Lab 2 Manual — Polymer Light Emitting Diode — ECE592S Fabrication and Characterization of a Polymer Light Emitting Diode ECE592S/492S– Soft Electronics: Organic Electronics & LCDs Lab 2 Manual Page 1 Dr. Michael Escuti Abstract In this lab experiment we will fabricate our own polymer-Light Emitting Diode (pLED). We will start with glass substrates coated with indium-tin-oxide (ITO) as the anode, and then spin-coat two polymer layers onto these. The polymers will be the light-emitting-polymer MEH-PPV, and the hole-transport-layer PEDOT:PSS. Finally, a silver (Ag) cathode will be applied on top of this structure to complete the pLED. For characterization, we will measure and observe several aspects, including the color of electroluminescence, color of photoluminescence, the turn-on voltage, the intensity vs voltage, the current-voltage curve, and the power efficiency. Station 1 Station 2 Station 3 Station 4 Coating the Polymer Layers Create The Cathode Template Creating the Cathode Device Characterization Write-up Instructions Your lab “report”, or lab write-up, will consist of the following elements for each station: A. Statement of experimental objective B. Sketches of experimental setup C. Record of all measurements D. All requested calculations The following lab manual will indicate specifically what to include and where. The purpose of this style write-up is to force you to keep a technical record of your experiments in the way that many engineers and scientists are required to do (in industry and universities). The lab TA will provide you with blank technical notebook sheets in the lab (and available on the website). You are expected to follow the lab notebook guidelines introduced by the lab TA (see Appendix and posted on the website), and your lab grade will depend both on your experimental procedure and on how well you follow these guidelines. Note that the same pages you use during the lab experiment should also be the ones you complete at home and hand-in as your write-up — no need to rewrite them.ECE592S/492S – Soft Electronics: Organic Electronics & LCDs Lab 2 Manual Page 2 Dr. Michael Escuti Figure 1. Comparison of luminous efficiency of different types of LEDs General Introduction A. Brief History Polymer Light Emitting Diodes (pLEDs) fall into the more general category of Organic Light Emitting Diodes (OLEDs). The earlier forms of OLEDs consisted of simply a light emitting organic semiconductor sandwiched between two electrodes, similar to the construction of inorganic LEDs. Several organic molecules and polymers were tested as candidates for the organic semiconductor. pLEDs have attracted interest because all the different device layers can be easily processed in solution and with economically-attractive coating techniques (as opposed to inorganic LEDs). Moreover, the interfaces between the different layers do not have to be structurally regular to the atomic level. Because of these reasons, several combinations of materials can be tested. Initial results by Burroughes and Friend et. al in 1990s were based on a structure that had the light-emitting-polymer poly(phenylene-vinylene) (PPV) enclosed between two electrodes. These devices were very inefficient, limited to only a few percent external quantum efficiency (EQE). Later several groups identified that including an additional organic layer between one of the electrodes and the light-emitting layer improved the efficiency dramatically. In particular a material called poly(ethylenedioxy)thiophene (PEDOT) was found to be very useful in this aspect. Also research on improving the interfaces between the electrodes and the organic materials resulted in more efficient pLEDs. Fig. 1 shows the improvement of pLEDs over time. High performance pLEDs can now rival the efficiency of conventional incandescent filament lamps. B. Basic Device Operation In this lab, we will build a pLED with the layout shown in Fig. 2(a). We will use MEH-PPV as the light-emitting polymer, which simultaneously serves as the Electron Transport Layer (ETL). MEH-PPV is a variation of PPV, which has additional alkyl groups attached to the basic phenylene ring as shown in Fig. 2(b). Generally these groups are added so that the material becomes soluble in common solvents such as xylene and thus can be spin-coated easily on a smooth surface. For the Hole-Transport Layer (HTL), we will use the mixture PEDOT/PSS with the chemical structure also shown in Fig. 2(b). Indium-tin-oxide (ITO) coated on glass will act as the anode while a silver-doped conducting paste (from DuPont) will be the cathode material.ECE592S/492S – Soft Electronics: Organic Electronics & LCDs Lab 2 Manual Page 3 Dr. Michael Escuti Figure 2. (a) pLED device layout. (b) Chemical structures for HTL and ETL materials. (c) Energy Band Diagram showing basic operation. Fig 2(c) shows the Energy Band Diagram with the different energy levels for the individual layers. Many materials may be used as the cathode, and this diagram denotes several options. The basic principle pLED operation has similarities to that of an inorganic LED, but is based on molecular electroluminescence (EL). Electrons from the cathode and holes from the anode are injected into the LEP layer, where they recombine and release the energy in the form of a photon. This process is also sometimes referred to as radiative recombination. The wavelength of light emitted depends upon the band gap (Eg) of the polymer semiconductor, which is defined as the energy difference between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) levels as indicated in the figure. It also depends somewhat on the HOMO energy of the HTL and the applied voltage. In order to observe emission from this device, a minimum voltage needs to be applied and this is called as the turn-ON voltage of the pLED. This voltage needs to be large enough for the electrons and holes to overcome the barriers to LUMO and HUMO energy levels, indicated by φe and φh respectively in Fig. 2(c): φe = φcathode – φLUMO φh = φanode – φHOMO The purpose of the HTL layer here is to ease