Lead-Free MA2CuClxBr4−x Hybrid Perovskites

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic−inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of twodimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)2CuClxBr4−x was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d−d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material’s properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices. ■ INTRODUCTION In the past two years, organic−inorganic lead halide perovskites have delivered breakthrough solar cell efficiencies. In particular, after the first pioneering works, record performances have been demonstrated in succession with methylammonium lead iodide CH3NH3PbI3 6−12 thanks to its long electron−hole diffusion length, high absorption coefficient, and low defect density. Power conversion efficiencies of 19.3% and NREL-certified 20.1% have been achieved, establishing perovskites as the most efficient solution-processed solar cells, and challenging the thin film and polycrystalline silicon solar cell paradigm. Recently, the application of hybrid perovskites have been extended with superlative results to light-emitting devices opening new frontiers for perovskitebased lasers, light-emitting diodes (LEDs), and field-effect light-emitting transistors (FETs). Despite these unprecedented advances and the versatility in fabrication processes, the lead content of these materials has raised concerns due to its toxicity and bioaccumulation in the ecosystem, thus hampering the perovskites’ pathway to commercialization. It is therefore of primary importance to study and develop alternative classes of lead-free perovskites for optoelectronic applications. In the photovoltaic community, first attempt to replace Pb involved substitution with Sn2+, and efficiencies as high as 6% were shown with CH3NH3SnI3. 32 However, Sn is easily oxidized into Sn, which is a stumbling block that frustrates reproducibility of highly efficient devices and limits practical use of these compounds. Typically, hybrid perovskite sensitizers are based on the three-dimensional (3D) structure with general formula AMX3, where X = Cl −, Br−, I−; A is an organic cation like methylammonium CH3NH3 + (MA) Received: August 25, 2015 Published: January 12, 2016 Article