Organic-Inorganic Halide Perovskite Semiconductors: Synthesis and Application in Photovoltaics
Abstract: The introduction of semiconducting halide perovskite compounds, AMX3 [A = Cs+, CH3NH3+ or HC(NH2)2+; M = Ge2+, Sn2+, Pb2+; X = Cl-, Br-, I-], in solid-state solar cells has engendered enormous attention from the photovoltaic community in the last five years. The efficiency of perovskite solar cells has increased dramatically from 6% in 2012 to above 20% up to date. Yet the most efficient material, methylammonium lead iodide (CH3NH3PbI3), suffers from two major drawbacks: the toxicity of lead and device instability.
This seminar will discuss strategies and progress in developing efficient, lead-free, and stable perovskite solar cells. In particular, lead is entirely replaced by tin. The valence band of tin, consisting of the 5s2 electrons, lies significantly higher in energy with respect to the 6s2 electrons of lead, leading to narrowing of the band gap, or in other words, better light absorption. On the downside, Sn2+ is easily oxidized to Sn4+ when exposed to ambient conditions, resulting in undesired properties.
To protect tin-based perovskites from moisture and oxygen, primary aliphatic or aromatic alkylammonium cations (such as n-C4H9NH3+) are intercalated into the three-dimensional framework of CH3NH3SnI3. Consequently, the crystallographic dimensionality is reduced to a two-dimensional layered structure due to steric hindrance. Two-dimensional perovskite solar cells are indeed more stable than those of their three-dimensional analogs. Two-dimensionsal perovskite materials are also tunable and offer flexibility in tailoring their physical and chemical properties, granting them applications beyond photovoltaics.