Thank you Ram.

The first challenge for determining the current of the MOSM diode is the energy alignment at the metal organic semiconductor interface. When the semiconductor material is doped then one can define for it a Fermi-level and accordingly the femi level will be constant at the interface similar to the metal- inorgenic semiconductor.

In case of intrinsic organic semiconductor it behaves as an insulator because of the relatively large bandgap. In this case, the alignant  will be according to the vacuum energy level. That is at equilibrium the energy levels align themselves to the vacuum level  Evac. Consequently, the electron transfer from one material to the other is governed by the energy difference between the two intended levels and their probability of occupation. Such simplifying assumption can help get mathematical expressions for the currents.

However, experimental measurenments showed that there is a potentail step at the metal organic semicondutor that may be due to unintentional doping of the material.

But here we will assume tht the material is free of doping.

Inspecting the literature concerned with the currents in the MOSM diodes with one charge type it is found that the current can be either injection limited from the M-S contact or it can be space charge limited. The space charge limited limited current exists when it is much less than injection current. The injection current is limited by the hight of the injection barrier at the contact which is more or less independent of the applied voltage except the image force barrier lowering and equal to the contact difference of potential for the ideal interface.

When the contact is injection rich, it can act as an Ohmic contact. So , in case of the Ohmic contacts the current will be space charge limited.

In the next post the contact limited current will be introduced.

Best wishes

Hi,

Are the Organic materials having a typical push-pull architecture? In my devices, typically MIM`s devices, the active materials have a Donor-acceptor architecture. This is believed to create trap states in the film (when the active layer is used). In my devices the current is limited by the "traps"  which are filled by the space charges that are injected by the respective electrodes. 

The band gap of my materials range from 1.83-2 eV most of the times. Sometimes in my recent study I have found 1.7 eV too!!. 

As such in the devices, I have observed that the absolute current is area dependent and the voltage required to fill these traps was thickness dependent.

The mode of charge transport was Space Charge Limited Current 

Traps can play a essential role in determining mode of charge transport..unless the materials is trap free...for a reference you could have a look at this paper of mine..I have tried to deal with mode of charge transport and have dealt its physical aspects

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.5b11509

Hope I have answered some aspect of this question. 

Cheers..:)

Thank you Ram.

The first challenge for determining the current of the MOSM diode is the energy alignment at the metal organic semiconductor interface. When the semiconductor material is doped then one can define for it a Fermi-level and accordingly the femi level will be constant at the interface similar to the metal- inorgenic semiconductor.

In case of intrinsic organic semiconductor it behaves as an insulator because of the relatively large bandgap. In this case, the alignant  will be according to the vacuum energy level. That is at equilibrium the energy levels align themselves to the vacuum level  Evac. Consequently, the electron transfer from one material to the other is governed by the energy difference between the two intended levels and their probability of occupation. Such simplifying assumption can help get mathematical expressions for the currents.

However, experimental measurenments showed that there is a potentail step at the metal organic semicondutor that may be due to unintentional doping of the material.

But here we will assume tht the material is free of doping.

Inspecting the literature concerned with the currents in the MOSM diodes with one charge type it is found that the current can be either injection limited from the M-S contact or it can be space charge limited. The space charge limited limited current exists when it is much less than injection current. The injection current is limited by the hight of the injection barrier at the contact which is more or less independent of the applied voltage except the image force barrier lowering and equal to the contact difference of potential for the ideal interface.

When the contact is injection rich, it can act as an Ohmic contact. So , in case of the Ohmic contacts the current will be space charge limited.

In the next post the contact limited current will be introduced.

Best wishes

Dear colleagues,

Welcome,

Now let us describe the current of the MOSM diode limited by the potential barrier between the metal and the organic semiconductor. 

Let us assume that the energy level alignment will be the vacuum level. Then at the metal semiconductor contact a potential energy difference phib between the metal and the organic semiconductor will be formed that can be expressed by the difference between the metal work function phim and the electron affinity exai, that is:

phib= phim-exai,

Let  us assume that the density of state in the organic material is No and the electrons will fill all these states. So Under the application of an electric field E that will be formed the  applied voltage dropped homogeneously on the semiconductor. Assuming the thickness of the semiconductor is d and the applied voltage is V, then the electric field will be = V/d. If the mobility of the material is mu, then the current will be drift current and can be expressed by:

J= q mu (No exp - phib/Vt) V/d

the term (No exp - phib/Vt))= n is the electrons that cross the barrier between the metal and the semiconductor.

For more information refer to chapter 6 of the thesis :Vertical Organic Field Effect Transistors Realized by Block-Copolymers Self-Assembly

Ariel J. Ben-Sasson

Best wishes

I wold like to thank my sincere colleagues who followed this question specially the friends Ioannis, Ram and Nilotpal. There fore i hope many others come to this question and contribute in it. So, for me i will continue to complete the answer and discuss the subject from the different view.

The current in metal organic semiconductor metal diodes are limited either by injection across the metal organic interface as i outline in my last post or it it may be limited by space charge  formation in the the bulk of the semiconductor. This current is termed space charge limited current SCL. In this case the injection rate is higher than the flow rate of charges in the semiconductor. Ohmic metal organic semiconductor contacts has unlimited injection capability and therefore whenever they exist the current will be SCL.

The origin of the space charge limitation is very low mobility of the organic semiconductors and and the unipolar behavior. That is the existence of the either mobile holes or electrons. We assumed such condition for conduction mechanisms which we treat in this post. I would like to bring to your memory that the unipolar performance stems from the intrinsic nature of many organic semiconductors. Intrinsically the organic semiconductor is more or less empty of mobile charge carries because of the its large bandgap.

Accordingly during their flow inside the material they accumulate in it and build space charge that impede the motion of the charges from the injector to the injecting contact to the collecting contact.

The space charge limited current is well known from the transport of electrons in vacuum and it is termed the three halves power low where the current is proportional to V^3/2, where V is the voltage across the diode. It also exists in the conduction by insulators, but with other forms:

For a 1D trap-free solid, the corresponding SCL current density is known as the Mott–Gurney (MG) law, given by

JMG =9/8 epsilon0 epsilonrl mu V^2/D^3 ; 

where epsilonr is the relative permittivity of the solid, and mu is the

electron mobility.

If the solid has an exponentially distributed traps (in energy), it is known as the Mark–Helfrich (MH) law,or the trap-limited SCL current density JMH

JMH= Nc mu^(1-l)[ epsilon l/ Nt(l+1)]^l  (2l+1/l+1)^l+1 (V^l+1/D^2l+1)

Here, Nc is the effective density of states corresponding to the energy at the bottom of the conduction band, Nt is the total trapped electron density, and l= Tt/T is the ratio of distribution of traps to the free carriers.

The current expressions are taken from the reference:Analytical re-derivation of space charge limited current in solids using

capacitor model,Y. B. Zhu1 and L. K. Ang1,2,a

Best wishes

Hi,

Thank you for the informative discussion. I loved the discussion and it helps me with my present research too. Just interested to know...a question

Assuming that there are traps in the organic semoconductor...and the traps originate from chemical defects(Since donor-acceptor architecture) do you think the deficient traps impede motion of holes or could be either way around? that way the current is trap limited? I found this feature to be interesting because of this most of the times I could run device tests without current compliance!! as the device does t experience a breakdown.

ALso, would like to know with certain paramters known (like thickness etc.,) Can I have an estimate of mobility from I-V curve since there is a SCLC transition happening due to charge accumalation? What rules could I put into for this calculation?

CHeers,

Ram

Dear Ram,

You are welcome.

The elctron traps lie  near the conduction band which is here the luma level while the hole traps lie near he valence band which is homo level. So, in principle there are hole traps and electron traps as there is electron mobility and hole mobility. There is also deep lying traps in the energy gap which can act as trap for both electrons and holes.

Most probable that they can act as  recombination centers affecting the concentration of the electrons and holes. While the electrons traps affect the mobility of one charge type.

As for the conduction mechanisms, every one has its characteristic I-V relation specially y the voltage dependence and the the thickness dependence of the active material in the diode, D. They can be used as indicators of the dominating mechanism.

Then by curve fitting one can determine the physical parameters of the material.

One can make additional measurements to verify some of the estimated parameters from the fitting such as the C-V measurements.

For more information please see the phd thesis: Device Physics of All-Polymer

Solar Cells,Maria Magdalena Mandoc

Best wishes

The contact metal - organic semiconductor metal MOSM diode current is limited by the action of an additional electric field arising due to the limitation contact surface with there to adjacent free surfaces of metal and semiconductor, virtually all real Schottky diodes.

 Professor Rasim,

would you please elaborate on your answer by giving examples of metal organic contacts behaving as Schottky diode. If the junction is Schottky it will limit the current through the device by formation of potential barrier at the interface which will be controlled by the applied voltage on the diode. 

This is also the case of metal - nonorganic semiconductor contact such that metal silicon schottky contacts.

What is the prevailing conditions in the organic semiconductor such that it behaves similar to the nonorganic ones.

Thank you for your  valuable contribution.

Best wishes

Dear colleagues,

In direct contact of the metal and semiconductor the work function (~ 4-5eV) of their free surfaces adjacent to the contact surfaces remain unchanged , while the potential barrier height on the contact surface becomes about of 0.1 - 1.0 eV. The appearance of the contact potential difference between the contact surface and the adjacent free surfaces of the metal and the semiconductor forms a additional electric field (AEF), directed from the contact surface to the free surfaces of metal and semiconductor. In all design structures Schottky contacts (SC), except  TMBS diode structure, the AEF extends around the contact and covers the peripheral near contact region of the wide  and completely the near contact region of the narrow contact surface. In the TMBS diode structure AEF generally exists in the contact region of the semiconductor and therefore it is the most suitable SC structure for clear description of the electronic processes associated with the AEF. Features occurrence AEF and its role in the formation of the potential barrier and  current in SC with TMBS diode structure studied in detail in [R.K. Mamedov, M.A. Yeganeh, Current Transport and Formation of Energy Structures in Narrow Schottky Diodes, J. Microelectronics Reliability, 52 (2)  (2012). 418 – 424 and R.K. Mamedov,  A.R. Aslanova, Additional electric field in TMBS diode, Superlattices and Microstructures,  92 (2016) 1–9]. Under the influence of AEF is a redistribution of free electrons in a semiconductor, arising by completely depletion of donor impurities  at the  certain temperatures. In near contact region of the semiconductor is formed the SCR from the positive ion impurities. In this region,  due to superposition of the AEF and the electric field SCR arises the resulting electric field,  which has a minimum value at the metal surface and the maximum value at a sufficiently large distance from the contact surface of the metal deeper into the semiconductor.

            Practically all thermo-, photo-, tensor- and other electrophysical characteristics of real contacts metal - semiconductor well interpreted using AEF.

Yours faithfully.

Dear Colleagues,

You are welcome!

I searched for the equilibrium between the metals and organic semiconductors and when and how a band bending occurs in the surface region organic semiconductor , i found that if the material behaves as a semi- insulator the energy bands of the metals and the organic material align to the vaccum level as one can not define a Fermi level for the organic material and because of the very bad conduction capability of the organic material.

In case of doped organic semiconductor either unintentionally or intentionally the material will possess free carries and has a definitive Fermi level.In this case the material behaves as an ordinary semiconductor and builds Schottky contacts.

In this case the M-S contacts will follow, the theory of Schottky junctions:

I = Is( exp V/nVt -1),

Where Is is the reverse saturation current, n is the ideality factor and Vt the thermal voltage. 

For the evidence for these findings please follow the Link: https://www.researchgate.net/profile/Michio_Niwano/publication/227998798_Kelvin_Probe_Study_of_Band_Bending_at_Organic_SemiconductorMetal_Interfaces_Examination

In the next post , i will make  a summary for the presented conduction models in this forum.

Chapter Kelvin Probe Study of Band Bending at Organic Semiconductor/...

Dear prof. Abdelhalim Zekry,

Greetings!

Thanks for the valuable and useful information and articles.

As you know, the fundamental energy model and the theory of Schottky thermionic emission current transport Bethe quite fundamentally interpret the electronic processes occurring in the idealized M-S contacts with homogeneous and unlimited the interface. At the same time, carry out a comprehensive and sufficiently detailed experimental studies using the most reliable and modern instrumentation parameters and characteristics of the Schottky contacts have convincingly shown that the features of real Schottky contacts having a generally inhomogeneous barrier height along the limited surface, differ markedly from ideal contacts. Existing discrepancy features real and ideal Schottky contacts were interpreted quite well considering the recently detected phenomenon of additional electric field (AEF) in real Schottky contacts by way of limitation, and the inhomogeneity the contact surface. You can see for yourself in this, if'll investigate the electrical properties of contacts metal - organic semiconductor with different diameters in the range of 1 micron to 1,000 microns.

You will see that the electrical properties of a wide Schottky contacts with a diameter greater than about 100 microns and a narrow Schottky contacts with diameters less than 100 microns are quite different, and this difference is well interpreted with the help of AEF (Ismayilov T. H., Aslanova A.R.,Influence of Peripheral Effects on the Electro Physical Properties of Schottky Diodes, International Journal of Innovative Technology and Exploring Engineering, 2015, v.5, n.2, pp.1-7 and Ismayilov T. H., Aslanova A.R., Electrophysical properties of Schottky diodes with inhomogeneous contact surface, J. Superlattices and Microstructures, 2016, v.90,  pp. 68-76).

Unfortunately, the discovery of a previously unknown, objectively existing phenomenon occurrence AEF in real Schottky contacts, not yet spread widely in the literature. It should be noted that the occurrence of AEF in the Schottky contact has enough fundamental scientific basis and is quite reliable experimental confirmation. Therefore, the study of electronic processes in Schottky contacts to obtain more valid scientific results need to pay special attention to the effect of AEF in real M-S contacts.

Yours faithfully.