Selected Publications

Graphene memristors based on humidity...

Elimination of Charge Carrier Trapping...

Electron Trapping in Conjugated Polymers

Efficient Blue Polymer Light‐Emitting Diodes

Performance investigation of supercapacitors with polyethylene oxide-based gel polymer and ionic liquid electrolytes: Molecular dynamics simulation

Due to the importance of supercapacitors in electronic storage devices, improving their efficiency is one of the topics that has attracted the attention of many researchers. Choosing the proper electrolyte for supercapacitors is one of the most significant factors affecting supercapacitors’ performance. In the present paper, we compare liquid electrolytes (ionic liquid electrolytes) and solid electrolytes (polymer electrolytes) by molecular dynamics simulation to summarize their pros and cons. We consider polymer electrolytes in linear and network configurations. The results show that although ionic liquid-based supercapacitors have a larger differential capacitance since they have a smaller operation voltage, the energy stored is less than polymer electrolyte-based supercapacitors. Also, our investigations indicate that polymer electrolyte-based supercapacitors have more mechanical stability. Therefore, they can be considered a very suitable alternative to liquid electrolyte-based supercapacitors since they do not have known liquid electrolyte problems and display better performance.

Authors: N. Eyvazi, D. Abbaszadeh, M. Biagooi, and SE. Nedaaee Oskoee
Journal of Applied Physics, 2023

Graphene memristors based on humiditymediated reduction of graphene oxide

Memristors have emerged as promising devices for neuromorphic applications, particularly as synaptic weight. Graphene oxide, a partially oxidised and electrically insulating form of graphene, has been employed in metal/insulator/metal devices, where resistance switching based on the filamentary growth of the contacting metals has been demonstrated. Here we demonstrate an alternative highly reproducible resistance switching mechanism based on solid-state reduction of GO thin-films mediated by adsorbed water. It is shown that distinguishable and highly stable resistance states can be controllably realised in graphene oxide metal/insulator/metal devices. We have unravelled the growth mechanism and determined the growth kinetic of reduced graphene oxide, which enables a deterministic way to tune the resistance in GO devices. The demonstration of highly reproducible memristors based on graphene oxide crossbar devices is very promising for the realisation of low-cost and environmentally benign solution-processable neuromorphic synaptic weight.

Authors: Fatemeh Haghshenas Gorgabi, Maria C. Morant-Min˜ana, Haniyeh Zafarkish, Davood Abbaszadeh, and Kamal Asadi
Journal of Materials Chemistry C, 2023

Electron Trapping in Conjugated Polymers

Electron trapping is a well-recognized issue inorganic semiconductors, in particular in conjugated polymers,leading to a significant electron mobility reduction in materialswith electron affinities smaller than 4 eV. Space-charge limitedcurrent measurements in diodes indicate that these traps havesimilar molecular origin, while calculations show that hydratedmolecular oxygen is a plausible molecular candidate, with the tailof the solid-state electron affinity distribution reaching values ashigh as 4 eV. By decreasing the trap density by mixingconjugated polymers with an insulating polymer matrix, one canfill the traps with charges and hence eliminate their effect onelectron mobility. Trap dilution not only improves transport butalso reduces trap-assisted recombination, boosting the efficiencyof polymer light emitting diodes.

Authors: Davood Abbaszadeh, Alexander Kunz, Naresh B. Kotadiya, Anirban Mondal, Denis Andrienko, Jasper J. Michels, Gert-Jan A. H. Wetzelaer, and Paul W. M. Blom
Chemistry of Materials, 2019.

Elimination of charge carrier trapping in diluted semiconductors

In 1962, Mark and Helfrich demonstrated that the current in a semiconductor containing traps is reduced by N/N t r, with N the amount of transport sites, N t the amount of traps and r a number that depends on the trap energy distribution. For r> 1, the possibility opens that trapping effects can be nearly eliminated when N and N t are simultaneously reduced. Solution-processed conjugated polymers are an excellent model system to test this hypothesis, because they can be easily diluted by blending them with a high-bandgap semiconductor. We demonstrate that in conjugated polymer blends with 10% active semiconductor and 90% high-bandgap host, the typical strong electron trapping can be effectively eliminated. As a result we were able to fabricate polymer light-emitting diodes with balanced electron and hole transport and reduced non-radiative trap-assisted recombination, leading to a doubling of their ...

Authors: Davood Abbaszadeh, Alexander Kunz, Gert-Jan A. H. Wetzelaer, Jasper J. Michels, N. Irina Crăciun, Kaloian Koynov, Ingo Lieberwirth, Paul W. M. Blom
Nature Materials, 2016

Efficient Blue Polymer Light‐Emitting Diodes with Electron‐Dominated Transport Due to Trap Dilution

As is common for many conjugated polymers used in light‐emitting diodes (PLEDs), the charge transport in blue‐emitting polyspirobifluorene (PSF) copolymerized with the hole transport unit – N,N,N′N′‐tetraaryldiamino (TAD) biphenyl – is dominated by holes. Although the free electron mobility is an order of magnitude higher than the hole mobility, the electron transport is strongly hindered by traps. By diluting PSF‐TAD with the wide band gap polymer poly(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl) (PFO), the effect of electron trapping can be nearly eliminated. As a result, the transport in the PSF‐TAD:PFO blend becomes electron dominated. Due to the higher electron mobility, PLEDs made from these blends exhibit higher current and light‐output as compared to hole‐dominated PLEDs made from pristine PSF‐TAD. The reduced amount of electron traps enhances their efficiency from 2 cd A−1 for the hole‐dominated …

Authors: Davood Abbaszadeh, Paul W. M. Blom
Advanced Electronic Materials, 2016

Efficient polymer light-emitting diode with air-stable aluminum cathode

The fast degradation of polymer light-emitting diodes (PLEDs) in ambient conditions is primarily due to the oxidation of highly reactive metals, such as barium or calcium, which are used as cathode materials. Here, we report the fabrication of PLEDs using an air-stable partially oxidized aluminum (AlOx) cathode. Usually, the high work function of aluminum (4.2 eV) imposes a high barrier for injecting electrons into the lowest unoccupied molecular orbital (LUMO) of the emissive polymer (2.9 eV below the vacuum level). By partially oxidizing aluminum, its work function is decreased, but not sufficiently low for efficient electron injection. Efficient injection is obtained by inserting an electron transport layer of poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3] thiadiazol-4,8-diyl)] (F8BT), which has its LUMO at 3.3 eV below vacuum, between the AlOx cathode and the emissive polymer. The intermediate F8BT layer not only serves as a hole-blocking layer but also provides an energetic staircase for electron injection from AlOx into the emissive layer. PLEDs with an AlOx cathode and F8BT interlayer exhibit a doubling of the efficiency as compared to conventional Ba/Al PLEDs, and still operate even after being kept in ambient atmosphere for one month without encapsulation.

Authors: Davood Abbaszadeh, Gert-Jan A. H. Wetzelaer, Nutifafa Y. Doumon, Paul W. M. Blom
Journal of Applied Physics, 2016

Effect of the layer thickness on the efficiency enhancement in bilayer polymer light-emitting diodes

To eliminate quenching of excitons at the metallic cathode of a polymer light-emitting diode (PLED) the emitting layer is separated from the cathode by a hole-blocking layer (HBL). We investigate a wide range of single-layer and bilayer PLEDs with different thicknesses consisting of a poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) emitting layer and a 20nm poly(9,9′-dioctylfluorene) (PFO) HBL. The highest efficiency for both single-layer and bilayer devices is achieved when the total polymer layer thickness is ∼90nm. As a result, addition of an HBL to reduce cathode quenching is only effective when the luminescence enhancement due to microcavity effects in PLEDs is restored. The relative efficiency enhancement in bilayer devices as compared to single-layer devices varies from 283% for a 30nm active layer to 20% for a 250nm device.

Authors: Davood Abbaszadeh, Nutifafa Y. Doumon, Gert-Jan A.H. Wetzelaer, L. Jan Anton Koster, Paul W. M. Blom
Synthetic Metals, 2016

Exciton quenching at PEDOT: PSS anode in polymer blue-light-emitting diodes

The quenching of excitons at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) anode in blue polyalkoxyspirobifluorene-arylamine polymer light-emitting diodes is investigated. Due to the combination of a higher electron mobility and the presence of electron traps, the recombination zone shifts from the cathode to the anode with increasing voltage. The exciton quenching at the anode at higher voltages leads to an efficiency roll-off. The voltage dependence of the luminous efficiency is reproduced by a drift-diffusion model under the condition that quenching of excitons at the PEDOT:PSS anode and metallic cathode is of equal strength. Experimentally, the efficiency roll-off at high voltages due to anode quenching is eliminated by the use of an electron-blocking layer between the anode and the light-emitting polymer.

Authors: Davood Abbaszadeh, Gert-Jan A. H. Wetzelaer, Herman T. Nicolai, Paul W. M. Blom
Journal of Applied Physics, 2014

Effect of arylamine hole-transport units on the performance of blue polyspirobifluorene light-emitting diodes

The operation of blue light-emitting diodes based on polyspirobifluorene with a varying number of N,N,N′,N′ tetraaryldiamino biphenyl (TAD) hole-transport units (HTUs) is investigated. Assuming that the electron transport is not affected by the incorporation of TAD units, model calculations predict that a concentration of 5% HTU leads to an optimal efficiency for this blue-emitting polymer. However, experimentally an optimum performance is achieved for 10% TAD HTUs. Analysis of the transport and recombination shows that polymer light-emitting diodes with 5%, 7.5%, and 12.5% TAD units follow the predicted behavior. The enhanced performance of the polymer with 10% TAD originates from a decrease in the number of electron traps, which is typically a factor of three lower than the universal value found in many polymers. This reduced number of traps leads to a reduction of nonradiative recombination and exciton quenching at the cathode.

Authors: Davood Abbaszadeh, Herman T. Nicolai, N. Irina Craciun, and Paul W. M. Blom
Physical Review B, 2014

Measurement of the acoustic-to-optical phonon coupling in multicomponent systems

Authors: Antonio Caretta, , Michiel C. Donker, Diederik W. Perdok, Davood Abbaszadeh, Alexey O. Polyakov, Remco W. A. Havenith, Thomas T. M. Palstra, Paul H. M. van Loosdrecht,
Physical Review B, 2015

Measurement of the acoustic-to-optical phonon coupling in multicomponent systems

Authors: Antonio Caretta, , Michiel C. Donker, Diederik W. Perdok, Davood Abbaszadeh, Alexey O. Polyakov, Remco W. A. Havenith, Thomas T. M. Palstra, Paul H. M. van Loosdrecht,
Physical Review B, 2015

Who we are?!

The Device-Physics of Electronic Materials (D-POEM) group is the experimental condensed matter branch of the Physics Department at the IASBS. We study electron transport in organic and hybrid organic-inorganic nanostructured materials using experimental methods. Beside this, to deploy the physics of these materials we try to model the obtained measured results numerically. We aim to work on the state-of-the-art topics and materials that are used to make the new types of photovoltaics, light-emitting diodes, memristors, supecapacitors, etc. For this, we have made several collaborations with some very prestigious groups in Iran and outside of Iran. Want to join? Contact us!