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The majority of organic semiconductors have a low relative dielectric constant (ϵr < 6), which is an important limitation for organic solar cells (OSCs). A high dielectric constant would reduce the exciton binding energy, reduce charge carrier recombination losses, and thereby enhance the overall device performance of OSCs. However, the development of organic/polymeric semiconductors with higher relative dielectric constants (ϵr > 6) has attracted a very limited attention. Moreover, high performance OSCs based on high dielectric constant photovoltaic materials are still in their infancy. Herein, we report an oligoethylene oxide side chain-containing non-fullerene acceptor (ITIC-OE) with a high relative dielectric constant of ϵr ≈ 9.4, which is two times larger than that of its alkyl chain-containing counterpart ITIC. Encouragingly, the OSCs based on ITIC-OE show a high power conversion efficiency of 8.5%, which is the highest value for OSCs that employ high dielectric constant materials. Nevertheless, this value is lower than those of ITIC-based control devices. The less phase-separated morphology in blend films due to the reduced crystallinity of ITIC-OE and the too good miscibility between PBDB-T and ITIC-OE are responsible for the lower device performance. This work suggests additional prerequisites to make high dielectric constants play a significant role in OSCs.

Polymer solar cells based on PDPP5T and PCBM as donor and acceptor materials, respectively, were processed from aqueous nanoparticle dispersions. Careful monitoring and optimization of the concentration of free and surface-bound surfactants in the dispersion, by measuring the conductivity and ζ-potential, is essential to avoid aggregation of nanoparticles at low concentration and dewetting of the film at high concentration. The surfactant concentration is crucial for creating reproducible processing conditions that aid in further developing aqueous nanoparticle processed solar cells. In addition, the effects of adding ethanol, of aging the dispersion, and of replacing [60]PCBM with [70]PCBM to enhance light absorption were studied. The highest power conversion efficiencies (PCEs) obtained are 2.0% for [60]PCBM and 2.4% for [70]PCBM-based devices. These PCEs are limited by bimolecular recombination of photogenerated charges. Cryo-TEM reveals that the two components phase separate in the nanoparticles, forming a PCBM-rich core and a PDPP5T-rich shell and causing a nonoptimal film morphology.

We report the results of a systematic study of the interplay of triplet-polaron quenching (TPQ) and triplet-triplet annihilation (TTA) on the efficiency roll-off of organic light-emitting diodes (OLEDs) with increasing current density. First, we focus on OLEDs based on the green phosphorescent emitter tris[2-phenylpyridine]iridium(III) (Ir(ppy)3) and the red phosphorescent dye platinum octaethylporphyrin. It is found that the experimental data can be reproduced using kinetic Monte Carlo (kMC) simulations within which TPQ and TTA are due to a nearest-neighbor (NN) interaction, or due to a more long-range Förster-type process. Furthermore, we find a subtle interplay between TPQ and TTA: decreasing the contribution of one process can increase the contribution of the other process, so that the roll-off is not significantly reduced. Furthermore, we find that just analyzing the shape of the roll-off is insufficient for determining the relative role of TPQ and TTA. Subsequently, we investigate the wider validity of this picture using kMC simulations for idealized but realistic symmetric OLEDs, with an emissive layer containing a small concentration of phosphorescent dye molecules in a matrix material. Whereas for NN-interactions the roll-off can be reduced when the dye molecules act as shallow hole and electron traps, we find that such an approach becomes counterproductive for long-range TTA and TPQ. Developing well-founded OLED design rules will thus require that more quantitative information is available on the rate and detailed mechanism of the TPQ and TTA processes.

We investigate the effect of introducing thiazole π-spacers in wide bandgap semiconducting copolymers based on benzothiadiazole with either benzodithiophene or indacenodithiophene. Thiazole-containing polymers show similar optical bandgaps but drastically downshifted frontier orbital levels compared to the corresponding thiophene derivatives as a result of the more electronegative imine nitrogen. Consequently, they provide an enhanced open-circuit voltage (Voc) of about 0.2 V in bulk heterojunction polymer solar cells. Notably, the Voc approaches 0.95 V for polymers with an optical bandgap below 1.75 eV. Despite the favourable energetics, the performance of the thiazole-containing polymers in solar cells is currently restricted by the relatively low molecular weight, which results in a sub-optimal bulk heterojunction morphology.

The effect of side chain length on the photovoltaic properties of conjugated polymers is systematically investigated with two sets of polymers that bear different alkyl side chain lengths based on benzodithiophene and benzo[2,1,3]thiadiazole or 5,6-difluorobenzo[2,1,3]thiadiazole. Characterization of the photovoltaic cells reveals a strong interdependency between the side chain length of conjugated polymers and photovoltaic performances (power conversion efficiency, short-circuit current, and fill factor) of the resulting bulk-heterojunction (BHJ) solar cells. Charge carrier transport and external quantum efficiency (EQE) measurements in combination with morphology characterization suggest that too long side chains lead to deteriorated charge transport, suboptimal BHJ morphology, considerable bimolecular recombination, and consequently poor photovoltaic performances. On the other hand, when the side chains are too short, they cannot afford a high enough solubility and molecular weight for the resulting polymers and produce poor solar cell performance as well. This study shows that side chain optimization is of significant importance to maximize the potential of photovoltaic active conjugated polymers, which indicates the fruitful molecular design rules toward highly efficient BHJ polymer solar cells.

Tandem polymer solar cells are used for light-driven electrochemical water splitting. To attain a high enough electrochemical potential a new wide band gap electron donor polymer (PTPTIBDT-OD) is developed and used in combination with [70]PCBM as an electron acceptor in a tandem device architecture with two identical photoactive layers. This homo-tandem device comprises an intermediate ZnO/PEDOT:PSS/MoO3 charge recombination layer to connect the two subcells electrically and optically. The homo-tandem solar cell has an open-circuit voltage of 1.74 V and reaches a power conversion efficiency (PCE) of 5.3%. In combination with RuO2 as the electrocatalyst for oxygen evolution and RuO2 or Pt catalysts for hydrogen evolution, sunlight-driven electrochemical water splitting occurs with a solar-to-hydrogen conversion efficiency of ηSTH = 4.3%. Owing to the very high fill factor of the polymer tandem cell (0.73), water splitting takes place near the maximum power point of the homo-tandem solar cell. As a consequence, the difference between PCE and ηSTH is only due to the overpotential losses.

Photoelectrochemical water splitting is demonstrated in an organic artificial leaf composed of a triple junction polymer solar cell for light absorption and charge generation and low-overpotential catalytic electrodes for hydrogen and oxygen evolution. For small area solar cells (

Polymer nanowires based on two diketopyrrolopyrrole conjugated polymers with similar chemical structures are shown to have distinct "edge-on" and "face-on" configurations in addition to high well-balanced hole and electron mobilities of 5.47 and 5.33 cm(2) V(-1) s(-1) in field-effect transistors.

The characteristic doping process in polymer light-emitting electrochemical cells (LECs) causes a tradeoff between luminescence intensity and efficiency. Experiments and numerical modeling on thin film polymer LECs show that, on the one hand, carrier injection and transport benefit from electrochemical doping, leading to increased electron-hole recombination. On the other hand, the radiative recombination efficiency is reduced by exciton quenching by polarons involved in the doping. Consequently, the quasi-steady-state luminescent efficiency decreases with increasing ion concentration. The transient of the luminescent efficiency shows a characteristic roll-off while the current continuously increases, attributed to ongoing electrochemical doping and the associated exciton quenching. Both effects can be modeled by exciton polaron-quenching via diffusion-assisted Förster resonance energy transfer. These results indicate that the tradeoff between efficiency and intensity is fundamental, suggesting that the application realm of future LECs should be sought in high-brightness, low-production cost devices, rather than in high-efficiency devices. A fundamental tradeoff between emission intensity and efficiency in light-emitting electrochemical cells is reported. The admixed ions in LECs on the one hand improve charge transport by electrochemical doping, but on the other hand reduce the luminescent efficiency by quenching of excitons for KCF3SO3 densities of >1025 m-3.

New alternating copolymers based on benzo[1,2-b:4,5-b′]dithiophene and thieno[3,4-d]thiazole derivatives have been synthesized by using different combinations of side chains and spacers in order to study their effect on the optical, electrochemical and photovoltaic properties. Optical and electrochemical characterization were performed and the resulting copolymers exhibit optical bandgaps ranging from 1.66 to 1.80 eV, HOMO energy levels between −5.19 and −5.30 eV and LUMO energy levels between −3.60 and −3.72 eV. Photovoltaic devices were fabricated and characterized after a careful optimization of the active layer fabrication by using different solvents, additives and acceptors. Following this process, PCEs up to 4.4% were obtained.

Diketopyrrolopyrrole-based conjugated polymers bridged with thiazole units and different donors have been designed for polymer solar cells. Quantum efficiencies above 50% have been achieved with energy loss between optical band gap and open-circuit voltage below 0.6 eV.

Four different thiazole-flanked diketopyrrolopyrrole-based polymers were applied as an electron acceptor in bulk heterojunction solar cells with poly(3-hexylthiophene) as an electron donor. Power conversion efficiencies of 1.5% to 3.0% were achieved with a spectral response from 350 to 950 nm.

In ternary bulk heterojunction solar cells based on a semiconducting biphenyl-dithienyldiketopyrrolopyrrole copolymer donor and two different fullerene acceptors that distinctly differ in electron affinity, the open-circuit voltage is found to depend in a slightly sublinear fashion on the relative ratio of the two fullerenes in the blend. Similar effects have previously been observed and have been attributed to the formation of an alloyed fullerene phase possessing electronic levels that are the weighted average of the two components. By analyzing the contribution of the charge transfer (CT)-state absorption to the external quantum efficiency of the ternary blend solar cells as a function of composition, we find no evidence for a CT state formed between the polymer and an alloyed fullerene phase. Rather, the results are consistent with the presence of two distinct CT states, one for each polymer–fullerene combination. The two-state CT model does not, however, explain the sublinear behavior of the open-circuit voltage as a function of the blend composition.

The performance of organic light-emitting diodes (OLEDs) is determined by a complex interplay of the optoelectronic processes in the active layer stack. In order to enable simulation-assisted layer stack development, a three-dimensional kinetic Monte Carlo OLED simulation method which includes the charge transport and all excitonic processes is developed. In this paper, the results are presented of simulations including degradation processes in idealized but realizable phosphorescent OLEDs. Degradation is treated as a result of the conversion of emitter molecules to non-emissive sites upon a triplet-polaron quenching (TPQ) process. Under the assumptions made, TPQ provides the dominant contribution to the roll-off. There is therefore a strong relationship between the roll-off and the lifetime. This is quantified using a "uniform density model", within which the charge carrier and exciton densities are assumed to be uniform across the emissive layer. The simulations give rise to design rules regarding the energy levels, and are used to study the sensitivity of the roll-off and lifetime to various other materials parameters, including the mobility, the phosphorescent dye concentration, the triplet exciton emissive lifetime and binding energy, and the type of TPQ process.

The influence of cross-conjugation on the optical and electrochemical properties of donor–acceptor copolymers is investigated. Isoindigo, substituted at the 6,6' or the 5,5' positions, and thieno[3,2-b]thiophene and thieno[2,3-b]thiophene are taken as conjugated and cross-conjugated electron-deficient and electron-rich building blocks from which four isomeric donor–acceptor polymers were synthesized. Introducing cross-conjugation in isoindigo has only a small effect on the electrochemical band gap and on the onset of the absorption, which remains in the near-infrared. The optical absorption spectra, however, differ strongly because cross-conjugation strongly reduces the absorption coefficient. DFT calculations confirm that the transition to the lowest excited singlet state has a small oscillator strength in cross-conjugated isoindigo model compounds. Cross-conjugation in thienothiophene exerts a different effect. It causes a moderate but distinct blue-shift of the optical absorption and a deeper HOMO energy level.

Layer deposition of organometal halide perovskites for solar cells usually involves tedious experimentation, in which numerous devices are tested to determine the ideal processing conditions. One of the important issues is determining the optimum time and temperature for thermal annealing of the perovskite layer. Here we demonstrate that in-situ photoluminescence allows to determine the optimal annealing procedure without fabricating complete solar cells. We use a deposition method in which dense layers of perovskite crystals are formed within seconds in ambient air by hot casting a mixture of lead acetate, lead chloride, and methylammonium iodide. The as-cast perovskite layers are highly luminescent because charge carriers are unable to reach the charge extraction layers that quench the photoluminescence. Thermal annealing enhances charge transport and quenches the photoluminescence, but deteriorates the photovoltaic performance via decomposition of the perovskite if applied for a too long time. We demonstrate that the optimal annealing time coincides with the time required for the in-situ measured photoluminescence intensity to reach its baseline value for annealing temperatures in the range of 80-100 °C. This results in efficient (>14%) perovskite solar cells and shows that in-situ photoluminescence is a simple but powerful tool for in-line quality monitoring of perovskite films.

A dual-gate organic field-effect transistor is investigated for electrically pumped lasing. The two gates can independently accumulate electrons and holes, yielding current densities exceeding the lasing threshold. Here, the aim is to force the electrons and holes to recombine by confining the charges in a single semiconducting film. It is found that independent hole and electron accumulation is mutually exclusive with vertical recombination and light emission. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The nature of the solubilizing alkyl side chains has a strong effect on the performance of semiconducting diketopyrrolopyrrole polymers in organic solar cells with fullerene acceptors. The effect relates to the width of semicrystalline polymer fibrils that form in these blends. If the width of the fibril is wider than the exciton diffusion length, fewer charges form and the efficiency drops. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We study the occurrence and effect of intrachain homocoupling defects in alternating push-pull semiconducting PDPPTPT polymers based on dithienyl-diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT-TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5-5.6% for polymers containing 20% to 5% defects. © 2014 American Chemical Society.

Isomeric random and regular alternating p-conjugated terpolymers comprising diketopyrrolopyrrole (DPP), thienopyrrolodione (TPD), and bithiophene (2T) were synthesized to study the effect of the sequential distribution of monomeric units on the semiconducting properties. The optical and electrochemical properties and the performance in photovoltaic cells of the random and regular terpolymers are found to be significantly different. DPP2T-rich sections in the random terpolymer cause higher HOMO and deeper LUMO energy levels and a smaller optical band gap compared to the regular terpolymer. The randomization of DPP and TPD units along the chain has a negative effect on the photovoltaic performance, resulting in power conversion efficiencies of merely 1.0% for the random terpolymer while a more favorable efficiency of 5.3% is obtained for the regular terpolymer when combined with a fullerene acceptor.

An important loss mechanism in organic electroluminescent devices is exciton quenching by polarons. Gradual electrochemical doping of various conjugated polymer films enabled the determination of the doping density dependence of photoluminescence quenching. Electrochemical doping was achieved by contacting the film with a solid electrochemical gate and an injecting contact. A sharp reduction in photoluminescence was observed for doping densities between 1018 and 1019 cm 3. The doping density dependence is quantitatively modeled by exciton diffusion in a homogeneous density of polarons followed by either Förster resonance energy transfer or charge transfer. Both mechanisms need to be considered to describe polaron-induced exciton quenching. Thus, to reduce exciton-polaron quenching in organic optoelectronic devices, both mechanisms must be prevented by reducing the exciton diffusion, the spectral overlap, the doping density, or a combination thereof. © 2014 American Physical Society.

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes. Funding Agencies|NanoNextNL [06D.03]

Top-illuminated, indium tin oxide (ITO)-free, tandem polymer solar cells are fabricated on opaque substrates in an inverted device configuration. In the tandem cell, a wide band gap subcell, consisting of poly[N-9'- heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2', 1',3'-benzothiadiazole)] (PCDTBT) blended with [70]PCBM is combined with a small band gap subcell consisting of a mixture of poly[{2,5-bis(2- hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}-alt-{[2, 2'-(1,4-phenylene)bisthiophene]-5,5'-diyl}] (PDPPTPT) and [60]PCBM. Compared to the more common bottom-illuminated inverted tandem polymer solar cells on transparent ITO substrates, the front and back cells must be reversed when using opaque substrates and a transparent and conductive top contact must be employed to enable top illumination. A high conductive poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer in combination with Ag lines surrounding the active area as current collection electrode is used for this purpose. The tandem polymer solar cell on an opaque glass/metal substrate yields a power conversion efficiency of 6.1% when the thicknesses of the photoactive layers are balanced for optimum performance. This is similar to the equivalent inverted tandem device fabricated on a transparent glass/ITO substrate. © 2014 American Chemical Society.

Three isomeric p-conjugated molecules based on diketopyrrolopyrrole and bithiophene (DPP2T) substituted with hexyl side chains in different positions are investigated for use in solution-processed organic solar cells. Efficiencies greater than 3% are obtained when a mild annealing step is used. The position of the side chains on the DDP2Ts has a major influence on the optical and electronic properties of these molecules in thin semicrystalline films. By combining optical absorption and fluorescence spectroscopy, with microscopy (AFM and TEM) and scattering techniques (GIWAXS and electron diffraction), we find that the position of the side chains also affects the morphology and crystallization of these DPP2Ts when they are combined with a C70 fullerene derivative in a thin film. The study demonstrates that changing the side chain position is an additional, yet complex, tool to influence behavior of conjugated molecules in organic solar cells.

Developing novel materials that tolerate thickness variations of the active layer is critical to further enhance the efficiency of polymer solar cells and enable large-scale manufacturing. Presently, only a few polymers afford high efficiencies at active layer thickness exceeding 200 nm and molecular design guidelines for developing successful materials are lacking. It is thus highly desirable to identify structural factors that determine the performance of semiconducting conjugated polymers in thick-film polymer solar cells. Here, it is demonstrated that thiophene rings, introduced in the backbone of alternating donor-acceptor type conjugated polymers, enhance the fill factor and overall efficiency for thick (>200 nm) solar cells. For a series of fluorinated semiconducting polymers derived from electron-rich benzo[1,2-b: 4,5-b'] dithiophene units and electron-deficient 5,6-difluorobenzo[2,1,3] thiazole units a steady increase of the fill factor and power conversion efficiency is found when introducing thiophene rings between the donor and acceptor units. The increased performance is a synergistic result of an enhanced hole mobility and a suppressed bimolecular charge recombination, which is attributed to more favorable polymer chain packing and finer phase separation.

The effects of cold and hot processing on the performance of polymer-fullerene solar cells are investigated for diketopyrrolopyrrole (DPP) based polymers that were specifically designed and synthesized to exhibit a strong temperature-dependent aggregation in solution. The polymers, consisting of alternating DPP and oligothiophene units, are substituted with linear and second position branched alkyl side chains. For the polymer-fullerene blends that can be processed at room temperature, hot processing does not enhance the power conversion efficiencies compared to cold processing because the increased solubility at elevated temperatures results in the formation of wider polymer fibres that reduce charge generation. Instead, hot processing seems to be advantageous when cold processing is not possible due to a limited solubility at room temperature. The resulting morphologies are consistent with a nucleation-growth mechanism for polymer fibres during drying of the films. We thank Hans van Franeker and Fallon Colberts for TEM experiments. G. H. L. Heintges acknowledges the Agency for Innovation by Science and Technology in Flanders (IWT). Part of the work was performed in the framework of the TripleSolar (ERC Adv Grant No. 339031) project, and received funding from the Ministry of Education, Culture and Science (Gravity program 024.001.035).

The light collection in thin film polymer solar cells is substantially improved by application of a textured retroreflective foil, which reduces primary reflection and outcoupling of unabsorbed light. Consequently the external quantum efficiency (EQE) improves over the whole sensitivity range and the power conversion efficiency is improved by as much as 19%.

All-solution processed organic solar cells with a conventional device structure were demonstrated. The evaporated low work function LiF/Al electrode was replaced by a printed high work function silver electrode combined with an additional electron transport layer (ETL). Two electron transport layers were tested: (I) zinc oxide (ZnO) nanoparticles and (II) poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9, 9-dioctylfluorene)] (PFN). Devices with printed silver nanoparticle inks on top of the ZnO electron transport layer lead to crack formation in the silver layer during the drying and sintering. The crack formation was avoided by using PFN as electron transport layer. The sputtered high work function ITO electrode was substituted by a printed composite electrode containing inkjet-printed silver grids in combination with high conductivity poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS). All-solution processed solar cells demonstrated a power conversion efficiency of 1.94%. © 2013 Elsevier B.V.

Solution processed polymer:fullerene solar cells on opaque substrates have been fabricated in conventional and inverted device configurations. Opaque substrates, such as insulated steel and metal covered glass, require a transparent conducting top electrode. We demonstrate that a high conducting (900 S cm-1) PEDOT:PSS layer, deposited by a stamp-transfer lamination technique using a PDMS stamp, in combination with an Ag grid electrode provides a proficient and versatile transparent top contact. Lamination of large size PEDOT:PSS films has been achieved on variety of surfaces resulting in ITO-free solar cells. Power conversion efficiencies of 2.1% and 3.1% have been achieved for P3HT:PCBM layers in inverted and conventional polarity configurations, respectively. The power conversion efficiency is similar to conventional glass/ITO-based solar cells. The high fill factor (65%) and the unaffected open-circuit voltage that are consistently obtained in thick active layer inverted geometry devices, demonstrate that the laminated PEDOT:PSS top electrodes provide no significant potential or resistive losses.

A high-molecular-weight conjugated polymer based on alternating electron-rich and electron-deficient fused ring systems provides efficient polymer solar cells when blended with C60 and C70 fullerene derivatives. The morphology of the new polymer/fullerene blend reduces bimolecular recombination and allows reaching high fill factors and power conversion efficiencies for films up to 300 nm thickness.

For its application as transparent conductor in light-emitting diodes and photovoltaic cells, both the in-plane and out-of-plane conductivity of PEDOT:PSS are important. However, studies into the conductivity of PEDOT:PSS rarely address the out-of-plane conductivity and those that do, report widely varying results. Here a systematic study of the out-of-plane charge transport in thin films of PEDOT:PSS with varying PSS content is presented. To this end, the PEDOT:PSS is enclosed in small interconnects between metallic contacts. An unexpected, but strong dependence of the conductivity on interconnect diameter is observed for PEDOT:PSS formulations without high boiling solvent. The change in conductivity correlates with a diameter dependent change in PEDOT:PSS layer thickness. It is suggested that the order of magnitude variation in out-of-plane conductivity with only a 3-4-fold layer thickness variation can quantitatively be explained on basis of a percolating cluster model. Reported measurements of the anisotropy in the conductivity of spin coated films of PEDOT:PSS vary widely. A systematic study of the out-of-plane charge transport in thin films of PEDOT:PSS with varying PSS content is presented. It is suggested that the orders of magnitude change in anisotropy can be quantitatively explained on the basis of a percolating cluster model. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We compare the opto-electronic and photovoltaic properties of two diketopyrrolopyrrole (DPP) based semiconducting polymers in which the DPP unit alternates along the chain with a conjugated bis(dithienyl)phenylene (4TP) unit. The two polymers differ only in the solubilizing substituents on the thiophene rings which are either alkyl (PDPP4TP) or alkoxy (PDPP4TOP) groups. We show that alkoxy groups lower the optical band gap and increase the ionization potential compared to the alkyl groups. As a result, PDDP4TOP provides a significantly higher charge generation efficiency and concomitant higher short-circuit current, 18.0 mA cm-2 vs. 12.4 mA cm-2, compared to PDPP4TP in optimized devices with [6,6]phenyl-C71-butyric acid methyl ester ([70]PCBM) as acceptor, but a simultaneous decrease in open circuit voltage, 0.51 vs. 0.67 V. The increased current arises from a higher external quantum efficiency and a wider spectral coverage. The net result is a small increase in power conversion efficiency from 5.8% for PDPP4TP to 6.0% for the PDPP4TOP in optimized devices. The optimized processing conditions and bulk heterojunction morphology are virtually identical for both photoactive layers. The study demonstrates that the side chains enable effective method for rationally designing new photoactive semiconducting polymers.

Light emission from ambipolar organic field-effect transistors (OFETs) is often observed when they are operated in the unipolar regime. This is unexpected, the light emission should be completely suppressed, because in the unipolar regime only one type of charge carrier is accumulated. Here, an electroluminescent diketopyrrolopyrrole copolymer is investigated. Local potential measurements by scanning Kelvin probe microscopy reveal a recombination position that is unstable in time due to the presence of injection barriers. The electroluminescence and electrical transport have been numerically analyzed. It is shown that the counterintuitive unipolar light emission is quantitatively explained by injection of minority carriers into deep tail states of the semiconductor. The density of the injected minority carriers is small. Hence they are relatively immobile and they recombine close the contact with accumulated majority carriers. The unipolar light output is characterized by a constant efficiency independent of gate bias. It is argued that light emission from OFETs predominantly originates from the unipolar regime when the charge transport is injection limited.

The performance of solution processed polymer:fullerene thin film photovoltaic cells is largely determined by the nanoscopic and mesoscopic morphology of these blends that is formed during the drying of the layer. Although blend morphologies have been studied in detail using a variety of microscopic, spectroscopic, and scattering techniques and a large degree of control has been obtained, the current understanding of the processes involved is limited. Hence, predicting the optimized processing conditions and the corresponding device performance remains a challenge. We present an experimental and modeling study on blends of a small band gap diketopyrrolopyrrole- quinquethiophene alternating copolymer (PDPP5T) and [6,6]-phenyl-C 71-butyric acid methyl ester ([70]PCBM) cast from chloroform solution. The model uses the homogeneous Flory-Huggins free energy of the multicomponent blend and accounts for interfacial interactions between (locally) separated phases, based on physical properties of the polymer, fullerene, and solvent. We show that the spinodal liquid-liquid demixing that occurs during drying is responsible for the observed morphologies. The model predicts an increasing feature size and decreasing fullerene concentration in the polymer matrix with increasing drying time in accordance with experimental observations and device performance. The results represent a first step toward a predictive model for morphology formation. © 2013 American Chemical Society.

For polymer tandem solar cells it is not easy to determine the individual internal electric fields across each of the two individual photoactive layers because the intermediate contact is often buried in the device stack and cannot be contacted. Here we explore electroabsorption (EA) spectroscopy as a possible tool to investigate these electric fields in polymer tandem solar cells. EA spectroscopy is a noninvasive electro-optical technique that can be used to probe internal electrical fields in organic thin film devices. The tandem cell investigated consists of a wide band gap front cell and a small band gap back cell, connected via an intermediate recombination contact. We demonstrate that the EA spectra of the individual subcells and their dependence on applied external bias can be distinguished in the EA spectrum of the tandem cells but that a quantitative interpretation of the EA spectra in terms of individual internal fields is hampered by a nonlinear bias dependence of the EA intensity on the applied external bias, especially for the small band gap cell. By studying the corresponding single junction wide and small band gap cells, we establish that the nonlinear bias dependence finds its origin in a spectral shift of the EA signal due to a change in the ratio of two different components contributing to the EA spectrum. The two components have been identified as being due to the Stark effect and to charges induced into the active layer in the EA experiment.

Four structurally related molecules consisting of two diketopyrrolopyrrole (DPP) units linked via a terthiophene aromatic p-bridge were synthesized and blended with [70]PCBM in solution-processed small-molecule organic solar cells. The four bis-DPP molecules possess nearly identical optical band gaps and energy levels, but their solubility differs significantly. The processing conditions, such as the solvent, processing additive, and total concentration, have a significant effect on the device performance. The bis-DPP derivative with the lowest solubility gives the highest power conversion efficiency (PCE) of 4.6% when blended with [70]PCBM, compared to 3.6–4.0% for the other three. The results show that subtle changes and tailoring of the molecular structure can strongly affect the solubility and, in turn, the processing conditions leading to the optimized device performance and its ultimate PCE.

We investigate light-driven electrochemical water splitting with series-connected polymer solar cells using a combined experimental and modeling approach. The expected maximum solar-to-hydrogen conversion efficiency (ηSTH) for light-driven water splitting is modeled for two, three, and four series-connected polymer solar cells. In the modeling, we assume an electrochemical water splitting potential of 1.50 V and a polymer solar cell for which the external quantum efficiency and fill factor are both 0.65. The minimum photon energy loss (Eloss), defined as the energy difference between the optical band gap (Eg) and the open-circuit voltage (Voc), is set to 0.8 eV, which we consider a realistic value for polymer solar cells. Within these approximations, two series-connected single junction cells with Eg = 1.73 eV or three series-connected cells with Eg = 1.44 eV are both expected to give an ηSTH of 6.9%. For four series-connected cells, the maximum ηSTH is slightly less at 6.2% at an optimal Eg = 1.33 eV. Water splitting was performed with series-connected polymer solar cells using polymers with different band gaps. PTPTIBDT-OD (Eg = 1.89 eV), PTB7-Th (Eg = 1.56 eV), and PDPP5T-2 (Eg = 1.44 eV) were blended with [70]PCBM as absorber layer for two, three, and four series-connected configurations, respectively, and provide ηSTH values of 4.1, 6.1, and 4.9% when using a retroreflective foil on top of the cell to enhance light absorption. The reasons for deviations with experiments are analyzed and found to be due to differences in Eg and Eloss. Light-driven electrochemical water splitting was also modeled for multijunction polymer solar cells with vertically stacked photoactive layers. Under identical assumptions, an ηSTH of 10.0% is predicted for multijunction cells.

The spatial charge distribution in diketopyrrolopyrrole-containing ambipolar polymeric semiconductors embedded in dual-gate field-effect transistors (DGFETs) was investigated. The DGFETs have identical active channel layers but two different channel/gate interfaces, with a CYTOP™ organic dielectric layer for the top-gate and an octadecyltrichlorosilane (ODTS) self-assembled monolayer-treated inorganic SiO2 dielectric for the bottom-gate, respectively. Temperature-dependent transfer measurements of the DGFETs were conducted to examine the charge transport at each interface. By fitting the temperature-dependent measurement results to the modified Vissenberg-Matters model, it can be inferred that the top-channel interfacing with the fluorinated organic dielectric layers has confined charge transport to two-dimensions, whereas the bottom-channel interfacing with the ODTS-treated SiO2 dielectric layers has three-dimensional charge transport.

In our recent Correspondence (Nature Photon. 9, 478-479; 2015), we presented a set of general rules for characterizing tandem organic solar cells to achieve reliable and comparable device performance data, especially cell efficiencies. In a reply to our Correspondence, Bahro et al. (Nature Photon. 10, 354-355; 2016) present a useful supplement to our analysis. They provide external quantum efficiency (EQE) data for an organic tandem solar cell without bias illumination, showing a case where the cell's EQE does not follow the lower envelope of the EQE spectra of the two subcells due to the specific intensity of the probe light. We agree that this can be a practically relevant issue when one of the subcells has a low shunt resistance and thank Bahro et al. for pointing this out. Furthermore, Bahro et al. comment on rule 3 of our general rules for characterizing tandem organic solar cells. They point out that when the EQE of a tandem cell without bias illumination is not following the lower envelope of the subcells' EQEs, measurements under bias light can deliver a correct result in certain cases. Actually, we feel we have already covered this circumstance, albeit not highlighted specifically, in our Supplementary Information (in the last paragraph of Supplementary Section 3.1). For the purposes of simplicity and transparency, we suggest to keep rule 3 unchanged. Our Supplementary Information combined with the reply of Bahro et al. make the correct characterization procedures clear.

Highly efficient hybrid multijunction solar cells are constructed with a wide-bandgap amorphous silicon for the front subcell and a low-bandgap polymer for the back subcell. Power conversion efficiencies of 11.6% and 13.2% are achieved in tandem and triple-junction configurations, respectively. The high efficiencies are enabled by deploying effective optical management and by using photoactive materials with complementary absorption.

Optical excitation of a polymer-fullerene solar cell via the charge-transfer transition at the donor-acceptor interface, leads to generation of charge carriers displaying dynamics that are indistinguishable from those generated by excitation of the conjugated polymer at higher photon energy. This indicates that excess energy liberated in the photoinduced electron transfer between polymer and fullerene is not essential for the charge carrier generation.

Despite the poor screening of the Coulomb potential in organic semiconductors, excitons can dissociate efficiently into free charges at a donor–acceptor heterojunction, leading to application in organic solar cells. A kinetic Monte Carlo model that explains this high efficiency as a two-step process is presented. Driven by the band offset between donor and acceptor, one of the carriers first hops across the interface, forming a charge transfer (CT) complex. Since the electron and hole forming the CT complex have typically not relaxed within the disorder-broadened density of states (DOS), their remaining binding energy can be overcome by further relaxation in the DOS. The model only contains parameters that are determined from independent measurements and predicts dissociation yields in excess of 90% for a prototypical heterojunction. Field, temperature, and band offset dependencies are investigated and found to be in agreement with earlier experiments. Whereas the investigated heterojunctions have substantial energy losses associated with the dissociation process, these results suggest that it is possible to reach high dissociation yields at low energy loss.

The operational mechanism of polymer light-emitting electrochemical cells (LECs) in sandwich geometry is studied by admittance spectroscopy in combination with numerical modeling. At bias voltages below the bandgap of the semiconducting polymer, this allows the determination of the dielectric constant of the active layer, the conductivity of mobile ions, and the thickness of the electric double layers. At bias voltages above the bandgap, p–n junction formation gives rise to an increase in capacitance at intermediate frequencies (˜10 kHz). The time and voltage dependence of this junction are successfully studied and modeled. It is shown that impedance measurements cannot be used to determine the junction width. Instead, the capacitance at intermediate biases corresponds to a low-conductivity region that can be significantly wider than the recombination zone. Finally, the long settling time of sandwich polymer LECs is shown to be due to a slow process of dissociation of salt molecules that continues after the light-emitting p–n junction has formed. This implies that in order to significantly decrease the response-time of LECs an electrolyte/salt combination with a minimal ion binding energy must be used.

We investigate the mechanism of charge transport in indium gallium zinc oxide (a-IGZO), an amorphous metal-oxide semiconductor. We measured the field-effect mobility and the Seebeck coefficient (S=ΔV/ΔT) of a-IGZO in thin-film transistors as a function of charge-carrier density for different temperatures. Using these transistors, we further employed a scanning Kelvin probe-based technique to determine the density of states of a-IGZO that is used as the basis for the modeling. After comparing two commonly used models, the band transport percolation model and a mobility edge model, we find that both cannot describe the full properties of the charge transport in the a-IGZO semiconductor. We, therefore, propose a model that extends the mobility edge model to allow for variable range hopping below the mobility edge. The extended mobility edge model gives a superior description of the experimental results. We show that the charge transport is dominated by variable range hopping below, rather than by bandlike transport above the mobility edge. © 2012 American Physical Society.

Using Stille and Suzuki polymerization reactions we incorporate thienoisoindigo (TII) as an acceptor co-monomer into a series of alternating p-conjugated copolymers with combinations of benzene, thiophene and carbazole as donor co-monomers. By changing the nature and length of the donor segments, the optical band gap of these soluble TII copolymers can be varied over a large range from 1.52 eV down to 0.87 eV. The semiconducting properties of the TII copolymers were established in bottom-gate bottom-contact field-effect transistors that provide hole mobilities for these materials in the range of 10-3 to 10-2 cm2 V-1 s-1.

The spectral responsivity, S, and the related spectrally resolved photon-to-electron external quantum efficiency, EQE, are standard device characteristics of organic solar cells and can be used to determine the short-circuit current density and power conversion efficiency under standardized test conditions by integrating over the spectral irradiance of the solar emission. However, in organic solar cells S and EQE can change profoundly with light intensity as a result of processes that vary non-linearly with light intensity such as bimolecular recombination of electrons and holes or space charge effects. To determine the S under representative solar light conditions, it is common to use modulated monochromatic light and lock-in detection in combination with simulated solar bias light to bring the cell close to 1 sun equivalent operating conditions. In this paper we demonstrate analytically and experimentally that the S obtained with this method is in fact the differential spectral responsivity, DS, and that the real S and the experimental DS can differ significantly when the solar cells exhibit loss processes that vary non-linearly with light intensity. In these cases the experimental DS will be less than the real S. We propose a new, simple, experimental method to more accurately determine S and EQE under bias illumination. With the new method it is possible to accurately estimate the power conversion efficiency of organic solar cells.

The scaling of the characteristic frequencies of electronic ratchets operating in a flashing mode is investigated by measurements and numerical simulations. The ratchets are based on organic field effect transistors operated in accumulation mode. Oscillating potentials applied to asymmetrically spaced interdigitated finger electrodes embedded in the gate dielectric create a time-dependent, spatially asymmetric perturbation of the transistor channel potential. As a result a net DC current can flow between source and drain despite zero source-drain bias. The frequency at current maximum is linearly dependent on the charge carrier density and the charge carrier mobility and inversely proportional to the squared length of the ratchet period, which can be related to the RC-time of one asymmetric unit. Counter-intuitively, it is independent of driving amplitude. Furthermore, the frequency at current maximum depends on the asymmetry of the ratchet potential whereas the frequency of maximum charge pumping efficiency does not.

We investigate the efficiency and emission color of small-molecule based double-layer organic light-emitting diodes (OLEDs) based on 4,4′-bis[1-naphthyl (phenyl) amino]-1,1′-biphenyl (α-NPD) and aluminum (III) bis (2-methyl-8-quinolinato)4-phenylphenolato (BAlq) by studying the charge transport and photophysics near the organic–organic interface between the emitting layers. For that purpose, the light-emission profile is reconstructed from full angle, wavelength and polarization dependent electroluminescence spectra. By increasing the thickness of the BAlq layer from 100 to 300 nm, at a fixed 160 nm α-NPD layer thickness, the emission color is found to vary from deep blue to green, yellow-green, white and back to blue. We demonstrate that this is due to a gradual emission profile shift, in combination with a wavelength and layer thickness dependent light outcoupling efficiency. The emission profile shift, from an approximately 20 nm-wide zone on the α-NPD-side of the interface to a very narrow zone on the BAlq-side of the interface, gives rise to a changing balance between the contributions from BAlq excitons, α-NPD excitons and charge-transfer excitons. It also contributes to a pronounced layer thickness dependence of the external quantum efficiency. The shift of the emission profile is explained by a charge transport and recombination model.

A series of diketopyrrolopyrrole (DPP)-based small band gap polymers has been designed and synthesized by Suzuki or Stille polymerization for use in polymer solar cells. The new polymers contain extended aromatic π-conjugated segments alternating with the DPP units and are designed to increase the free energy for charge generation to overcome current limitations in photocurrent generation of DPP-based polymers. In optimized solar cells with [6,6]phenyl-C(71)-butyric acid methyl ester ([70]PCBM) as acceptor, the new DPP-polymers provide significantly enhanced external and internal quantum efficiencies for conversion of photons into collected electrons. This provides short-circuit current densities in excess of 16 mA cm(-2), higher than obtained so far, with power conversion efficiencies of 5.8% in simulated solar light. We analyze external and internal photon to collected electron quantum efficiencies for the new polymers as a function of the photon energy loss, defined as the offset between optical band gap and open circuit voltage, and compare the results to those of some of the best DPP-based polymers solar cells reported in the literature. We find that for the best solar cells there is an empirical relation between quantum efficiency and photon energy loss that presently limits the power conversion efficiency in these devices.

A major drawback of light-emitting electrochemical cells (LECs) is the long time scale associated with switching, during which ions redistribute in the active layer. We present a numerical modeling study that gives fundamental insight in the dynamics during turn-on. The characteristic response of LECs to an applied bias is the electrochemical doping of the active layer by doping fronts moving across the active layer. Formation and motion of such doping fronts are shown to be intimately related to both the electronic and ionic mobility and therefore provide useful information regarding these two quantities in LECs. In particular, it is shown that the switch-on time in LECs is directly related to the time an ion needs to cross approximately half the device, enabling the extraction of the ion mobility from the switch-on time.