From Macroscopic to Nanoscopic Current Hysteresis Suppressed by Fullerene in Perovskite Solar Cells

et al. 2020

Adv Materials Inter

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All‐inorganic halide perovskite (AIHP) is becoming one of the most promising generation materials of perovskite photovoltaics for commercialization due to its thermodynamic stability and soared efficiency. Depending on material properties, grain boundary (GB) has detrimental or beneficial effect on device photovoltaic performance. However, less attention is paid to GB behavior in AIHPs. Herein, it is concluded that the microscopic GBs are the major sites for photocarrier generation and transport, as well as the ionic pathway dominating the current hysteresis behavior in AIHP solar cells. Kelvin probe force microscopy (KPFM) measurements reveal a lower surface potential at GBs as compared to grain interiors (GIs), suggesting a significant upward band bending around GBs. Conductive atomic force microscopy (c‐AFM) measurements show a higher current flow in the vicinity of GBs, indicating enhanced carrier separation and collection taking place at GBs. Furthermore, the existence of ion motion is evidenced both by the single‐point c‐AFM measurement and voltage‐controlled KPFM measurement, which accounts for the common current hysteresis behavior in AIHP solar cells. These investigations provide an advanced understanding of the role of GBs in AIHP and further inspiration on how to optimize device performance up to the levels of organic–inorganic hybrid perovskite.

Section: Resultssupporting

confidence: 90%

Comprehensive Elucidation of Grain Boundary Behavior in All‐Inorganic Halide Perovskites by Scanning Probe Microscopy

Wang

et al. 2020

Adv Materials Inter

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“…This hysteresis mainly originates from the ionic motion within the perovskite. 49 In addition, the unbalanced charge transport in the PSC with a perovskite thickness of 250 nm results in the larger current hysteresis. 50 The smaller hysteresis in the PSCs with a perovskite thickness of 400 nm should be attributed to reduced interface-accumulated charges due to fast charge transfer from the perovskite to FTO.…”

Section: Resultsmentioning

confidence: 99%

“…All these devices show non-negligible current hysteresis. This hysteresis mainly originates from the ionic motion within the perovskite . In addition, the unbalanced charge transport in the PSC with a perovskite thickness of 250 nm results in the larger current hysteresis .…”

Section: Resultsmentioning

confidence: 99%

Charge Carrier Dynamics in Electron-Transport-Layer-Free Perovskite Solar Cells

Wang

ACS Appl. Electron. Mater.

et al. 2019

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Better understanding of the carrier dynamics process in electron-transport-layer (ETL)-free perovskite solar cells (PSCs) enables further exploration of the perovskite potential and corresponding device structure design. Herein, an ETL-free perovskite/transparent conductive film SnO2:F(FTO) contact with different perovskite thicknesses illuminated by varied light intensities were investigated by scanning probe microscopy technology. Strong charge transfer occurs at the perovskite/FTO contact, evidenced by the variations of both the surface contact potential and local current value. Mott–Schottky analysis based on capacitance–voltage measurements suggests that the interface property of the perovskite/FTO contact is similar to that of the perovskite/TiO2/FTO structure, making it ideal for fabricating ETL-free PSCs. The cross-sectional surface potential drop was found to be mainly located at the CH3NH3PbI3/FTO heterojunction, suggesting a single diode junction in the ETL-free PSCs. Furthermore, a single-side abrupt p–n++ junction model is employed to illustrate the energy level alignment at the perovskite/FTO contact. This study will be helpful for understanding the carrier dynamics process and thus achieving high device efficiency in the ETL-free PSCs.

“…It has been confirmed that the PC 61 BM molecule undergoes an electron transfer reaction with the halogens from perovskite and binds them to the crystal GBs . When these I – ions are covalently attached to PC 61 BM molecules, their migration that drifted by an external field will be ineffective, so the current hysteresis is suppressed. − When excess MA + ions are in the PC 61 BM, the PC 61 BM can only absorb part of the MA + ions, or most of the I – ions are free to move, and even into bulk perovskite, so there will be an obvious phenomenon of light soaking. The schematic illustration for the origin of light-soaking behavior is shown in Figure .…”

Section: Resultsmentioning

confidence: 99%

Elimination of Light-Soaking Effect in Hysteresis-Free Perovskite Solar Cells by Interfacial Modification

Yang

et al. 2019

J. Phys. Chem. C

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The hysteresis and light-soaking effect have been observed in organo−metal halide perovskite solar cells (PSCs) under operating conditions, which inhibit the precise evaluation of power output. The mechanisms leading to these effects are little understood. Here, our studies have demonstrated that the light-soaking effect is related to the electron selective layer/perovskite interface in hysteresis-free PSCs. The introduction of [6,6]-phenyl C 61 -butyric acid methyl ester (PC 61 BM) doped with CH 3 NH 3 I molecules as the interfacial layer can effectively release or eliminate this effect due to the efficient charge transfer, accompanied with the best stable output efficiency of 19% and an ultrahigh fill factor over 84%. Capacitance−voltage and capacitance−frequency curves derived from electrochemical impedance spectroscopy demonstrate that the light-soaking effect in PSCs mainly originates from the charge accumulation at the PC 61 BM/ perovskite interface. The CH 3 NH 3 I doped in PC 61 BM forestalls the ion movement among the perovskite and thus eliminates the light-soaking effect. Furthermore, a model that combines ion migration and the charge accumulation process to interpret the light-soaking effect and performance improvement in PSCs is proposed.