Revealing the Degradation and Self‐Healing Mechanisms in Perovskite Solar Cells by Sub‐Bandgap External Quantum Efficiency Spectroscopy

Abstract

Ion dissociation has been identified to determine the intrinsic stability of perovskite solar cells (PVSCs), but the underlying degradation mechanism is still elusive. Herein, by combining highly sensitive sub‐bandgap external quantum efficiency (s‐EQE) spectroscopy, impedance analysis, and theoretical calculations, the evolution of defect states in PVSCs during the degradation can be monitored. It is found that the degradation of PVSCs can be divided into three steps: 1) dissociation of ions from perovskite lattices, 2) migration of dissociated ions, and 3) consumption of I− by reacting with metal electrode. Importantly, step (3) is found to be crucial as it will accelerate the first two steps and lead to continuous degradation. By replacing the metal with more chemically robust indium tin oxide (ITO), it is found that the dissociated ions under light soaking will only saturate at the perovskite/ITO interface. Importantly, the dissociated ions will subsequently restore to the corresponding vacancies under dark condition to heal the perovskite and photovoltaic performance. Such shuttling of mobile ions without consumption in the ITO‐contact PVSCs results in harvesting–rest–recovery cycles in natural day/night operation. It is envisioned that the mechanism of the intrinsic perovskite material degradation reported here will lead to clearer research directions toward highly stable PVSCs.

Self‐Healing Cs3Bi2Br3I6 Perovskite Wafers for X‐Ray Detection

“…[30] Self-healing occurs in perovskite solar cells based on CH 3 NH 3 PbI 3 after removal of water vapors, [31] or after switching off light irradiation. [32,33] Here we observe evidences for selfhealing for the Cs 3 Bi 2 Br 3 I 6 X-ray detectors under ambient conditions in darkness. After a long storage time of 1.5 years, we find a reduction of the samples dark current by one order of magnitude, combined with an increase of the current under X-ray pulses, a substantial decrease of the dark current drift and an improvement of the transient response.…”

Perovskite/Si tandem solar cells: Fundamentals, advances, challenges, and novel applications

“…Various transparent conductive materials have been employed as electrode for perovskite solar cells including thin metal layer, Ag nanowire, carbon nanotube, graphene, and TCOs 107–110 . Among these, TCOs such as ITO has been evidenced to be a promising top electrode for semitransparent perovskite cells by significantly improving the device lifetime 37 . Generally, the ITO made by industrial sputtering technique are widely used in Si‐based PV technology.…”

“…33,34 Previous calculation indicates that top cell with bandgap between 1.6 and 1.8 eV is perfect for crystalline Si (1.1 eV) bottom cell to construct a tandem device. 35,36 Furthermore, the perovskite materials exhibit ultralow subbandgap absorption, 37 which indicates that perovskite top cell is highly transparent at photon energies below its bandgap and reduces the optical loss for the bottom cell. Finally, since the perovskite layer can be made via solution-process or vacuum-based thermal evaporation, it is easy to fabricate perovskite top cell on the surface of Si bottom cell directly to form a 2T tandem device.…”

“…[107][108][109][110] Among these, TCOs such as ITO has been evidenced to be a promising top electrode for semitransparent perovskite cells by significantly improving the device lifetime. 37 Generally, the ITO made by industrial sputtering technique are widely used in Si-based PV technology. To achieve a highly transparent and conductive ITO layer, a hightemperature treatment at over 300 • C is always required during sputtering or after sputtering process.…”

Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport

“…The distinct features of electron and hole transport, which are crucial for the effective separation of free charges, remain to be properly identified. Importantly, the trap‐assisted recombination of free carriers can induce chemical bond breaking [ 41 ] and degradation of the active PV material. This paper provides the most detailed description of methylammonium lead iodide (MAPbI 3 ) defects existing to date, measuring their concentration, energy, capture cross‐section, and charge trapping/detrapping time by several highly sensitive spectroscopy methods.…”