Tiankai Zhang

Hybrid Halide Perovskite Solar Cell Precursors: Colloidal Chemistry and Coordination Engineering behind Device Processing for High Efficiency

The precursor of solution-processed perovskite thin films is one of the most central components for high-efficiency perovskite solar cells. We first present the crucial colloidal chemistry visualization of the perovskite precursor solution based on analytical spectra and reveal that perovskite precursor solutions for solar cells are generally colloidal dispersions in a mother solution, with a colloidal size up to the mesoscale, rather than real solutions. The colloid is made of a soft coordination complex in the form of a lead polyhalide framework between organic and inorganic components and can be structurally tuned by the coordination degree, thereby primarily determining the basic film coverage and morphology of deposited thin films. By utilizing coordination engineering, particularly through employing additional methylammonium halide over the stoichiometric ratio for tuning the coordination degree and mode in the initial colloidal solution, along with a thermal leaching for the selective release of excess methylammonium halides, we achieved full and even coverage, the preferential orientation, and high purity of planar perovskite thin films. We have also identified that excess organic component can reduce the colloidal size of and tune the morphology of the coordination framework in relation to final perovskite grains and partial chlorine substitution can accelerate the crystalline nucleation process of perovskite. This work demonstrates the important fundamental chemistry of perovskite precursors and provides genuine guidelines for accurately controlling the high quality of hybrid perovskite thin films without any impurity, thereby delivering efficient planar perovskite solar cells with a power conversion efficiency as high as 17% without distinct hysteresis owing to the high quality of perovskite thin films.

Nonstoichiometric acid-base reaction as reliable synthetic route to highly stable CH3NH3PbI3 perovskite film.

Perovskite solar cells have received worldwide interests due to swiftly improved efficiency but the poor stability of the perovskite component hampers the device fabrication under normal condition. Herein, we develop a reliable nonstoichiometric acid–base reaction route to stable perovskite films by intermediate chemistry and technology. Perovskite thin-film prepared by nonstoichiometric acid–base reaction route is stable for two months with negligible PbI2-impurity under ∼65% humidity, whereas other perovskites prepared by traditional methods degrade distinctly after 2 weeks. Route optimization involves the reaction of PbI2 with excess HI to generate HPbI3, which subsequently undergoes reaction with excess CH3NH2 to deliver CH3NH3PbI3 thin films. High quality of intermediate HPbI3 and CH3NH2 abundance are two important factors to stable CH3NH3PbI3 perovskite. Excess volatile acid/base not only affords full conversion in nonstoichiometric acid–base reaction route but also permits its facile removal for stoichiometric purification, resulting in average efficiency of 16.1% in forward/reverse scans.

Rapid growth of high quality perovskite crystal by solvent mixing

High quality polycrystalline CH3NH3PbI3 was prepared by dripping the γ-butyrolactone (GBL) solution of CH3NH3PbI3 into toluene, dimethylformamide (DMF) and dimethylsilicone oil. In toluene, a compact CH3NH3PbI3 thin film was observed but with grain size smaller than 1 μm because of the rapid extraction of GBL. As CH3NH3PbI3 is soluble in DMF, the crystallization in DMF is understood by interdiffusion induced local and temporal supersaturated droplets, which allow for growth of irregular, separated single crystals with size of more than 0.1 mm. In dimethylsilicone oil, compact, yolk-like polycrystalline CH3NH3PbI3 was observed. Unlike the round grains observed in toluene, it is composed of single crystal grains with clear angular surfaces. The average crystal size is tens of μm and can be modulated by the solvent temperature. We suggest that the poor miscibility between GBL and dimethylsilicone oil, and the poor solubility of CH3NH3I, PbI2 and CH3NH3PbI3 in the dimethylsilicone oil, allow slow extraction of GBL while keeping the stoichiometry of CH3NH3PbI3. The results offer a feasible and high-yielding method for growing uniform CH3NH3PbI3 crystals in a few seconds.

Abnormal Synergetic Effect of Organic and Halide Ions on the Stability and Optoelectronic Properties of a Mixed Perovskite via In Situ Characterizations

The mixed cation lead mixed halide perovskite (MLMP) Csx FA1-x PbIy Br3-y is one of the most promising candidates for both single-junction and tandem solar cells due to its high efficiency and remarkable stability. However, the composition effect on thermal stability and photovoltaic performances has not yet been comprehensively investigated. Therefore, the interplay between composition, crystal structure, morphology, and optoelectronic properties under heat stress, is systematically elucidated here through a series of in situ characterizations. It is revealed for the first time that the FA+ and Br- release synchronously at first even under mild annealing. This leads to a serious FA- and Br-deficiency issue, with only 88.3% of Br and 90.2% of FA retained after annealing at 100 °C, which significantly magnifies the hysteresis, phase segregation, and instability issues. Finally, a trace amount of FA+ and Br- is introduced onto the post-annealed MLMP surface to compensate for the deficiency through vacancy filling. The degradation lifetime to 80% of the initial efficiency (t80 ) is improved from 504 to 1056 h and the hysteresis issue is also well resolved. This work highlights the importance of the synergetic composition effect of the organic cation and halide anion on stability and efficiency optimization for long-term applications.