Predicting vertical phase segregation in polymer-fullerene bulk heterojunction solar cells by free energy analysis

Intermolecular Interactions, Morphology, and Photovoltaic Patterns in p-i-n Heterojunction Solar Cells With Fluorine-Substituted Organic Photovoltaic Materials

During the layer-by-layer (LBL) processing of polymer solar cells (PSCs), the swelling and molecule interdiffusion are essential for achieving precise, controllable vertical morphology, and thus efficient PSCs. However, the influencing mechanism of material properties on morphology and correlated device performance has not been paid much attention. Herein, a series of fluorinated/non-fluorinated polymer donors (PBDB-T and PBDB-TF) and non-fullerene acceptors (ITIC, IT-2F, and IT-4F) are employed to investigate the performance of LBL devices. The impacts of fluorine substitution on the repulsion and miscibility between the donor and acceptor, as well as the molecular arrangement of the donor/acceptor and the vertical distribution of the LBL devices are systematically explored by the measurement of donor/acceptor Flory-Huggins interaction parameters, spectroscopic ellipsometry, and neutron reflectivity, respectively. With efficient charge transfer due to the ideal vertical and horizon morphology properties, devices based on PBDB-TF/IT-4F exhibit the highest fill factors (FFs) as well as champion power conversion efficiencies (PCEs). With this guidance, high-performance LBL devices with PCE of 17.2%, 18.5%, and 19.1% are obtained by the fluorinated blend of PBDB-TF/Y6, PBDB-TF/L8-BO, and D18/L8-BO respectively.

How to Choose an Interfacial Modifier for Organic Photovoltaics Using Simple Surface Energy Considerations

Organic photovoltaics are typically composed of at least four different materials, including the donor and acceptor components of the bulk heterojunction, the interfacial layers at each electrode, the electrodes themselves, and solution additives that may persist in the final sandwich structure. The interplay of surface energies between these different layers is profoundly complex, as the deposition and annealing of one layer on top of another may be influenced by the surface energy of these interfaces. While the energy levels of one layer with respect to adjacent layers are important to facilitate charge separation and collection at the electrodes, the relative surface energies of the interfaces in contact with the multicomponent bulk heterojunction can be beneficial or disadvantageous, or be neutral, with respect to the performance of the OPV device. Because the bulk heterojunction is a mixture of donor and acceptor polymers and/or small molecules, the accumulation of one of the components on the underlying electrode interface can be driven by surface energy considerations. A donor- or acceptor-rich interface may affect charge carrier flow to the electrode, thus affecting the overall efficiency. Here, ITO/PEDOT:PSS electrodes in forward organic photovoltaic devices are treated with five different thin interfacial layers to change the relative surface energy of this electrode with respect to the adjacent bulk heterojunction. Contact angle measurements with four probe liquids enable calculation of the surface energies, and the results are compared with the performance of forward-biased organic photovoltaic devices. Time-of-flight secondary ion mass spectrometry results substantiate the predictions of gradients in the bulk heterojunction layers, and grazing-incidence wide-angle X-ray scattering measurements show the impact on the polymer crystallites. Thus, a simple algorithm based on surface energy considerations may inform which interfacial layer for a given bulk heterojunction in an organic photovoltaic device can be the most appropriate.

Nanographene-Osmapentalyne Complexes as a Cathode Interlayer in Organic Solar Cells Enhance Efficiency over 18

Interface engineering is a critical method by which to efficiently enhance the photovoltaic performance of nonfullerene solar cells (NFSC). Herein, a series of metal-nanographene-containing large transition metal involving d_π -p_π conjugated systems by way of the addition reactions of osmapentalynes and p-diethynyl-hexabenzocoronenes is reported. Conjugated extensions are engineered to optimize the π-conjugation of these metal-nanographene molecules, which serve as alcohol-soluble cathode interlayer (CIL) materials. Upon extension of the π-conjugation, the power conversion efficiency (PCE) of PM6:BTP-eC9-based NFSCs increases from 16% to over 18%, giving the highest recorded PCE. It is deduced by X-ray crystallographic analysis, interfacial contact methods, morphology characterization, and carrier dynamics that modification of hexabenzocoronenes-styryl can effectively improve the short-circuit current density (J_sc ) and fill factor of organic solar cells (OSCs), mainly due to the strong and ordered charge transfer, more matching energy level alignments, better interfacial contacts between the active layer and the electrodes, and regulated morphology. Consequently, the carrier transport is largely facilitated, and the carrier recombination is simultaneously impeded. These new CIL materials are broadly able to enhance the photovoltaic properties of OSCs in other systems, which provides a promising potential to serve as CILs for higher-quality OSCs.

All-Polymer Solar Cells Approaching 12% Efficiency with a New π-Conjugated Polymer Donor Enabled by a Nonhalogenated Solvent Process

High efficiency and nonhalogenated solvent processing are important issues for commercial application of all-polymer solar cells (all-PSCs). In this regard, we increased the photovoltaic performance of all-PSCs to a benchmark power conversion efficiency (PCE) of 11.66% by manipulating the pre-aggregation of a new π-conjugated polymer donor (Nap-SiBTz) using toluene as a solvent. This use of Nap-SiBTz enhanced the absorption coefficient (λ_max = 9.30 × 10⁴ cm^-1), increased charge carrier mobility, suppressed trap-assisted recombination, improved bulk heterojunction morphology, and resulted in high PCEs of all-PSCs with an active layer thickness of 200 nm. To overcome severe charge recombination and energy losses, a 1-phenylnapthalene additive was used to achieve a well-ordered microstructure and molecular packing that inherently improved the device performances. The resulting encapsulation-free devices exhibited good ambient and thermal stabilities. The results of this study augur well for the future of the roll-to-roll production of all-PSCs.

Preaggregation Matching of Donors and Acceptors in Solution Accounting for Thermally Stable Non-Fullerene Solar Cells

The mechanism of how the solvent type influences photovoltaic performance and thermal stability of non-fullerene organic solar cells remains unexplored. In this article, the well-known PTB7-Th was selected as a donor, while F8IC was used as an acceptor. The PTB7-Th:F8IC processed from chloroform (CF) exhibited a superiorly higher power conversion efficiency (PCE) of 10.5%, in contrast to the specimen processed from chlorobenzene (CB) of 6.8%. In addition, upon thermal annealing at 160 °C for 120 min, the device processed from CF was more stable than that processed from CB. The incorporation of perylene diimide derivative TBDPDI-C₁₁, serving as the third additive, could also obviously improve the PCE value and thermal stability of PTB7-Th:F8IC processed from CB. According to ultraviolet spectroscopy, atomic force microscopy, transmission electron microscopy, and grazing incidence wide-angle X-ray scattering analyses, the enhanced photovoltaic performance and thermal stability are mainly attributed to formation of PTB7-Th nanofibers and appropriate aggregation of F8IC. The interaction free energy calculated using water and diiodomethane contact angles reveals that PTB7-Th well disperses in CB and tends to aggregate in CF, while F8IC aggregates strongly in CB. The preaggregation matching of the donor and acceptor in solution is essential for the optimization of morphology, efficiency, and thermal stability. The findings in this article could provide useful guidelines to fabricate efficient and thermally stable organic solar cells simply by analyzing the surface energy of components processed from different solvents.

Quinonoid Zwitterion: An Amphiphilic Cathode Interlayer with Initial Thickness-Insensitive and Self-Organizing Properties for Inverted Polymer Solar Cells

Orthogonal solvent processability is generally considered as one of the key requirements for an efficient interfacial material. Here, we showed that in inverted polymer solar cells (PSCs), solvent orthogonality is not required for an effective and reliable cathode interlayer. A quinonoid zwitterionic molecule with amphiphilic property [dissolved in both methanol and o-dichlorobenzene (o-DCB)] named ZW-Bu was first applied as the cathode interlayer in inverted PSCs. For three different photoactive systems, the devices with ZW-Bu cathode buffer layers (CBLs) exhibited better performance than those with commonly used ZnO CBLs. Most importantly, the device efficiency was fairly insensitive to the initial thickness of ZW-Bu. In addition, due to the high surface energy of the ZW-Bu film, it was successfully used as a self-organized CBL in poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) systems, yielding a desirable efficiency compared to the PSCs fabricated via the layer-by-layer deposition method.

Probing the morphology evolution of chemically anisotropic colloids prepared by homopolymerization- and copolymerization-induced phase separation

Chemically anisotropic colloids prepared by polymerization-induced phase separation during seeded emulsion polymerization with non-crosslinked seeds reveals tunability in both surface and interior properties based on the morphology evolution.

Impact of Donor-Acceptor Interaction and Solvent Additive on the Vertical Composition Distribution of Bulk Heterojunction Polymer Solar Cells

The vertical composition distribution of a bulk heterojunction (BHJ) photoactive layer is known to have dramatic effects on photovoltaic performance in polymer solar cells. However, the vertical composition distribution evolution rules of BHJ films are still elusive. In this contribution, three BHJ film systems, composed of polymer donor PBDB-T, and three different classes of acceptor (fullerene acceptor PCBM, small-molecule acceptor ITIC, and polymer acceptor N2200) are systematically investigated using neutron reflectometry to examine how donor-acceptor interaction and solvent additive impact the vertical composition distribution. Our results show that those three BHJ films possess homogeneous vertical composition distributions across the bulk of the film, while very different composition accumulations near the top and bottom surface were observed, which could be attributed to different repulsion, miscibility, and phase separation between the donor and acceptor components as approved by the measurement of the donor-acceptor Flory-Huggins interaction parameter χ. Moreover, the solvent additive 1,8-diiodooctane (DIO) can induce more distinct vertical composition distribution especially in nonfullerene acceptor-based BHJ films. Thus, higher power conversion efficiencies were achieved in inverted solar cells because of facilitated charge transport in the active layer, improved carrier collection at electrodes, and suppressed charge recombination in BHJ solar cells.

Dispersion of Multi-Walled Carbon Nanotubes Stabilized by Humic Acid in Sustainable Cement Composites

Multi-walled carbon nanotubes (MWCNTs) are promising nanoreinforcing materials for cement-based composites due to their superior material properties. Dispersion of MWCNTs is key for achieving the most effective way of enhancing efficiency, which is challenging in an alkaline cementitious environment. In this study, humic acid (HA) was used to stabilize the degree of dispersion of MWCNTs in an alkaline environment. The efficiency of HA in stabilizing MWCNT dispersion in cement composites was characterized using an ultraviolet spectrophotometer. The influences of HA on the workability and mechanical properties of ordinary Portland cement (OPC) reinforced with MWCNTs were evaluated, and the results revealed that the addition of HA can improve the stability of MWCNT dispersion in an alkaline environment. A concentration of 0.12 wt.% HA/S added to MWCNT suspensions was found to perform the best for improving the dispersion of MWCNTs. The addition of HA results in a decreased workability of the OPC pastes but has little influence on the strength performance. HA can affect the mechanical properties of OPC reinforced with MWCNTs by influencing the dispersion degree of the MWCNTs. An optimum range of HA (0.05⁻0.10 wt.%) is required to achieve the optimum reinforcing efficiency of MWCNTs.

Constructing Desired Vertical Component Distribution Within a PBDB-T:ITIC-M Photoactive Layer via Fine-Tuning the Surface Free Energy of a Titanium Chelate Cathode Buffer Layer

Rationally controlling the vertical component distribution within a photoactive layer is crucial for efficient polymer solar cells (PSCs). Herein, fine-tuning the surface free energy (SFE) of the titanium(IV) oxide bis(2,4-pentanedionate) (TOPD) cathode buffer layer is proposed to achieve a desired perpendicular component distribution for the PBDB-T:ITIC-M photoactive layer. The Owens-Wendt method is adopted to precisely calculate the SFE of TOPD film jointly based on the water contact angle and the diiodomethane contact angle. We find that the SFE of TOPD film increases as the annealing temperature rises, and the subtle SFE change causes the profound vertical component distribution within the bulk region of PBDB-T:ITIC-M. The results of secondary-ion mass spectroscopy visibly demonstrate that the TOPD film with an SFE of 48.71 mJ/cm2, which is very close to that of the ITIC film (43.98 mJ/cm2), tends to form desired vertical component distribution. Consequently, compared with conventional bulk heterojunction devices, the power conversion efficiency increases from 9.00 to 10.20% benefiting from the short circuit current density increase from 14.76 to 16.88 mA/cm2. Our findings confirm that the SFE adjustment is an effective way of constructing the desired vertical component distribution and therefore achieving high-efficiency PSCs.

Vertical Stratification Engineering for Organic Bulk-Heterojunction Devices

High-efficiency organic solar cells (OSCs) can be produced through optimization of component molecular design, coupled with interfacial engineering and control of active layer morphology. However, vertical stratification of the bulk-heterojunction (BHJ), a spontaneous activity that occurs during the drying process, remains an intricate problem yet to be solved. Routes toward regulating the vertical separation profile and evaluating the effects on the final device should be explored to further enhance the performance of OSCs. Herein, we establish a connection between the material surface energy, absorption, and vertical stratification, which can then be linked to photovoltaic conversion characteristics. Through assessing the performance of temporary, artificial vertically stratified layers created by the sequential casting of the individual components to form a multilayered structure, optimal vertical stratification can be achieved. Adjusting the surface energy offset between the substrate results in donor and acceptor stabilization of that stratified layer. Further, a trade-off between the photocurrent generated in the visible region and the amount of donor or acceptor in close proximity to the electrode was observed. Modification of the substrate surface energy was achieved using self-assembled small molecules (SASM), which, in turn, directly impacted the polymer donor to acceptor ratio at the interface. Using three different donor polymers in conjunction with two alternative acceptors in an inverted organic solar cell architecture, the concentration of polymer donor molecules at the ITO (indium tin oxide)/BHJ interface could be increased relative to the acceptor. Appropriate selection of SASM facilitated a synchronized enhancement in external quantum efficiency and power conversion efficiencies over 10.5%.

Critical Role of Vertical Phase Separation in Small-Molecule Organic Solar Cells

An inverted device structure is a more stable configuration than a regular device structure for solution-processed organic solar cells (OSCs). However, most of the solution-processed small-molecule OSCs (SM-OSCs) reported in the literature used the regular device structure, and a regular device normally exhibits a higher efficiency than an inverted device. Herein, a representative small-molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage ( V_oc) and fill factor (FF) in the inverted device were studied. The reduced V_oc and FF is due to the vertical phase separation with excess [6,6]-phenyl-C₇₁-butyric acid methyl ester near the air/blend surface, which leads to a reduction in build-in voltage and unbalanced charge transport in the inverted device. Another reason for the reduced FF is the unfavorable DR3TBDTT crystallite orientation distribution along the film thickness, which is preferential edge-on crystallites in the top layer of the blend film and the increased population of face-on crystallites in the bottom layer of the blend film. This study illustrates that the morphology plays a key role in photovoltaic performance difference between regular and inverted devices and provides useful guidelines for further optimization of the morphology of solution-processed SM-OSCs.

Surface-Segregation-Induced Nanopapillae on FDTS-Blended PDMS Film and Implications in Wettability, Adhesion, and Friction Behaviors

Polymer composites have been extensively used to tune the surface property (e.g., wettability, friction, and adhesion) for its advantages of cost-effectiveness, high efficiency, and ease of fabrication. In this work, different amount of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FDTS) was added into poly(dimethylsiloxane) elastomer to prepare polymer composite films and were selected as a model to illustrate the effects of surface segregation on surface topology, wettability, friction, and adhesion. The results show that the added FDTS forms aggregations and increasing the content of FDTS leads to the difficulty of air bubble elimination, increase in viscosity, and drop in transparency. Driven by the differences of chemical potential, FDTS aggregations migrate to the air-polymer interface, resulting in surface enrichment and formation of nanopapillae (1-200 nm). This phenomenon becomes more significant with the increment in FDTS. The change in surface composition and structure generates profound effects on wettability, friction, and adhesion. The addition of FDTS makes the surface relatively oleophobic and further increasing the content of FDTS does not helpful in improving the oleophobicity due to the notable aggregation. Friction forces first grow with the increasing content of FDTS and then decline after the maximum point at 1.0 wt % of FDTS, which is attributed to the generated regular larger nanopappillae at high concentration. However, these larger nanopapillae lead to the increase in adhesion because more interactions are formed. The findings demonstrate the behaviors of FDTS in polymer composites and provide important guidance for controlling the formation of nanostructures via aggregation and phase segregation and exploring their implications on surface properties.

Dynamics of Additive Migration to Form Cathodic Interlayers in Organic Solar Cells

Migration of additives to organic/metal interfaces can be used to self-generate interlayers in organic electronic devices. To generalize this approach for various additives, metals, and organic electronic devices it is first necessary to study the dynamics of additive migration from the bulk to the top organic/metal interface. In this study, we focus on a known cathode interlayer material, polyethylene glycol (PEG), as additive in P3HT:PC₇₁BM blends and study its migration to the blend/Al interface during metal deposition and its effect on organic solar cell (OSC) performance. Using dynamic secondary ion mass spectroscopy (DSIMS) depth profiles and X-ray photoelectron spectroscopy surface analysis (XPS), we quantitatively correlate the initial concentration of PEG in the blend and sequence of thermal annealing/metal deposition processes with the organic/Al interfacial composition. We find that PEG is initially distributed within the film according to the kinetics of the spin coating process, i.e., the majority of PEG accumulates at the bottom substrate, while the minority resides in the film. During electrode evaporation, PEG molecules kinetically "trapped" near the film surface migrate to the organic/Al interface to reduce the interfacial energy. This diffusion-limited process is enhanced with the initial concentration of PEG in the solution and with thermal annealing after metal deposition. In contrast, annealing the film before metal deposition stalls PEG migration. This mechanism is supported by corresponding OSC devices showing that V_oc increases with PEG content at the interface, up to a saturation value associated with the formation of a continuous PEG interlayer. Presence of a continuous interlayer excludes the driving force for further migration of PEG to the interface. Revealing this mechanism provides practical insight for judicious selection of additives and processing conditions for interfacial engineering of spontaneously generated interlayers.

Prediction of the Wetting Behavior of Active and Hole-Transport Layers for Printed Flexible Electronic Devices Using Molecular Dynamics Simulations

Molecular dynamics (MD) simulations were used to predict the wetting behavior of materials typical of active and hole-transport layers in organic electronics by evaluating their contact angles and adhesion energies. The active layer (AL) here consists of a blend of poly(3-hexylthiophene) and phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM), whereas the hole-transport layer (HTL) consists of a blend of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) (PEDOT:PSS). Simulations of the wetting of these surfaces by multiple solvents show that formamide, glycerol, and water droplet contact angle trends correlate with experimental values. However, droplet simulations on surfaces are computationally expensive and would be impractical for routine use in printed electronics and other applications. As an alternative, contact angle measurements can be related to adhesion energy, which can be calculated more quickly and easily from simulations and has been shown to correlate with contact angles. Calculations of adhesion energy for 16 different solvents were used to rapidly predict the wetting behavior of solvents on the AL and HTL surfaces. Among the tested solvents, pentane and hexane exhibit low and similar adhesion energy on both of the surfaces considered. This result suggests that among the tested solvents, pentane and hexane exhibit strong potential as orthogonal solvent in printing electronic materials onto HTL and AL materials. The simulation results further show that MD can accelerate the evaluation of processing parameters for printed electronics.

Conjugated-Polymer Blends for Organic Photovoltaics: Rational Control of Vertical Stratification for High Performance

The photoactive layer of bulk-heterojunction organic solar cells, in a thickness range of tens to hundreds of nanometers, comprises phase-separated electron donors and acceptors after solution casting. The component distribution in the cross-section of these thin films is found to be heterogeneous, with electron donors or acceptors accumulated or depleted near the electrode interfaces. This vertical stratification of the photovoltaic blend influences device metrics through its impact on charge transport and recombination, and consequently plays an important role in determining the power conversion efficiency of photovoltaic devices. Here, different techniques, e.g., surface analysis and sputter-assisted depth-profiling, reflectivity modeling, and 3D imaging, that have been employed to characterize vertical stratification in bulk-heterojunction photovoltaic blends are reviewed. The origins of vertical stratification are summarized, including thermodynamics, kinetics, surface free energy, and selective dissolubility. The impact of correct and wrong vertical stratification to device metrics of solar cells are highlighted. Examples are then given to demonstrate how desired vertical stratification can be controlled with properly aligned device architecture to enable solar cells with high efficiency.

A hybrid organic–inorganic three-dimensional cathode interfacial material for organic solar cells

An alcohol soluble hybrid organic–inorganic three-dimensional material POSS-FN has been synthesized and assessed as a cathode interlayer within organic solar cells consisting of a PBDT-BT:PC₆₁BM bulk heterojunction.

Alloy Acceptor: Superior Alternative to PCBM toward Efficient and Stable Organic Solar Cells

The alloy acceptor (indene-C₆₀ bis-adduct (ICBA)/[6,6]-phenyl-C₇₁ -butyric acid-methyl-ester (PC₇₁ BM)) is employed to replace the widely used fullerene acceptor (PC₇₁ BM) in organic solar cells based on five different polymer donors, which exhibit a higher efficiency and much better device stability than the PC₇₁ BM counterpart.

Role of Interfacial Layers in Organic Solar Cells: Energy Level Pinning versus Phase Segregation

Organic photovoltaics (OPVs) are assembled from a complex ensemble of layers of disparate materials, each playing a distinct role within the device. In this work, the role of the interface that bridges the transparent anode and the bulk heterojunction (BHJ) in an OPV device was investigated. The surface characteristics of the electrode interface affect the energy level alignment, phase segregation, and the local composition of the bulk heterojunction (BHJ), which is in close contact. The commonly used ITO/PEDOT:PSS electrode was tailored with a thin, low-band-gap polymer overlayer, called PBDTTPD-COOH, a variant of the established donor polymer, PBDTTPD. Three BHJs that were composed of a donor polymer and PC71BM, were examined, including the donor polymers PBDTTPD, PCDTBT, and PTB7, within the following OPV device stack: ITO/(interfacial layer or layers)/BHJ/LiF/Al/Mg. It was found that modification of the ITO/PEDOT:PSS electrode with PBDTTPD-COOH resulted in statistically significant increases of power conversion efficiency for the PBDTTPD- and PCDTBT-based donor polymer:PC71BM BHJs, but not for the PTB7:PC71BM BHJ. Ultraviolet photoelectron spectroscopy (UPS) enabled determination of the respective energy level diagrams for these three different polymers relative to the ITO/PEDOT:PSS/PBDTTPD-COOH electrode, and revealed no injection barrier in all three polymer/substrate pairs. The observed differences of efficiency were not, therefore, electronic in origin. ToF-SIMS depth profiling and detailed experiments to determine surface energies strongly suggested that the greatest factor influencing device performance was a significant change of the local composition of the BHJ at this interface. When favorable accumulation of the donor polymer at the

PEDOT

PSS/interfacial layer was observed, the result was higher OPV device efficiencies. These results suggest that for each BHJ, the surface energies of the electrodes need to be carefully considered, as they will influence the local composition of the BHJ and resulting device performance.

Interfacial Characteristics of Efficient Bulk Heterojunction Solar Cells Fabricated on MoOx Anode Interlayers

The role of the interface between an MoOx anode interlayer and a polymer:fullerene bulk heterojunction is investigated. Processing differences in the MoOx induce large variations in the vertical stratification of the bulk heterojunction films. These variations are found to be inconsistent in predicting device performance, with a much better gauge being the quantity of polymer chemisorbed to the anode interlayer.

Understanding properties of engineered catalyst supports using contact angle measurements and X-ray reflectivity

There is significant interest in broadening the type of catalyst substrates that support the growth of high-quality carbon nanotube (CNT) carpets. In this study, ion beam bombardment has been utilized to modify catalyst substrates for CNT carpet growth. Using a combination of contact angle measurements (CAMs) and X-ray reflectivity (XRR) for the first time, new correlations between the physicochemical properties of pristine and engineered catalyst substrates and CNT growth behavior have been established. The engineered surfaces obtained after exposure to different degrees of ion beam damage have distinct physicochemical properties (porosity, layer thickness, and acid-base properties). The CAM data were analyzed using the van Oss-Chaudhury-Good model, enabling the determination of the acid-base properties of the substrate surfaces. For the XRR data, a Fourier analysis of the interference patterns enabled extraction of layer thickness, while the atomic density and interfacial roughness were extracted by analyzing the amplitude of the interference oscillations. The dramatic transformation of the substrate from "inactive" to "active" is attributed to a combined effect of substrate porosity or damage depth and Lewis basicity. The results reveal that the efficiency of catalyst substrates can be further improved by increasing the substrate basicity, if the minimum surface porosity is established. This study advances the use of a non-thermochemical approach for catalyst substrate engineering, as well as demonstrates the combined utility of CAM and XRR as a powerful, nondestructive, and reliable tool for rational catalyst design.

Enhanced Ultraviolet Stability of Air-Processed Polymer Solar Cells by Al Doping of the ZnO Interlayer

Photostability of organic photovoltaic devices represents a key requirement for the commercialization of this technology. In this field, ZnO is one of the most attractive materials employed as an electron transport layer, and the investigation of its photostability is of particular interest. Indeed, oxygen is known to chemisorb on ZnO and can be released upon UV illumination. Therefore, a deep analysis of the UV/oxygen effects on working devices is relevant for the industrial production where the coating processes take place in air and oxygen/ZnO contact cannot be avoided. Here we investigate the light-soaking stability of inverted organic solar cells in which four different solution-processed ZnO-based nanoparticles were used as electron transport layers: (i) pristine ZnO, (ii) 0.03 at %, (iii) 0.37 at %, and (iv) 0.8 at % aluminum-doped AZO nanoparticles. The degradation of solar cells under prolonged illumination (40 h under 1 sun), in which the ZnO/AZO layers were processed in air or inert atmosphere, is studied. We demonstrate that the presence of oxygen during the ZnO/AZO processing is crucial for the photostability of the resulting solar cell. While devices based on undoped ZnO were particularly affected by degradation, we found that using AZO nanoparticles the losses in performance, due to the presence of oxygen, were partially or totally prevented depending on the Al doping level.

The effect of donor content on the efficiency of P3HT:PCBM bilayers: optical and photocurrent spectral data analyses

A numerical analysis of optical absorption and photocurrent data reveals extensive interdiffusion in P3HT:PCBM bilayer devices.

Formation of cathode buffer layer by surface segregation of fluoroalkyl-modified ZnO for polymer solar cells

A cathode buffer layer, formed by surface segregation of fluoroalkyl modified ZnO, was present in polymer solar cells based on P3HT:PCBM.

First step into space: performance and morphological evolution of P3HT:PCBM bulk heterojunction solar cells [corrected] under AM0 illumination

P3HT (poly(3-hexylthiophene-2,5-diyl)):PC61BM ([6,6]-phenyl-C61 butyric acid methyl ester) bulk heterojunction solar cells are fabricated and characterized as a function of solar intensity, temperature, and aging at vacuum conditions under illumination with AM0 illumination for testing potential use in space applications. The evolution of the inner film morphology is probed with grazing incidence X-ray scattering techniques and correlated with the evolution of the efficiency during aging. Grazing incidence wide-angle X-ray scattering shows almost no change of the crystalline structure of the P3HT:PCBM films due to aging. In contrast, the morphological evolution on the mesoscale extracted from grazing incidence small-angle X-ray scattering can explain the observed decay of the overall efficiency. The behavior at high solar intensities as well as elevated temperatures suggests that organic solar cells have high potential for space applications in the future.

High-molecular-weight insulating polymers can improve the performance of molecular solar cells

The addition of small quantities of polystyrene (PS) is a simple and economically viable process that improves the power conversion efficiency of one of the most efficient small molecule donors. Addition of PS increases the solution viscosity, thereby providing thicker layers, and allows the formation of a desirable bulk heterojunction morphology. Moreover, the PS spontaneously accumulates as phase separated domains, away from the electrodes, so as not to interfere with charge extraction.

[6,6]-phenyl-C₆₁-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)amino)-ethyl ester as an acceptor and cathode interfacial material in polymer solar cells

An amine-based, alcohol-soluble fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)amino)-ethyl ester (PCBDAN) with 4-fold electron mobility of 6,6-phenyl-C61-butyric acid methyl ester (PCBM) is applied successfully as an acceptor and cathode interfacial material in polymer solar cells ITO/P3HT:PCBDAN/MoO3/Ag, where indium tin oxide (ITO) alone is used as the cathode and poly(3-hexylthiophene) (P3HT) is used as a donor. The X-ray photoelectron spectroscopy (XPS) depth profile confirming a favorable vertical phase separation is formed where P3HT is rich at the air/active blend interface and PCBDAN is rich at the buried interface with ITO and, thus, reduces the work function of ITO for use as the cathode. A moderate power conversion efficiency (PCE) of 3.1% is achieved. The slightly low PCE could be due to unoptimized morphology and low structure ordering of P3HT in the blends. However, this result demonstrates that the amine-based fullerene could be used as the acceptor and cathode interfacial material, which eliminated the multilayer device fabrication process. Because PCBDAN has high electron mobility, it would have potential applications in nano-structured organic solar cells. In the near future, alcohol-processable, high-efficient organic/polymer solar cells can be anticipated.

Self-organization and nanostructural control in thin film heterojunctions

In spite of more than two-decades of studies of molecular self-assembly, the achievement of low cost, easy-to-implement and multi-parameter bottom-up approaches to address the supramolecular morphology in three-dimensional (3D) systems is still missing. In the particular case of molecular thin films, the 3D nanoscale morphology and function are crucial for both fundamental and applied research. Here we show how it is possible to tune the 3D film structure (domain size, branching, etc.) of thin film heterojunctions with nanoscale accuracy together with the modulation of their optoelectronic properties by employing an easy two-step approach. At first we prepared multi-planar heterojunctions with a programmed sequence of nanoscopic layers. In a second step, thermal stimuli have been employed to induce the formation of bulk heterojunctions with bicontinuous and interdigitated phases having a size below the exciton diffusion length. Importantly, the study of luminescence quenching of these systems can be considered as a useful means for the accurate estimation of the exciton diffusion length of semiconductors in nanoscale blends. Finally, nearly a thousand times lower material consumption than spin coating allows a drastic reduction of material wasting and a low-cost implementation, besides the considerable possibility of preparing thin film blends also by employing materials soluble in different solvents.

Self-organization of amine-based cathode interfacial materials in inverted polymer solar cells

We present a strategy to fabricate polymer solar cells in inverted geometry by self-organization of alcohol soluble cathode interfacial materials in donor-acceptor bulk heterojunction blends. An amine-based fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)-ethyl)(methyl)amino)ethyl ester (PCBDAN) is used as an additive in poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) blend to give a power conversion efficiency of 3.7% based on devices ITO/P3HT:PCBM:PCBDAN/MoO3/Ag where the ITO alone is used as the cathode. A vertical phase separation in favor of the inverted device architecture is formed: PCBDAN is rich on buried ITO surface reducing its work function, while P3HT is rich on air interface with the hole-collecting electrode. The driving force of the vertical phase separation is ascribed to the surface energy and its components of the blend compositions and the substrates. Similar results are also found with another typical alcohol soluble cathode interfacial materials, poly[(9,9-bis(3'-(N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), implying that self-organization may be a general phenomenon in ternary blends. This self-organization procedure could eliminate the fabrication of printing thin film of interlayers or printing on such thin interlayers and would have potential application for roll-to-roll processing of polymer solar cells.

Construction of planar and bulk integrated heterojunction polymer solar cells using cross-linkable D-A copolymer

An integrated device architecture was constructed via vertical combination of planar and bulk heterojunctions by solution processing, where a cross-linked D-A copolymer (PBDTTT-Br25) was inserted between a PEDOT:PSS layer and the blended photoactive layer. PBDTTT-Br25 can readily undergo photo crosslinking to form an insoluble robust film via ultraviolet irradiation after solution-deposition, which enables the subsequent solution processing of a photoactive layer on the robust surface. The insertion of a pure PBDTTT-Br25 layer to build an integrated heterojunction could provide an additional donor/acceptor interface, which enables hole transport to the anode without interruption, thereby reducing the charge carrier recombination probability. The power conversion efficiency (PCE) of the polymer solar cell (PSC) with the integrated architecture reaches 5.24% under an AM1.5G illumination of 100 mW/cm(2), which is increased by 65%, in comparison with that of the reference single heterojunction device (3.17%), under the same experimental conditions.