DOI: https://doi.org/10.29363/nanoge.inform.2019.064
Publication date: 8th January 2019
Metal halide perovskite solar cells are now effectively competing with their inorganic counterparts in terms of power conversion efficiencies. However, state of the art perovskite solar cells still suffer from limited fill factor (FF) and open circuit voltage (Voc), due to non-radiative recombination processes happening in the device. We found that selective charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, the CTL/perovskite interfaces induce additional non-radiative recombination pathways which limit the Voc of the cell. In order to harvest the full thermodynamic potential of the perovskite absorber the interfaces of both the electron and hole transport layers (ETL/HTL) must be properly addressed and improved. Here, we show a significant improvement of the Voc and FF of pin-type perovskite solar cells by employing a novel surface treatment to a of triple cation perovskite Cs5(MA0.17FA0.83)95)95Pb(I0.83 Br0.17)3 using a polyionic liquid (PIL) material. The resulting solar cell devices show outstanding FF values of up to 83% and Voc of 1.17V, which lead an extraordinarily PCE of 21.5%. Through combined photoluminescence and electroluminescence studies we found that the PIL reduces the non-radiative recombination at perovskite surface by acting as a defect passivating agent of the bare perovskite surface and limiting the recombination of charges across the perovskite/C60 interface. Photoemission spectroscopy (XPS/UPS) and conductive atomic force microscopy (CAFM) show how the ionic nature of the PIL induces a specific charge redistribution at the perovskite surface, going along with improved extraction of the electrons at the perovskite/C60 interface. Ultimately, the hydrophobic nature of the PIL provides a shielding coverage of the perovsktie which reduces degradation due to moisture and air. The PIL modified devices show exceptionally long dark storage stability and enhanced maximum power point tracking (MPP) lifetimes. In conclusion, our work proposes a novel approach to efficiently suppress non-radiative recombination of charges, promote the charge extraction and improve the stability at the same time. Additionally, given the simplicity of this post treatment, our results can be representative of a more general methodology for device modification, and therefore potentially applicable to other compositions and cell architecture, opening doors for e new class of materials to be implemented in perovskite solar cells.
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