Organic Electronic Devices Week 5: Organic Light-Emitting Devices and Emerging Technologies Lecture 5.5: Course Review and Summary Bryan W. Boudouris Chemical Engineering Purdue University 1
Understanding Device Operation Requires Knowledge of Materials Organic Light-emitting Device (OLED) Displays Thin and Lightweight Flexible Transparent Sony Samsung Organic Photovoltaic (OPV) Devices Polytron Large Area Production Portable Applications Conformal Coverage Konarka Konarka Konarka
Commonly-used Polymeric Organic Semiconductors Primarily Hole Transporting (p-type) Polymer Semiconductors MEH-PPV P3AT PBDTTT-C PDTP-DFBT PEDOT:PSS Primarily Electron Transporting (n-type) Polymer Semiconductors CN-MEH-PPV BBL P(NDI2OD-T2) PT01 Further Reading: Boudouris, B. W. Curr. Opin. Chem. Eng. 2013, 2, 294.
Electron Donor-Electron Acceptor Copolymers Species that are Electron Rich are capable of donating some of the local electron density. Species that are Electron Poor (or Deficient) are capable of accepting some of the local electron density. Combination of these two effects allows for a lowered energy and a lower bandgap energy Further Reading: Zhang, Z-G.; Wang, J. J. Mater. Chem. 2012, 22, 4178.
Classic Semiconducting Polymer Crystal Structure: P3HT P3AT Crystal Schematic alkyl stacking direction, (h00) chain axis direction, (00l) π-π stacking direction, (0k0) Further Reading: Prosa, T. J.; Moulton, J.; Heeger, A. J. Macromolecules 1999, 32, 4000.
Example of Conjugated Bonding in 1,3-Butadiene 1,3-Butadiene 4 Carbons with sp 2 Hybridized Orbitals 4 Frontier p z Orbitals for π-bonding π 4 * π 3 * π 2 Energy p z sp 2 sp 2 sp 2 p z p z p z p z π 1 Here, π 2 is the Highest Occupied Molecular Orbital (HOMO) Energy Level and π 3 * is the Lowest Unoccupied Molecular Orbital (LUMO) Energy Level. Image Reproduced From: Organic Semiconductor World. http://www.iapp.de/orgworld/.
The Schrödinger Equation The 1D-Time-Independent Schrödinger Equation 2 2 ψ ( x) 2 2m x + V ( x) ψ ( x) = Eψ ( x) Here, We Are Given a Free Electron (i.e., V(x) = 0) Confined to a Box V (x) V (x) V ( x) = 0 Because of the infinite potential energy outside of the box (x < 0, x > L), the electron is confined to the box where the potential energy is 0 everywhere. x = 0 x = L Solving for the Energy with Respect to Different Integer Values Yields: E n = 2 2 n h 8mL 2
The Fermi-Dirac Distribution is a Function of Temperature At Temperatures Above 0 K There is a Non-Zero Probability Associated with f(e)
Plots of D(E), f(e), n, and p as a Function of Energy Density of States Fermi-Dirac Distribution Electron and Hole Densities D(E) f(e) n(e), p(e) Image Reproduced From: The Dissertation of Robert Wittmann. http://www.iue.tuwien.ac.at/phd/wittmann/diss.html.
Localized Charges Exist in Organic Semiconductors Imagine a Polymer Chain with Three Chromophoric Units An extra electron is added to the first chromophoric unit such that the electron density on the black portion of the polymer chain is higher than that of the blue and the red segments. The black, blue, and red segments of the polymer chain are all one molecule. However, some chemical or structural defect causes them to have a break in conjugation between the segments. Within each chromophoric unit, however, there is going to be electronic coupling, so we can think of the electron as being delocalized across a single chromophoric unit in the polymer chain. The non-equilibrium electron distribution, causes the first segment to pass the extra charge to the second segment. Then, the second segment may pass the electron density to the third segment, or the charge may be passed back to the first segment. Now, we need to develop a mathematical model that will allow us to describe the rate of charge transport using this type of system.
Doping Introduces New Energy Levels in Semiconductors Imagine a Traditional Inorganic Semiconductor, Silicon Intrinsic Si n-doped Extrinsic Si Si Si Si Si Si Si Si Si Si P Has 5 Valence Atoms Si Si Si Si Si Si P Si Si Si Intrinsic Si Band Diagram Doped Si Band Diagram E C E C E D E i E c E D << E g Band Gap Energy (E g ) E g = 1.1 ev E V Band Gap Energy (E g ) E g = 1.1 ev E V
Introduction to the Multiple Trap and Thermal Release (MTR) Model The key difference between band transport in traditional solid state physics and organic electronic devices is the LARGE AMOUNT OF DISORDER in most organic electronic systems relative to inorganic semiconductors. In some organic materials, transport is limited by localized states induced by defects and unwanted impurities. These defects and impurities are referred to by the catchall term of traps. Imagine a Transport Band with Impurity States E C Energy E T,1 E T,2 E T,3 Localized states with trap energies at 3 distinct energy levels There is a finite probability that the carrier will be trapped in (and that the carrier will be released from) one of the lower energy trap states available due to the defects and impurities.
Variable Range Hopping (VRH) Model The VRH model is in place when the semiconductor is highly disordered both in terms of space and energy. That is, there are no crystal unit cells, no periodic potentials, and in a regime where band theory does not hold well. In this system, the states are localized with: 1. An Energy Distribution that describes the density of states, which provides the probability for a certain binding energy of a charge in a trap site. 2. A Spatial Distribution that accounts for the variable spacings of the trap sites with respect to the ideal repetitive points in a crystal. Crystal Lattice Point Localized Spots Not Necessarily on the Crystal Lattice Conduction Happens between Hopping from State (or Site) to State (or Site)
OFET Device Structure and Operating Mechanism Organic Field-effect Transistors (OFETs) Are Useful Devices for Testing the Properties of Organic Semiconductors OFETs Are Three Electrode Devices Operating Mechanism of an OFET in Hole Transporting Mode S Source Electrode D Drain Electrode Gate Gate Electrode V G Voltage Between Source and Gate V D Voltage Between Source and Drain L Channel Length, Distance between Source and Drain Electrodes (Typical Value ~200 µm) W Channel Width, Distance of the Source and Drain Electrodes in the Direction Orthogonal to the Channel Length Direction (Typical Value ~2,000 µm)
Rapid Efficiency Increases in Laboratory-scale OPV Devices As compiled by the National Renewable Energy Laboratory (NREL)
Nanoscale Morphology and Interfaces Are Crucial in OPVs hν + Domain Sizes and Interfaces Critical
Current Active Layer Microstructures are History Dependent
Multiple Junction Solar Cells Extend Efficiencies to 11% Triple Junction Device Structure Triple Junction Device J-V Curves Further Reading: Chen, C.-C.; et al. Adv. Mater. 2014, 26, 5670.
An OLED Has a Similar Device Structure to an OPV General OLED Structure J-V Response and Emission Curves In OPV Devices, Light Was Input to Generate a Voltage In OLED Devices, a Voltage Bias is Applied to Generate the Emission of Light Holes are Injected from the Anode Electrons are Injected from the Cathode Recombination and Photoemission Occurs in the Organic Active Layer Further Reading: Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
Thermoelectric Materials Convert Heat to Electricity Thermoelectric Device Schematic Efficiency Increases with Increasing ZT Figure of Merit η max = T HOT ZT = T T HOT COLD σ 2 S κ T ZT is Related to Efficiency 1+ ZT 1 T 1+ ZT T COLD HOT Goal is to MAXIMIZE Electrical Conductivity (σ) and Thermopower (S) while MINIMIZING Thermal Conductivity (κ)
Organic Electronics Have a Bright Future Organic Light-emitting Device (OLED) Displays Thin and Lightweight Flexible Transparent Sony Samsung Organic Photovoltaic (OPV) Devices Polytron Large Area Production Portable Applications Conformal Coverage Konarka Konarka Konarka