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The Cooke Corporation (Romulus, Mich.)
Institute for Solar Energy Research Hameln (Emmerthal, Germany)
The Imaging Challenge
Solar cells are large-area semiconductor devices with typical side lengths of 15.6cm. Local loss mechanisms often reduce the energy to current conversion efficiency of solar cells. Optical characterization techniques that are capable of providing spatially resolved information about the performance of a solar cell are therefore of great importance, not only for research and development, but also as a process control tool in the solar cell production lines.
Unfortunately, most of the present characterization techniques providing spatially resolved information are very slow because the final image needs to be generated from point-by-point measurements. Recently, Fuyuki et al. (2005) introduced the camera-based electroluminescence (EL) imaging technique, which allows a rapid solar cell characterization with a high spatial resolution. The Institute for Solar Energy Research Hameln (ISFH) set up this new technique and investigated its potential for solar cell characterization.
Electroluminescence is the emission of light resulting from a forward bias voltage being applied to the solar cell. The electrons injected into the solar cell recombine radiatively with the available holes by transferring their excess energy to an emitted photon. The intensity of the luminescence radiation (IEL) is determined by the product of the electron and hole concentrations.
The image captured with the CCD camera shows the intensity distribution of the luminescence radiation. While a homogeneous intensity distribution would be expected for an ideal solar cell, electroluminescence images of real solar cells always show inhomogeneities. Due to the finite resistance of the front grid and the emitter sheet resistance the local voltage and thus the electroluminescence signal is considerably higher at the contact grid in comparison to a point midway between the contact fingers. Generally, all effects resulting in a local reduction of the carrier concentration are visible on the electroluminescence image. Even though the reason for such a local reduction in the carrier concentration can be manifold, they can be clearly distinguished in most cases. Thus, local variations in the bulk carrier lifetime like those in multi-crystalline silicon are clearly visible on the electroluminescence image.
The photo above shows the measurement system set up at the Institute for Solar Energy Research Hameln (ISFH). The electroluminescence images are captured with a cooled 12 bit CCD camera system (sensicam qe). Since the distance between the camera and the solar cell is freely adjustable, solar cells of any size and even complete modules can be analyzed. A more detailed analysis of particular areas with a higher spatial resolution can easily be achieved by reducing the distance between camera and sample and by choosing appropriate lenses. A meaningful image can be captured in a couple of seconds.
The Tools Used
Cooke pco.1300 solar cooled digital CCD camera system.
The Difference it Made
Camera-based electroluminescence measurements enable the depiction of the impact of different loss mechanisms in solar cells with high spatial resolution and within very short measurement times. This new characterization technique has become an indispensable tool in the daily research and development work at ISFH and has already contributed to promoting the research activities in the fields of wafer-based cell development and the integrated module assembly of silicon thin film solar cells.