原子力显微镜案例分享 | 钙钛矿的光电滞回研究
本文把卤素钙钛矿的离子迁移、极化和PV回滞性在纳米尺度上联系起来。通过牛津仪器Asylum Research的专利技术DART PFM实验可以看出，光照下极化显著增强（下文图1），离子迁移性没有显著增加；在样品上加正向和反向的电压，纳米光电流图像没有明显区别（下文图2）。宏观CsFAMA电池实验表明回滞系数只有3%，本文将离子迁移、极化和回滞联系起来，光电流增强了极化，然而并不导致回滞增强。本文用到了MPF-3D上的光电测量技术。
U.S. and Chinese researchers used nanoscale electrical and strain-based SPM techniques to investigate photoinduced processes in perovskite materials. The results elucidate the greater and lesser roles of ion migration and spontaneous polarization, respectively, in photocurrent hysteresis.
Halide perovskite materials are exciting candidates for next-generation photovoltaics due to their high conversion efficiencies and relative ease of manufacture. However, large-scale production of perovskite solar cells requires a better understanding of current hysteresis and its impact on device efficiency and stability.
A research team from Univ. Washington–Seattle (U.S.), the Chinese Academy of Sciences–Shenzhen, and several Chinese universities has gained insight into this topic. They applied scanning probe microscopy (SPM) techniques to measure electrical and electromechanical behavior on the nanoscale in both light and dark conditions. Samples contained the triple-cation mixed-halide perovskite CsFAMA [Csx(FAyMA1−y)1−xPb(IzBr1−z)3], where FA=NH2CHNH2 (formamidinium) and MA=CH3NH2 (methylammonium).
The SPM data showed that illumination induced minimal hysteresis but strongly enhanced polarization strain. These findings indicate that photovoltaic hysteresis in ferroelectric halide perovskites is caused primarily by ion migration and not spontaneous polarization. The nanoscale information was further leveraged to construct a macroscale CsFAMA solar cell with 20.11% efficiency and hysteresis index as low as 3%.
The results establish nanoscale correlations between ion migration, polarization, and hysteresis in perovskites for the first time. In so doing, they advance understanding of photovoltaic processes in perovskites and could accelerate development of commercial perovskite-based solar cells.
Instrument used
MFP-3D AFM
Techniques used
Dynamic-strain-based SPM experiments were performed on an MFP-3D AFM in both illuminated and dark conditions. The Photovoltaic Option for MFP-3D Infinity AFMs provides a turnkey solution for experiments such as these that require high-resolution measurements with simultaneous bottom-sample illumination. The strain-based experiments used either piezoresponse force microscopy (PFM) or a custom, pointwise measurement of first and second harmonic response. For enhanced sensitivity, PFM was performed in Dual AC Resonance Tracking mode (DART PFM). Images of photocurrent were also obtained at different forward and reverse voltage biases with conductive AFM (CAFM) on an MFP-3D with the ORCA module. Image acquisition time was minimized by using the MacroBuild high-level GUI to automatically adjust the DC bias while continuing to scan.
Citation: G. Xia, B. Huang, Y. Zhang et al., Nanoscale insights into photovoltaic hysteresis in triple-cation mixed-halide perovskite: Resolving the role of polarization and ionic migration. Adv. Mater. 31, 1902870 (2019).
https://doi.org/10.1002/adma.201902870
Note: The data shown here are reused under fair use from the original article, which can be accessed through the article link above.
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