A bright idea: Scanning Kelvin Probe (SKP) for the investigation of photovoltaic systems

The worldwide dependence on green energy sources from wind to solar power continues to grow. Of particular interest is the use of photovoltaics whose use continues to grow exponentially. Advances in existing and future photovoltaic technologies will allow these devices to play a crucial role in the growing shift to renewable energy.

In the development of novel photovoltaic devices, it is important to accurately measure the work function of the components of the device, in particular that of the electrodes. The respective work functions of the positive and negative electrodes, and their relation to each other, influence the efficiency of the final device. For example, increasing the work function of the negative electrode allows for better alignment of its energy state with that of the transport layer, which improves power conversion efficiency. Furthermore, this increase in the work function of the negative electrode causes a larger potential difference with the positive electrode, improving the splitting of electrons and holes, causing further improvement in the device. It is important, therefore, that work function is considered in the design of novel and existing photovoltaic devices.

How can work function be measured?

Scanning Kelvin Probe (SKP) has been demonstrated as a method of determining the average work function of a photovoltaic electrode [1]. Using SKP non-contact, non-destructive, work function measurements of the highest energy resolution can be performed. SKP also has the particular advantage that it can also be used to map the local variations in work function across a sample.

How does Scanning Kelvin Probe measure work function?

In SKP the sample and probe are in direct electrical contact to form a parallel plate capacitor. As the probe and conducting or semiconducting sample have differing work functions a flow of charge from one material to the other occurs. This flow of charge results in the equilibration of the Fermi levels of the two materials, and the development of a surface charge on the two materials. The result is a parallel plate capacitor.

To measure the difference in potential between the probe and the sample, known as the Volta or contact potential, the SKP probe vibrates perpendicular to the sample. This vibration in the presence of a surface charge gives rise to an ac current, which is measured through the use of a Lock-In Amplifier (LIA). The application of a backing potential to the system until the point where zero ac current is measured allows the difference between the probe and sample potentials, or the Kelvin potential, to be found. The Kelvin potential is equal but opposite to the Volta potential. By calibrating the probe against a sample of known work function it is possible to convert the Volta potential measurement to the work function value of interest.

Planning to use SKP in your own photovoltaic measurements?

Learn more about the SKP-M470 system

[1] X. Liu, H. Zheng, J. Zhang, Y. Xiao, Z. Wang, J. Mater. Chem. A 1 (2013) 10703