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）是新一代太阳能电池极具潜力的一类光吸收材料，目前已报道的光电转换效率已超过22%。采用此类材料制备的太阳能电池普遍存在一个问题：其光电效率随循环测试历史发生变化，该效应被称为电流-电压（current–voltage, J–V）迟滞现象。近年来，钾掺杂（potassium doping）被证实为克服这一不利效应的普适性策略。尽管目前尚未完全阐明J-V迟滞现象的原子级起源，但阐明各类抑制该效应的工艺方案所产生的原子层面调控机制，仍是至关重要的研究方向。本研究通过21.1特斯拉场强下的39K魔角旋转核磁共振谱（39K MAS NMR），首次实现了多阳离子多阴离子卤化铅钙钛矿经碘化钾（KI）掺杂后形成的含钾物相的原子级表征。研究发现，在近期报道的此类材料中，并无钾离子掺入三维钙钛矿晶格的证据。反之，我们观测到体系中形成了富钾物相与未反应碘化钾（KI）的混合相。对于经碘化钾掺杂的含溴卤化铅钙钛矿体系，最终会形成碘化钾与溴化钾（KBr）的混合相，从而导致钙钛矿物相中的溴/碘（Br/I）比例相较于未掺杂样品发生改变。同时进行铯（Cs）与钾掺杂时，则会生成非钙钛矿型的铯/钾碘化铅物相。