Blend Thin Films Research Articles

(Invited) High Performance Ternary Organic Solar Cells Based on Non-Fullerene Acceptors

Organic solar cells (OSCs) comprising of a photoactive layer of electron-donor and electron-acceptor have attracted great attention for decades due to their low-cost, large-area, solution-processed in flexible substrates, and indoor light energy harvester [1,2]. Their power conversion efficiencies (PCEs) have been boosted to over 18% via material design, device engineering, and theoretical studies [3]. One of the most promising methods to improve device performance is adding a third component into a binary system, resulting in a ternary blend. Ternary blends can form an even larger variety of morphologies, depending on the ratio and the interactions between the components. In a ternary blend, the third components not only provide a broadened light harvesting and optimizing the film morphology, but also facilitating exciton dissociation and charge transport. As a consequence, it is improving the device performance of OSCs, i.e., the fill factor (FF), short-circuit current density (Jsc), open-circuit voltage (Voc) and power conversion efficiency (PCE). On the other hand, the rapid progress of highly efficient NFAs materials has offered a new opportunity for studying ternary OSCs due to the great tunability of chemical structures, optical properties and electronic properties of NFAs, and their ability to phase separate into nanoscopic domains in blended thin films [4].In this study, we have been focusing first on the optimization of binary systems using fullerene and non-fullerene acceptors. We performed several strategies to improve the device performance and stability of these binary systems: engineering the electron transport layers, annealing treatment, using inkjet printing and spray pyrolysis deposition methods, and photoactive materials engineering. As a result, we achieved PCEs from ~3% using P3HT as donor polymer, ~10% PCE using PTB7-Th donor polymer up to more than 17% efficiency using PM6 donor polymer [5–10]. To the next level, we shift the direction of our work using ternary system through careful selection of the third component in donor-acceptor materials. However, blending three light-absorbing semiconductor materials is not a trivial work. A huge materials combination, their compatibility, relative-energies of their frontier orbitals and solubility need to be considered to achieve a successful high performance ternary bulk heterojunction OSCs.To date, we have been able to improve device performance by using new phthalocyanines derivatives as third components in binary PTB7-Th:PC70BM due to their high extinction coefficients, stability, and energy band gaps well-matched to the incident solar spectrum [11]. The proper choice of the central atom in pthalocyanine compound were studied to know their effect to the electrical parameters and absorption spectra of OSCs. Moreover, we also compare the effect of fullerene PC70BM and non-fullerene ITIC-M as third components in the bulk heterojunctions of PM6:Y7-based devices. Acknowledgements: This work was supported by the Ministerio de Ciencia, Innovación y Universidades (MICINN/FEDER) RTI2018-094040-B-I00, by the Agency for Management of University and Research Grants ref. 2017-SGR-1527 and by the Catalan Institution for Research and Advanced Studies (ICREA) under the ICREA Academia Award References[1] L. Duan, A. Uddin, Adv. Sci. 2020, 1903259.[2] A. A. A. Torimtubun, J. G. Sánchez, J. Pallarès, L. F. Marsal, Sustain. Energy Fuels 2020, 4, 3378.[3] Q. Liu, Y. Jiang, K. Jin, J. Qin, J. Xu, W. Li, J. Xiong, J. Liu, Z. Xiao, K. Sun, S. Yang, X. Zhang, L. Ding, Sci. Bull. 2020, 65, 272.[4] L. Xiao, B. He, Q. Hu, L. Maserati, Y. Zhao, B. Yang, M. A. Kolaczkowski, et al., Joule 2018, 2, 2154.[5] J. G. Sanchez, A. A. A. Torimtubun, V. S. Balderrama, M. Estrada, J. Pallares, L. F. Marsal, IEEE J. Electron Devices Soc. 2020, 8, 421.[6] A. A. A. Torimtubun, J. G. Sanchez, J. Pallares, L. F. Marsal, in 2020 IEEE Lat. Am. Electron Devices Conf., IEEE, San Jose, Costa Rica, 2020, pp. 1–4.[7] J. G. Sánchez, V. S. Balderrama, S. I. Garduño, E. Osorio, A. Viterisi, M. Estrada, J. Ferré-Borrull, J. Pallarès, L. F. Marsal, RSC Adv. 2018, 8, 13094.[8] V. S. Balderrama, J. G. Sánchez, G. Lastra, W. Cambarau, S. Arias, J. Pallarès, E. Palomares, M. Estrada, L. F. Marsal, J. Mater. Chem. A 2018, 6, 22534.[9] V. S. Balderrama, M. Estrada, A. Cerdeira, B. S. Soto-Cruz, L. F. Marsal, J. Pallares, J. C. Nolasco, B. Iñiguez, E. Palomares, J. Albero, Microelectron. Reliab. 2011, 51, 597.[10] E. Osorio, J. G. Sánchez, L. N. Acquaroli, M. Pacio, J. Ferré-Borrull, J. Pallarès, L. F. Marsal, ACS Omega 2017, 2, 3091.[11] A. A. A. Torimtubun, J. Follana-Berná, Á. Sastre‐Santos, J. Pallarès, L. F. Marsal, to be Publ. 2020.

Förster energy transfer mechanism and color tunability in three binary conjugated polymer blends

The energy transfer mechanism from poly(9,9-dioctylfluorene-2,7-diyl) (PFO) to poly 9,9-dioctylfluorene-alt-benzothiadiazole (F8BT) and to poly(2-methoxy-5(2-ethylhexyl)-1,4 -phenylenevinylene (MEH-PPV), and from F8BT to MEH-PPV were comprehensively investigated. The concentrations of the donors in their binary blends were kept constant, while the acceptor concentration was carefully varied before reaching the complete emission quenching. The binary blend thin films were prepared using the solution blending method before they were spin-coated onto glass substrate. The UV–Vis absorption and emission spectra showed color emission shift towards the acceptor emission, confirming the energy transfer. These spectra were used to calculate important energy transfer parameters, such as the Förster radius (Ro), the distance between donor and acceptor molecules (RDA), energy transfer rate (kET), energy transfer efficiency (η), energy transfer probability (PDA), quenching constant (Ksv) and quenching rate constant (kq). The spectra also indicated that at high acceptor content the creation of excimer due to the molecule aggregation resulted in a red shift of the acceptor emission. In addition, the CIE coordinates proved the tunability of the color-emission. Finally, based on the calculated parameters, energy transfer mechanism in the three binary blends were suggested.

Thin film growth of phase-separating phthalocyanine-fullerene blends: A combined experimental and computational study

Blended organic thin films have been studied during the last decades due to their applicability in organic solar cells. Although their optical and electronic features have been examined intensively, there is still lack of detailed knowledge about their growth processes and resulting morphologies, which play a key role for the efficiency of optoelectronic devices such as organic solar cells. In this study, pure and blended thin films of copper phthalocyanine (CuPc) and the Buckminster fullerene (C60) were grown by vacuum deposition onto a native silicon oxide substrate at two different substrate temperatures, 310 K and 400 K. The evolution of roughness was followed by in-situ real-time X-ray reflectivity. Crystal orientation, island densities and morphology were examined after the growth by X-ray diffraction experiments and microscopy techniques. The formation of a smooth wetting layer followed by rapid roughening was found in pure CuPc thin films, whereas C60 shows a fast formation of distinct islands at a very early stage of growth. The growth of needle-like CuPc crystals loosing their alignment with the substrate was identified in co-deposited thin films. Furthermore, the data demonstrates that structural features become larger and more pronounced and that the island density decreases by a factor of four when going from 310 K to 400 K. Finally, the key parameters roughness and island density were well reproduced on a smaller scale by kinetic Monte-Carlo simulations of a generic, binary lattice model with simple nearest-neighbor interaction energies.

Influence of ZnO nanoparticles on the performance of LED based on oligomer thin films

In this work, a set of thin films of ZnO nanoparticles doped with a conjugated oligomer {9,9,9′,9′,9″,9″-hexakis(hexyl)-2,7′;2′,7″-trifluorene} has been prepared. The optical and electrical measurements on pure oligomer, ZnO nanoparticles, and blended thin films have been investigated. The absorption spectra of the pure oligomer in thin films showed a single band at 350 nm. When the thickness was increased, there was no new band detected at the end of the spectrum. This is a strong indication that the oligomer cannot be present in the dimeric state. On the other hand, the photoluminescence (PL) spectra of the oligomer showed two distinguished peaks at 400 and 420 nm due to the monomer and excimer states, respectively. Furthermore, the results showed that the PL spectrum of the oligomer was affected by the thickness. For the blended thin films with different ratios of ZnO nanoparticles, the PL spectra showed an enhancement in intensity by increasing the concentration of ZnO nanoparticles. For the electrical properties, the addition of ZnO nanoparticles to oligomer had increased the current and luminescence. The luminescences of the pure and blend film at 10 V had reached 1000 and 4200 cd/m2, respectively; and the turn-on voltage was reduced from 10.5 to 5.8 V. The light emission in the device is due to the Auger-assisted energy up-conversion process. The lifetime of the blend’s diodes measured under standard atmospheric conditions was greatly improved.

Enhancement Efficiency of Active Layer P3HT:POT:DBSA/PCBM Photoactive Solar Cell Device

In this work we have investigated thin film blend produced from regioregular poly(3-hexylthiophene) (rr-P3HT), Poly(o-toluidine) doped with dodecylbenzene sulphonic (POT:DBSA) and [6,6]-phenyl-C61 butyric acid methylester (PCBM) materials by spin coating method for application as active layer in photoactive solar cell (PSCs). A thin film uniform rr-P3HT:PCBM and rr-P3HT:POT:DBSA:PCBM were successfully deposited on the poly (3,4 ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) buffer layer substrate. The absorption spectra of the films were studied by using UV–vis spectrophotometer and Raman spectroscopy. the morphology of films is evaluated by X-ray diffraction (XRD) and atomic force microscopy (AFM) patterns. We have seen that, the efficiency enhancement for the device with a rr-P3HT:POT:DBSA:PCBM film is more significant than for the device with rr-P3HT:PCBM.

Nanoparticle Induced Morphology Modulation in Spin Coated PS/PMMA Blend Thin Films.

The influence of adding nanoparticles on the ascast morphology of spin coated immiscible polystyrene/poly(methyl methacrylate) (PS/PMMA) thin films of different thickness (hE) and composition (RB, volume ratio of PS to PMMA) has been explored in this article. To understand the precise effect of nanoparticle addition, the morphology of PS/PMMA thin blend films spin cast from toluene on a native oxide covered silicon wafer substrate was first investigated. It is seen that in particle free films, the generic morphology of the films remains nearly unaltered with increase in hE, for RB = 3:1 and 1:3. In contrast, strong hE dependent morphology transformation is observed in films with RB = 1:1. Subsequently, thiol-capped gold nanoparticles (AuNP) containing films with different particle concentrations (CNP) were cast from the same solvent along with the polymer mixture. We observe that addition of AuNPs barely alters the generic morphology of the films with RB = 3:1. In contrast, the presence of the particles significantly influences the morphology of the films with RB = 1:1 and 1:3, particularly at higher CNP (≈10.0%). X-ray photoelectron spectroscopy and X-ray reflectivity of some samples reveal that the AuNPs tend to migrate to the free surface through the PS phase, thereby stabilizing this layer partially or fully (depending on CNP) against dewetting over a surface of adsorbed PMMA layer and influencing the ascast morphology as a function of CNP. The work is fundamentally important in understanding largely overlooked implications of nanoparticle addition on the morphology of PS/PMMA blend thin films which forms the fundamental basis for future interesting studies involving dynamics of nanoparticles within the blend thin films.

Role of Morphology and Förster Resonance Energy Transfer in Ternary Blend Organic Solar Cells

Organic solar cells (OSCs) fabricated from ternary blend thin film absorbers are designed to maximize the range of absorption in the solar spectrum and thus increase the short-circuit current densi...

Enhancement Efficiency of Active Layer P3HT:POT:DBSA/PCBM Photoactive Solar Cell Device

In this work we have investigated thin film blend produced from regioregular poly(3-hexylthiophene) (rr-P3HT), Poly(o-toluidine) doped with dodecylbenzene sulphonic (POT:DBSA) and [6,6]-phenyl-C61 butyric acid methylester (PCBM) materials by spin coating method for application as active layer in photoactive solar cell (PSCs). A thin film uniform rr-P3HT:PCBM and rr-P3HT:POT:DBSA:PCBM were successfully deposited on the poly (3,4 ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) buffer layer substrate. The absorption spectra of the films were studied by using UV–vis spectrophotometer and Raman spectroscopy. the morphology of films is evaluated by X-ray diffraction (XRD) and atomic force microscopy (AFM) patterns. We have seen that, the efficiency enhancement for the device with a rr-P3HT:POT:DBSA:PCBM film is more significant than for the device with rr-P3HT:PCBM.

Analysis of Homopolymer Distribution in a Polymer Blend Thin Film by Anomalous Grazing Incidence Small-Angle X-ray Scattering at the Bromine K-Edge

Polystyrene-block-poly(4-hydroxystyrene) (PS–PHS)/poly(4-hydroxystyrene) brominated (Br-PHS) blend thin films were analyzed using anomalous grazing incidence small-angle X-ray scattering (anomalous...

Role of DIO vis-à-vis microstructural kinetics during thermal annealing on the performance of PTB7:PC71BM organic solar cells

Thermal annealing is an important process to improve the power conversion efficiency (PCE) of bulk-hetero junction organic solar cells (BHJ OSCs) by inducing micro-structural changes within the active layer blend’s constituents leading to amelioration of charge transfer/transport dynamics and interfacial properties. This work investigates the impact of thermal annealing on the photovoltaic performance ofPoly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-Phenyl C71-butyric acidmethyl ester(PC71BM) (PTB7:PC71BM) thin film blend in the presence of DIO (1,8 –Diiodooctane) additive, at different annealing temperatures (40, 60 and 80 °C) and correlates with the performance of DIO-free devices investigated under similar annealing conditions. Our results show that DIO-containing (w/DIO)blends, annealed at moderate temperature (40 °C) for 1 h exhibit significantly improved short circuit current (Jsc)as well as fill factor (FF) and hence 25% improvement in PCE over bench dried (unannealed) DIO-containing blend films. Interestingly, thermal treatment of DIO containing blend films at a higher temperature of 80 °C results in significant reduction of FF. However, for without DIO (w/o DIO) devices, FF remains unchanged while decrease in Jsc is significant. The inferior performance of both (w/DIO and w/o DIO) the devices at higher temperatures ~80 °C emanates from an unfavorable distribution of the polymer- fullerene network, as shown by morphological and surface potential analyses. Specifically, in w/DIO devices, the decreased performance is possibly induced by the temperature driven, accelerated loss of DIO additive that produces the evolution of undesirable morphology thus deteriorating the performance.

Thermoresponsive Poly(vinyl methyl ether) (PVME) Retained by 3-Aminopropyltriethoxysilane (APTES) Network

Thermoresponsive polymers (TRP)s have been widely used for various applications from controlling membrane fouling in separation to cell/cell sheet harvesting in regenerative medicine. While poly(N-isopropylacrylamide) (pNIPAAm) is the most commonly used TRP, less expensive and easily processed poly(vinyl methyl ether) (PVME) also shows a hydrophilic to hydrophobic transition at 32-35 °C, near physiological conditions. In this study, we investigated the processing conditions for retaining a stable layer of PVME thin film on silica surfaces via entrapment in a 3-aminopropyltriethoxysilane (APTES) network. In addition, the thermoresponsive behaviors (TRB) of the retained PVME films were evaluated. Blend thin films of PVME/APTES with 90:10 and 50:50 mass ratios were spin-coated from their solutions in ethanol under ambient conditions and then annealed in a vacuum oven at 40, 60, 80, or 120 °C for 1, 2, or 3 days. The annealed films were then thoroughly rinsed with room temperature water and then soaked in water for 3 days. Our results showed that annealing at a temperature of ≥40 °C was necessary for retaining a PVME film on the surface. The higher annealing temperature led to greater film retention, probably due to the formation of a tighter APTES network. Regardless of processing conditions, all retained PVME films showed TRB, determined by water contact angles below and above the transition temperature of PVME. Additionally, particle attachment and protein adsorption on retained PVME films showed lower attachment or adsorption at room temperature as compared to that at 37 °C, and a greater difference was observed for the 90:10 blend where more PVME was consisted. Furthermore, human mesenchymal stem cells attached and proliferated on the retained PVME surfaces at 37 °C and rapidly detached at room temperature. These results illustrated the potential applications of PVME surfaces as thermoresponsive supports for low-fouling applications and noninvasive cell harvesting.

Enhancing Long‐Term Device Stability Using Thin Film Blends of Small Molecule Semiconductors and Insulating Polymers to Trap Surface‐Induced Polymorphs

AbstractThe lack of long‐term stability in thin films of organic semiconductors can often be caused by the low structural stability of metastable phases that are frequently formed upon deposition on a substrate surface. Here, thin films of 2,7‐dioctyloxy[1]benzothieno[3,2‐b]benzothiophene (C8O‐BTBT‐OC8) and blends of this material with polystyrene by solution shearing are fabricated. Both types of films exhibit the metastable surface‐induced herringbone phase (SIP) in all the tested coating conditions. The blended films reveal a higher device performance with a field‐effect mobility close to 1 cm2 V−1 s−1, a threshold voltage close to 0 V, and an on/off current ratio above 107. In situ lattice phonon Raman microscopy is used to study the stability of the SIP polymorph. It is found that films based on only C8O‐BTBT‐OC8 slowly evolve to the Bulk cofacial phase, significantly impacting device electrical performance. In contrast, the blended films stabilize the SIP phase, leading to devices that maintain a high performance over 1.5 years. This work demonstrates that blending small‐molecule organic semiconductors with insulating binding polymers can trap metastable polymorphs, which can lead to devices with both improved performance and long‐term stability.

Improving Photophysical Properties of White Emitting Ternary Conjugated Polymer Blend Thin Film via Additions of TiO2 Nanoparticles.

The effect of TiO2 nanoparticles on the photophysical properties of ternary conjugated polymer (CP) blends of poly(9,9-dioctylfluorene-2,7-diyl) (PFO), poly 9,9-dioctylfluorene-alt-benzothiadiazole (F8BT) and poly(2-methoxy-5(2-ethylhexyl)-1,4 -phenylenevinylene (MEH-PPV) thin films was investigated. This ternary blend used a fixed amount of PFO as the donor with MEH-PPV and F8BT in various ratios as the acceptors. The solution-blending method and the spin-coating technique were used to prepare the blends and the thin films, respectively. Through efficient Förster Resonance Energy Transfer (FRET), the desired white emission was achieved with PFO/0.3 wt.% F8BT/0.5 wt.% MEH-PPV ternary blend thin film. Additions of nanoparticles up to 10 wt.% dramatically intensified the white emission which then dimmed at higher contents due to agglomerations. The current density–voltage characteristics of the nanocomposite thin films exhibited dependency on the content and distributions of the nanoparticles. Finally, a possible underlying mechanism for the intensification of emission is proposed.

Structural, Optical and Decay Properties of Zinc(II) 8-Hydroxyquinoline and Its Thin Film

Zinc(II) 8-hydroxyquinoline (Znq2) green luminescent material and its blended thin film in a poly(methyl methacrylate) (PMMA) matrix have been prepared. XRD analysis confirms the formation of the compound and its presence in the PMMA blended thin film. The orbital molecular geometry, quantum chemical calculations and band gap energy of Znq2 phosphor have been confirmed from the DFT study. The morphology of the phosphor was shown by the SEM images. UV–Vis absorption, photoluminescence and decay analysis of zinc(II) 8-hydroxyquinoline phosphor confirms its suitability as a green emitter for organic light-emitting diodes. Uniform distribution of Znq2 powder in the PMMA matrix and its roughness was estimated by AFM analysis. Luminescence decay study was utilized to demonstrate its lifetime analysis by the time-correlated single-photon counting (TCSPC) system.

Staining-free observation of polymer blend thin films on transmission extreme ultraviolet microscopy

Transmission extreme ultraviolet microscopy is applied to the staining-free observation of a poly(styrene–methyl methacrylate) (PS/PMMA) blend. At a photon energy of 92 eV, the imaginary part of the atomic scattering factor for oxygen, which represents the absorption, is four-times larger than that of carbon, and microstructures can be visualized by the contrast resulting from the presence of oxygen. Based on the signal to noise ratio of the images, we consider the optimum photon energy and sample thickness for common polymer blends. Finally, a practical high contrast of 30% is successfully demonstrated for the PS/PMMA thin film.

Suppressed crystallization and enhanced gas permeability in thin films of cellulose acetate blends

Film thickness and blending can affect crystallization and chain dynamics of semi-crystalline polymers, particularly for submicron films. Herein we apply these insights to elucidate a long-standing puzzle for industrial asymmetric membranes for CO2 removal from natural gas, which are based on blends of cellulose diacetate (CDA with a degree of acetylation or DS of 2.45) and cellulose triacetate (CTA with a DS of 2.87). The asymmetric membranes comprise thin selective layers (~100 nm) and have been reported to exhibit gas permeability much higher than the bulk blends. We hypothesize that the film thickness and blending confine crystallization and decrease the crystallinity, increasing gas permeability while retaining high selectivity. Specifically, we systematically determined crystallinity for CTA and two blends with various thicknesses ranging from ≈500 nm to ≈20 μm. Adding 75 mass% CTA in CDA (CDA-CTA75) decreases the crystallinity from 34 vol% to 23 vol% at ≈ 20 μm (accompanied by CO2 permeability increase from 3.9 Barrer to 7.1 Barrer), while decreasing the film thickness of CDA-CTA75 from ≈20 μm to ≈ 1.0 μm decreases the crystallinity from 23 vol% to 13 vol% and increases CO2 permeability from 7.1 Barrer to 14 Barrer. Understanding polymer chain dynamics and crystallization in thin films can be instrumental in designing high-performance membranes for industrial gas separations.

Solvo-selective imprinting of a thin polymer blend film for creating multi-length scale patterns

Controlling and spatial ordering of the phase separation in ultra-thin polymer-blend films is a viable approach to create complex surface structures with unique properties. This study reports a facile solvo-selective patterning technique in a phase-separated polymer blend thin film comprising of immiscible polymers polystyrene (PS) and poly(methyl methacrylate) (PMMA). The pattern replication was achieved using capillary force lithography by imprinting the blend films with cross-linked polydimethylsiloxane stamp with grating patterns. The replication was done in the presence of 1-chloropentane or acetic acid vapours, which are selective solvents for PS and PMMA, respectively. This engendered capillary flow of only one phase while the domains of the second phase remain rigid. Depending on the initial as-cast morphology of the film which is a function of the relative proportion of PS and PMMA, a variety of structures were obtained by combining the phase-separated domains with the structure imposed by the stamp. This method can be considered as a simple, one-step technique for creating hierarchical patterns that are likely to find applications in modulating properties of soft surfaces.

Analysis of Surface Segregation of Bottlebrush Polymer Additives in Thin Film Blends with Attractive Intermolecular Interactions

The surface chemistry of polymer films and coatings is relevant to a wide range of applications, and recent work has shown that bottlebrush polymers can be used as additives to modify film interfac...

Optimizing the performance of P3HT-based photodetector by tuning the composition of OXCBA

In this study, the performance of organic-based photodetectors using poly(3-hexylthiophene) (P3HT) and o-xylenyl-C60-bisadduct (OXCBA) as the donor and acceptor material, respectively, is reported. Four different ratios of 1:0.25, 1:0.50, 1:1.00 and 1:1.50 of P3HT:OXCBA binary blended thin films are used as photoactive layers for organic photodetectors. Optimizing the ratio of photoactive layers dramatically enhances the performance of photodetectors, with rise and decay time of 445 and 309 ms, respectively, are recorded from 1:0.50 ratio. The responsivity of the optimized device is 252.69 mA/W while its specific detectivity is 1.85 × 101°Jones at −1 V, and has shown good stability for over 336 h. External quantum efficiency of 52.77 % and low surface roughness of 0.596 nm are obtained from the 1:0.50 P3HT:OXCBA device. These results imply a high detection potential of P3HT:OXCBA-based photodetector.

Circumventing Macroscopic Phase Separation in Immiscible Polymer Mixtures by Bottom-up Deposition

Achieving intimate mixing in immiscible polymer blend thin films is of paramount importance for maximizing their performance in numerous applications such as membranes, electronics, and energy devi...