Ammonium cations with high pKa in perovskite solar cells for improved high-temperature photostability

Half a century after the synthesis of the first subporphyrinoid, the study of tripyrrole and trisoindole porphyrin analogues constitutes a fervent and rapidly expanding research area. The outstanding structural, electronic and optical features of these cone-shaped aromatic macrocycles render them attractive candidates for a wide variety of applications, ranging from optoelectronics to biomedicine. To tune their properties and exploit their functionalities, the development of novel methodologies for the synthesis and post-functionalization of these contracted porphyrinoids, as well as a deep understanding of their supramolecular organization and their implementation into multicomponent systems of increasing complexity are of paramount importance. Herein, a review of the most recent advances in the fundamentals and applications of subporphyrinoids is presented, which comprehensively cover the last decade of discoveries. The final aim is to highlight the chemical versatility and intriguing physicochemical features of subporphyrinoids, while providing an updated overview of their most promising applications.

Recent advances in subphthalocyanines and related subporphyrinoids.

A new set of donor-acceptor (D-A) conjugates capable of undergoing ultrafast electron transfer were synthesized using 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-substituted phenothiazine, SM1-SM3, by a Pd-catalyzed Sonogashira cross-coupling reaction and a [2+2] cycloaddition-electrocyclic ring-opening reaction. The incorporation of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (abbreviated as DCNQ=dicyanodiquinodimethane) in BODIPY-substituted phenothiazine resulted in significant perturbation of the optical and electronic properties. The absorption spectrum of both SM2 and SM3 showed red shifted absorption as compared to SM1. Additionally, both SM2 and SM3 exhibited a distinct intramolecular charge-transfer (ICT) transition in the near-IR region more so for SM3. The electrochemical study revealed multi-redox processes due to the presence of redox-active phenothiazine, BODIPY, TCBD or DCNQ entities. Using data from spectral, electrochemical and computational studies, an energy-level diagram was established to witness excited-state electron-transfer events. Finally, evidence of electron transfer and their kinetic information was secured from studies involving a femtosecond transient absorption technique. The time constants for excited-state electron-transfer events in the case of SM2 and SM3 were less than 5 ps revealing ultrafast processes.

Excited-State Electron Transfer in 1,1,4,4-Tetracyanobuta-1,3-diene (TCBD)- and Cyclohexa-2,5-diene-1,4-diylidene-Expanded TCBD-Substituted BODIPY-Phenothiazine Donor-Acceptor Conjugates.

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

https://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.2c00843

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Two subphthalocyanines (SubPcs) decorated at their peripheral (SubPc 1) or peripheral and axial (SubPc 2) positions with tetracyanobuta-1,3-diene (TCBD)–aniline moieties have been prepared as novel electron donor–acceptor (D–A) conjugates. In 1 and 2, the multiple functionalization of C3-symmetric SubPcs by TCBD moieties, each of them having a chiral axis, results in the formation of several stereoisomers. Variable temperature 1H-NMR studies in chlorinated solvents suggest that these latter species, which are detected at low temperatures, rapidly interconvert – on the NMR timescale – into each other at room temperature. Beside their unique structural and stereochemical features, 1 and 2 present interesting physicochemical properties. Steady-state absorption and fluorescence, as well as electrochemical studies on 1 and 2 clearly point to an important degree of electronic communication between the SubPc, the TCBD and the aniline subunits. Moreover, in both derivatives, photoexcitation of the SubPc moiety yields charge transfer products involving the electron-rich SubPc moiety and the electron-withdrawing TCBD fragment. Interestingly, such polarized excited state species evolve in 1 and 2 in different ways. While in the former compound, it directly decays to the ground state, the fourth axial TCBD moiety in 2 leads to the formation of an intermediate fully charge separated state prior to the ground state deactivation.

/pdf/subphthalocyanine-tetracyanobuta-1-3-diene-aniline-1vrv1rjqnw.pdf

Subphthalocyanine–tetracyanobuta-1,3-diene–aniline conjugates: stereoisomerism and photophysical properties

One of the open challenges of spintronics is to control the spin relaxation mechanisms. Layered metal-halide perovskites are an emerging class of semiconductors which possess a soft crystal lattice that strongly couples electronic and vibrational states and show promise for spintronic applications. Here, we investigate the impact of such strong coupling on the spin relaxation of excitons in the layered perovskite BA2FAPbI7 using a combination of cryogenic Faraday rotation and transient absorption spectroscopy. We report an unexpected increase of the spin lifetime by two orders of magnitude at 77 K under photoexcitation with photon energy in excess of the exciton absorption peak, and thus demonstrate optical control over the dominant spin relaxation mechanism. We attribute this control to strong coupling between excitons and optically excited phonons, which form polaronic states with reduced electron-hole wave function overlap that protect the exciton spin memory. Our insights highlight the special role of exciton-lattice interactions on the spin physics in the layered perovskites and provide a novel opportunity for optical spin control. 

/pdf/optical-control-of-exciton-spin-dynamics-in-layered-metal-1yb3adhb.pdf

Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation

Spin-orbit coupling in the electronic states of solution-processed hybrid metal halide perovskites forms complex spin-textures in the band structures and allows for optical manipulation of the excited state spin-polarizations. Here, we report that motional narrowing acts on the photoexcited spin-polarization in CH3NH3PbBr3 thin films, which are doped at percentage-level with Mn2+ ions. Using ultrafast circularly polarized broadband transient absorption spectroscopy at cryogenic temperatures, we investigate the spin population dynamics in these doped hybrid perovskites and find that spin relaxation lifetimes are increased by a factor of 3 compared to those of undoped materials. Using quantitative analysis of the photoexcitation cooling processes, we reveal increased carrier scattering rates in the doped perovskites as the fundamental mechanism driving spin-polarization-maintaining motional narrowing. Our work reports transition-metal doping as a concept to extend spin lifetimes of hybrid perovskites.

Motional Narrowing Effects in the Excited State Spin Populations of Mn-Doped Hybrid Perovskites.

Boron subphthalocyanines (SubPcs) are attractive as light harvesting materials in photovoltaic devices. Here we present the synthesis, optical and electrochemical properties, and device performance ...

Donor−Acceptor‐Functionalized Subphthalocyanines for Dye‐Sensitized Solar Cells

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Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation

Motional Narrowing Effects in the Excited State Spin Populations of Mn-Doped Hybrid Perovskites.

Donor−Acceptor‐Functionalized Subphthalocyanines for Dye‐Sensitized Solar Cells