Trap concentration determination

Many lifetime measuring methods as QSSPC, µPCD or CDI, as well as MDP suffer from an anomalous high measured lifetime at very low injections. This effect is due to trapping centers in the sample. These trapping centers are very important, in order to understand the behavior of carriers in the material and can also have an effect on solar cells. Hence it is desirable to measure the trap density and the activation energy of these trap centers with a high resolution.

With the MDPmap and MDPingot it is possible to measure the photoconductivity as well as the minority carrier lifetime with one measurement and fully automated in a wide injection range. A clever algorithm allows the determination of the trap concentration in the sample.

From the injection dependent lifetime curve the lifetime at low injection τLLI can be determined and the photoconductivity is fitted with the slightly modified model of HORNBECK and HAYNES. The trapping density NT and the activation energy EA are the fitting parameters.

First measurement results were obtained on mc- and Cz-Si wafers and a correlation between the trap density and the dislocation density could be confirmed.

Photoconductivity versus Gopt for different trap densities and fit of measured photoconductivity curve

MDPmap allows to measure injection dependent photoconductivity and lifetime curves with a high resolution, so that the trap density and activation energy of trapping centers can be determined. With this it is possible to investigate the origin of traps and there influence on for example the efficiency of solar cells.

For more information read:
[1] J. A. Hornbeck and J. R. Haynes, Physical Review 97, 311-321 (1955)[2] D. Macdonald and A. Cuevas, Applied Physics Letters 74, 1710 - 1712 (1999)
[3] N. Schüler, T. Hahn, K. Dornich, J.R. Niklas, 25th PVSEC Valencia (2010) 343-346