Origin of Photoluminescence in Organolead Halide Perovskites: The Role of Mid-Gap Electronic States

Samuel Stranks^a, Tomas Leijtens^a, James Ball^a, Henry Snaith^a, Victor Burlakov^b, Alain Goriely^b, Ian McPherson^c

^aDepartment of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom

^bMathematical Institute, University of Oxford, OCCAM, Woodstock Road, Oxford, OX2 6GG

^cDepartment of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)

Ecublens, Switzerland, 2014 May 11th - 14th

Organizers: Michael Graetzel and Mohammad Nazeeruddin

Oral, Samuel Stranks, presentation 165
Publication date: 1st March 2014

Organic-inorganic perovskites have been attracting an enormous amount of recent attention for their use in high-performance solar cells^1-4. Nevertheless, there have been few studies focusing on device function and fundamental properties^5,6, and a complete understanding of the recombination pathways in these materials is crucial for further improvement of devices. Here, we provide a comprehensive study of the mechanisms of recombination and photoluminescence (PL) in CH₃NH₃PbI_3-xCl_x perovskite absorbers. We demonstrate the presence of mid-gap electronic trap states and introduce a robust model describing the dynamic interrelations between excitons, free charges and these mid-gap states. The model reveals insight into the nature of the dominant photoexcited species in the devices across a range of charge densities. We are also able to use the model to describe the perovskite PL decays (Figure a) by considering high and low fluence regimes where the recombination mechanism is bimolecular and monomolecular, respectively (Figure b). We discuss the origin of the mid-gap states and show their importance to emission and device function.

(a) Photoluminescence decays from the perovskite, detected at the peak emission wavelength (780nm), following pulsed excitation with various pulse fluences. Solid lines are fits to the data from the model. (b) Schematics to illustrate recombination mechanisms for the low fluence regime, leading to monomolecular recombination, and the high fluence regime, which leads to bimolecular recombination. m respresents the filled trap concentration, n_e the free electron concentration in the conduction band.
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. Kim, H.-S. et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2. 3. Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316-319 4. Liu, M., Johnston, M. B. & Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395-398. 5. Stranks, S. D. et al. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, 341-344. 6. Xing, G. et al. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344-347.

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