Organic Semiconductor

Topic: Interface processes controlled by technology as
a way to optimize the properties of thin metal oxide -
organic semiconductor hybrid layers.
PhD Applicant: Preeti Choudhary
chaudharypreeti1997@gmail.com
Applicant ID: 10591

Content
• Organic and inorganic semiconductor
• Thin metal oxide/organic semiconductor
• What are the Motivation, Challenges and Overcome
• Improvement in organic electronics
• Advantage of organic semiconductor/metal oxide
• Application and future scopes
• Conclusion

Inorganic vs. organic semi-conductors
Inorganic semi-conductors:
Free carriers in form of
electrons and holes
Wannier-Mott type
excitons (small binding
energies)
Much smaller effective
mass of carriers of that in
organic.
Higher mobility for charge
transport.
Band mechanism for
charge transport
Organic semi-conductors
Do not support free
electrons and holes
Frenkel type excitons
(large binding energies)
Much larger effective
mass of carriers of that in
inorganic.
Lower mobility for
charge transport.
Hopping mechanism for
charge transport.

Organic semiconductor
• An organic semi-conductor is an organic
material that demonstrates an unusually high
conductivity along with many other
characteristics of semiconductors.
• This conductivity shown is often enhanced
when certain gases are present.
• Organic semi-conductors include organic
dyes, aromatic compounds, polymers with
conjugated bonds, charge-transfer molecular
complexes, and ion radical salts.

Thin metal oxide/organic semiconductor
• There are no intrinsic charge carriers in a typical organic semiconductor, all
charges in the device must be injected from electrode/organic interfaces,
whose energetic structure consequentially dictates the performance of
devices.
• The energy barrier at the interface depends critically on the work function
of the electrode. For this reason, various types of thin-film metal oxides
can be used as a buffer layer to modify the electrode work function.
• Metal oxides in organic devices enables the creation of electronic devices
that have both the advantages of organic materials, such as flexibility and
light weight, as well as those of metal oxide materials, such as optical
transparency and stability against ambient air.
• Developing high-performance organic/metal-oxide hybrid devices is
challenging, because the deposition of a metal oxide onto an organic
semiconductor layer severely damages the device for reasons that are not
well understood.
Ref: (1)Suemori, K., Ibaraki, N., & Kamata, T. (2021). Importance of internal stress control in organic/metal-oxide hybrid devices. Applied Physics Letters, 119(1), 013502.
(2) Greiner, M. T., & Lu, Z. H. (2013). Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces. NPG Asia Materials, 5(7), e55-e55.

Motivation
• Organic devices are essential components of next-generation soft and
lightweight electronics such as film-like displays sensors for internet
of things, and smart skins.
• The high operational stability can be achieved by over-coating the
organic devices with a passivation film of a metal oxide, such as Al2O3,
TiOx, and SiOn to prevent the diffusion of O2 and H2O molecules from
the ambient air to the organic layers.
• Cost effective and biodegradable (being made from carbon).

Challenges
• The metal oxide films are fabricated by sputtering, because their
sublimation temperatures are too high for thermal evaporation.
degradation of device performance caused by formation of the metal oxide
layer is generally referred to as “sputtering damage.”
• Exposure of the organic underlayer by the plasma, electrons, and particles
with a high kinetic energy formed during the sputtering of metal oxide
causes the breakdown of the organic molecules in the organic underlayer
surface, leading to significant degradation of the device performance.
• The fabrication of organic/metal oxide hybrid devices without sputtering
damage remains challenging
• Metal oxide films generally contain the high internal stress. The internal
stress of a metal oxide incorporated in an organic device may notably affect
the morphological and electrical characteristics of the device.

Overcome
• To prevent the breakdown of the organic molecules due to such
processes, a facing target sputtering system, which has a suitable
target configuration that enables the preservation of the organic
underlayer by preventing its exposure to plasma, electrons, and high-
energy particles, is generally used for metal oxide deposition.
• The degradation of the electrical characteristics of a device caused by
sputtering damage persists even when the facing target sputtering
technique is reviewed.
• By depositing an internal-stress-controlled metal oxide, we fabricated
an organic/ metal-oxide hybrid device without the deterioration of
the device performance.
Ref: Suemori, K., Ibaraki, N., & Kamata, T. (2021). Importance of internal stress control in organic/metal-oxide hybrid

Improvement in organic semiconductor device
• OLED displays are arguably the most mature of the organic
semiconductor technologies. Ultra-high-resolution OLED displays— with
high brightness, extreme contrast ratios, rich color reproduction and
extended operating lifetimes—are already available in the consumer
electronics market.
• OPVs have also seen great progress in recent years, with power
conversion efficiencies increasing upto 17.5%.
• Major developments have also been seen with OFETs—a technology that
enables computer processors and display devices to take the form of
flexible plastic sheets, using inexpensive production techniques.

• The alignment between donor and acceptor levels is often referred to as energy-level
alignment (ELA).
• For an organic semiconductor, the donor level is the highest-occupied molecular orbital
(HOMO) and acceptor level is the lowest unoccupied molecular orbital (LUMO). For
conductive electrodes, the Fermi level (EF) serves as both the donor and acceptor level.
• The ELA criteria vary slightly for different types of organic devices: In OLEDs, there are
two types of charge injection contacts; electron-injecting and hole-injecting contacts.
Low-resistance electron-injecting contacts require that the donor level of the electrode
(that is, the cathode) be closely aligned with the LUMO level of the organic
semiconductor. Conversely, low-resistance hole-injecting electrodes (that is, anodes)
require that the electrode’s acceptor level (that is, Fermi level) is closely matched with
the organic’s HOMO level.
• In p-OFETs, there are hole-injecting contacts, where the Fermi level of the source
electrode should align with the HOMO of the organic semiconductor to minimize the
hole-injection barrier. In n-OFETs, there are electron-injecting contacts, where the Fermi
level of the source electrode should align with the LUMO of the organic semiconductor
to minimize the electron-injection barrier.
• OPV’s, the scenario is slightly different. One often refers to hole-collecting and electron-
collecting electrodes, rather than hole-injecting and electron-injecting electrodes. A
hole-collecting electrode should have its donor level closely aligned with the organic’s
HOMO level, and an electron-collecting electrode should have its acceptor level closely
aligned with the organic’s LUMO level to minimize energy losses.
Representation of organic electronic device
Ref: Greiner, M. T., & Lu, Z. H. (2013). Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces. NPG Asia Materials, 5(7), e55-e55.

Energy-level diagram of (a) a two-layered OLED, (b) a two-layered OPV, (c) an one-layer p-OFET and (d) an one-layer n-
OFET.

Transition metal oxide Buffer layer
• Transition metal oxides can possess a wide range of work functions, spanning
from extreme low of 3.5 eV for defective ZrO2 to the extreme high of 7.0 eV
for stoichiometric V2O5.
• High-work function metal oxides are often used as hole-injecting buffer layers
for anodes including MoO3 ,NiO, CuO, V2O5, Fe3O4 and Ag2O.
• The low-work-function transition metal oxides—such as TiO2, ZnO and ZrO2
are used as electron-injection buffer layers for cathodes.
• Some oxides can be evaporated at relatively mild temperatures—such as
MoO3, WO3 and V2O5—and are used for devices that are fabricated in
vacuum. Other oxides can be solution deposited—such as NiO,V2O5, TiO2,
WO3, ZnO,MoO3 and sub-stoichiometric MoOx —making them convenient to
use as buffer layers in solution-processed organic devices.
• There has also been some use of sub stoichiometric oxides, such as WO3 and
MoO3 which tend to be metallic and can provide low-resistance buffer layers.

Comparison between organic Electronics & Silicon
Cost
Fabrication Cost
Device Size
Material
Required Conditions
Process
Organic Electronic
$5 / ft2
Low Capital
10 ft x Roll to Roll
Flexible Plastic Substrate
Ambient Processing
Continuous Direct Printing
Silicon
$100 / ft2
$1-$10 billion
< 1m2
Rigid Glass or Metal
Ultra Cleanroom
Multi-step Photolithography

Advantages metal oxide/organic semiconductor
• Organic electronics are lighter, more flexible and metal
oxide having transparency, air ambient.
• Low-Cost Electronics
• No vacuum processing
• No lithography (printing)
• Low-cost substrates (plastic, paper, even cloth…)
• Direct integration on package (lower insertion costs)
• They are also biodegradable (being made from carbon).
• This opens the door to many exciting and advanced new
applications that would be impossible using copper or
silicon

Disadvantages of metal-oxide/organic semiconductor
• Internal stress of metal oxide affect the device performance.
• Deposition of metal oxide layer without damage still challenging.
• Conductive polymers have high resistance and therefore are not good
conductors of electricity.
• Because of poor electronic behavior (lower mobility), they have much
smaller bandwidths.
• Shorter lifetimes and are much more dependent on stable environment
conditions than inorganic electronics would be.

Future of metal/oxide organic Semiconductor
• Smart Textiles
• Lab on a chip
• Portable compact screens
• Skin Cancer treatment

Conclusion
• High-performance organic/metal-oxide hybrid devices without the
damage may be key devices that open up electronics with features.
• Metal-oxides in organic devices enables the creation of electronic
devices that have both the advantages of organic materials, such as
flexibility and light weight, as well as those of metal oxide materials,
such as optical transparency and stability against ambient air.

Thank You for your kind attention


Organic Semiconductor

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