Comparison to µ-PCD and QSSPC

Among carrier lifetime experiments, it is still one of the biggest problems to understand widely contradicting results as obtained by different experimental methods. With our novel simulation tool it is possible to simulate steady state and non steady state measurements for any given material leading to a quantitative interpretation of the results.

Comparison to µ-PCD and QSSPC

Besides MDP the two most important contact less lifetime measuring methods are QSSPC (quasi steady state photoconductivity) and µ-PCD (microwave detected photoconductive decay). Currently one of the biggest problems in the photovoltaic industry is to make the deviating lifetime measurement results of the different methods comparable. With our novel simulation tool it is possible to simulate steady state and non steady state measurements, so that a comparison is possible.

µ-PCD

The µ-PCD method typically operates at very high injections with a very short light pulse of only 200 ns. The minority carrier lifetime is determined via the photoconductive decay, similar to MDP. µ-PCD detects the photoconductivity by measuring the reflection of a microwave at the sample, which makes this method less sensitive than MDP.

QSSPC

QSSPC detects the changes in permeability of the sample and therefore the conductance via the coupling of the sample by a coil to a radio-frequency bridge. The exciting light is tuned down slowly, so that the sample is always in a quasi steady state. A further difference to MDP is the use of a flash-light with a hole light spectrum, in stead of monochrome laser light as the excitation source.

Figure 1 displays the injection ranges in which the different lifetime measuring methods typically operate. It becomes clear that MDP surpasses the other methods, because it enables to measure over 7 decades of injection.

If the measurement results of these different methods are compared, the injection, excitation wavelength, penetration depth of the microwave, the carrier profile and the different behaviour of traps, that depends on the duration of the light pulse, has to be taken into consideration.

For more details read:

[1] T. Hahn, Thesis, TU Bergakademie, 2009

Fig. 1: Comparison of MDP, µPCD and QSSPC with respect to their typical injection ranges