Organic semiconductors differ from inorganic semiconductors. In organic semiconductors the molecules are held together by weak van der Waals interactions and in inorganic semiconductors by covalent bonds. So the bonds are different

Thank you Hatem for opening the answers.

What are the consequences of this weak inter molecular bonding on the electrical properties of the organic semiconductors?

The first consequence is 'energy level'. The covalent, periodic structure of inorganic S/C results in band energy --> conduction band and valence band. In general, organic S/C don't have bands but discrete energy levels (HOMO, LUMO with respect to the bottom of valence and conduction band). Another consequence is doping effect. In inorganic S/C, atomic doping normally just increases carrier density and does not interfere the band structure. On the other hand, molecular doping in (weak Van der Waals interaction) organic S/C affects the molecular packing and assembly, hence the morphology and electronic properties of the film such as mobility, density of state distribution...

Thank you Hung for the vital extension of the of the answer.

Let us continue step by step, WHAT about the concept of the intrinsic concentration? Wheat about the effective mass of electrons and holes?

I invite all colleagues having experience with semiconductor materials to share!

Dear Hung Phan, could you please give one basic article to get these idea related to inorganic semiconductor and organic semiconductor

To be honest, I do not know any paper that clearly compare inorganic and organic semiconductors. I am interested more in charge transport between the twos, and this book chapter of Bassler is a good start to study charge transport in organic S/C. He did mention a little bit about inorganic S/C.

http://link.springer.com/chapter/10.1007/128_2011_218

Just to add to your discussion. There are even intermediate materials - such as Quantum Dots - that are somewhere in between the organic and inorganic world. We work with these materials for PV applications, and find that they are closer to inorganic materials in that they can form substantial depletion regions, you can dope them, and measure some effective mobility/diffusion length. However, there is also an organic component to the films we make with these particles (each nanoparticle can be surrounded by an organic ligand shell), and it is likely that the functional groups on the organics play some role in transport and the general energy landscape in these materials.

@Abdelhalim Zekry: I haven't seen organic S/C researchers emphasizing the impact of effective mass of electron and hole in their study. In thin film organic S/C, there is no well-defined band structure, then I don't know how to define or calculate the effective mass.

Intrinsic carrier density is thermal physics parameters, so I think it just depends on energy gaps (HOMO-LUMO).

Can someone explain what affect the effective mass of hole and electron in inorganic S/C?

The concept of the effective mass describes the effects of the internal periodic forces on the electron motion under the effect of an external electric field. Whenever one defines a mobility of charge carriers in a solid material, one can assign an effective mass to that charge carriers. In the literature of organic semiconductors, values for the effective masses and mobilities are given. It is noticed that the effective masses in organic materials are much greater than that of the inorganic materials probably because of the strong bonding inside t he molecules and weak inermolecular bonding among the molecules so that the electrons are largely localised in the molecules. This is in contrary to inorganic materials where the interatomic forces are large causing strong overlapping of the atomic orbitals and strong electron delocalization.

From the principle point of view, energy bands are formed in solid materials even when the interatomic or intermolecular forces are even weak. The overlap of the atomic orbitals may be small but is not zero.

Intrinsic concentration is caused by thermal generation and depends on the temperature, the energy gap and density of states in the valence band and conduction band.

For more details on this issue please refer to the link:

http://www.iitk.ac.in/directions/directions_dec07/3jan~DEEPAK.pdf

Dear Colleagues,

It is intended to continue the discussion in this question about the comparison of the organic and metallic semiconductors. 

The first criterion of the comparison is the chemical bonding among the constituting units of the material:

The metallic semiconductors are characterized by covalent  bonds between their constituting atoms such as silicon and thereby they crystalize in diamond or zencbelende structure. The organic semiconductors are molecular in nature and thereby form van der val forces between them. Such bond is much weker than the covalent bod. Both type of  materials can be crystalline, polycrystalline  or amorphous.

Let us assume that the two types of materials are crystalline. Then it is the chemical bond that affect the difference between the two materials.

Because of the molecular nature of the organic material and their weak bond the energy level splitting of the molecular orbitals will be small and there fore the bands will be narrow. The valence band in the organic semiconductor is termed homo band and the conduction band is termed lumo band. The density of state in every band is assumed Gaussian.

In the metallic semiconductor crystalls the splitting in very band is much larger leading to wide energy bands. The density of state is parabolic near the band edges. 

The band gap in the organic material is direct as it preserves the molecular nature while the bandgap can be direct like gallium arsenide or indirect like silicon.

The energy band structure affects the electrical and the optical performance of the material.

So, the organic materials has much larger effective mass than the metallic semiconductor and therefore much less mobility as the mobility is inversely proportional to the effective mass.

If any material is polycrystalline the mobility will be further reduced as the grain size decreases. In the limit if the material becomes amorphous, not only the mobility will be degraded to its minimum value but also the effective energy gap increases.

It is so that the organic materials are prepared using low temperature processes such as solution based method resulting in either amorphous or at the best polycrystalline  materials compared to crystalline or semicrystalline metallic materials.

This contributes to further lowering of mobility of the organic material. However one has not to forget that these material production methods runs at low temperature and with much simpler apparatus rendering them very cost effective.

The other important property of the organic semiconductor that relay on their unique energy band structure is that they have a very large absorption coefficient  of light  such that only a fraction of a micrometer of the material is sufficient to fully absorb the radiation in their absorption range. This makes them a strong alternative as a solar cell material. 

Also, the direct energy gap make such materials very suitable for light emitting diodes. The organic LEDS are now commercial while the organic solar cells still under development. 

Best wishes

Dear Question followers,

I would like that you actively participate in this discussion about this important topic to crystallize the most important similarities and difference between the the two classes of the semiconductors; the metallic and organic semiconductors.I would be happy if you invite colleagues who are interested in the topic.

Now we continue the discussion; this time about the mobile carrier concentrations and the and their sources in the materials.

Generally in any semiconductor their are two types of mobile charges, the electrons in the conduction band and the holes in the valence band,

If the material is pure and intrinsic the source of the electrons and holes will be the thermal generation of electron hole pairs. The concentration of the electrons will be equal to that of the holes n0=p0=ni. According to the mass action law, n0 p0= ni^2,

The intrinsic concentration ni^2= Nc Nv exp - Eg/kT, where Nv, Nc are the effective density of states , Eg is energy gap and T is the absolute temperature. This law is a general law and applicable for all semiconductors.

Organic semiconductor materials have relatively high energy gaps compared to the most common semiconductor metallic material, the silicon Si. The effective density of states according to the data in the litterateur are not much different from those of silicon, therefore the intrinsic concentration of the organic materials is very very small. Accordingly, organic materials approach the insulators rather than the semiconductors. In the sense, practically in their intrinsic state they behave as insulator.

As a matter of purity of the material metallic semiconductor are produced with high purity but the organic semiconductors are produced with less purity such they contain impurities leading to make them either p-type or n-type conducting.

Once doped either intentionally or unintentional one one can define for them a Fermi level as the in the metallic material. Otherwise one describes them as insulators in the sense their energy levels are aligned to the vacuum level. If the material is doped the energy level through a system of materials is aligned to Fermi level at thermal equilibrium. This is a very important point and i investigated it intensively in the literature and discussed it in a separate post in the researchgate:https://www.researchgate.net/post/How_is_the_current_limited_in_metal_organic_semiconductor_metal_MOSM_diode

The doping of the metallic semiconductors is by atomic substitution while in the organic material it is by adding doping molecules. There is donor molecules and acceptor ones.

Both materials react to the incident light with the proper wavelength  by generating excitons in the materials. The excitons dissociate directly after their generation because of the larger screening between the their electrons and holes as the dielectric constant of the metallic semiconductors are much larger than those of organic materials. So, can speaks of photogeneration of electron hole pairs.

In contrary, the excitons are strongly bound in organic materials and need assistance to be dissociated before they recombine again. This assistance by an electric field or by donor acceptor interfacing.

Electrons and holes can be injected by forming junctions with different materials, This is valid for both type of materials.

This concerns the mobile charge carriers and their main sources in the both classes of materials.

Dear colleagues,

Welcome again,

One day some scientists will pass through these posts and benefit from them.

In the last post i spoke about the sources of the mobile charge carriers in semiconductors. The organic and metallic semiconductors.

Now , i will try to compare the current conduction in the two classes of materials.

 The mobile charge carriers can drift under the influence of the applied electric field and can also diffuse under the concentration gradients. So, there will be the drift current and the diffusion current with their known formulations:

Jdrift= sigma E, where sigma is the electric conductivity and E is the intensity of the electric field, and 

Jdiff= - qDp dp/dx where q is the electron charge, D the diffusion constant and dp/dx is the concentration gradient for holes. Similar equation can be written for electrons

In organic semiconductors, assuming they are free of charge carriers they behave as insulators where the current in them will be either limited by injection , injection limited current , or limited by building up space charge in the bulk of the organic material. This is because the mobility of organic materials is very low.

I devoted a separate post to the current conduction in the organic materials you can follow it in the link:https://www.researchgate.net/post/How_is_the_current_limited_in_metal_organic_semiconductor_metal_MOSM_diode

The next post will be about recombination

Best wishes

From one of my main topics - longevity aka persistence - there are 2 aspects not considered here until now:

• Organic semiconductors are significantly sensible vs. the attack of humidity as well as vs. oxygen. Shielding them from both permanently is a real challenge.
• And they are quite sensible vs. high temperatures - due to the properties of the compounds.

These are aspects where anorganic semiconductors perform significantly better.

Anyway, both have their right to exist. But the decision for one or the other has to be based on quite a number of influencing factors - making 'things' not exactly simpler. On the other hand: it is good to have some choice.

Dear colleague Dreher, thank you for your valuable contribution. You touched a very important issue which concerns the reliability of such materials and pointed out the the challenges with such materials as solar cells. In fact, it was planned to come to them at the end of the treatment.

Now let us resume the comparison this ti me about the recombination mechanisms in both types of materials.

The recombination is the disappearance of a mobile electron in a hole. This process leads to the loss of mobile charge carriers and thereby affect much the electrical characteristics of the  semiconductor devices including solar cells and light emitting diodes.

The recombination mechanisms were studied intensively in metallic semiconductors and to less extent in the organic semiconductors.

From the conceptual point of view, the recombination mechanisms occurring in the the metallic semiconductor also occur in the organic semiconductors.These mechanisms can be classified into radiative and nonradative types. Radiative recombination is a consequence of the direct fall of electrons from the conduction band to the valence band band while the nonradiative one when the fall of the electrons occurs through trap levels in the bandgap. These trap levels are called recombination centers.

Every recombination mechanism has its specific dependence on the hole and electron concentrations and its rate constants.

For more details on such recombination mechanisms, please refer to the book in the link:https://www.researchgate.net/publication/236003006_Electronic_Devices

There is a recombination mechanism which exists in the organic semiconductors.

It is the Langevin mechanism which occurs as direct consequence of an electron and holes come in the field of the other.  They get this chance when they meet while moving with low mobility.  Its recombination rate r lv can be expressed by the expression:

r lv=q (mun+mup) pn/ epsilon,

where mun, mup are the electron and hole mobilities, p,n are the hole and electron concentrations, epsilon is the permitivity and q is the electron charge.

It is found that this mechanism is applicable in case of the organic LEDs, where it must be reduced by an appreciable factor when applied to organic solar cells.

Generally the rate can be expressed by r lv= kr pn, where kr is the recombination rate constant that can be determined experimentally.

Best wishes

Book Electronic Devices

Thank-you Prof. Zekry

Dear Prof. Zekry, dear Members, very interesting conversation; according to this theme my question on the subject is what  organic materials in combination with silicon (like SIO modulators) also for high-speed telecommunication in the area of near-infrared were suitable and also also have a very short rise time < 2 ns and a high transition rate.(~ 100 Gbit/s)? The electron mobility charge are an important factor and to increase the mobility, I think about  doping with phtalocyanides the organic materials.  I know that a lot of informations about high speed OPD in combination of Silicon / Organic (like DSTMS, OH1, DAst, etc.) are reported and that these materials have a very good performance. I need these information for an telecommunication project. Are an comparisation table with listed electrooptical parameters are available?!   Thanks for possible support. Best greetings and wishes

It looks interesting, which is why I also want to write a little about it:

Molecular orbital theory provides a delocalized perspective on chemical bonding such that any electron wave function in a molecule can be found anywhere in the molecule, as quantum mechanical conditions allow electrons in permitted eigenstates to travel under the influence of an arbitrarily large, or very limited, number of nuclei. Concepts of highest occupied molecular orbital (HOMO) with the filled molecular orbital highest in energy and lowest unoccupied molecular orbital (LUMO) with the empty molecular orbital just above HOMO are conventionally applied to covalent or organic compounds. They therefore tend to mean for insulating or non-conducting materials. However, in a complex organic molecule or conducting polymers, electronic orbitals may spread out as molecular orbitals over long distances in a molecule or electrons that are completely delocalized over arbitrary distances to move freely allowing to conduct electricity in the atomic sheet plane, as if they are in a metal

In contrast, in a metal or a semiconductor (usually, inorganic elements, compounds or intermetallic materials), which are crystalline, on the other hand, atom after atom forms three-dimensional array with sharing of valence electrons, which take part in bonding spreading throughout the specimen (wherein valence electron in free atom, say in Na, becomes conduction electron in metal, forming partially filled (10^23 atoms /cm^3) overlapping higher energy conduction band (CB) and lower energy valence band (VB). Hence, (s-band) electrons are free to move throughout the specimen. In a semiconductor, CB and VB is separated by a small energy gap, which in turn is large in insulators. Thus, HOMO and LUMO are molecular (0.5 - 10 nm) while VB and CB are macroscopic (~ or >1 mm). Or one may also say that the former is chemist’s and latter is physicist’s way of looking at the same thing as in electron spectroscopy for chemical analysis (ESCA) or x-ray photoelectron spectroscopy (XPS).

PS: In both cases, doping changes or tends to change the band structure or MOs, leading to consequent change material properties in the doped material, irrespective of the material is organic or inorganic. This is the reason why the whole lot of new research in materials science is proliferating for every passing hour or day…….

Just found this thread after searching about organic/inorganic material behaviors for photoelectronic devices optimization. I'm really grateful for all the scientists who share the info to start with and want to thanks Prof. Abdelhalim Zekry very much.

Dear Dinu and colleagues!

Welcome!

Thank you very much. I would like to pay your attention to my paper on the researchgate link: Data Advanced solar cell materials and solar cells analytical modeling

You will find it useful. It complements the discussion thread here.

It will appear in short time as ieee conference paper.

Best wishes

Thanks to all colleagues, especially Prof Zekry for all knowledge and a valuable answer. Since I've been thru to the topic I just to now why conductive polymerer is much better than any other material that had been used in thermoelectric?

For electronic transition to have non-zero probability, it is necessary to have spin conservation as we find while studying spectroscopic physics of molecular organic semiconductors. Is there any literature discussing the conservation of spin in inorganic direct bandgap semiconductors while radiative recombination occurs? Does the ratio of 1:3 for singlets and triplets hold for inorganic LEDs too?

Dear Dhruvajyoti Barah ,

In the transition among the different electronic sates the energy and momentum must be conserved. For example when electrons are transferred from a sate with energy E1, momentum 1p and spin s1 to another state with of E2,p2 and s2 it must be acquire of loose the difference in energy, momentum and spin.

If an electron is falling from the bottom of the conduction band to the top of the valence band having the same momentum, that is a direct band transition, it will only loose an energy eqaul to the energy gap.

This is the first part of your question.

The other part is that in the metallic LEDs with direct bandgap materials there is only one dominant type of transition which is the direct transition.

It is so that before the emission process the free electrons and holes must form an exciton. The formation of exciton is very probable as its dissociation. This is because the binding energy of the excitons is in the range of the energy of the phonons. There is no need to form a triplet level to promote the exciton formations as in the organic materials.

This is my explanations as I understood the behavior of the two material types.

After my contribution you have enough key words to search for the topic.

As for the recombination in the metallic semiconductors please follow the book:

Book Electronic devices with physical insight

After collecting and understanding you are invited to discuss my answer to your question.

Best wishes

Abdelhalim abdelnaby Zekry Thank you very much for your response.

If we try to give an analogy between the recombination processes in these two types of materials (organic and inorganic), can we say that the e-h pair in a direct bandgap inorganic semiconductor is analogous to the e-h pair forming a singlet exciton in an organic semiconductor?

According band gap, organic semiconductor has lower than inorganic semiconductor , in addition that, in case of inorganic semiconductor there is conduction band and valence band whereas organic conductor has HOMO &LUMO. inorganic semiconductor has low absorbance.