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Physics World Discovery

Printed Electronics

Jessica Wade, Joseph Razzell Hollis and Sebastian Wood

1 IntroductionIn recent years, the development of organic and hybrid semiconductors has enableda totally new paradigm for manufacturing electronics, in the form of printableelectronics. Ever since the invention of the solid-state transistor in the middle of thelast century, electronics depended almost exclusively on inorganic semiconductorssuch as silicon, which is now produced in multi-billion-dollar foundries. Printableelectronics promise to revolutionise the modern world, by providing low-cost, cleanenergy solutions; roll-up large-area lighting; smart packaging and even cheapmedical sensors. Adaptable device architectures, highly customisable materials,and a wide variety of printing-based manufacturing techniques will enable a broaderrange of electronic applications than has previously been possible. The globalmarket for printed electronics has been estimated by market research companyIDTechEx to reach $73 billion by 2027.

Whilst printed electronics has become a commercial technology, it is still an activeresearch topic with some important unanswered questions. We know that thephysical properties of semiconductors—their molecular packing, crystalline orderand defect density—are determined by both chemical structure and depositionconditions, and these properties determine the electronic performance. However,there remains a debate over the precise mechanisms at play, and the complexrelationships between chemical and physical properties, and device performance.Elucidating these relationships requires detailed characterisation of devices, in termsof their chemical structure, morphology, and optoelectronic properties down to thenanoscale. Such requirements challenge the limitations of state-of-the-art metrologyand have led to the ongoing development of new measurement techniques.

Further improvement in both manufacturing and performance is to be expectedas research and development in plastic electronics continues. Recently the focus ofresearch has shifted towards the problems of operational stability, as well as scaling-up towards print-based manufacturing. The performance of many printable semi-conductors tends to drop over time when exposed to oxygen, water, heat or light.Intense research efforts are focused on strategies to increase stability, designing newmaterials and device architectures that resist degradation.

doi:10.1088/978-0-7503-1608-8ch1 1 ª IOP Publishing Ltd 2018

Printed electronics is a truly interdisciplinary area of research; requiring expertinput on subjects from electronic engineering to synthetic chemistry, and frommaterials science to computational physics. The research landscape also remainsdynamic with new topics of investigation rapidly opening up, such as the recentemergence of stretchable sensors, implantable electronics, and perovskite solar cells.This book is intended to be a primer on the fascinating, and fast-changing, field ofprinted electronics, highlighting three key topics: novel materials, characterisationtechniques, and device stability.

2 BackgroundA typical electronic device comprises three separate classes of materials: conductor,insulator (dielectric), and semiconductor. Therefore, a key requirement for fullyprintable electronics is that suitable materials and processes are developed to depositthese different classes of material from a solution (ink). Breakthrough publicationsin 1977 (Alan Heeger, Alan MacDiarmid, and co-workers at University ofPennsylvania) and 1990 (Andrew Holmes, Richard Friend, and co-workers atUniversity of Cambridge), demonstrated high conductivity and light-emitting semi-conductor properties in polymeric materials, respectively. This led to concentratedresearch efforts and rapid expansion of the field. Whilst polymers are attractivematerials from a printing and processing perspective, there are in fact broad classes ofelectronic materials suitable for printed electronics: organic and inorganic.

Inorganic electronic materials

Although inorganic materials are generally insoluble, making them unsuitable forprinting directly, their electronic properties make them very attractive. Variousinorganic materials are employed either as an active layer, conductive electrode, oras dielectrics. Some semiconducting devices require light to enter or leave the device(e.g. solar cells or light-emitting diodes), and so a transparent electrical conductor isrequired. Transparent conducting oxides, such as indium tin oxide (ITO), are highlyconductive, and polycrystalline layers are produced by sputtering. Whilst this is nota printing process, ITO-coated glass and plastics are readily available and continueto be widely used in printed electronics. Other metal oxides can be deposited fromsolution, but the high temperatures required are incompatible with plastic sub-strates. There are several other materials being investigated as indium is a rare andexpensive metal.

For truly printed inorganic materials, suitable inks must be formulated which arelow-cost, non-toxic, and environmentally friendly, while also having the appropriatesurface tension, viscosity and wettability. Metallic nanoparticles are good candidatesfor inorganic functional inks. They can be grown via two routes: top-down, where abulk metal is broken down into a medium; or bottom-up, using ion-precursors andreducing agents. The bottom-up approach is generally preferred as the nanoparticlesize and shape can be carefully controlled, determining their electronic properties.Owing to its remarkable conductivity and relatively low-cost, silver is the mostcommonly used metal.

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Whilst inorganic materials have established uses as conductive layers, the highsintering temperatures required for inorganic semiconductors have largely renderedthem unsuitable for printed electronics. Despite the processing challenges, there areseveral inorganic oxides (e.g. ZnO, TiO2), hybrid organic–inorganic crystals(perovskites), and 2D inorganic nanomaterials (e.g. MoS2) that have shownpotential in printed electronic devices.

Organic electronic materials

The use of organic materials in printed electronics is largely based on the class ofcarbon-based compounds that have a conjugated backbone: a system of alternatingsingle and double bonds resulting in delocalised π-electrons (see figure 1). For a detailedexplanation of how carbon bonding results in semiconducting properties, see workslisted in the Additional Resources section. Organic semiconductors come in manydifferent forms, from allotropes of pure carbon (such as nanotubes and fullerenes), tolong chain polymers and small molecules. The majority of these compounds comprise acentral, conjugated backbone that is responsible for optoelectronic properties, and

Figure 1. (a) Molecular orbital diagrams of high-energy anti-bonding (π*) and low energy bonding (π)molecular orbitals. (b) σ and π bonds in ethane, the simplest conjugated system (c) Energy levels of aπ-conjugated molecule.

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alkyl side-chains. These chains are added to improve solubility in organic solvents, ormodify the way in which the molecules pack together in the solid state.

Though organic semiconductors only emerged a few decades ago, high-performanceelectroluminescent organic materials have already made their way into commercialproducts in the form of organic light-emitting diode (OLED) displays. Less maturetechnologies include organic photovoltaic cells (OPVs), and organic field-effecttransistors (OFETs). As of 2017, the efficiency of OPVs has exceeded 13%, comparedto 20% for silicon, and have an unencapsulated lifetime of over 5000 hours. OFETs,on the other hand, have achieved hole mobilities of almost 20 cm2 V−1 s−1, comparedto 1 cm2 V−1 s−1 for amorphous silicon.

While the electronic performance of organic semiconductors is generally notexpected to surpass that of mature technologies based on silicon, organic semi-conductors offer numerous other advantages. Their optical absorption and emissionproperties are tunable over a broad range, they are also soluble in common solvents,and mechanically flexible. By formulating organic inks containing different semi-conductors, complex multi-layered devices can be printed using well-establishedlow-temperature solution-based techniques under ambient conditions. Their proper-ties are easily tuned by altering their chemical structure—adding or removingorganic multi-ring molecular building blocks. After decades of work, the library ofknown organic semiconductors is truly enormous, with thousands of uniquestructures reported. This continues to expand as new, higher-performance materialsare synthesised based on computational models and a growing understanding of therules that determine their electrical properties.

Alongside carbon-based small molecules and polymers, carbon nanomaterials(such as graphene and carbon nanotubes) can be printed onto flexible substrates astransparent conductors, or as semiconductors. For carbon nanotubes, separatingconductive and semiconducting nanotubes is a key challenge, and for graphene,control of film quality is a crucial limitation. Carbon/metal hybrids are beingdeveloped for improved performance, but there is a lack of research into theirconductive mechanisms.

Printed electronic devices

While early organic semiconductor devices were relatively simple—consisting of oneor two layers of semiconductor—state-of-the-art devices can have complex archi-tectures comprising several different layers. Each layer performs specific functions(electron/hole transport, charge transfer/recombination, light absorption/emission,etc) in order to maximise overall efficiency. The structures are compared in figure 2.

OLEDs were the first organic devices to go to market, and can now be found inmobile phones, digital cameras, laptops and televisions. Whilst conventional LEDsstill use liquid-crystalline filters with backlighting, OLED displays have individualpixels of organic materials that emit their own light, making them more energy-efficient and improving their colour range. In July 2017, LG announced the 88 inch8k flexible organic light-emitting diode display.

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State-of-the-art OPV devices are based on a bulk heterojunction (see figure 3),meaning that the light-absorbing ‘active’ layer comprises a mixture of two differentsemiconductors, known as the electron donor and electron acceptor. Any photo-generated excited states (excitons) are dissociated into free-charge carriers (i.e.photocurrent) at the interface. In a bulk heterojunction, the semiconductors aremixed at the nanoscale to create a large interface distributed through the layer thatcompensates for the short diffusion length (∼10 nm) of the exciton. The first bulkheterojunction OPVs represented a significant step up in efficiency compared tobilayer cells, and that efficiency has continued to rise. The German companyHeliatek—which specialises in organic flexible solar-films—achieved record OPVefficiency of 13.2%, thanks to the development of new materials and better controlover heterojunction structure. Current excitement surrounds new applications forprinted photovoltaics beyond outdoor power generation, such as low-cost photo-detectors and indoor photovoltaics, which could be important enablers for theInternet of Things (IoT). Printable photodetectors are of great interest for

Figure 2. Diagrams showing organic electronic device structures discussed in text.

Figure 3. Illustration of heterojunctions for OPV devices. The first heterojunction was (a) planar bilayer ofdonor and acceptor materials. To increase the active volume of the OPV, heterojunctions must exist within anexciton diffusion length such as in an (b) bulk heterojunction or (c) an ordered interdigitated heterojunction.

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applications in lab-on-a-chip diagnostics, where low-cost sensors with colour-tuneable absorption would be particularly attractive.

Transistors are the fundamental component of all conventional electronics. Acommonly-used type is the FET, which is a three-terminal device (source, drain andgate), where the active layer is a semiconducting material whose conductivitychanges upon application of an electric field. Current flows through the semi-conducting layer from a source to a drain electrode, when a voltage is applied to thegate electrode. The gate electrode is insulated from the active channel by a dielectricmaterial. The operational performance depends on the FET switching speed, on–offcurrent ratio, low voltage operation, and minimal ‘off’-current leakage. To improvethe ‘on’-current, efforts have focussed on reducing the channel length, increasing thegate capacitance, and engineering high mobility organic semiconductor materials.Significant progress in these areas is required for printable organic FETs to havesuitable performance for applications such as fully flexible displays, and radiofrequency identification (RFID) tags.

Printing and post-printing processes

Printing technology is well established and widely employed in a range of contexts,where high-throughput at low-cost is desirable. As a result, a large number ofprinting techniques exist with unique capabilities, some of which lend themselvesparticularly well to use with electronic inks.

Screen printing—where a mesh-based stencil is used to transfer ink to a substrate,which can be anything from plastic to fabric—is the most well-established printingtechnique for use with electronic materials. This technique is already used commer-cially for printing conducting layers and dielectric materials, though not yet forsemiconductors.

One limitation of screen printing is that it is a batch process, and so not suitablefor the largest scales of manufacturing, where a continuous process is required. Thisis often described as ‘roll-to-roll’ (R2R) printing, encompassing a broad family ofprinting and coating techniques where pre-patterned (or blank) substrates are loadedonto a roller, passed through a manufacturing line, and re-rolled at the end as afinished product.

Slot-die coating is among the simplest R2R-compatible techniques, relying on theinjection of solution from a narrow nozzle onto a moving substrate. To control thedrying process of the ink on the substrate, both the substrate and nozzle can beindependently heated. Film formation is driven by capillary forces acting betweenthe slot-die nozzle and the substrate, which is only a few hundreds of micrometresbelow the nozzle. The parameters of the ink, distance to the substrate, speed of themoving substrate and nozzle dimensions determine the microstructure of the filmthat is created, which is critical for printing semiconducting inks. Slot-die coatinghas no meaningful spatial resolution, but is well-suited to the printing of organicsolar cells, where uniform coverage over a large area is required.

R2R printing with high resolution requires techniques such as gravure or inkjetprinting. ‘Drop-on demand’ (DOD) is the most widely used form of inkjet printing;

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where a printer head ejects drops of ink on the prompt of a digital signal. Bydepositing a large number of overlapping drops, a continuous line or area ofelectronically active material can be produced. Whether or not an ink is compatiblewith DOD printing depends on its viscosity and surface tension, requiring consid-erable efforts in ink formulation to produce functional devices. Spray coating isanother printing technique that has been demonstrated to be faster than inkjetprinting, but also requires the formulation of low viscosity functional materials.

After the initial deposition of ink, other processes are often carried out to controlthe drying process, and in the case of printed electronics, these post-depositionprocesses are important for optimising the electronic performance. Typically, thismight include light-soaking, thermal annealing, or solvent vapour annealing. Whilstthese processes are readily achievable in small batch processes, scaling them up forR2R printing is not trivial. For organic semiconductors, these processes usuallyincrease the crystallinity and ensure the uniform orientation of ordered conductingpathways through the material. A significant body of literature concentrates onunderstanding how processing such conditions impacts the crystallisation of com-plex blend systems (a combination of two or more semiconductors), and how thisinfluences their optoelectronic properties.

Research into printed electronics has focused on increasing the efficiency ofdevices and improving their operational lifetime. The continuous development ofnew materials with superior optoelectronic properties has led to significant increasesin performance. Optimisation of device architectures has also contributed to betterperformance, through the use of layers with specialised functions. To achieve this,we must consider how the materials are organised within each layer duringdeposition, known as morphology. Chemically identical compounds can exhibitstrikingly different electronic properties depending on their organisation in the solidstate, which in turn depends on deposition conditions.

The difficulty arises in understanding exactly how deposition conditions, orchemical structure, influence the resulting morphology. As a rule, gradual depositionleads to more ordered structures that are associated with better performance: regularclose-packing of conjugated backbones aids in the transport of charge carriersbetween molecules and increases delocalisation within molecules through greater co-planarity. The most efficient morphology, or conditions required to obtain it, will beunique to each set of materials used, and thus careful characterisation of morphol-ogy will always be an essential part of optimising new devices based on novelmaterial.

3 Current directionsNovel materials

For almost four decades, organic semiconductors have been used in organic light-emitting diodes, photovoltaic cells and transistors. The performance of these devicesdepends on the use of materials with carefully chosen properties. To an extent theseproperties can be modified by processing conditions, but primarily, they are

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determined by the chemical structures of the materials themselves. As a result, thedevelopment of new materials is a key activity (figure 4).

Designing polymers for photovoltaicsTo illustrate the challenge of designing new materials, we consider the case of anOPV device, whose operational mechanism is represented by figure 5. The photo-excitation of organic semiconductors does not produce the free electrons of theirinorganic counterparts, but bound electron–hole pairs known as Frenkel excitons.Excitons have a large binding energy (0.1–0.3 eV) and so tend to recombine afterphotoexcitation rather than generate separated electrical charges. In photovoltaicdevices, two different organic semiconductors with different energy levels (ionisation

Figure 4. The evolution of polymer molecular structure (left to right) from polyacetylene to poly(p-phenylenevinylene) (PPV), poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO or F8)and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) This shows a clear evolution in terms of chemicalcomplexity over time.

Figure 5. Energy level diagrams illustrating the operational steps of a heterojunction OPV device: (a) lightabsorption, (b) exciton splitting, (c) charge transport, and (d) charge extraction.

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potential and electron affinity) are usually blended together, such that it is energeti-cally favourable for one electrical charge (typically the electron) to overcome thebinding energy and transfer across the organic interface. The exciton diffusionlength (distance an exciton can travel before recombining) in organic semiconduc-tors is ∼10 nm, so interfaces must be very close to one another. The photoexcitedelectron then moves through the ‘acceptor’ material and the hole moves through the‘donor’ material towards their respective electrodes. There are several challenges forefficient solar cell operation, such as charge recombination before the exciton is split,recombination of separated charges with one another, and optimising the opticalabsorption of the materials. As a result, the materials used must have carefullyselected energy levels, strong light absorption, high mobilities for electrons andholes, and sufficient solubility for printing. Understanding these needs guides theprocess of new material development.

Until recently, the most effective electron accepting materials were derivatives ofthe carbon fullerene (structure shown in figure 6), but these do not absorb visiblelight effectively and have an undesirable tendency to migrate and aggregate withinthe device. Efforts to develop non-fullerene acceptors have recently proved fruitful:achieving high efficiencies of over 12%.

The energy levels of organic materials are largely controlled by the extent ofπ-conjugation, and the energy gap can be reduced by planarising (reducing torsionbetween adjacent monomer units) the polymer backbone through chemical design.In addition, the donor–acceptor motif is an established strategy for producing smallenergy-gaps (hence long wavelength absorption/emission), where electron-rich(donor) and electron-poor (acceptor) chemical groups are joined together resultingin ‘charge transfer’ absorption bands. This strategy offers tuneable energy levels andprecise control of the semiconductor energy gap. Chemists in many research groupsand companies are continually developing new molecules with increasingly complexstructures that further refine the optical and electronic properties. The placement ofside chains and functional groups can offer further control of molecular packing,which in turn dictates charge transport and recombination.

Figure 6. A C60 fullerene molecule.

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Organometallic halide perovskitesOrganometallic halide perovskite materials (usually just called perovskites) haverecently and dramatically changed the paradigm of solar cell research. Emerging outof the more established organic photovoltaic and dye-sensitised solar cell fields,perovskites offer an attractive combination of a highly crystalline semiconductorwith the possibility of solution deposition. The crystal lattice contains organiccations (A), metal cations (B) and halide anions (X), described by the chemicalformula ABX3—as illustrated by figure 7. Their intrinsic photophysical andelectronic properties are very promising; exhibiting efficient charge-transport,tuneable optical absorption, weakly bound excitons, intrinsic ferroelectric polar-isation and spin-dependent responses. The experimental efficiencies of perovskitesolar cells now exceed 20%, but these devices are typically very small, have shortoperational lifetimes, and have raised environmental concerns over their use oflead. Current research is seeking to develop a lead-free alternative, as well as tounderstand and overcome the degradation mechanisms. Beyond photovoltaics,perovskites are also now being developed as the active layer in light-emitting diodesand lasing applications.

Printed bioelectronicsThe excitement surrounding nanomedicine, brain–machine interfaces, and medicalrobotics indicates a growing area of research into the interaction between biologyand technology. The inherent soft and stretchable nature of printable materials isparticularly advantageous for applications involving soft biological matter.Implantable bioelectronics must be biocompatible, have good mechanical propertiesfor their application, long shelf lives, mimic body tissue and form safe degradationproducts.

The first steps in printed bioelectronics have been taken in the form of organicelectrochemical transistors based on poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS), a conductive polymer, that can change conductivity basedon neural activity—amplifying the small electronic signal of neurons. Alongsideproviding a high ion mobility, PEDOT: PSS also helps the implant to integrate with

Figure 7. Perovskite crystal structure and schematic structure of a perovskite solar cell.

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nearby tissue. Other conjugated polymers, developed previously for OPV applica-tions, have been used in cellular scaffolds, neural probes and actuators for drugrelease. This includes the recent application of polymers to trigger neuron firing andrestore light sensitivity to retinas with light-induced photoreceptor degeneration.

Characterisation techniques

The functional properties of printed electronic materials are critically dependent ontheir nanoscale structure. This dependence is often referred to as the ‘structure–function relationship’ but is highly complex and unique to each material system. Akey difficulty is the requirement to characterise the sample in terms of structure,chemical composition, electronic and optical property over a range of length-scalesfrom centimetres (or larger) down to nanometres and even the molecular scale.Further complexity is introduced by variation in the material properties over time,due to controlled or unintentional stress conditions, requiring techniques that can beused in real time to observe these changes.

Here we focus on a number of exciting developments that allow characterisationof printed electronic devices with the highest spatial resolution and the ability tomeasure the evolution of these properties in situ during important steps of devicefabrication and operation.

Scanning probe microscopyOne of the most significant challenges for thoroughly characterising flexibleelectronic devices is the sensitivity of device performance to variations in nano-and micro-structure within the semiconducting film. These structures exist at scalesbelow the diffraction limit for optical resolution (∼200 nm), so alternatives to opticalmicroscopy are required.

Scanning probe microscopy (SPM), and the sub-category atomic force microscopy(AFM) in particular, is a long-established method for obtaining topographical mapsof materials with nanometre accuracy. Figure 8 illustrates the technique: an extremelysharp ‘tip’ is scanned across the surface, tracing any changes in height through direct

Figure 8. Diagram illustrating the operation of atomic force microscopy (AFM).

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contact or the near-contact electrostatic interaction. Surface topography informationcan be useful but advanced modes of SPM are capable of simultaneously measuringelectrical properties such as surface potential, conductivity or photoconductivity, all ofwhich are useful for understanding properties of electronic devices. Conductive-AFMhas been employed to study the impact of grain boundaries on charge transportthrough poly-crystalline films used in organic transistors and perovskite solar cells.Photoconductive AFM (PC-AFM) is remarkably sensitive to sub-surface structures,like nanowires of highly-organised polymer, that are not always apparent in top-ography but are detectable through their efficient transport of charge carriersgenerated within the film under illumination.

The combined measurement of both electrical properties and topographicalfeatures with nanometre resolution makes these SPM techniques ideal for studyingthe semiconducting films used in flexible electronic devices, and we can expectfurther improvements in functionality as instrumentation continues to advance,especially in the light of developments in plasmonics, as described below.

Plasmonic spectroscopyPlasmonics is a well-established field and provides some important tools forspectroscopic characterisation of materials at the nanoscale. Aside from its limitedspatial resolution, optical spectroscopy is a powerful family of techniques forprobing the chemical and optoelectronic properties of functional materials.However, the use of plasmonic structures offers a way to perform spectroscopywith nanometre resolution. These techniques exploit localised surface plasmonresonances (LSPR), where oscillations in electron density on metallic nanostructuresresult in an intense, highly localised electric field. This leads to a strong enhancementof the interaction of nearby molecules with light, and hence enables spectroscopicmeasurements. The effect is typically confined to within ∼10 nm of the metal surfaceand, unlike SPM techniques, the probed metal/organic interface can be buried deepwithin the device as long as it is optically accessible. Depending on the metallicnanostructure used, LSPR has been shown to enhance both Raman scattering andfluorescence from organic molecules by factors of up to 1011, allowing detection ofsingle molecules. When applied to blends of conjugated polymers, surface-enhancedRaman spectroscopy (SERS) is able to distinguish differences in chemical compo-sition between the bulk film and the buried metal/polymer interface.

A further advance is to combine plasmonically-enhanced spectroscopy with SPM,by developing suitable tips (typically based on a silver or gold coating layer) thatsupport an LSPR. This technique is referred to as ‘tip-enhanced optical spectro-scopy’ (TEOS) and can be combined with various modes of spectroscopy forchemical sensitivity with impressive lateral resolution down to a few nanometres.The performance and reproducibility of plasmonic tips presents an ongoingchallenge, as well as producing quantitative results.

In situ characterisationAnother exciting advance in material and device characterisation is the rise of in situmeasurement set-ups: the combination of rapid data collection with specialised

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sample chambers (with in-built heating elements, electrical contacts, etc) permittingdirect observation of how the properties of semiconducting films change over time.This offers incredibly rich information on the dynamics of processes such as filmformation, response to treatment, and stability during device operation.

In printed electronics, the ideal thin-film device is one that forms an optimal filmstructure during deposition from solution, with no need for further treatment thatmight disrupt the roll-to-roll printing process. In situ techniques can shed light onhow semiconducting materials crystallise when the solution evaporates, directlyrevealing the relationship between deposition conditions and resulting film structure.Recent demonstrations have shown that in situ ellipsometry, an optical method thatidentifies layered structures from how they polarise reflected light, can be used toobserve how organic blend films phase-separate during deposition. Similarly, in situwide-angle x-ray scattering (WAXS) can be used to identify crystalline domainsduring perovskite deposition from their diffraction patterns.

Understanding the dynamics of morphological changes during heating, or whileapplying an electric current, may help us ascertain why organic and hybrid devicesexhibit limited stability. The evolution of surface roughness, film thickness, andother parameters (measured via in situ SPM or ellipsometry) during heating revealthat some organic blends undergo important phase transitions. They begin toreorganise and deviate from their optimised morphology above temperaturescomparable to the expected operating conditions for certain classes of device(20 °C–50 °C). Application of electric current can also have an impact on stability:chemical structures are altered by the presence of charge carriers and rendered moresusceptible to irreversible oxidation that breaks their conjugation, which can beinvestigated in situ with an electrochemical cell during prolonged charge injection.

Device stability

Performance is not the only figure of merit for electronic devices—it is one keyaspect, along with manufacturing cost and operational lifetime (illustrated by the‘critical triangle’ in figure 9). As the performance of printed electronics has improvedto the point of commercial viability, attention is increasingly focused on improvingthe lifetime of devices based on organic and hybrid semiconductors. It is generally

Figure 9. Cost, performance, lifetime triangle, proposed by Christoph Brabec in 2004.

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considered that costs will be driven down by large-scale printing processes, when thetechnology itself is ready.

Organic materials degrade much more readily than their inorganic counterpartsand even the most state-of-the-art structures can become damaged, which meansthat their performance degrades over time. The current understanding is that OPVlifetimes need to exceed five years for commercial viability, but many lab-scaledevices last only hours. There are several factors that can affect the stability oforganic and hybrid devices, including environmental stresses (light, oxygen, etc) andthe operation of the device itself (electric bias, deformation). In this section weprovide a short summary of the main factors that reduce stability, and outline effortsto counteract them.

Oxidation is one of the main forms of degradation for organic and hybridsemiconductors, and typically occurs when the material is exposed to a combinationof light, oxygen and water (especially pertinent to outdoor solar cell applications).The resulting change in the chemical structure typically disrupts the conjugatedbackbone (for organics) or the 3D lattice (hybrid perovskites) that are intrinsicallylinked to their semiconducting properties. The precise mechanisms of oxidation aredifferent in each case, but the result is obvious: loss of charge mobility and reducedlight absorption, which correlate with exposure and the appearance of oxidisedproducts. Figure 10 shows the primary photo-oxidation mechanism elucidated forthe case of polythiophene. Different organic semiconductors show very differentreaction-rates under oxidising conditions, as certain side-chains, atoms and mono-mers are more sensitive. The group of Frederik Krebs at Technical University ofDenmark has done extensive work to overcome degradation and to determine whichchemical units should be avoided due to their susceptibility to degradation.

During prolonged operation, electronic devices can be expected to reach hightemperatures: solar cells in direct sunlight regularly can reach above 50 °C, and eventhe least stringent operating temperature standards specified for commercialelectronics require operation up to 70 °C. While this is far below the thermaldecomposition temperature of most organic molecules, it can still cause irreversiblechanges in other structural parameters such as the morphology. Polymer films canexhibit glass transition temperatures (Tg) as low as 10 °C, above which the polymer-

Figure 10. Proposed photo-oxidation mechanism of polythiophene.

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chains become mobile, gradually altering the organisation of the layer away from itspreviously-optimised morphology, and potentially hindering performance. This hasbeen observed in pure films (as the growth of overly-large crystals), blend films(excessive phase separation) and at interfaces (delamination). The thermal stabilityof a material can be improved by increasing Tg through the use of additives,specifically vitrifying agents, or by cross-linking individual molecules once arrangedin the desired morphology. Oxidation can also impact inorganic layers, especiallylow-work function electrodes such as aluminium, forming insulating oxide barrierswhich limit charge injection, conduction and collection.

The flexibility of organic semiconductors that is crucial for large-scale roll-to-rolldevice fabrication leaves them susceptible to stress and strain. Achieving well-ordered conjugated polymer morphologies has proven very difficult, and moleculesare understood to migrate over time, which adds further complications. Themorphological stability of the active layer depends strongly on the startingmorphology, and the best performance is rarely from the most thermodynamicallystable structure.

Printed electronic devices are usually multilayer structures, which makes thestructure susceptible to catastrophic mechanical degradation by delamination,where one layer peels off another. This can occur in two ways, between two layers(adhesive) or within one layer (cohesive). In organic solar cells, adhesion andcohesion occur at sites of high concentrations of acceptor molecules—the mechan-ically weakest components. Structural defects in the active layer material orimpurities in any one of the layers can trigger the formation of microscale cracks,which increase the device resistance and alter the electrical properties. It has beenreported that mechanical properties can also be improved by blending the semi-conductor with a more durable insulating polymer and that, with the rightmorphology, electrical conductivity can be retained at semiconductor concentra-tions as low as 5% by weight.

There is a growing body of evidence about the factors that determine stability,and how to improve it. Generally speaking, the most effective intervention to slowchemical degradation is encapsulation of the device with barriers impermeable towater and oxygen. However, this makes devices more difficult to manufacture andmuch more expensive. The long-term goal of stability studies remains the develop-ment of high-performance semiconductors with chemical structures that intrinsicallyresist oxidation. Mechanical stability will depend on finding the optimal combina-tion of chemical structure, molecular weight, and blend composition of the activelayer to provide both the desired electronic properties and the necessary durability.

4 OutlookThe importance of electronic devices to the modern world cannot be overstated. Todate, the production of electronic components has been limited to primarilyinorganic materials manufactured via complex, costly methods, hindered by poormechanical properties. The incredible functionalities of modern materials offer anew paradigm in device design, with the potential to revolutionise the way

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electronics integrate into everyday life. Low-cost mass production of flexibleelectronic paper, large area lighting, biosensors, photovoltaics and wearabletechnology are all applications of the research introduced in this book. This isonly the beginning: printed electronics will change all our futures.

If the development of new materials, device architecture and applicationscontinues at its current pace, the prediction of a printed electronics industry worth$73 billion by 2027 will be an underestimation. The rapid rise of OLED displays inthe consumer market, an early starter in plastic-electronics research, and now acommon technology in phones and TV screens, shows just how successful thesedevices can be.

Research is focused on overcoming the commercialisation challenges for organicand hybrid materials—their relatively poor electronic properties compared toinorganic counterparts and their operational stability. Thanks to an interdisciplinaryteam of international researchers, incredible progress has been made to improvethese properties by orders of magnitude. Such developments can only continue witha better grasp of the relationships between the chemical design, material science andfundamental physics involved in the operation of printed electronics. This under-standing is built on the thorough investigation of structure–property relationships atmany different scales. For each new application where these materials are consid-ered, careful multi-disciplinary work must be done to ensure that the best possibleperformance is achieved.

It is difficult to summarise in so few words all the exciting developments that ourresearch field has to offer, but we hope that our contribution to the Physics WorldDiscovery collection will stimulate further interest in printed electronics, and helppeople to keep abreast of this fast-paced, ever-advancing topic.

Additional resourcesIntroduction and background:

• Das R, Ghaffarzadeh K, Chansin G and He X 2017 Printed, organic &flexible electronics: forecasts, players & opportunities 2017–2027 IDTechReport

• Chiang C K, Fincher C R Jr, Park Y W, Heeger A J, Shirakawa H, Louis E J,Gau S C and MacDiarmid A G 1977 Electrical conductivity in dopedpolyacetylene Phys. Rev. Lett. 39 1098

• Burroughes J H, Bradley D D C, Brown A R, Marks R N, MacKay K,Friend R H, Burns P L and Holmes A B 1990 Light-emitting diodes based onconjugated polymers Nature 347 539

• Köhler A and Bässler H 2015 Electronic Processes in OrganicSemiconductors: An Introduction (Wiley-VCH)

• Klauk H 2006 Organic Electronics. Materials, Manufacturing andApplications (Wiley-VCH)

• Heliatek sets new organic photovoltaic world record efficiency of 13.2%,Heliatek Press-release 8 February 2016

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• Berggren M, Nilsson D, Robinson N D 2007 Organic materials for printedelectronics Nat. Mater. 6 3

• Wu W 2017 Inorganic nanomaterials for printed electronics: a reviewNanoscale 9 7342

Novel materials:• Li S, Liu W, Li C-Z, Shi M, Chen H 2017 Efficient organic solar Cells withnon-fullerene acceptors Small 1701120

• Zhou H, Yang L and You W 2012 Rational design of high performanceconjugated polymers for organic solar cells Macromolecules 45 607

• Stranks S D and Snaith H J 2015 Metal-halide perovskites for photovoltaicand light-emitting devices Nat. Nanotechnol. 10 391

• T Someya T, Bao Z and Malliaras G 2016 The rise of bioelectronics Nature540 379

Characterisation techniques:• Pingree L S C, Reid O G and Ginger D S 2009 Electrical scanning probemicroscopy on active organic electronic devices Adv. Mater. 21 19

• Le Ru E C and Etchegoin P G 2009 Principles of Surface-enhanced RamanSpectroscopy (Amsterdam: Elsevier B.V)

• Deckert-Gaudig T, Taguchi A, Kawata S and Deckert V 2017 Tip-enhancedRaman spectroscopy—from early developments to recent advances Chem.Soc. Rev. 46 4077

• Richter L J, DeLongchamp D M and Amassian A 2017 Morphologydevelopment in solution-processed functional organic blend films: an in situviewpoint Chem. Rev. 117 6332

Device Stability:• Brabec C J 2004 Organic photovoltaics: technology and market Sol. EnergyMater. Sol. Cells 83 273

• Lee J U, Jung J W, Jo J W and Jo W H 2012 Degradation and stability ofpolymer-based solar cells J. Mater. Chem., 22, 24265

• Gevorgyan S A, Madsen M V, Roth B, Corazza M, Hösel M, SøndergaardR R, Jørgensen M and Krebs F C 2016 Lifetime of organic photovoltaics:status and predictions Adv. Energy Mater. 6 1501208

• Cheng P and Zhan X 2016 Stability of organic solar cells: challenges andstrategies Chem. Soc. Rev. 45 2544

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