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Classroom Experiments and Teaching Materials on OLEDs with Semiconducting Polymers

Supplementary Material for Online Publication, Journal of Chemical Education Amitabh Banerji, Michael W. Tausch, Ullrich Scherf

Notes for the Instructor

The following experiments use similar methods and techniques as described in the supplemental materials of Sevian et al. [1]. For this reason we refer to their paper at the corresponding positions. We developed four OLED experiments, which all use a FTO1 coated glass or PET-foil as anode, MEH-PPV as emitter and Galinstan (respectively gallium-indium eutectic) as cathode. For all three components it is difficult to get small quantities as required for the experiments. Therefore try to motivate many colleges for the experiments, so you can distribute the materials and costs to a wider group. Following calculation example considers an order for 40 participants. (The amounts for each lesson will suffice to realize all four OLED-variations in one class.) Material Amount to order Total costs Amount needed Cost per party MEH-PPV 1g adds up to 200

mL solution 500 US$ 5 mL each lesson 12,50 US$

FTO-glass 400 pieces 500 US$ 10 each lesson 12,50 US$ Galinstan 80 units 1500 US$ 2 each lesson 37,50 US$

Total costs for each lesson 62,50 US$ (All prices are estimated roughly without considering vats, duty or shipping costs.)

Propose for curricular integration For a curricular integration we designed a possible scenario, which needs five periods each with 50 min of time. Optionally 1-2 additional periods can be attached for a deeper attendance of the topic.

1. 1st Lab-period (experiment 2) For introduction into the topic we recommend to show the Easy-OLED in a demonstration experiment at first. This makes the students familiar with the components of the OLED and motivates them to build their own one. The instructor should point out the disadvantages of the Easy-OLED, which are:

a. All emission-spots use a common cathode, this is unsuitable for display applications. b. The liquid alloy is not fixed into the corpus, which makes the OLED less agile. c. The construction with a metal plate and clips makes the OLED bulky and unattractive.

With this preparation the students should be introduced into experiment 1, the Standard-OLED. Use the Flash-tool for this task (access information is provided in the student handout).

2. Home-work for preparation to the experiment 1 (video tutorial and work sheet 1) Hand out the worksheet 1 and assign the students to watch the construction video (access information is given in the handouts) and to accomplish the tasks on the worksheet. This preparation is important, as the students will be better prepared for the experiment and thus be able to carry it out within the estimated time.

1 Although the used conductive glass is based on FTO (fluorine doped tin oxide), we recommend establishing the term ITO to the students. Indium doped tin oxide (ITO) is still the standard material for transparent electrodes in optoelectronics and the acronym is widely spread in literature and school books.

Banerji, Tausch, Scherf OLEDs – Notes fort he instructor J. Chem. Ed.

3. 2nd Lab-period (experiment 1; steps 1 – 3) The instructor should point out, that a concentrated and precise working is crucial for the success of the experiment. For this reason we recommend to split the construction process in two lab-periods. In the first period the students will carry out the first three steps of the Standard-OLED.

4. 3rd Lab-period (experiment 1; steps 4 – 6) In the following lab-period the students will complete the rest of the steps for the Standard-OLED and record I/V-curves (current/voltage-curves) for each emission spot.

5. 1st Classroom-period (evaluation using work sheet 2) The instructor should schedule at least two period of time for the evaluation of the experiment. Before considering electroluminescence in conjugated polymers, it is crucial to clarify the structural precondition for conductivity in polymer molecules. For this intention we considered a simple model presented in work sheet 2, which focuses on the main carbon-chain of a polymer with its valence electrons. According to the electron gas model of metals the electrons of conjugated double bonds should be defined as delocalized. In this way the model clearly illustrates, that molecules without any conjugation (a) as well as partially conjugated molecules (b) cannot continuously transport electrons along their backbones. Only fully conjugated molecules2 (c) are capable of this and will show an intrinsic conductivity. This level of understanding may be quite superficial, but it is appropriate for undergraduate students and suitable for a further dealing of electroluminescence.

6. 2nd Classroom-period (evaluation using the Flash-tool and work sheet 3) For explaining the processes of electroluminescence we recommend our Flash-tool, which is freely available. The work sheets 3 will guide the students and help them to investigate the Flash-tool self-employed. The instructor can decide how deep she or he wants to proceed into the topic. The evaluation of the experiment should at least consider all elementary processes in the OLED. If current-voltage-data have been collected, additionally the (fictitious3) efficiencies of the OLED spots can be determined by the students.

7. 4th Lab-period (experiment 3 + 4 and work sheet 3) If the instructor wishes to go further into the topic, we recommend to introduce the Flexi-OLED. As an appetizer you can show the students a youtube-video presenting a flexible OLED display (i.e. from Samsung). Just search for the key-words “flexible oled” or “flexible display”. The Flexi-OLED should be pointed out as a low-cost precursor to this leading technology. After this introduction the weak-point of the Flexi-OLED have to be centered. To achieve flexibility it uses an ITO-PET-foil instead of the ITO-glass, which lowers the performance of the OLED due to a higher electrical resistance of the foil. For this reason the requirement of a hole injection layer (HIL) can be suggested and justified to the students. Use the exercise 7 in work sheet 3 to discuss the theoretical background of the HIL first, before building the Flexi-OLED in a demonstration experiment or by the students.

2 Technically conjugated polymers aren’t entirely conjugated due to molecular defects or geometrical distortions. Their efficient conjugation length typically varies between 9 and 15 repetition units [2]. We recommend omitting this point for reasons of clarity. 3 Since the light densities of the OLED spots are not captured, it is not possible to calculate real efficiencies. Anyhow, in exercise 9 (work sheet 3) the students can roughly estimate their light densities and thus calculate estimated efficiencies.


Experimental section

Hazards There are no significant hazards reported for MEH-PPV or Superyellow. However the solutions are based on noxious solvents, so spin-coating must be carried out in a fume hood. The plastic flask does not only protect from splashes, but also catches the glass if it is released during spin-coating. So never spin-coat without protection. Cover the work space with news paper before working with Galinstan. Spills of the alloy can be carefully sucked back into the syringe or cleaned up with soap and water. There are no significant hazards reported for Galinstan or the gallium-indium eutectic.

PEDOT:PSS is corrosive (C), but harmless in the amounts as used in the experiments. Sources of supply and CAS numbers Material Purchased from CAS-Nr./ other information

1. FTO-glass (TEC 7) Sigma-Aldrich, p-nr: 735167 EC: 242-159-0

2. ITO PET-foil Sigma-Aldrich, p-nr: 639303 MDL: MFCD00171662 3. Galinstan®

or gallium-indium eutectic

Geratherm Germany Sigma-Aldrich, p-nr: 495425

http://www.geratherm.com MDL: MFCD00144387

4. MEH-PPV (Mn = 150.000 – 350.000) or Superyellow®

Sigma-Aldrich, p-nr: 536512 Merck KgaA, Germany

CAS Number 138184-36-8 no information available

5. Chloroform Sigma-Aldrich, p-nr: C2432 CAS Number 67-66-3 6. PEDOT:PSS (2.8 wt% in water) Sigma-Aldrich, p-nr: 560596 MDL: MFCD07371079

7. Self adhesive copper-foil www.ebay.com search item “copper tape” Preparation of the MEH-PPV solution Required time: 10 minutes for preparation, 6 hours for dissolving Use an analytical balance to weight approx. 20 mg of the polymer into a 5 mL vial. Add 5 mL chloro-form and stir the mixture over night. You should receive a clear and deep-orange colored solution. The amount is enough for 25 – 40 OLEDs, depending on how much solution per coating is used. Store the solution airproof (use parafoil if needed) in the fridge. It should be stable for at least one year. Experiment 1: Standard OLED Required time: 2 lab periods of 50 min 1. period: step 1 – 3 2. period: steps 4 – 6


Materials to provide at each lab station: 1 FTO-glass slide 3 pieces of double sided tape (2.5 x 3.5 cm) 3 small pieces of self adhesive copper-foil Cellophane tape 0,5 mL Galinstan in a syringe Power supply (0 – 12V DC) Multimeter 2 wire leads with alligator clips Lens paper or tissues Small quantity of acetone for cleaning

Materials/Reagents to provide at a central location: Glass vial containing the MEH-PPV-solution 1 mL syringe Drilling machine (3000 rpm) PET-flask for splash-protection

(The following numbered notes correspond to the steps in the procedure outlined for students in the student handout.)

1. Preparing the FTO-glass • Advice the students to clean the glass well. Particles of dust or dirt may dysfunction the OLED. • Sevian et al. [1], instructor notes 1 and 2 (p. 1-2)

2. Spin-coating MEH-PPV

• Mount the drilling machine in a fume hood with the cone end pointing to the ceiling. Close the drilling chuck completely. Alternatively you can use an angle grinder, which can be handled easier. But you may have problems finding one with the required rotation speed of 3000 rpm.

• Press the FTO-glass well to the double sided tape to ensure a good fixation. Replace the tape every tenth spin-coating process.

• The emitter solution has to be applied carefully to the centre of rotation, otherwise the liquid will disperse unevenly and you will get an inhomogeneous layer.

• When the students are trained to target the rotation centre, the emitter amount can be reduced to 0.1 mL to save some chemicals.

• It is not important to coat the whole glass surface. But make sure, that the layer is sufficient to cover the three spots of the prepared mask (see point 3).

• Failed layers can be washed out with acetone and then reapplied by spin-coating. • For further notices refer to Sevian et al. [1], instructor notes 3 and 5 (p. 2)

3. Preparing the cathodes:

• Carpet tape is really difficult to handle. It can be cut easier if you wet the scissor with acetone. To save some time we advice to cut the pieces in the demanded size (3 x 2.5 cm) in advance. The students then just have to stick the three pieces together.

• Remove the cover of the hole-puncher, so the students can see the anchors from the bottom side. If you wet the anchors with acetone, the tape will stick less to the hole-puncher, which makes the punching process much easier.

• Advice the Students to mark the positions of the holes with a pen before punching. This will help to get the holes aligned.

• Take care, that the ends of the copper-foils do not extend to far into the holes, otherwise they may damage the emitter layer. The foils should be aligned to the edge of the holes in best case.

• The piece of adhesive tape for enclosing the cavities should extend the FTO-glass. Fold the tape to the other side of the glass to get a better fixation.


4. Injection of Galinstan:

• Cover the work-space with newspaper. Spills of Galinstan often leave dark spots, which can be washed out with soap and water.

• The students may have problems handling the syringe. For a better handling you may cut the needle of the syringe down to approx. 2 mm. This lowers the risk getting to deep into the cavity and injuring the emitter layer.

• This step seems to be the bottle-neck of the experiment. Advise the students to be very carefully and to take some time for the injection.

• Sevian et al. [1], instructor notes 8 (p. 2) 5. Connecting the OLED and measuring an I/V-curve:

• Assist the students to connect the OLED right into the circuit. Set the multimeter to a range of 0-400 mA.

• Normally the current starts at about 1 mA and then rises up to 100 mA with increasing voltage. Luminescence usually can be observed from approx. 4 V and reaches a maximum at 8 or 9 V.

• If a spot has a short-circuit it will show currents of >> 100 mA from the beginning. In some cases even though luminescence can be observed at higher voltages (> 10 V).

• Only a minority of the OLEDs will show luminescence from all three emission spots. In average at least one of the spots will have a short-circuit and give no light.

6. Disassembling and chemical disposal

• If you want to save the Galinstan, use a separate syringe to suck it back. Notice that the used alloy has a less quality due to oxidative processes.

• ITO-glass can be reused, but notice, that with every application the ITO layer will degrade. • Sevian et al. [1], instructor notes “Disposal Instructions” (p. 2)

Experiment 2: Easy-OLED Required time: 1 lab period of 50min Materials to provide at each lab station: FTO-glass slide Retort clamp 2 Matches or toothpicks as spacers Metal plate (e.g. a copper or iron electrode) 0.5 mL Galinstan in a syringe 9 V battery 2 wire leads with alligator clips Lens paper or Kimwipes Small quantity of acetone for cleaning 2 Small foldback clips

Materials/Reagents to provide at a central location: Glass vial containing the MEH-PPV-solution 1 mL syringe Drilling machine (3000 rpm) PET-flask for splash-protection

The injection of Galinstan is the bottleneck of the experiment 1. Any injury of the polymer layer may lead to dysfunction of the OLED due to short circuits. The following Easy-OLED does not require an injection process and can be built very simply and quickly. But notice that the OLED is less agile (because the Galinstan is not enclosed) and the emission spots cannot be controlled individually.


1. Preparing the cathode

• To save materials you can substitute the matches with thinner materials like small pieces of double sided tape.

2. Applying Galinstan • When removing the glass after operation, normally the Galinstan drops remain on the cathode.

If you remove the thin oxidative-skins on the drops (use the needle of the syringe for this), you can use them for another coated glass.

3. Connecting the OLED:

• Take care that the alligator clip connecting the FTO-glass does not touch the cathode, otherwise a short-circuit may be caused.

Experiment 3: Applying a HIL (Power-OLED) Required time: 2 lab periods of 50 min (for Standard-variation) 1 lab period of 50 min (for Easy-variation) Materials to provide at each lab station: In addition to the materials provided with the Standard- or Easy-OLED Coverslip

Materials/Reagents to provide at a central location: In addition to the materials provided with the Standard- or Easy-OLED PEDOT:PSS dispersion Hair-dryer Small plastic stick or pipette

A hole injection layer (HIL) can be applied in the Standard- as well as in the Easy-version. Students may want to build both versions, so you can tell one half of the group to build the Easy-variation while the other half is building the Standard-variation with a HIL. The instructor should successively go to the student groups and assist putting a small quantity of PEDOT:PSS dispersion as described onto the FTO-glass. Use a plastic stick or a pipette for this step. A good layer is nearly transparent. If the layer is apparently defective or to thick, wash the glass under flowing water and start from the beginning. Experiment 4: Flexi-OLED Required time: 2 lab periods of 50 min 1. period: until step 3 of exp. 1 2. period: steps 4 – 6 of exp. 1. Materials to provide at each lab station: In addition to the materials provided with the Standard- and Power-OLED ITO-PET-foil,

Materials/Reagents to provide at a central location: In addition to the materials provided with the Standard- and Power-OLED Propanol

The delivered ITO-sheet can be easily cut with a scissor into the required size of 3 x 3 cm. The application of propanol and PEDOT:PSS should be done by the instructor. Try just to wet the foil with the alcohol. The dragging of the coverslip should do the students themselves.


Further Experiments For further ideas and investigation hints for the low-cost OLED we refer to Sevian et al. [1]. Literature: [1] H. Sevian, S. Müller, H. Rudmann, M. F. Rubner, Learning About Materials Science by Making Organic Light-Emitting Electrochemical Thin Film Devices, http://pubs.acs.org/doi/suppl/10.1021/ed081p1620/suppl_file/ jce2004p1620w.pdf, access: 24.03.2012

[2] J. Shinar, V. Savvateev, Introduction to Organic Light-Emitting Devices, In: J. Shinar (Editor), Organic Light-Emitting Devices – A Survey, S. 1 – 41, Springer-Verlag, New York, 2004

[3] B. Geffroy, P. le Roy, C. Prat, Organic light-emitting diode (OLED) technology: materials, devices and display technologies, Polym. Int. 55, 6, p. 572-582, 2005

Banerji, Tausch, Scherf Notes for the instructor according the work sheets for the students J. Chem. Ed.

Hand this work sheet out for homework and preparation to the experiment 1 (before entering the 2nd lab-period)

Assist the students to find the right link for downloading the video.

Anticipated solutions for the tasks 1-3

1. Preparing the ITO-glass To dos: - clean well with acetone - Identify the conductive side Materials: - ITO-glass, tissue, acetone, multimeter wit leads, adhesive tape

2. Spin-coating the emitter To dos: - This has to be done by the instructor - Inject the emitter to the middle of… Materials: - drilling-machine, splash-protection, prepared ITO-glass, emitter-solution

3. Preparing the cathode-layer To dos: - Use three layers of double sided tape - Align the cu-foils to the hole-edges Materials: - double sided tape, copper-foil, adhesive tape

4. Injection of Galinstan To dos: - Don’t injure the emitter layer - Fill the holes completely up with… Materials: - Ito-glass with cavities, syringe with Glainstan, adhesive tape

5. Driving the OLED To dos: - Connect the plus-pole to the ITO… - Connect the minus-pole to the Cu… Materials: - 9V battery, alligator-wire, OLED


Hand this work sheet out in the 1st classroom period

Anticipated solutions for the tasks 1-2 1 a)

1 b)

- PMMA has no conjugated double bonds.

2. Example for non conjugated: PMMA Example for partially conjugated: PET


Example for fully conjugated: Polypyrrole


Hand this work sheet out in the 2nd classroom period

Assist the students to find the right link for downloading the flash-tool.

Anticipated solutions for the tasks 1-5

1 a) Poly-methoxy-5-(2-ethylhexyloxy)-

1,4-phenylenevinylene

1 b) The side groups (methoxy-group and ethylhexyloxy-group) are missing. These groups don’t influence the electrical properties of the molecule and therefore they have been omitted. The substituents increase the solubility of the polymer for better processing. (This is a simplification for better clarity, in fact even the side groups can fine-tune optical or electrical properties of the polymer.)

2. - Let the students count the total amounts of C-, O and H-atoms in one repetition unit. - C: 17, O: 2, H: 24 - Now add the atom masses: M = 17x12 g/mol + 2x16 g/mol + 24 g/mol M = 260 g/mol - Divide the average molar mass by M n = (200.000g/mol) : (260g/mol) ≈ 769

3. - The left rectangle is the anode. - The right rectangle is the cathode. - The hole drifts through the conjugated system to the cathode. - The electron drifts through the conjugated system to the anode.

4. The simple model shows polymer molecules reaching from the anode to the cathode. The molecules are strictly ordered. The detailed model shows an unordered polymer layer with several molecules lying between the electrodes.

5. 1. Recombination under light emission 2. Hole hopping 3. Electron hopping 4. Recombination under thermal… 5. Short-circuit discharge at cathode 6. Short-circuit discharge at anode


Hand this work sheet out in the Classroom period

Anticipated solutions for the tasks 6-7 6)

1) Electron injection, 2) hole injection, 3) electron hopping 4) hole hopping, 5) recombination under light emission

The numeration of the processes may be of course different.

7. - For hole injection an electron from the HOMO of MEH-PPV have to overcome an energetic barrier (respectively 0.7 eV) to get into the Fermi-level of the anode (ITO). - PEDOT:PSS has a HOMO level of -4.9 eV, which lies between the HOMO level of MEH-PPV and the Fermi-level of ITO. - If PEDOT:PSS is introduced between the emitter and the ITO, the hole injection is subdivided into two injection processes each of them with a smaller energetic barrier. This facilitates the hole injection and increases the performance of the OLED. (Probably you will need to assist the students with this exercise. Take time to discuss the diagram and to clarify the meanings of the symbols and values. In case of need, give them the first information.)


Hand this work sheet out in the Classroom period

Anticipated solutions for the tasks 8-9 8) - Assign the students to plot three separate diagrams if the maximum currents of the spots vary more than 100mA. - Spot with short-circuits should be neglected it in the diagram.

Example diagram

9. - Advice the students to use a spread sheet (i.e. Excel) for this assignment. - The equation given for the power-efficiency is from [3]. - The factor 0,28 cm² is the surface of the emission spots of our OLED. It transfers the current into the current density. - The factor 1/10 balances the units (centi²/milli). - The luminescence efficiencies for the low-cost OLEDs lie in the region of 0.001 to 0.3 lm/W. Professional fabricated OLEDs reach

0

50

100

150

200

250

300

-1 1 3 5 7 9 11 13

I [m

A]

U [V]


ηP>100 lm/W (state of the art 2012).

Banerji, Tausch, Scherf OLEDs – Student hand out J. Chem. Ed.

Classroom Experiments and Teaching Materials on OLEDs with Semiconducting Polymers

Supplementary Material for Online Publication, Journal of Chemical Education Amitabh Banerji, Michael W. Tausch, Ullrich Scherf

Notes for the Students

Wok sheet 1: Homework for preparation to the experiment 1

For the following exercises goto http://www.chemiedidaktik.uni-wuppertal.de/material/interactive/index.htm and type “oled”

in the search field. You will get to the corresponding flash-animation. Download the video file “Video tutorial to the Standard-OLED” onto your desktop and extract the file “oledassembly.wmv”. Watch the video and assign following tasks.

1. Entitle the five steps for building the Standard-OLED as shown in the video. 2. Assign for each building step two tasks from the list in the lower right corner. 3. Watch the video again and note which materials are respectively used in the according steps.

The five steps to the Standard-OLED

1. ______________________________________________ To dos:

Materials:

2. ______________________________________________

To dos:

Materials:

3. ______________________________________________ To dos:

Materials:

4. ______________________________________

To dos:

Materials:

5. ______________________________________________ To dos:

Materials:

• Don’t injure the emitter layer. • This has to be done by the instructor. • Clean well with acetone. • Connect the plus pole to the ITO-glass. • Use three layers of double sided tape. • Identify the conductive side. • Align the copper-foils to the hole-edges. • Fill the holes completely up with Galinstan. • Inject the emitter to the middle of the glass. • Connect the minus pole to the copper-feeds.

Banerji, Tausch, Scherf OLEDs – Handout for the students J. Chem. Ed.

Experimental section Experiment 1: Standard-OLED on ITO-glass with three individually controllable emission spots. 1) Preparing the ITO-glass:

Clean the ITO-glass first with water and then with a Kleenex and acetone. From this point on avoid touching the surface of the glass with your fingers (carry it only at the edges). Measure the electrical resistance of both sides of the glass holding the multimeter leads (in a distance of 1 cm) on the surface of the glass. The conducting side shows a resistance of 30-40 Ω·cm. Put a strip of adhesive tape on one end of the conducting side to mask the area, where later the anode will be connected (Fig. 1a).

Notes:

Fig1a: Prepared ITO-glass

2) Spin-coating the emitter: Warning! During spin-coating chloroform will evaporate. For this reason this step should be done in a fume hood and under supervision of a teacher. Use double sided tape to fix the ITO-glass with the conducting side upside onto the drilling chuck. Cut a 0.5 L PET-flask to a tube and put it over the construction to protect from splashes. Inject about 0.15-0.2 mL MEH-PPV solution on the middle of the ITO-glass with the syringe (Fig. 1b). Close the front door of the fume hood as far as possible. Start the machine with the full rotation force (3000 rpm) and spin-coat for about 20 sec. You should get a thin and homogeneous layer of the orange polymer onto your ITO-glass (Fig. 1c). Remove the adhesive tape.

Notes:

Fig1b: low-cost spin-coater

Fig1c: covered ITO-glass

3) Preparing the cathode-layer: Stick three pieces of double sided tape (about 3cm x 2.5cm) together and punch three holes into the layer using a hole puncher (hint: wet the pins with acetone). Put the tape onto the MEH-PPV layer, but don’t remove the rear protection sheet. Now fix three thin pieces of adhesive copper-foil as following: one end should just extend into one of the holes, while the other end is folded to the opposite side of the glass


(Fig. 1d). Enclose the holes with a piece of tape to gain three cavities. Fig1d: tape-layer & Cu-feeds

4) Injection of Galinstan: Warning! Avoid touching the emitter layer with the syringe, this may cause short circuits later. Move the tip of the syringe carefully into the first cavity and gently fill the cavity up with Galinstan (Fig. 1e). If the alloy spills out, it indicates that the hole is completely filled up. Suck the excessive alloy back into the syringe. Repeat this step with the other cavities. Now close the pinholes with a piece of adhesive tape, but don’t press to strongly, otherwise Galinstan may spill out (Fig. 1f).

Notes:

Fig1e: Injection of Galinstan

Fig1f: Galinstan applied

5) Driving the OLED and recording an I/V-curve:

Use an alligator clip to connect the positive pole of the power supply to the uncoated part of the ITO-glass. Connect a multimeter to the negative pole of the power supply and regulate the voltage to 2.0 V. Set the multimeter to current measurement and connect it successively to all three copper-feeds of the OLED (Fig. 1g)4. Dim the ambient light. Now increase the voltage in 1V-steps and note the corresponding currents of all three emission spots into the following table. Use the symbols given at the bottom of the table to estimate the observed luminescence. You will need this information to determine the efficiency of your OLED. The strongest luminescence is normally observed at 8-9 V (Fig. 1h). End the measurement at 12 V. Disassemble the OLED as described in 6) and plot the I/V-diagrams of all three emission spots.

Fig1g: Connecting circuit

Fig1h: OLED at about 9V

U [V] 2 3 4 5 6 7 8 9 10 11 12

I1 [mA]

I2 [mA]

I3 [mA] - = no luminescence; o = weak luminescence; oo = medium luminescence; + = good luminescence; ++ = very good luminescence

6) Disassembly of the OLED and material disposal: Remove the upper adhesive tape first and wipe out the Galinstan with a tissue. It can be disposed of in the household garbage. Now remove the double sided tape including the copper feeds and wipe the

4 Alternatively you can drive the OLED with a 9 V battery, if you do not wish to record an I/V-curve.


ITO-glass under running water using your fingers. Dry the glass and clean it with acetone.

Experiment 2: Easy-OLED within 10 minutes.

Prepare the ITO-glass as described in experiment 1 and spin-coat it with the emitter.

1) Fixing the metal plate to the retort clamp: Fix the metal plate as shown in Fig. 2a to the retort clamp and place the matches on the edges of the metal plate as spacers.

Notes: Fig2a: Preparing the cathode

2) Applying Galinstan and fixing the ITO-glass: Put some drops of Galinstan on the metal plate between the two matches exceeding their height (Fig. 2b). Put the ITO-glass with the coated side onto the matches, so that a close contact between Galinstan and the polymer is achieved. The uncoated part of the ITO-glass should extend the metal plate (Fig. 2c). Now fix the glass with the foldback clips.

Notes:

Fig2b: Applying Galinstan

Fig2c: covered ITO-glass

3) Connecting the Easy-OLED: Use the alligator clips to connect the positive pole of the 9 V battery with the uncoated part of the ITO-glass and the negative pole with the metal plate. Dim the ambient light and observe the luminescence for at least 30 sec. The contact spots between Galinstan and the polymer should begin to glow (Fig. 2d).

Notes:

Fig2d: Connecting the Easy-OLED. In this picture a Superyellow-OLED is shown.


Experiment 3: Power-OLED.

Prepare the ITO-glass as described in experiment 1. Before spin-coating it with the emitter put a small quantity of PEDOT:PSS dispersion along the edge of the adhesive tape, which you have fixed on the ITO surface (Fig. 3 – left). Use the coverslip and drag the dispersion along the complete surface to get a thin layer of PEDOT:PSS (Fig 3 – right). Dry the layer with a hair-dryer and then continue with spin-coating the emitter. You can complete the Power-OLED either in the Standard-variation (experiment 1) or in the Easy-variation (experiment 2). Notes:

Fig3: Applying the hole injector PEDOT:PSS with the doctor blade technique

Experiment 4: Flexi-OLED.

Remove the protection sheet of the ITO-foil. Put a piece of adhesive tape on one edge of the foil. Before applying the PEDOT:PSS layer with the doctor-blade-technique wet the edge of the attached tape with propanol. The alcohol mediates the interaction between the hydrophilic PEDOT:PSS dispersion and the relatively hydrophobic ITO-PET-foil. In this way a continuous and homogeneous layer is gained. Dry the layer with a hair-dryer and continue with building the Standard-variation, but apply only one emission hole. The OLED on PET is flexible up to an angel of about 120°, even while operating (Fig. 4). Notes:

Fig4: Flexi-OLED with Superyellow at 9 V DC.


Work sheet 2: Conductivity in polymers 1. a) Use square brackets to mark the repetition units of each polymer formulated in Fig 1.

b) Check the structures for segments of conjugated double bonds and highlight them with a color.

Fig. 1: Structural sections of three different polymers

2. Fig. 2 shows a simple model for explaining conductivity in polymer molecules. It focuses on the main carbon-chain of a

polymer with its valence electrons. Read the explanation text first and then assign the three polymers from Fig. 1 to the three models in Fig. 2.

a) In polymer molecules without conjugated double bonds all electrons are localized. Such molecules are not able to transport charges and don’t show any conductivity.

Example:_____________________________

b) Partially conjugated polymer molecules exhibit segments, in which the electrons of the double bonds are delocalized. Such molecules show a very low conductivity in the region of insulators.

Example:_____________________________

Fig. 2: Model for explaining conductivity in polymer molecules

localized electron delocalized electron area of delocalization

c) Fully conjugated polymer molecules feature electrons, which are delocalized over the whole polymer chain. Such molecules show a relatively high conductivity in the region of semiconductors.

Example:_____________________________


Work sheet 3: Electroluminescence in OLEDs

For the following exercises goto http://www.chemiedidaktik.uni-wuppertal.de/material/interactive/index.htm and type “oled” in the search field. You will get to the corresponding flash-animation. Download the flash-tool on your desktop, extract the containing file “oled.exe” and start the program. Use the button in the upper left corner to switch the language to English.

(Note that you can also operate the flash-animation online, this is recommended for users with MAC or LINUX system).

Investigate the simple model first

1. The electroluminescent polymer used in our experiment is called MEH-PPV, which stands for: Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] a. Assign the underlined name-fractions to the structure of the molecule in Fig. 3 using three different colors. b. The animation shows the molecule in a reduced form. Designate which parts are missing in the illustration

and suggest, for what reason this simplification have been made.

Fig. 3: Structure of MEH-PPV

2. The average molecular weight of MEH-PPV used in our experiment is approx. 200.000 g/mol. Calculate the average number of repetition units n. Use the following atomic masses: M(C) = 12 g/mol, M(H) = 1 g/mol, M(O) = 16 g/mol.

3. Fig. 4 shows the situation in the PPV molecule just after charge injection. Complete the illustration by determining the anode and cathode and pointing out the pathways of the injected charges with arrows.

Fig. 4: Device structure of the OLED with special focus on the polymer layer. The electrodes are illustrated as rectangles.

Now investigate the detailed model 4. Focus on the polymer layer and explain the main difference between the simple model and the detailed model.

5. Name the six elementary processes occurring at the “hot-spots”:

1)______________________________________ 2)___________________________________________ 3)______________________________________ 4)___________________________________________ 5)______________________________________ 6)___________________________________________


6. There are five relevant elementary processes which lead to electroluminescence. Investigate these processes in the

detailed model one by one. Name all five processes and assign them in both parts of the following illustration.

1)______________________________________ 2)___________________________________________

3)______________________________________ 4)___________________________________________

5)______________________________________

Fig. 5: The five elementary processes of electroluminescence in a structure model (upper) and an energy model (lower).

7. For higher performance of the OLED a hole injection layer (HIL) can be introduced. The most used HIL is PEDOT:PSS

(Fig. 6 left), which is a highly conductive polymer. Investigate the energy diagram of the OLED with HIL (Fig. 6 right)

and explain, how the HIL facilitates the injection of holes into the emission layer (MEH-PPV).

Fig. 6: left: structure of PEDOT: PSS; right: energy diagram of an OLED with HIL

Ef: Fermi-level (highest occupied energy level in inorganic conductors) All energy data are given in relation to the vacuum level.


8. Plot the measured currents in three different colors each for one spot of the OLED into the following diagram.

Scale the y-axis as required for your results and label both axes correctly.

0 1 2 3 4 5 6 7 8 9 10 11 12 13

9. To calculate the efficiencies of your OLED-spots, you have to measure the light densities, which is a complex operation,

if you don’t have the special equipment. Therefore assume the following light densities for the observed luminescence:

(-) = 0 Cd/m2 (o) = 50 Cd/m2 (oo) = 100 Cd/m2 (+) = 200 Cd/m2 (++) = 400 Cd/m2

Now you are able to determine estimated power-efficiencies ηP (given in Lumen per Watt ) for your three OLED-spots.

For this task you need to insert the I/U-pairs together with the estimated light densities into the following equation:

ηP = !∙"∙0,28 #$²%∙&∙10 [lm/W]

L: light density [Cd/m2]; I: current [mA]; U: voltage [V]; The factor π converts Candela to Lumen

Insert your results into the following table (round to the 4th decimal place)

and mark the points of maximum efficiency for each spot in the diagram above.

U [V] 2 3 4 5 6 7 8 9 10 11 12

η1

η2

η3

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