Ion Migration and the Role of Preconditioning Cycles in the Stabilization of the J–V Characteristics of Inverted Hybrid Perovskite Solar Cells

Abstract

groups have recently and independently demonstrated that, by applying an electric fi eld across a pristine fi lm of 3D hybrid perovskites of different chemical composition, a self-sustained fi eld is induced in the semiconductor as a consequence of ion migration toward the electrode regions. [10][11][12][13] The formation of a self-sustained internal fi eld upon device polarization is also in good agreement with the observations reported by Tan et al. when testing perovskite-based light emitting diodes. [ 14 ] This concept has also been the base of the explanation proposed by Tress et al. for the rate-dependent hysteresis seen in current-voltage scans of solar cells. [ 15 ] So far reports suggest that transient electrical characteristics are due to a polarization response of the perovskite active layer that results in changes in the photocurrent extraction effi ciency of the device. [ 11,15 ] However, it must be noted that a variety of dynamics have been reported, which differ in magnitude and time scale, depending both on the specifi c device architecture and, in particular, on the adopted charge extraction layer. [ 7,9 ] This indicates that contact interfaces have a considerable effect on transients in perovskite based devices.In this Communication we investigate the role played by charge extracting layers on the slow transient behavior of CH 3 NH 3 PbI 3 perovskite based solar cells. Such transients, which typically affect both short-circuit currents and open circuit voltage of hysteretic devices, are found to notably modify the open circuit voltage also in the very fi rst J -V scans of so-called "hysteresis-free" devices integrating a phenyl-C61-butyric acid methyl ester (PCBM) charge extraction layer. Here a preconditioning of the device, i.e., a repetition of J -V scans, is needed to achieve completely stable J -V characteristics under illumination. In particular, we fi nd that under device operation, iodide ions migrate to the electron extracting layer. We fi rst show that the use of an organic extraction layer such as PCBM, albeit not hampering ions motion, evidently improves charge extraction with respect to interfaces involving compact TiO 2 , in agreement with what is suggested in other seminal investigations, [ 16,17 ] and makes the short-circuit current density virtually insensitive to the transient phenomena related to ions migration. Moreover, while self-doping of the perovskite fi lm close to the contact has been generally put forward in the study of transient phenomena, [ 12,13 ] here we show that ions can specifi cally interact with the organic electron extracting layer, inducing electronic doping and that such I − /PCBM interaction is at the origin of the preconditioning requirement for stabilizing the device and for improving its open circuit voltage with respect to the fi rst scan.Solution-processable hybrid perovskite semiconductors have risen to the forefront of photovoltaics research, offering the potential to combine low-cost fabrication with high-powerconversion effi ciency. Originally u...

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

“…These accumulated ions result in both a change of the internal field and a concomitant modulation of interfacial barriers, which ultimately results in the hysteretic behavior. [121] This model can be simply depicted as an interfacial mechanism, as shown in Figure 8.…”

“…[171] Recent evidence consistently suggests that c) the migration of iodide ions plays the major role in the hysteretic behavior of PSCs. [121] Xiao et al [173] proposed that positively charged iodide vacancies, V I , were responsible for the n-type doping in PSC under an external electrical field. Furthermore, long-term electrical biasing in devices with lateral electrodes led to an obvious degradation at the anode side.…”

“…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. [119] d) The hysteresis strongly depends on the external scanning parameters, [120][121][122] such as scan rate, amplitude of external electrical field, scanning direction, pre-scanning conditions, etc. During the last years, different mechanisms have been proposed to be responsible including 1) ferroelectricity, 2) charge trapping/detrapping, and 3) ionic migration.…”

“…[76,121] Several groups have made efforts regarding the construction of a band diagram, based on indirect evidence, such as transient photocurrent [171] and surface potential characterization. [180] On the other hand, electroabsorption spectroscopy [185,186] has proven to be a powerful approach to study the internal electric field and to characterize the interfacial barrier modulation under working conditions.…”

Hysteresis Analysis of Organolead Halide Perovskite Solar Cells by Transient Current Measurement

“…There have been a lot of reports dealing with the issue since perovskite solar cells became a hot topic. Several components are considered for the origin of hysteresis such as ferroelectricity, [10][11][12][13] ion migration, [14][15][16][17][18][19][20][21][22] carrier trapping 15,26 and capacitance component at the interfaces. [27][28][29][30][31][32] The effect of ferroelectricity is analyzed as the response of bias poling.…”