Hybrid multi-junction solar cells based on a monolithic integration of a wide-bandgap organo-metal-halide perovskite and low-gap organic polymer sub-cells – (MUJUPO)

Tandem solar cells based on a serial connection of wide-gap and low-gap sub-cells allow to minimize losses due to thermalization and thereby unlock elevated efficiencies. In organic multi-junctions the wide-gap cell (energy-gap about 1.8 eV), which should simultaneously provide a high Voc and high Jsc, currently states the main limitation. Even in the best organic wide-gap devices the voltage loss, i.e. 1/q * Eg - Voc, is unsatisfactorily high (about 0.8-1 V).In this project we intend to design and realize hybrid multi-junction solar cells where the wide-gap sub-cell is based on an organo-metal halide perovskite absorber, which allows for a voltage loss as low as 0.3-0.4 V. Reports of single junction perovskite cells with an efficiency >20% are accompanied by serious concerns about the stability of established perovskites like methyl ammonium lead iodide (MAPbI3). Perovskites based on mixed cations (e.g. MA and Cs) and mixed halides (e.g. I and Br), such as MA1-xCsxPb(I(1-y)Bry)3, bear the potential of enhanced stability. In general, the addition of Cs cations, which are smaller than MA, as well as the addition of Br, both lead to a widening of the bandgap of the perovskite, which is favorable for their use in a tandem cell. Regarding the sub-cell with low energy gap (1.2-1.3 eV), organic photo-active materials are available and some systems will be provided by the group of Prof. Janssen (TU Eindhoven) for this project. As of yet, no multi-junction devices of wide-gap perovskite cells based on MA1-xCsxPb(I(1-y)Bry)3 and low-gap organic cells have been reported. In this project we will first identify an optimum wide-gap perovksite material along with a robust preparation protocol. Alongside, the careful analysis of its electronic structure by photoelectron spectroscopy (PES) will be of paramount importance. Until now these studies are lacking for perovskites like MA1-xCsxPb(I(1-y)Bry)3. The outcome of this research states the prerequisite for the selection of optimum interfacial materials that not only improve charge extraction but at the same time enable enhanced stability of the entire cell. As an example, microporous TiO2 is an established electron extraction material, that has to be prepared at high temperatures (>400°C) and its photocatalytic nature is frequently associated with reliability issues in perovskite cells. Opposed to that, we aim to use cross-linkable organic semiconductors or metal-oxides that can be prepared at temperatures below 100°C. In a combined approach of PES with dedicated device testing (e.g. unipolar electron/hole-only), we aim to identify optimum charge extraction layers for the selected wide-gap perovskite. These interfacial materials will also be the platform for the design of an interconnect, which must allow the loss-free monolithic integration of the sub-cells. We expect to achieve long-term stable hybrid tandem cells prepared at low temperatures (<100°C) with an efficiency > 20%.

Duration:
01.11.2018 - 31.10.2020

Projectleader:
Prof. Dr. T. Riedl
Prof. Dr. H.-C. Scheer

Budget: 201.700 €