Charge Recombination and Transport Phenomena in Lead-Free Quantum Dot Solar Cells Probed by Impedance Spectroscopy
Authors: Treber Fiona , De Grande Elke , Cappel Ute , Johansson Erik 
Published: December 2024

Due to their unique tunability and solution processability, the implementation of nanoscale semiconductor particles into optoelectronic devices is of increasing interest for future applications.1 Regarding the incorporation into solar cells, one of the most well-studied materials in form of quantum dots (QDs) is lead sulphide.1–3 However, due to its toxicity, replacing lead by a more environmentally friendly material would be immensely advantageous. In that context, ternary metal chalcogenides attracted a lot of interest, as many of them also provide a suitable band gap and high absorbance coefficients. One of the more prominent examples of that material class is AgBiS2.4 It has already been implemented in QD solar cells with remarkably high efficiencies, exceeding 8%.5 Nevertheless, detailed understanding of the charge recombination and transport processes in these devices still needs to be improved, especially with respect to the effect differences in surface passivation have on the aforementioned parameters.

At the same time, impedance spectroscopy is a versatile technique that allows to probe the inductive, capacitive and resistive properties of a material or device over a wide range of time scales.6 However, it is a rarely employed method in this specific field of research.

Therefore, it has been utilized here to study the recombination and transport properties in AgBiS2 QD solar cells to gain new insights into the underlying dynamics. Additionally, transient photovoltage and photocurrent measurements were employed to obtain an initial model and thus be able to reliably interpret the data from the impedance spectroscopy.

As a result, it was found that changing the surface passivating treatment of the AgBiS2 QDs from iodide-containing compounds to bromide or chloride would improve the charge carrier lifetime and recombination resistance, while simultaneously the corresponding transport times exhibited a much less pronounced response. However, there were subtle differences found in the related transport resistances, which give rise to the hypothesis that there might be divergent mechanisms involved or even limiting charge transport depending on the exact surface composition of the QDs.

Consequently, this study highlights the crucial importance of understanding the interplay of QD-ligand interaction in those hybrid systems, since it will significantly impact the overall material properties and thereby any device based on it. Furthermore, it demonstrates the suitability and value of employing impedance spectroscopy as a tool to probe for charge transport and recombination processes in this material.

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