University of Groningen

Towards Self-Healing Organic ElectronicsOostra, Antoon

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CHAPTER 1

Motivation

1

1. Motivation

1.1 Next-generation lighting

The European Union and the U.S.A are maintaining policies to reduce greenhouse gasemissions cost-effectively. Reducing the energetic footprint of lighting is an essentialstep in these policies. A recent study by the International Energy Agency (IEA) showedthat 20% of the world’s energy consumption is represented by lighting.[1] The associatedgreenhouse gas emissions are equivalent to approximately 70% of the emission of pas-senger vehicles in the world.

Almost all governments that are a member of the Organization for Economic Co-oper-ation and Development (OECD) and many non-OECD governments have been in the pro-gress of phasing out incandescent light bulbs since 2007.[2] These inefficient light bulbsconvert only 5% of energy into light.[3] Replacing these light sources with more efficientalternatives is a first step towards reducing the energy consumption by lighting. How-ever, in 2012, roughly 25% of all light sources used in households consisted of incandes-cent light bulbs.[4] Apparently, some households still prefer the use of old, inefficient lightbulbs. The exact reasons why some households prefer the use of incandescent lightingare complex and have been found to be related to geographical aspects, initial purchasecosts and preconceptions about the used technology.[5,6]

One of the factors that influence whether someone will buy a lamp is the perceivedcolor of the light source, e.g. residents of European countries and the U.S.A. are morelikely to purchase light sources with warm colors.[5] This color, or temperature, of lightingis related to the correlated color temperature (CCT). The CCT is defined as “the absolutetemperature of a blackbody whose chromaticity most nearly resembles that of the lightsource”.1 Light sources with warm yellowish-red colors have a low temperature (2700 –3200 K), whereas emitters with a cold color have higher temperatures (>5000 K). How ac-curately a light source reveals the colors of objects in comparison with an ideal or naturallight source, is called the color rendering index (CRI). The CRI is 100 for a black body radi-ator. Both of these factors are used to characterize light sources. The incandescent lightbulb, which is almost a black body radiator, has a CRI rating of 100 and a CCT of∼2800 K.They therefore emit (the generally by Europeans preferred) warm colors with high colorrevealing accuracy.

The compact fluorescent lamp (CFL) is a more efficient alternative to the classic lightbulb. This type of light source is based on a phosphor-coated light bulb filled with a mer-cury/argon gas. The mercury vapor in the bulb is excited by passing a current throughthe gas mixture. The excited mercury vapor emits ultraviolet (UV) light, which is incid-ent on the phosphorescent coating. The coating is then excited by the UV-photons, andemits light of a visible wavelength. CFLs with different CTTs and CRIs can be fabricatedthrough careful selection of the coating material. CFLs are on average 75% more efficient(20% efficiency) than incandescent lighting.[3] On the downside, CFLs contain mercury asenvironment-unfriendly materials. They are furthermore highly sensitive to temperatureand mounting position, i.e. only at a mercury vapor pressure of 6×10−3 hPa is the optimalluminous efficacy (35–80 lm/W) achieved, which is at∼25 °C when the bulb is mounted inan upright position. At higher or lower temperatures, and with different mounting posi-

1definition by the Illuminating Engineering Society of North America (IESNA)

2

1.2 Renewable energy

tions, the internal temperature changes, in turn decreasing luminous efficacy.[7,8] Humid-ity also affects the internal temperature, and therefore its luminous efficacy. The lampshave a slight "warm up" time, and their lifetime is drastically reduced when it is turnedon for only a few minutes at a time.[3] So, although the efficiency of CFLs is higher thanthat of incandescent lighting, there is still room for improvement.

Fortunately, there is a relatively new light source that is also capable of emitting warmlight: the light-emitting diode (LED). LEDs are fabricated from very thin layers of crystal-line semiconducting material that emit light through the process of electroluminescence.They are often considered to be the next generation of lighting as they exhibit high effi-ciency (30 %) and low power consumption (100 lm/W efficacy),[3] while being technicallymore robust than CFLs, i.e. they do not suffer from low temperatures, humidity or re-duced lifetime when turned on for brief periods.

White LEDs can be fabricated following two different methods. The first approachis similar to the fabrication of a CFL: a LED capable of emitting UV-light excites a phos-phorescent coating. The excited coating then emits the visible, white, light. The secondapproach uses three different colored LEDs (red, green, blue). The light output of thesethree LEDs is combined, which results in the emission of white light.

White LEDs have only recently been commercialized due to the relative difficulty offabricating a blue LED. Although the first LED emitting visible light (red) was alreadydemonstrated in 1962,[9] and a green LED was developed a few years later, it took overthree decades before a LED was developed that was capable of emitting blue light.[10] Afterthe invention of the blue LED, which was awarded the 2014 Nobel prize in Physics, it wasfinally possible to develop white LEDs using the aforementioned methods.

Unfortunately LEDs also have a few downsides. LEDs require expensive driving mod-ules that control voltage and current. They furthermore are sensitive to high ambienttemperatures and require adequate heat sinking to maintain high lifetime. These mod-ules increase the price per watt of LEDs. Furthermore, to illuminate large areas, multipleLEDs need to be used, or elaborate luminaires. Nonetheless, when the cost of ownershipis considered, LEDs far surpass other light sources.[11]

1.2 Renewable energy

In order to reduce greenhouse gas emission, it is also necessary to explore renewable en-ergy sources.[12,13] Of all renewable energy sources, solar energy is the most abundantone.[14] It is therefore an important source of energy to consider as alternative to fossilfuels.

The first “solar module” capable of converting sun light into energy was based on sel-enium. In 1873 Willoughby Smith discovered that once exposed to sunlight, the resist-ance of this material was lowered to an extent depending on the intensity of the incidentlight.[15] This effect was studied in more depth by Professor William Grylss Adams andRichard Evans Day who found that an electric current started to flow when selenium wasexposed to candle light. They named this current “photoelectric”. The first photoelectricmodule was developed a few years later by Charles Fritts in 1883.[16] He covered a thinlayer of selenium, spread on a wide metal plate, with a thin semi-transparent gold film.

3

1. Motivation

This module produced a current when exposed to sunlight. Already then he predictedthat photoelectric devices may compete with fossil fuel based power plants.

In 1941, Russel Ohl discovered that monocrystalline silicon doped with impuritieswas also capable of converting light into electrical energy.[17] The efficiency of these testdevices was however low, approximately 1 %. The first practical solar cell was invented in1954 at the Bell Laboratories by Daryl Chapin, Calvin Fuller and Gerald Pearson. It had anefficiency of 6% and was capable of powering small devices such as radio transmitters.[18]

Since then, solar cells have been studied in depth, and now efficiencies of 25% can beachieved for cells based on monocrystalline silicon, and much higher efficiencies (>40%)are achieved using other materials in triple and quadruple junction devices.[19] Unfortu-nately though, the fabrication of solar cells is expensive, due to required energy neededto purify silicon, as well as the low production throughput.[20]

A newcomer in the field of solar energy is the perovskite-based solar cell (PSC). Thesesolar cells have shown an unprecedented growth in efficiency. In 2009 the first PSC wasdeveloped, with an efficiency of 3.8%.[21]Due to the relatively low efficiency, it went largelyunnoticed. However, in 2012, its efficiency was doubled to over 10% efficiency.[22] And inthe last 3 years, this efficiency has again been doubled to 20.1% in 2014 for smalldevices,[23] and 15% for larger devices (> 1 cm2, 2015).[24] What is more, these solar cellscan be fabricated relatively easily with low production costs, using earth-abundantmaterials.[23] These benefits make PSCs a serious contender to ’standard’ silicon solarcells. There are however still a few challenges that need to be overcome for PSCs, i.e. theirenvironmental and photo-stability should be improved.[25] When proper encapsulationtechnology for PSC is developed, and the technology is scaled up, PSCs will certainly playa very important role in satisfying future energy demands.[26]

1.3 Organic semiconductors

The initial purchase cost of LEDs and solar cells is a significant issue that may preventconsumers from buying these devices. However, this problem may be circumvented byfabricating LEDs and solar cells using semiconducting organic materials (hydrocarbon-based molecules). The advantage of using organic materials compared to crystalline sil-icon are chemical tenability and easy processing. This allows for i) low temperature pro-cessing, ii) solution processing, and iii) thermal evaporation. These factors reduce theproduction costs, as devices can theoretically be fabricated using manufacturing meth-ods from the printing industry. A simple light-emitting diode or solar cell, fabricatedfrom organic materials (e.g. polymers) has a device structure as shown in Figure 1.1. Thedevice consists of a thin (∼100 nm) organic (active) material, sandwiched between twoelectrodes. One of these electrodes is transparent to allow emission and absorbance oflight for light-emitting diodes and solar cells respectively.

For an organic light-emitting diode (OLED) the active material is capable of emittinglight. OLEDs are broadband emitters and generally have very good CRI values (>90), highefficacy (> 100 lm/W at 1000 cd/m2), and CTT (3000 K) values comparable to those ofincandescent light sources.[27] A large difference between OLEDs and (inorganic) LEDs,besides the fabrication methods, is that the active material in OLEDs have a disordered

4

1.3 Organic semiconductors

Cathode

Active material ( ~ 100 nm)

Transparent anode

Transparent substrate

Figure 1.1: Schematic overview of the device structure of organic light-emitting diodes and solar cells.The active area of these devices can exceed 1m2 when fabricated at large scale.

(a) (b)

Figure 1.2: a) White light-emitting diodes. b) white organic light-emitting diode.

molecular structure, whereas in LEDs the active material has a crystalline structure. Dueto this disordered structure, the entire light-emitting layer is capable of emitting light.Therefore, when processed at large scale, it is possible to design OLEDs with a light-emit-ting surface area that is orders larger than that of inorganic LEDs (see Figure 1.2).2 Fur-thermore, the light-emitting materials in OLEDs can easily be chemically modified toemit different types of colors. They can be processed on flexible substrates, and do notrequire additional use of luminaires to distribute light. If they are successfully commer-cialized, OLEDs may proof to be the actual next generation light source.

Organic solar cells (OSCs) have similar benefits as those of OLEDs, as they can also befabricated using low-cost, high-throughput methods. By significantly reducing the costsof producing solar cells, it may open up the market to easy-to-make, low-fabrication-cost,efficient solar cells. Efficiencies exceeding 10% have been reported for small area (< 1cm2) OSCs.[28] These values are similar to those of inorganic thin film photovoltaics. Un-fortunately though, it has not yet been possible to translate these efficiencies to large areamodules, as for these devices the highest efficiency achieved is approximately 3%.[29] Thisis a significant challenge that needs to be solved prior to opening up the market for OSCs.However, as long as production costs of OSCs are sufficiently low, there may still be aneconomic “sweetspot” for low-efficiency solar cells at which fabrication is worthwhile,even though there are more efficient technologies such as silicon- and perovskite-basedsolar cells. To ensure low-cost production of OSCs and OLEDs, it is necessary to fabricatethe devices at very large scale, with high production yield.

2In inorganic LEDs only the relatively small PN-junction (explained in Chapter 2) emits light.

5

1. Motivation

1.4 Organic thin film device fabrication

OLEDs are typically fabricated either by thermal evaporation of small molecule-basedemissive and charge-transport materials,[30] or by solution-based processing of small mo-lecule or polymeric semiconductors using methods such as inkjet-printing,[31,32] blade-coating,[33] and spin-coating. OSCs are typically only fabricated by solution-based pro-cessing methods. Depending on the type of processing-line, e.g. roll-to-roll processing orsheet-to-sheet processing,[34] different deposition methods can be used. These methodsallow for high-throughput with relatively low production costs. However, as large areas (>1 m2) need to be covered with very thin organic layers (< 100 nm), there are also signific-ant challenges when these deposition methods are used. Any particle or inhomogeneityon the substrate during deposition of the thin active layer can easily result in small areaswhere the bottom electrode is not properly covered with the active material. Depositionof the top-electrode then easily results in short-circuits between the electrodes enclosingthe active material. These short-circuits give rise to catastrophic device failure, loweredproduction yield and increased production costs. Prevention of these short-circuits isnot straight-forward. However, when this challenge is overcome, OLEDs and OSCs canbe fabricated at large scale, with high production yield, and low production costs.

1.5 Scope of this thesis

In this thesis we will introduce new additional processing steps that prevent, or intrinsic-ally repair, short-circuits and other defects within organic thin film devices such as OLEDsand OSCs that are processed at large scale. We hope that the proposed methods can beused to improve the production yield of OSCs/OLEDs, so that lower production costs,along with the aforementioned benefits of organic devices, make these products more vi-able to consumers. Widespread adoption of OLEDs and OSCs by consumers can then bean efficient step towards the reduction of greenhouse gas emissions.

In Chapter 2 we will describe the device physics of OLEDs and OSCs. We will explainthe functionality of the different layers that compose the organic device, and show thedifferent issues that are encountered during solution processing of these devices. Finallywe will give a brief overview of the methods that are currently applied to “repair” defectsin OLEDs and OSCs.

In Chapter 3, we explain the over-oxidation reaction of high conductivepoly(3,4-ethyl-enedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), a commonlyused organic anode material, by aqueous sodium hypochlorite (bleach). We show thata percolating network of PEDOT filaments in PEDOT:PSS is slowly over-oxidized, so thatthe conductivity of PEDOT:PSS layers is disrupted. We show that this process is diffusionlimited for low hypochlorite concentrations, and that a residual layer is left after completede-activation of the PEDOT:PSS layers. Finally we obtain the minimal treatment time thatallows for complete deactivation of PEDOT:PSS.

In Chapter 4 we propose a method to prevent microscopic short-circuits in OLEDs byover-oxidizing areas of the uncovered PEDOT:PSS anode with sodium hypochlorite. Weshow that conductive PEDOT:PSS agglomerates can penetrate the light-emitting layer in

6

REFERENCES

OLEDs, and that these agglomerates create parasitic current pathways and microscopicshort-circuits. We show that light-emitting polymers, when briefly immersed in aqueoussodium hypochlorite, are hardly affected by the treatment. Together with the results fromChapter 3, a process window is obtained in which uncovered areas of PEDOT:PSS canbe deactivated, without affecting the light-emitting polymer. Finally we show, that aftercomplete immersion of OLED precursors in hypochlorite, prior to evaporation of thecathode, parasitic currents in OLEDs are reduced, the occurrence of bright spots in OLEDsis eliminated, without loss of device lifetime.

In Chapter 5 we use the proposed treatment from Chapter 4 to deactivate macroscopicdefects in OSCs. However, using contact-angle measurements, we show that the photo-active material used for bulk-heterojunction OSCs (P3HT-PCBM) is sensitive to oxidationby aqueous hypochlorite. Therefore, it is necessary to locally treat defects with hypo-chlorite, rather than fully immersing OSC precursors in the oxidizing solution. After localtreatment of macroscopic defects, similar efficiency is found for intentionally defectedand subsequently treated OSCs and not defected reference cells.

In Chapter 6, we study the effect of PEDOT:PSS-cathode short-circuits on OSC deviceperformance with standard lithium fluoride - aluminum (LiF/Al) bi-layer cathodes. Weshow, that when the resistance of a short-circuit between the two electrodes is increasedfrom 5 to 1000Ω, equal device performance is obtained for short-circuited and not-short-circuited solar cells. We study the junction resistance of PEDOT:PSS-LiF/Al short-circuits,and show that a small increase in LiF thickness rapidly increases the junction resistance.We propose a mechanism to self-heal PEDOT:PSS-cathode short-circuits, by increasingthe evaporated LiF thickness. We proof that the proposed method functions as expected,and that macroscopic short-circuits in OSCs can successfully be deactivated by slightlyincreasing the evaporated LiF thickness, without suffering from significant loss in deviceperformance.

Finally, in Chapter 7, we deviate from the prevention/repair of short-circuits in or-ganic devices, to the repair of silver grids in organic electronics. We show, that when silverwires are separated by a small distance, that they can be repaired by means of electro-deposition. Dendritic growth of silver is able to restore part of the initial conductivity of asilver track. The restored conductivity is however not yet enough. Fortunately, by apply-ing a short high-voltage pulse to the dendrite-covered silver track, it is possible to restorethe conductivity of the track to nearly 10% of its original value.

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10