Polymer-fullerene Blends Research Articles

Charge Carrier Formation following Energy Gap Law in Photo-Activated Organic Materials for Efficient Solar Cells

The charge carrier formation and transport in the pristine polymers as well as in the polymer–fullerene blend is still a hot topic of discussion for the scientific community. In the present work, the carrier generation in some prominent organic molecules has been studied through ultrafast transient absorption spectroscopy. The identification of the exciton and polaron lifetimes of these polymers has led to device performance-related understanding. In the Energy Gap Law, the slope of the linear fit gradient (γ) of lifetimes vs. bandgap are subjected to the geometrical rearrangements experienced by the polymers during the non-radiative decay from the excited state to the ground state. The value of gradient (γ) for excitons and polarons is found to be −1.1 eV−1 and 1.14 eV−1, respectively. It suggests that the exciton decay to the ground state is likely to involve a high distortion in polymer equilibrium geometry. This observation supports the basis of Stokes shift found in the conjugated polymers due to the high disorder. It provides the possible reasons for the substantial variation in the exciton lifetime. As the bandgap becomes larger, exciton decay rate tends to reduce due to the weak attraction between the holes in the HUMO and electron in the LUMO. The precise inverse action is observed for the polymer–fullerene blend, as the decay of polaron tends to increase as the bandgap of polymer increases.

Thermal and optical properties of P3HT:PC70BM:ZnO nanoparticles composite films

The results of studies on the influence of zinc oxide nanoparticles (ZnO-NPs) on the structural, thermal and optical properties of thin films of mixtures of phenyl-C71-butyric acid methyl ester (PCBM) with poly[3-hexylthiophene] (P3HT) of various molecular weights are described in this article. The structural properties of the layers of: polymers, mixtures of polymers with fullerenes and their composites with ZnO-NPs were investigated using X-ray diffraction. Whereas their glass transition temperature and optical parameters have been determined by temperature-dependent spectroscopic ellipsometry. The presence of ZnO-NPs was also visible in the images of the surface of the composite layers obtained using scanning electron microscopy. These blends and composite films have also been used as the active layer in bulk heterojunction photovoltaic structures. The molecular weight of P3HT (Mw = 65.2; 54.2 and 34.1 kDa) and the addition of nanoparticles affected the power conversion efficiency (PCE) of the obtained solar cells. The determined PCE was the highest for the device prepared from the blend of P3HT:PCBM with the polymer of the lowest molecular weight. However, solar cells with ZnO-NPs present in their active layer had lower efficiency, although the open-circuit voltage and fill factor of almost all devices had the same values whether they contained ZnO-NPs or not. It is worth noting that thermal studies carried out using temperature-dependent ellipsometry showed a significant effect of the presence of ZnO-NPs on the value of the glass transition temperature, which was higher for composite films than for films made of a polymer-fullerene blend alone.

Variable Temperature Spectroscopic Ellipsometry as a Tool for Insight into the Optical Order in the P3HT:PC70BM and PC70BM Layers.

Two combined ellipsometric techniques-variable angle spectroscopic ellipsometry (VASE) and variable temperature spectroscopic ellipsometry (VTSE)-were used as tools to study the surface order and dielectric properties of thin films of a poly(3-hexylthiophene-2,5-diyl) (P3HT) mixture with a fullerene derivative (6,6-phenyl-C71-butyric acid methyl ester) (PC70BM). Under the influence of annealing, a layer of the ordered PC70BM phase was formed on the surface of the blend films. The dielectric function of the ordered PC70BM was determined for the first time and used in the ellipsometric modeling of the physical properties of the P3HT:PC70BM blend films, such as their dielectric function and thickness. The applied ellipsometric optical model of the polymer-fullerene blend treats the components of the blend as a mixture of optically ordered and disordered phases, using the effective medium approximation for this purpose. The results obtained using the constructed model showed that a layer of the ordered PC70BM phase was formed on the surface of the layer of the polymer and fullerene mixture. Namely, as a result of thermal annealing, the thickness of the layer of the ordered fullerene phase increased, while the thickness of the underlying material layer decreased.

Assembly of galvinoxyl doped in polymer-fullerene photovoltaic blends.

Galvinoxyl (Gx) is a stable free radical used as a dopant in active layers of organic solar cells. Here, the nanoscale arrangement of Gx molecules in a composite of the PCDTBT polymer and modified C60 fullerene, PCBM, was studied using a two-pulse electron spin echo (ESE) technique. The results show that the Gx molecules assemble into clusters, which can be described by the model of 8 molecules on the surface of a sphere with a radius of 2.0 nm. Such a structure can arise due to the octahedral packing of 6 PCBM molecules surrounded by 8 Gx molecules. ESE decays also indicate that these clusters repel each other, forming a quasi-regular nanostructure in the matrix. The Gx concentration of 2 wt% at which clusters appear correlates with the literature data on the Gx-induced enhancement of photocurrent, which provides structural insight into the possible molecular mechanism of this enhancement.

Effect of Solvent Removal Rate and Annealing on the Interface Properties in a Blend of a Diketopyrrolopyrrole-Based Polymer with Fullerene

We study the effect of solvent-free annealing and explicit solvent evaporation protocols in classical molecular dynamics simulations on the interface properties of a blend of a diketopyrrolopyrrole (DPP) polymer with conjugated substituents (DPP2Py2T) and PCBM[60]. We specifically analyze the intramolecular segmental mobility of the different polymer building blocks as well as intermolecular radial and angular distribution functions between donor and acceptor. The annealing simulations reveal an increase of the glass-transition temperature of 45 K in the polymer–fullerene blend compared to that of pure DPP2Py2T. Our results show that the effective solvent evaporation rates at room temperature only have a minor influence on the segmental mobility and intermolecular orientation, characterized in all cases by a preferential arrangement of PCBM[60] close to the electron-donating substituents in DPP2Py2T. In contrast, solvent-free annealing from a liquid yields clustering of the fullerene close to the electron-withdrawing DPP, generally considered to be detrimental for application in organic solar cells. We find that the difference can be attributed to differences in the behavior of 2-hexyldecyl side-chains, which collapse toward DPP when solvent is explicitly removed, thereby blocking access of PCBM[60].

Fullerene-Functionalized Poly(3-hexylthiophene) Additive Stabilizes Conjugated Polymer–Fullerene Blend Morphologies

Fullerene-Functionalized Poly(3-hexylthiophene) Additive Stabilizes Conjugated Polymer–Fullerene Blend Morphologies

Strongly Correlated Molecular Magnet with Curie Temperature above 60 K

Summary Two-dimensional crystals with ferromagnetic ordering would lead to unprecedented possibilities for magnetic, magneto-optic, and magnetoelectric applications. However, long-range ferromagnetic ordering in two-dimensional van der Waals materials is strongly diminished by thermal fluctuation, while three-dimensional systems show the stiffness of magnetic ordering temperature to magnetic fields. Here, we report the discovery of intrinsic ferromagnetic ordering temperature of 60 K in an iron/tetracyanoquinodimethane layer via superconducting precursor. In such a molecular ferromagnet, an unprecedented control of ferromagnetic fluctuation temperature by ∼200% improvement is realized through magnetic fields. We demonstrate magnetodielectric coupling with a concomitant transition in magnetic ordering. A small magnetic field of 0.01 T enables an effective route to realize a sizable transition shift in magnetodielectric states. Stimulated by photoirradiation, it manifests a hidden metallic conducting state, declaring a metallic ferromagnet. The molecular ferromagnet and photoirradiation-induced hidden metallic phase present a “holy grail” in the field of molecular conducting magnets.

Making triplets from photo-generated charges: observations, mechanisms and theory

Triplet formation by charge recombination is a phenomenon that is encountered in many fields of the photo-sciences and can be a detrimental unwanted side effect, but can also be exploited as a useful triplet generation method, for instance in photodynamic therapy. In this Perspective we describe the various aspects that play a role in the decay of charge separated states into local triplet states. The observations and structures of a selection of (pre-2015) molecular electron donor-acceptor systems in which triplet formation by charge recombination occurs are reported. An overview is given of some more recent systems consisting of BODIPY dimers, and BODIPYs attached to various electron-donor units displaying this same triplet formation process. A selection of polymer–fullerene blends in which triplet formation by (non-geminate) charge recombination has been observed, is presented. Furthermore, in-depth information regarding the mechanistic aspects of triplet formation by charge recombination is given on spin dephasing, through hyperfine interactions, as well as on spin–orbit coupling occurring simultaneously with charge recombination. The limits and constraints of these factors and their role in intersystem crossing are discussed. A pictorial view of the two mechanisms is given and this is correlated to aspects of the selection rules for triplet formation, the so-called El-Sayed rules. It is shown that the timescale of triplet formation by charge recombination is indicative for the mechanism that is responsible for the process. The relatively slow rates (CRkT ~ 1 × 108 s−1 or slower) can be correlated to proton hyperfine interactions (also called the radical pair mechanism), but substantially faster rates (CRkT ~ 1 × 109 up to 2.5 × 1010 s−1 or faster) have to be correlated to spin-orbit coupling effects. Several examples of molecular systems showing such fast rates are available and their electron donor and acceptor orbitals display an orthogonal relationship with respect to each other. This orientation of (the nodal planes of) the π-orbitals of the donor and acceptor units is correlated to the mechanisms in photodynamic agents and photovoltaic blends

Effects of π-spacer and fluorine loading on the optoelectronic and photovoltaic properties of (X-DADAD)n benzodithiophene-based conjugated polymers

Four new conjugated polymers based on benzodithiophene, thiophene and 2,1,3-benzothiadiazole or 5,6-difluoro-2,1,3-benzothiadiazole were synthesized and investigated as absorber materials for organic photovoltaics. The effect of (bi)thiophene π-spacers and fluorine substitution on the physicochemical and optoelectronic properties of the polymers was revealed and correlations were drawn with their electrical characteristics in organic solar cells. In particular, introducing either thiophene spacers or fluorine substituents does not affect much the photovoltaic performance of the polymers, while the combination of both routes was found to be a promising strategy for improving the charge carrier mobilities and morphology of the polymer-fullerene blends as well as light power conversion efficiency in solar cells based on these materials. The bulk heterojunction organic solar cells based on the π-bridged and fluorinated polymer P4 showed the highest short-circuit current density and power conversion efficiency of 7 %, which is an inspiring value for fullerene-based organic photovoltaics. Most importantly, our findings provide important insights into rational design of high-performance conjugated polymers while pursuing a combination of two efficient backbone functionalization strategies based on introduction of fluorine substituents and π-spacers to control the geometry and electronic characteristics of the polymer chains.

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Summary Fabrication of non-fullerene (NF)-based polymer solar cells (PSCs) has been an arduous task involving various approaches to optimize the morphology of blend film for superior performance. In this work, unique exothermic peaks can be observed in differential scanning calorimetry first heating scan of the blend films, which are assigned to the cold crystallization temperature (TCC) of NF acceptors when blended with different degree of crystallinity polymers, and TCC contains the phase information at quenched stage of the film development. A linear correlation between the phase separation parameters and TCC was established, which helps to select the appropriate donor for a specific acceptor. Utilizing multiple polymer-NF blend systems, we report an expectation TCC value between 210°C and 230°C, which would promise balanced domain size, purity, and consequentially superior performance. Further investigation revealed that TCC can also be used to evaluate the thermal stability.

Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells.

We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium conceptsappear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends.

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

Effects of thermally cross-linkable polymeric additive in the photoactive layer of polymer solar cells

Abstract Performance and stability are the most important issues to be considered to propel and advance the polymer solar cell (PSC) technology to be realized in commercial market. Herein, we propose a new concept of stabilizing polymer-fullerene morphology for PSCs and simultaneously improve device performance by introducing thermally cross-linkable polymer as an additive in the photoactive layer. The cross-linker, poly[9,9-bis(6-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis(3-ethyl(oxetane-3-ethyloxy)hexyl)-fluorene] (PFN-ox), forms a three-dimensional network embedded in the polymer blend after thermal treatment and plays various roles including reducing the carrier recombination at the donor/acceptor interface, improving the electrical property of PSCs, enhancing the hydrophobicity of the photoactive layer, slightly decreasing fullerene domain size, and retarding the crystal growth of fullerenes. Those characteristics lead to the improvement of device performance from the moderate improved fill factor and stable device operation under ambient condition (>85% efficiency retention after 2 months). The stability effects of PFN-ox are not only valid in polymer-fullerene blends, but also in polymer-polymer systems, showing its general characteristics. We believe that our finding can advance the development of stable PSCs and other optoelectronic devices based on organic materials.

Bilayer–Ternary Polymer Solar Cells Fabricated Using Spontaneous Spreading on Water

AbstractA new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk‐heterojunction layers obtained using spontaneous spreading of polymer–fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer–fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin‐coated devices of similar thickness. Next, bilayer–ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer–fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer–fullerene layer by transfer from a water surface. These novel bilayer–ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near‐infrared bias light illumination. Drift‐diffusion calculations confirm that near‐infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends.

Degradation kinetics in different polymer–fullerene blends investigated by electron spin resonance

Abstract

Organic solar cells based on anthracene-containing PPE–PPVs and non-fullerene acceptors

Lately, non-fullerene acceptors (NFAs) have received increasing attention for use in polymer-based bulk-heterojunction (BHJ) organic solar cells (OSCs), as improved photovoltaic performance compared to classical polymer–fullerene blends could be demonstrated. In this study, polymer solar cells based on a statistically substituted anthracene-containing poly(p-phenylene ethynylene)-alt-poly(p-phenylene vinylene)s (PPE–PPVs) copolymer (AnE-PVstat) as donor in combination with a number of different electron accepting materials were investigated. Strong photoluminescence quenching of the polymer donor indicates intimate intermixing of both materials. However, the photovoltaic performances were found to be poor compared to blends that use fullerene as acceptor. Time-delayed collection field (TDCF) measurements demonstrate: charge generation is field-independent, but bimolecular recombination processes limit the fill factor and thus the efficiency of devices.

Simulating Phase Separation during Spin Coating of a Polymer–Fullerene Blend: A Joint Computational and Experimental Investigation

During spin coating of the photoactive layer of a bulk heterojunction organic solar cell, phase separation between the donor (D) and acceptor (A) components is triggered by solvent evaporation. The morphology of the resulting layer is one of the main determinants of the device efficiency and critically depends on processing conditions such as the spinning speed, D–A mixing ratio, and choice of solvents. It is crucial to understand how these conditions influence the nanostructure of the photoactive layer. Optical experiments have a limited spatial resolution and cannot probe the short length scales of phase separation. In this work, we present three-dimensional simulations of evaporation-induced phase separation in a diketopyrrolopyrrole–fullerene D–A blend, where we derive the simulation parameters from in situ laser interference and contact angle experiments. Depending on the drying rate, phase separation initiates in different regions of the thinning film. From a linear stability analysis, we estimate t...

Introducing a New Potential Figure of Merit for Evaluating Microstructure Stability in Photovoltaic Polymer-Fullerene Blends

A theoretical understanding of the microstructure of organic semiconducting polymers and blends is vital to further advance the optoelectronic device performance of organic electronics. We outline the theoretical framework of a combined numerical approach based on polymeric solution theory to study the microstructure of polymer:small molecule blends. We feed the results of ab initio density functional theory quantum chemistry calculations into an artificial neural network for the determination of solubility parameters. These solubility parameters are used to calculate Flory–Huggings intermolecular parameters. We further show that the theoretical values are in line with experimentally determined data. On the basis of the Flory–Huggings parameters, we establish a figure of merit as a relative metric for assessing the phase diagrams of organic semiconducting blends in thin films. This is demonstrated for polymer:fullerene blend films on the basis of the prototypical polymers poly(3-hexylthiophene-2,5-diyl) (...

Determining Material-Specific Morphology of Bulk-Heterojunction Organic Solar Cells Using AFM Phase Imaging

A technique for determining the material-specific morphology of a polymer–fullerene blend is presented. This technique is applied to solution processed bulk-heterojunction organic solar cells with different weight ratios of polymer–fullerene blend using the PTB7:PCBM material system. Optical and electrical characterizations show that the light absorption increases for larger polymer (PTB7) content, while the fill factor of the fabricated solar cells is improved for larger fullerene (PCBM) content. The material-specific morphologies of polymer–fullerene bulk-heterojunctions are measured by employing AFM phase imaging. The measured AFM phase images reveal that fullerene material forms flake-like clusters which are embedded in the polymer network. The size of the flakes is increasing with larger content of fullerene material. By correlating the optical and electrical properties with the measured bulk-heterojunction morphologies, the relation between bulk-heterojunction structure and solar cell performances i...

Factors affecting tacticity and aggregation of P3HT polymers in P3HT:PCBM blends

Optimising the performance of P3HT:PCBM (polymer–fullerene, P3HT – poly(3-hexylthiophene), PCBM – [6, 6]-phenyl-C61-butyric acid methyl ester) blends for solar cell applications is currently impeded by our lack of understanding of the effect of processing conditions on the resulting morphology of P3HT and hence its performance. The literature has generally overlooked studying the link between morphology and tacticity, which, in turn, affects charge mobility. The conformational change from trans to cis between adjacent thiophene rings in the P3HT backbone determines the tacticity of the polymer molecules. We used all-atom molecular dynamics simulations at 400 K, matching typical experimental processing temperatures, to unravel the molecular-level configuration changes that P3HT polymers undergo during processing, focusing here on temperature and shear. The effect of shear was included since it improves charge transport and polymer aggregation in polymer–fullerene blends. Our simulations implicate the side chains in initiating the polymer backbone twisting and curvature that we observe and, consequently, altering the trans-to-cis ratio (and hence mobility). Complementary density functional theory (DFT) calculations indicate that such tacticity conversions follow a rugged energy surface. The simulations also help explain the origin of disorder in the aggregation of polymer chains and shorter conjugation length, all of which are observed in experiments. Each individual polymer molecule’s response to local thermodynamic changes and the kinetics of neighbouring polymers guides its morphological evolution. Thus, we put forward the prominent role of tacticity in determining both kinetic and thermodynamic properties of this prototypical polymer–fullerene blend. As a result, we uncover two types of processing condition effects, one that directly affects polymer conformation (e.g. high temperature), and the other that directly changes the alignment between polymers (e.g. shear).