I joined the theory group at the Max Planck Institute for Polymer Research as a Postdoctoral fellow in October 2016. My current research focusses on the design of efficient host/guest systems for OLEDs.
I obtained my Ph.D. at Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), India under the supervision of Prof. S. Balasubramanian. My Ph.D. research interest was focussed on Ionic Liquids using, molecular modeling for their use in energy and environment applications. I obtained my Master of Science degree, in Material Science, at the JNCASR, in 2013. I received my Bachelor of Science (Honours) in Chemistry, from Visva-Bharati University, India in 2010.
The influence of impurities on the charge carrier mobility of small molecule organic semiconductors
P. Friederich, A. Fediai, J. Li, A. Mondal, N. B. Kotadiya, F. Symalla, G.-J. A. H. Wetzelaer, D. Andrienko, X. Blase, D. Beljonne, P. W. M. Blom, J.-L. Bredas, W. Wenzel
Electron trapping in conjugated polymers
D. Abbaszadeh, A. Kunz, N. B. Kotadiya, A. Mondal, D. Andrienko, J. J. Michels, G.‐J. A. H. Wetzelaer, P. W. M. Blom
Electron trapping is a well-recognized issue in organic semiconductors in particular in conjugated polymers leading to a significant electron mobility reduction in materials with electron affinities smaller than 4 eV. Space-charge limited current measurements in diodes indicate that these traps have similar molecular origin while calculations show that hydrated molecular oxygen is a plausible molecular candidate with the tail of the solid-state electron affinity distribution reaching values as high as 4 eV. By decreasing the trap density by mixing conjugated polymers with an insulating polymer matrix one can fill the traps with charges and hence eliminate their effect on electron mobility. Trap-dilution does not only improve transport but also reduces trap-assisted recombination boosting the efficiency of polymer light emitting diodes.
Perspectives of Unicoloured Phosphor-sensitised Fluorescence (UPSF)
L. Paterson, A. Mondal, P. Heimel, R. Lovrincic, F. May, C. Lennartz, D. Andrienko
Adv. Electron. Mater.,
Unicoloured phosphor-sensitised fluorescence (UPSF) is a dual emitting concept proposed for improving efficiencies and operational lifetimes of blue organic light emitting diodes (OLEDs). To overcome the limitations of the individual emitters it uses a phosphorescent donor to sensitise a fluorescent acceptor. To quantify the potential of the concept we develop a multiscale model of a UPSF OLED. We start from atomistic morphologies parameterise the rates of all processes on the available experimental data and solve the respective master equation with the help of the kinetic Monte Carlo algorithm. Our simulations show that the energy transfer between donor molecules is essential to reproduce the results of the time-resolved photoluminescence experiment. We then expand the scope of experiment by studying the effect of the acceptor concentration as well as Förster and (parasitic) Dexter energy transfer from the donor to acceptor on the characteristics of the UPSF OLED. Our study shows that an appropriate material design can further improve efficiency by more than 30\% and at the same time achieve radiative decay times below 0.02 µs thus significantly extending OLED operational lifetime.
A window to trap-free charge transport in organic semiconducting thin films
N. B. Kotadiya, A. Mondal, P. W. M. Blom, D. Andrienko, G.‐J. A. H. Wetzelaer
Organic semiconductors which serve as the active component in devices such as solar cells light-emitting diodes and field-effect transistors often exhibit highly unipolar charge transport meaning that they predominantly conduct either electrons or holes. Here we identify an energy window inside which organic semiconductors do not experience charge trapping for device-relevant thicknesses in the range of 100 to 300 nm leading to trap-free charge transport of both carriers. When the ionization energy of a material surpasses 6 eV hole trapping will limit the hole transport whereas an electron affinity lower than 3.6 eV will give rise to trap-limited electron transport. When both energy levels are within this window trap-free bipolar charge transport occurs. Based on simulations water clusters are proposed to be the source of hole trapping. Organic semiconductors with energy levels situated within this energy window may lead to optoelectronic devices with enhanced performance. However for blue-emitting light-emitting diodes which require an energy gap of 3 eV removing or disabling charge traps will remain a challenge.
Self-organization and charge transport properties of selenium and tellurium analogues of polythiophene
Sh. Ye, L. Janasz, W. Zajaczkowski, J. G. Manion, A. Mondal, T. Marszalek, D. Andrienko, K. Muellen, W. Pisula, D. S. Seferos
Macromol. Rapid Commun.,
A series of conjugated polymers comprising polythiophene polyselenophene and polytellurophene with branched 37‐dimethyloctyl side chains well‐matched molecular weight dispersity and regioregularity is synthesized. The ionization potential is found to vary from 5.14 to 5.32 eV with polytellurophene having the lowest potential. Field‐effect transistors based on these materials exhibit distinct hole transport mobility that varies by nearly three orders of magnitude with polytellurophene having the highest mobility (2.5 × 10−2 cm² V−1 s−1). The large difference in mobility demonstrates the significant impact of heteroatom substitution. Although the series of polymers are very similar in structure their solid‐state properties are different. While the thin film microstructure of polythiophene and polyselenophene is identical polytellurophene reveals globular features in the film topography. Polytellurophenes also appear to be the least crystalline even though their charge transport properties are superior to other samples. The torsional barrier and degree of planarity between repeat units increase as one moves down group‐16 elements. These studies show how a single atom in a polymer chain can have a substantial influence on the bulk properties of a material and that heavy group‐16 atoms have a positive influence on charge transport properties when all other variables are kept unchanged.
Rigorous characterization and predictive modelling of hole transport in amorphous organic semiconductors
N. B. Kotadiya, A. Mondal, S. Xiong, P. W. M. Blom, D. Andrienko, G.-J. A. H. Wetzelaer
Adv. Electron. Mater.,
Amorphous small-molecule hole-transporting materials are commonly used in organic light-emitting diodes and perovskite solar cells. Characterization of their main functionality hole transport has been complicated by the presence of large contact barriers. Using a recently developed technique to establish Ohmic hole contacts we investigate the bulk hole transport in a series of molecules with a broad range of ionization energies. The measured charge-carrier mobility dependence on charge concentration electric field and temperature is used to extract the energetic disorder and molecular site spacing. Excellent agreement of these parameters as well as ionization energies with simulation results paves the way to predictive charge-transport simulations from the molecular level.
Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs
P. Heimel, A. Mondal, F. May, W. Kowalsky, C. Lennartz, D. Andrienko, R. Lovrincic
Improving lifetimes and efficiencies of blue organic light-emitting diodes is clearly a scientific challenge. Towards solving this challenge we propose a unicolored phosphor-sensitized fluorescence approach with phosphorescent and fluorescent emitters tailored to preserve the initial color of phosphorescence. Using this approach we design an efficient sky-blue light-emitting diode with radiative decay times in the submicrosecond regime. By changing the concentration of fluorescent emitter we show that the lifetime is proportional to the reduction of the radiative decay time and tune the operational stability to lifetimes of up to 320 h (80 % decay initial luminance of 1000 cd/m2). Unicolored phosphor-sensitized fluorescence provides a clear path towards efficient and stable blue light-emitting diodes helping to overcome the limitations of thermally activated delayed fluorescence.
Universal strategy for Ohmic hole injection into organic semiconductors with high ionization energies
N. B. Kotadiya, H. Lu, A. Mondal, Y. Ie, D. Andrienko, P. W. M. Blom, G.-J. A. H. Wetzelaer
Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices such as organic light-emitting diodes solar cells and field-effect transistors. Here we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.