an entire wafer. For example if a measurement takes 0.1 seconds and a resolution of 100 µm is required for a 1500x1500 array of data points then the time to map an entire wafer will be 62 hours. The long sample times usually makes wafer mapping unsuitable for in-line characterization. An example of wafer mapping is an LBIC system where a laser is scanned across the surface of the wafer and the current is read out of each data point. due to the times required for two-dimensional images mapping techniques are often simplified by using line scans.
3. Imaging
   Imaging away for is somewhat similar to taking a photograph the measurement technique uses a sensor array to sample multiple points simultaneously. the advantage of imaging techniques is that large arrays of data points can be taken in quite small short periods of time. The disadvantage is the cost of the sensors. The role of imaging techniques is increasing due to the appearance of low cost silicon CCD cameras similar to those used in digital cameras.

Electrical Characterisation

The first section discusses the electrical measurement of solar cells with the three basic techniques:

• Illuminated IV, where the cell is illuminated at one sun and basic cell parameters measured
• Dark IV, where the cell is in the dark and the cell IV characteristic traced
• Jsc Voc where the Voc and Isc are recorded at different illumination levels

These three techniques have numerous names and technical varations.