Functional π-Gelators and Their Applications

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping ...

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

The efficiencies of perovskite solar cells have gone from single digits to a certified 22.1% in a few years' time. At this stage of their development, the key issues concern how to achieve further improvements in efficiency and long-term stability. We review recent developments in the quest to improve the current state of the art. Because photocurrents are near the theoretical maximum, our focus is on efforts to increase open-circuit voltage by means of improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring. The challenges associated with long-term perovskite solar cell device stability include the role of testing protocols, ionic movement affecting performance metrics over extended periods of time, and determination of the best ways to counteract degradation mechanisms.

http://science.sciencemag.org/content/sci/358/6364/739.full.pdf

Promises and challenges of perovskite solar cells

All highly-efficient organic–inorganic halide perovskite (OIHP) solar cells to date are made of polycrystalline perovskite films which contain a high density of defects, including point and extended imperfections. The imperfections in OIHP materials play an important role in the process of charge recombination and ion migration in perovskite solar cells (PSC), which heavily influences the resulting device energy conversion efficiency and stability. Here we review the recent advances in passivation of imperfections and suppressing ion migration to achieve improved efficiency and highly stable perovskite solar cells. Due to the ionic nature of OIHP materials, the defects in the photoactive films are inevitably electrically charged. The deep level traps induced by particular charged defects in OIHP films are major non-radiative recombination centers; passivation by coordinate bonding, ionic bonding, or chemical conversion have proven effective in mitigating the negative impacts of these deep traps. Shallow level charge traps themselves may contribute little to non-radiative recombination, but the migration of charged shallow level traps in OIHP films results in unfavorable band bending, interfacial reactions, and phase segregation, influencing the carrier extraction efficiency. Finally, the impact of defects and ion migration on the stability of perovskite solar cells is described.

/pdf/imperfections-and-their-passivation-in-halide-perovskite-mfrto61u3p.pdf

Imperfections and their passivation in halide perovskite solar cells

Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.

Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction

We study the effects of a series of post-deposition ligand treatments on the photoluminescence (PL) of polycrystalline methylammonium lead triiodide perovskite thin films. We show that a variety of Lewis bases can improve the bulk PL quantum efficiency (PLQE) and extend the average PL lifetime, ⟨τ⟩, with large enhancements concentrated at grain boundaries. Notably, we demonstrate thin-film PLQE as high as 35 ± 1% and ⟨τ⟩ as long as 8.82 ± 0.03 μs at solar equivalent carrier densities using tri-n-octylphosphine oxide-treated films. Using glow discharge optical emission spectroscopy and nuclear magnetic resonance spectroscopy, we show that the ligands are incorporated primarily at the film surface and are acting as electron donors. These results indicate it is possible to obtain thin-film PL lifetime and PLQE values that are comparable to those from single crystals by control over surface chemistry.

http://pubs.acs.org/doi/pdf/10.1021/acsenergylett.6b00236

Photoluminescence Lifetimes Exceeding 8 μs and Quantum Yields Exceeding 30% in Hybrid Perovskite Thin Films by Ligand Passivation

Efficient noncentrosymmetric arrangement of nonlinear optical (NLO) chromophores with high first-order hyperpolarizability (β) for increased electro-optical (EO) efficiency has proven challenging as strong dipolar interactions between the chromophores encourage antiparallel alignment, attenuating the macroscopic EO effect. This work explores a novel approach to simultaneously achieve large β values while providing an adjustable dipole moment by linking a strong neutral-ground-state (NGS) NLO chromophore with positive β to a zwitterionic (ZWI) chromophore with negative β in an antiparallel fashion. It is proposed that the overall β of such a structure will be the sum of the absolute values of the two types of chromophores while the dipole moment will be the difference. Molecules 1−3 were synthesized to test the feasibility of this approach. Molecular dynamics calculations and NMR data supported that the NGS chromophore component and the ZWI chromophore component self-assemble to an antiparallel conformatio...

Antiparallel-aligned neutral-ground-state and zwitterionic chromophores as a nonlinear optical material.

In suspensions of Nafion beads and of cationic gel beads, NMR spectroscopy showed two water−proton resonances, one representing intimate water layers next to the polymer surface, the other corresponding to water lying beyond. Both resonances show notably shorter spin−lattice relaxation times (T1) and smaller self-diffusion coefficients (D) indicating slower dynamics than bulk water. These findings confirm the existence of highly restricted water layers adsorbed onto hydrophilic surfaces and dynamically stable water beyond the first hydration layers. Thus, aqueous regions on the order of micrometers are dynamically different from bulk water.

Impact of Hydrophilic Surfaces on Interfacial Water Dynamics Probed with NMR Spectroscopy.

A glutathione-based hydrogel and its site-selective interactions with water.

Dimeric glutathione S-transferases (GSTs) are pharmacological targets for several diseases, including cancer. Isoform specificity has been difficult to achieve due to their overlapping substrate selectivity. Here we demonstrate the utility of bivalent GST inhibitors and their optimization via combinatorial linker design. A combinatorial library with dipeptide linkers emanating symmetrically from a central scaffold (bis-3,5-aminomethyl benzoic acid, AMAB) to connect two ethacrynic acid moieties was prepared and decoded via iterative deconvolution, against the isoforms GSTA1-1 and GSTP1-1. The library yielded high affinity GSTA1-1 selective inhibitors (70−120-fold selectivity) and with stoichiometry of one inhibitor:  one GSTA1-1 dimer. Saturation Transfer Difference (STD) NMR with one of these inhibitors, with linker structure (Asp-Gly-AMAB-Gly-Asp) and KD = 42 nM for GSTA1-1, demonstrates that the Asp-Gly linker interacts tightly with GSTA1-1, but not P1-1. H/D exchange mass spectrometry was used to map t...

Rajan K. Paranji

Papers

Photoluminescence Lifetimes Exceeding 8 μs and Quantum Yields Exceeding 30% in Hybrid Perovskite Thin Films by Ligand Passivation

Antiparallel-aligned neutral-ground-state and zwitterionic chromophores as a nonlinear optical material.

Impact of Hydrophilic Surfaces on Interfacial Water Dynamics Probed with NMR Spectroscopy.

A glutathione-based hydrogel and its site-selective interactions with water.

Optimization of bivalent glutathione S-transferase inhibitors by combinatorial linker design.