Using time-resolved photoluminescence and transient absorption measurements at room temperature, we report excitation-intensity-dependent photocarrier recombination processes in thin films made from ...the organo-metal halide perovskite semiconductor CH3NH3PbI3 for solar-cell applications. The photocarrier dynamics are well described by a simple rate equation including single-carrier trapping and electron–hole radiative recombination. This result provides clear evidence that the free-carrier model is better than the exciton model for interpreting the optical properties of CH3NH3PbI3. The observed large two-carrier recombination rate suggests the promising potential of perovskite semiconductors for optoelectronic device applications. Our findings provide the information about the dynamical behaviors of photoexcited carriers that is needed for developing high-efficiency perovskite solar cells.
Abstract
The toxicity of lead perovskite hampers the commercialization of perovskite-based photovoltaics. While tin perovskite is a promising alternative, the facile oxidation of tin(II) to tin(IV) ...causes a high density of defects, resulting in lower solar cell efficiencies. Here, we show that tin(0) nanoparticles in the precursor solution can scavenge tin(IV) impurities, and demonstrate that this treatment leads to effectively tin(IV)-free perovskite films with strong photoluminescence and prolonged decay lifetimes. These nanoparticles are generated by the selective reaction of a dihydropyrazine derivative with the tin(II) fluoride additive already present in the precursor solution. Using this nanoparticle treatment, the power conversion efficiency of tin-based solar cells reaches 11.5%, with an open-circuit voltage of 0.76 V. Our nanoparticle treatment is a simple and broadly effective method that improves the purity and electrical performance of tin perovskite films.
We investigated the exciton–phonon couplings and exciton binding energy in CH3NH3PbI3 (MAPbI3) single crystals using temperature-dependent photocurrent (PC) and photoluminescence (PL) spectroscopy. ...The PC and PL data provide clear evidence of the existence of excitons in orthorhombic-phase MAPbI3. The temperature-dependent PC data were found to be less influenced by the bound excitons than the PL data, and thus the PC data reflect the intrinsic scatterings of excitons. We observed that the exciton–phonon couplings were strong in MAPbI3 and determined the longitudinal optical phonon energy to be 16.1 meV. Moreover, on the basis of the temperature dependences of the PC and PL data, we evaluated the exciton binding energy to be 12.4 meV for orthorhombic-phase MAPbI3 single crystals. Our findings pave a way for using simultaneous PC and PL measurements to determine precisely fundamental properties of perovskites.
We utilized time-resolved photoluminescence (PL) spectroscopy to investigate the photocarrier recombination dynamics in CH3NH3PbI3 thin films as a function of the time elapsed from the film’s ...fabrication. We found that the PL lifetime gradually increased and began to level out once the age of the film reached ∼30 h. Even under weak excitation, the PL dynamics depended on the excitation intensity in the fresh sample, while the mature sample displayed no excitation-intensity dependence associated with the PL dynamics. We submit that this can be explained by the fact that a significant number of defects are initially formed in CH3NH3PbI3 thin films fabricated by the sequential method and are spontaneously reduced by room-temperature annealing. Our results provide important insights for reducing the nonradiative recombination centers, which improves the power conversion efficiency of perovskite solar cells.
The dynamic optical properties of perovskite CH3NH3PbI3 single crystals were studied by means of time-resolved photoluminescence (PL) spectroscopy at room temperature. The PL peak under one-photon ...excitation exhibits a red-shift with elapsing time, while two-photon PL is time-independent and appears at lower energy levels. The low-energy two-photon PL can be attributed to emissions from the localized states because of strong band-to-band absorption and photon re-absorption of the emitted light in the interior region. We revealed that the PL behaviors can be explained by the diffusion of photocarriers generated in the near-surface region to the interior region. The excitation fluence dependence of the one-photon PL dynamics is also discussed in terms of the electron–hole radiative recombination and carrier diffusion effects.
Even though the Scholl reaction is one of the most powerful processes for the synthesis of polycyclic aromatic hydrocarbons (PAHs), its mechanism still remains a subject of discussion. Herein, we ...report a unique twofold Scholl cyclization of a 5,11‐dinaphthyltetracene. Single‐crystal X‐ray diffraction analysis of the cyclization product revealed that unsymmetric cyclizations of the two naphthyl groups resulted in the formation of fully unsaturated pentagonal and hexagonal rings. The thus obtained product exhibits a twisted π‐surface and an absorption band that reaches up to 950 nm. A combined experimental and theoretical study showed that such unsymmetric Scholl cyclizations can be rationalized in terms of a mechanism that involves dicationic intermediates, which stands in contrast to previously reported pathways based on radical cations and arenium ions.
Doubly positive: The twofold Scholl cyclization of a 5,11‐dinaphthyltetracene furnished an unsymmetrically cyclized product with a twisted π‐surface and an absorption band reaching up to 950 nm. A combined experimental and theoretical study showed that such unsymmetric Scholl cyclizations can be rationalized by a mechanism that involves dicationic intermediates.
Seeing the light: Tuning the electron‐donating ability of the π‐conjugated framework of bithiophene has resulted in intense solid‐state emissions with maxima ranging over a wide visible region (see ...picture). Even a deep‐red fluorescence with a large Stokes shift close to 200 nm, arising from the intramolecular charge‐transfer (CT) transition from the twisted bithiophene π framework to the boron center, can be obtained.
Organic–inorganic hybrid perovskite solar cells are attracting much attention due to their excellent photovoltaic properties. In these multilayered structures, the device performance is determined by ...complicated carrier dynamics. Here, we studied photocarrier recombination and injection dynamics in CH3NH3PbI3 perovskite solar cells using time-resolved photoluminescence (PL) and photocurrent (PC) measurements. It is found that a peculiar slowdown in the PL decay time constants of the perovskite layer occurs for higher excitation powers, followed by a decrease of the external quantum efficiency for PC. This indicates that a carrier-injection bottleneck exists at the heterojunction interfaces, which limits the photovoltaic performance of the device in concentrator applications. We conclude that the carrier-injection rate is sensitive to the photogenerated carrier density, and the carrier-injection bottleneck strongly enhances recombination losses of photocarriers in the perovskite layer at high excitation conditions. The physical origin of the bottleneck is discussed based on the result of numerical simulations.
Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the ...performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices.
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