2D/3D Hybrid Cs2AgBiBr6 Double Perovskite Solar Cells: Improved Energy Level Alignment for Higher Contact-Selectivity and Large Open Circuit Voltage
Florian Wolf a, Rik Hooijer a, Melina Armer b, Firouzeh Ebadi c, Mahdi Mohammadi c, Clement Maheu d, Andreas Weis a, Bas T. van Gorkom e, Sebastian Häringer a, Rene A. J. Janssen e, Thomas Mayer d, Vladimir Dyakonov b, Wolfgang Tress c, Thomas Bein a
a University of Munich (LMU), Department of Chemistry and Center for Nanoscience (CeNS), 81377 Múnich, Alemania, Múnich, Germany
b Experimental Physics VI, Julius Maximilian University of Würzburg, 97074 Würzburg, Germany
c ZHAW – Institute of Computational Physics, 8401 Winterthur, Switzerland
d TU Darmstadt, Dept. of Materials Science, Surface Science, Jovanka-Bontschits-Str. 2, Darmstadt, 64287, Germany
e Molecular Materials and Nanosystems, Eindhoven University of Technology, The Netherlands, Eindhoven Station, 5612 AZ Eindhoven, Países Bajos, Eindhoven, Netherlands
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Florian Wolf, presentation 121
DOI: https://doi.org/10.29363/nanoge.hopv.2022.121
Publication date: 20th April 2022

Since their introduction in 2017, the efficiency of lead-free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%, although theoretical limits predict maximum efficiencies of up to 7.9%.[1] The limiting bottlenecks are attributed to a low electron diffusion length, self-trapping events and poor selectivity of the contacts, leading to large non-radiative VOC losses.[2] Here, we introduce the strategy of creating a 2D/3D perovskite double layer, for double perovskite solar cells to overcome some of the bottlenecks (Figure 1 left). The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. We compared the organic spacer cations butylammonium (BA), phenethylammonium (PEA) and 4-fluorophenethylammonium (4FPEA) and systematically investigated the reasons for improvement on a model system with (PEA)4AgBiBr8/Cs2AgBiBr6 as the 2D/3D double perovskite layer. The improvement is mainly due to a VOC increase by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of PEA precursor spin-coating solutions. These voltages are among the highest reported VOC values for Cs2AgBiBr6 solar cells. We attribute this effect to a change in recombination behavior within the full device and a better selectivity at the interface toward the hole transporting material (HTM). This explanation is supported by the voltage-dependent external quantum efficiency, and by results from photoelectron spectroscopy, revealing a better energy level alignment and thus a better hole-extraction and improved electron blocking at the HTM interface (Figure 1 right).[3]

The authors acknowledge funding from the German Federal Ministry of Education and Research (BMBF) under the agreement number 03SF0516B, the Bavarian Ministry of the Environment and Consumer Protection, the Bavarian Network “Solar Technologies Go Hybrid”, the German Science Foundation (DFG) focus program SPP 2196 and the DFG Excellence Cluster e-conversion (EXC 2089/1-390776260). The authors thank Dr. Steffen Schmidt for performing the SEM measurements.

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