Zahrah Almutawah1 Suneth Watthage1 Ramez Ahangharnejhad1 Fadhil Alfadhili1 Geethika Liyanage1 Niraj Shrestha1 Adam Phillips1 Randy Ellingson1 Michael Heben1

1, University of Toledo, Toledo, Ohio, United States

Organic-inorganic metal halide (OIMH) perovskite absorbers such as methyl ammonium lead iodide (CH3NH3PbI3, MAPbI3) have been used to prepare high efficiency solar cells when they are paired with appropriate electron and hole transport materials (ETMs and HTMs). The power conversion efficiency (PCE) of OIMH perovskites solar cells has improved to more than 22% [1]. The tremendous progress in improving PCE is mainly due to a unique combination of the intrinsic optical and electrical properties of OIMH perovskites, such as large absorption coefficient, long carrier diffusion lengths, desirable optical band gaps, and fast carrier collection rates, and the existence of cost effective and facile preparation methods. Among several deposition methods, the single-step solution deposition coupled with the anti-solvent dropping technique has been widely used in the fabrication of high efficiency perovskite solar cells [2]. Although, this method produces a dense, uniform, and conformal perovskite film, the size of grains is extremely small (200 nm). The small grains form a large number of grain boundaries that increases the density of charge carrier traps resulting in higher non-radiative recombination of carriers. By fabricating larger grains with a high degree of crystallinity, the non-radiative recombination in perovskite films can be reduced and device performance improved.
Here, we adapt a technique demonstrated for the two-step deposition process [3] to enhance the grain size and crystallinity by adding small amounts of metal cations (Cd2+, Zn2+, and Fe2+) during the single-step solution deposition process. This metal-additive-assisted single-step deposition fabrication process resulted in grains that are ~1 μm in diameter, which is an increase of a factor of 5. In addition to the improved grain size, the degree of crystallinity of the grains is also improved with the metal cation inclusion, as determined by the X-ray spectroscopy analysis. These results suggest that the process may be an effective and facile method to fabricate high efficiency perovskite photovoltaic devices.

[1] NREL, "Solar Cell Efficiency Chart". [] accessed: October 2017.
[2] Song, Z.; Watthage, S. C.; Phillips, A. B.; Heben, M. J. “Pathways toward High-Performance Perovskite Solar Cells: Review of Recent Advances in Organo-Metal Halide Perovskites for Photovoltaic Applications.” J. Photon. Energy. 2016, 6, 022001.
[3] S. C. Watthage, Z. Song, N. Shrestha, A. B. Phillips, G. K. Liyanage, P. J. Roland, R. J. Ellingson, and M. J. Heben, "Enhanced Grain Size, Photoluminescence, and Photoconversion Efficiency with Cadmium Addition During the Two-Step Growth of CH3NH3PbI3," ACS Applied Materials & Interfaces, 2016.