Prof. Dr. Gerhard Gobsch Optical, Electronic and Structural Properties of Semiconductor Nanostructures and Optoelectronic Devices Inorganic Semi-conductors & Devices (Compounds of III-V, I-III-VI2, II-IV-V2) Organic Semi-conductors & Devices (Polymers & Functional Polymers) Solar Components & Systems (Photovoltaic und Solar Thermal) Department of Experimental Physics I

Organic Optoelectronics An Introduction Materials, Processing, Concepts and Devices ACOPhys, St. Petersburg, Sept 2006

Molecular Energy Levels and Charges Materials and Processing Devices Organic Optoelectronics - Outline Introduction Molecular Energy Levels and Charges Materials and Processing Devices Summary

Organic Optoelectronics Organic Optoelectronics deals with semiconductor devices in which the semiconductor is an organic material. There are light emitting diodes (OLED), thin film field effect transistors (OFET), solar cells, lasers, detectors, sensors... Beyond the scope of the present lecture is the field of “Organic Electronics ↔ Polymer Electronics“. Theoretical basics of electronic properties of organic materials were given in the lecture of Prof. Runge.

Organic Optoelectronics                       The Nobel Prize in Chemistry 2000 "for the discovery and development of conductive polymers"                                 Alan J. Heeger Alan G. MacDiarmid Hideki Shirakawa     1/3 of the prize USA USA and New Zealand Japan University of California Santa Barbara, CA, USA University of Pennsylvania Philadelphia, PA, USA University of Tsukuba Tokyo, Japan b. 1936 b. 1927 (in Masterton, New Zealand) “for the discovery and developement of conductive polymers“ A new material class!

“In creating and expanding the 4th generation of polymers, we attempted to understand nature with sufficient depth that we could achieve materials with novel and unique properties, that are not otherwise available. This was (and is) an elegant and somewhat dangerous exercise; elegant because it requires the synthesis of knowledge from chemistry, physics and materials science, and dangerous because when working on the boundary of three disciplines, one is always pushing beyond the knowledge and experience of this background. To our research in this interdisciplinary field has had sufficient impact on chemistry to be recognized by the Nobel Prize gives us, therefore, particular satisfaction.” Alan J. Heeger (on occasion of giving him the Nobel Prize for Chemistry in 2000)

Pros and Cons of polymers for electronic applications Advantages Metallic and semiconducting properties by doping Combination of plastic with electronic properties Property engineering Solubility in organic solvents, variable processibility Use of printing technologies No vacuum and no high temperature processes  Low-cost production Disadvantages Low integrated devices and circuits (in the very near future) Degeneration in O2- and H2O-atmosphere  Long-term stability is still a critical point

Application of Functional Polymers Organic field-effect transistor (OFET) Polymerelektronik made by semiconducting and dielectric polymers Integrated polymerelectronic circuits (IPC) Polymer actors for microsystem technology Organic solar cells Photovoltaic & optoelectronic devices made by polymer photoconductors Polymer batteries Functional polymers with special / selected electronic properties Organic light emitting diodes (OLED) and lasers Fuel cells Super capacities Polymer sensors Optical polymers with optimized spectral transparency Special elektronic polymers for Planar antennas Temperature sensors Humidity sensors Chemo- & biosensors Pressure / force sensors

Organic Semiconductors More conjugated polymers...

Conductivity of materials

Charge carrier mobilities comparably small (FET) mobilities*: low mobilities & large absorption coefficients  thin absorber *C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001

Charge carrier mobilities of thin films

Absorption overlap with solar spectrum comparably small absorption range:  only a small fraction of the sunlight is used today! Silicon

Solar Spectrum and Absorption

Molecular Energy Levels and Charges Materials and Processing Devices Organic Optoelectronics Introduction Molecular Energy Levels and Charges Materials and Processing Devices Summary

-bands in conjugated polymers

Molecular Energy Levels Optical properties and transitions: Absorption Emission LUMO HOMO absorption fluorescence phosphorescence luminescence (photo-, or electro-)

Molecular Energy Levels For oligomers a shift of HOMO or LUMO levels towards smaller band-gaps is observed upon increasing the repeat unit. Compare with quantum-mechanical particle-in-a-box problem. longer chain/wider box  decrease in „band-gap“

Organic Semiconductors: Charges Charge carriers on Polyacetylene: H H H Negative Soliton: C C C C H H H H (degenerate groundstate) H H H Positive Soliton: C H H H H

Charge transport in conjugated polymers Polymer chain with characteristic defects (a); schematic energy diagram for positive polarons without electric field (b) and charge transport under applied electric field (c).

Electron transfer in a conjugated polymer after optical excitation calculated within the td-DFT by means of the GAUSSIAN quantum chemistry package

Charge carrier mobilities comparably small (FET) mobilities*: low mobilities & large absorption coefficients  thin absorber *C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001

Molecular Energy Levels and Charges Materials and Processing Devices Organic Optoelectronics Introduction Molecular Energy Levels and Charges Materials and Processing Devices Summary

Organic Semiconductors: Processing Solution processing Evaporation (polymers): (small molecules):

Molecular Materials: Pigments & Fullerenes „p-type“: e.g. phthalocyanines ZnPc „n-type“: perylenes & fullerenes used for evaporation Me-Ptcdi C60

Conjugated Polymers/ substituted Fullerenes Solution processing: „p-type“ conjugated polymers: MDMO-PPV P3HT PFB „n-type“ conjugated polymers/fullerenes: Spin coating CN-MEH-PPV PCBM F8BT Doctor blading

Molecular Energy Levels and Charges Materials and Processing Devices Organic Optoelectronics Introduction Molecular Energy Levels and Charges Materials and Processing Devices Summary

MIM picture of device function Very thin (low mobility) absorbers of ~ 50 – 300 nm  MIM a.) short circuit condition: solar cell b.) open circuit condition (VOC): current = 0 c.) reverse bias: photodiode d.) forward bias: light emitting diode (a) (b) (c) (d) ITO Al + – – + VOC Strong (up to 105 V/cm) internal electric fields drive charge transport

Organic Semiconductor Devices Field effect transistor: bottom gate (usually Si/SiO2)

Organic Semiconductor Devices: OFET Field effect transistor: top gate (polymeric insulator) W. Fix, A. Ullmann, J. Ficker, and W. Clemens, Applied Physics Letters 81, 9, p. 1735 (2002)

Organic Semiconductor Devices: OFET Influence of molecular order on charge carrier mobility: *C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001

Organic Field Effect Transistors: RF ID tags OFET: Applications and Products Organic Field Effect Transistors: RF ID tags

Organic Semiconductor Devices: OLED Organic light emitting diode (OLED): single layer device 1.) charge injection 2.) charge transport 3.) charge recombination  exciton formation 4.) light emission

Organic Semiconductor Devices: OLED OLED display structure: electrode bars one pixel = three devices

Organic Semiconductor Devices: OLED Comparison: advantages of OLED versus LCD (STN/TFT) full color (24 bit) high contrast (3000:1) wide viewing angle (170°C) lower power consumption faster response time less complicated architecture ( low cost, thinner device)

Organic Light Emitting Displays: OLED: Applications and Products Organic Light Emitting Displays: (2002, polymer) (2005, molecule)

Organic Light Emitting Displays: OLED: Applications and Products Organic Light Emitting Displays: (2003) (2004)

Wide viewing angle! Organic Light Emitting Displays: OLED: Applications and Products Organic Light Emitting Displays:                                                           Wide viewing angle!

Organic Semiconductor Devices: OSC Organic Solar Cell: device structure (“plastic solar cell“): Aluminum Evaporation Spin coating, doctor blading, printing Etching, laser-etching LiF (6 Å) Active layer ITO PEDOT:PSS Glass active layer: conjugated polymer / fullerene blend selective contacts: electrons: Al/LiF holes: ITO/ PEDOT:PSS (poly[3,4-(ethylenedioxy) thiophene] : poly(styrene sulfonate)) or flexible plastic substrates

Organic Semiconductor Devices: OSC Photoinduced Charge Transfer, dissociates the exciton: E  Evac LUMO Charge separated state! LUMO HOMO HOMO PPV C60 *N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258, 1474 (1992)

Organic Semiconductor Devices: OSC The power conversion efficiency h is a function of: State of the art: h ~ 5% Where: 1.) VOC is the “open circuit voltage“ 2.) ISC is the “short circuit current“ 3.) FF is the “fill factor“

Organic (plastic) solar cell

OSC: Applications and Products Organic Solar Cells                                

OSC: Sensor applications Photodiode (arrays)  sensors Combination of OLED illumination and photodiode array detection for personal identification.

Organic Semiconductor Devices: Laser Organic Laser (optically pumped): From left to right: Organic laser emitting red light; structure of the first organic laser using external optical excitation; typical laser emission spectra spanning the visible from the blue at a wavelength of 450nm, to the infrared at 700nm.

Molecular Energy Levels and Charges Materials and Processing Devices Organic Optoelectronics Introduction Molecular Energy Levels and Charges Materials and Processing Devices Summary

Organic Optoelectronics Summary Why “ORGANIC“? Disadvantages: light weight flexible low cost large area “tailor-made“ properties more colors printing production enviromental instability - requires encapsulation lower performance (charge carrier mobility) The challenge of today!