Interface-Dependent Ion Migration/Accumulation Controls Hysteresis in MAPbI₃ Solar Cells

Abstract

Hysteresis in the current−voltage characteristics of hybrid organic−inorganic perovskite-based solar cells is one of the fundamental aspects of these cells that we do not understand well. One possible cause, suggested for the hysteresis, is polarization of the perovskite layer under applied voltage and illumination bias, due to ion migration within the perovskite. To study this problem systemically, current−voltage characteristics of both regular (light incident through the electron conducting contact) and so-called inverted (light incident through the hole conducting contact) perovskite cells were studied at different temperatures and scan rates. We explain our results by assuming that the effects of scan rate and temperature on hysteresis are strongly correlated to ion migration within the device, with the rate-determining step being ion migration at/across the interfaces of the perovskite layer with the contact materials. By correlating between the scan rate with the measurement temperature, we show that the inverted and regular cells operate in different hysteresis regimes, with dif ferent activation energies of 0.28 ± 0.04 eV and 0.59 ± 0.09 eV, respectively. We suggest that the differences observed between the two architectures are due to different rates of ion migration close to the interfaces, and conclude that the diffusion coefficient of migrating ions in the inverted cells is 3 orders of magnitude higher than in the regular cells, leading to different accumulation rates of ions near the interfaces. Analysis of V OC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells.

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…b) Selective contact materials, [76,116] i.e., hole and electron transport layers (TLs), play crucial roles in this behavior, including the material and the morphology (mesoporous or planar) of the TLs. c) The time scale of hysteresis is in the range of ≈10 to 100 s, [117,118] which is in great contrast to the typical charge generation/recombination processes (≈ns) in PSCs.…”

Migration of cations induces reversible performance losses over day/night cycling in perovskite solar cells

“…25,30,31,[34][35][36][37][38][39][40][41][42][43] Ion migration on timescales from 10 -1 to 10 2 s has been widely investigated to explain the hysteresis of current density-voltage (J-V) curves. 36,37,40,[44][45][46][47][48] However, the impact of X and potentially A and/or B defect formation and migration on PSC performance on timescales above 10 3 s, which are indicative of long-term stability, remains unknown. 49 Little experimental evidence exists on this subject since separating reversible ion migration from any non-reversible long-term degradation is complex in real device working conditions, i.e.…”

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI3

“…35,36 According to recent reports, the charge/ion accumulation at TiO 2 /perovskite interface is intimately responsible for the hysteresis response of the devices. 37,38 Before exploring the charge/ion accumulation mechanism in the devices, the key parameters of solar cells such as series resistance (R S ), diode ideality factor (m) and dark reverse saturation Considering the fact that hysteresis phenomena may be related to the interfacial charge accumulation and is more prominent in MAPbI 3 (s) than MAPbI 3 (m) devices, impedance spectroscopy (IS) measurements were performed at zero bias under dark and illumination to investigate the ionic and electronic charge accumulation. As shown in Figure S8, Nyquist spectra in the dark exhibit a high frequency semicircle related to the electronic transport followed by the low frequency line due to ionic diffusion or ionic charge accumulation.…”