Vygintas Jankauskas

Enhancing Thermal Stability and Lifetime of Solid-State Dye-Sensitized Solar Cells via Molecular Engineering of the Hole-Transporting Material Spiro-OMeTAD

Thermal stability of hybrid solar cells containing spiro-OMeTAD as hole-transporting layer is investigated. It is demonstrated that fully symmetrical spiro-OMeTAD is prone to crystallization, and growth of large crystalline domains in the hole-transporting layer is one of the causes of solar cell degradation at elevated temperatures, as crystallization of the material inside the pores or on the interface affects the contact between the absorber and the hole transport. Suppression of the crystal growth in the hole-transporting layer is demonstrated to be a viable tactic to achieve a significant increase in the solar cell resistance to thermal stress and improve the overall lifetime of the device. Findings described in this publication could be applicable to hybrid solar cell research as a number of well-performing architectures rely heavily upon doped spiro-OMeTAD as hole-transporting material.

High hole mobilities in carbazole-based glass-forming hydrazones

The properties of a series of carbazole-based dihydrazones are reported. The dependence of their thermal and glass-forming properties on their chemical structure is discussed. The hydrazones having phenyl substituents at the N atom of the hydrazine moiety form glasses and their amorphous films on the glass or polyester substrates can be prepared by casting from solutions. The ionization potentials of the synthesized hydrazones measured by the electron photoemission technique range from 5.24 to 5.50 eV. Hole drift mobilities of some newly synthesized carbazole-based dihydrazones appproach 10−2 cm2 V−1 s−1 at an electric field of 6.4×105 V cm−1, at 22 °C.

Air stable electron-transporting and ambipolar bay substituted perylene bisimides

Four new perylene bisimides containing carbazolyl and triphenylamino electron-donor groups in the bay region have been designed, synthesized and characterized. The materials possess high thermal stability and form uniform films. They display a wide absorption window extending to the near infrared region of the spectrum and demonstrate efficient photoinduced intramolecular electron transfer. Ionization potential values of these perylene bisimide derivatives measured by photoelectron spectroscopy range from 5.8 eV to 6 eV. Charge-transporting properties were investigated by the xerographic time of flight (XTOF) technique. Complementary ambipolar charge-transport was observed in differently linked carbazolyl substituted perylene bisimides while the triphenylamino substituted material exhibited competent electron drift mobility (>10−3 cm2 V−1 cm−1) under ambient conditions. Density functional theory (DFT) calculations were performed for carbazolyl bay substituted perylene bisimides in order to understand the complementary ambipolar charge transport as well as the difference in the optical properties.

Molecular engineering of face-on oriented dopant-free hole transporting material for perovskite solar cells with 19% PCE

Through judicious molecular engineering, novel dopant-free star-shaped D–π–A type hole transporting materials coded KR355, KR321, and KR353 were systematically designed, synthesized and characterized. KR321 has been revealed to form a particular face-on organization on perovskite films favoring vertical charge carrier transport and for the first time, we show that this particular molecular stacking feature resulted in a power conversion efficiency over 19% in combination with mixed-perovskite (FAPbI3)0.85(MAPbBr3)0.15. The obtained 19% efficiency using a pristine hole transporting layer without any chemical additives or doping is the highest, establishing that the molecular engineering of a planar donor core, π-spacer and periphery acceptor leads to high mobility, and the design provides useful insight into the synthesis of next-generation HTMs for perovskite solar cells and optoelectronic applications.

Hole-transporting molecular glasses based on carbazole and diphenylamine moieties

Abstract Various molecular designs based on carbazole and diphenylamine moieties are found to constitute novel hole transporting amorphous molecular materials, as characterized by differential scanning calorimetry and time of flight method. Hole drift mobility of synthesized materials were in the range 10 −4 –10 −6 cm 2 V −1 s −1 at an applied field of 3.6×10 5 V cm −1 .

Synthesis and Properties of Methoxyphenyl-Substituted Derivatives of Indolo[3,2-b]carbazole

The synthesis and full characterization of new derivatives of indolo[3,2-b]carbazole with differently substituted phenyl groups at nitrogen atoms is reported. Comparative study on their thermal, optical electrochemical, and photoelectrical properties is presented. The synthesized compounds are electrochemically stable. Their highest occupied molecular orbital energy values range from -5.14 to -5.07 eV. The electron photoemission spectra of the films of synthesized materials revealed the ionization potentials of 5.31-5.47 eV. Hole drift mobility of the amorphous film of 5,11-bis(3-methoxyphenyl)-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole exceed 10(-3) cm(2)/V·s at high electric fields, as it was established by xerographic time-of-flight technique. In contrast to diphenylamino substituted derivatives of carbazole, no effect of the position of methoxy groups on the photoelectrical properties was observed for the synthesized methoxyphenyl-substituted derivatives of indolo[3,2-b]carbazole. The indolo[3,2-b]carbazole core has a larger resonance structure that includes 3 phenyl rings, and thus the energy gap of the HOMO and LUMO π orbitals is lower as compared to that of carbazoles. With a larger energy difference between the phenyl substituents and the core moiety, the indolo[3,2-b]carbazole derivatives studied all have a weaker coupling between the phenyl group and a much weaker dependence of the molecular properties on the position of substituents on the phenyl groups as compared to those observed in substituted carbazoles.

Hole-Transporting Glass-Forming 3,3′-Dicarbazyl-Based Hydrazones

ABSTRACT 3,3′-Di(9-(4-butylphenyl)carbazyl) and 3,3′-di(9-ethylcarbazyl)-based hydrazones were synthesized by the multi-step synthetic route including oxidative dimerization of 9-(4-butylphenyl)carbazole and 9-ethylcarbazole, formylation of the dimmers obtained and the reactions of the formyl derivatives with different hydrazines. The chemical structure of the compounds was confirmed by 1HNMR, IR and mass spectroscopy. The hydrazones synthesized form stable glasses with the glass transition temperatures exceeding 140°C. The values of ionisation potentials measured by electron photoemission technique are in the range of 5,21–5,4 eV. The hole drift mobilities in the films of the 50% solid solutions of the hydrazones in bisphenol Z polycarbonate established by the xerographic time-of-flight technique exceed 10−5 cm2/Vs at high electric fields.

Phenylethenyl-Substituted Triphenylamines: Efficient, Easily Obtainable, and Inexpensive Hole-Transporting Materials

Star-shaped charge-transporting materials with a triphenylamine (TPA) core and various phenylethenyl side arm(s) were obtained in a one-step synthetic procedure from commercially available and relatively inexpensive starting materials. Crystallinity, glass-transition temperature, size of the π-conjugated system, energy levels, and the way molecules pack in the solid state can be significantly influenced by variation of the structure of these side arm(s). An increase in the number of phenylethenyl side arms was found to hinder intramolecular motions of the TPA core, and thereby provide significant enhancement of the fluorescence quantum yield of the TPA derivatives in solution. On the other hand, a larger number of side arms facilitated exciton migration through the dense side-arm network formed in the solid state and, thus, considerably reduces fluorescence efficiency by migration-assisted nonradiative relaxation. This dense network enables charges to move more rapidly through the hole-transport material layer, which results in very good charge drift mobility (μ up to 0.017 cm(2) V (-1) s(-1)).

Can hydrogen bonds improve the hole-mobility in amorphous organic semiconductors? Experimental and theoretical insights

Five hole-transporting triphenylamine derivatives containing methoxy and methyl groups are synthesized and investigated. The hole-mobility increases in the presence of methyl and methoxy substituents, exceeding 10−2 cm2 V−1 s−1 in the case of methyl groups. Quantum mechanical calculations on these compounds indicate very different dipole moments and intermolecular interaction strengths, with intriguing correlations with the trend in hole-mobility. Temperature dependent hole-mobility measurements indicate disorder dominated hole transport. The values of the energetic disorder parameter (σ) decrease upon methyl and methoxy substitutions despite the increase in dipole moments. This trend is discussed as a function of the interaction energy between adjacent molecules, the dipole moment, the molecular polarizability, and the conformational degree of freedom. Our results indicate that the global decrease of σ upon methyl and methoxy substitutions is dominated by the larger decrease in the geometrical randomness component of the energetic disorder. A direct correlation is established between the decrease in geometrical randomness and the increase in intermolecular interaction energies, mainly stemming from the additional C–H⋯π, O, N hydrogen bonds induced by methyl and methoxy groups.

Influence of methoxy groups on the properties of 1,1-bis(4-aminophenyl)cyclohexane based arylamines: experimental and theoretical approach

Three new isomeric cyclohexylidene linked triphenylamines containing methoxy groups in different positions were synthesized via Ullmann coupling from 1,1-bis(4-aminophenyl)cyclohexane and respective aryl iodides. Thermal behaviour, optical and photoelectrical properties of the obtained materials were investigated. Calculations based on the density functional methods (DFT) were also carried out in order to better understand structure–property relationships. Methoxy-substituted derivatives of 1,1-bis(4-aminophenyl)cyclohexane show lower ionization potentials and higher hole drift mobilities than the corresponding derivative having no methoxy groups. The ionization potentials of the solid samples of the methoxy-substituted compounds established by the electron photoemission technique are in the range of 5.34–5.55 eV. Hole-drift mobility values of the amorphous layers of the methoxy-substituted materials established by the time-of-flight technique range from 4.0 × 10−4 to 1.2 × 10−3 cm2 V−1 s−1 at the electric field of 106 V cm−1. The highest hole mobilities were observed for the para-substituted derivative. Theoretical results suggest that the hole mobility in the bulk materials is driven by the electronic coupling parameter whilst the reorganization energy parameter predicts the wrong mobility trend. The role of the methoxy groups is found to be related to the well-known mesomeric (π-donor) effect and the possibility to establish C–H⋯π(Ph) and C–H⋯X (X = O, N) hydrogen bonds. The effect of these properties on the enhanced Stokes shifts of the para-methoxy-substituted compound, on the decrease of the ionization potentials for the methoxy substituted compounds as compared to the non-substituted one, and on the enhanced possibility to establish considerable electronic couplings between adjacent molecules is discussed in detail.