Here, an overview will be provided of our recent findings of charge trapping and doping in CH3NH3PbI3-xClx perovskite films. We focus on the two structures that are most promising for solar cell applications: mesosuperstructured perovskite films where the perovskite is infiltrated into a mesoporous Al2O3 scaffold, and neat perovskite films such as those used in planar heterojunction solar cells.4,5 We probe the presence of sub gap electron acceptor states, or traps, in both neat and mesostructured perovskite films via a combination of X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) measurements.  We use these finding to complement photoconductivity, photoluminescence, mobility, and solar cell performance measurements to develop a clear picture of the role of charge trapping and doping in the two types of perovskite structures.

We find that the mesoporous scaffold n-dopes the perovskite and leads to a passivation of a high density of trap states, raising the photovoltage of the solar cells as compared to the neat planar heterojunction solar cells whose behavior is dominated by a high density of trap states.  We link this effect to the crystal size in the perovskite and surface effects at the oxide interfaces.  Still, neat perovskite films are far superior to the mesosuperstructured films in terms of charge transport (long range mobilities in excess of 20 cm2 V-1 s-1), making the more promising for optoelectronic applications if alternative methods can be found to passivate the trap sites and dope the material.This work gives new insights into dominant loss pathways in solution processed organometal trihalide perovskite solar cells, and can be used to direct future work towards overcoming these losses. The extremely high long range mobility of the neat perovskite films also demonstrates the potential of this class of materials in other applications such as solution processed transistors.                                                             

References

(1)           Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Sci. Rep. 2012, 2.

(2)           Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science (80-. ). 2012, 338, 643.

(3)           Wang, J. T.-W.; Ball, J. M.; Barea, E. M.; Abate, A.; Alexander-Webber, J. a; Huang, J.; Saliba, M.; Mora-Sero, I.; Bisquert, J.; Snaith, H. J.; Nicholas, R. J. Nano Lett. 2013.

(4)           Ball, J. M.; Lee, M. M.; Hey, A.; Snaith, H. J. Energy Environ. Sci. 2013.

(5)           Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Adv. Funct. Mater. 2013, Ahead of Print.