Impact of Interfaces and Laser Repetition Rate on Photocarrier Dynamics in Lead Halide Perovskites

Instant p-doping and pore elimination of the spiro-OMeTAD hole-transport layer in perovskite solar cells

An instant p-doping strategy employing 4-tert-butyl-2-chloropyridine and tert-butyl peroxybenzoate for the spiro-OMeTAD hole-transport layer (HTL) in perovskite solar cells (PSCs) is proposed to replace the conventional 4-tert-butylpyridine-doped HTL. The novel doping process eliminates the formation of pores in the HTL. Meanwhile, a 21.4% efficiency is achieved on the corresponding absolute methylammonium-free PSCs with significantly improved thermal and moisture stability.

Collective Motion Improves the Stability and Charge Carrier Lifetime of Metal Halide Perovskites: A Phonon-Resolved Nonadiabatic Molecular Dynamics Study

By implementing a novel algorithm that realizes the constraints of certain normal modes of interest and using nonadiabatic molecular dynamics for the CsPbBr₃, we explicitly demonstrate for the first time that the collective motion between the Cs atom and inorganic octahedra facilitates to delay the nonradiative recombination of negative and positive charges. The Cs atoms can instantaneously respond to the motion of Pb and Br atoms during normal molecular dynamics, maintain the perovskite structure, and homogenize the structural distortion caused by thermal fluctuations, thus decreasing nonadiabatic coupling and charge recombination. In contrast, the perovskite becomes unstable because geometry distortion is strongly localized when the normal modes of Cs atoms are constrained, which increases the nonadiabatic coupling and accelerates charge recombination. The study emphasizes the important role of correlated motion on the stability and charge-phonon dynamics in metal halide perovskites.

Are Shockley-Read-Hall and ABC models valid for lead halide perovskites?

Metal halide perovskites are an important class of emerging semiconductors. Their charge carrier dynamics is poorly understood due to limited knowledge of defect physics and charge carrier recombination mechanisms. Nevertheless, classical ABC and Shockley-Read-Hall (SRH) models are ubiquitously applied to perovskites without considering their validity. Herein, an advanced technique mapping photoluminescence quantum yield (PLQY) as a function of both the excitation pulse energy and repetition frequency is developed and employed to examine the validity of these models. While ABC and SRH fail to explain the charge dynamics in a broad range of conditions, the addition of Auger recombination and trapping to the SRH model enables a quantitative fitting of PLQY maps and low-power PL decay kinetics, and extracting trap concentrations and efficacies. However, PL kinetics at high power are too fast and cannot be explained. The proposed PLQY mapping technique is ideal for a comprehensive testing of theories and applicable to any semiconductor.

Efficient Energy Funnelling by Engineering the Bandgap of a Perovskite: Förster Resonance Energy Transfer or Charge Transfer?

Energy funnelling enables directional carrier transfer along cascaded energy levels, which can be employed to significantly improve energy transfer efficiency and photoelectronic performances. However, the exact mechanism is still under intensive debate on whether Förster resonance energy transfer (FRET) or charge transfer (CT) is playing the dominant role, hindering broad practical device design and applications. Herein, a spectroscopic method is developed to unveil the energy funnelling mechanism by comparing and modeling the photoluminescence (PL) spectra excited by pulsed and continuous-wave (CW) lasers. The applicability of this method is verified in a typical energy funnelling system constructed by engineering the bandgap of a perovskite. Composite hexagonal microplates (MPs) with FAPbBr₃, FAPb(Br_xI_1-x)₃, and FAPbI₃ (formamidinium = FA) at the surface, middle mezzanine, and bottom layers are synthesized by a two-step chemical vapor deposition (CVD) method, which introduces a directional energy funnelling from wide-bandgap FAPbBr₃ to narrow-bandgap FAPbI₃. By using the spectroscopic method developed in this work, we reveal that charge transfer is the dominant mechanism for energy funnelling in the FAPbBr₃/FAPb(Br_xI_1-x)₃/FAPbI₃ sandwich MP. This study not only provides novel insights into the energy funnelling in multiple-bandgap perovskite systems but also develops a widely applicable spectroscopic method to explore the energy funnelling mechanism in other graded bandgap systems.

Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their V_OC to values well below the Shockley-Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi-Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the V_OC of operational devices. These measurements prove that in state-of-the-art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump-probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome-paving the way to the thermodynamic efficiency limit.

Investigating the Role of the Organic Cation in Formamidinium Lead Iodide Perovskite Using Ultrafast Spectroscopy

Organic cation rotation in hybrid organic-inorganic lead halide perovskites has previously been associated with low charge recombination rates and (anti)ferroelectric domain formation. Two-dimensional infrared spectroscopy (2DIR) was used to directly measure 470 ± 50 fs and 2.8 ± 0.5 ps time constants associated with the reorientation of formamidinium cations (FA⁺, NH₂CHNH₂⁺) in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations reveal the FA⁺ agitates about an equilibrium position, with NH₂ groups pointing at opposite faces of the inorganic lattice cube, and undergoes 90° flips on picosecond time scales. Time-resolved infrared measurements revealed a prominent vibrational transient feature arising from a vibrational Stark shift: photogenerated charge carriers increase the internal electric field of perovskite thin films, perturbing the FA⁺ antisymmetric stretching vibrational potential, resulting in an observed 5 cm^-1 shift. Our 2DIR results provide the first direct measurement of FA⁺ rotation inside thin perovskite films, and cast significant doubt on the presence of long-lived (anti)ferroelectric domains, which the observed low charge recombination rates have been attributed to.