Ionic polarization-induced current–voltage hysteresis in CH3NH3PbX3 perovskite solar cells

Abstract

CH3NH3PbX3 (MAPbX3) perovskites have attracted considerable attention as absorber materials for solar light harvesting, reaching solar to power conversion efficiencies above 20%. In spite of the rapid evolution of the efficiencies, the understanding of basic properties of these semiconductors is still ongoing. One phenomenon with so far unclear origin is the so-called hysteresis in the current–voltage characteristics of these solar cells. Here we investigate the origin of this phenomenon with a combined experimental and computational approach. Experimentally the activation energy for the hysteretic process is determined and compared with the computational results. First-principles simulations show that the timescale for MA+ rotation excludes a MA-related ferroelectric effect as possible origin for the observed hysteresis. On the other hand, the computationally determined activation energies for halide ion (vacancy) migration are in excellent agreement with the experimentally determined values, suggesting that the migration of this species causes the observed hysteretic behaviour of these solar cells.

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

“…Based on first principle simulations (e.g., DFT), the time interval for the polarization switching was in the nanosecond region. [130] Leguy et al [131] estimated the expected time interval within a millisecond range, and noted that ferroelectric materials require an electric field (larger than the coercive field) to switch ferroelectric domains, which contradicts the observations in step-wise J-V curve measurements, exhibiting always a transition-like behavior. [132] Hence, although the contribution of ferroelectricity to hysteresis is still under debate, [133] the above discussion suggests that ferroelectric polarization may not play a dominant role in the J-V hysteretic behavior.…”

“…Due to the difficulty of directly characterizing the ionic movement, most of the description is on the basis of theoretical DFT calculations. [130,171,177] Herein, the ionic transport is described as a hopping mechanism, i.e., jumping between neighboring sites. [178] As shown in Figure 10a,b, i) MA + ions migrate into a nearby vacant cage, ii) Pb 2+ ions migrate via the diagonal direction, whilst iii) iodide ions move along an octahedron in the Pb-I plane.…”

“…This barrier results in an activation energy, E a . Detailed DFT calculations have been provided by Mosconi et al [177] and Meloni et al [130] The latter calculated the migrating pathways for each defect, including iodide vacancies, MA vacancies, Pb vacancies, and iodide interstitials, as shown in Figure 10c-e, respectively. The migration pathways of individual ions/defects are consistent with the ones proposed by Eames et al [171] Apart from hopping through point defects (Schottky and Frenkel defects) within the crystal lattice, [178] as shown in Figure 10f,g, a range of other possible ion migration channels can exist.…”

“…Adapted with permission. [130] Copyright 2016, Nature Publishing Group. Macroscopic schematic of ionic migration channels.…”

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode

“…The low defect formation energies in MAPbI 3 indicate that the energy cost for bond breaking and distortion is low, which further implies low defect diffusion barriers. Indeed, fast diffusion of native defects in MAPbI 3 14, 17, 18, 19, 20, 21, 22, 23, 24, 25 and the resulting phenomena (such as hysteresis in current–voltage curves,25, 26, 27, 28 giant dielectric constant,28, 29 switchable photovoltaic effect,17 photon‐induced phase separation,30 etc.) have been reported and extensively discussed in the literature.…”