Unprecedented ambient triplet‐mediated emission in core‐substituted naphthalene diimide (cNDI) derivatives is unveiled via delayed fluorescence and room temperature phosphorescence. Carbazole core‐substituted cNDIs, with a donor–acceptor design, showed deep‐red triplet emission in solution processable films with high quantum yield. This study, with detailed theoretical calculations and time‐resolved emission experiments, enables new design insights into the triplet harvesting of cNDIs; an important family of molecules which has been, otherwise, extensively been investigated for its n‐type electronic character and tunable singlet fluorescence.
We present experimental evidence showing that the effective carrier diffusion length L d and lifetime τ depend on the carrier density in MAPbBr 3 single crystals. Independent measurements reveal that both L d and τ decrease with an increase in photocarrier density. Scanning photocurrent microscopy is used to extract the characteristic photocurrent I ph decay-length parameter L d , which is a measure of effective carrier diffusion. The L d magnitudes for electrons and holes are determined to be ∼13.3 and ∼13.8 μm, respectively. A marginal increase in uniform light bias ( 5 × 10 15 photons/cm 2 ) increases the modulated photocurrent magnitude and reduces the L d parameter by a factor of 2 and 3 for electrons and holes, respectively, indicating that the recombination is not monomolecular. The L d variations are correlated to the features in photoluminescence lifetime studies. Analysis of lifetime variation shows intensity-dependent monomolecular and bimolecular recombination trends with recombination constants determined to be ∼9.3 × 10 6 s −1 and ∼1.4 × 10 −9 cm 3 s −1 , respectively. Based on the trends of L d and lifetime, it is inferred that the sub-band-gap trap recombination influences carrier transport in the low-intensity excitation regime, while bimolecular recombination and transport dominate at high intensity.
Sequential deposition route is widely investigated in fabricating perovskite thin films for state‐of‐the‐art perovskite photovoltaics. However, concerns such as lower morphological control, phase purity, and remnant unreacted salts methylammonium iodide (MAI and PbI2) are raised, which can significantly deteriorate optoelectronic properties, hence the operational durability of the devices. Herein, a facile two‐step method to prepare high‐quality perovskite thin films with reproducibility is reported, as‐spun PbI2 is annealed at varying thermal input under controlled rate, and a trend in converted perovskite film properties is noted. Specifically, PbI2 thin film annealed at 200 °CC results in 20x intensified crystallinity with pinholes free and a subsequent reduction in the crystal microstrain. In addition, it provides higher surface roughness to load more MAI [in iso‐propyl alcohol (IPA)]; therefore, a higher perovskite conversion is achieved. This method enables a significant efficiency enhancement in the treated sample (Pero@PbI2‐200 °C) as compared with controlled film; it retains around 90% initial efficiency after 384 h of ambient exposure. Furthermore, a facile intermediate solvent treatment method to gain the complete conversion of PbI2 into perovskite is also reported. This study highlights the importance of morphological control in governing optoelectronic properties, hence the efficiency and stability of perovskite solar cells.
Direct bandgap perovskite and indirect bandgap Si, which form the two active layers in a tandem solar cell configuration, have different optoelectronic properties and thicknesses. The charge-carrier dynamics of the two-terminal perovskite-on-Si tandem solar cell in response to a supercontinuum light pulse is studied using transient photocurrent (TPC) measurements. Spectral dependence of TPC lifetime is observed and can be classified into two distinct timescales based on their respective carrier generation regions. The faster timescale (∼500 ns) corresponding to the spectral window (300–750 nm) represents the top-perovskite sub-cell, while the slower timescale regime of ∼25 μs corresponds to the bottom-Si sub-cell (>700 nm). Additionally, under light-bias conditions, the transient carrier dynamics of the perovskite sub-cell is observed to be coupled with that of the Si sub-cell. A sharp crossover from the fast-response to a slow-response of the device as a function of the light-bias intensity is observed. These results along with a model based on transfer matrix formulation highlight the role of charge-carrier dynamics in accessing higher efficiencies in tandem solar cells. The carrier transit times and lifetimes in addition to their optical properties need to be taken into account for optimizing the performance.
Thermally activated delayed fluorescence (TADF) systems exhibit high emissive yield due to efficient back-conversion of nonemissive triplet states to emissive singlet states via reverse intersystem crossing (RISC). In this paper, both the charge carrier and triplet exciton dynamics are explored using transient electroluminescence (TrEL) measurements in the TADF molecule, 2,3,4,6-Tetra(9H-carbazol-9-yl)-5-fluorobenzonitrile (4CzFCN)-based devices. The analysis of the rising edge of the TrEL pulse indicates that the carriers follow multiple trapping, de-trapping, and exciton recombination dynamics. The trailing edge of the TrEL pulse provides insight into the monomolecular and bimolecular exciton dynamics. These studies along with a kinetic model reveal triplet harvesting processes in a 4CzFCN molecule via both RISC and triplet–triplet annihilation (TTA). Furthermore, at high temperatures, the analysis suggests that TADF processes are dominant with negligible contribution from TTA. The presence of bimolecular triplet processes acts as bottlenecks for accessing higher efficiencies in TADF organic light emitting diodes.
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