Stabilization of the Trigonal High Temperature Formamidinium Lead Iodide Phase

Thomas Bein^a, Pablo Docampo^a, Fabian Hanusch^a, Andreas Binek^a

^aLudwig-Maximilians-University

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Andreas Binek, presentation 089
Publication date: 5th February 2015

Hybrid perovskite-based solar cells have already surpassed some commercial photovoltaic devices such as amorphous silicon solar cells. At present, the highest efficiencies are achieved with methylammonium lead iodide (MAPbI₃) films. However, other variants such as those based on formamidinium lead iodide (FAPbI₃) have the potential to achieve even higher performance due to its slightly narrower bandgap, very near the optimum bandgap of a single junction device. Additionally, it has been observed that exchanging formamidinium for methylammonium results in a longer lifetime of the photoexcited species which is beneficial for overall device performance. The major drawback of FAPbI₃ is that the stable polymorph of formamidinium-based perovskites at room temperature is a hexagonal (P6₃mc) phase, which exhibits a rather wide bandgap of over 2 eV. For photovoltaic applications the main interest is focused on the trigonal phase (P3m1) exhibiting a bandgap of 1.52 eV, which is formed at higher temperature (130 °C). Here, we have developed planar hetero-junction formamidinium lead iodide solar cells, employing two step approaches were an initial lead iodide film is converted into the perovskite in a second immersion step. Our results shows that employing a mixture of 15 wt% mehtylammonium iodide and 85 wt% formamidinium iodide in the conversion solution results in the exchange of 13 % of the FA molecules with MA without any lattice shrinkage. Additionally, we show with temperature-dependent X-ray diffraction that the trigonal phase can be stabilized even at room temperature with this exchange due to the increase of Madelung energy of the lattice. Moreover, we show that the inclusion of this small amount of methylammonium also has a positive effect on the lifetime of the photoexcited species and results in higher performing devices.

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