Computational Study of Halide Perovskite-Derived A2_2BX6_6 Inorganic Compounds: Chemical Trends in Electronic Structure and Structural Stability

The electronic structure and energetic stability of A$_2$BX$_6$ halide compounds with the cubic and tetragonal variants of the perovskite-derived K$_2$PtCl$_6$ prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A$_2$BX$_6$ compounds with A = K, Rb and Cs, and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se and Te and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For variations in the B site in the (Ni, Pd, Pt) group and the (Se, Te) group, the band gap increases with increasing cation size. However, no observed chemical trend with respect to cation size for band gap was found for the (Si, Sn, Ge, Pb) group. The findings in this work provide guidelines for the design of halide A$_2$BX$_6$ compounds for potential photovoltaic applications

Low-dimensional metal halide perovskite phosphors for solid-state lighting

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.Doctor of Philosoph

2D black phosphorous nanosheets as a hole transporting material in perovskite solar cells

We demonstrate for the first-time liquid exfoliated few layers of 2D Black phosphorus (BP) nanosheets as a hole transporting material (HTM) for perovskite based solar cells. The photoelectron spectroscopy in air (PESA) measurements confirm the low laying valence band level of BP nanosheets (−5.2 eV) favourable for hole injection from CH3NH3PbI3 (MAPbI3). Our results show that ∼25% improvement in power conversion efficiency (PCE) of η = 16.4% for BP nanosheets + Spiro-OMeTAD as an HTM as compared to spiro-OMeTAD (η = 13.1%). When BP nanosheets are exclusively utilised as an HTM, a PCE of η = 7.88% is noted, an improvement over the 4% PCE values observed for HTM free devices. Photoluminescence (PL) quenching of MAPbI3 and impedance measurements further confirm the charge extraction ability of BP nanosheets. The structural and optical characterization of liquid exfoliated BP nanosheets is discussed in detail with the aid of transmission electron microscopy, Raman spectroscopy, absorption spectroscopy and photo-electron spectroscopy.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Dominant factors limiting the optical gain in layered two-dimensional halide perovskite thin films

Semiconductors are ubiquitous gain media for coherent light sources. Solution-processed three-dimensional (3D) halide perovskites (e.g., CH3NH3PbI3) with their outstanding room temperature optical gain properties are the latest members of this family. Their two-dimensional (2D) layered perovskite counterparts with natural multiple quantum well structures exhibit strong light–matter interactions and intense excitonic luminescence. However, despite such promising traits, there have been no reports on room temperature optical gain in 2D layered perovskites. Herein, we reveal the challenges towards achieving amplified spontaneous emission (ASE) in the archetypal (C6H5C2H4NH3)2PbI4 (or PEPI) system. Temperature-dependent transient spectroscopy uncovers the dominant free exciton trapping and bound biexciton formation pathways that compete effectively with biexcitonic gain. Phenomenological rate equation modeling predicts a large biexciton ASE threshold of ∼1.4 mJ cm−2, which is beyond the damage threshold of these materials. Importantly, these findings would rationalize the difficulties in achieving optical gain in 2D perovskites and provide new insights and suggestions for overcoming these challenges.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)ASTAR (Agency for Sci., Tech. and Research, S'pore)Published versio

Cubic NaSbS2 as an Ionic-Electronic Coupled Semiconductor for Switchable Photovoltaic and Neuromorphic Device Applications

Just Accepted - Advanced Material

Computational study of halide perovskite-derived A2BX6 inorganic compounds : chemical trends in electronic structure and structural stability

The electronic structure and energetic stability of A2BX6 halide compounds with the cubic and tetragonal variants of the perovskite-derived K2PtCl6 prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A2BX6 compounds with A = K, Rb, and Cs; and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se, and Te; and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For variations in the B site in the (Ni, Pd, Pt) group and the (Se, Te) group, the band gap increases with increasing cation size. However, no observed chemical trend with respect to cation size for band gap was found for the (Si, Sn, Ge, Pb) group. The findings in this work provide guidelines for the design of halide A2BX6 compounds for potential photovoltaic applications.NRF (Natl Research Foundation, S’pore)Accepted versio

Doping and Switchable Photovoltaic Effect in Lead Free Perovskites Enabled by Metal Cation Transmutation

Published in Advanced Materials : https://doi.org/10.1002/adma.20180208

Grain size modulation and interfacial engineering of CH3NH3PbBr3 emitter films through incorporation of tetraethylammonium bromide

Metal halide perovskites have demonstrated breakthrough performances as absorber and emitter materials for photovoltaic and display applications respectively. However, despite the low manufacturing cost associated with solution‐based processing, the propensity for defect formation with this technique has led to an increasing need for defect passivation. Here, we present an inexpensive and facile method to remedy surface defects through a postdeposition treatment process using branched alkylammonium cation species. The simultaneous realignment of interfacial energy levels upon incorporation of tetraethylammonium bromide onto the surface of CH3NH3PbBr3 films contributes favorably toward the enhancement in overall light‐emitting diode characteristics, achieving maximum luminance, current efficiency, and external quantum efficiency values of 11 000 cd m−2, 0.68 cd A−1, and 0.16 %, respectively.Accepted versio

Lead-free germanium iodide perovskite materials for photovoltaic applications

Computational screening based on density-functional-theory calculations reveals Ge as a candidate element for replacing Pb in halide perovskite compounds suitable for light harvesting. Experimentally, three AGeI3 (A = Cs, CH3NH3 or HC(NH2)2) halide perovskite materials have been synthesized. These compounds are stable up to 150 °C, and have bandgaps correlated with the A-site cation size. CsGeI3-based solar cells display higher photocurrents, of about 6 mA cm−2, but are limited by poor film forming abilities and oxidising tendencies. The present results demonstrate the utility of combining computational screening and experimental efforts to develop lead-free halide perovskite compounds for photovoltaic applications.NRF (Natl Research Foundation, S’pore)Accepted versio

Perovskite templating via a bathophenanthroline additive for efficient light-emitting devices

Identified as emerging light absorbers due to their plethora of unique optoelectronic properties, perovskites have also been touted as a promising candidate for light emission. However, despite the effortless transition of perovskites into the current organic light-emitting diodes (OLEDs), misalignment of energy levels at the hole transporting material (HTM) and perovskite interface limits the efficacy of interfacial charge transport. Herein, it is shown that by incorporating a small organic molecule, bathophenanthroline (BPhen), into the CH3NH3PbBr3 emitter via a solvent engineering technique, the energy band levels of the perovskite can be tailored and the energy mismatch at the HTM/perovskite interface can be ameliorated through the formation of a graded emitter layer and accompanying morphological improvements. With a BPhen concentration of 0.500 mg mL−1, more than ten-fold enhancement of device luminance and efficiency was achieved, thus demonstrating a facile and viable approach for fabricating high-performance perovskite light-emitting diodes (PeLEDs).NRF (Natl Research Foundation, S’pore)Accepted versio