Efficient Polythiophene/Polyfluorene Copolymer Bulk Heterojunction Photovoltaic Devices: Device Physics and Annealing Effects

Here the influence of annealing on the operational efficiency of all‐polymer solar cells based on blends of the polymers poly(3‐hexylthiophene) (P3HT) and poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthiophen‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2″‐diyl) (F8TBT) is investigated. Annealing of completed devices is found to result in an increase in power conversion efficiency from 0.14 to 1.20%, while annealing of films prior to top electrode deposition increases device efficiency to only 0.19% due to a lowering of the open‐circuit voltage and short‐circuit current. By studying the dependence of photocurrent on intensity and effective applied bias, annealing is found to increase charge generation efficiency through an increase in the efficiency of the separation of bound electron‐hole pairs following charge transfer. However, unlike many other all‐polymer blends, this increase in charge separation efficiency is not only due to an increase in the degree of phase separation that assists in the spatial separation of electron‐hole pairs, but also due to an order of magnitude increase in the hole mobility of the P3HT phase. The increase in hole mobility with annealing is attributed to the ordering of P3HT chains evidenced by the red‐shifting of P3HT optical absorption in the blend. We also use X‐ray photoelectron spectroscopy (XPS) to study the influence of annealing protocol on film interface composition. Surprisingly both top and bottom electrode/blend interfaces are enriched with P3HT, with the blend/top electrode interface consisting of more than 95% P3HT for as‐spun films and films annealed without a top electrode. Films annealed following top electrode deposition, however, show an increase in F8TBT composition to ～15%. The implications of interfacial composition and the origin of open‐circuit voltage in these devices are also discussed.     

Correlating Emissive Non‐Geminate Charge Recombination with Photocurrent Generation Efficiency in Polymer/Perylene Diimide Organic Photovoltaic Blend Films

Evidence for a correlation between the dynamics of emissive non‐geminate charge recombination within organic photovoltaic (OPV) blend films and the photocurrent generation efficiency of the corresponding blend‐based solar cells is presented. Two model OPV systems that consist of binary blends of electron acceptor N′‐bis(1‐ethylpropyl)‐3,4,9,10‐perylene tetracarboxy diimide (PDI) with either poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) or poly(9,9‐dioctylindenofluorene‐co‐benzothiadiazole) (PIF8BT) as electron donor are studied. For the F8BT:PDI and PIF8BT:PDI devices photocurrent generation efficiency is shown to be related to the PDI crystallinity. In contrast to the F8BT:PDI system, thermal annealing of the PIF8BT:PDI layer at 90 °C has a positive impact on the photocurrent generation efficiency and yields a corresponding increase in PL quenching. The devices of both blends have a strongly reduced photocurrent on higher temperature annealing at 120 °C. Delayed luminescence spectroscopy suggests that the improved efficiency of photocurrent generation for the 90 °C annealed PIF8BT:PDI layer is a result of optimized transport of the photogenerated charge‐carriers as well as of enhanced PL quenching due to the maintenance of optimized polymer/PDI interfaces. The studies propose that charge transport in the blend films can be indirectly monitored from the recombination dynamics of free carriers that cause the delayed luminescence. For the F8BT:PDI and PIF8BT:PDI blend films these dynamics are best described by a power‐law decay function and are found to be temperature dependent.     

Compositional and Morphological Studies of Polythiophene/Polyflorene Blends in Inverted Architecture Hybrid Solar Cells

This work investigates the composition and morphology of films of poly(3‐hexylthiophene) (P3HT), polyfluorene co‐polymer poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2″‐diyl) (F8TBT) and blends thereof that are used in efficient all‐polymer solar cells. Ultraviolet photoemission spectroscopy (UPS) and X‐ray photoemission spectroscopy (XPS) studies on thin polymer and blend films on ZnO substrates reveal the existence of a 1–2 nm thick P3HT layer at the top surface of the blend films. XPS depth profiling studies reveal a density wave (λ ≈ 70 nm) originating from the air interface. As no preferential accumulation is observed at the bottom interface with ZnO, the composition at this interface is consistent with the original composition of the blend solution prior to spin‐coating. The morphology of this buried interface was studied by means of atomic force microscopy (AFM) and revealed that upon annealing the average domain size increases slightly (from 27 nm to 40 nm). It is observed that the photovoltaic performance of such inverted hybrid device improves upon annealing, however we believe this to mostly be a result of increased crystallinity in the P3HT domains leading to improved charge transport in the device, rather than changes in the blend phase separation.     

Comparison of the Operation of Polymer/Fullerene,Polymer/Polymer,and Polymer/Nanocrystal Solar Cells: A Transient Photocurrent and Photovoltage Study

We utilize transient techniques to directly compare the operation of polymer/fullerene, polymer/nanocrystal, and polymer/polymer bulk heterojunction solar cells. For all devices, poly(3‐hexylthiophene) (P3HT) is used as the electron donating polymer, in combination with either the fullerene derivative phenyl‐C₆₁‐butyric acid methyl ester (PCBM) in polymer/fullerene cells, CdSe nanoparticles in polymer/nanocrystal cells, or the polyfluorene copolymer poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2,2‐diyl) (F8TBT) in polymer/polymer cells. Transient photocurrent and photovoltage measurements are used to probe the dynamics of charge‐separated carriers, with vastly different dynamic behavior observed for polymer/fullerene, polymer/polymer, and polymer/nanocrystal devices on the microsecond to millisecond timescale. Furthermore, by employing transient photocurrent analysis with different applied voltages we are also able to probe the dynamics behavior of these cells from short circuit to open circuit. P3HT/F8TBT and P3HT/CdSe devices are characterized by poor charge extraction of the long‐lived carriers attributed to charge trapping. P3HT/PCBM devices, in contrast, show relatively trap‐free operation with the variation in the photocurrent decay kinetics with applied bias at low intensity, consistent with the drift of free charges under a uniform electric field. Under solar conditions at the maximum power point, we see direct evidence of bimolecular recombination in the P3HT/PCBM device competing with charge extraction. Transient photovoltage measurements reveal that, at open circuit, photogenerated charges have similar lifetimes in all device types, and hence, the extraction of these long‐lived charges is a limiting process in polymer/nanocrystal and polymer/polymer devices.     

Local Probing of Photocurrent and Photoluminescence in a Phase‐Separated Conjugated‐Polymer Blend by Means of Near‐Field Excitation

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 on a binary system consisting of poly[(9,9‐dioctylfluorene)‐alt‐benzothiadiazole] (F8BT) and poly[(9,9‐dioctylfluorene)‐alt‐(bis(N,N′‐(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 μm 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 an 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.     

Improved Performance of Polymer:Polymer Solar Cells by Doping Electron‐Accepting Polymers with an Organosulfonic Acid

The performance of polymer:polymer solar cells that are made using blend films of poly(3‐hexylthiophene) (P3HT) and poly(9,9‐dioctylfluorene‐co‐ benzothiadiazole (F8BT) is improved by doping the F8BT polymer with an organosulfonic acid [4‐ethylbezenesulfonic acid (EBSA)]. The EBSA doping of F8BT, to form F8BT‐EBSA, is performed by means of a two‐stage reaction at room temperature and 60°C with various EBSA weight ratios. The X‐ray photoelectron spectroscopy measurement reveals that both sulfur and nitrogen atoms in the F8BT polymer are affected by the EBSA doping. The F8BT‐EBSA films exhibit huge photoluminescence quenching, ionization potential shift toward lower energy, and greatly enhanced electron mobility. The short‐circuit current density of solar cells is improved by ca. twofold (10 wt.% EBSA doping), while the open‐circuit voltage increases by ca. 0.4 V. Consequently, the power conversion efficiency was improved by ca. threefold, even though the optical density of the P3HT:F8BT‐EBSA blend film is reduced by 10 wt.% EBSA doping due to the nanostructure and surface morphology change.     

Intermolecular Interactions of Perylene diimides in Photovoltaic Blends of Fluorene Copolymers: Disorder Effects on Photophysical Properties,Film Morphology and Device Efficiency

In the present work, we correlate the photophysical and photovoltaic properties with the respective film morphologies of three different blends made of the fluorene copolymers poly(9,9′‐dioctylfluorene‐co‐benzothiadiazole) (F8BT), poly[9,9′‐dioctylfluorene‐co‐N‐(4‐butylphenyl)diphenylamine] (TFB), and poly[9,9′‐dioctyfluorene‐co‐bis‐N,N′‐(4‐butylphenyl)‐bis‐N,N‐phenyl‐1,4‐phenylenediamine] (PFB) when blended with a perylene tetracarboxylic diimide (PDI) derivative. Additional photophysical studies in reference PDI blends of the electronically inert poly(styrene) matrix address the enhanced PDI intermolecular solid‐state interactions. We resolve the process of resonance energy transfer from excited polymer hosts to PDI and the process of photoinduced hole transfer from PDI to the polymer hosts. We deduce the efficiency of charge‐transfer PDI photoluminescence (PL) quenching and we discuss the power‐law PL kinetics seen in the as‐spun systems. Next we determine the dependence of the device external quantum efficiency (EQE) of these blends, in a range of annealing temperatures and PDI loadings. Differential scanning calorimetry enables precise selection of annealing temperatures. Optical microscopy shows that annealing enhances the order characteristics in the PDI aggregates in the F8BT:PDI system. In the case of the TFB:PDI and PFB:PDI blends, AFM studies suggest the formation of PDI‐rich domains on the film/air interface. The degree of order in the π–π stacking of the PDI monomers is inferred by the UV–Vis and PL spectra of the blends. The extent of order characteristics in PDI aggregates is correlated with the thermal properties of the hosts that control PDI molecular mobility upon annealing. The efficient dispersion of disrupted PDI crystallites is proposed to form appropriate percolation networks that favor balanced extraction of photogenerated carriers.     

Influence of Annealing and Interfacial Roughness on the Performance of Bilayer Donor/Acceptor Polymer Photovoltaic Devices

Through controlled annealing of planar heterojunction (bilayer) devices based on the polyfluorene copolymers poly(9,9‐dioctylfluorene‐co‐bis(N,N′‐(4,butylphenyl))bis(N,N′‐phenyl‐1,4‐phenylene)diamine) (PFB) and poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) we study the influence of interface roughness on the generation and separation of electron–hole pairs at the donor/acceptor interface. Interface structure is independently characterized by resonant soft X‐ray reflectivity with the interfacial width of the PFB/F8BT heterojunction observed to systematically increase with annealing temperature from 1.6 nm for unannealed films to 16 nm with annealing at 200 °C for ten minutes. Photoluminescence quenching measurements confirm the increase in interface area by the three‐fold increase in the number of excitons dissociated. Under short‐circuit conditions, however, unannealed devices with the sharpest interface are found to give the best device performance, despite the increase in interfacial area (and hence the number of excitons dissociated) in annealed devices. The decrease in device efficiency with annealing is attributed to decreased interfacial charge separation efficiency, partly due to a decrease in the bulk mobility of the constituent materials upon annealing but also (and significantly) due to the increased interface roughness. We present results of Monte Carlo simulations that demonstrate that increased interface roughness leads to lower charge separation efficiency, and are able to reproduce the experimental current‐voltage curves taking both increased interfacial roughness and decreased carrier mobility into account. Our results show that organic photovoltaic performance can be sensitive to interfacial order, and heterojunction sharpness should be considered a requirement for high performance devices.     

Charge transport properties and microstructure of polythiophene/polyfluorene blends

We present a combined charge transport and X-ray diffraction study of blends based on regioregular poly(3-hexylthiophene) (P3HT) and the polyfluorene co-polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT) that are used in efficient all-polymer solar cells. Hole mobility is observed to increase by nearly two orders of magnitude from less than 10^?7 cm² V^?1 s^?1 for as spin-coated blends to 6 × 10^?6 cm² V^?1 s^?1 for blends annealed at 453 K at a field of 2.7 × 10⁵ V/cm, but still significantly below the time-of-flight mobility of unblended P3HT of 1.7 × 10^?4 cm² V^?1 s^?1. The hole mobility of the blends also show a strong negative electric-field dependence, compared with a relatively flat electric-field dependence of unblended P3HT, suggestive of increased spatial disorder in the blends. X-ray diffraction measurements reveal that P3HT/F8TBT blends show a phase separation of the two components with a crystalline part attributed to P3HT and an amorphous part attributed to F8TBT. In as-spun and mildly annealed blends, the measured d-values and relative intensities of the 100, 200 and 300 P3HT peaks are noticeably different to unblended P3HT indicating an incorporation of F8TBT in P3HT crystallites that distorts the crystal structure. At higher anneal temperatures the blend d-values approach that of unblended P3HT suggesting a well separated blend with pure P3HT crystallites. P3HT crystallite size in the blend is also observed to increase with annealing from 3.3 to 6.1 nm, however similar changes in crystallite size are observed in unblended P3HT films with annealing. The lower mobility of P3HT/F8TBT blends is attributed not only to increased P3HT structural disorder in the blend, but also due to the blend morphology (increased spatial disorder). Changes in hole mobility with annealing are interpreted in terms of the need to form percolation networks of P3HT crystallites within an F8TBT matrix, with a possible contribution due to the intercalation of F8TBT in P3HT crystallites acting as defects in the as-prepared state.     

n‐Doping of a Low‐Electron‐Affinity Polymer Used as an Electron‐Transport Layer in Organic Light‐Emitting Diodes

n‐Doping electron‐transport layers (ETLs) increases their conductivity and improves electron injection into organic light‐emitting diodes (OLEDs). Because of the low electron affinity and large bandgaps of ETLs used in green and blue OLEDs, n‐doping has been notoriously more difficult for these materials. In this work, n‐doping of the polymer poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)] (F8BT) is demonstrated via solution processing, using the air‐stable n‐dopant (pentamethylcyclopentadienyl)(1,3,5‐trimethylbenzene)ruthenium dimer [RuCp*Mes]₂. Undoped and doped F8BT films are characterized using ultraviolet and inverse photoelectron spectroscopy. The ionization energy and electron affinity of the undoped F8BT are found to be 5.8 and 2.8 eV, respectively. Upon doping F8BT with [RuCp*Mes]₂, the Fermi level shifts to within 0.25 eV of the F8BT lowest unoccupied molecular orbital, which is indicative of n‐doping. Conductivity measurements reveal a four orders of magnitude increase in the conductivity upon doping and irradiation with ultraviolet light. The [RuCp*Mes]₂‐doped F8BT films are incorporated as an ETL into phosphorescent green OLEDs, and the luminance is improved by three orders of magnitude when compared to identical devices with an undoped F8BT ETL.     

Room‐Temperature Phase Demixing in Bulk Heterojunction Layers of Solution‐Processed Organic Photodetectors: the Effect of Active Layer Ageing on the Device Electro‐optical Properties

Herein, we address the reduction in the external quantum efficiency (EQE) of solution‐processed organic photodetectors caused by the room temperature phase demixing of components in the composite material of the photoactive layer. The reduction takes place under ambient conditions and after the completion of device fabrication. As a model system, we study photoactive blend films that consist of the electron acceptor N,N’‐bis(alkyl)‐3,4,9,10‐perylene tetracarboxylic diimide) (PDI) and the electron donor polymer poly(9,9’‐dioctylfluorene‐co‐benzothiadiazole) (F8BT). The ambient ageing of these photo­active layers is a consequence of the PDI component segregation; however, the final PDI domain size remains smaller than the resolution limit of optical microscopy. We find that the photophysical properties of the aged F8BT:PDI layer and the EQE of the aged device are significantly altered. The fabrication of F8BT:PDI layers from solvents of increasing boiling point allows for the spectroscopic monitoring of the ageing‐induced phase segregation (a‐PSG) process. For each solvent used, the extent of a‐PSG is correlated with the PDI dispersion in the F8BT matrix as received immediately after layer deposition. The tendency for room temperature phase demixing becomes stronger as PDI is more finely dispersed in the freshly spun F8BT:PDI layer. The evolution of the room temperature phase segregation of PDI has a negative impact on the photophysical processes that are essential for charge photogeneration in the F8BT:PDI photoactive layer.     

Nanoporous Silver Thin Films: Multifunctional Platforms for Influencing Chain Morphology and Optical Properties of Conjugated Polymers

Disordered nanoporous silver (NPAg) thin films fabricated by a thermally assisted dewetting method are employed as a platform to influence chain alignment, morphology, and optical properties of three well‐known conjugated polymers. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurements show that the porous structure of the metal induces close π–π stacking of poly(3‐hexylthiophene) (P3HT) chains and extended, planar chain conformations of poly(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl) (PFO) and poly[(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)] (F8BT). A greater degree of vertically‐oriented P3HT chains are found on NPAg compared with planar Ag. However, PFO and F8BT chain alignment is only affected when pore size is large. The optical properties of NPAg films are investigated by transmission and back‐scattering spectroscopies. Strong back‐scattering is observed for all NPAg morphologies, especially for NPAg with small pore sizes. Photoluminescence spectroscopy of conjugated polymer layers on NPAg showed pronounced emission enhancements (up to factors of 26) relative to layers on glass. The enhancements are attributed primarily to: 1) redistribution of conjugated polymer emission by Ag; 2) redirection of emission by polymer‐filled nanopores; and 3) local electromagnetic field effects. This work demonstrates the potential of NPAg‐thin films to influence molecular chain morphology and to improve light‐extraction in organic optoelectronic devices.     

Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells

We investigate thin poly(3‐hexylthiophene‐2,5‐diyl)/[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT/PCBM) films, which are widely used as active layers in plastic solar cells. Their structural properties are studied by grazing‐incidence X‐ray diffraction (XRD). The size and the orientation of crystalline P3HT nanodomains within the films are determined. PCBM crystallites are not detected in thin films by XRD. Upon annealing, the P3HT crystallinity increases, leading to an increase in the optical absorption and spectral photocurrent in the low‐photon‐energy region. As a consequence, the efficiency of P3HT/PCBM solar cells is significantly increased. A direct relation between efficiency and P3HT crystallinity is demonstrated.     

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.     

Origins of Improved Hole‐Injection Efficiency by the Deposition of MoO3 on the Polymeric Semiconductor Poly(dioctylfluorene‐alt‐benzothiadiazole)

The electronic structure of the interfaces formed after deposition of MoO₃ hole‐injection layers on top of a polymer light‐emitting material, poly(dioctylfluorene‐alt‐benzothiadiazole) (F8BT), is studied by ultraviolet photoelectron spectroscopy (UPS), X‐ray photoelectron spectroscopy and metastable atom electron spectroscopy. Significant band bending is induced in the F8BT film by MoO₃ “acceptors” that spontaneously diffuse into the F8BT “host” probably driven by kinetic energy of the deposited hot MoO₃. Further deposition leads to the saturation of the band bending accompanied by the formation of MoO₃ overlayers. Simultaneously, a new electronic state in the vicinity of the Fermi level appears on the UPS spectra. Since this peak does not appear in the bulk MoO₃ film, it can be assigned as an interface state between the MoO₃ overlayer and underlying F8BT film. Both band bending and the interface state should result from charge transfer from F8BT to MoO₃, and they appear to be the origin of the hole‐injection enhancement by the insertion of MoO₃ layers between the F8BT light‐emitting diodes and top anodes.     

Mapping of Domain Orientation and Molecular Order in Polycrystalline Semiconducting Polymer Films with Soft X‐Ray Microscopy

We utilize scanning transmission X‐ray microscopy (STXM) to study the domain structure of polycrystalline films of the semiconducting polymer poly(9,9’‐dioctylfluorene‐co‐benzothiadiazole) (F8BT). By taking several images at different orientations of the film with respect to the polarization of the X‐ray beam, we are able to compute quantitative maps of molecular alignment/order and molecular orientation, including both the backbone direction and phenyl ring plane orientation, as well as the in‐plane and out‐of‐plane components. We show that polycrystalline F8BT films consist of well‐ordered micron‐sized domains with the transition from one domain orientation to another characterized either by a smooth transition of orientation or by ～ 200 nm wide disordered domain boundaries. The morphology of the disordered domain boundaries resemble the electroluminescence patterns observed previously in F8BT light‐emitting field‐effect transistors suggesting that charge trapping at these disordered domain boundaries facilitates charge recombination in ambipolar operation. A relatively narrow distribution of local average tilt angles is observed that correlates with film structure, with the ordered domains in general showing a higher tilt angle than the disordered domain boundaries. We also use secondary electron detection to image the surface domain structure of polycrystalline F8BT films and demonstrate that the polycrystalline structure extends to the film/air interface. Finally, we calculate ideal NEXAFS spectra corresponding to a perfect F8BT crystal oriented with the 1s – π^* transition dipole moment parallel and perpendicular to the electric field vector of a perfectly linearly polarized X‐ray beam.     

Surface‐Directed Phase Separation of Conjugated Polymer Blends for Efficient Light‐Emitting Diodes

The ability to control organic‐organic interfaces in conjugated polymer blends is critical for further device improvement. Here, we control the phase separation in blends of poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) (F8BT) and poly(9,9‐di‐n‐octylfluorene‐alt‐(1,4‐phenylene‐((4‐sec‐butylphenyl)imino)‐1,4‐phenylene) (TFB) via chemical modification of the substrate by microcontact printing of octenyltrichlorosilane molecules. The lateral phase‐separated structures in the blend film closely replicate the underlying micrometer‐scale chemical pattern. We found nanometer‐scale vertical segregation of the polymers within both lateral domains, with regions closer to the substrate being substantially pure phases of either polymer. Such phase separation has important implications for the performance of light‐emitting diodes fabricated using these patterned blend films. In the absence of a continuous TFB wetting layer at the substrate interface, as typically formed in spin‐coated blend films, charge carrier injection is confined in the well‐defined TFB‐rich domains. This confinement leads to high electroluminescence efficiency, whereas the overall reduction in the roughness of the patterned blend film results in slower decay of device efficiency at high voltages. In addition, the amount of surface out‐coupling of light in the forward direction observed in these blend devices is found to be strongly correlated to the distribution of periodicity of the phase‐separated structures in the active layer.     

Evolution of Structure,Optoelectronic Properties,and Device Performance of Polythiophene:Fullerene Solar Cells During Thermal Annealing

We use spectroscopic ellipsometry to study the evolution of structure and optoelectronic properties of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) photovoltaic thin film blends upon thermal annealing. Four distinct processes are identified: the evaporation of residual solvent above the glass transition temperature of the blend, the relaxation of non‐equilibrium molecular conformation formed through spin‐casting, the crystallization of both P3HT and PCBM components, and the phase separation of the P3HT and PCBM domains. Devices annealed at 150 °C for between 10 and 60 min exhibit an average power conversion efficiency of around 4.0%. We find that the rate at which the P3HT/PCBM is returned to room temperature is more important in determining device efficiency than the duration of the isothermal annealing process. We conclude that the rapid quenching of a film from the annealing temperature to room temperature hampers the crystallization of the P3HT and can trap non‐equilibrium morphological states. Such states apparently impact on device short circuit current, fill factor and, thus, operational efficiency.     

Host Exciton Confinement for Enhanced Förster‐Transfer‐Blend Gain Media Yielding Highly Efficient Yellow‐Green Lasers

This paper reports state‐of‐the‐art fluorene‐based yellow‐green conjugated polymer blend gain media using Förster resonant‐energy‐transfer from novel blue‐emitting hosts to yield low threshold (≤7 kW cm^?2) lasers operating between 540 and 590 nm. For poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) (15 wt%) blended with the newly synthesized 3,6‐bis(2,7‐di([1,1′‐biphenyl]‐4‐yl)‐9‐phenyl‐9H‐fluoren‐9‐yl)‐9‐octyl‐9H–carbazole (DBPhFCz) a highly desirable more than four times increase (relative to F8BT) in net optical gain to 90 cm^?1 and 34 times reduction in amplified spontaneous emission threshold to 3 µJ cm^?2 is achieved. Detailed transient absorption studies confirm effective exciton confinement with consequent diffusion‐limited polaron‐pair generation for DBPhFCz. This delays formation of host photoinduced absorption long enough to enable build‐up of the spectrally overlapped, guest optical gain, and resolves a longstanding issue for conjugated polymer photonics. The comprehensive study further establishes that limiting host conjugation length is a key factor therein, with 9,9‐dialkylfluorene trimers also suitable hosts for F8BT but not pentamers, heptamers, or polymers. It is additionally demonstrated that the host highest occupied and lowest unoccupied molecular orbitals can be tuned independently from the guest gain properties. This provides the tantalizing prospect of enhanced electron and hole injection and transport without endangering efficient optical gain; a scenario of great interest for electrically pumped amplifiers and lasers.     

Organic Light‐Emitting Diodes Based on Poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2)

A study of the optical properties of poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2) is reported, identifying this polymer as one that possesses a desirable combination of charge transport and light emission properties. The optical and morphological properties of a series of polymer blends with F8T2 dispersed in poly(9,9‐dioctylfluorene) (PFO) are described and almost pure‐green emission from light emitting diodes (LEDs) based thereon is demonstrated. High luminance green electroluminescence from LEDs using only a thin film of F8T2 for emission is also reported. The latter demonstration for a polymer previously primarily of interest for effective charge transport constitutes an important step in the development of emissive materials for applications where a union of efficient light emission and effective charge transport is required.