Marco Gruber

Pentacene appended to a TEMPO stable free radical: the effect of magnetic exchange coupling on photoexcited pentacene.

Understanding the fundamental spin dynamics of photoexcited pentacene derivatives is important in order to maximize their potential for optoelectronic applications. Herein, we report on the synthesis of two pentacene derivatives that are functionalized with the [(2,2,6,6-tetramethylpiperidin-1-yl)oxy] (TEMPO) stable free radical. The presence of TEMPO does not quench the pentacene singlet excited state, but does quench the photoexcited triplet excited state as a function of TEMPO-to-pentacene distance. Time-resolved electron paramagnetic resonance experiments confirm that triplet quenching is accompanied by electron spin polarization transfer from the pentacene excited state to the TEMPO doublet state in the weak coupling regime.

Unexpected Michael Additions on the Way to 2.3,8.9-Dibenzanthanthrenes with Interesting Structural Properties.

The synthesis and properties of a new polycyclic aromatic hydrocarbon containing eight annulated rings and based on the anthanthrene core is described. An unexpected, nucleophile-dependent Michael addition to a dibenzanthanthrene-1,7-dione is found, giving a product with three triisopropylsilylacetylene units and a remarkable solid-state structure (as determined by X-ray crystallography).

Singlet Fission for Photovoltaics with 130 % Injection Efficiency

A novel pentacene dimer (P2) and a structurally analogous monomer (P1) were synthesized for use in n-type dye-sensitized solar cells. In P2, the triplet excited states formed by the rapid, spin-allowed process singlet fission were expected to enable carrier multiplication in comparison to the slow, spin-forbidden intersystem crossing seen in P1. A meta-positioning of the two pentacenes and the carboxylate anchor were chosen in P2 to balance the intramolecular dynamics of singlet fission and electron injection. Electron injection from energetically low-lying triplet excited states of pentacene units necessitated the intrinsic and extrinsic lowering of the Fermi level of the semiconductor. Indium-zinc oxide in the presence of Li+ was found to be the optimum choice for the photoelectrodes. Efficient electron injection from the triplet excited states of P1 and P2 was found, with a carrier multiplication of nearly 130u2009%.

Acenequinocumulenes: Lateral and vertical π-extended analogs of tetracyanoquinodimethane (TCNQ)

We have designed a series of molecules and developed synthetic methodology that allows for the inclusion of structural diversity along both the lateral and vertical axes of the basic TCNQ skeleton. In the lateral direction, benzoannulation extends the π-system through (hetero)acene formation, whereas incorporation of a [3]cumulene increases delocalization vertically. The potential of these new molecules as semiconductors is explored through UV/Vis spectroscopy, cyclic voltammetry, X-ray crystallography, thin-film formation, and mobility measurements (using space charge limited current measurements).

Tuning Intramolecular Förster Resonance Energy Transfer and Activating Intramolecular Singlet Fission

The synergy of panchromatic absorption throughout most of the visible range of the solar spectrum and intramolecular singlet fission (SF) has been realized in a series of conjugates featuring different light-harvesting subphthalocyanines (SubPcs) and an energy accepting pentacene dimer (Pnc2 ). At the focal point was a modular SubPc approach, which was based on decorating the SubPc core with different peripheral substituents to tailor and fine-tune their optical properties. Transient absorption measurements assisted in corroborating that the SubPcs act as energy-transfer antennas by means of unidirectional and quantitative intramolecular Förster resonance energy transfer (FRET) to the Pnc2 , where an intramolecular SF affords triplet quantum yields reaching unity.

Deciphering the role of impurities in methylammonium iodide and their impact on the performance of perovskite solar cells(Conference Presentation)

Solution based perovskite solar cell fabrication typically involves rather complex processing sequences to yield highest performance. While most studies concentrate on the exploration of processing conditions, we have investigated the purity levels of common perovskite precursor solutions and found a number of impurities which are most critically controlling the crystallization of perovskites. Moreover, we identified these impurities at different level of concentrations is all commercially available precursors. In detail, we present a detailed chemical study on the nature of the various impurities in CH3NH3I and explored their impact on the crystal formation. The detrimental role of the impurities is best demonstrated by comparing perovskite solar cell devices fabricated from impurity free precursors vs precursors containing different concentrations of impurities. Most interestingly, we revealed that a certain concentration of impurities is detrimental to facilitate the growth of large grained crystals. This study gives valuable insight into the rate determining steps of perovskite crystal growth and provides the basis for developing reliable and reproducible high performance recipes for Perovskite solar cell processing.