Submolecular Resolution Imaging of P3HT:PCBM Nanostructured Films by Atomic Force Microscopy: Implications for Organic Solar Cells

The efficiency of organic bulk-heterojunction (BHJ) solar cells depends greatly on both the bulk and surface structure of the nanostructured bicontinuous interpenetrating network of materials, known as the active layer. The morphology of the top layer of a coated film is often resolved at the scale of a few nanometers, but fine details of the domains and the order within them are more difficult to identify. Here, we report a high-resolution atomic force microscopy (AFM) investigation of various stoichiometries of the well-studied poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) active layer mixture. Images of the surface were obtained using AC-mode AFM exciting higher-order resonance frequencies of a standard silicon probe, a promising technique for acquiring real-space images of organic-based thin films with nanoscale and even submolecular resolution. We provide firm evidence of the nanoscale organization of the P3HT polymer and of the P3HT:PCBM stoichiometric mixtures at the surface-air interface of the BHJ architecture. Our study shows the characteristic periodicity of the regioregular P3HT identified in the nanoscale domain areas with submolecular resolution. Such areas are then distorted in place when adding different quantities of PCBM forming stoichiometric mixtures. When the samples were exposed to ambient light, the morphologies were very different, and submolecular resolution was not achieved. This approach is shown to provide a precise view of the active layer's nanostructure and will be useful for studies of other materials as a function of various parameters, with particular attention to the role of the acceptor in tuning morphology for understanding optimum performance in organic photovoltaic devices.

Thermal Insolubilization of Electrically n-Doped Films Achieved Using 7-Alkoxy-Benzocyclobutene-Substituted Fullerene and Dopant Molecules

Insoluble electrically n-doped fullerene-containing films have been obtained by thermal annealing of a fullerene compound and a 1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazole n-dopant moiety, both of which are functionalized with a 7-butoxybenzocyclobutene group. The covalent tethering and electrical doping reactions are studied by mass spectrometry as well as electron paramagnetic resonance. Optical absorption spectra on BBCB-N-DMBI-H-doped BBCBP indicate films heated at 150 °C for 10 min are unaffected by immersion for 10 min in ortho-dichlorobenzene. Although films containing a 10 mol % loading of the dopant showed electrical conductivity values of 1.1 × 10^-5 ± 3.4 × 10^-7 S cm^-1 prior to heating, the thermal insolubilization process led to values around two orders-of-magnitude lower. However, the thermal insolubilization also leads to immobilization of the dopant molecule and the corresponding cation, reducing their ability to diffuse into an adjacent layer of a stronger electron acceptor.

A Cross-Linkable Electron-Transport Layer Based on a Fullerene-Benzoxazine Derivative for Inverted Polymer Solar Cells

The synthesis, optoelectronic characterization and device properties of a cross-linkable fullerene derivative, [6,6]-phenyl-C₆₁ -butyric benzoxazine ester (PCBB) is reported. PCBB shows all the basic photophysical and electrochemical properties of the parent compound [6,6]-phenyl-C₆₁ -butyric methyl ester (PCBM). Thermal cross-linking of the benzoxazine moiety in PCBB resulted in the formation of cross-linked, solvent resistive adhesive films (C-PCBB). Atomic force microscopy (AFM) and optical microscopic studies showed dramatic reduction in the roughness and aggregation behaviour of P3HT-PCBM polymer blend film upon incorporation of C-PCBB interlayer. An inverted bulk heterojunction solar cell based on the configuration ITO/ZnO/C-PCBB/P3HT-PCBM/V₂ O₅ /Ag achieved 4.27 % power conversion efficiency (PCE) compared to the reference device ITO/ZnO/P3HT-PCBM/V₂ O₅ /Ag (PCE=3.28 %). This 25 % increase in the efficiency is due to the positive effects of C-PCBB on P3HT/C-PCBB and PCBM/C-PCBB heterojunctions.

Diffusion-Limited Crystallization: A Rationale for the Thermal Stability of Non-Fullerene Solar Cells

Organic solar cells are thought to suffer from poor thermal stability of the active layer nanostructure, a common belief that is based on the extensive work that has been carried out on fullerene-based systems. We show that a widely studied non-fullerene acceptor, the indacenodithienothiophene-based acceptor ITIC, crystallizes in a profoundly different way as compared to fullerenes. Although fullerenes are frozen below the glass-transition temperature T_g of the photovoltaic blend, ITIC can undergo a glass-crystal transition considerably below its high T_g of ∼180 °C. Nanoscopic crystallites of a low-temperature polymorph are able to form through a diffusion-limited crystallization process. The resulting fine-grained nanostructure does not evolve further with time and hence is characterized by a high degree of thermal stability. Instead, above T_g, the low temperature polymorph melts, and micrometer-sized crystals of a high-temperature polymorph develop, enabled by more rapid diffusion and hence long-range mass transport. This leads to the same detrimental decrease in photovoltaic performance that is known to occur also in the case of fullerene-based blends. Besides explaining the superior thermal stability of non-fullerene blends at relatively high temperatures, our work introduces a new rationale for the design of bulk heterojunctions that is not based on the selection of high- T_g materials per se but diffusion-limited crystallization. The planar structure of ITIC and potentially other non-fullerene acceptors readily facilitates the desired glass-crystal transition, which constitutes a significant advantage over fullerenes, and may pave the way for truly stable organic solar cells.

Toward Improved Environmental Stability of Polymer:Fullerene and Polymer:Nonfullerene Organic Solar Cells: A Common Energetic Origin of Light- and Oxygen-Induced Degradation

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.

N-Alkylation vs. O-alkylation: influence on the performance of the photovoltaic cells based on a tetracyclic lactam polymer donor

Lactam-containing acceptors, which could provide two potential alkylation positions (N-alkylation and O-alkylation), are important building blocks for polymeric donors in high performance polymer solar cells (PSCs). However, the influence of alkylation positions on the PSC performance has seldom been studied. Herein, we investigated the influence of O-alkylation and N-alkylation on a novel bislactam acceptor, namely dibenzonaphthyridinedione (DBND), on the physical properties of the corresponding polymers and hence their PSC performance. Besides O-alkylated and N-alkylated DBND, half-N-alkylated-half-O-alkylated DBND (N,O-DBND) was also prepared and copolymerized with stannyl bithiophene (2T). It was found that by varying the alkylation positions, the optical, crystalline and aggregation properties of the corresponding polymers were greatly altered. In comparison with P(N-DBND-2T) and P(O-DBND-2T), P(N,O-DBND-2T) shows both better solubility and shorter π–π stacking distance. By blending with PC₇₁BM, P(N,O-DBND-2T) forms better nano-fibrillar phase separation so that less charge recombination is observed, thus leading to a much better power conversion efficiency (PCE) around 5%, which is the highest value of the conjugated system based on N,O-alkylated acceptors. The results show that the asymmetric N,O-alkylation protocol is a promising way to adjust the properties of the bislactam-containing conjugated polymers.

The alkylation positions (N-alkylation & O-alkylation) of a bis-lactam containing acceptor has a strong influence on the solar cell performance of the corresponding polymers.

The influence of the configuration of the (C70)2 dimer on its rovibrational spectroscopic properties: a theoretical survey

A study of the spectroscopic properties of the buckyball dimer (C₇₀)₂ was performed, which involved mapping the potential energy curve of this system. The spectroscopic constants of the system were obtained using theoretical Dunham and discrete variable representation methods, as well as the Rydberg analytical function expanded to the sixth degree. Because the fullerenes in the dimer have both hexagonal and pentagonal faces, the properties of (C₇₀)₂ were examined for different system configurations. The fullerene dimerization process involves a weak interaction, possibly mediated by short-range components such as van der Waals forces. The differences between the spectroscopic constants of the various (C₇₀)₂ configurations and between their dissociation energies D_e were found to be rather small, which can be attributed to the dominant influence of the hexagonal faces of the fullerenes on the interaction between the fullerenes. These results should aid our understanding of the process of fullerene dimer formation and hopefully facilitate the development and application of new materials based on these dimers. Graphical Abstract Comparison of the potential energy curve and a schematic representation for the all (C₇₀)₂ fullerenes dimers configurations.

Recent Advances in Morphology Optimization for Organic Photovoltaics

Organic photovoltaics are an important part of a next-generation energy-harvesting technology that uses a practically infinite pollutant-free energy source. They have the advantages of light weight, solution processability, cheap materials, low production cost, and deformability. However, to date, the moderate photovoltaic efficiencies and poor stabilities of organic photovoltaics impede their use as replacements for inorganic photovoltaics. Recent developments in bulk-heterojunction organic photovoltaics mean that they have almost reached the lower efficiency limit for feasible commercialization. In this review article, the recent understanding of the ideal bulk-heterojunction morphology of the photoactive layer for efficient exciton dissociation and charge transport is described, and recent attempts as well as early-stage trials to realize this ideal morphology are discussed systematically from a morphological viewpoint. The various approaches to optimizing morphologies consisting of an interpenetrating bicontinuous network with appropriate domain sizes and mixed regions are categorized, and in each category, the recent trends in the morphology control on the multilength scale are highlighted and discussed in detail. This review article concludes by identifying the remaining challenges for the control of active layer morphologies and by providing perspectives toward real application and commercialization of organic photovoltaics.

Toward Environmentally Robust Organic Electronics: Approaches and Applications

Recent interest in flexible electronics has led to a paradigm shift in consumer electronics, and the emergent development of stretchable and wearable electronics is opening a new spectrum of ubiquitous applications for electronics. Organic electronic materials, such as π-conjugated small molecules and polymers, are highly suitable for use in low-cost wearable electronic devices, and their charge-carrier mobilities have now exceeded that of amorphous silicon. However, their commercialization is minimal, mainly because of weaknesses in terms of operational stability, long-term stability under ambient conditions, and chemical stability related to fabrication processes. Recently, however, many attempts have been made to overcome such instabilities of organic electronic materials. Here, an overview is provided of the strategies developed for environmentally robust organic electronics to overcome the detrimental effects of various critical factors such as oxygen, water, chemicals, heat, and light. Additionally, molecular design approaches to π-conjugated small molecules and polymers that are highly stable under ambient and harsh conditions are explored; such materials will circumvent the need for encapsulation and provide a greater degree of freedom using simple solution-based device-fabrication techniques. Applications that are made possible through these strategies are highlighted.

Enhanced thermal stability of a polymer solar cell blend induced by electron beam irradiation in the transmission electron microscope

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.

Direct-Write Optical Patterning of P3HT Films Beyond the Diffraction Limit

Doping-induced solubility control is a patterning technique for semiconducting polymers, which utilizes the reduction in polymer solubility upon p-type doping to provide direct, optical control of film topography and doping level. In situ direct-write patterning and imaging are demonstrated, revealing sub-diffraction-limited topographic features. Photoinduced force microscopy shows that doping level can be optically modulated with similar resolution.

Soluble fluorene–benzothiadiazole polymer-grafted graphene for photovoltaic devices

Herein, a polymer based on fluorene and benzothiadazole (PFBT) was covalently grafted on brominated graphene (G-PFBT) sheets via Suzuki coupling reaction and characterized by Fourier transform infrared (FTIR), ultraviolet-visible (UV-vis), fluorescence emission (FL), and ¹H-NMR spectroscopy.

Enhanced thermal stability of a polymer solar cell blend induced by electron beam irradiation in the transmission electron microscope

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.

Bulk-Heterojunction Organic Solar Cells: Five Core Technologies for Their Commercialization

The past two decades of vigorous interdisciplinary approaches has seen tremendous breakthroughs in both scientific and technological developments of bulk-heterojunction organic solar cells (OSCs) based on nanocomposites of π-conjugated organic semiconductors. Because of their unique functionalities, the OSC field is expected to enable innovative photovoltaic applications that can be difficult to achieve using traditional inorganic solar cells: OSCs are printable, portable, wearable, disposable, biocompatible, and attachable to curved surfaces. The ultimate objective of this field is to develop cost-effective, stable, and high-performance photovoltaic modules fabricated on large-area flexible plastic substrates via high-volume/throughput roll-to-roll printing processing and thus achieve the practical implementation of OSCs. Recently, intensive research efforts into the development of organic materials, processing techniques, interface engineering, and device architectures have led to a remarkable improvement in power conversion efficiencies, exceeding 11%, which has finally brought OSCs close to commercialization. Current research interests are expanding from academic to industrial viewpoints to improve device stability and compatibility with large-scale printing processes, which must be addressed to realize viable applications. Here, both academic and industrial issues are reviewed by highlighting historically monumental research results and recent state-of-the-art progress in OSCs. Moreover, perspectives on five core technologies that affect the realization of the practical use of OSCs are presented, including device efficiency, device stability, flexible and transparent electrodes, module designs, and printing techniques.

Thermally stable and efficient polymer solar cells based on a novel donor-acceptor copolymer

We report high photovoltaic performance of a novel donor-acceptor (D-A) conjugated polymer poly[2,6[4,8-bis(2-ethyl-hexyl)benzo[1,2-b;4,5-b']dithiophene-co-2,5-thiophene-co-4,7[5,6-bis-octyloxy-benzo[1,2,5]thiadiazole]-co-2,5-thiophene] (PBDTTBTZT) in bulk heterojunctions with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). A power conversion efficiency (PCE) of more than 7% is obtained for optimized charge-extracting electrodes. Upon application of thermal stress via annealing, a superior thermal stability is demonstrated as compared to poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT).

Inverted organic photovoltaic cells

The advance in lifestyle, modern industrialization and future technological revolution are always at high expense of energy consumption. Unfortunately, there exist serious issues such as limited storage, high cost and toxic contamination in conventional fossil fuel energy sources. Instead, solar energy represents a renewable, economic and green alternative in the future energy market. Among the photovoltaic technologies, organic photovoltaics (OPVs) demonstrate a cheap, flexible, clean and easy-processing way to convert solar energy into electricity. However, OPVs with a conventional device structure are still far away from industrialization mainly because of their short lifetime and the energy-intensive deposition of top metal electrode. To address the stability and cost issue simultaneously, an inverted device structure has been introduced into OPVs, bridging laboratory research with practical application. In this review, recent progress in device structures, working mechanisms, functions and advances of each component layer as well their correlations with the efficiency and stability of inverted OPVs are reviewed and illustrated.

Interfacial self-assembly of nanoporous C60 thin films

Self-assembled stabilized nanoporous C₆₀ films offer an enhanced active interfacial area.

Toward Improved Lifetimes of Organic Solar Cells under Thermal Stress: Substrate-Dependent Morphological Stability of PCDTBT:PCBM Films and Devices

Morphological stability is a key requirement for outdoor operation of organic solar cells. We demonstrate that morphological stability and lifetime of polymer/fullerene based solar cells under thermal stress depend strongly on the substrate interface on which the active layer is deposited. In particular, we find that the stability of benchmark PCDTBT/PCBM solar cells under modest thermal stress is substantially increased in inverted solar cells employing a ZnO substrate compared to conventional devices employing a

PEDOT

PSS substrate. This improved stability is observed to correlate with PCBM nucleation at the 50 nm scale, which is shown to be strongly influenced by different substrate interfaces. Employing this approach, we demonstrate remarkable thermal stability for inverted PCDTBT:PC70BM devices on ZnO substrates, with negligible (<2%) loss of power conversion efficiency over 160 h under 85 °C thermal stress and minimal thermally induced "burn-in" effect. We thus conclude that inverted organic solar cells, in addition to showing improved environmental stability against ambient humidity exposure as widely reported previously, can also demonstrate enhanced morphological stability. As such we show that the choice of suitable substrate interfaces may be a key factor in achieving prolonged lifetimes for organic solar cells under thermal stress conditions.

Fullerene-based materials for solar cell applications: design of novel acceptors for efficient polymer solar cells--a DFT study

Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Polymer solar cells (PSCs) hold promise for their potential to be used as low-cost and efficient solar energy converters. PSCs have been commonly made from bicontinuous polymer:fullerene composites or so-called bulk heterojunctions. The conjugated polymer donors and the fullerene derivative acceptors are the key materials for high performance PSCs. In the present study, we have performed density functional theory calculations to investigate the electronic structures and magnetic properties of several representative C60 fullerene derivatives, seeking ways to improve their efficiency as acceptors of photovoltaic devices. In our survey, we have successfully correlated the LUMO energy level as well as chemical hardness, hyper-hardness, nucleus-independent chemical shift, and static dipole polarizability of PC60BM-like fullerene derivative acceptors with the experimental open circuit voltage of the photovoltaic device based on the P3HT:fullerene blend. The obtained structure-property correlations allow finding the best fullerene acceptor match for the P3HT donor. For this purpose, four new fullerene derivatives are proposed and the output parameters for the corresponding P3HT-based devices are predicted. It is found that the proposed fullerene derivatives exhibit better photovoltaic properties than the traditional PC60BM acceptor. The present study opens the way for manipulating fullerene derivatives and developing promising acceptors for solar cell applications.

Highly sensitive and selective liquid-phase sensors based on a solvent-resistant organic-transistor platform

Liquid-phase sensing of various organic solvents is performed for the first time, using a solvent-resistant organic-transistor platform. Sensors composed of a cross-linked poly(3-hexylthiophene) (P3HT)-azide co-polymer and a calixarene derivative exhibit highly sensitive and selective sensing behavior, owing to the selective binding effects of the liquid analytes with the calixarene-functionalized P3HT-azide, extending the range of their use in practical applications.

Low band-gap polymers based on easily synthesized thioester-substituted thieno[3,4-b]thiophene for polymer solar cells

The polymers based on thioester-substituted thieno[3,4-b]thiophene (TTS) exhibit a low band gap (∼1.5 eV) and desirable HOMO and LUMO energy levels relative to the fullerene acceptors.

Nanoscale phase separation control in rationally designed conjugated polymer solar cells processed using co-additives

A conjugated polymer based on benzo[1,2-b:4,5-b′]dithiophene with a thiophene-conjugated side chain andN-alkylthieno[3,4-c]pyrrole-4,6-dione was synthesized.

Thermal stabilisation of polymer-fullerene bulk heterojunction morphology for efficient photovoltaic solar cells

A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported. The concept is demonstrated with various polymers: fullerene derivatives systems enable highly thermally stable polymer solar cells.

Morphological stability and performance of polymer-fullerene solar cells under thermal stress: the impact of photoinduced PC60BM oligomerization

We report a general light processing strategy for organic solar cells (OSC) that exploits the propensity of the fullerene derivative PC60BM to photo-oligomerize, which is capable of both stabilizing the polymer:PC60BM active layer morphology and enhancing the device stability under thermal annealing. The observations hold for blends of PC60BM with an array of benchmark donor polymer systems, including P3HT, DPP-TT-T, PTB7, and PCDTBT. The morphology and kinetics of the thermally induced PC60BM crystallization within the blend films are investigated as a function of substrate and temperature. PC60BM nucleation rates on SiOx substrates exhibit a pronounced peak profile with temperature, whose maximum is polymer and blend-composition dependent. Modest illumination (<10 mW/cm(2)) significantly suppresses nucleation, which is quantified as function of dose, but does not affect crystalline shape or growth, in the micrometer range. On PEDOT:PSS substrates, thermally induced PC60BM aggregation is observed on smaller (≈ 100 nm) length scales, depending upon donor polymer, and also suppressed by light exposure. The concurrent thermal dissociation process of PC60BM oligomers in blend films is also investigated and the activation energy of the fullerene-fullerene bond is estimated to be 0.96 ± 0.04 eV. Following light processing, the thermal stability, and thus lifetime, of PCDTBT:PC60BM devices increases for annealing times up to 150 h. In contrast, PCDTBT:PC70BM OSCs are found to be largely light insensitive. The results are rationalized in terms of the suppression of PC60BM micro- and nanoscopic crystallization processes upon thermal annealing caused by photoinduced PC60BM oligomerization.

25th anniversary article: progress in chemistry and applications of functional indigos for organic electronics

Indigo and its derivatives are dyes and pigments with a long and distinguished history in organic chemistry. Recently, applications of this 'old' structure as a functional organic building block for organic electronics applications have renewed interest in these molecules and their remarkable chemical and physical properties. Natural-origin indigos have been processed in fully bio-compatible field effect transistors, operating with ambipolar mobilities up to 0.5 cm(2) /Vs and air-stability. The synthetic derivative isoindigo has emerged as one of the most successful building-blocks for semiconducting polymers for plastic solar cells with efficiencies > 5%. Another isomer of indigo, epindolidione, has also been shown to be one of the best reported organic transistor materials in terms of mobility (∼2 cm(2) /Vs) and stability. This progress report aims to review very recent applications of indigoids in organic electronics, but especially to logically bridge together the hereto independent research directions on indigo, isoindigo, and other materials inspired by historical dye chemistry: a field which was the root of the development of modern chemistry in the first place.