Research

We investigate thin film solar cells based on chalcopyrites Cu(In,Ga)Se2 or Cu(In,Ga)S2 . They are stable and highly efficient. We are also interested in new materials for such solar cells.

Our main tool to investigate the electronic structure of semiconductor films and solar cells is photoluminescence (PL). From these measurements we can predict how good a new material is for use in a solar cell. We can also understand for existing material where their limitations are – and propose ways to improve them. We study how many of the electrons that are generated by the light are actually available for energy conversion in the solar cell. When we study PL at low temperatures we can see the electronic defects in the semiconductor, that are either beneficial, because the help the conductivity of the material by doping, or they are detrimental, because they lead to recombination, i.e. the loss of light-generated electrons.

 

When a laser beam hits the semiconductor, it excites electrons from the valence band to the conduction band. These electrons are used in a solar cell to generate electricity. In a PL experiment we measure the light, the luminescence, that is emitted when these electrons fall back to the valence band. Thereby we can study how many of them are available for the solar cell and how many are lost by (non-radiative) recombination. By intensity and spectrally calibrated PL, which allows photon counting, we can determine the internal voltage of the semiconductor, the so-called quasi-Fermi level splitting. By doing this intensity dependent we can predict the whole current-voltage characteristics of the solar cell. We use time-resolved PL with nanosecond resolution, to measure how long the light-generated electrons remain in their excited state and are available for the solar cell. We can do this also spatially resolved with a micrometer resolution, for example to see where defects are located that lead to recombination and a reduction of the internal voltage.

We make the films and devices ourselves, starting from the bare glass substrate. Making the devices ourselves gives us the possibility to control the growth parameters and investigate their influence on fundamental properties and efficiencies of solar cells. We use co-evaporation to prepare the absorber films of our solar cells. For details on our solar cell baseline, please check under equipment (link to equipment tab). To understand the role of grain boundaries better, we also prepare by epitaxy single crystalline films or films with a controlled (low) number of grain boundaries.

 

With our finished devices we measure the current-voltage characteristics, which tells us the efficiency of the solar cell. Comparing this to our PL measurements helps us understand additional losses at interfaces and contact layers. Capacitance spectroscopy gives us a further tool to study defects, in particular those that were created during the solar cell making.

The solar cells we study are of interest in tandem devices. No solar cell can use the whole solar spectrum efficiently. They lose completely the light with too low photon energies, below the bandgap of the absorber. And the light with high energies is not used efficiently, only the bandgap energy is used. By combining two (or more) solar cells, we can reduce both of these losses: the top solar cell (with a high bandgap) makes more efficient use of the high energy photons and lets the lower energy ones pass to the bottom solar cell (with a lower band gap) that uses more of them than a stand-alone cell. Therefore we study materials that have high bandgaps, like sulphide chalcopyrites Cu(In,Ga)S2 , and those with a low bandgap, like CuInSe2 .

 

For our latest results, please, check our publications