Phase Separation and Molecular Intermixing in Polymer-Fullerene Bulk Heterojunction Thin Films

Deuteration-enhanced neutron contrasts to probe amorphous domain sizes in organic photovoltaic bulk heterojunction films

An organic photovoltaic bulk heterojunction comprises of a mixture of donor and acceptor materials, forming a semi-crystalline thin film with both crystalline and amorphous domains. Domain sizes critically impact the device performance; however, conventional X-ray scattering techniques cannot detect the contrast between donor and acceptor materials within the amorphous intermixing regions. In this study, we employ neutron scattering and targeted deuteration of acceptor materials to enhance the scattering contrast by nearly one order of magnitude. Remarkably, the PM6:deuterated Y6 system reveals a new length scale, indicating short-range aggregation of Y6 molecules in the amorphous intermixing regions. All-atom molecular dynamics simulations confirm that this short-range aggregation is an inherent morphological advantage of Y6 which effectively assists charge extraction and suppresses charge recombination as shown by capacitance spectroscopy. Our findings uncover the amorphous nanomorphology of organic photovoltaic thin films, providing crucial insights into the morphology-driven device performance.

Small-angle neutron scattering of long-wavelength magnetic modulations in reduced sample dimensions

Magnetic small-angle neutron scattering (SANS) is ideally suited to providing direct reciprocal-space information on long-wavelength magnetic modulations, such as helicoids, solitons, merons or skyrmions. SANS of such structures in thin films or micro-structured bulk materials is strongly limited by the tiny scattering volume vis a vis the prohibitively high background scattering by the substrate and support structures. Considering near-surface scattering just above the critical angle of reflection, where unwanted signal contributions due to substrate or support structures become very small, it is established that the scattering patterns of the helical, conical, skyrmion lattice and fluctuation-disordered phases in a polished bulk sample of MnSi are equivalent for conventional transmission and near-surface SANS geometries. This motivates the prediction of a complete repository of scattering patterns expected for thin films in the near-surface SANS geometry for each orientation of the magnetic order with respect to the scattering plane.

Unraveling the Photovoltaic, Mechanical, and Microstructural Properties and Their Correlations in Simple Poly(3-pentylthiophene) Solar Cells

The power conversion efficiency of polythiophene organic solar cells is constantly refreshed. Despite the renewed device efficiency, very few efforts have been devoted to understanding how the type of electron acceptor alters the photovoltaic and mechanical properties of these low-cost solar cells. Herein, we conduct a thorough investigation of photovoltaic and mechanical characteristics of a simple yet less explored polythiophene, namely poly(3-pentylthiophene) (P3PT), in three different types of organic solar cells, where ZY-4Cl, PC₇₁ BM, and N2200 are employed as three representative acceptors, respectively. Compared with the reference P3HT-based solar cells, P3PT-based devices all perform more efficiently. Particularly, the P3PT:ZY-4Cl blend exhibits the highest efficiency (nearly 10%) among the six combinations and outperforms the prior top-performance system P3HT:ZY-4Cl. Furthermore, the blend films based on N2200 exhibit a high crack-onset strain of ∼38% on average, which is approximately 15 and 17 times higher than those of ZY-4Cl and PC₇₁ BM, respectively. The microstructural origins for the above difference are well elucidated by detailed grazing incidence X-ray scattering and microscopy analysis. This work not only underlines the potential of P3PT in prolific solar cell research but also demonstrates the superior tensile properties of polythiophene-based all-polymer blends for the preparation of stretchable solar cells. This article is protected by copyright. All rights reserved.

Scattering techniques for mixed donor-acceptor characterization in organic photovoltaics

Precise control of the complex morphology of organic photovoltaic bulk heterojunction (BHJ) active layers remains an important yet challenging approach for improving power conversion efficiency. Of particular interest are the interfacial regions between electron donor and acceptor molecules where charge separation and charge recombination occur. Often, these interfaces feature a molecularly mixed donor-acceptor phase. This mixed phase has been extensively studied in polymer:fullerene systems but is poorly understood in state-of-the-art polymer:non-fullerene acceptor blends. Accurate, quantitative characterization of this mixed phase is critical to unraveling its importance for charge separation and recombination processes within the BHJ. Here, we detail X-ray and neutron scattering characterization techniques and analysis methods to quantify the mixed phase within BHJ active layers. We then review the existing literature where these techniques have been successfully used on several different material systems and correlated to device performance. Finally, future challenges for characterizing non-fullerene acceptor systems are addressed, and emerging strategies are discussed.

Resonant Grazing-Incidence Small-Angle X-ray Scattering at the Sulfur K-Edge for Material-Specific Investigation of Thin-Film Nanostructures

Scattering techniques are a powerful tool for probing thin-film nanomorphologies but often require additional characterization by other methods. We applied the well-established grazing-incidence small-angle X-ray scattering (GISAXS) technique for a selection of energies around the absorption edge of sulfur to exploit the resonance effect (grazing incidence resonant tender X-ray scattering, GIR-TeXS) of the sulfur atoms within a poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) sample to gain information about the composition of the film morphology. With this approach, it is possible not only to identify structures within the investigated thin film but also to link them to a particular material combination.

Kinetics of Polymer-Fullerene Phase Separation during Solvent Annealing Studied by Table-Top X-ray Scattering

Solvent annealing is an efficient way of phase separation in polymer-fullerene blends to optimize bulk heterojunction morphology of active layer in polymer solar cells. To track the process in real time across all relevant stages of solvent evaporation, laboratory-based in situ small- and wide-angle X-ray scattering measurements were applied simultaneously to a model P3HT:PCBM blend dissolved in dichlorobenzene. The PCBM molecule agglomeration starts at ∼7 wt % concentration of solid content of the blend in solvent. Although PCBM agglomeration is slowed-down at ∼10 wt % of solid content, the rate constant of phase separation is not changed, suggesting agglomeration and reordering of P3HT molecular chains. Having the longest duration, this stage most affects BHJ morphology. Phase separation is accelerated rapidly at concentration of ∼25 wt %, having the same rate constant as the growth of P3HT crystals. P3HT crystallization is driving force for phase separation at final stages before a complete solvent evaporation, having no visible temporal overlap with PCBM agglomeration. For the first time, such a study was done in laboratory demonstrating potential of the latest generation table-top high-brilliance X-ray source as a viable alternative before more sophisticated X-ray scattering experiments at synchrotron facilities are performed.

The Effect of Molecular Structure and Environment on the Miscibility and Diffusivity in Polythiophene-Methanofullerene Bulk Heterojunctions: Theory and Modeling with the RISM Approach

Although better means to model the properties of bulk heterojunction molecular blends are much needed in the field of organic optoelectronics, only a small subset of methods based on molecular dynamics- and Monte Carlo-based approaches have been hitherto employed to guide or replace empirical characterization and testing. Here, we present the first use of the integral equation theory of molecular liquids in modelling the structural properties of blends of phenyl-C61-butyric acid methyl ester (PCBM) with poly(3-hexylthiophene) (P3HT) and a carboxylated poly(3-butylthiophene) (P3BT), respectively. For this, we use the Reference Interaction Site Model (RISM) with the Universal Force Field (UFF) to compute the microscopic structure of blends and obtain insight into the miscibility of its components. Input parameters for RISM, such as optimized molecular geometries and charge distribution of interaction sites, are derived by the Density Functional Theory (DFT) methods. We also run Molecular Dynamics (MD) simulation to compare the diffusivity of the PCBM in binary blends with P3HT and P3BT, respectively. A remarkably good agreement with available experimental data and results of alternative modelling/simulation is observed for PCBM in the P3HT system. We interpret this as a step in the validation of the use of our approach for organic photovoltaics and support of its results for new systems that do not have reference data for comparison or calibration. In particular, for the less-studied P3BT, our results show that expectations about its performance in binary blends with PCBM may be overestimated, as it does not demonstrate the required level of miscibility and short-range structural organization. In addition, the simulated mobility of PCBM in P3BT is somewhat higher than what is expected for polymer blends and falls into a range typical for fluids. The significance of our predictive multi-scale modelling lies in the insights it offers into nanoscale morphology and charge transport behaviour in multi-component organic semiconductor blends.

Interfacial Structure of a Liquid Crystal/Nanoparticle Nanocomposite Studied by X-ray Scattering: Indirect Evidence for the Role of Faceting of the Nanoparticles

The interfacial structure in a liquid crystal/nanoparticle nanocomposite is dictated by the type of nanoparticle and its functionalization compound. Nanocomposites consisting of smectic liquid crystals and nanoparticles have been studied for their applications in devices such as photovoltaics and to model biological devices. With the use of a model system, this paper presents evidence of an interfacial structure close to the vicinity of the nanoparticles that is more disordered than that of the bulk liquid crystal but is still in the smectic phase, and it seems to follow the faceting of the structure the nanoparticles adopt when they coalesce or recluster after the liquid crystal is added.

Effect of Blend Composition and Additives on the Morphology of PCPDTBT:PC71BM Thin Films for Organic Photovoltaics

The use of solvent additives in the fabrication of bulk heterojunction polymer:fullerene solar cells allows to boost efficiencies in several low bandgap polymeric systems. It is known that solvent additives tune the nanometer scale morphology of the bulk heterojunction. The full mechanism of efficiency improvement is, however, not completely understood. In this work, we investigate the influences of blend composition and the addition of 3 vol % 1,8-octanedithiol (ODT) as solvent additive on polymer crystallization and both, vertical and lateral morphologies of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] and [6,6]-phenyl C71-butyric acid methyl ester (PCPDTBT:PC71BM) blend thin films processed from chlorobenzene-based solutions. The nanoscale morphology is probed with grazing incidence small- and wide-angle X-ray scattering as well as X-ray reflectivity and complemented with UV/vis spectroscopy. In PCPDTBT:PC71BM films the use of ODT is found to lower the solubility of fullerene in the polymer matrix and to promote polymer crystallization, both vertical and lateral microphase separation with morphological coarsening, and formation of a fullerene-rich topping layer. The enhanced photovoltaic performance is explained by these findings.

Effect of alcohol treatment on the performance of PTB7:PC71BM bulk heterojunction solar cells

The effect of an environmentally friendly alcohol treatment on bulk heterojunction (BHJ) polymer solar cells using the low-bandgap copolymer based on thieno[3,4-b]thiophene-alt-benzodithiophene units and [6,6]-phenyl-C71-butyric acid methyl ester is systematically investigated. Different alcohols are tested, and besides the most commonly used methanol treatment, other alcohols such as ethanol, 2-propanol, and 1-butanol also improve the device performance to certain extents as compared to the untreated solar cells. Changes of the surface structure caused by the alcohol treatment are probed with atomic force microscopy, and the modification of inner film morphology is probed by time-of-flight-grazing incidence small-angle neutron scattering (TOF-GISANS). UV/vis measurements show that the thickness of all BHJ films remains unchanged by the different solvent treatments. Thus, the enhanced device performance induced by the alcohol treatments is correlated to the reconstruction of the inner film structures probed with TOF-GISANS and the modified energy levels at the interfaces between the BHJ layer and the aluminum electrodes, evident by the enhanced short-circuit current and open-circuit voltage of the I-V curves.

A close look at charge generation in polymer:fullerene blends with microstructure control

We reveal some of the key mechanisms during charge generation in polymer:fullerene blends exploiting our well-defined understanding of the microstructures obtained in pBTTT:PCBM systems via processing with fatty acid methyl ester additives. Based on ultrafast transient absorption, electro-absorption, and fluorescence up-conversion spectroscopy, we find that exciton diffusion through relatively phase-pure polymer or fullerene domains limits the rate of electron and hole transfer, while prompt charge separation occurs in regions where the polymer and fullerene are molecularly intermixed (such as the co-crystal phase where fullerenes intercalate between polymer chains in pBTTT:PCBM). We moreover confirm the importance of neat domains, which are essential to prevent geminate recombination of bound electron-hole pairs. Most interestingly, using an electro-absorption (Stark effect) signature, we directly visualize the migration of holes from intermixed to neat regions, which occurs on the subpicosecond time scale. This ultrafast transport is likely sustained by high local mobility (possibly along chains extending from the co-crystal phase to neat regions) and by an energy cascade driving the holes toward the neat domains.

The active layer morphology of organic solar cells probed with grazing incidence scattering techniques

Grazing incidence X-ray scattering (GIXS) provides unique insights into the morphology of active materials and thin film layers used in organic photovoltaic devices. With grazing incidence wide angle X-ray scattering (GIWAXS) the molecular arrangement of the material is probed. GIWAXS is sensitive to the crystalline parts and allows for the determination of the crystal structure and the orientation of the crystalline regions with respect to the electrodes. With grazing incidence small angle X-ray scattering (GISAXS) the nano-scale structure inside the films is probed. As GISAXS is sensitive to length scales from nanometers to several hundred nanometers, all relevant length scales of organic solar cells are detectable. After an introduction to GISAXS and GIWAXS, selected examples for application of both techniques to active layer materials are reviewed. The particular focus is on conjugated polymers, such as poly(3-hexylthiophene) (P3HT).

Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells

The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC₇₁BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10–50 nm wide and 200–400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.

The morphology of organic solar cells is crucial to their performance but is difficult to measure. Using a variety of probes, Hedley et al. map the morphology of polymer-fullerene solar cells and find that elongated fibre-like polymer- and fullerene-rich domains are desirable for high performance.

Characterization and distribution of poly(3-hexylthiophene) phases in an annealed blend film

The characteristic absorption spectra of three kinds of phases, the isolated, ordered, and disordered phases, in a solvent-vapor annealed poly(3-hexylthiophene)/[6,6]-phenyl-C61 -butyric acid methyl ester (P3HT/PCBM) blend film were studied by means of spectroelectrochemistry (SEC) and time-resolved absorption spectroscopy (TAS). The results reveal that the content of three phases are 12 % isolated, 37 % ordered, and 51 % disordered for the annealed P3HT neat film, and 25 % isolated, 31 % ordered, and 44 % disordered for the annealed P3HT/PCBM blend film. The vertical distribution of the different phases in the blend film was studied by SEC, and the results show that the ordered and isolated phases are mainly distributed in the top and in the bottom of the annealed films, respectively, while the disordered phase is mainly distributed in the middle and the bottom of the films.

A direct evidence of morphological degradation on a nanometer scale in polymer solar cells

In situ measurement of a polymer solar cell using micro grazing incidence small angle X-ray scattering (μGISAXS) and current-voltage tracking is demonstrated. While measuring electric characteristics under illumination, morphological changes are probed by μGISAXS. The X-ray beam (green) impinges on the photo active layer with a shallow angle and scatters on a 2d detector. Degradation is explained by the ongoing nanomorphological changes observed.

Spin-cast bulk heterojunction solar cells: a dynamical investigation

Spin-coating is extensively used in the lab-based manufacture of organic solar cells, including most of the record-setting solution-processed cells. We report the first direct observation of photoactive layer formation as it occurs during spin-coating. The study provides new insight into mechanisms and kinetics of bulk heterojunction formation, which may be crucial for its successful transfer to scalable printing processes.

Controlling colloidal morphologies by critical Casimir forces

Active control over the assembly of colloidal and nanoparticles has important applications for the design of new nanostructured materials, but it is a difficult task. Here, a new method is presented to control the morphology of colloidal aggregates using critical Casimir forces. Via direct temperature control of critical Casimir forces, the particles are assembled into aggregates with well-defined architecture.

High-Resolution Photocurrent Imaging of Bulk Heterojunction Solar Cells

Images obtained from photocurrent scanning of organic bulk heterojunction solar cell devices provide a direct measure of correlation of the morphology to the performance parameters. The peripheral photocurrent induced from light coupled to probe tips in the near-field regime of bulk heterojunction layers permits in situ scanning of active solar cells with asymmetric electrodes. We present a methodology involving a combination of atomic force microscopy, near-field optical microscopy, and near-field photocurrent microscopy to decipher the carrier generation and transport regions in the bulk heterojunction layer. The angular Fourier transformation technique is implemented on these images to rationalize the optimum blend concentration in crystalline and amorphous donor systems and provide insights into the role of the bulk heterojunction morphology.