P3HT Domains Research Articles

Effect of short chain iodoalkane solvent additives on photovoltaic performance of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester based bulk heterojunction solar cells

In this work the effect of short chain iodoalkane solvent additives such as iodobutane, iodoethane, diiodomethane and iodomethane on the photovoltaic parameters of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) (1:1) based bulk heterojunction (BHJ) solar cells was studied in an ambient air conditions. Devices processed in 2% (v/v) of diiodomethane, iodobutane and iodoethane showed improved power conversion efficiency (PCE) of 2.40, 2.29 and 2.04%, respectively as compared to the efficiency of pristine (without additive) devices (1.93%), while devices made using iodomethane exhibit PCE of 1.66%. The UV–vis absorption spectra of devices showed that the presence of these additives results the growth of enhanced local structure with improved crystalline and order of P3HT domain. Furthermore, UV–vis absorption response of the solar cells before and after soaked in the aforementioned solvents indicates that each additive has selective solubility for PCBM except iodomethane in which both P3HT and PCBM showed solubility.

Morphology‐Limited Free Carrier Generation in Donor/Acceptor Polymer Blend Solar Cells Composed of Poly(3‐hexylthiophene) and Fluorene‐Based Copolymer

The charge generation and recombination dynamics in polymer/polymer blend solar cells composed of poly(3‐hexylthiophene) (P3HT, electron donor) and poly[2,7‐(9,9‐didodecylfluorene)‐alt‐5,5‐(4′,7′‐bis(2‐thienyl)‐2′,1′,3′‐benzothiadiazole)] (PF12TBT, electron acceptor) are studied by transient absorption measurements. In the unannealed blend film, charge carriers are efficiently generated from polymer excitons, but some of them recombine geminately. In the blend film annealed at 160 °C, on the other hand, the geminate recombination loss is suppressed and hence free carrier generation efficiency increases up to 74%. These findings suggest that P3HT and PF12TBT are intermixed within a few nanometers, resulting in impure PF12TBT and disordered P3HT domains. The geminate recombination is likely due to charge carriers generated on isolated polymer chains in the matrix of the other polymer and at the domain interface with disordered P3HT. The undesired charge loss by geminate recombination is reduced by both the purification of the PF12TBT‐rich domain and crystallization of the P3HT chains. These results show that efficient free carrier generation is not inherent to the polymer/fullerene domain interface, but is possible with polymer/polymer systems composed of crystalline donor and amorphous acceptor polymers, opening up a new potential method for the improvement of solar cell materials.

Molecular design of near-IR dyes with different surface energy for selective loading to the heterojunction in blend films.

We have synthesized three silicon phthalocyanine dyes with different hydrophobic substituents in order to control surface energy in the solid state, aiming at selective loading of the dyes into blend films of poly(3-hexylthiophene) (P3HT) and polystyrene (PS). These three dyes are differently located at P3HT domains, at P3HT/PS interface, and at PS domains, respectively, which are fully consistent with the locations predicted by the wetting coefficient derived from the surface energy of each material.

Morphology, structure, and photovoltaic properties of poly(3‐hexylthiophene) and [6,6]‐phenyl‐C61‐butyric acid methyl ester‐based ternary blends doping with polystyrene of different tacticities

ABSTRACTThe influence of the polystyrene of different tacticities on the morphology, phase structure, and photovoltaic properties of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blend has been extensively investigated. The atactic polystyrene (aPS) immiscible with P3HT tended to form the phase‐separated and columnar structure at low aPS weight ratio. Besides, the aPS could migrate to the surface of the films with PCBM phase distributing in the interfaces between P3HT and aPS domains at high aPS weight ratio of 75 wt %. The syndiotactic polystyrene (sPS) immiscible with P3HT could induce the crystallization of P3HT at low weight ratio of 3 wt %. The device based on aPS/P3HT/PCBM ternary blend showed of power conversion efficiency (PCE) of 1.2% even at aPS weight ratio of 50 wt %. However, the device based on sPS/P3HT/PCBM exhibited a sharp decrease in PCE value from 2.3% to 0.6% at sPS weight ratio of 3 wt %, due to the change in film morphology. The performance of the solar cell is believed to be determined by the morphology and phase structure of the ternary blends as revealed by the atomic force microscopy and UV‐vis spectra analysis. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41823.

Solution processing of polymer semiconductor: Insulator blends—Tailored optical properties through liquid–liquid phase separation control

ABSTRACTIt has been demonstrated that the 0‐0 absorption transition of poly(3‐hexylthiophene) (P3HT) in blends with poly(ethylene oxide) (PEO) could be rationally tuned through the control of the liquid–liquid phase separation process during solution deposition. Pronounced J‐like aggregation behavior, characteristic for systems of a low exciton band width, was found for blends where the most pronounced liquid–liquid phase separation occurred in solution, leading to domains of P3HT and PEO of high phase purity. Since liquid–liquid phase separation could be readily manipulated either by the solution temperature, solute concentration, or deposition temperature, to name a few parameters, our findings promise the design from the out‐set of semiconductor:insulator architectures of pre‐defined properties by manipulation of the interaction parameter between the solutes as well as the respective solute:solvent system using classical polymer science principles. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 304–310

Ternary Blend Organic Solar Cells Based on P3HT/TT-TTPA/PC61BM

Organic solar cells (OSCs) are a promising cost-effective alternative for utility of solar energy, and possess advantages such as low cost, light weight and flexibility. Much attention has been focused on the development of OSCs (synthesis of new donor or acceptor materials, control of morphology of active layer and fabrication of new device structures) which have seen a dramatic rise in efficiency over the last decade. Since the bulk heterojunction concept was reported in 1995, two-component (binary) blend has been predominant as active layer and achieved great success with power conversion efficiencies (PCEs) up to ca. 10%. Ternary blend of active layer consisting of a donor material, an acceptor material and a third component has received increasing attention in recent five years. The third components include light-absorbing polymers or small molecules, fullerene or non-fullerene acceptors, inorganic nanomaterials (quantum dots, metal nanomaterials or carbon-based nanomaterials) and organic nonvolatile additives (polymers or small molecules). Compared with binary blend, ternary blend may present some advantages: broader and stronger absorption; more efficient charge transfer and charge transport, better morphology and improved stability. In this work, we fabricated ternary blend OSCs based on P3HT/TT-TTPA/PC61BM blend. TT-TTPA is a conjugated small molecule with thiazolothiazole as acceptor unit, triphenyl-amine as donor unit and thiophene as bridge. TT-TTPA has good miscibility with PC61BM and the phase separation scale of TT-TTPA/PC61BM blend is very small. During solvent annealing and thermal annealing, the small amount of TT-TTPA in P3HT domains can move from P3HT domains to PC61BM domains, thus increase the phase purity of P3HT domains that can undergo crystallization. After optimization of the weight ratio of TT-TTPA in ternary blend, we achieved better PCE (4.41%) relative to binary blend (3.85%). Effects of TT-TTPA on absorption, crystallinity and morphology of P3HT:PC61BM blend films were investigated by UV-vis, X-ray diffraction and atomic force microscopy.

Morphological investigation of P3HT/PCBM heterojunction and its effects on the performance of bilayer organic solar cells

Organic solar cells utilizing a P3HT/PCBM bilayer (BL) as their photoactive layer are fabricated using a sequential solution process. The film morphology and solar cell performance are investigated by changing the thickness of PCBM layer. The power conversion efficiency (PEC) of a BL solar cell strongly depends on the thickness of the PCBM layer; this value increases significantly after thermal annealing above the glass transition temperature of the P3HT. Based on the water contact angle and photoluminescence (PL) measurements, most of the PCBM was diffused into the amorphous P3HT region after thermal annealing. The external quantum efficiency spectrum reveals that the morphology of the P3HT in the BL solar cell is crystalline, even without thermal annealing, in contrast to the bulk heterojunction films. Moreover, thermal annealing improves charge collection efficiency by generating crystalline P3HT and PCBM domains. The transient photovoltage experiments suggest that the morphology of P3HT and PCBM in the BL solar cell reduces the charge recombination better than that in the BHJ solar cell. The BL solar cell exhibits a PEC of 3.54%, similar to that of the BHJ solar cell.

Effects of incorporation of copper sulfide nanocrystals on the performance of P3HT: PCBM based inverted solar cells

It has been reported that performance of bulk heterojunction organic solar cells can be improved by incorporation of an additive like metal and semiconducting nanoparticles in the active layer. Here in, we have synthesized Cu2S nanocrystals (NCs) by chemical route and studied its dispersion in poly (3-hexylthiophene) [6, 6]-phenyl C61-butyric acid methyl ester (P3HT: PCBM) matrix. Variation in the performance parameters with change in the concentration of Cu2S NCs into the P3HT: PCBM matrix has also been studied and it was found that the inverted geometry device with concentration of 20wt% of Cu2S NCs and having the structure ITO/ZnO (NPs)/P3HT: PCBM:Cu2S NCs/MoO3/Al has shown maximum efficiency of 3.39% which is more than 100% increase in comparison with devices without Cu2S NCs. Photoluminescence measurements studies unveiled that the incorporation of Cu2S NCs into a P3HT: PCBM matrix has helped in quenching photoluminescence which suggests more effective exciton dissociation at the interfaces between the P3HT and PCBM domains. The Nyquist plots obtained from impedance spectroscopy at 1V bias in the dark has suggested the effective lifetime and global mobilities for P3HT: PCBM as 0.267ms and 1.17×10−3cm2/V-S and for P3HT: PCBM:Cu2S NCs (20wt%) systems as 0.156ms and 2.02×10−3cm2/V-S respectively. Based on observed photoluminescence quenching, calculated effective lifetime and global mobility, we have tried to explain the possible reason for improvement in the efficiency with the very well dispersion of Cu2S NCs into the P3HT: PCBM matrix.

Role of structural order at the P3HT/C60 heterojunction interface

The influence of structural order on the electronic and optical properties of C60/P3HT bulk heterojunctions (BHJs) was studied using first principles DFT and TDDFT methods. The electronic levels alignment between the two phases in the BHJs is mainly controlled by the interfacial dipole moment that shifts the P3HT electronic levels towards higher binding energies with respect to C60 levels. An increasing order translates into an increasing P3HT domains size, for which we considered different stacks of P3HT oligomers. A significant decrease of both the electronic (HOMOP3HT–LUMOC60) and the optical (HOMOP3HT–LUMOP3HT) band gap is observed with an increasing P3HT domain size. TDDFT approach was used to identify the orbitals involved in the electronic transitions, and to reveal that the reduction of the BHJ optical band gap cannot simply be predicted from the variation of the rrP3HT band gap. The lowest electronic transition in rrP3HT becomes optically forbidden due to the formation of H-aggregates.

Ferroelectric coupling effect on the energy-band structure of hybrid heterojunctions with self-organized P(VDF-TrFE) nanomatrices.

Ferroelectric coupling effects on the energy-band structure of hybrid heterojunctions are investigated using hybrid photovoltaic devices with poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). The self-organized P(VDF-TrFE):P3HT photoactive layer forms a novel architecture consisting of P3HT domains in a P(VDF-TrFE) matrix. The energy-band structure at the interface of the p-n heterojunction is tuned by artificial control of the ferroelectric polarization of the P(VDF-TrFE) material, consequently modulating the photovoltaic performance of the hybrid photovoltaic devices.

Solvent polarity and nanoscale morphology in bulk heterojunction organic solar cells: A case study

Organic bulk heterojunction solar cells were fabricated under identical experimental conditions, except by varying the solvent polarity used for spin coating the active layer components and their performance was evaluated systematically. Results showed that presence of nitrobenzene-chlorobenzene composition governs the morphology of active layer formed, which is due to the tuning of solvent polarity as well as the resulting solubility of the P3HT:PCBM blend. Trace amount of nitrobenzene favoured the formation of better organised P3HT domains, as evident from conductive AFM, tapping mode AFM and surface, and cross-sectional SEM analysis. The higher interfacial surface area thus generated produced cells with high efficiency. But, an increase in the nitrobenzene composition leads to a decrease in cell performance, which is due to the formation of an active layer with larger size polymer domain networks with poor charge separation possibility.

Positioning lithium ions by host–guest chemistry combined with self‐assembly using a thiophene‐based all‐conjugated amphiphilic block copolymer

ABSTRACTSelf‐assembly of poly(3‐hexylthiophene) (P3HT) driven by π–π stacking, combined with “Host‐Guest Chemistry” of ethylene glycol oligomer and lithium ion is demonstrated using a thiophene‐based all conjugated amphiphilic block copolymer, containing 93 mol % of P3HT and 7 mol % of poly(3‐(2‐(2‐{2‐[2‐(2‐methoxy‐ethoxy)‐ethoxy]‐ethoxy}‐ethyl))thiophene), P3EGT blocks. An ion chelating ability of ethylene glycol oligomers with lithium ions in the P3EGT block is confirmed using 1H‐NMR spectrometry. This method could allow positioning lithium ions at the interface between P3HT domains and PC61BM clusters, confirmed using XRD and photoluminescence quenching experiments. The compact lamellar P3HT domains by side repulsion driven self‐assembly of amphiphilic block copolymer and the molecular engineering of the interface with an optimized lithium contents are resulted in the improvement of photovoltaic performance in an organic solar cell (2.1–3.0%). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1068–1074

Synthesis and morphology of all‐conjugated donor–acceptor block copolymers based on poly(3‐hexylthiophene) and poly(naphthalene diimide)

ABSTRACTA series of all‐conjugated diblock and triblock copolymers comprised of poly(naphthalene diimide) (PNDI)‐based n‐type and the poly(3‐hexylthiophene) (P3HT) segments could be synthesized via the Kumada catalyst‐transfer polycondensation process. The crystalline structures and chain orientation of the block copolymer thin films were systematically studied by grazing incident wide‐angle X‐ray scattering (GIWAXS). The GIWAXS results indicated that both the P3HT and PNDI segments in the block copolymers form exclusive crystalline domains in which the P3HT domain aligns with an edge‐on rich orientation, and the PNDI domain aligns with a face‐on rich orientation. In contrast, the blend films of the P3HT and PNDI homopolymers also show two distinguished crystalline domains in which the P3HT domain aligns with an edge‐on rich orientation, and the PNDI domains align in different ways depending on the chemical structure of n‐type polymers, that is, PNDI1Th is isotropically dispersed, while PNDI2Th aligns with a face‐on rich orientation. In addition, the effect of thermal annealing on the crystalline behavior of the block copolymers is reported. The GIWAXS results indicated that thermal annealing increases the crystallinity of both segments without affecting their chain orientation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1139–1148

Low-boiling-point solvent additives can also enable morphological control in polymer solar cells

Processing organic photovoltaic (OPV) blend solutions with high-boiling-point solvent additives has recently been used for morphological control in bulk-heterojunction OPV cells. Here we show that even low-boiling-point solvents can be effective additives. When P3HT:PCBM OPV cells were processed with a low-boiling-point solvent tetrahydrafuran as an additive in parent solvent o-dichlorobenzene, charge extraction increased leading to fill factors as high as 69.5%, without low work-function cathodes, electrode buffer layers or thermal treatment. This was attributed to PCBM demixing from P3HT domains and better vertical phase separation, as indicated by photoluminescence lifetimes, hole mobilities, and shunt leakage currents. Dependence on solvent parameters and applicability beyond P3HT system was also investigated.

Influence of Nickel(II) Oxide Nanoparticle Addition on the Performance of Organic Field Effect Transistors

Here, the improved performance of organic field effect transistors (OFET) by doping inorganic nanoparticles into a semiconducting polymer as a channel layer is briefly reported. Nickel(II) oxide nanoparticle (NiOnp) was used as an inorganic dopant while regioregular poly(3-hexylthiophene) (P3HT) was used as a matrix polymer for the channel layer in the OFETs. The doping ratio of NiOnp was made 1 wt.% so that it would minimally influence the nanostructure of the P3HT channel layer. The results showed that the optical absorption spectrum of the P3HT film was slightly red-shifted by the NiOnp doping, which reflects the improved crystallinity of the P3HT domains in the P3HT:NiOnp films. The drain current of the OFETs with the P3HT:NiOnp films was significantly enhanced ca. three-to-seven fold by the NiOnp doping under appying gate voltages while the hole mobility of the OFETs P3HT:NiOnp films was improved as much as three fold by the NiOnp doping. The enhanced performance has been assigned to the role of NiOnp that has relatively higher hole mobility than the P3HT polymer.

Resonance Raman overtones reveal vibrational displacements and dynamics of crystalline and amorphous poly(3-hexylthiophene) chains in fullerene blends

Resonance Raman spectra of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester blend thin films display progressions of overtone and combination bands (up to two harmonics) involving the dominant symmetric C=C backbone stretching mode of P3HT that encode excited state vibrational displacements and dynamics information. Contributions from both crystalline (aggregated) and amorphous (unaggregated) P3HT domains are resolved and intensities are analyzed using the time-dependent theory of spectroscopy. Raman spectra, excitation profiles, and absorption spectra are simulated with the same parameters using a single electronic state description for each P3HT form. Time-dependent wavepacket overlaps expose vibrational coherence on sub-100 fs timescales, which is usually difficult to extract from conventional ultrafast pump-probe spectra and transients of polymer∕fullerene blends. The results demonstrate the potential of simpler CW resonance Raman approaches to uncover excited state geometry changes and early vibrational dynamics from distinct morphological forms in polymer∕fullerene blends.

The Effect of Solvatochromism on the Interfacial Morphology of P3HT-CdS Nanowire Nanohybrids

Increasing performance in organic/inorganic bulk heterojunctions hybrid photovoltaic systems hinges not only on the structure of the inorganic component but also on the morphology of the polymeric component. Changing the morphology of the organic component is a facile way of changing the morphology of the interface between the inorganic and organic components in the bulk heterojunction system. Engineering this interface to more efficiently split photogenerated excitons and transport these carriers to electrodes can increase the efficiency of photovoltaic devices. In this report, we investigate the effect of solvent quality on the morphology of the poly(3-hexylthiophene)-2,5-diyl (P3HT) polymer-semiconductor interface (solvatochromism). We have found that creating more order within the P3HT main chain in solution and prior to deposition has a profound effect on the nature of the P3HT-CdS nanowire interface. Solvents with a larger difference in solubility parameter, Δδ, relative to P3HT, such as methanol and isopropanol, create larger rod domains in the P3HT main chain and result in larger domains of crystalline P3HT at the interface. Solvents with similar solubility parameters as P3HT, such as pyridine and hexanol, create relatively shorter rod domains in the main chain and, as a result, nanohybrids with reduced crystallinity. The results of this paper further cement the importance of manipulating the rod-coil transition in the conducting polymers such as P3HT to improve the crystallinity at the polymer-semiconductor interface that can easily be scaled up to improve the efficiency of bulk heterojunction photovoltatic systems.

Large-Scale Simultaneous Orientation of CdSe Nanorods and Regioregular Poly(3-hexylthiophene) by Mechanical Rubbing

Highly oriented hybrid thin films composed of rod-shaped CdSe nanocrystals and poly(3-hexylthiophene) are prepared by mechanical rubbing. The orientation distribution of both CdSe nanorods and crystalline P3HT domains is determined by a combination of transmission electron microscopy (electron diffraction, low dose high-resolution TEM, and tomography), grazing incidence X-ray diffraction measurements, and UV–vis spectroscopy. After rubbing, P3HT crystalline domains show a preferential face-on orientation ((010) contact plane) and the chains align parallel to the rubbing direction. CdSe nanorods also align parallel to the rubbing direction, but the level of alignment is a function of their concentration in the polymer matrix: the lower their concentration, the higher the level of in-plane orientation of both the nanorods and the polymer chains. GIXD and electron diffraction suggest that CdSe nanorods adopt a preferential contact plane on the substrate after rubbing. The high in-plane alignment of the nanor...

Correlating dilute solvent interactions to morphology and OPV device performance

Solvent additives have been explored as a reliable way to control the morphology in bulk-heterojunction (BHJ) layers for improved device performance. We show that the choice of solvent additives has direct implications on morphological evolution, i.e. poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM) BHJ films processed with a small amount of 1,8-diiodooctane or 1-chloronaphthalene have more crystalline PCBM domains compared to crystalline P3HT domains, while the opposite is true for films cast with nitrobenzene additive and films cast purely from chlorobenzene. The BHJ film cross-links when annealed at 300°C in the presence of 1,8-diiodooctane. Cross-linking is found to occur even in pristine P3HT and PCBM films annealed under similar conditions. NMR spectroscopy is presented as a viable technique for quantitative analysis of the amount of solvents left in the BHJ films before and after heat treatment. Despite differences in the ways the additives affect the morphology of the BHJ layer, device performance remained stable over 300h for all additives tested.

Direct Correlation of the Organic Solar Cell Device Performance to the In‐Depth Distribution of Highly Ordered Polymer Domains in Polymer/Fullerene Films

AbstractThis study correlates the device performance of organic solar cells and the electronic charge transport within polymer/fullerene films, directly to the optical order of the polymer. The optical order was measured by spectroscopic ellipsometry and evaluated by our previously derived model. We were able to determine the in‐depth distribution of higher and lower ordered poly(3‐hexylthiophene) (P3HT) domains within an [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) matrix. The over the film thickness integrated volume fraction of highly ordered P3HT domains could be directly correlated to the corresponding solar cell device performance. We are able to describe various thermally annealing conditions between room‐temperature and 200 °C.