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Towards the Photonic Solar Cell - In-Situ Defect Characterization in Metal-Halide Perovskites

Periodic Reporting for period 1 - PhotSol (Towards the Photonic Solar Cell-In-Situ Defect Characterization in Metal-Halide Perovskites)

Reporting period: 2019-07-01 to 2021-06-30

The transformation of our energy economy from a fossil-fuel based system to a solar-energy based one is one of the major challenges of this century. Only such a transformation will allow for a sustainable life style and secure equal opportunities for the generations to come. One important pillar of this transformation is efficient and low-cost photovoltaics with a small ecological footprint. Recently, the pool of available technologies (mainly silicon solar cells) have been enlarged by a newcomer to the photovoltaic field: Perovskites. Solar cells based on these perovskite materials have reached similarly high efficiencies than silicon solar cells, however, so far on the lab scale only. Perovskite solar cells require little material and are simple to process. These properties make them promising contenders for the large-scale photovoltaic market. However, long-term stability issues are still a big challenge. Additionally, there is room for further efficiency improvements.
This project focusses on the physics of these solar cells, and in particular on electronic defects that are a source of performance losses. Origins of these defects could be surfaces, crystal defects, grain boundaries, or impurities. The objectives of this projects are to better characterize the role of these defects on the photovoltage of the solar cell. This goal is achieved by combining numerical modelling and experimental data, for instance to better comprehend the transient photovoltage signal. To better understand the role of (unintentional and unwanted) impurities, an approach has been selected to systematically introduce a certain impurity with a controlled concentration.
During this project, it has been found that certain impurities are highly relevant. Furthermore, common characterization methods have been critically evaluated.
A second part of this project is dedicated to a further major drawback of the common high-efficiency perovskite solar cells: They contain the toxic element lead. This project started looking into the defect physics of alternative materials (double perovskites).
During the ten months of the project, research and teaching tasks have been performed. Based on the gained expertise, insights have been disseminated in conference talks and lectures to students. A study has been performed and published regarding transient photovoltage measurements on perovskite solar cells with varied defect concentrations, introduced by Bismuth impurities. Initial characterization of absorption and luminescence properties of lead-free double perovskites have been performed to better understand the so-far low performance of these systems.
The project gave insights on how to better characterize defects in perovskite solar cells. As defects are limiting device performance, the project results constitute a contribution towards a commercialization of this technology. Furthermore, initial investigations on loss mechanisms of lead-free alternatives have been undertaken. A breakthrough in these less-toxic materials might bring this photovoltaic technology closer to a real “green” technology, which is, however, still a long way to go.