(Invited) Optoelectronic and Dielectric Properties of Organic Cation Tin-Based Perovskite Solar Cells
Authors: Saeki Akinori 
Published: July 2021

Tin iodide perovskite (ASnI3) is one of the most feasible, less toxic alternatives to lead halide perovskite (APbX3), owing to its readily formed three-dimensional (D) structure and more suited bandgap (1.2–1.4 eV) of the solar cell application than APbX3 (1.5–2.3 eV). Regardless of the skyrocketing boost in Pb perovskite solar cells throughout this decade, Sn-based counterparts have remained constrained because of the intrinsically unstable Sn2+ that is easily oxidized to Sn4+. The resultant p-doping causes electron trapping and a shorting of the diode circuit, which is partly mitigated by SnF2 addition. Nevertheless, the power conversion efficiency (PCE) values of MASnI3 (MA: methylammonium cation) based on a normal cell structure (mesoporous TiO2/active layer/hole transport layer: HTL) have been suppressed due to the p-doping and low film quality. Notably, progress in an inverted cell structure (HTL/active layer/electron transport layer: ETL) has been reported for FASnI3 (FA: formamidinium cation) mixed with either MA, PEA (phenylethylamine cation), EDA (ethylenediammonium dication), or GA (guanidium cation). In addition to this A-site cation mixing strategy associated with 2D/3D perovskites, the use of {en} (ethylenediammonium), SnX2-(pyrazine, trimethylamine, or solvent) complexes, hot anti-solvent treatment (HAT), poly(vinyl alcohol) (PVA) addition, and GeI2 dopingreportedly showed improvement in PCEs. Thus, the updated PCE in the forefront of the Sn perovskite solar cell is ~9–10%, which is enabled by MA/FA mixing, FA/PEA mixing, GA/FA mixing, as well as the addition of 4-fuorobenzohydrazide (FBH), 2-cyano-3-[5-[4-(diphenylamino)phenyl]-2-thienyl]-propenoic acid (CDTA), or passivation with ethylenediamine (edamine). Although the mixing of A-site cations, along with additives, has been found to smooth the film surface, decrease the Sn4+ impurities, and tune the valence band maximum (VBM), their effects on charge carrier dynamics and morphological changes are the subject of ongoing debate.

We characterized the solar cells composed of ternary A-site cation-mixed Sn perovskite: ASnI3 [A = (GAxFA1–x)0.9PEA0.1, the ratios are the precursor stoichiometry] by changing the GA content (x = 0–1) and investigated their charge carrier dynamics, surface morphologies, and energetics.[1] The inclusion of 10 mol% PEA was found necessary for performing these evaluations over a wide range of GA:FA ratios. Electrode-less, time-resolved microwave conductivity (TRMC) measurements[2] were applied to a pristine ASnI3 film, double layers of HTL/ASnI3 and ASnI3/ETL, as well as a triple layer of HTL/ASnI3/ETL, where PEDOT:PSS and thermally evaporated fullerene (C60) were used as the HTL and ETL, respectively. These comparative samples with different GA contents discerned the contributions of hole and electron mobilities in the TRMC transients. Notably, the change in electron mobilities corresponded to the PCE dependence on the GA content, highlighting the pivotal role of electron mobility in solar cell performance. We further examined the atomic force microscopy (AFM) surface images, VBM measured by photoelectron spectroscopy (PYS), and crystalline features measured by X-ray diffraction (XRD), which supports the dramatic change of charge carrier mobility and lifetime.

On one hand, the dielectric properties of organic-inorganic hybrid perovskite materials have remained poorly understood, despite probably influencing delayed charge recombination and device capacitance. We characterize the unprecedented dielectric behavior of MHPs comprising methylammonium cations, Pb/Sn as metals, and Br/I as halides using TRMC measurements.[3] At specific compositions, the above MHPs exhibit negative real and positive imaginary photoconductivities, the polarities of which are opposite to those observed for conventional photogenerated charge carriers. Comparing the observed TRMC kinetics with that of inorganic perovskites (SrTiO3 and BaTiO3) and characterizing its dependence on temperature, frequency and near-infrared second push pulse, we conclude that the above behavior is due to the trapping of polaronic holes/electrons by oriented dipoles of organic cations, which opens a hitherto unexplored route to the dynamical control of dielectric permittivity by photoirradiation.

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