In this paper, we present high-efficiency perovskite light-emitting diodes (PeLEDs) using high-quality green emissive colloidal CsPbBr3 nanocrystals (NCs) of size 11 ± 0.7 nm. The surface chemistry ...of the NCs was optimized to achieve both reasonably high luminescence and charge transport in NC film. The thickness of the active layer film was controlled by a layer-by-layer deposition technique, where each layer is being deposited and washed using methyl acetate (MeOAc) before we add another layer of NCs. MeOAc being a low surface tension solvent enters into the NC layers and partially removes the long-chain organic ligands oleylamine and oleic acid. The optical and structural properties were measured as a prescreening for these device-grade NCs. Steady-state photoluminescence (PL) and electroluminescence (EL) fwhm (full width at half maxima) are in the range of 20 ± 0.5 nm, suggesting a high color purity feature of these materials suitable for higher color gamut in the display applications. An ITO/PEDOT:PSS/CsPbBr3–NCs/TPBi/Ca/Al (TPBi is 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) device structure is fabricated with various thicknesses of the NC layer, and we demonstrated a maximum of ∼5.2 cd/A luminance efficiency for optimal thickness of the CsPbBr3 NC layer.
Triplet sensitization is an important aspect for enhancing the performance of organic light-emitting diodes (OLEDs). The triplet–triplet annihilation (TTA) process is known to increase the efficiency ...of OLEDs. In this case, the time scale of triplet diffusion is an important parameter. In this paper, we have studied the triplet energy-transfer process as a function of time scale and temperature in pure and phosphorescent molecule FIrpic (bis2-(4,6-difluorophenyl)pyridinato-C2,N(picolinato)iridium(III))-doped F8BT (poly(9,9-dioctylfluorene-alt-benzothiadiazole)) films. We have investigated the dynamics of triplet energy-transfer processes by probing the corresponding delayed fluorescence (DF) spectra (of the F8BT host system) generated from TTA. We have noticed that the dynamics of DF depends on the diffusion kinetics of the triplet excitonic state. We find that the successful triplet transfer to the host polymer system is directly proportional to the triplet lifetime of the guest molecule. We have further implemented doped F8BT films in polymer LEDs (PLEDs). A high triplet absorption (TA) signal (of the host triplets, i.e., F8BT) has been observed in doped PLEDs under electrical excitation. The high TA signal of the host triplets indicates the successful triplet transfer process from the FIrpic guest molecule to the F8BT host polymer system.
Quasi‐2D‐layered perovskites have more exotic optical properties than their 3D halide perovskites congener, such as prominent excitonic features, higher photoluminescence (PL) yield, and ...composition‐induced energy transfer among different phases. However, there are known challenges in quasi‐2D perovskite where by default one gets a mixture of various phases (n = 1, 2, 3….3D) in one composition. Energy transfer processes occur among various phases, and optimization is needed to ensure one can get the best optical properties by targeting optimal phase purity. Herein, a detailed analysis of structural and optical properties of different perovskites (2D perovskite, quasi‐2D perovskite, and 3D perovskite) of (PEA)2(FAPbBr3)n−1PbBr4 series is done. The n = 3 composition with the least phase impurity is prepared, which provides a higher PL emission intensity than members of this series. Hence, the n = 3 composition (emissive layer) based perovskite light‐emitting diode (PeLED) is fabricated. Poly(9‐vinyl carbazole) (PVK) as an additional hole‐injection layer with poly(3,4‐ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) helps to suppress (1) the leakage current (effective blocking of electrons) and (2) the non‐radiative channels at the interface. Optimized thickness of emissive layer in PeLED (100 nm) gives a luminance efficiency of 15.4 cd A−1 and external quantum efficiency (EQE) of 4.32%.
Quasi‐2D perovskites are mixtures of self‐assembled multiple‐quantum wells. Composition and processing‐induced optimal phase purity are essential for high photoluminescence quantum yield (PLQY) from these thin films. The n = 3 composition with optimal phases, give higher PLQY than other members (n ≠ 3) in (PEA)2(FAPbBr3)n−1PbBr4. Use of n = 3 composition with optimized device engineering give significant external quantum efficiency number.
Temperature‐dependent time‐resolved photoluminescence (TRPL) spectroscopy of high optical gain CsPbBr3 nanocrystals (NCs) films along with temperature‐dependent steady‐state absorption and ...photoluminescence (PL) spectra is studied. A red shift in the optical band edge and the corresponding PL peak is observed by lowering the temperature from room temperature, analogous to hybrid perovskite semiconductors. However, the faster PL decay of CsPbBr3 NCs film at a low temperature as compared to room temperature is in contrast with many other traditional colloidal quantum dots. The TRPL spectroscopy as a function of temperature suggests the presence of a red‐shifted (≈75 ± 15 meV) defect‐induced emission peak at longer times. Temperature‐dependent PL intensity study shows an Arrhenius type of character with two activation energies: ≈77 ± 18 and ≈12 ± 3 meV. A qualitative model is developed where interaction between the excitonic level and these observed localized states as a function of temperature can qualitatively explain the contrasting PL decay mechanism of CsPbBr3 NCs film. To ensure the quality of in‐house synthesized CsPbBr3 NCs with a mixture of oleylamine and octylamine ligands, the amplified spontaneous emission from CsPbBr3 NCs film as a planar waveguide medium is studied, which shows state‐of‐art optical gain coefficient of ≈51cm−1 and a loss coefficient of ≈15cm−1 at room temperature.
The temperature‐dependent emission kinetics in CsPbBr3 nanocrystals with energy landscape as shown in the figure dictates a contrasting decay dynamics, where the photoluminescence (PL) decays faster at lower temperatures. Delayed emission spectroscopy and steady‐state PL as function of temperature assign the energy values of localized states with respect to free excitonic states.
In this work, a unique comprehensive and comparative analysis of photoinduced charge generation, recombination kinetics, and energy losses has been carried out to study the effect of different ...fullerene-based acceptors (FBAs) and nonfullerene acceptors (NFAs) on the performance of organic solar cells (OSCs). For this, different FBAs, specifically ICBA, PC60BM, and PC70BM, and NFAs, namely, ITIC, IT-4F, and IEICO-4F, were employed independently along with a particular donor polymer, PBDB-T, to fabricate bulk heterojunction OSCs and their performances have been compared. This donor molecule is known to give similar power conversion efficiency (PCE) with FBAs and NFAs and hence is ideal for comparative studies. The origin of the higher PCE of NFA-based OSCs vs FBA-based OSCs is analyzed in terms of spectral coverage, charge generation, recombination, and energy loss. It is found that the energy loss (ΔE loss) is ∼0.8 to 1 eV for FBA-based OSCs, while it is 0.5–0.7 eV for NFA-based OSCs. Interestingly, for the PBDB-T:IEICO-4F-based system, energy losses due to charge generation (ΔE CT) are ∼0 eV and therefore this system has minimum ΔE loss among all of the studied devices. Providing a systematic, comprehensive, and comparative outlook, our study may further be extended to new upcoming NFA systems and beyond the donor system used in this work.
A study of hybrid light‐emitting diodes (HyLEDs) fabricated with and without solution‐processible Cs2CO3 and Ba(OH)2 inorganic interlayers is presented. The interlayers are deposited between a zinc ...oxide electron‐injection layer and a fluorescent emissive polymer poly(9‐dioctyl fluorine–alt‐benzothiadiazole) (F8BT) layer, with a thermally evaporated MoO3/Au layer used as top anode contact. In comparison to Cs2CO3, the Ba(OH)2 interlayer shows improved charge carrier balance in bipolar devices and reduced exciton quenching in photoluminance studies at the ZnO/Ba(OH)2/F8BT interface compared to the Cs2CO3 interlayer. A luminance efficiency of ≈28 cd A−1 (external quantum efficiency (EQE) ≈ 9%) is achieved for ≈1.2 μm thick single F8BT layer based HyLEDs. Enhanced out‐coupling with the aid of a hemispherical lens allows further efficiency improvement by a factor of 1.7, increasing the luminance efficiency to ≈47cd A−1, corresponding to an EQE of 15%. The photovoltaic response of these structures is also studied to gain an insight into the effects of interfacial properties on the photoinduced charge generation and back‐recombination, which reveal that Ba(OH)2 acts as better hole blocking layer than the Cs2CO3 interlayer.
A solution‐processible barium hydroxide interlayer is used in between ZnO and (9,9′‐dioctylfluorene)‐co‐ benzothiadiazole (F8BT) in highly efficient inverted hybrid light‐emitting‐diode structures. With the aid of an index matched hemispherical lens, luminance and power efficiency reach up to 47 cd A−1 and 15 lm W−1, respectively, for a polymer film thickness of ≈1.2 μm.
We report on the structural, morphological and optical properties of AB(Br1–x Cl x )3 (where, A = CH3NH3 +, B = Pb2+ and x = 0 to 1) perovskite semiconductor and their successful demonstration in ...green and blue emissive perovskite light emitting diodes at room temperature. The bandgap of perovskite thin film is tuned from 2.42 to 3.16 eV. The onset of optical absorption is dominated by excitonic effects. The coulomb field of the exciton influences the absorption at the band edge. Hence, it is necessary to explicitly account for the enhancement of the absorption through the Sommerfield factor. This enables us to correctly extract the exciton binding energy and the electronic bandgap. We also show that the lattice constant varies linearly with the fractional chlorine content satisfying Vegards law.
In this Letter, we investigate the temperature dependence of the optical properties of methylammonium lead iodide (MAPbI3 = CH3NH3PbI3) from room temperature to 6 K. In both the tetragonal (T > 163 ...K) and the orthorhombic (T < 163 K) phases of MAPbI3, the band gap (from both absorption and photoluminescence (PL) measurements) decreases with decrease in temperature, in contrast to what is normally seen for many inorganic semiconductors, such as Si, GaAs, GaN, etc. We show that in the perovskites reported here, the temperature coefficient of thermal expansion is large and accounts for the positive temperature coefficient of the band gap. A detailed analysis of the exciton line width allows us to distinguish between static and dynamic disorder. The low-energy tail of the exciton absorption is reminiscent of Urbach absorption. The Urbach energy is a measure of the disorder, which is modeled using thermal and static disorder for both the phases separately. The static disorder component, manifested in the exciton line width at low temperature, is small. Above 60 K, thermal disorder increases the line width. Both these features are a measure of the high crystal quality and low disorder of the perovskite films even though they are produced from solution.
PDF
