Photocurrent measurement technique by interferometry (SSPG)

The SSPG technique (measurement of photocurrents created by an interference system) was first proposed in 1986 by D. Ritter, E. Zeldov and K. Weiser [1] to determine the ambipolar carrier scattering length in a highly resistive semiconductor material. Weiser [1] to determine the ambipolar scattering length of carriers in a highly resistive semiconductor material. This technique has been successfully applied to many materials, mainly thin films (e.g. hydrogenated amorphous silicon, microcrystalline silicon, Sb2S3, P3HT-PCBM, perovskites) but also bulk crystalline materials (GaAs). 

However, one of the shortcomings of this technique, in these early designs, is that the realization of interference illuminating the sample to be studied requires the manual adjustment of mirrors to set the interrange.

In order to save time we have devised a system where the position of the mirrors is predefined and a movable mirror allows us to choose the pitch of the interference that is created in the sample. The figure below gives a synoptic diagram of the system developed at GeePs. The measurement benches obtained from this diagram allow a measurement to be made automatically in a few minutes for scattering lengths ranging from 50 nm to 2 µm [2].

 

 

 

The second advantage of this system is that the sample to be studied is fixed and can therefore be placed under vacuum in a cryostat, which is particularly important for materials that are likely to evolve when placed in the ambient atmosphere (organic compounds, perovskites). 

As an example, the figure below shows the evolution of the ambipolar scattering length measured on a perovskite thin film between the study in vacuum and the study in air at room temperature (295 K). After keeping the sample under secondary vacuum (< 10-5 mbar) for several hours, air is allowed to enter the cryostat (Vacuum OFF). An increase in the diffusion length is observed, reaching a saturation value after 250 minutes. When the vacuum is turned back on, the diffusion length decreases rapidly and returns to its original value after about 150 minutes. The demonstration of such a behaviour was only possible thanks to the rapid measurement of the diffusion length provided by our measuring bench [3].

 

Evolution of the diffusion length of a triple-cation perovskite thin film, MaFaCs, prepared at the Institut Photovoltaïque d'Ile de France (IPVF, Palaiseau, France). At room temperature and in air, the scattering length increases and decreases to its original value when the sample is returned to vacuum.

 

Références

[1] D. Ritter, E. Zeldov and K. Weiser, Appl. Phys. Lett. 49, 791 (1986).
[2] C. Longeaud, Rev. Sci. Instrum. 84, 055101 (2013). http://dx.doi.org/10.1063/1.4803006.
[3] C. Longeaud, F. J. Ramos, A. Rebai, J. Rousset, Sol. RRL 1800192 (2018). https://doi.org/10.1002/solr.201800192.


Contact : longeaud@geeps.centralesupelec.fr