| Summary: | Artificial lighting accounts for close to 20% of the electricity used world-wide and solid-state light emitting diodes (LEDs) have the potential to reduce this usage by up to 80%. While inorganic phosphors have been widely used to enable white light emission, new class of materials that could be inexpensive, made from earth-abundant materials, processed at low temperatures, and solution deposited is of great interest. Metal-halide perovskite have taken the optoelectronics community by storm by delivering more than 25.2% solar cell efficiencies and external quantum efficiencies in excess of 20% in light emitting diodes. Early reports have also indicated the applicability of perovskites as broad band emitting phosphors, however to-date, perovskite phosphors have seen very limited research breadth and the understanding of the broad band emissions emanating from these phosphors is much limited. In addition to the promise of perovskites as exceptional semiconducting materials, the compositional and structural variability of these materials suggests upto 102 to 106 variants in is archetypal 3D perovskite, ABX3, with organic cations (CH3NH3), divalent metals (Pb, Sn, Ge), and halides corresponding to the A, B, X sites respectively. By the choice of the organic cation, perovskites can be synthesized with layered (2D) or molecularly lower dimensionalities (e.g. 1D, 0D); with the lower dimensionalities resulting in a more deformable structure and larger binding energies; potential more amenable to broad band emissions.
While uncovering the fundamental reasons for broad band emission, this thesis undertook a comparative study of lower dimensional perovskites by exploring substituents of the A-site cation to yield 2D, 1D, and 0D structures, metal substituents to explore both lead and lead-free systems, and halide substitutions to understand the role of composition on structure as well as on emission properties.
In the first study, a 2D lead chloride perovskite was prepared using a phenethylammonium (PEA) cation. The single crystal X-ray diffraction data, time-resolved photophysical measurements, temperature-dependent photoluminescence measurements, and density functional theory (DFT) calculations were used to demonstrate that broad band emission is arising from strong exciton-phonon coupling with the organic lattice, which is independent of the morphology of the perovskite. The phenethylammonium lead chloride (PEPC) perovskite exhibited broad band emission that closely mimicked artificial lighting from cool white light LEDs, and thermal & photo stability showing suitability for phosphor applications. However, photoluminescence quantum yields (PLQY) were less than 1% and lower dimensional perovskites were subsequently explored.
Substituting the A-site cations by a larger m-xylylenediammonium (m-XDA) ion enabled formation of 0D perovskites with lead bromide (m-XDALB) or chloride (m-XDALC) octahedra separated by these organic moieties. These exhibited metal-centered s-p transitions along with structural deformations of the metal halide octahedra at low temperatures, although they are not emissive at room temperature. Addition of excess PbBr2, of up to one mole, resulted in 1D perovskites with broad band, white light emission matching that of cool white light emission.
Tin substituted (m-XDATB) perovskite showed strongly Stokes-shifted broadband emission, from 400 to 650 nm, with a peak maximum at 505 nm, a full-width at half-maximum (FWHM) of 95 nm, and a high PLQY of 60%. White-light-emitting devices could be fabricated using this cyanm-XDATB emitter with a sulfoselenide phosphor. Germanium substituted, m-XDAGB, showed yellow emission (450 – 700 nm), with PLQY of 20%. White-light-emitting devices fabricated using yellow-emitting m-XDAGB with a barium aluminate phosphor displayed emission suitable for daytime white light applications. Several other materials were also explored with alkyl chains (e.g. octyl and butylammonium, OA and BA respectively) for A-site cations and some of them showed promising applications for light emission; but also, for down conversion phosphors, luminescent solar cell collectors, and for UV radiation harvesting solar cells.
Typically, 3D perovskites show very narrow emissions due to weak exciton-phonon couplings associated with large deformation energies of the metal halide octahedra. This thesis has shown that reducing the dimensionality of hybrid organic-inorganic perovskites from 3D interconnected polyanionic inorganic networks into lower-dimensional 2D, 1D, or molecular 0D octahedra results in localized electronic states, narrower conduction and valence bands. Reducing the dimensionality of perovskites localize the charge carriers which leads to short-range lattice deformations which results in self-trapping of excitons as well as enhanced broadband emission. Using transient absorption and transient photoluminescence (TRPL) studies, and raman spectroscopy, the broad band emission observed from films and also nanoparticles of PEPC were shown to originate from self-trapped excitons predominantly in the organic lattice. This contrasts with the commonly attributed origins of self-trapped excitons only in the inorganic metal halide lattice. These findings highlight the importance of rational selection of both the organic and inorganic components in 2D perovskites for white-light emission.
Molecular zero-dimensional (0D) perovskites could be fabricated using newly synthesized m-XDA organic cations, where the metal halide octahedral anions were separated by the large m-XDA cationic matrix. PLQY trend observed was m-XDATB > m-XDAGB > m-XDALB; with highest PLQY of 60% seen in Sn; but the highest Stokes shift observed in Ge perovskites. These differences are yet to be fully understood; however mechanistic as well as theoretical analysis pointed to the broadband emissions attributable to exciton self-trapping. Replacing the m-XDA organic cations with alkylammonium (BA and OA) results in larger Stokes shift and much higher PL quantum yield, where the OATB could reach almost 100 % PLQY.
The exciton couples strongly to the lattice and generates lattice distortions that can be defined as “excited-state defects”. These structural deformations would able to stabilize the exciton, allowing significant broadband emission with large Stokes shift. Low-dimensional perovskites could show strong exciton-phonon couplings as the deformation energies of the metal halide octahedra decrease with reduction of the dimensionality of the perovskite. Additionally, phosphors (e.g. m-XDATB, m-XDALB) based on metal ions with s-p electronic transition are less explored compared to phosphors with other electronic transitions like d-d, f-d, and f-f transitions. Study of the transition from the excited singlet to the triplet states aided by the spin-orbit coupling that occurs due to the heavy atoms such as Sn, Pb, Br was also made possible in these low dimensional perovskites.
It has clearly been established in this thesis that factors such as structural deformations, rigidity of the structure, are very important criteria in terms of creating self-trapped excitons and broad band emission. Furthermore, the thesis has also highlighted that low dimensional perovskites represent a great potential with its vast parametric space that is opportune for implementation of rational design methodology to identify the right material combinations that could be considered as attractive alternatives for currently used phosphors.
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