Loading Fullerene into a Conjugated Polymer Without Chemical Modification

Conjugated polymers doped with fullerenes show excellent photoconversion efficiencies. However, due to the poor solubility of pure C₆₀ in common organic solvents, it was thought that some chemical modifications to C₆₀ were required in order to load conjugated polymers with comparable weights of fullerene. Here, we report a novel way to load large amounts of unmodified C₆₀ fullerene into the conjugated polymer poly(2‐methoxy‐5‐(2′‐ethylhexoxy)‐1,4‐phenylenevinylene), MEH‐PPV. The principle of our approach is to separate the film‐deposition stage from the material‐solidification stage. That is, materials are quickly solidified from dilute solution into colloidal particles, which are large enough to be collected by electrophoretic deposition. Deposition from a mixture of separate suspensions of MEH‐PPV and C₆₀ fullerene resulted in films consisting of isolated particles. In contrast, by using a suspension made of a dilute solution of MEH‐PPV and C₆₀, which was so dilute that traditional techniques such as spin‐coating were not applicable, an MEH‐PPV film doped with 20 mol‐% C₆₀, corresponding to an almost 1:1 ratio by weight of molecularly dispersed C₆₀, was easily obtained. To the best of our knowledge, this is the first report on the successful loading of an equivalent weight ratio of unmodified C₆₀ molecules into MEH‐PPV.     

The Effect of Polymer Optoelectronic Properties on the Performance of Multilayer Hybrid Polymer/TiO2 Solar Cells

We report a study of the effects of polymer optoelectronic properties on the performance of photovoltaic devices consisting of nanocrystalline TiO₂ and a conjugated polymer. Three different poly(2‐methoxy‐5‐(2′‐ethylhexoxy)‐1,4‐phenylenevinylene) (MEH‐PPV)‐based polymers and a fluorene–bithiophene copolymer are compared. We use photoluminescence quenching, time‐of‐flight mobility measurements, and optical spectroscopy to characterize the exciton‐transport, charge‐transport, and light‐harvesting properties, respectively, of the polymers, and correlate these material properties with photovoltaic‐device performance. We find that photocurrent is primarily limited by the photogeneration rate and by the quality of the interfaces, rather than by hole transport in the polymer. We have also studied the photovoltaic performance of these TiO₂/polymer devices as a function of the fabrication route and device design. Including a dip‐coating step before spin‐coating the polymer leads to excellent polymer penetration into highly structured TiO₂ networks, as was confirmed through transient optical measurements of the photoinduced charge‐transfer yield and recombination kinetics. Device performance is further improved for all material combinations studied, by introducing a layer of poly(ethylene dioxythiophene) (PEDOT) doped with poly(styrene sulfonic acid) (PSS) under the top contact. Optimized devices incorporating the additional dip‐coated and PEDOT:PSS layers produced a short‐circuit current density of about 1 mA cm^–2, a fill factor of 0.50, and an open‐circuit voltage of 0.86 V under simulated AM 1.5 illumination (100 mW cm^–2, 1 sun). The corresponding power conversion efficiency under 1 sun was ≥ 0.4 %.     

Directed Self‐Assembly of Gradient Concentric Carbon Nanotube Rings

Hundreds of gradient concentric rings of linear conjugated polymer, (poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐ phenylenevinylene], i.e., MEH‐PPV) with remarkable regularity over large areas were produced by controlled “stick‐slip” motions of the contact line in a confined geometry consisting of a sphere on a flat substrate (i.e., sphere‐on‐flat geometry). Subsequently, MEH‐PPV rings were exploited as a template to direct the formation of gradient concentric rings of multiwalled carbon nanotubes (MWNTs) with controlled density. This method is simple, cost effective, and robust, combining two consecutive self‐assembly processes, namely, evaporation‐induced self‐assembly of polymers in a sphere‐on‐flat geometry, followed by subsequent directed self‐assembly of MWNTs on the polymer‐templated surfaces.     

Stabilization of Semiconducting Polymers with Silsesquioxane

Polyhedral oligomeric silsesquioxanes (POSS) anchored to poly(2‐methoxy‐5‐(2‐ethylhexyloxy)‐1.4‐phenylenevinylene) (MEH‐PPV) (MEH‐PPV–POSS), and to poly(9,9‐dihexylfluorenyl‐2,7‐diyl) (PFO) (PFO–POSS) were synthesized. Compared with the corresponding parent polymers, MEH‐PPV and PFO, MEH‐PPV–POSS and PFO–POSS have better thermal stability. MEH‐PPV–POSS and MEH‐PPV have identical absorption and photoluminescent (PL) spectra, both in solution and as thin films. They also have identical electroluminescent (EL) spectra. Devices made from MEH‐PPV–POSS exhibit higher brightness (1320 cd m^–2 at 3.5 V) and higher external quantum efficiency (η_ext = 2.2 % photons per electron) compared to MEH‐PPV (230 cd m^–2 at 3.5 V and η_ext = 1.5 % ph el^–1). Compared with PFO in the same device configuration, PFO–POSS has improved blue EL emission and higher η_ext.     

Influence of the Relative Humidity on the Performance of Polymer/TiO2 Photovoltaic Cells

Hydrolysis of titanium(IV ) isopropoxide (TTIP) is a well‐known method for the fabrication of TiO₂. Normally it is made via a sol–gel reaction in the presence of water. In this paper we report on the preparation of flat TiO₂ films for conjugated polymer/TiO₂ photovoltaic cells, from a TTIP/isopropanol solution. It is shown that the morphological structure of the TiO₂ film is strongly dependent on the relative humidity during spin‐coating of the TTIP/isopropanol solution. In bilayer devices consisting of TiO₂/poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV), a low relative humidity (< 25 %, room temperature) is needed in order to form smooth, transparent TiO₂ films. Increasing the relative humidity results in porous TiO₂ films with a high surface roughness, which leads to shunted devices. Apart from bilayer devices, bulk‐heterojunction (BHJ) hybrid TiO₂:MDMO‐PPV photovoltaic cells have been made, by spin‐coating a mixture of TTIP and MDMO‐PPV in toluene. Again a strong relation was found between the relative humidity during spin‐coating and the current–voltage characteristics of the devices. However, in contrast to the bilayer devices, the best BHJ devices were made at higher relative humidity. The observed performance dependence on relative humidity is discussed in relation to the TiO₂ morphology.     

Roll‐to‐Roll Printed Silver Nanowire Semitransparent Electrodes for Fully Ambient Solution‐Processed Tandem Polymer Solar Cells

Silver nanowires (AgNWs) and zinc oxide (ZnO) are deposited on flexible substrates using fast roll‐to‐roll (R2R) processing. The AgNW film on polyethylene terephthalate (PET) shows >80% uniform optical transmission in the range of 550–900 nm. This electrode is compared to the previously reported and currently widely produced indium‐tin‐oxide (ITO) replacement comprising polyethylene terephthalate (PET)|silver grid|poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)|ZnO known as Flextrode. The AgNW/ZnO electrode shows higher transmission than Flextrode above 490 nm in the electromagnetic spectrum reaching up to 40% increased transmission at 750 nm in comparison to Flextrode. The functionality of AgNW electrodes is demonstrated in single and tandem polymer solar cells and compared with parallel devices on traditional Flextrode. All layers, apart from the semitransparent electrodes which are large‐scale R2R produced, are fabricated in ambient conditions on a laboratory roll‐coater using printing and coating methods which are directly transferrable to large‐scale R2R processing upon availability of materials. In a single cell structure, Flextrode is preferable with active layers based on poly‐3‐hexylthiophene(P3HT):phenyl‐C61‐butyric acid methylester (PCBM) and donor polymers of similar absorption characteristics while AgNW/ZnO electrodes are more compatible with low band gap polymer‐based single cells. In tandem devices, AgNW/ZnO is more preferable resulting in up to 80% improvement in PCE compared to parallel devices on Flextrode.     

Design of Multilayered Nanostructures and Donor–Acceptor Interfaces in Solution‐Processed Thin‐Film Organic Solar Cells

Multilayered polymer thin‐film solar cells have been fabricated by wet processes such as spin‐coating and layer‐by‐layer deposition. Hole‐ and electron‐transporting layers were prepared by spin‐coating with poly(3,4‐ethylenedioxythiophene) oxidized with poly(4‐styrenesulfonate) (PEDOT:PSS) and fullerene (C₆₀), respectively. The light‐harvesting layer of poly‐(p‐phenylenevinylene) (PPV) was fabricated by layer‐by‐layer deposition of the PPV precursor cation and poly(sodium 4‐styrenesulfonate) (PSS). The layer‐by‐layer technique enables us to control the layer thickness with nanometer precision and select the interfacial material at the donor–acceptor heterojunction. Optimizing the layered nanostructures, we obtained the best‐performance device with a triple‐layered structure of PEDOT:PSS|PPV|C₆₀, where the thickness of the PPV layer was 11 nm, comparable to the diffusion length of the PPV singlet exciton. The external quantum efficiency spectrum was maximum (ca. 20%) around the absorption peak of PPV and the internal quantum efficiency was estimated to be as high as ca. 50% from a saturated photocurrent at a reverse bias of ?3 V. The power conversion efficiency of the triple‐layer solar cell was 0.26% under AM1.5G simulated solar illumination with 100 mW cm^?2 in air.     

Triplet Exciton and Polaron Dynamics in Phosphorescent Dye Blended Polymer Photovoltaic Devices

The triplet exciton and polaron dynamics in phosphorescent dye (PtOEP) blended polymer (MEH‐PPV) photovoltaic devices are investigated by quasi‐steady‐state photo‐induced absorption (PIA) spectroscopy. According to the low‐temperature PIA and photoluminescence (PL) results, the increase in strength of the triplet‐triplet (T₁‐T_n) absorption of MEH‐PPV in the blend system originates from the triplet‐triplet energy transfer from PtOEP to MEH‐PPV. The PtOEP blended MEH‐PPV/C₆₀ bilayer photovoltaic device shows a roughly 30%–40% enhancement in photocurrent and power‐conversion efficiency compared to the device without PtOEP. However, in contrast to the bilayer device results, the bulk heterojunction photovoltaic devices do not show a noticeable change in photocurrent and power‐conversion efficiency in the presence of PtOEP. The PIA intensity, originating from the polaron state, is only slightly higher (within the experimental error), indicating that carrier generation in the bulk heterojunction is not enhanced in the presence of PtOEP. The rate and probability of the exciton dissociation between PtOEP and PCBM is much faster and higher than that of the triplet‐triplet energy transfer between PtOEP and MEH‐PPV.     

Nanostructure‐Dependent Vertical Charge Transport in MEH‐PPV Films

The correlation between morphology and charge‐carrier mobility in the vertical direction in thin films of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV) is investigated by a combination of X‐ray reflectivity (XRR), field‐emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), fluorescence optical microscopy (FOM), photoluminescence spectroscopy (PL), photoluminescence excitation spectroscopy (PLE), as well as time‐of‐flight (TOF) and transient electroluminescence (TrEL) techniques. The mobility is about two orders of magnitude greater for drop‐cast films than for their spin‐cast counterparts. Drop‐casting in the presence of a vertical static electric field (E‐casting) results in films with an additional increase in mobility of about one order of magnitude. While PL and PLE spectra vary with the method of film preparation, there is no correlation between emission spectra and charge‐carrier mobility. Our XRR measurements on spin‐cast films indicate layering along the film depth while no such structure is found in drop‐cast or E‐cast films, whereas FESEM examination indicates that nanodomains within drop‐cast films are eliminated in the E‐cast case. These observations indicate that carrier transport is influenced by structure on two different length scales. The low mobility observed in spin‐cast films is a direct result of a global layered structure with characteristic thickness of ca. 4 nm: in the absence of this layered structure, drop‐cast films with inherent nanoscale heterogeneities (ca. 20 nm in size) exhibit much better hole mobility. Elimination of nanodomains via electric‐field alignment results in further improved charge mobility.     

Phase Segregation in Thin Films of Conjugated Polyrotaxane– Poly(ethylene oxide) Blends: A Scanning Force Microscopy Study

Scanning force microscopy (SFM) is used to study the surface morphology of spin‐coated thin films of the ion‐transport polymer poly(ethylene oxide) (PEO) blended with either cyclodextrin (CD)‐threaded conjugated polyrotaxanes based on poly(4,4′‐diphenylene‐vinylene) (PDV), β‐CD–PDV, or their uninsulated PDV analogues. Both the polyrotaxanes and their blends with PEO are of interest as active materials in light‐emitting devices. The SFM analysis of the blended films supported on mica and on indium tin oxide (ITO) reveals in both cases a morphology that reflects the substrate topography on the (sub‐)micrometer scale and is characterized by an absence of the surface structure that is usually associated with phase segregation. This observation confirms a good miscibility of the two hydrophilic components, when deposited by using spin‐coating, as suggested by the luminescence data on devices and thin films. Clear evidence of phase segregation is instead found when blending PEO with a new organic‐soluble conjugated polymer such as a silylated poly(fluorene)‐alt‐poly(para‐phenylene) based polyrotaxane (THS–β‐CD–PF–PPP). The results obtained are relevant to the understanding of the factors influencing the interfacial and the intermolecular interactions with a view to optimizing the performance of light‐emitting diodes, and light‐emitting electrochemical cells based on supramolecularly engineered organic polymers.     

Controlled Pattern Formation of Poly[2‐methoxy‐5‐(2′‐ethylhexyloxyl)–1,4‐phenylenevinylene] (MEH–PPV) by Ink‐Jet Printing

A detailed survey on the processing of poly[2‐methoxy‐5‐(2′‐ethylhexyloxyl)–1,4‐phenylenevinylene] (MEH–PPV) solutions via ink‐jet printing and the subsequent characterization of the resulting films is reported. The printability of MEH–PPV dissolved in different solvents, and with varied concentrations, is studied. Limitations of the printability of highly concentrated polymer solutions are overcome by using ultrasonication. The pattern formation of the resulting lines is explained in relation to the contact angle formed by the MEH–PPV solution on the substrate and interchain interactions. A uniform thickness distribution of MEH–PPV films is obtained when toluene is used as the solvent. Further improvement on the surface quality of the lines is achieved by optimizing the printing parameters. The line stability as a function of the print‐head velocity is also studied. Additionally, current–voltage (I–V) characteristics and the morphology of the MEH–PPV films, as determined by atomic force microscopy, are discussed.     

Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface

A cationic and water‐soluble polythiophene [poly[3‐(6‐pyridiniumylhexyl)thiophene bromide] (P3PHT⁺Br^?)] is synthesized and used in combination with anionic poly(3,4‐ethylenedioxythiophene):poly(p‐styrenesulfonate) (PEDOT:PSS)^? to produce hybrid coatings on indium tin oxide (ITO). Two coating strategies are established: i) electrostatic layer‐by‐layer assembly with colloidal suspensions of (PEDOT:PSS)^?, and ii) modification of an electrochemically prepared (PEDOT:PSS)^? film on ITO. The coatings are found to modify the work function of ITO such that it could act as a cathode in inverted 2,5‐diyl‐poly(3‐hexylthiophene) (P3HT)/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) polymer photovoltaic cells. The interfacial modifier created from the layer‐by‐layer assembly route is used to produce efficient inverted organic photovoltaic devices (power conversion efficiency ～2%) with significant long‐term stability in excess of 500 h.     

Efficient Conjugated‐Polymer Optoelectronic Devices Fabricated by Thin‐Film Transfer‐Printing Technique

The fabrication of functional multilayered conjugated‐polymer structures with well‐defined organic‐organic interfaces for optoelectronic‐device applications is constrained by the common solubility of many polymers in most organic solvents. Here, we report a simple, low‐cost, large‐area transfer‐printing technique for the deposition and patterning of conjugated‐polymer thin films. This method utilises a planar poly(dimethylsiloxane) (PDMS) stamp, along with a water‐soluble sacrificial layer, to pick up an organic thin film (～20 nm to 1 µm) from a substrate and subsequently deliver this film to a target substrate. We demonstrate the versatility of this transfer‐printing technique and its applicability to optoelectronic devices by fabricating bilayer structures of poly(9,9‐di‐n‐octylfluorene‐alt‐(1,4‐phenylene‐((4‐sec‐butylphenyl)imino)‐1,4‐phenylene))/poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) (TFB/F8BT) and poly(3‐hexylthiophene)/methanofullerene([6,6]‐phenyl C₆₁ butyric acid methyl ester) (P3HT/PCBM), and incorporating them into light‐emitting diodes (LEDs) and photovoltaic (PV) cells, respectively. For both types of device, bilayer devices fabricated with this transfer‐printing technique show equal, if not superior, performance to either blend devices or bilayer devices fabricated by other techniques. This indicates well‐controlled organic‐organic interfaces achieved by the transfer‐printing technique. Furthermore, this transfer‐printing technique allows us to study the nature of the excited states and the transport of charge carriers across well‐defined organic interfaces, which are of great importance to organic electronics.     

Controllable Molecular Doping and Charge Transport in Solution‐Processed Polymer Semiconducting Layers

Here, controlled p‐type doping of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) deposited from solution using tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) as a dopant is presented. By using a co‐solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH‐PPV‐based hole‐only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4‐TCNQ dopant per 600 repeat units of MEH‐PPV leads to a free carrier density of 4 × 10²² m^?3. Neglecting the density‐dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4‐TCNQ doped MEH‐PPV and a silver electrode.     

Direct Photopatterning of Electrochromic Polymers

Propylenedioxythiophene (ProDOT) polymers are synthesized using an oxidative polymerization route that results in methacrylate substituted poly(ProDOTs) having a M_n of 10–20 kDa wherein the methacrylate functionality constitutes from 6 to 60% of the total monomer units. Solutions of these polymers show excellent film forming abilities, with thin films prepared using both spray‐casting and spin‐coating. These polymers are demonstrated to crosslink upon UV irradiation at 350 nm, in the presence of an appropriate photoinitiator, to render the films insoluble to common organic solvents. Electrochemical, spectroelectrochemical, and colorimetric analyses of the crosslinked polymer films are performed to establish that they retain the same electrochromic qualities as the parent polymers with no detriment to the observed properties. To demonstrate applicability for multi‐film processing and patterning, photolithographic patterning is shown, as is desired for fully solution processed and patterned devices.     

Solvent‐Induced Morphology in Polymer‐Based Systems for Organic Photovoltaics

Studies on the influence of four different solvents on the morphology and photovoltaic performance of bulk‐heterojunction films made of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) via spin‐coating for photovoltaic applications are reported. Solvent‐dependent PCBM cluster formation and P3HT crystallization during thermal annealing are investigated with optical microscopy and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) and are found to be insufficient to explain the differences in device performance. A combination of atomic force microscopy (AFM), X‐ray reflectivity (XRR), and grazing‐incidence small‐angle X‐ray scattering (GISAXS) investigations results in detailed knowledge of the inner film morphology of P3HT:PCBM films. Vertical and lateral phase separation occurs during spin‐coating and annealing, depending on the solvent used. The findings are summarized in schematics and compared with the IV characteristics. The main influence on the photovoltaic performance arises from the vertical material composition and the existence of lateral phase separation fitting to the exciton diffusion length. Absorption and photoluminescence measurements complement the structural analysis.     

Efficient Polymer Solar Cells Based on Poly(3‐hexylthiophene):Indene‐C70 Bisadduct with a MoO3 Buffer Layer

Polymer solar cells (PSCs) with poly(3‐hexylthiophene) (P3HT) as a donor, an indene‐C₇₀ bisadduct (IC₇₀BA) as an acceptor, a layer of indium tin oxide modified by MoO₃ as a positive electrode, and Ca/Al as a negative electrode are presented. The photovoltaic performance of the PSCs was optimized by controlling spin‐coating time (solvent annealing time) and thermal annealing, and the effect of the spin‐coating times on absorption spectra, X‐ray diffraction patterns, and transmission electron microscopy images of P3HT/IC₇₀BA blend films were systematically investigated. Optimized PSCs were obtained from P3HT/IC₇₀BA (1:1, w/w), which exhibited a high power conversion efficiency of 6.68%. The excellent performance of the PSCs is attributed to the higher crystallinity of P3HT and better a donor–acceptor interpenetrating network of the active layer prepared under the optimized conditions. In addition, PSCs with a poly(3,4‐ethylenedioxy‐thiophene):poly(styrenesulfonate) (PEDOT:PSS) buffer layer under the same optimized conditions showed a PCE of 6.20%. The results indicate that the MoO₃ buffer layer in the PSCs based on P3HT/IC₇₀BA is superior to that of the PEDOT:PSS buffer layer, not only showing a higher device stability but also resulting in a better photovoltaic performance of the PSCs.     

Impedance Spectroscopy of π‐Conjugated Organic Materials

AC electrical properties of organic light‐emitting diodes with poly(2‐methoxy‐5‐(2'‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV), poly[2,5‐bia(dimethyloctylsilyl)‐1,4‐phenylene‐vinylene] (BDMOS‐PPV), and tris‐(8‐hydroxyquinolate)‐aluminum (AlQ₃) as light‐emitting materials are studied. The frequency‐dependent real and imaginary parts of impedance were fitted using an equivalent circuit. We found that the conduction mechanism is a space‐charge limited current with exponential trap distribution.     

Lateral Phase Separation Gradients in Spin‐Coated Thin Films of High‐Performance Polymer:Fullerene Photovoltaic Blends

In this study, it is demonstrated that a finer nanostructure produced under a rapid rate of solvent removal significantly improves charge separation in a high‐performance polymer:fullerene bulk‐heterojunction blend. During spin‐coating, variations in solvent evaporation rate give rise to lateral phase separation gradients with the degree of coarseness decreasing away from the center of rotation. As a result, across spin‐coated thin films the photocurrent at the first interference maximum varies as much as 25%, which is much larger than any optical effect. This is investigated by combining information on the surface morphology of the active layer imaged by atomic force microscopy, the 3D nanostructure imaged by electron tomography, film formation during the spin coating process imaged by optical interference and photocurrent generation distribution in devices imaged by a scanning light pulse technique. The observation that the nanostructure of organic photovoltaic blends can strongly vary across spin‐coated thin films will aid the design of solvent mixtures suitable for high molecular‐weight polymers and of coating techniques amenable to large area processing.     

Polymer Light‐Emitting Electrochemical Cells: Doping Concentration,Emission‐Zone Position,and Turn‐On Time

Direct optical probing of the doping progression and simultaneous recording of the current–time behavior allows the establishment of the position of the light‐emitting p–n junction, the doping concentrations in the p‐ and n‐type regions, and the turn‐on time for a number of planar light‐emitting electrochemical cells (LECs) with a 1 mm interelectrode gap. The position of the p–n junction in such LECs with Au electrodes contacting an active material mixture of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV), poly(ethylene oxide), and a XCF₃SO₃ salt (X = Li, K, Rb) is dependent on the salt selection: for X = Li the p–n junction is positioned very close to the negative electrode, while for X = K, Rb it is significantly more centered in the interelectrode gap. Its is demonstrated that this results from that the p‐type doping concentration is independent of salt selection at ca. 2 × 10²⁰ cm^–3 (ca. 0.1 dopants/MEH‐PPV repeat unit), while the n‐type doping concentration exhibits a strong dependence: for X = K it is ca. 5 × 10²⁰ cm^–3 (ca. 0.2 dopants/repeat unit), for X = Rb it is ca. 9 × 10²⁰ cm^–3 (ca. 0.4 dopants/repeat unit), and for X = Li it is ca. 3 × 10²¹ cm^–3 (ca. 1 dopants/repeat unit). Finally, it is shown that X = K, Rb devices exhibit significantly faster turn‐on times than X = Li devices, which is a consequence of a higher ionic conductivity in the former devices.