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OLET : Organic Light Emitting Transistor

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Published in: Physics
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This is a seminar presentation which include: Introduction to OLED  OLET vs. OLED OFET OLEFET Review of researches Applications Conclusion

Miziya K / Sharjah

14 years of teaching experience

Qualification: M.Tech

Teaches: Electronics, Matlab, Engineering, Electrical Technology, SAT, Maths, GATE, Computer Science, Mathematics, Physics

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  1. OLET Presented by : Miziya
  2. CONTENTS Introduction OLET vs. OLED OFET OLEFET Review of researches Applications Conclusion
  3. I,INTRODUCTION Organic Semiconductors Why organic electronics Organic Semiconductors are molecules and polymers. based on pi-conjugated
  4. Why organic electronics Low cost Ease Of processing on a wide range of substrates. Charge carrier transport, luminescence, environmental and thermal stability depend on their molecular structure which can be tuned by chemical synthesis. Unlimited choice of materials. Multifunctionality. Ability to transport ions. Integration with biological systems. Organic electronics is not meant to replace Si electronics but to complement and extend it.
  5. 2,OLET Vs,OLED OLED: Advantages and Disadvantages Comparison Of OLET with OLED Advantages Of OLET over OLED.
  6. Advantages Easier to produce Can be made on large area Large field of view about 1700 OLED Disadvantages Shorter lifetime Not very stable easily Can contaminated by water and oxygen.
  7. OLED Vertical Device structure Charge transport occurs perpendicular organic to layers(Bulk Charge Transport) Through the contacts When light travels through the contacts, there is usually a loss of efficiency of 20 to 30 percent Mobility Minority Carriers travel only a few tens of nanometers to recombine radiatively. OLET Planar device structure Charge transport horizontally is (Field Effect Charge transport) The light is emitted as a stripe along the emissive layer The transistor does not suffer loss Under typical biasing condition electron and hole mobility can be about four orders of magnitude higher Longer distances typically tens of micrometers
  8. Advantages Virtually higher electroluminescence quantum efficiency (device structure). Reduced exciton quenching due to interaction with externally applied electric field and with metal contacts. Presence Of third electrode helps to achieve a balanced charge carrier current, further reducing EQ Highly Integrated device structure. Position Of emission zone can be controlled. The shape of the emitted light would also make it easier to couple it into waveguides and other structures. "We show that the same organic emitting layer leads to more efficient device emission when it is incorporated in the OLET structure than in the OLED one" says Muccini.
  9. 3,OFET 1.Fundamenta1s of OFET 2.Device structure 3,0perating regimes 4.Energy level Analysis 5.Classification
  10. 3, I Fundamentals of OFET os Gate oxide Vg What ? Components -Y > Organic semiconductor Dielectric layer > Three electrodes Tetracene, pentacene , ruberene Gate, Substrate -5 highly doped Si Gold, Silver , Calcium Dielectric -->SiO Si N A1203,organic insulators
  11. BC/BG Wce Semicondwtor Dielectric Göte 3.2, Device structure Doped Si layer serves as back gate. Then dielectric layer is grown on top of substrate. S and D , fabricated by photolithography and metal deposition process. Finally Organic semiconductor is deposited. Advantage Easy to fabricate High efficiency due to photolithography. Disadvantage High injection resistance as the interface between the electrodes and semiconductor are limited to sidewalls of the electrodes.
  12. b) Source Dielectric TC/BG Dr*n Electrode material Stencil Top contacts Nanoflbers Si02 (gate dielectric) Sib (backgate) Semiconductor should be deposited first. S and D are fabricated by Stencil technology. Advantage The injection resistance is decreased Disadvantage Process is more complicated and less efficient. Metal deposition can cause damage.
  13. c) BC/TG Gate Oieleetric Source Semiconductor Advantages The injection resistance is decreased Process is not complicated . Dr*n Disadvantage The only issue is Si02 cannot be grown on substrate as dielectric layer, as it has to form after the semiconductor deposition. So organic dielectric layer is applied by spin coating process.
  14. regimes Current -voltage output characteristics curves: Ids Linear regime Saturation regime -Ids Linear regime Vg -0 Vds a) for n-type semiconductors Saturation regime -Vg -Vds b) for p-type semiconductors
  15. w BC/BG structure a) Vd Drain Gate Vg b) Vd
  16. 3.4, Energy level Analysis VacuumLevel .5.1ev 2.76ev LUMO 4.83ev 5.1ev HOMO pprrpp HOMO -5 Conduction band LUMO Valance band The current is formed by electrons jumping from one molecule to another. S and D electrodes with appropriate work functions should be selected to enable carrier injection.
  17. 3.4, Energy level Analysis VacuumLevel .5.1ev 2.76ev LUMO 4.83ev 5.1ev HOMO pprrpp IfVgs = +ve , energy level shift downwards. IfVd = +ve , energy level shift downwards. P-channel - Electrode energy level is closer to HOMO N-channel - electrode energy level is closer to HOMO
  18. Classification Organic semiconductors are used in high purity. Even for small dopants it is difficult to directly observe the difference of transport properties of electrons. If the material has more hole accumulation behaviour and less electrone affinity (+ve vg) -5 P-Channel . Both p-channel and n-channel OFETs are called unipolar OFETs,
  19. Ambipolar OFET Organic Semiconductors which can accumulate and transfer both holes and electrons are known as ambipolar OLETs. Let Vg= Vd and source is grounded, If Vg > Vt, electrons are injected from source and induced through the semiconductor layer by positive drain voltage. If Vg < V t, no electrons are injected from source If Vg is reduced further, IVgl> IVtl of holes, holes will get injected from drain.
  20. 4.OLEFET 1. OLEFET, Unipolar 2.DC gated ambipolar OLEFET 3.AC gated ambipolar OLEFET
  21. OLEFET A device which can combine electrical functions with optical functions, as it integrates the switching property of transistors with emission properties of LED. S,D -5 hole injecting and electron injecting electrode. Unipolar -Y weak light emission in the close proximity of the drain. Ideally ambipolar device -5 strong light emission Location of the light emission region within the channel can be varied by changing Vg.
  22. OLEFET Operation Applied electric field drives holes at the energy level of HOMO and electrons to LUMO. When opposite charge carriers meet ,they form exciton. A part of excitons annihilate radiatively while rest give up energy by heat. Challenges High resistance of organic semiconductor leads to heating energy losses and the injection barriers are often significant. Both leads to high operating voltages which may cause melting of electrodes. Low energy efficiency.
  23. gated ambipolar OLEFET and vgVte, electrons are injected from S to OSCO (ambipolar) Electrons and holes recombine radiatively.
  24. gated ambipolar OLEFET light emission and S ova-vg
  25. 4.3, I gated ambipolar OLEFET Source Drain Organic semiconductor Gate dielectric: silicon oxide Gate Gate: doping silicon When Vg = -ve max, channel is occupied by +ve charge carriers. When Vg=Vs ,reaches pinch off point and hole density near source electrode becomes zero As Vg increases ,electrons starts to inject into the channel from S High frequency gate is needed to inject electrons quickly enough before the hole started to drift to S.
  26. 4.3, I gated ambipolar OLEFET Vgs = Negative peak Vgs becomes positive b)Vd- vg>lvthl light emission
  27. gated ambipolar OLEFET Another method to operate AC gated OLEFETs is demonstrated by Xuhai in 2009. The device can be lit up even with Vd=Vs=oV. The light emission has nothing to do with the channel length if the frequency of gate voltage is high enough. The transistor can emit the light, even if only one electrode is (D or S) connected to circuit as the electrode can inject both holes and electrons.
  28. gated ambipolar OLEFET When Vg swings to negative value, the electrical field will attract the holes out of the metal electrodes (no matter S When Vg swings to positive value, electrons are injected from the electrodes. If the frequency is high enough, the holes still stay in the semiconductor and meet the subsequently injected electrons, contributing to the light emission. a) Metal electrode Gate dielectric: silicon oxide Gate: doping silicon - b) Metal electrode Gate dielectric: silicon oxide Gate: doping silicon Organic semiconductor Gate— Organic semiconductor Gate
  29. 5, Of researches Unipolar OLEFETs based on tetracene. based on oligocenes other than tetracene, based on oligothiophenes. based on oligofluorene derivatives. Towards Ambipolar LEFETs based on n-type and P-tYPe Ambipolar OLEFETs. Light Emission from carbon nanotube FET
  30. 5, Unipolar OLEFETs based on tetracene The first OLET was reported by Hepp et al in 2003. Tetracene good charge carrier transport and EL properties. fabricated on Si/Si02 substrates bottom-gate bottom-contact configuration gold electrodes Unipolar OLET (p-channel) Weak light emission. Green electroluminescence, only near to drain Hepp et al, introduced an empirical model in which they supposed different injection mechanisms at the S or D electrodes as a consequence of the thin-film physical imperfections.
  31. 5, Unipolar OLEFETs based on tetracene Santato et al., (2004) introduced another model. Assumed that the voltage drop at D electrode, caused by a contact barrier, induces a distortion of HOMO and LUMO (EQE) is proportional to drain-source voltage (Vds), but it is independent from gate voltage (Vg) Increasing Vg leads to an overall increase of the electroluminescence (EL) but leaves unaffected the EQE.
  32. 5, 1 , 2 Unipolar OLEFETs Based on Oligoacenes other than tetracene, In 2005, Omayada et al. showed a new single layer unipolar OLET with a blend of lwt%-rubrene doped tetraphenylpyrene (TPPy) EL was improved. The quantum efficiency of OLEFETs is increased. PL efficiency as high as 99 0/0. Rubrene emission intensity increased with increasing I Vgl The maximum quantum efficiency was observed when MgAl was used for the electron and Au for the hole injecting electrode
  33. 5, Unipolar OLEFETs Based on Oligothiophenes Oyamada et al., 2005 demonstrated how the channel length could affect EQE using thiophene-derivative. Strong photoluminescence (P L) as well as EL in OLED configuration was found for films based on dithienothiophene S. LEFETs based on solution processed and evaporated films had similar FET mobility (about cm2V-1 s-l) and EL. Evaporated films showed higher ON/OFF ratio and lower threshold voltages. The only single crystal OLEFET reported to date is based on the phenylthiophene derivative BP3T
  34. 5, 1.4 OLEFETs Based on Oligofluorene Derivatives Oligo( 9,9-diarylfluorene) derivatives have great potential for OLEFETs and organic lasers. Five oligo(9,9-diarylfluorene) derivatives were investigated as active materials in OLEFETs. Exhibit high PL efficiencies from deep blue to near Exhibit reasonably high mobilities for both holes and electrons (about 10-3 cm2V-1 s-1) Thermally stable. Vacuum sublimed films of oligo(9,9-diarylfluorene) derivatives exhibit transistor behavior (unipolar p- type) and high PL quantum efficiencies.
  35. DRAWBACKS Despite the enhancement in EL in unipolar devices, the EQE achieved is still too low for any practical application Some problems arisen from the type of architecture, limited drastically its potentiality. EL emission takes place in proximity to the metal electrode. Weak emissions. Unipolar OLETs suffer from the same negative effects of OLEDs.
  36. Towards Ambipolar LEFETs based on n- type and p-type OLEFETs showing ambipolar electrical characteristics have been realized combining p-type and n-type molecular semiconductors using three different approaches: Bulk heterojuction two componentlayered structures heterostructures confined within the transistor channel. All these devices used the perylene derivative P 13 as n-type semiconductor and Au hole and electron injecting electrodes.
  37. 5.2 Towards Ambipolar LEFETs based on n- type and p-type The first OLEFET with ambipolar electrical characteristics was based on the bulk heterojunction approach The active material was a co-evaporated film of P 13 and the oligothiophene a-T5 (p-type semiconductor), in the ratio 1:1. Bulk heterojunction engineering was later employed to tune the optoelectronic properties of OLEFETs based on the same materials using different ratios of the two components.
  38. Towards Ambipolar LEFETs based on n-type and p-type Three different working regimes were identified: (i) ambipolar operation and no light emission for an excess of a-T5; (ii) ambipolar operation and light emission for 1:1 ratio;(iii) unipolar operation and light emission for an excess of P 13 PL spectra revealed that when the concentration of P 13 was reduced from 100 to 25 %, PL intensity decreased of about two orders of magnitude.
  39. Ambipolar OLEFETs, The first ambipolar OLEFETs were based on solution processed active layers of polyphenylene vinylene (PPV) derivatives The presence of the organic dielectric is a key feature to realize ambipolar OFETs since it limits electron trapping at the semiconductor/dielectric interface, thus enabling ambipolar transport in semiconductors believed to act exclusively as p-type. Swensen et al. used an active layer of the PPV derivative "Superyellow" and Ag and Ca contacts for hole and electron injecting electrodes.
  40. 5,3Ambipolar OLEFETs, Zaumseil et al. used an active layer of OCIC10-PPV and Au and Ca electrodes for hole and electron injection Vg = —90 V and Vds varying from —79 to —100 V. A narrow line of light appears close to the Ca contact as soon as Vds exceeds the threshold bias for the formation of an electron accumulation layer. With increasing I Vds I the emission zone moves gradually from the Ca electrode toward the Au electrode.
  41. 5.3 Ambipolar OLEFETs, in 2004, T. Sakanoue et al observed orange light emission with OLEFET based MEH-PPV EL in MEH-PPV based OLEFETs was improved by a factor of 20 using dissimilar electrodes, i.e., Au/Cr hole injecting and Al electron injecting electrodes. Smits et al reported light emission in the near infrared region when squarylium dye (SQI) was used with Au electrodes.
  42. Light Emission from carbon nanotube FET The electronic structures of carbon nanotubes (NTs) and organic molecular semiconductors have the common feature of an extended pi conjugation of electrons. A carbon NT light emitting FET and an OLEFET have a similar device structure. Charge injection in carbon NTS light emitting FETs was found to occur via tunnelling, through a Schottky barrier.
  43. , ,continued In 2003, infrared light emission from a carbon nanotube FET was reported. The first ambipolar light emitting FET based on carbon NTS used titanium electrodes. The location of light emission region in devices with long channel lengths (—50 um) is possible to be controlled varying biasing conditions Emitted light is polarized along the nanotube axis Wavelength of the emitted radiation in principle is tunable with the diameter of the NT.
  44. 01 D[NMARK NanoSYD group NANüSYD Nowadays, para-hexaphenylene(p6P) as the light-emitting organic semiconductor has been studied a lot in OLEFETs in the NanoSYD group. It can emit polarized, blue light up on UV excitation. (PPTTPP) has rarely been investigated for applying on the transistors. The properties of PPTTPP still need to be investigated.
  45. continued In 2012 Cai Liang etal successfully realized AC gated OLEFET using PPTTPP nanofibers on BC/BG configuration Future: multi-color transistor by the deposition of p6P and PPTTPP nanofibers on the same substrate, active them to emit different color of light, so the multi- color transistor is obtained.
  46. 7,APPLICATIONS
  47. Organic Lasers As exciton-charge interactions and photon loss at the electrodes reduce the efficiency of OLEDs. They are not suitable for organic laser applications Organic laser once if developed with OLET, it will allow very high efficiency and no loss of light. These lasers could then be used in fibre optics applications in the telecommunications industry.
  48. Biodegradable electronic circuits Plastics from electronic waste are currently a problem Christopher Bettinger from Stanford University in California, US, set out to fabricate a biodegradable and biocompatible transistor They could be implanted into the body for a short period of time before being broken down and absorbed without the need for a second operation to remove the implant.
  49. Optical interconnects OLETs are faster and might make better on-chip optical interconnects A transistor-based light source would switch much faster than a Processor diode, and because of its planar design it could be more easily integrated onto computer chips, providing faster data transmission across chips than copper wire.
  50. 7.4 Displays Displays in mobile phones, MP3 players, digital cameras. With more efficient and cheaper OLED technologies it will possible to make ultra flat, very bright and power- saving OLED televisions, windows that could be used as light source at night, and large-scale organic solar cells They do not require a backlight to function and therefore require less power to operate; also, since they are thinner than comparable LEDs, they can be printed onto almost any substrate. Exciton quenching and photon loss processes still limit OLED efficiency and brightness.
  51. Conclusion OLETs could open a new era in organic optoelectronics and serve as test beds to address general fundamental optoelectronic and photonic issues. The focus of OLET development is the possibility to enable new display/ light source technologies, and exploit a transport geometry to suppress the deleterious photon losses and exciton quenching mechanisms inherent in the OLED architecture.'
  52. ΤΗΑΝΚ γου

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