High Performance Perovskite Solar Cells Based on Amphiphilic Spacer Molecules
Michael Graetzel a
a Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, Lausanne, Switzerland
nanoGe Perovskite Conferences
Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO20)
Sevilla, Spain, 2020 February 23rd - 25th
Organizer: Hernán Míguez
Invited Speaker, Michael Graetzel, presentation 072
Publication date: 25th November 2019

Hybrid perovskites have the AMX3 stoichiometry (Figure 1) and are composed of a monovalent cation A (e.g. Cs+, methylammonium (MA) CH3NH3+ or formamidinium (FA) CH(NH2)2+), a metal M (Pb2+,  Sn2+ or Ge2+), and a halide X (Cl–, Br–, or I–). Pb-based perovskites with mixed MA/FA cations and Br/I halides show the most remarkable optoelectronic properties.They can be improved further upon addition of inorganic cations, such as Rb+ or K+. While this approach allowed reaching the present perovskite solar cell (PSC) solar-to-electric power conversion efficiency (PCE) record exceeding 25%, the instability of perovskites towards sunlight, oxygen, and moisture, as well as the environmental impact of lead content, continue to hamper industrial applications. Unlike three-dimensional (3D) perovskites, their layered two-dimensional (2D) analogues have demonstrated promising stabilities against environmental factors. Layered 2D hybrid perovskites are defined by a general formula S2An–1MnX3n+1 (S is a monovalent organic spacer cations,), which represents a layered structure of 3D perovskite slabs separated by the organic spacer layers. In addition to increasing environmental stabilities, 2D layered perovskites act as versatile platforms for realizing lead-free perovskite solar cell architectures that could reduce the detrimental environmental impact of lead. This inspired us to develop a new generation of 2D/3D structures featuring judiciously engineered amphiphilic spacer molecules that exhibit outstanding optoelectronic properties by suppressing non-radiative recombination and achieving very high open-circuit voltages, while retaining excellent operation stability. Several layered 2D perovskite solar cell architectures will be presented which have surpassed the performance of the state-of-the-art. These findings exemplify that molecular engineering based on noncovalent interactions can lead to improved performances, durability, and scalability of perovskite solar cells. Furthermore, the studies on molecular modulation and layered 2D PSCs induced us to monitor atomic-level interactions within hybrid perovskite materials using solid-state 2D-NMR as a powerful tool.

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