Non-radiative recombination is one of the solar cells biggest enemies. However, in contrast to radiative recombination, non-radiative recombination can be avoided and is therefore a key objective of our research group. Non-radiative recombination originates from traps and defect states in the bandgap allowing charges to relax to the ground state through the interaction with phonons and without the emission of photons. Unfortunately, however, the location of these defects and the dominant non-radiative recombination pathway is often unknown, which complicates a systematic optimization of the devices.
To quantify, and understand the complex non-radative recombination processes and mechanism in the multi-layered perovskite cells, the Perowskite Group applies various electro-optical measurement techniques, in particular photoluminescence (PL) measurements. While the quality of a light emitting diode is measured by applying a voltage to the cell and detecting the number of emitted photons (i.e. the electroluminescence), the PL is measured by applying light to the cell. The assement of the solar cell via PL, is based on the fact that emission and absorption are inherently linked via the principals of detail balance and the black body radiation. It also means, a good solar cell must be a good LED! The advantage of photoluminescence is that it can be measured on incomplete or partial cells. For example, the neat material, or the neat material in combination with any of the transport layers or the electrodes. PL measurements therefore enable insights into the electro-optical quality of certain parts of the cells that are otherwise not accessible in measurements of complete cells. In principal, this allows to quantify how much non-radiative recombination is caused by each layer of the device. Perovskites are particularly suited for PL characterizations due to their sharp absorption onsets and low exciton binding energies.
The Perovskite group at the University of Potsdam is using these principals to indentify the limiting cell components and assess the quality of single and multi-junction perovskite cells and we have recently demonstrated several advanced PL characterization techniques. This includes, quasi-Fermi level splitting (or internal voltage) measurements, intensity and voltage dependent PL and the quantification of the efficiency potential of neat perovskite films and partial stacks. Contact us for more information!