Fast view @ solar cell quality

The solar cell quality can be fast accessed with an optimized CCD camera in the NIR.

Solar cells are large area semiconductor devices with typical side lengths of 15.6 cm. Local loss mechanisms such as locally reduced diffusion lengths or parallel resistances often reduce the energy conversion efficiency of solar cells. Characterization techniques which 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 line.

Image of a mono-crystalline solar cell, which is operated under forward bias condition and additionally iluminated by 950nm NIR LEDs (the bright spots show reflections of these LEDs). The electro luminescence (EL) of the cell below the bus bar and the grid electrodes is clearly visible. The image was recorded by a pco.1300 solar at 2s exposure time. The darker areas indicate areas of minor quality within the solar cell.

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 electro luminescence (EL) imaging technique which allows a rapid solar cell characterization with a high spatial resolution. The ISFH has set up this new technique and investigated its potential for solar cell characterization.

The photo shows the measurement system set up at the Institute for Solar Energy Research Hameln (ISFH). The electro luminescence 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. An meaningful image can be captured in a couple of seconds.

Electro luminescence 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. Simplified, the product of the electron and hole concentration increases exponentially with the externally applied bias voltage.

Potential profile between the two bus bars of an industrial mono-crystalline silicon solar cell. The potential profile is calculated directly from the electroluminescence measurement. The upper part the figure shows the corresponding electroluminescence image. A comparison with a numerical solar cell simulation reveals a very good correspondence between the expected and the measured potential profile.

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 electro luminescence signal is considerably higher at the contact grid in comparison to a point midway between the contact fingers. The figure above shows the local voltage between the two bus bars of a mono-crystalline silicon solar cell. Generally, all effects resulting in a local reduction of the carrier concentration are visible on the electro luminescence 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 next figure shows impressively shows the impact of to much pressure when applying the grid electrodes, as all the dark finger structures and black areas show, that the solar cell panel is damaged at these locations.

Electroluminescence (EL) image of a solar cell panel operated at nominal forward bias current. The dark areas indicate damages in the cells crystalline structure, which would not contribute to the generation of photo voltaic current.

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. Therefore, 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.

...if you have further questions, please contact:
Institute for Solar Energy Research Hameln - ISFH, Am Ohrberg 1, D-31860 Emmerthal, Germany, fon: +49 (0) 5151 999 100, fax: +49 (0) 5151 999 400, www.isfh.de

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