Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Efficiencies of perovskite solar cell devices based on methylammonium lead iodide have skyrocketed during the past two years and efficient thin film solar cells have been demonstrated.1 The most efficient solution-processed methylammonium lead iodide thin films to date are derived from a mix of lead chloride with three equivalents of methylammonium iodide. For these thin films long photoluminescence lifetimes indicative of long charge carrier diffusion lengths have been measured exceeding those of perovskite thin-films deposited from equimolar lead iodide/methylammonium iodide solutions.2
Chlorine is thought to act as a dopant in methylammonium lead iodide thin films.3We investigate the atomic composition of solution-processed perovskite films using X-ray photoelectron spectroscopy. While we do find chlorine in films prior annealing, the amount of chlorine in fully annealed films is under the detection limit of our instrument.
In this work, we emphasize the role and importance of lead chloride in the formation of highly crystalline and oriented perovskite thin films. Using in-situ UV-Vis reflectometry, spectral and time-resolved photoluminescence and X-ray diffraction, we are able to follow the formation of the perovskite absorber thin film. From in-situ X-ray diffraction annealing experiments and X-ray photoelectron spectroscopy we conclude, that the perovskite absorber phase grows slowly from a distinct precursor phase containing methylammonium, lead, iodide and chloride. A chlorine-rich organo-halide perovskite phase forms intermittently but is consumed in favor of methylammonium lead iodide. Because of the slow growth, large crystal-grains and highly oriented films can be obtained via this preparation procedure.
Long photoluminescence lifetimes and charge carrier diffusion lengths are a consequence of large crystal grains. Crystal domain size increase is observed as a continuous red-shift of the absorption maximum during annealing. In our in-situ photoluminescence study we also find, that the photoluminescence quantum yield is the highest for thin films only annealed for 30 minutes while annealing time of 45 minutes are necessary to achieve high solar cell device efficiencies.
We establish that thin film preparation techniques that lead to perovskite films with large crystal-grain size are a prerequisite for efficient thin film perovskite-absorber solar cells. As organo-metal halide perovskite materials are of interest for optoelectronic applications other than solar cells, it is important to understand the relation between morphology and optoelectronic properties of perovskite thin films.
1. Liu, M., Johnston, M. B. & Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395–398. 2. Stranks, S. D. et al. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, 341–344. 3. Colella, S. 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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