[ODCAD] Junction and Vacuum Level in Organic Devices

[ODCAD] Junction and Vacuum Level in Organic Devices
The electrical performance of a device is effected by the junctions present. The energy barrier is a major factor to consider in modeling the junction. In device physics, the vacuum level is regarded as the common reference point to calculate the energy barrier. In inorganic semiconductors based on Si material, it is always regarded as truth that the vacuum level is the same for all of contacting layers (materials). This is called Vacuum level alignment.

In organic devices, it is common that function layer is organic semiconductor, and electrode layers are inorganic materials. Scientists found that this vacuum level alignment may not be applicable in the junction between organic layer and inorganic layer. For example, the junctions Ag/Alq3, Ag/Almq3 have been observed that the vacuum levels are not aligned for difference about 1.1 eV [1].

This non aligned vacuum level can happen for system with or without chemical bond, and doped and undoped. It means that it is not due to chemical reaction in the junction. Scientists explained that there is dipole behaving like internal field across the junction. The source of the dipole is due to significant difference between two materials. The molecules in the junction try to arrange their position to have minimum free energy of the whole system. This junction and the dipole are different from p-n or Schottky junction in terms of size and field. They will be discussed in the other topic.

The consequence of different vacuum levels is critical for the energy barrier. For example, the vacuum level of organic layer is usually reduced by a amount (say 0.5 eV). For hole injection from the junction into the organic layer, the energy barrier is increased by that amount, while the energy barrier is reduced by that amount for electron injection. That amount of energy barrier change can result in significant effect to electrical performance of the device.


1. I. G. Hill, Appl. Phys., Vol. 84, 3236 (1998)


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[ODCAD] US Defence Dept.- Organic Photovotaics (PV)

Effective solar cell device definitly has great application in any electronic products that require power supply. Inorganic semiconductors like Si currently rule the market. Organic materials potentially have advantage of smaller size (thinner), light weight, effcient, flexible (bend without breaking), and low cost. Many experts, and labs bet the future of photovotaics (PV) on organic materials.

US Dept. of Defence may be the biggest investor in this technology. Lynne Samuelson, a researcher at the US Army's Natick Soldier Center, in Massachusetts, claimed that his lab is starting to make proto-type PV devices to try out in the field. That Lab teamed up with a chemistry lab at Univ. of Massachusetts Lowell. The structure and materials used in their cell may contain Titania (? it may mean Ti)-TiO2 particles/Polymer. Titania particle can be as small as 20nm. The polymer is polyethylene terephthalate. Its effciency has not been disclosed. It is estimated that it may be >4%. This lab is expecting 20% effciency in five years.

Part of Information is from Spectrum of IEEE.
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[ODCAD] Coming Events in May, 2004

May 4, Singapor: Semicon 2004
Location: Singapore International Convention and Exhibition Centre (SICEC)
When :May 4-6, 2004
More Info:http://www.semi.org/wps/portal/_pagr/103/_pa.103/259?
id=19040&startRow=1

May 4 US: The 15th Annual IEEE/SEMI Advanced Semiconductor
Manufacturing Conference
Location: Seaport Hotel,Boston, Massachusetts, USA
Mre Infor: http://www.semi.org/wps/portal/_pagr/103/_pa.103/259?
id=18967&startRow=1

Ma 7 Bay Area, CA, US:IEEE EDS/SCV Evening Seminar in Bay area, CA
Location: National Semiconductor, Building 31, 955 Kifer Rd.
Sunnyvale, CA
Topic: Compact Modeling
When: 6-8 PM
Admission: Free

May 17 NY, US: Nanobusiness Conference
Location: Marriott Financial Center, NY
More Info :snemeth@penton.com

May 23-28 Seattle, US: SID Display 2004
Location: Washtington Sate Convetion and Trade Center
More info :http://www.sid.org/conf/sid2004/sid2004.html

May 25 Pennsylvania, US:Pennsylvania Nanotechnology Conference 2004
Location: The Pennsylvania Convention Center, Philadelphia, PA
More Info: http://www.pananoconference.org/

To check events in June or later, visit
http://groups.yahoo.com/group/OrganicDevice/, chect its calendar, and
add interesting event to yours.

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[ODCAD] Schottky Effect: Junction with Organic Semiconductor
Electric current vs voltage of an organic device such as OLED is important factor to decide the device performance. When field is at medium strength and the device is at junction control, one factor in device model is Schottky effect.

This effect reduces energy barrier for charge carrier injection from the electrode. This is due to the image force while charge carrier leaving electrode. The equation can be expressed as

[qE/(4 Pi e0)]^1/2 where e0 is dielectric constant of the semiconductor.

In the following discussion, we assume organic device has structure of Electrode/Organic semiconductor/electrode. Also, we assume one junction is ohmic.

In terms of equation, there is nothing new for organic semiconductor (compared with Si technology). A few points we have to pay attention when you use the model to describe your device behavior.

1. The derivation of the equation assumes that the field in the semiconductor is uniform. This may not be true if the organic material has high impurity such as ions that can redistribute under field effect. In such case, the field E may need to be carefully related with external electric voltage.

2. This is effect for junction control. Do not apply it when the device is at bulk control. It is easy to be confused with Poole Frenkel Model.

3. It is applicable when the field is relatively strong say E>10^3 V/cm, but not too strong say E~10^6V/cm. At very strong field, see if it is tunneling in control.

4. Compared with Si material, Schottky effect is much stronger for organic semiconductor. This is because the organic material has much smaller dielectric constant (ususally it is ~3,much less than ~11 of Si).

This article is from Organic Device group http://groups.yahoo.com/group/OrganicDevice/.

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