Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Hybrid organolead halide perovskites (CH3NH3PbX3, X= halogen)are attracting increasing attention as high-efficiency light sensitizers in all-solid-state solar cells due to their outstanding performances and ease of fabrication [1, 2].
The electronic activities of CH3NH3PbX3 absorbers are strongly correlated with their crystalline structures. For example, adjusting the component I/Br ratio in the CH3NH3Pb(I3-xBrx) (0≤x≤ 3) mixed halide perovskites allows the band gap to be gradually tuned so that the optical absorption can be adjusted to cover almost the whole visible spectrum, while the crystal structure transforms from the tetragonal structure of CH3NH3PbI3 to the cubic structure of CH3NH3PbBr3 [3]. In the case of mixed I/Cl perovskites, the large difference in the halogen ionic radii inhibits the formation of a continuous solid solution. However, even if the material band gap remains substantially unchanged, the incorporation of Cl as a dopant dramatically improves the charge transport within the perovskite layer [4].
Although many papers have been published dealing with the device photovoltaic performances as determined by the use of either different perovskites, synthesis procedures, or solar cell structures, very few information is available about the possible relations existing between synthetic procedures and properties of the obtained perovskites. In particular, many papers reported XRD spectra where the hybrid perovskite features are accompanied by PbI2 peaks, which is a telltale sign of incompletely reacted perovskites, but it is not clear if this depends on the process used to prepare the material.
Aiming at a deeper control of the perovskite synthesis procedures as a tool to enhance solar cell performances, in this work we report about the influence of the synthesis parameters (e.g. solution composition, annealing temperature and duration) on the properties of organolead I/Br and I/Cl mixed-halide perovskites prepared as thin films on glass substrates. In particular, we show by XRD measurements that optimized synthesis procedures give pure-phase perovskite films. Besides, based on diffuse reflectance measurements, the perovskite band-gap energies are determined and compared with reference to the synthesis parameters, while the steepness of the absorbance onset near the bandgap is used as a marker of the film quality.
[1] Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647 [2] Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, R. P.; Nazeeruddin, M. K.; Graetzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. [3] Noh, J. H.; Im, S. H.; Heo, J. H.; T.N. Mandal, T. N.; Seok, S. I.Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13,1764−1769. [4] Colella, S.; Mosconi, E.; Fedelli, P.; Listorti, A.; Gazza, F.; Orlandi, F.; Ferro, P.; Besagni, T.; Rizzo, A.; Calestani, F.; et al. MAPbI3‑xClx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25, 4613−4618.
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