Dual length morphological model for bulk‐heterojunction,polymer‐based solar cells

We present a dual length morphological model for the active layer of bulk‐heterojunction, polymer‐based solar cells using results from neutron and X‐ray scattering techniques. Two critical characteristic lengths are found in the mixtures composed of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM). A characteristic length at 15 nm is the local characteristic of the P3HT crystals and PCBM agglomerations, which is independent of the bulk composition upon relaxation by thermal annealing. Conversely, a larger bicontinuous structure described by Teubner–Strey model with phase distances between 23 and 35 nm forms only after thermal annealing, which is highly correlated to the bulk compositions. These results suggest phase separation between the polymer and fullerene can only be partially manipulated by simple processing techniques such as coating conditions and annealing, and a more rigorous design of the morphology should be implemented in the future. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym. Phys. 2014 , 52, 387–396     

Effect of polydispersity on the bulk‐heterojunction morphology of P3HT:PCBM solar cells

Bulk heterojunctions (BHJs) based on semiconducting electron–donor polymer and electron–acceptor fullerene have been extensively investigated as potential photoactive layers for organic solar cells (OSCs). In the experimental studies, poly‐(3‐hexyl‐thiophene) (P3HT) polymers are hardly monodisperse as the synthesis of highly monodisperse polymer mixture is a near impossible task to achieve. However, the majority of the computational efforts on P3HT: phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM)‐based OSCs, a monodisperse P3HT is usually considered. Here, results from coarse‐grained molecular dynamics simulations of solvent evaporation and thermal annealing process of the BHJ are shared describing the effect of variability in molecular weight (also known as polydispersity) on the morphology of the active layer. Results affirm that polydispersity is beneficial for charge separation as the interfacial area is observed to increase with higher dispersity. Calculations of percolation and orientation tensors, on the other hand, reveal that a certain polydispersity index ranging between 1.05 and 1.10 should be maintained for optimal charge transport. Most importantly, these results point out that the consideration of polydispersity should be considered in computational studies of polymer‐based OSCs. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 895–903     

Effects of amorphous poly(3‐hexylthiophene) on active‐layer structure and solar cells performance

A key challenge to the development of polymer‐based organic solar cells is the issue of long‐term stability, which is mainly caused by the unstable time‐dependent morphology of active layers. In this study, poly(3‐hexylthiophene) (P3HT)/[6,6]‐phenyl C60‐butyric acid methyl ester (PCBM) blend is used as a model system to demonstrate that the long‐term stability of power conversion efficiency can be significantly improved by the addition of a small amount of amorphous regiorandom P3HT into semicrystalline regioregular one. The optical properties measured by UV–vis absorption and photoluminescence reveal that regiorandom P3HT can intimately mix with PCBM and prevent the segregation of PCBM. In addition, X‐ray scattering techniques were adopted to evidence the retardation of phase separation between P3HT and PCBM when regiorandom P3HT is added, which is further confirmed by optical microscopy that shows a reduction of large PCBM crystals after annealing at high temperature in the presence of regiorandom P3HT. The improvement of the long‐term stability is attributed to the capability of amorphous P3HT to be thermodynamically miscible with PCBM, which allows the active layer to form a more stable structure that evolves slower and hence decelerates the device decay. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 975–985     

In situ film characterization of thermally treated microstructured conducting polymer films

Based on a low‐cost fabrication routine microstructured conducting polymer films of poly (dioctylfluorene‐co‐benzothiadiazole) (F8BT) are prepared without any heat treatment or vacuum steps. The influence of thermal annealing at temperatures below the glass transition temperature of F8BT on such microstructured channel structures is investigated. In the applied structuring routine, a F8BT film is spin coated on a channel‐type hard master structure and afterwards floated on a flat support. Thereby, the properties of the final polymeric structures, for example channel width and height, can be tuned by simply varying the polymer concentration in solution and using the same master structure. With in situ grazing incidence small angle X‐ray scattering and imaging ellipsometry the installed channel structure and the influence of thermal treatment are probed. A complex interplay between a macroscopic polymer flow (reduced channel heights) and a molecular rearrangement (formation of mesoscopic crystallites) takes place during thermal annealing. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Quantifying the efficiency of förster‐assisted optical gain in semiconducting polymer blends by excitation wavelength selective amplified spontaneous emission

We study the correlation between Förster resonance energy transfer (FRET) and optical gain properties in conjugated polymer blends based on regioregular poly(3‐hexylthiophene) (P3HT) and poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT). First, FRET dynamics are investigated with femtosecond transient absorption spectroscopy observing a sub‐picosecond energy transfer from F8BT to P3HT (550 fs) at medium doping levels (40% wt P3HT in F8BT). Amplified spontaneous emission (ASE) is then characterized in blends upon exciting predominantly the host and guest polymers, respectively. The corresponding density of absorbed photons at threshold conditions is compared upon host or guest photoexcitation as a method to quantitatively determine the FRET‐assisted ASE efficiencies. We observe a reduction in ASE efficiency upon host photoexcitation of 20%, in the best case, respect to guest photoexcitation. Our results confirm that even in strongly coupled host:guest mixtures delayed exciton population by energy transfer is subtle to losses ascribed to exciton–exciton annihilation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2311–2317.     

Photoluminescence Study on Charge Transfer Behavior of Poly(3‐hexylthiophene) and PCBM Blends

Charge transfer behavior of Poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl eser (PCBM) in solutions and in films were examined by photoluminescence (PL) spectroscopy. PL study in solutions indicated that separation distance between P3HT and PCBM affected charge transfer efficiency more seriously than the interface area issue between P3HT and PCBM. P3HT/PCBM film showed very effective photo‐induced charge transfer before post‐thermal annealing on the bi‐layer P3HT/PCBM film. Charge transfer efficiency was gradually diminished by the annealing‐induced phase separation between P3HT and PCBM as revealed by increasing PL emission intensity of P3HT.     

Novel alkoxyanthracene donor and benzothiadiazole acceptor for organic thin film transistor and bulk heterojunction organic photovoltaic cells

Novel alkoxy anthracene (ODA)‐based polymeric semiconductors were designed for polymer solar cell applications. Alkoxyanthracene, which contains many π electrons and electron donating group, was easily synthesized. The copolymers, poly(alkoxy anthracene‐alt‐thiophene benzothiadiazole thiophene) poly(ODA‐TBT) and poly(alkoxy anthracene‐alt‐benzothiadiazole) poly(ODA‐BT), have been obtained by Suzuki coupling polymerization. Both polymers have ODA unit as a donor and benzothiadiazole as an acceptor. ODA‐TBT has thiophene linkages between ODA and benzothiadiazole. The optical, thermal, and electrochemical properties have been investigated by UV–visible absorption, thermal gravimetric analysis, differential scanning calorimetry, and CV. Organic thin‐film transistor using polymers showed that the hole mobility of poly(ODA‐alt‐TBT) was around 3.6 × 10^?3 cm²/Vs with on/off ratio of 9.91 × 10⁵ while that of poly(ODA‐alt‐BT) was around 1.21 × 10^?2 cm²/Vs with on/off ratio of 2.64 × 10⁶. Organic photovoltaic performance based on polymers were evaluated with a configuration of ITO/PEDOT:PSS/active layer/LiF/Al. Poly(ODA‐TBT) exhibits a short circuit current (J_sc) of 3.9 mA/cm² and power conversion efficiency (PCE) of 1.4%, and poly(ODA‐BT) exhibits the J_sc of 6.4 mA/cm² and PCE of 2.2%. The better device performance of poly(ODA‐BT) is attributed to its charge transfer ability and enhanced mobility and crystallinity although poly(ODA‐BT) does not have extended π‐conjugation due to twisted structure compared with poly(ODA‐TBT). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1306–1314     

Exploring the synthesis and impact of end‐functional poly(3‐hexylthiophene)

In recent years, end‐functional poly(3‐hexylthiophene) (P3HT) has proven to be instrumental in the continued development and innovation within the broad conjugated polymer arena, enabling a variety of applications, particularly in organic electronics. The availability of P3HT with controlled molecular weights, low polydispersity, and importantly, a wide range of reactive end‐groups not only serves as a key building block for the preparation of conjugated block copolymers but also facilitates the development of hybrid nanocomposite materials via inorganic surface modification strategies. This Highlight focuses on the synthetic approaches to end‐functional P3HT and the impact of these systems in emerging technologies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 831–841     

Alternating copolymers of electron‐rich arylamine and electron‐deficient 2,1,3‐benzothiadiazole: Synthesis,characterization and photovoltaic properties

A series of alternating copolymers of electron‐rich arylamine and electron‐deficient 2,1,3‐benzothiadiazole (BT), PV‐BT, DP‐BT, and TP‐BT, were synthesized by Heck coupling reaction. UV–vis absorption and fluorescence spectra show that the copolymerization of electron‐rich diphenylamine (DP), triphenylamine (TP), MEH‐PV (PV), and electron‐deficient BT results in low‐bandgap conjugated polymers. Within the three copolymers of PV‐BT, DP‐BT, and TP‐BT, TP‐BT possesses the highest hole mobility of 4.68 × 10^{? 5} cm²/V, as determined from the space charge limited current (SCLC) model. The bulk heterojunction‐typed polymer solar cells (PSCs) were fabricated with the blend of the copolymers and PCBM as the photosensitive layer. The power conversion efficiencies (PCE) of the PSCs based on PV‐BT, DP‐BT, and TP‐BT reached 0.26%, 0.39%, and 0.52%, respectively, under the illumination of AM 1.5, 100 mW/cm². The results indicate that TP‐BT is a promising photovoltaic polymer for PSCs. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3861–3871, 2007     

Degradation mechanisms of polyfluorene‐based organic semiconductor lasers under ambient and oxygen‐free conditions

In this communication we investigate the degradation mechanisms of different highly fluorescent polyfluorenes for applications as active organic semiconductor material in laser devices. Using various analytical methods, like Ultraviolet‐Visible (UV‐Vis) absorption spectroscopy, Fourier‐transform infrared spectroscopy (FT‐IR) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), we investigate photo‐induced degradation mechanisms. It is shown that the photo‐oxidation rate decreases with an increasing number of benzothiadiazole units within the conjugated polymer. Photooxidation is much more distinct for poly[9,9‐dioctylfluorenyl‐2,7‐diyl] (PFO) than for poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐alt‐co‐(1,4‐benzo‐{2,1′,3}‐thiadiazole)] (F8BT). The influence of the photooxidation on the lifetime of the organic laser devices is not as profound as previously assumed, since the laser shuts down before any evidence of photo‐oxidation in F8BT manifests. We observe that the solubility of the material is different at various degradation levels and we consider chain scission of excited bonds and cross‐linking as dominant degradation factors. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1029–1034     

Poly(3‐hexylthiophene)‐b‐poly(3‐cyclohexylthiophene): Synthesis,microphase separation,thin film transistors,and photovoltaic applications

We report the synthesis, characterization, microphase separation, field‐effect charge transport, and photovoltaic properties of regioregular poly(3‐hexylthiophene)‐b‐poly(3‐cyclohexylthiophene) (P3HT‐b‐P3cHT). Two compositions of P3HT‐b‐P3cHT (HcH63 and HcH77) were synthesized with weight‐average molecular weights of 155,500 and 210,800 and polydispersity indices of 1.45 and 1.57, respectively. Solvent‐casted HcH77 was found to self‐assemble into nanowires with a width of 12.5 ± 0.9 nm and aspect ratios of 50–120, as observed by TEM imaging. HcH77 and HcH63 annealed 280 °C were observed by small angle X‐ray scattering (SAXS) and wide angle X‐ray scattering (WAXS) to be microphase‐separated with characteristic length scales of 17.0–21.7 nm. The microphase‐separated domains were shown to be crystalline with interlayer backbone (100) d‐spacings of 1.69 and 1.40 nm, which correspond to the P3HT and P3cHT blocks, respectively. Field‐effect transistors fabricated from P3HT‐b‐P3cHT thin films showed a mobility of holes (0.0019 cm²/Vs) which is independent of thermal annealing. Bulk heterojunction solar cells based on HcH77/fullerene (PC₇₁BM) blend thin films had a maximum power conversion efficiency of 2.45% under 100 mW/cm² AM1.5 solar illumination in air. These results demonstrate that all‐conjugated block copolymers are suitable semiconductors for applications in field‐effect transistors and bulk heterojunction solar cells. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 614–626, 2010     

Annealing effect on performance and morphology of photovoltaic devices based on poly(3‐hexylthiophene)‐b‐poly(ethylene oxide)

Novel block copolymers, poly(3‐hexylthiophene)‐b‐poly(ethylene oxide) (P3HT‐b‐PEO) were synthesized via Suzuki coupling reaction of P3HT and PEO homopolymers. The copolymers were characterized by NMR, gel permeation chromatography, differential scanning calorimeter, and UV–vis measurements. A series of devices based on the block copolymers with a fullerene derivative were evaluated after thermal or solvent annealing. The device using P3HT‐b‐PEO showed higher efficiency than using P3HT blend after thermal annealing. Phase‐separated structures in the thin films of block copolymer blends were investigated by atomic force microscopy to clarify the relationship between morphologies constructed by annealing and the device performance. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Influence of the bridging atom in fluorene analogue low‐bandgap polymers on photophysical and morphological properties of copper indium sulfide/polymer nanocomposite solar cells

This contribution presents the correlation between structural, morphological, and fluorescence properties as well as device performance of nanocomposite solar cells comprising two low‐band gap polymers, poly[[9‐(1‐octylnonyl)?9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl] (PCDTBT) and poly[2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl(9,9‐dioctyl‐9H‐9‐silafluorene‐2,7‐diyl)?2,5‐thiophenediyl] (PSiF‐DBT) and copper indium sulfide (CIS). It shows that, in analogy to organic solar cells, the device efficiency is strongly determined by different polymer structures leading to a different packing of the polymer chains and consequently to diverse morphologies. X‐ray diffraction investigation indicates increased semicrystallinity in PSiF‐DBT compared with the nitrogen analogue PCDTBT. The photoluminescence (PL) quenching of this polymer indicates that the higher photogeneration achieved in PSiF‐DBT based films can be correlated to a favorable donor‐acceptor phase separation. Transmission electron microscopy studies of PCDTBT:CIS blended films suggest the formation of polymer agglomerates in the layer resulting in a decreased PL quenching efficiency. For the considered polymer:CIS system, the combination of these effects leads to an enhanced overall device efficiency. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2013, 51, 1400–1410     

Monte Carlo morphological modelling of a P3HT:PCBM bulk heterojunction organic solar cell

To deepen the understanding of morphology evolution in bulk heterojunction P3HT:PCBM organic photovoltaics system by thermal treatment, domain‐size‐dependent interfacial energies were first determined by coarse‐grained molecular dynamics modelling and then used in Monte Carlo simulations of the morphology evolution. Thereby initial conditions associated with optimal interfacial surface area, continuous volume, as well as domain sizes, and spatial distributions of the phase separated domains were identified. In line with earlier studies, a 1:1 P3HT:PCBM blend ratio is found to exhibit the most efficient morphology for exciton dissociation and charge transport. Our simulations reveal that preseeding of P3HT crystal at the anode side prior to the annealing process will be instrumental to pin the formation of P3HT at the favorable electrode especially when seeding exceeds a threshold of 10% surface coverage, whereas denser seeding patterns beyond the threshold did not improve the active layer morphology further. The observed trilayer depth profile (in the absence of preseeded P3HT crystals) implies that the commonly used thickness 100 nm of the active layer is not ideal for ensuring that donor and acceptor phases dominate at opposite ends of the active layer. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 270–279     

Random poly(3‐hexylthiophene‐co‐3‐cyanothiophene‐co‐3‐(2‐ethylhexyl)thiophene) copolymers with high open‐circuit voltage in polymer solar cells

For the purpose of developing poly(3‐hexylthiophene) (P3HT) based copolymers with deep‐lying highest occupied molecular orbital (HOMO) levels for polymer solar cells with high open‐circuit voltage (V_oc), we report a combined approach of random incorporation of 3‐cyanothiophene (CNT) and 3‐(2‐ethylhexyl)thiophene (EHT) units into the P3HT backbone. This strategy is designed to overcome CNT content limitations in recently reported P3HT‐CNT copolymers, where incorporation of more than 15% of CNT into the polymer backbone leads to impaired polymer solubility and raises the HOMO level. This new approach allows incorporation of a larger CNT content, reaching even lower‐lying HOMO levels. Importantly, a very low HOMO level of ?5.78 eV was obtained, representing one of the lowest HOMO values for exclusively thiophene‐based polymers. Lower HOMO levels result in higher V_oc and higher power conversion efficiencies (PCE) compared to the previously reported P3HT‐CNT copolymers containing only 3‐hexylthiophene and CNT units. As a result, solar cells based on P3HT‐CNT‐EHT(15:15) , which contains 70% of P3HT, 15% of CNT and 15% of EHT, yield a V_oc of 0.83 V in blends with PC₆₁BM while preserving high fill factor (FF) and high short‐circuit current density (J_sc), resulting in 3.6% PCE. Additionally, we explored the effect of polymer number‐average molecular weight (M_n) on the optoelectronic properties and solar cell performance for the example of P3HT‐CNT‐EHT(15:15). The organic photovoltaic (OPV) performance improves with polymer M_n increasing from 3.4 to 6.7 to 9.6 kDa and then it declines as M_n further increases to 9.9 and to 16.2 kDa. The molecular weight study highlights the importance of not only the solar cell optimization, but also the significance of individual polymer properties optimization, in order to fully explore the potential of any given polymer in OPVs. The broader ramification of this study lies in potential application of these high band gap copolymers with low‐lying HOMO level in the development of ternary blend photovoltaics as well as tandem OPV. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1526–1536     

Classification of semiconducting polymeric mesophases to optimize device postprocessing

Semiconducting polymers form a variety of phases and mesophases that respond differently to postdeposition solvent or thermal treatments. Here it is shown that classification of these materials into their appropriate mesophases can be a useful tool to optimize their thermal postdeposition treatments. Calorimetry is used to quantify differences between materials having similar molecular structures, using a well‐established framework based on the kinetics and thermodynamics of phase changes. By way of example, this classification scheme is used to identify differences in three polymers, poly(3‐hexylthiophene‐2,5‐diyl) and two isomeric bithiophene–thienothiophene copolymers. It is demonstrated that poly(3‐hexylthiophene) is a “normal” polymer crystal and that the two bithiophene–thienothiophene copolymers have liquid crystalline phases. The different phase structure is notable in light of the molecular similarity of the three polymers and has an impact on the thermal postprocessing conditions that maximize field effect charge carrier mobility in thin film transistor devices. Strong superheating effects are demonstrated for the two bithiophene–thienothiophene copolymers and the impact on annealing is demonstrated using grazing incidence X‐ray diffraction. Some suggestions are also put forth for what post‐processing should be employed for each class of polymer. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1641–1653     

Improving charge transport in poly(3‐hexylthiophene) transistors via blending with an alkyl‐substituted phenylene–thiophene–thiophene–phenylene molecule

A prototypical semiconducting bicomponent system consisting of a conjugated polymer, that is, poly(3‐hexylthiophene) (P3HT), blended with a small thiophene containing conjugated molecule, that is, an alkyl‐substituted bisphenyl‐bithiophene [phenylene–thiophene–thiophene–phenylene (PTTP)], has been used as an electroactive active layer in field‐effect transistors (FETs). The self‐assembly of this bicomponent system at surfaces has been studied at different length scales, from the nanoscale to the macroscale, and compared with the behavior of monocomponent films of PTTP and P3HT. The correlation between morphology and electric properties of the semiconducting material is explored by fabricating prototypes of FETs varying the relative concentrations of the two‐component blend. The maximum charge carrier mobility value, achieved with a few percent of PTTP component, is not simply due to a uniform dispersion of the molecules in the polymer matrix, but rather to the generation of very long percolation paths, whose composition and electrical properties can be tuned with the PTTP concentration. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

High‐efficiency polymer solar cells based on phenylenevinylene copolymer with BF2‐azopyrrole complex and CN‐PC70BM with solvent additive

Power conversion efficiency (PCE) of phenylenevinylene‐based copolymer with BF₂ azopyrrole complex (PB)/modified PC₇₀BM, that is, CN‐PC₇₀BM bulk heterojunction solar cells improves from 2.16 to 4.90% using a processing additive and drying condition. The results demonstrate that a processing additive and drying condition provides an effective means to control both the surface roughness and finer interpenetrating networks to enhance the exciton dissociation into free charge carriers, charge transportation, and collection. Taking into the account of simple device fabrication process without thermal annealing, the PCE of the polymer solar cell can further improved by chloronapthalene (CN) additive under the fast drying condition. The average carrier lifetimes extracted from the impedance spectra and found to correlate with measured PCEs. At short circuit conditions and illumination, the average charge carrier lifetime was found vary from 16.8 to 32 μs with power conversion efficiencies ranging from 3.0 to 4.9%. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Unravelling the Correlation between Charge Mobility and Cocrystallization in Rod–Rod Block Copolymers for High‐Performance Field‐Effect Transistors

Cocrystallization involving two or more components aggregating into cocrystals allows the preparation of materials with markedly improved charge mobility. This approach however, is little explored in all‐conjugated block copolymers (BCPs). Herein, we report the first investigation into the correlation between cocrystals and charge mobility in a series of new all‐conjugated BCPs: poly(3‐butylthiophene)‐b‐poly(3‐hexylselenophene) (P3BT‐b‐P3HS) for high‐performance field‐effect transistors. These rationally synthesized rod–rod BCPs self‐assemble into cocrystals with high charge mobilities. Upon one‐step thermal annealing, their charge mobilities decrease slightly despite their increased crystallinities. After two‐step thermal annealing, P3BT‐b‐P3HS (P3BT/P3HS=2:1) and (1:1) cocrystals disappear and phase separation occurs, leading to greatly decreased charge mobilities. In contrast, P3BT‐b‐P3HS (1:2) retains its cocrystalline structure and its charge mobility.