Many semi­conducting materials are possible candidates for solar cells. In recent years, perovskite semiconductors in particular have attracted attention, as they are both inexpensive and easy to process and enable high effi­ciencies. Now a study with 15 participating research insti­tutions shows how terahertz (TRTS) and microwave spectroscopy (TRMC) can be used to reliably determine the mobility and lifetime of the charge carriers in new semi­conducting materials. Using these measurement data it is possible to predict the potential efficiency of the solar cell in advance and to classify the losses in the finished cell.  

The most important properties of a semi­conductor to be used as a solar cell include the mobility and lifetime of electrons and holes. Both quantities can be measured without contacts with spectroscopic methods using terahertz or microwave radiation. However, measurement data found in literature often differ by orders of magnitude. This has made it difficult to use them for reliable assessments of material quality. We wanted to get to the bottom of these differences, and contacted experts from a total of 15 inter­national labora­tories to analyse typical sources of error and problems with the measurements,” says Hannes Hempel from the HZB team led by Thomas Unold. The physicists sent reference samples produced by the team of Martin Stolterfoht at University Potsdam to each labora­tory with the perovskite semi­conductor compound (Cs,FA,MA)Pb(I,Br)₃) optimised for stability.

One result of the joint work is the signi­ficantly more precise deter­mination of the transport properties with terahertz or microwave spectro­scopy. “We could identify some neuralgic points that have to be considered before the actual measurements takes place, which allows us to arrive at signi­ficantly better agreement of the results,” Hempel emphasises. And with reliable measurement data and a more advanced analysis, the characteristics of the solar cell can also be calculated more precisely. “We believe that this analysis is of great interest for photo­voltaic research, because it predicts the maximum possible efficiency of the material in a solar cell and reveals the influence of various loss mechanisms, such as transport barriers,” says Unold. This applies not only to the material class of perovskite semi­conductors, but also to other new semi­conducting materials, which can thus be tested more quickly for their potential suitability. (Source: HZB)

Reference: H. Hempel et al.: Predicting Solar Cell Performance from Terahertz and Microwave Spectroscopy, Adv. En. Mat., online 26 February 2022; DOI: 10.1002/aenm.202102776

Link: Structure and Dynamics of Energy Materials, Helmholtz Center for Materials and Energy HZB, Berlin, Germany