Effective design of A-D-A small molecules for high performance organic solar cells via F atom substitution and thiophene bridge

Three novel small molecules with acceptor-donor-acceptor (A-D-A) configuration, SBDT1, SBDT2 and SBDT3, where 4,8-bis(octyloxy)benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron-donating core connecting to thiophene-substituted benzothiadiazole (BT) as electron-withdrawing are reported. The effects of fluorine atoms on the photophysical properties by introducing different fluorine atoms into the benzothiadiazole unit were investigated. These SBDTs exhibit good thermal stability, excellent panchromatic absorption in solution and film. SBDT2 and SBDT3 with fluorine-substituted BT possess a relatively deeper the highest occupied molecular orbital (HOMO). These A-D-A type molecules were treated as donor and PC₇₁BM as acceptor in bulk heterojunction (BHJ) small-molecule organic solar cells (SMOSCs). Among them, device based on SBDT2 gave the best device performance with a PCE of 5.06% with J_sc of 10.56 mA/cm², V_oc of 0.85 V, fill factor (FF) of 56.4%. These studies indicate that proper incorporation of fluorine atoms is an effective way to increase the efficiency of organic solar cells.     

High electron mobility fluorinated indacenodithiophene small molecule acceptors for organic solar cells

Indacenodithiophene (IDT) derivatives are kinds of the most representative and widely used cores of small molecule acceptors (SMAs) in organic solar cells (OSCs). Here we systematically investigate the influence of end-group fluorination density and position on the photovoltaic properties of the IDT-based SMAs IDIC-nF (n = 0, 2, 4). The absorption edge of IDIC-nF red-shifts with the π-π stacking and crystallinity improvement, and their electronic energy levels downshift with increasing n. Due to the advantages of J_sc and FF as well as acceptable V_oc, the difluorinated IDIC-2F acceptor based OSCs achieve the highest power conversion efficiency (PCE) of 13%, better than the OSC devices based on IDIC and IDIC-4F as acceptors. And the photovoltaic performance of the PTQ10: IDIC-2F OSCs is insensitive to the active layer thickness: PCE still keep high values of 12.00% and 11.46% for the devices with active layer thickness of 80 and 354 nm, respectively. This work verifies that fine and delicate modulation of the SMAs molecular structure could optimize photovoltaic performance of the corresponding OSCs. Meanwhile, the thickness-insensitivity property of the OSCs has potential for large-scale and printable fabrication technology.     

Fluorination strategy enables greatly improved performance for organic solar cells based on polythiophene derivatives

The power conversion efficiencies(PCEs) of organic solar cells(OSCs) have reached 18% recently,which have already met the demand of practical application.However,these outstanding results were generally achieved with donor-acceptor(D-A) type copolymer donors,which can hardly fulfill the low-cost largescale production due to their complicated synthesis processes.Therefore,developing polymer donors with simple chemical structures is urgent for realizing low-cost OSCs.Polythiophene(PT) derivatives are currently regarded as promising candidates for such kind of donor materials,which has been illustrated in many works.In this work,two new alkylthio substituted PT derivatives,P301 and P302,were synthesized and tested as donors in the OSCs using Y5 as the accepto r.In comparison,the introduction of fluorine atoms on the backbone of P302 can not only downshift the energy levels,but also greatly improve the phase separation morphologies of the active layers,which is ascribed to the enhanced aggregation effect and the reduced miscibility with the non-fullerene acceptor.As a result,the P302:Y5-based OSC exhibits a significantly improved PCE of 9.65% than that of P301:Y5-based one,indicating the important role of fluorination in the construction of efficient PT derivative donors.     

Recent developments of di-amide/imide-containing small molecular non-fullerene acceptors for organic solar cells

Non-fullerene organic solar cells have received increasing attentions in these years, and great progresses have been made since 2013. Among them, aromatic di-amide/imide-containing frameworks have shown promising applications. The outstanding properties of them are highly associated with their unique electronic and structural features, such as strong electron-withdrawing nature, broad absorption in UV-visible region, tunable HOMO/LUMO energy levels, easy modifications, and excellent chemical, thermal and photochemical stabilities. In this review, we give an overview of recent developments of aromatic diamide/imide-containing small molecules used as electron acceptors for organic solar cells.     

Substituent effect of fluorine atom on benzothiadiazole bridging unit in dye sensitized solar cells

Dye sensitized solar cells performances using two organic dyes with fluorinated-benzothiadiazole spacer, 3-{5-[7-(5-{4-[Bis(9,9-dimethyl-9H-fluoren-2-yl)-amino]-5-fluoro-phenyl}thiophen-2-yl)benzo-[1,2,5]thiadiazol-4-yl]thiophen-2-yl}-2-cyano acrylic acid (JK-311) and 3-{5-[7-(5-{4-[Bis(9,9-dimethyl-9H-fluoren-2-yl)-amino]-5,6-difluorophe-nyl}thiophen-2-yl)benzo-[1,2,5]thiadiazol-4-yl]thiophen-2-yl}-2-cyano acrylic acid (JK-312), were systematically investigated by solar simulation equipment, stepped light-induced transient measurements of photocurrent and voltage, and electrochemical impedance spectroscopy. To investigate substituent effect of fluorine atom on benzothiadiazole, molecular orbital calculations of two dyes using a time dependent density functional theory model with B3LYP/3-31G* were also carried out. JK-312 showed a unique electronic transition from HOMO-1 to LUMO. Short circuit current and open-circuit voltage in DSSCs performances were increased by the introduction of fluorine atom into spacer segment, compared to fluorine-free dyes.     

Molecular design of organic dyes based on vinylene hexylthiophene bridge for dye-sensitized solar cells

Three donor-(π-spacer)-acceptor (D-π-A) organic dyes, containing different groups (triphenylamine, di(p-tolyl)phenylamine, and 9-octylcarbazole moieties) as electron donors, were designed and synthesized. Nanocrystalline TiO₂ dye-sensitized solar cells were fabricated by using these dyes. It was found that the variation of electron donors in the D-π-A dyes played an important role in modifying and tuning photophysical properties of organic dyes. Under standard global AM 1.5 solar condition, the DSSC based on the dye D2 showed the best photovoltaic performance: a short-circuit photocurrent density (J _sc ) of 13.93 mA/cm², an open-circuit photovoltage (V _oc ) of 0.71 V, and a fill factor (FF) of 0.679, corresponding to solar-to-electric power conversion efficiency (η) of 6.72%. Supported by the Key Project of Hunan Province of China (Grant No. 2008FJ2004), Natural Science Foundation of Hunan Province of China (Grant Nos. 09JJ3020 & 09JJ4005), and Scientific Research Fund of Hunan Provincial Education Department (Grant No. 08C888).     

Thieno[3,4-c]pyrrole-4,6-dione based copolymers for high performance organic solar cells and organic field effect transistors

Donor-acceptor type copolymers have wide applications in organic field-effect transistors and organic photovoltaic devices. Thieno[3,4-c]pyrrole-4,6-dione (TPD), as an electron-withdrawing unit, has been widely used in D-A type copolymers recently. Till now, the highest power conversion efficiency and mobility of TPD-based copolymers are over 8% and 1.0 cm² V^-1 s^-1 respectively. In this review, the recent progress of TPD-based copolymers in organic solar cells and organic transistors is summarized.     

Application of direct (hetero)arylation in constructing conjugated small molecules and polymers for organic optoelectronic devices

Direct (hetero)arylation, as a sustainable, atom-economic and environmentally benign synthetic protocol compared to conventional coupling techniques, has been extensively applied to the sustainable preparation of π-conjugated materials for organic optoelectronic devices. In this review, we will highlight recent advances made in direct arylation for conjugated small molecules and polymers toward high performance organic optoelectronic devices. Some important insights in direct arylation for synthesizing organic optoelectronic materials are given, together with the challenges and outlook in this significant and hot research field.     

Furan-containing conjugated polymers for organic solar cells

Development of organic semiconductors is one of the most intriguing and productive topics in material science and engineering. Many efforts have been made on the synthesis of aromatic building blocks such as benzene, thiophene and pyrrole due to the facile preparation accompanied by the intrinsic environmental stability and relatively efficient properties of the resulting polymers. In the past, furan has been less explored in this field because of its high oxidation potential. Recently, furan has attracted obsession due to its weaker aromaticity, the greater solubilities of furan-containing π-conjugated polymers relative to other benzenoid systems and the accessibility of furan-based starting materials from renewable resources. This review elaborates the advancements of organic photovoltaic polymers containing furan building blocks. The uniqueness and advantages of furan-containing building blocks in semiconducting materials are also discussed.     

Diketopyrrolopyrrole‐based conjugated polymers and small molecules for organic ambipolar transistors and solar cells

The present highlight discusses major work in the synthesis of low bandgap diketopyrrolopyrrole ( DPP )‐based polymers with donor–acceptor–donor ( D–A–D ) approach and their application in organic electronics. It examines the past and recent significant advances which have led to development of low bandgap DPP ‐based materials with phenyl and thiophene as donors. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4241–4260     

Recent development of perylene diimide-based small molecular non-fullerene acceptors in organic solar cells

This review summarizes the recent progress of perylene diimide (PDI) derivatives used as the acceptor materials in non-fullerene organic solar cells. The resulting structure-property correlations and design strategies of this type of acceptors are discussed and commented, which will help to constructing high-performance PDI-based acceptor materials in the future. The problems at present and the effort direction are also pointed out in this review.     

D-A structural protean small molecule donor materials for solution-processed organic solar cells

This review summarizes the high performance small molecule donors of organic solar cells in various classes of typical donor-acceptor (D-A) structures and discusses their relationships briefly.     

Metal-free organic dyes with benzothiadiazole as an internal acceptor for dye-sensitized solar cells

We synthesized three metal-free organic dyes (H11–H13) consisting of a 3,6-disubstituted carbazole, benzothiadiazole, and cyanoacrylic acid. All the dyes exhibited high molar extinction coefficients and suitable energy levels for electron transfer from the electrolyte to the TiO₂ nanoparticles. Under standard AM 1.5G solar irradiation, the device using dye H13 with co-adsorbed chenodeoxycholic acid (CDCA) displayed the best performance: an open-circuit voltage (V_oc) of 0.71 V, a short-circuit current density (J_sc) of 12.69 mA cm⁻², a fill factor (FF) of 0.71, and a power conversion efficiency (PCE) of 6.32%. The PCE was ∼79% of that for commercially available N719 cells (8.02%) under the same conditions.     

Novel small oligothiophene molecules with phenylene and naphthalene cores as promising absorber materials for organic solar cells

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The evolution of organic materials for efficient dye-sensitized solar cells

In the past three decades, dye-sensitized solar cells (DSSCs) have gained increased recognition as a potential substitute for inexpensive photovoltaic (PV) devices, and their maximum efficiency has grown from 7% to 14.3%. Recent developments in DSSCs have attracted a plethora of research activities geared at realizing their full potential. DSSCs have seen a revival as the finest technology for specific applications with unique features such as low-cost, non-toxic, colourful, transparent, ease of fabrication, flexibility, and efficient indoor light operation. Several organic materials are being explored and employed in DSSCs to enhance their performance, robustness, and lower production costs to be viable alternatives in the solar cell markets. This review provides a concise summary of the developments in the field over the past decade, with a special focus on the incorporation of organic materials into DSSCs. It covers all elements of the DSSC technology, including practical approaches and novel materials. Finally, the emerging applications of DSSCs, and their future promise are also discussed.     

Naphthodithiophene-based donor materials for solution processed organic solar cells

As an emerging donor building block, naphthodithiophene (NDT) is causing more concerns in the field of organic semiconductors. With the rigid and coplanar molecule structure, NDT will exhibit more application space relying on its own advantage for facilitating the charge carrier transport. In this review article, we have summarized the development progress on the NDT-based donor materials for solution processed organic solar cells. Discussions and comments on those representative NDT type materials about structure and property are also presented.     

Theoretical characterization and design of small molecule donor material containing naphthodithiophene central unit for efficient organic solar cells

To seek for high‐performance small molecule donor materials used in heterojunction solar cell, six acceptor–donor–acceptor small molecules based on naphtho[2,3‐b:6,7‐b′]dithiophene ( NDT ) units with different acceptor units were designed and characterized using density functional theory and time‐dependent density functional theory. Their geometries, electronic structures, photophysical, and charge transport properties have been scrutinized comparing with the reported donor material NDT(TDPP)₂ ( TDPP = thiophene‐capped diketopyrrolopyrrole). The open circuit voltage (V_oc), energetic driving force(ΔE_L‐L), and exciton binding energy (E_b) were also provided to give an elementary understanding on their cell performance. The results reveal that the frontier molecular orbitals of 3–7 match well with the acceptor material PC₆₁BM , and compounds 3–5 were found to exhibit the comparable performances to 1 and show promising potential in organic solar cells. In particular, comparing with 1 , system 7 with naphthobisthiadiazole acceptor unit displays broader absorption spectrum, higher V_oc, lower E_b, and similar carrier mobility. An in‐depth insight into the nature of the involved excited states based on transition density matrix and charge density difference indicates that all S₁ states are mainly intramolecular charge transfer states with the charge transfer from central NDT unit to bilateral acceptor units, and also imply that the exciton of 7 can be dissociated easily due to its large extent of the charge transfer. In a word, 7 maybe superior to 1 and may act as a promising donor candidate for organic solar cell. © 2013 Wiley Periodicals, Inc.     

Structural engineering of dipolar organic dyes with an electron-deficient diphenylquinoxaline moiety for efficient dye-sensitized solar cells

A series of new organic dyes containing an electron-deficient diphenylquinoxaline moiety was synthesized and employed as the photosensitizers in dye-sensitized solar cells (DSSCs). The multiple phenyl rings in the peripheral positions of the dye structure provide a hydrophobic barrier to slow down the charge recombination. The photophysical and electrochemical properties of these dyes were investigated in detail. The cell performance and the associated photophysical and electrochemical properties can be easily tuned by the modification of the aromatic fragments within the π spacer. Dye CR204-based DSSC reached the best energy conversion efficiency of 6.49% with an open-circuit voltage of 666 mV, a short-circuit photocurrent density of 14.9 mA cm⁻², and a fill factor of 0.66. The IPCE of CR204-based DSSC covers the light-harvesting to NIR region.     

A-D-A型小分子电子给体光伏材料的端基修饰及其光伏性能

有机太阳能电池(OSCs)活性层中的给体材料主要包括共轭聚合物与有机小分子,由于有机小分子给体具有结构确定、易于提纯、重复性高等独特的优势,近年来受到研究工作者的广泛关注。本工作中,我们采取具有良好共平面性的三联苯并二噻吩(TriBDT-T)为推电子(D)中心共轭单元,分别以罗丹宁(RN)、氰基罗丹宁(RCN)和1,3-茚二酮(IDO)为拉电子(A)共轭端基,设计并合成了三种具有A-D-A型结构的小分子给体材料TriBDT-T-RN、TriBDT-T-RCN和TriBDT-T-IDO。我们对比研究了三种端基对其热分解温度、吸收光谱和分子能级等基本性能的影响,并分别将三种小分子给体与非富勒烯型受体材料IT-4F共混制备器件,详细研究了活性层形貌与光伏性能之间的关系。结果表明,不同的A型端基对小分子给体材料的光学性能、电化学性能、光伏器件中活性层的微观形貌以及能量转换效率(PCE)产生显著影响。基于TriBDTT-RN:IT-4F、TriBDT-T-RCN:IT-4F和TriBDT-T-IDO:IT-4F的光伏器件的能量转换效率分别为9.25%、6.31%和6.18%。     

Incremental optimization in donor polymers for bulk heterojunction organic solar cells exhibiting high performance

The power conversion efficiency of an organic solar cell has now exceeded the 10% mark, which is a significant improvement in the last decade. This has been made possible due to the development of low-band-gap polymers with tunable electron affinity, ionization potential, solubility, and miscibility with the fullerene acceptor, and the improved understanding of the factors affecting the critical device parameters such as the V_OC and the J_SC. This review examines the latest strategies, results, and trends that have evolved in the design of solar cells with better efficiency and durability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012