Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Publication date: 21st February 2018
Bismuth-based absorber materials are promising candidates for non-toxic but stable and low-cost solar cells. The ternary halide Cs3Bi2I9 has a bandgap of ≈2 eV which correlates to a theoretically maximum power conversion efficiency (PCE) of 22.6%.[1,2] However, despite of the encouraging properties of Bi-based absorber materials, up to now only 1.09% PCE has been achieved for Cs3Bi2I9 by Park et al.[3] and 4.3% PCE for the rudorffite Ag3BiI6 [4], respectively. Noticeable, these are the best results, in most publications the efficiencies for A3Bi2I9 (A = Cs or MA) are <0.1% PCE and limited by poor short current densities.
In this work we tried to optimize the coverage of Cs3Bi2I9 and its mixed cation analogues with formamidinium and/or methylammonium (FAMACs)3Bi2I9. For this purpose, we used on one hand antisolvent dripping (chlorobenzene, ethyl acetate) or a 2-step protocol and on the other hand we investigated different ETLs (TiO2, SnO2, PCBM) as well as different annealing times and temperatures.
The absorber layers were characterized by optical microscopy, atomic force microscopy, UV/VIS spectroscopy, X-ray diffraction spectroscopy and time-of-flight secondary ion mass spectrometry. Additionally, I-V curves of the resulting solar cells have been recorded and the bandgaps were determined by external quantum efficiency.
Although challenging, the best results for homogeneous film formation were obtained by using Cs3Bi2I9 as absorber processed by spin coating together with an ethyl acetate dripping step or a FA1.5Cs1.5Bi2I9 absorber also spin coated from solution with ethyl acetate or chlorobenzene dripping. Unfortunately, coverage and adhesion of the absorber layer on the substrate are still poor and therefore the resulting solar cells suffer from very low short circuit currents. The best obtained Bi-based solar cell has only 0.02% PCE, 641 mV open-circuit voltage, 0.07 mA/cm2 short circuit current density and 46% fill factor.
[1] W. Shockley, H. J. Queisser, J. Appl. Phys. 1961, 32, 510.
[2] S. Rühle, Sol. Energy 2016, 130, 139.
[3] B.-W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo, E. M. J. Johansson, Adv. Energy Mater. 2015, 27, 6806.
[4] I. Turkevych, S. Kazaoui, E. Ito, T. Urano, K. Yamada, H. Tomiyasu, H. Yamagishi, M. Kondo, S. Aramaki, ChemSusChem 2017, 10, 3754.
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