Control of Charge Recombination in Perovskites by Oxidation State of Halide Vacancy

Abstract

Advances in perovskite solar cells require development of means to control and eliminate the nonradiative charge recombination pathway. Using ab initio nonadiabatic molecular dynamics, we demonstrate that charge recombination in perovskites is extremely sensitive to the charge state of the halogen vacancy. A missing iodine anion in MAPbI3 has almost no effect on charge losses. However, when the vacancy is reduced, the recombination is accelerated by up to 2 orders of magnitude. The acceleration occurs due to formation of a deep hole trap in the singly reduced vacancy, and both deep and shallow hole traps for the doubly reduced vacancy. The shallow hole involves a significant rearrangement of the Pb–I lattice, leading to a new chemical species: a Pb–Pb dimer bound by the vacancy charge, and under-coordinated iodine bonds. Hole trapping by the singly reduced iodide vacancy operates parallel to recombination of free electron and hole, accelerating charge losses by a factor of 5. The doubly reduced vacancy acts by a sequential mechanism-free hole, to shallow trap, to deep trap, to free electron, and accelerates the recombination by a factor of 50. The study demonstrates that iodine anion vacancy can be beneficial to the performance, because it causes minor changes to the charge carrier lifetime, while increasing charge carrier concentration. However, the neutral iodine and iodine cation vacancies should be strongly avoided. The detailed insights into the charge carrier trapping and relaxation mechanisms provided by the simulation are essential for development of efficient photocatalytic, photovoltaic, optoelectronic and related devices.

Passivating Detrimental DX Centers in CH3NH3PbI3 for Reducing Nonradiative Recombination and Elongating Carrier Lifetime

“…The calculated lifetime of excited electrons in conduction band for the DX center in MAPbI 3 is 3.2 ns, agreeing excellently with the previously calculated theoretical value. [22] The lifetime increases significantly by a factor of 6 after Br passivation. The increased carrier lifetime can be ascribed to i) the reduced defect states, i.e., recombination channels, as shown in Figure 4c; ii) decreased EPC due to the breakdown of Pb-Pb dimer and saturation of I-DBs.…”

“…In the V I 1− system, two Pb 2+ ions near the vacancy site move close to form a strong covalent bond (abbreviated as Pb dimer) and create deep levels in the bandgap, [19,20] which act as electron-hole recombination centers and are one of the bottlenecks for further optimization of cell performance. [22] In order to further enhance PCE, it is necessary to eliminate the nonradiative recombination pathways in MAPbI 3 created by deep defects like iodine vacancy. [22] In order to further enhance PCE, it is necessary to eliminate the nonradiative recombination pathways in MAPbI 3 created by deep defects like iodine vacancy.…”

“…The results are shown in Figure 4b,d, respectively. [22] The lifetime increases significantly by a factor of 6 after Br passivation. [22] The lifetime increases significantly by a factor of 6 after Br passivation.…”

“…[16,21] Recently, Li et al revealed that charge recombination was extremely sensitive to the charge state of iodine vacancy, which significantly accelerates electron-hole recombination rate due to the formation of hole trap. [22] In order to further enhance PCE, it is necessary to eliminate the nonradiative recombination pathways in MAPbI 3 created by deep defects like iodine vacancy.…”

Indirect but Efficient: Laser-Excited Electrons Can Drive Ultrafast Polarization Switching in Ferroelectric Materials

“…For example, tuning the FE polarization can vary the tunneling resistance over several orders of magnitude [1], enabling technological advancements such as non-volatile memory in digital electronic devices, [2] memristors, [3] and integrated neuromorphic networks [4,5]. In addition, by enabling a steady-state photocurrent, the use of FE polarization can substantially increase light-harvesting efficiency, particularly in hybrid organic-inorganic halide perovskite solar cells [6][7][8][9][10][11][12][13][14][15][16][17]. Finally, the variation in FE polarization on surfaces can also dramatically change the adsorption energetics in catalytic systems and could be further harnessed to enable other polarization-dependent surface mechanisms [18,19].All of these applications are intrinsically associated with FE polarization and can, therefore, be further enhanced by tuning and controlling this intrinsic material property.…”