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.     

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.     

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.     

Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

Here, studies on the evolution of photophysics and device performance with annealing of blends of poly(3‐hexylthiophene) with the two polyfluorene copolymers poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2′′‐diyl) (F8TBT) and poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) are reported. In blends with F8TBT, P3HT is found to reorganize at low annealing temperatures (100 °C or below), evidenced by a redshift of both absorption and photoluminescence (PL), and by a decrease in PL lifetime. Annealing to 140 °C, however, is found to optimize device performance, accompanied by an increase in PL efficiency and lifetime. Grazing‐incidence small‐angle X‐ray scattering is also performed to study the evolution in film nanomorphology with annealing, with the 140 °C‐annealed film showing enhanced phase separation. It is concluded that reorganization of P3HT alone is not sufficient to optimize device performance but must also be accompanied by a coarsening of the morphology to promote charge separation. The shape of the photocurrent action spectra of P3HT:F8TBT devices is also studied, aided by optical modeling of the absorption spectrum of the blend in a device structure. Changes in the shape of the photocurrent action spectra with annealing are observed, and these are attributed to changes in the relative contribution of each polymer to photocurrent as morphology and polymer conformation evolve. In particular, in as‐spun films from xylene, photocurrent is preferentially generated from ordered P3HT segments attributed to the increased charge separation efficiency in ordered P3HT compared to disordered P3HT. For optimized devices, photocurrent is efficiently generated from both P3HT and F8TBT. In contrast to blends with F8TBT, P3HT is only found to reorganize in blends with F8BT at annealing temperatures of over 200 °C. The low efficiency of the P3HT:F8BT system can then be attributed to poor charge generation and separation efficiencies that result from the failure of P3HT to reorganize.     

One‐Step Macroscopic Alignment of Conjugated Polymer Systems by Epitaxial Crystallization during Spin‐Coating

The one‐step preparation of highly anisotropic polymer semiconductor thin films directly from solution is demonstrated. The conjugated polymer poly(3‐hexylthiophene) (P3HT) as well as P3HT:fullerene bulk–heterojunction blends can be spin‐coated from a mixture of the crystallizable solvent 1,3,5‐trichlorobenzene (TCB) and a second carrier solvent such as chlorobenzene. Solidification is initiated by growth of macroscopic TCB spherulites followed by epitaxial crystallization of P3HT on TCB crystals. Subsequent sublimation of TCB leaves behind a replica of the original TCB spherulites. Thus, highly ordered thin films are obtained, which feature square‐centimeter‐sized domains that are composed of one spherulite‐like structure each. A combination of optical microscopy and polarized photoluminescence spectroscopy reveals radial alignment of the polymer backbone in case of P3HT, whereas P3HT:fullerene blends display a tangential orientation with respect to the center of spherulite‐like structures. Moreover, grazing‐incidence wide‐angle X‐ray scattering reveals an increased relative degree of crystallinity and predominantly flat‐on conformation of P3HT crystallites in the blend. The use of other processing methods such as dip‐coating is also feasible and offers uniaxial orientation of the macromolecule. Finally, the applicability of this method to a variety of other semi‐crystalline conjugated polymer systems is established. Those include other poly(3‐alkylthiophene)s, two polyfluorenes, the low band‐gap polymer PCPDTBT, a diketopyrrolopyrrole (DPP) small molecule as well as a number of polymer:fullerene and polymer:polymer blends.     

Fullerene‐Free Polymer Solar Cells with Open‐Circuit Voltage above 1.2 V: Tuning Phase Separation Behavior with Oligomer to Replace Polymer Acceptor

This study has proposed to use a well‐defined oligomer F4TBT4 to replace its analogue polymer as electron acceptor toward tuning the phase separation behavior and enhancing the photovoltaic performance of all‐polymer solar cells. It has been disclosed that the oligomer acceptor favors to construct pure and large‐scale phase separation in the polymer:oligomer blend film in contrast to the polymer:polymer blend film. This gets benefit from the well‐defined structure and short rigid conformation of the oligomer that endows it aggregation capability and avoids possible entanglement with the polymer donor chains. The charge recombination is to some extent suppressed and charge extraction is also improved. Finally, the P3HT:F4TBT4 solar cells not only output a high V_OC above 1.2 V, but also achieve a power conversion efficiency of 4.12%, which is two times higher than the P3HT:PFTBT solar cells and is comparable to the P3HT:PCBM solar cells. The strategy of constructing optimum phase separation with oligomer to replace polymer opens up new prospect for the further improvement of the all‐polymer solar cells.     

On the correlation between morphology and Amplified Spontaneous Emission properties of a polymer: Polymer blend

We investigate the Amplified Spontaneous Emission (ASE) properties of a prototypical host-guest polymer polymer blend, namely poly(9,9-dioctylfluorene) (PF8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) blend, with different concentration ratio. We show that the initial F8BT content increase causes an increase of the F8BT ASE threshold, even leading to ASE suppression for F8BT contents between 25% and 75%. ASE is then recovered upon further increase of the F8BT relative content. We demonstrate that the ASE properties of the PF8:F8BT are dominated by morphology effects, like submicrometric phase segregation, determining the net gain of the active waveguides.     

Improved Thermal Stability of Polymer Solar Cells by Incorporating Porphyrins

Thermal stability has been the important issue in organic solar cell, especially for the large scale fabrication and application in the future. In this work, a new strategy involving the introduction of porphyrin compound (BL) is proposed to prevent the [6,6]‐phenyl C61 butyric acid methyl ester (PC₆₁BM) aggregation. The supramolecular interactions between PC₆₁BM and BL are first demonstrated in PC₆₁BM:BL binary blend, and then the effect of BL on P3HT:PC₆₁BM blend is qualitatively and quantitatively studied by differential scanning calorimetry, UV–vis absorption spectroscopy, atomic force microscopy, optical microscopy, and fluorescence techniques. It is found that the BL addition not only stabilizes the morphology of P3HT:PC₆₁BM blend films, but also shows a good ability to maintain the electron mobility by depressing the PC₆₁BM crystallization. And the thermal stability of the devices based on P3HT:PC₆₁BM:BL ternary blend films is therefore greatly improved. For example, 8 wt% BL doping drops the power conversion efficiency by 10.5% relative to its peak value after 48 h of annealing at 130 °C, while 71.5% of decrease is obtained for the device without BL after only 3 h of annealing. This strategy is preliminarily proved to be universal and will show great potentials in future commercialization of polymer solar cells.     

“Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3‐hexylthiophene) and Methanofullerenes

The self‐organization of the polymer in solar cells based on regioregular poly(3‐hexylthiophene) (RR‐P3HT):[6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) is studied systematically as a function of the spin‐coating time t_s (varied from 20–80 s), which controls the solvent annealing time t_a, the time taken by the solvent to dry after the spin‐coating process. These blend films are characterized by photoluminescence spectroscopy, UV‐vis absorption spectroscopy, atomic force microscopy, and grazing incidence X‐ray diffraction (GIXRD) measurements. The results indicate that the π‐conjugated structure of RR‐P3HT in the films is optimally developed when t_a is greater than 1 min (t_s ～ 50 s). For t _s < 50 s, both the short‐circuit current (J_SC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < t_s < 55 s, the J_SC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered π‐conjugated structure of RR‐P3HT to 1.2 % for a less‐ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon‐to‐electron conversion efficiency spectrum of the less‐ordered solar cell shows a clear loss in both the overall magnitude and the long‐wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under “solvent annealing” conditions, the polymer is seen to be ordered even at 67 wt % PCBM loading. The open‐circuit voltage (V_OC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.     

Abrupt Morphology Change upon Thermal Annealing in Poly(3‐Hexylthiophene)/Soluble Fullerene Blend Films for Polymer Solar Cells

The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₁ (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.     

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.     

Generation of Compositional‐Gradient Structures in Biodegradable,Immiscible, Polymer Blends by Intermolecular Hydrogen‐Bonding Interactions

A biodegradable, immiscible poly(butylenes adipate‐co‐butylenes terephthalate) [P(BA‐co‐BT)]/poly(ethylene oxide) (PEO) polymer blend film with compositional gradient in the film‐thickness direction has been successfully prepared in the presence of a low‐molecular‐weight compound 4,4′‐thiodiphenal (TDP), which is used as a miscibility‐enhancing agent. The miscibilities of the P(BA‐co‐BT)/PEO/TDP ternary blend films and the P(BA‐co‐BT)/PEO/TDP gradient film were investigated by differential scanning calorimetry (DSC). The compositional gradient structure of the P(BA‐co‐BT)/PEO/TDP (46/46/8 w/w/w) film has been confirmed by microscopic mapping measurement of Fourier‐transform infrared spectra and dynamic mechanical thermal analysis. We have developed a new strategy for generating gradient‐phase structures in immiscible polymer‐blend systems by homogenization, i.e., adding a third agent that can enhance the miscibility of the two immiscible polymers through simultaneous formation of hydrogen bonds with two component polymers.     

Effect of Alkyl Side‐Chain Length on Photovoltaic Properties of Poly(3‐alkylthiophene)/PCBM Bulk Heterojunctions

The morphological, bipolar charge‐carrier transport, and photovoltaic characteristics of poly(3‐alkylthiophene) (P3AT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side‐chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3‐butylthiophene) (P3BT, m = 4), poly(3‐pentylthiophene) (P3PT, m = 5), and poly(3‐hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo‐current yield (exceeding 10 mA cm^?2) irrespective of side‐chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole‐only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field‐effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.     

Efficient Polymer Solar Cells with Surface Relief Gratings Fabricated by Simple Soft Lithography

Polymer‐based photovoltaic cells, with periodic sub‐micrometer structures as an efficient light‐trapping scheme, are investigated to improve the performance of organic solar cells based on poly(3‐hexylthiophene) and 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐(6,6)C₆₁. A soft lithographic approach that uses photoresponsive azo polymer films as masters and poly(dimethylsiloxane) as stamps is used to form surface relief gratings (SRGs) on the active layers. The effect of periodic gratings on solar cell performance is precisely investigated according to various grating conditions such as period, depth, and dimension. The solar cells with 1D and 2D SRGs present improved incident‐photon‐to‐current conversion efficiencies and an overall increase in power conversion efficiencies, primarily resulting from the enhancement of short‐circuit current density, indicating that periodic structures induce further photon absorption in the active film.     

Efficiency improvement of polymer solar cells by iodine doping

A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C₆₀ butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I₂) doping concentrations. The short circuit current density (J_sc) was increased to 8.7 mA/cm² from 4 mA/cm², meanwhile the open circuit voltage (V_oc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage.     

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.     

Hybrid Solar Cells from a Blend of Poly(3‐hexylthiophene) and Ligand‐Capped TiO2 Nanorods

Hybrid bulk heterojunction solar cells based on nanocrystalline TiO₂ (nc‐TiO₂) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3‐hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active‐layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer–metal oxide interaction at the nc‐TiO₂/P3HT interface, resulting in a six‐fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc‐TiO₂ particle surface is found to be crucial for charge separation. Ligand‐exchange procedures applied on the TOPO‐capped nc‐TiO₂ nanorods with an amphiphilic ruthenium‐based dye are found to further improve the charge‐separation yield at the polymer–nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor–acceptor bulk heterojunction systems, we conclude that charge transport in the nc‐TiO₂:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc‐TiO₂ particles.     

Ternary mixing: A simple method to tailor the morphology of organic solar cells

We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier mobility and organic solar cell performance. We investigate ternaries consisting of regio random poly(3-hexylthiophene) (P3HT), regio regular P3HT and soluble fullerene derivative, PCBM. By means of absorption, photoluminescence, atomic force microscopy and X-ray diffraction, we demonstrate that the structure of ternary films consists of crystallites of regular P3HT embedded into a random polymer matrix acting as a soft scaffolding where PCBM can only form nanoscale aggregates but cannot grow the detrimental micron-sized structures often observed in the conventional regular P3HT:PCBM case upon annealing. The ternary films exhibit higher degree of crystallinity than the conventional blends, but with smaller crystallite sizes. Moreover, we show that the addition of the random polymer chains does not prevent good charge carrier transport for regio random P3HT concentrations up to 50% of the total polymer content. Finally, we prove that solar cells based on the ternary systems have a similar short circuit current than the conventional binary, but improved open circuit current (by 100 mV), which leads to an overall enhancement of power conversion efficiency.