Paul M. Lahti

Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability

Solar cells fabricated using alkyl ammonium metal halides as light absorbers have the right combination of high power conversion efficiency and ease of fabrication to realize inexpensive but efficient thin film solar cells. However, they degrade under prolonged exposure to sunlight. Herein, we show that this degradation is quasi-reversible, and that it can be greatly lessened by simple modifications of the solar cell operating conditions. We studied perovskite devices using electrochemical impedance spectroscopy (EIS) with methylammonium (MA)-, formamidinium (FA)-, and MA(x)FA(1-x) lead triiodide as active layers. From variable temperature EIS studies, we found that the diffusion coefficient using MA ions was greater than when using FA ions. Structural studies using powder X-ray diffraction (PXRD) show that for MAPbI3 a structural change and lattice expansion occurs at device operating temperatures. On the basis of EIS and PXRD studies, we postulate that in MAPbI3 the predominant mechanism of accelerated device degradation under sunlight involves thermally activated fast ion transport coupled with a lattice-expanding phase transition, both of which are facilitated by absorption of the infrared component of the solar spectrum. Using these findings, we show that the devices show greatly improved operation lifetimes and stability under white-light emitting diodes, or under a solar simulator with an infrared cutoff filter or with cooling.

A Cobalt Pyrenylnitronylnitroxide Single‐Chain Magnet with High Coercivity and Record Blocking Temperature

Coordination of a [Co(hfac)2] moiety (hfac = hexafluoroacetylacetonate) with a nitronylnitroxide radical linked to bulky, rigid pyrene (PyrNN) gives a helical 1:1 chain complex, in which both oxygen atoms of the radical NO(·) groups are bonded to Co(II) ions with strong antiferromagnetic exchange. The complex shows single-chain magnet (SCM) behavior with frequency-dependent magnetic susceptibility, field-cooled and zero-field-cooled susceptibility divergence with a high blocking temperature of around 14 K (a record among SCMs), and hysteresis with a very large coercivity of 32 kOe at 8 K. The magnetic behavior is partly related to good chain isolation induced by the large pyrene units. Two magnetic relaxation processes have been observed, a slower one attributable to longer, and a faster one attributable to short chains. No evidence of magnetic ordering has been found.

Phospholes with reduced pyramidal character from steric crowding III NMR and X-ray diffraction studies on 1-(2,4,6-tri-isopropylphenyl)-3-methylphosphole

Abstract The 2,4,6-tri-isopropylphenyl substituent was placed on the phosphorus of a phosphole to reduce the pyramidal character. That this was accomplished was revealed by single crystal X-ray diffraction analysis; with respect to the plane of C2PC5 in the phosphole ring, the ipso carbon of the benzene ring was deflected by only 58.0°, whereas the deflection is 66.9° in the uncrowded 1-benzylphosphole. This proves that the concept of reducing the pyramidal character (with the goal of increasing the electron delocalization) through steric crowding can be realized. In the crystal the two rings are in orthogonal planes, but this relation is not retained in solution; NMR studies show that the two edges of the benzene ring, as well as the 2,6-isopropyl groups, are identical.

Structure–property relationships for metal-free organic magnetic materials

Abstract This chapter gives an introduction to the basic concepts of magnetism in organic paramagnetic soft matter materials. Key concepts are emphasized using example case studies. Detailed analysis covers radicals functionalized with phenols and with benzimidazole functionalities, which induce various degrees of crystal self-assembly, depending on specific structures. A review with over 200 references and notes.

A single-chain magnet with a very high blocking temperature and a strong coercive field.

Two isostructural 1D complexes, [M(hfac)2NaphNN]n [M = Mn(II) (1) or Co(II) (2); NaphNN = 1-naphthyl nitronylnitroxide], were synthesized and exhibit very strong antiferromagnetic metal-radical exchange coupling. Compound 2 shows slow magnetic relaxation behavior with a high blocking temperature (TB ≈ 13.2 K) and a very high coercive field of 49 kOe at 4.0 K.

Electronic and Molecular Structures of Quintet Bisnitrenes as Studied by Fine-Structure ESR Spectra from Random Orientation: All the Documented ZFS Constants Correct?

Abstract A quintet ground-state bisnitrene, 1,3-dinitreno-5-nitrobenzene, was studied by randomly oriented ESR spectroscopy as part of our project to study purely organic magnetism and supramolecular functionality expected for high spin systems. An observed ESR spectrum of the bisnitrene was interpreted using S=2, g=2.003, |D| = 0.224 cm−1, and |E| = 0.038 cm−1. The determined fine-structure parameter D and E values are in accord with our semiempirical treatment for the quintet state in terms of the fine-structure tensor of the triplet state of p-nitrophenylnitrene. In this work, we emphasize that all the documented fine-structure parameters D and E for quintet bisnitrenes do not reproduce the corresponding observed fine-structure ESR spectra from random orientation.

Polymerization of 1,4-bis(tetrahydrothiopheniomethyl)-2-cyano-5-methoxybenzene dibromide: synthesis of electronically ‘push-pull’ substituted poly(p-phenylene vinylene)s

Abstract The base-induced polymerization of 1,4-bis(tetrahydrothiopheniomethyl)-2-cyano-5-methoxybenzene dibromide in aqueous acetonitrile yields a soluble polyelectrolyte which may be thermally eliminated to give the electronically ‘push-pull’ substituted poly(2-cyano-5-methoxy-1,4-phenylene vinylene). Ultraviolet-visible and infra-red spectral studies showed production of the conjugated polymer, with a band gap of ca. 2.6–3.0eV (410–480 nm) that varied somewhat as a function of elimination conditions. Elemental analysis indicates some retention of tetrahydrothiophenium side groups in the polyelectrolyte polymer, despite the use of a variety of elimination conditions. The retention of side chains in this synthesis is possibly due to a significant degree of irregularity in the positions of the cyano and methoxy substitutents in the eliminated polymer, resulting from non-regioselectivity in this polymerization. This reaction demonstrates the potential in the Wessling process for polymerization of p-xylylenes that have electronically very asymmetric (‘push-pull’) substitution patterns.

Multiscale Active Layer Morphologies for Organic Photovoltaics Through Self-Assembly of Nanospheres

We address here the need for a general strategy to control molecular assembly over multiple length scales. Efficient organic photovoltaics require an active layer comprised of a mesoscale interconnected networks of nanoscale aggregates of semiconductors. We demonstrate a method, using principles of molecular self-assembly and geometric packing, for controlled assembly of semiconductors at the nanoscale and mesoscale. Nanoparticles of poly(3-hexylthiophene) (P3HT) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with targeted sizes. Nanoparticles containing a blend of both P3HT and PCBM were also fabricated. The active layer morphology was tuned by the changing particle composition, particle radii, and the ratios of P3HT:PCBM particles. Photovoltaic devices were fabricated from these aqueous nanoparticle dispersions with comparable device performance to typical bulk-heterojunction devices. Our strategy opens a revolutionary pathway to study and tune the active layer morphology systematically while exercising control of the component assembly at multiple length scales.

Efficient Charge Transport in Assemblies of Surfactant-Stabilized Semiconducting Nanoparticles

Charge transport through a semiconducting nanoparticle assembly is demonstrated. The hole mobility of low and high molecular weight and regioreglular poly(3-hexylthiophene) (P3HT) nanoparticles is on the order of 2 × 10(-4) to 5 × 10(-4) cm(2) V(-1) s(-1) , which is comparable to drop-cast thin films of pristine P3HT. Various methods are employed to understand the nature and importance of the nanoparticle packing.

Correction to “2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, A Hydrogen-Bonded Organic Quasi-1D Ferromagnet”

The title radical (F4BImNN) is a stable nitronylnitroxide that forms hydrogen-bonded NH... ON chains in the solid state. The chains assemble the F4BImNN molecules to form stacked contacts between the radical groups, in a geometry that is expected to exhibit ferromagnetic (FM) exchange based on spin polarization (SP) models. The experimental magnetic susceptibility of F4BImNN confirms the expectation, showing 1-D Heisenberg chain FM exchange behavior over 1.8-300 K with an intrachain exchange constant of Jchain/k = +22 K. At lower temperatures, ac magnetic susceptibility and variable field heat capacity measurements show that F4BImNN acts as a quasi-1-D ferromagnet. The dominant ferromagnetic exchange interaction is attributable to overlap between spin orbitals of molecules within the hydrogen-bonded chains, consistent with the SP model expectations. The chains appear to be antiferromagnetically exchange coupled, giving cusps in the ac susceptibility and zero field heat capacity at lower temperatures. The results indicate that the sample orders magnetically at about 0.7 K. The magnetic heat capacity ordering cusp shifts to lower temperatures as external magnetic field increases, consistent with forming a bulk antiferromagnetic phase below a Neel temperature of TN(0) = 0.72 K, with a critical field of Hc approximately 1800 Oe. The interchain exchange is estimated to be zJ/k congruent with (-)0.1 K.