Phase Segregation in Potassium-Doped Lead Halide Perovskites from ³⁹K Solid-State NMR at 21.1 T

Organic–inorganic lead halide perovskites are a promising family of light absorbers for a new generation of solar cells, with reported efficiencies currently exceeding 22%. A common problem of solar cells fabricated using these materials is that their efficiency depends on their cycling history, an effect known as current–voltage (J–V) hysteresis. Potassium doping has recently emerged as a universal way to overcome this adverse phenomenon. While the atomistic origins of J–V hysteresis are still not fully understood, it is essential to rationalize the atomic-level effect of protocols that lead to its suppression. Here, using 39K MAS NMR at 21.1 T we provide for the first time atomic-level characterization of the potassium-containing phases that are formed upon KI doping of multication and multianion lead halide perovskites. We find no evidence of potassium incorporation into 3D perovskite lattices of the recently reported materials. Instead, we observe formation of a mixture of potassium-rich phases and unreacted KI. In the case of Br-containing lead halide perovskites doped with KI, a mixture of KI and KBr ensues, leading to a change in the Br/I ratio in the perovskite phase with respect to the undoped perovskite. Simultaneous Cs and K doping leads to the formation of nonperovskite Cs/K lead iodide phases.有机-无机卤化铅钙钛矿（organic–inorganic lead halide perovskites）是一类极具应用前景的新一代太阳能电池用光吸收材料，目前已报道的光电转换效率已突破22%。采用此类材料制备的太阳能电池普遍存在一个共性问题：其光电效率随循环测试历程发生变化，该效应被称为电流-电压（J–V）滞后现象。近年来，钾掺杂被证实为克服这一不利效应的通用策略。尽管J–V滞后效应的原子级起源尚未完全阐明，但阐明可抑制该效应的调控工艺的原子层面作用机制至关重要。本研究借助21.1 T场强下的39K魔角旋转核磁共振（39K MAS NMR）技术，首次实现了对多阳离子多阴离子卤化铅钙钛矿经KI掺杂后形成的含钾物相的原子级表征。研究未发现钾离子掺入已报道的新型三维钙钛矿晶格的证据；与之相反，体系中形成了富钾物相与未反应KI的混合物。对于掺KI的含溴卤化铅钙钛矿体系，最终会生成KI与KBr的混合物，这会导致钙钛矿相中的Br/I比例相较于未掺杂样品发生改变。同时进行Cs与K掺杂则会形成非钙钛矿相的Cs/K基碘化铅物相。