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Electron transport layers (ETLs) have been instrumental in breaking the efficiency boundaries of solution-processed photovoltaics. In particular, bilayer ETLs with an MgO top component have afforded tremendous success in various solution-processed systems, such as perovskite photovoltaics, however, their application in the promising technology of organic photovoltaics is limited. In this work, we fabricate organic photovoltaic devices incorporating a “bilayer” ZnO/MgO ETL instead of a single ZnO ETL, so as to reduce the leakage current and boost the power conversion efficiency. The ZnO/MgO ETL is shown to have a more uniform top surface and a lower work function compared to the single ZnO ETL which is expected to be beneficial to electron extraction. Furthermore, we demonstrate that insertion of the thin (≲ 10 nm) MgO interlayer in devices leads to a reduced leakage current and an increase in the shunt resistance. Application of the MgO interlayer boosts the short circuit current density and fill factor, and enhances the power conversion efficiency by ∼10% (relative increase) thereby demonstrating a facile approach to push the efficiency of organic photovoltaics to higher levels.

Advanced optical materials 10, (2021). doi:10.1002/adom.202101116, Published by Wiley-VCH, Weinheim

We present planar perovskite solar cells incorporating thin SnO2/Al2O3 double electron transport layers between the perovskite and an indium tin oxide bottom electrode. When measured under 1 sun illumination, we obtained a maximum power conversion efficiency (PCE) of 20.1% and a steady state efficiency of 17.8% for the best cell. These values were ∼20%–30% higher in relative terms than those of cells with SnO2 only (i.e., a maximum PCE of 15.3% and a steady state PCE of 14.9%). Insertion of the thin UV-irradiated solution-processed nanoparticle Al2O3 interlayer effectively enhanced the wettability of the electron transport layer, provided enhanced interface area, as well as a lower work function, leading to improved charge extraction. Incorporation of an Al2O3 layer between the perovskite and SnO2 layers also improved the rectification ratios of the diodes as well as both series and shunt resistances. Our devices are fabricated using fully solution-processed transport and active semiconducting layers processed at low temperatures (≤150 °C).

The high power-per-weight ratio displayed by metal-halide perovskite solar cells is a key advantage of these promising devices for applications that require low payload, such as in space and avionics. However, little is known about the effect of the outer space radiation environment on these devices. Here, we report the first in operando study on fast neutron irradiation of perovskite solar cells. We show the remarkable resilience of these devices against one of the most hazardous forms of radiation that can be found at flight altitude and in space. In particular, our results highlight a comparable in operando degradation pattern between light soaked and light + neutron irradiated devices. However, whereas light-induced degradation is fully reversible, fast neutrons lead to permanent effects likely originating from atomic displacement in the active material. We also propose that such irreversible worsening is alleviated by the formation of neutron-induced shallow traps, which act as dopants and contribute to the increase of open circuit voltage and the decrease of leakage current in light + neutron irradiated devices. The high radiation dose that perovskite-based solar cells can potentially withstand renders these devices highly appealing for space and avionic applications.

Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided., Graphical abstract, Highlights • Telecommunications and biomedical sensors will hugely benefit from the development of non-toxic NIR LEDs. • Both organic and hybrid materials reported remarkable performance in the 700- to 1000-nm range. • Aggregation and the energy gap law represent the toughest hurdles to overcome for efficient NIR emission in organic materials. • Leveraging AIE, TADF, and hybrid materials have proven successful in improving efficiencies.

Visible light communication (VLC) is a wireless technology that relies on optical intensity modulation and is potentially a game changer for internet-of-things (IoT) connectivity. However, VLC is hindered by the low penetration depth of visible light in non-transparent media. One solution is to extend operation into the “nearly (in)visible” near-infrared (NIR, 700–1000 nm) region, thus also enabling VLC in photonic bio-applications, considering the biological tissue NIR semitransparency, while conveniently retaining vestigial red emission to help check the link operativity by simple eye inspection. Here, we report new far-red/NIR organic light-emitting diodes (OLEDs) with a 650–800 nm emission range and external quantum efficiencies among the highest reported in this spectral range (>2.7%, with maximum radiance and luminance of 3.5 mW/cm2 and 260 cd/m2, respectively). With these OLEDs, we then demonstrate a “real-time” VLC setup achieving a data rate of 2.2 Mb/s, which satisfies the requirements for IoT and biosensing applications. These are the highest rates ever reported for an online unequalised VLC link based on solution-processed OLEDs., Long-wavelength OLEDs: visible light communication Highly efficient organic light-emitting diodes (OLEDs) that operate in the red and near-infrared region bring new opportunities for applications involving visible light communication (VLC). Alessandro Minotto and coworkers from the UK, Poland and Italy have designed and fabricated heavy-metal-free OLEDs featuring a polymeric light-generating active region composed of a fluorescent 𝜋-expanded diketopyrrolopyrrole dye called eDPP blended in a host charge-transport matrix called F8BT. The devices emit light at 650–800 nm with an external quantum efficiency of around 2.7% and a radiance of 3.5 mW/cm2. When employed in a data communications link, error-free data rates of ~2.2 Mb/s were achieved making them suitable for a variety of internet-of-things (IoT) applications such as data-enabled biosensing implants where shorter-wavelength visible light cannot be used due to the issue of strong absorption.

In this article, we experimentally demonstrate a novel expanded nonorthogonal multiband super-Nyquist carrier-less amplitude and phase ( m -ESCAP) modulation for bandlimited organic visible light communication (VLC) systems. The proposed scheme has the same bandwidth requirement as the conventional m -CAP while breaking the orthogonality between subcarriers by purposely overlapping them. We compare m -ESCAP with the conventional m -CAP and a compressed nonorthogonal version of m -CAP ( m -SCAP) in terms of measured bit error rate (BER) performance, bit rates, and spectral efficiencies. We show that the m -ESCAP system offers improvement in the bit rate of $\sim$ 10% and 20% compared to the m -CAP and m -SCAP, respectively, and in the spectral efficiency of $\sim$ 20% compared to m -CAP. These gains are achieved at the cost of increased BER, which, however, remains below the 7% forward error correction limit.

This work reports the first observation of circularly polarized electroluminescence (CPEL) in thin films of self-organized oligothiophenes. Four new 1,4-phenylene and 9H-carbazole-based oligothiophenes were ad hoc designed to ensure efficient spontaneous formation of chiral supramolecular order. They were easily synthesized and their chiroptical properties in thin films were measured. Circularly polarized luminescence (CPL) spectra revealed glum in the order of 10−2 on a wide wavelengths range, originating from their self-organized chiral supramolecular organization. These molecules have reasonable properties as organic semiconductors and for this reason they can constitute the active layer of circularly-polarized organic light-emitting diodes (CP-OLEDs). Thus, we could investigate directly their electroluminescence (EL) and CPEL, without resorting to blends, but rather in a simple multilayer device with basic architecture. This is the first example of a CP-OLED with active layer made only of a small organic compound.

In this paper we experimentally demonstrate, for the first time, staggered carrier-less amplitude and phase (sCAP) modulation for visible light communication systems based on polymer light-emitting diodes emitting at ~639 nm. The key advantage offered by sCAP in comparison to conventional multiband CAP is its full use of the available spectrum. In this work, we compare sCAP, which utilises four orthogonal filters to generate the signal, with a conventional 4-band multi-CAP system and on-off keying (OOK). We transmit each modulation format with equal energy and present a record un-coded transmission speed of ~6 Mb/s. This represents gains of 25% and 65% over the achievable rate using 4-CAP and OOK, respectively., 6 pages, 9 figures, IEEE ICC 2019 conference

This paper investigates the use of organic polymer light emitting diodes (PLEDs) for the use in visible light communications (VLC). We prepared blue and green emitting PLEDs using commercial light-emitting polymers, and then characterised the device emission (spectrum and power), and extracted their circuit parameters for their electrical equivalent model for driving with small signals. In addition, we characterised the bandwidth ( $\boldsymbol{B_{mod}}$ ) of the devices over a period of continuous driving (∼ 4 h) and found that for the blue PLEDs the $\boldsymbol{B_{mod}}$ decreased from an initial 750 kHz to a steady state of ∼250 kHz. The green-emitting devices were found to benefit from an extended $\boldsymbol{B_{mod}}$ of ∼1.5 MHz at the beginning of the test, which then stabilised to ∼850 kHz. Furthermore, with the addition of a first order RC filter we show that, the steady state $\boldsymbol{B_{mod}}$ of the blue PLED cane be increased by a factor of ∼3, thus allowing > 1 Mbps non-return to zero on-off keying (NRZ OOK) data transmission in a complete VLC system.

Visible light communication systems often suffer from high frequency attenuation when transmitting out-of-band. This effect has been ameliorated by multi-band modulations such as multi-band carrier-less amplitude and phase (m-CAP), which minimises the effect of decreased high frequency magnitude and maximises signal-to-noise ratio-per-sub-band. On the other hand, in the pass-band region, super-Nyquist CAP (SCAP) can offer throughput improvements with no additional complexity at the receiver, at the cost of bit error rate. We propose, for the first time, a new hybrid SCAP modulation format that takes advantageous of both SCAP (i.e. overlapped sub-bands within the modulation bandwidth) and conventional m-CAP (orthogonally spaced bands outside the modulation bandwidth) while maintaining isolation between noise sources. We show higher baud rates within the passband region whilst supporting out-of-band transmission at lower error vector magnitudes.

International audience; Organic light-emitting transistors are pivotal components for emerging opto- and nanoelectronics applications, such as logic circuitries and smart displays. Within this technology sector, the integration of multiple functionalities in a single electronic device remains the key challenge. Here we show optically switchable organic light-emitting transistors fabricated through a judicious combination of light-emitting semiconductors and photochromic molecules. Irradiation of the solution-processed films at selected wavelengths enables the efficient and reversible tuning of charge transport and electroluminescence simultaneously, with a high degree of modulation (on/off ratios up to 500) in the three primary colours. Different emitting patterns can be written and erased through a non-invasive and mask-free process, on a length scale of a few micrometres in a single device, thereby rendering this technology potentially promising for optically gated highly integrated full-colour displays and active optical memory.

We present the synthesis and characterization of a series of encapsulated diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials display extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Both the absorption and emission spectra show clearer, more defined features compared to their naked counterparts demonstrating the suppression of inter and intramolecular aggregation. We find that the encapsulation results in decreased energetic disorder and a dramatic increase in backbone colinearity as evidenced by scanning tunnelling microscopy. This study paves the way for diketopyrrolopyrrole to be used in emissive solid state applications and demonstrates a novel method to reduce structural disorder in conjugated polymers.

We present new architectures in CH3NH3PbI3 based planar perovskite solar cells incorporating solution processed SnO2/MgO composite electron transport layers that show the highest power outputs ever reported for photovoltaic cells under typical 200–400 lx indoor illumination conditions. When measured under white OSRAM LED lamp (200, 400 lx), the maximum power density values were 20.2 µW/cm2 (estimated power conversion efficiency, PCE = 25.0%) at 200 lx and 41.6 µW/cm2 (PCE = 26.9%) at 400 lx which correspond to a ∼ 20% increment compared to solar cells with a SnO2 layer only (even at standard 1 sun illumination, where the maximum PCE was 19.0%). The thin MgO overlayer leads to more uniform films, reduces interfacial carrier recombination, and leads to better stability. All layers of the cells, except for the two electrodes, are solution processed at low temperatures for low cost processing. Furthermore, ambient indoor conditions represent a milder environment compared to stringent outdoor conditions for a technology that is still looking for a commercial outlet also due to stability concerns. The unparalleled performance here demonstrated, paves the way for perovskite solar cells to contribute strongly to the powering of the indoor electronics of the future (e.g. smart autonomous indoor wireless sensor networks, internet of things etc).

Large area surface microstructuring is commonly employed to suppress light reflection and enhance light absorption in silicon photovoltaic devices, photodetectors, and image sensors. To date, however, there are no simple means to control the surface roughness of III-V semiconductors by chemical processes similar to the metal-assisted chemical etching of black Si. Here, we demonstrate the anisotropic metal-assisted chemical etching of GaAs wafers exploiting the lower etching rate of the monoatomic Ga111and311planes. By studying the dependence of this process on different crystal orientations, we propose a qualitative reaction mechanism responsible for the self-limiting anisotropic etching and show that the reflectance of the roughened surface of black GaAs reduces up to ∼50 times compared to polished wafers, nearly doubling its absorption. This method provides a new, simple, and scalable way to enhance light absorption and power conversion efficiency of GaAs solar cells and photodetectors.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. The Copyright line for this article was changed on 16 July 2018 after original online publication., Due to the so‐called energy‐gap law and aggregation quenching, the efficiency of organic light‐emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light‐emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn‐on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole‐based emitter and a novel indacenodithiophene‐based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution‐processed active layer, not leveraging triplet‐assisted emission., European Community's Seventh Framework Programme. Grant Numbers: FP7/2007‐2013, 607585 H2020 ETN MSCA. Grant Number: 643238 EPSRC. Grant Number: EP/P006280/1 International Science Programme Uppsala University Royal Society Wolfson Merit Award holder

In this review, we summarise methods towards achieving 10 Mb/s connectivity for visible light communications links utilising organic polymer based light-emitting diodes as the transmitter. We present two different methods; on-off keying supported by least mean squares equalisation and orthogonal frequency division multiplexing without equalisation.

The presence of energetically low-lying triplet states is a hallmark of organic semiconductors. Even though they present a wealth of interesting photophysical properties, these optically dark states significantly limit optoelectronic device performance. Recent advances in emissive charge-transfer molecules have pioneered routes to reduce the energy gap between triplets and "bright" singlets, allowing thermal population exchange between them and eliminating a significant loss channel in devices. In conjugated polymers, this gap has proved resistant to modification. Here, we introduce a general approach to reduce the singlet-triplet energy gap in fully conjugated polymers, using a donor-orthogonal acceptor motif to spatially separate electron and hole wave functions. This new generation of conjugated polymers allows for a greatly reduced exchange energy, enhancing triplet formation and enabling thermally activated delayed fluorescence. We find that the mechanisms of both processes are driven by excited-state mixing between π-π*and charge-transfer states, affording new insight into reverse intersystem crossing.

The rapid development of the science and technology of organic semiconductors has already led to mass application of organic light-emitting diodes (OLEDs) in television monitors of outstanding quality as well as in a large variety of smaller displays found in smartphones, tablets, and other gadgets, while introduction of the technology to the illumination sector is imminent. Notably, the requirements of all such applications for emission in the visible range of the electromagnetic spectrum are well tuned to the optical and electronic properties of typical organic semiconductors, thereby representing relatively “low-hanging fruits,” in terms of material development and exploitation. However, the question arises as to whether developing materials suited for efficient near-infrared (NIR, 700–1000 nm) emission is possible, and, crucially, desirable to enable new classes of applications spanning from through-space, short-range communications to biomedical sensors, night vision, and more generally security applications to name but a few. Here, the major fundamental hurdles to be overcome to achieve efficient NIR emission from organic π-conjugated systems are discussed, recent progress is reviewed, and an outlook for further development of both materials and applications is provided.

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.A series of new near-infrared (NIR) emitting copolymers, based on a low band gap 6-(2-butyloctyl)-4,8-di(thiophen-2-yl)-[1,2,3]triazolo[4',5':4,5]benzo[1,2-c]-[1,2,5]thiadiazole (TBTTT) fluorophore copolymerized into a high band gap poly[3,3'-ditetradecyl-2,2'-bithiophene-5,5'-diyl-alt-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl] (P2TTPD) host backbone, for polymer light-emitting diode (PLED) applications is reported. PLEDs fabricated from the host polymer (P2TTPD-0) show external quantum efficiencies (EQEs) up to 0.49% at 690 nm, with turn-on voltage (Von) at only 2.4 V. By incorporating the TBTTT segments into the host polymer backbone, pure NIR emission peaking at ca. 900 nm is obtained with Von remaining below 5 V. This work demonstrates that such a low Von can be attributed to efficient intrachain energy and/or charge transfer to the TBTTT sites. When the NIR emitting copolymer (P2TTPD-10) is blended with P2TTPD-0, the TBTTT are confined to well-separated polymer chains. As a result, the EQE from the blend is lower and the Von higher than that obtained from the pure copolymer (P2TTPD-1.0) with equal content of TBTTT. An analogous copolymer (P4T-1.0), consisting of poly[3,3'-ditetradecyl-2,2':5',2'':5'',2'''-quaterthiophene-5,5'''-diyl] (P4T) as the host and 1% TBTTT as the NIR emitter, further demonstrates that pure NIR emission can be obtained only through optimized molecular orbital energy levels, as in P2TTPD-1.0, which minimizes chances for either charge trapping or exciton splitting.

White electroluminescence and fine-tuning of the emission color from binary blends of a blue-emitting polymer and a green/yellow-emitting threaded molecular wire consisting of a conjugated polymer supramolecularly encapsulated by functionalized cyclodextrins are demonstrated. Encapsulation controls the minimum intermolecular distance on the nanoscale, resulting in suppressed energy-transfer between the blend constituents and reduced formation of interchain charge-transfer complexes. The use of a green-emitting polyrotaxane significantly improves the electrical properties with respect to blends of a blue electroluminescent polyrotaxane and leads to a significant reduction in the turn-on voltage required for achieving white electroluminescence (V ON = 3V), with only 20% by weight of the encapsulated material. Blends of a blue-emitting polymer and a green/yellow-emitting threaded molecular wire emit white luminescence due to control of the minimum intermolecular distance at the nanoscale, which results in suppressed energy transfer between the blend constituents and reduced formation of interchain charge-transfer complexes. Light-emitting diodes incorporating only 20% by weight of the encapsulated material display white electroluminescence with a turn-on voltage of just 3 V. Copyright © 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Indium tin oxide (ITO), Au and Pt are materials of interest as high work function contacts for organic semiconductor devices. In this paper the relative energy level line-up of these materials is investigated both as bare surfaces or part of a polymer/conductor interface. Kelvin probe (KP) measurements show that the estimated work function for Au and Pt surfaces, evaporated under normal high vacuum (HV) fabrication conditions and measured in air, can be significantly lower (by ∼0.2 to 0.9 eV) than those of clean surfaces evaporated and measured in the more demanding and clean ultra high vacuum (UHV), that are often used as reference values. The work function of all surfaces increases significantly (from ∼0.4 to >1 eV) after an oxygen plasma, but then decreases upon air exposure, with different rates for different materials. The effect of the plasma wears off most rapidly for Au whilst it is more resilient for ITO. Most interestingly, via KP and electroabsorption measurements of the built-in potential on polymer/conductor and conductor/polymer/conductor structures, we demonstrate that a plasma-induced enhancement of the work function is “frozen in” by the application of a polymer semiconductor layer over the plasma-treated surfaces and can be made to last for years by proper device encapsulation. These results have strong implications on the understanding, fabrication, design and stability of organic semiconductor devices.

Highly oriented luminescent films are produced by stretching a 30-μm-thick polyvinyl alcohol matrix doped with water-soluble polyrotaxanes and their unthreaded analogues. Photoluminescence experiments reveal that over 95% of the emitted light is polarized along the orientation direction. A hybrid organic-inorganic light-emitting diode is built to investigate the possibility of using these films as polarizing filters for solid-state lighting and display technology. © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

The fabrication of high-resolution nanostructures in both poly(p-phenylene vinylene), PPV, and a crosslinkable derivative of poly(9,9'-dioctylfluorene), F8, using scanning near-field optical lithography, is reported. The ability to draw complex, reproducible structures with 65000 pixels and lateral resolution below 60 nm (< lambda/5) is demonstrated over areas up to 20 mu m x 20 mu m. Patterning on length-scales of this order is desirable for realizing applications both in organic nanoelectronics and nanophotonics. The technique is based on the site-selective insolubilization of a precursor polymer under exposure to the confined optical field present at the tip of an apertured near-field optical fiber probe. In the case of PPV, a leaving-group reaction is utilized to achieve insolubilization, whereas the polyfluorene is insolubilized using a photoacid initiator to create a crosslinked network in situ. For PPV, resolubilization of the features is observed at high exposure energies. This is not seen for the crosslinked F8 derivative, r-F8Ox, allowing us to pattern structures up to 200 nm in height.

Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkable upon exposure to an acid. Upon addition of a photoacid they become photo-cross-linkable if exposed to UV light. Optical and electrochemical properties of these copolymers, both in the soluble and cross-linked forms are compared. We further show that the use of this type of copolymer allows the fabrication of multilayer structures without restriction of the organic solvent used to deposit an upper layer. In particular, we fabricate multilayer LEDs with enhanced luminous efficiency using one of the copolymers. The possibility to photo-cross-link these materials is also used to prepare micrometer size patterns, using a transmission electron microscope grid as a shadow mask. Furthermore, the use of a scanning near-field optical microscope allows submicron patterns to be prepared. We propose that this approach, which can be extended to a wide range of conjugated polymers, can be useful for the fabrication of optical, electronic and optoelectronic structures.

The development of semiconducting polymers with high solid-state luminescence efficiencies has enabled the fabrication of efficient polymer light-emitting diodes. Luminescence is often quenched in well-ordered molecular solids, as a result of inter-molecular dipolar coupling, and the general observation of efficient luminescence in semiconducting polymers is unexpected. We report here the synthesis and characterisation of a series of model 'glassy' poly(arylenevinylene)s where we control the cis to trans ratio about the vinylene linkage and also the phenylene linkage geometry. Photoluminescence efficiency is enhanced for more disordered materials, with highest values for 50 : 50 cis : trans ratios, when it exceeds 50%. We also find that the free volume associated with these glassy disordered polymers allows conformational relaxation of the excitonic state, via ring rotation at the vinylene linkage, causing a large Stokes' shift of the emission. We propose that high luminescence efficiency in these glassy polymers is due to emission from the more disordered regions, and that two effects due to disorder are simultaneously required: firstly that these regions are luminescent (prevention of aggregation by disorder), and, secondly, that the electronic excited state (exciton) can lower its energy below its value in more ordered regions by means of the ring-rotational coupling.

The vast majority of perovskite solar cell research has focused on organic-inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current-voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current-voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current-voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices.

Conventional CMOS technologies require high production costs, which are largely due to masks production and high specification equipment. Interestingly, conjugated polymers and plastic semiconductors offer potential substitutes to lower the production expenses. Such materials can be used in applications such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaics (OPVs). However, electrical reliability, and especially potential damage by electrostatic discharge (ESD), is a major issue in the production cycle and cost, and also during operation. Here we present investigation of the ESD sensitivity of OFETs based on poly (3-hexylthiophene-2,5-diyl) (P3HT), which has so far received little or no attention in the literature.

In this work, a novel and straightforward technology for the fabrication of two-dimensional (2D) photoluminescent light source arrays by selective infiltration of conjugated luminescent polymers into three-dimensional (3D) silicon microstructures is presented. Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) integration into 3D silicon microstructures is investigated by means of three different deposition techniques, namely, spin-coating, dip-coating and drop-casting/slow solvent evaporation. The microstructure is fabricated by electrochemical micromachining (ECM) technology and integrates 2D arrays of square holes with different sizes (about 40 and 4 μm), spatial periods (about 70 and 10 μm), and aspect ratios (ARs) (about one and ten). Notably, square holes with higher AR can be selectively filled with polymer using spin-coating and drop-casting techniques, whereas dip-coating technique allows selective polymer filling of square holes with lower AR. Independently of size, period and AR, each polymer-infiltrated hole behaves as a single light source, thus enabling the effective synthesis of 2D photoluminescent light source arrays.

We report our investigations of the photophysical properties of a so-called “donor-acceptor” (D-A) electroluminescent polymer incorporating N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide (DI) as the electron acceptor and benzo[1,2-b:4,5-b′]dithiophene (BDT) acceptor units. Polymer light-emitting diodes (PLEDs) based on this copolymer give the best efficiency (external quantum efficiency EQE∼0.04%) and show a pure near-infrared (NIR) electroluminescence (EL) with a peak at 870 nm.

In this paper scanning near-field microscopy is used to characterize polymer blends for photovoltaic applications, and fluorescence imaging and photoconductivity are combined to elucidate the spatial distribution and relative efficiency of current generation and photoluminescence in different domains of compositionally heterogeneous films. Focus is placed oil a binary system consisting of poly [(9,9-dioctylfluorene) -alt-benzothiadiazole] (F8BT) and poly[(9,9-dioctylfluorene)-alt-(bis(NN-(4-butylphenyl))-bis(N,N'-phenyl-1,4-phenylenediamine))] (PFB), spun from xylene solutions, so as to obtain phase separation on micrometer and nanometer length scales. Protruding regions with diameters of about 5 mu m in the topography image coincide with regions of high photocurrent (PC) and luminescence; these regions are identified as being F8BT rich. A general method to estimate the photoluminescence efficiency in the different domains of phase-separated blends is proposed. As expected, lack of enhancement of the PC signal at the boundaries between protruding and lower-lying phases indicate that these microscale boundaries play a small role in the charge generation by exciton splitting. This is consistent with the domains compositional inhomogeneity, and thus with finer phase separation within the domains. We also provide all analysis of the extent to which the metallized probe perturbs the near-field photocurrent signal by integrating Poisson's equation. Finally, by using a Bethe-Bouwkamp, model, the energy absorbed by the polymer film in the different regions is estimated.

We present the electro-optical properties of light-emitting diodes (LEDs) based on two blue-emitting polyfluorenes, with aluminum cathodes and having indium-tin oxide (ITO) anodes modified by self-assembled monolayers (SAM) of benzenesulphenyl chloride derivatives. The modification of ITO surface was confirmed by contact angle measurements. Electroabsorption measurements were used to assess the modification of ITO work function upon SAM formation. We find that the use of a benzenesulphenyl chloride derivative containing a nitro group is as effective at improving LEDs characteristics as the use of a hole-injection layer of well known PEDOT:PSS.

We report the fabrication of a two-dimensional periodic structure made from the conjugated, fluorescent polymer poly(p-phenylene vinylene), PPV, by direct-write near-field lithography of a photosensitive precursor of the conjugated polymer. The precursor film is illuminated with ultraviolet radiation using the apertured fibre of a home-built scanning near-field optical microscope (SNOM), and unexposed areas are subsequently dissolved in methanol. The fully conjugated polymer is obtained by thermal conversion under vacuum. The structure is approximately 10 mu m x 10 mu m, and consists of 30 nm tall pillars on a 330 nm periodicity lattice. We present a calculation of the photonic properties and propose that structures made with the method investigated here are promising candidates for photonic crystal lasers. We also report the results of preliminary investigations aimed at increasing the pillar height.

We describe the application of scanning near-field optical microscopy (SNOM) to the study of the photophysical and self-organization properties of thin films of blends of conjugated polymers, and also to the lateral nanoscale patterning of conjugated-polymer structures. Such thin-film plastic semiconductor nanostructures offer significant potential for use in opto-electronic devices. The implementation of SNOM we employ is the most established form in which a probe with a sub-wavelength aperture is scanned in close proximity to the sample surface. We consider the nature of the near-field optical distribution, which decays within the first ca. 100 nm of these semiconductor materials, and address the identification of topographic artefacts in near-field optical images. While the topographic information obtained simultaneously with optical data in any SNOM experiment enables an easy comparison with the higher-resolution tapping-mode atomic force microscopy, the spectroscopic contrast provided by fluorescence SNOM gives an unambiguous chemical identification of the different phases in a conjugated-polymer blend. Both fluorescence and photoconductivity SNOM indicate that intermixing of constituent polymers in a blend, or nanoscale phase separation, is responsible for the high efficiency of devices employing these materials as their active layer. We also demonstrate a scheme for nano-optical lithography with SNOM of conjugated-polymer structures, which has been employed successfully for the fabrication of poly(-phenylene vinylene) nanostructures with 160 nm feature sizes.

Oxygen-plasma treatment of indium-tin oxide (ITO) anodes is now widely used as one of the most effective ways to improve the device performance of organic light-emitting diodes (LEDs). However, the role of oxygen-plasma treatment has not been clearly understood. We have performed detailed studies of the surface and bulk of the ITO thin films exposed to oxygen-plasma. We employed a multitude of experimental techniques, including X-ray and ultraviolet photoelectron spectroscopies, atomic force microscopy, dynamic contact angle measurement, four-point probe and Hall measurements to investigate the changes induced by the plasma. We have also analyzed the device characteristics of polymer LEDs fabricated with these anodes. We found significant modifications of the physico-chemical, morphological, transport and optical properties of the oxygen-plasma treated ITO. Although oxygen-plasma does not show any measurable etching effect, it induces considerable changes leading to an increase in work function, electron carrier concentration and conductivity. It also increases the surface energy and polarity. We relate these modifications to enhancement of the device performance, such as electroluminescence efficiency and lifetime, through their effects on hole injection, and interface structure and stability. Finally, we show that even in the presence of a hole-transport layer such as a poly(styrene sulphonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) inserted between the anode and the emissive polymer layer, oxygen-plasma treatment of the ITO anodes is still beneficial for the devices.

To fully exploit the properties of light-emitting polymers (LEPs) in electroluminescent applications, it is of paramount importance to develop efficient electrical contacts. An ideal electrode is highly conductive, stable, provides a low barrier to carrier injection, and does not degrade the LEP upon contact. It is difficult to find a single homogeneous material that satisfies all of these requirements. Hence, contact optimization has often required the development of multilayer structures. In particular, indium tin oxide covered by a film of poly(ethylene-dioxythiophene):poly(styrene sulfonic acid) {ITO/PEDOT:PSS} has become a favorite combination for the transparent anode, and heterostructures of LiF and CsF with metals (Al and Ca) have proven to be efficient electron-injecting contacts. Here we review our progress in the understanding of the operation of light-emitting diodes incorporating such contacts, in particular by gauging the materials' energy-level lineup via electroabsorption measurements. Among the series of LEDs investigated, using a high-energy-gap blue polyfuorene polymer, CsF/Ca/Al and LiF/Ca/Al electrodes lead to the best improvements in electron injection. The most promising performance for applications, where a high luminance (∼1600 cd/m2 at 5 V) is also accompanied by a high maximum efficiency (∼3 lm/W), was obtained with LiF/Ca/Al cathodes and ITO/PEDOT:PSS anodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2649–2664, 2003

The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor device physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we review our progress in the understanding of the energy level line-up in finished, blue-emitting, polyfluorene-based light-emitting diodes, which exploit LiF and CsF thin films in combination with Ca and Al to obtain cathodes with low injection barriers. We have used electroabsorption measurements, as they allow the noninvasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/cathode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky–Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g., Bardeen model), except for electrodes that extensively react with the polymer, and introduce deep gap states. In addition, we show results that validate the approximation of rigid tilting of polymer energy levels with bias (for biases for which no significant injection of carriers occurs). To investigate further the consequences of the electronic line-up on device operation, we complemented the electroabsorption measurements with characterization of the emissive and transport properties of the light-emitting diodes, and confirmed that the cathodic barrier lowering in CsF/Ca/Al and LiF/Ca/Al electrodes leads to the best improvements in electron injection. We found that luminance and overall current are greatly affected by the barrier-reducing cathodes, indicating a truly bipolar transport, with comparable electron and hole currents. We also found significant indications of CsF/Ca/Al cathodes strongly reacting with the polymer, which is suggestive of CsF dissociation and diffusion in the bulk of the polymer.

We study the effect of indium-tin oxide surface modification by self assembling of highly polar molecules on the performance of single-layer light-emitting diodes (LEDs) fabricated with polyfluorene blends and aluminium cathodes. We find that the efficiency and light-output of such LEDs is comparable to, and sometimes better than, the values obtained for LEDs incorporating a hole injection layer of poly(3,4-ethylene dioxythiophene) doped with polystyrene sulphonic acid. This effect is attributed to the dipole-induced work function modification of indium-tin oxide.

A series of alternating copolymers of the type A- alt -B, where A is 9,9-bis(2′-ethylhexyl)fluorene and B is a thiophene-based moiety, was synthesised via the palladium-catalysed Suzuki coupling. These copolymers were characterised by optical and electrochemical methods and used in the fabrication of light-emitting diodes. We find that upon increasing the conjugation length of the B moiety, from a thiophene unit to 2,5-bis(2′-vinyl thienyl)thiophene, the fluorescence quantum yields are reduced both in solution (from 51 to 11%) and in solid state. Both absorption and emission are red-shifted, with the solid state emission maximum increasing from 476 nm (green) to 600 nm (orange). When the B unit is changed from thiophene to thiophene- S , S -dioxide, there is a significant increase in both the ionisation potential and electron affinity, and a concurrent reduction of the solution fluorescence efficiency (down to 16%). These polymers appear promising materials for optoelectronic applications.

We present picosecond time-resolved studies of thin films of two polyfluorene blends, combining a relatively high energy-gap poly(9,9-dioctylfluorene), PFO, and a polyfluorene derivative with a smaller gap. These studies show both efficient energy transfer from the donor to the acceptor, as well as spatial confinement of the excitons in the acceptor polymers. The optoelectronic characteristics of light-emitting diodes fabricated with such blends reflect the contribution of these two processes. The electroluminescence is almost exclusively from the acceptor polyfluorenes and we observe a significant improvement of the electroluminescence efficiency of the light-emitting diodes fabricated with such blends in comparison to those based on neat films of the polyfluorenes.

We report the incorporation of graphene-oxide quantum dots (GOQDs) into films, diluted solutions, and light-emitting diodes (LEDs) as part of a water-soluble derivative of poly(p-phenylene vinylene), or PDV.Li, to investigate their impact on the light-emission properties of this model conjugated polymer. Despite the well-known ability of graphene and graphene oxide to quench the photoluminescence of nearby emitters, we find that the addition of GOQDs to diluted solutions of PDV.Li does not significantly affect the photoluminescence (PL) dynamics of PDV.Li, bringing about only a modest quenching of the PL. However, loading the polymer with GOQDs led to a substantial decrease in the turn-on voltage of LEDs based on GOQD–PDV.Li composites. This effect can be attributed to either the improved morphology of the host polymer, resulting in an increase in the charge mobility, or the enhanced injection through GOQDs near the electrodes.

Conjugated polyrotaxanes are conjugated polymeric semiconductors engineered at a supramolecular level by threading the conjugated moiety into molecular macrocycles, such as cyclodextrins (CD). CD-threaded rotaxanes thus provide a class of model compounds with reduced interchain interactions which enable us to explore the influence of such interactions on the fundamental photophysics of conjugated semiconductors. CD rotaxination also endows these materials with additional sites for functionalization, thus resulting in extremely versatile structures. Our current understanding of the photophysics of these materials is reviewed, both in solid/liquid solutions and in neat films, as a function of the relevant parameters, such as the threading ratio and the concentration, and with the help of rotaxanes incorporating a variety of different backbones.

We present a newly designed polymer light-emitting diode with a bandwidth of similar to 350 kHz for high-speed visible light communications. Using this new polymer light-emitting diode as a transmitter, we have achieved a record transmission speed of 10 Mb/s for a polymer light-emitting diode-based optical communication system with an orthogonal frequency division multiplexing technique, matching the performance of single carrier formats using multitap equalization. For achieving such a high data-rate, a power pre-emphasis technique was adopted. (C) 2014 Optical Society of America