First we have demonstrated a convenient synthetic procedure for producing deep blue emissive ultrasmall InP QDs by utilizing two different surface ligands and using the less toxic and cost-effective tris(dimethylamino)phosphine.
The morphological analysis of the QDs was carried out using high-resolution transmission electron microsco-py (HRTEM).The as-synthesized QDs then have been characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy (HRTEM) to determine their purity and crystallinity.
The synthesized Mn (II) bromide perovskites using monovalent (perovskite 1, P1) and bivalent (perovskite 2, P2) alkyl interlayer spacers were characterized with powder X-ray diffraction (PXRD), electron spin paramagnetic resonance (EPR), steady-state, and time-resolved emission spectroscopy. The EPR experiments suggest octahedral coordination in P1 and tetrahedral coordination for P2.
We have tried to understand the charge carrier dynamics i.e. charge generation, charge separation, charge transport, charge collection, and charge recombination in the QD and perovskite systems by employing primarily the femtosecond pump–probe technique.
Combining picosecond time resolved measurements, femtosecond fluorescence up-conversion, and transient absorption measurements, we unravel the photophysical behavior by deciphering the charge carrier dynamics of the deep blue emissive InP QDs. The results reveal that the bare InP QDs with low PLQY undergo ultrafast electron and hole trapping along with efficient Auger recombination. However, the significant increase in the PLQY upon successive surface passivation with ZnS shells is found to be associated with slower hot electron and hole relaxation times. ZnS coating effectively enhances the lifetimes of the hot electrons from 2 to 8 ps and of the holes from sub-picosecond to 0.5 ps. Surface passivation with ZnS also demonstrated increments of the lifetimes of the relaxed electrons and holes from 6 ps to 66 ps and from 1 ps to 4 ps, respectively. The rates for the trap-assisted Auger recombination show a decrease by an order of magnitude following coating with two ZnS shells.Therefore the results highlight the effect of ZnS coating on electron and hole dynamics which may hold the key thereof for the rational design of highly efficient blue emissive LEDs composed of Cd- and Pb-free InP QDs.
Two Mn (II)-based perovskites using a monovalent (ethyl ammonium (EA) bromide) and a bivalent (ethyl diammonium (EDA) dibromide) interlayer spacer cation was synthesized and we tried to gain an insight into the photobehavior of these perovskites. The perovskite with monovalent spacer cation (P1) exhibited orange emission whereas the perovskite with bivalent spacer cation (P2) exhibited green emission. In the diffuse reflectance and in the excitation spectra of P1 the peaks were found to be located at around 366, 377 (a shoulder), 427, 438 (a shoulder) and 524 nm. These arise from the 6A1(S) → 4E(D), 6A1(S)→ 4T2(D), 6A1(S) → (4A1, 4E(G)), 6A1(S) →4T2(G) and 6A1(S) → 4T1(G) transitions of the Mn2+ ions, respectively (Figure 2A). Most importantly, the peaks at 427 and 524 nm are typical features of octahedrally coordinated Mn2+ ions. For P2 the corresponding peaks were located at around 362, 371, 389, 435, 449 and 465 nm. The positions of these peaks were consistent with the energy states splitting for Mn2+ in a tetrahedral environment.
Both the perovskite showed biexponential decay and P2 exhibited higher average lifetime (τavg) than P1 which agrees well with their respective photoluminescence quantum yield (PLQY) values (3% for P1 and 26% P2). The fast component most likely originated from interacting Mn-Mn pairs whereas the slower component instigated not interacting Mn2+ ions, which eliminates the direct spin−spin interaction.
Work Package 3
In work package 3, we tried to fabricate the blue emissive InP/ZnS/ZnS QDs based deep blue emitting light emitting diodes (LEDs) through secondments at INAM, UJI, Spain, where I worked in close collaboration with the team of Prof. I. Mora-Sero. We have tried different parameters like concentration, power and different metal electrode aiming to optimize the LED´s performance. However, we did not get our desired EQE value for our prepared deep blue emissive LED. The results are demonstrated in Figure 3.
