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.     

Vacuum-processed small molecule solar cells based on terminal acceptor-substituted low-band gap oligothiophenes

Novel A-D-A type oligothiophenes incorporating benzothiadiazole (BTDA) and thiadiazolopyridine (TDAPy) as terminal acceptor groups have been developed for small molecule organic solar cells (SMSC). In vacuum-processed planar heterojunction solar cells the TDAPy-based oligomer showed a power conversion efficiency of 3.15% and a high fill factor of 0.67.     

D-π-A-π-A结构有机光敏染料的合成及其在太阳能光电转化和光解水制氢中的应用

选择N,N'-二甲基苯胺作为电子给体,乙炔基和苯基作为π桥键,苯并噻二唑基团作为辅助受体,氰基丙烯酸为电子受体设计合成了一个具有D-π-A-π-A结构的有机染料OD2。对该染料的光谱性能和电化学性能进行了研究,并将其用作光敏剂引入太阳能光电转化和光解水制氢领域。当OD2应用于光伏领域：在AM1.5（100 mW·cm^-2）的光强下,OD2敏化的电池的光电转化效率（η）为4.40%（短路电流密度（J_sc）=10.58 mA·cm^-2,开路电压（V_oc）=630 mV,填充因子（FF）=0.65）;当OD2应用于染料敏化可见光催化制氢领域：在300 W氙灯光源可见光照射10 h,OD2敏化的Pt/TiO₂在pH=7.0,10%（φ,体积分数）三乙醇胺水溶液中的催化转化数（TON）为140,相应的表观制氢量子产率（Φ_H2）仅为0.42%。显然,OD2在光电转化领域比可见光催化分解水制氢领域更具有应用潜力。     

Structure,Thermal Behavior and Characterization of a New Hybrid Iron Fluoride FeF4(2,2'-bipyridine)(H2O)2

New crystal of FeF₄(2,2'-bipyridine)(H₂O)₂ was prepared by hydrothermal synthesis. Crystalline structure determination is performed from single crystal X-ray diffraction data. The unit cell is monoclinic space group P2₁/n, with cell parameters a=0.9046(5) nm, b=0.7502(5) nm, c=1.9539(5) nm, β=93.307(5)°, V=1.3238(12) nm³ and Z=4. The structure of FeF₄(2,2'-bipyridine)(H₂O)₂ is built up from FeF₄N₂ octahedra coordinated by two nitrogen atoms of the 2,2'-bipyridine molecules, and four fluorine atoms as well as uncoordinated H₂O molecules. Thermal analysis of the title compound showed that the decomposition introduced four steps. IR spectra confirmed the presence of 2,2'-bipyridine molecules. The optical absorption was measured at the corresponding l_max using UV-Vis diffuse reflectance spectrum.     

Fluorination-substitution effect on all-small-molecule organic solar cells

Due to the strong crystallinity and anisotropy of small molecules, matched molecular photoelectric properties and morphologies between small molecules and non-fullerene acceptors are especially important in all-small-molecule organic solar cells(OSCs).Introducing fluorine atoms has been proved as an effective strategy to achieve a high device performance through tuning molecular energy levels, absorption and assembly properties. Herein, we designed a novel benzodithiophene-based small molecule donor BDTF-CA with deep highest occupied molecular orbital(HOMO) energy level. All-small-molecule OSCs were fabricated by combing non-fullerene acceptor IDIC with different fluorine-atom numbers. Two or four fluorine atoms were introduced to the end-capped acceptor of IDIC, which are named as IDIC-2 F and IDIC-4 F, respectively. With the increase of fluorination from IDIC to IDIC-4 F, the open circuit voltage(V_(oc)) of the devices decreased, while hole and electron mobilities of the active layers increased by one order of magnitude. Contributed to the most balanced V_(oc), short-circuit current(J_(sc)) and fill factor(FF), the device based on BDTF-CA/IDIC-2 F achieved the highest power conversion efficiency of 9.11%.     

The effect of end-capping groups in A-D-A type non-fullerene acceptors on device performance of organic solar cells

A series of novel wide bandgap small molecules(IFT-ECA, IFT-M, IFT-TH and IFT-IC) based on the A-D-A structure with indenofluorene core, thiophene bridge, and different electron-deficient end-capping groups, were synthesized and used as non-fullerene acceptors in organic solar cells. The influences of end-capping groups on the device performance were studied.The four materials exhibited different physical and chemical properties due to the variation of end-capping groups, which further affect the exciton dissociation, charge transport, morphology of the bulk-heterojunction films and device performance. Among them, IFT-IC-based device delivered the best power conversion efficiency of 7.16% due to proper nano-scale phase separation morphology and high electron mobility, while the devices based on the other acceptors achieved lower device performance(4.14% for IFT-TH, 1% for IFT-ECA and IFT-M). Our results indicate the importance of choosing suitable electron-withdrawing groups to construct high-performance non-fullerene acceptors based on A-D-A motif.     

Electronic Structure Effect of Polythiophene Derivatives and Nano N-Zn-Ag/TiO2 on Performance of Organic Thin Film Solar Cell

Nanocrystal N-Zn-Ag/TiO₂ powders were prepared with N-Zn/TiO₂ by photo deposition method. A series of pure polymers P3HT[poly(3-hexylthiophene)], P3OT[poly(3-octylthiophene)], P3DT[poly(3-decylthiophene)] and P3DDT[poly(3-dodecylthiophene)], was synthesized, which were used to synthesize p-n type semiconductor materials P3HT/N-Zn-Ag-TiO₂, P3OT/N-Zn-Ag-TiO₂, P3DT/N-Zn-Ag-TiO₂ and P3DDT/N-Zn-Ag-TiO₂ by in situ che-mical method. X-Ray diffraction(XRD) and infrared(IR) spectroscopy showed the structure of the polymers and complexes. Ultraviolet-visible(UV-Vis) spectra and cyclic voltammograms(CV) showed the optical and electronic performance of the polymers and complexes. Two new single and double organic thin film heterojunction solar cells were prepared with the above mentioned synthesized powders as raw materials. Current-voltage(I-V) measurements indicate that the conversion efficiency of the single organic thin film heterojunction solar cell is higher than that of the double organic thin film heterojunction solar cells. Single organic thin film heterojunction solar cells based on P3DT/N-Zn-Ag-TiO₂ can get a photoelectric conversion efficiency of 0.0408%. The performance of electronic transform between electron donor and acceptor on organic thin film solar cells was researched.     

Side chain engineering investigation of non-fullerene acceptors for photovoltaic device with efficiency over 15%

Science China Chemistry - Side chain engineering plays a substantial role for high-performance organic solar cells (OSCs). Herein, a series of non-fullerene acceptor (NFA) molecules with A-D-A...     

Additive-free non-fullerene organic solar cells with random copolymers as donors over 9% power conversion efficiency

The PBDB-TBT1:ITIC-based device obtains PCE of 9.09%, and is insensitive to additive and thermal annealing, and forms microstructural morphology.     

Enhanced Charge Transfer,Transport and Photovoltaic Efficiency in All‐Polymer Organic Solar Cells by Polymer Backbone Fluorination

We successfully designed and synthesized two BDT‐BT‐T (BDT=benzo[1,2‐b:4,5‐b']dithiophene, BT‐T=4,7‐dithien‐2‐yl‐2,1,3‐benzothiadiazole) based polymers as the electron donor for application in all‐polymer solar cells (all‐PSCs). By adopting N2200 as the electron acceptor, we systematically investigated the impact of fluorination on the charge transfer, transport, blend morphology and photovoltaic properties of the relevant all‐PSCs. A best power conversion efficiency (PCE) of 3.4% was obtained for fluorinated PT‐BT2F/N2200 (BT2F=difluorobenzo[c][1,2,5]thiadiazole) all‐PSCs in comparison with that of 2.7% in non‐fluorinated PT‐BT/N2200 (BT=benzothiadiazole) based device. Herein, all‐polymers blends adopting either non‐fluorinated PT‐BT or fluorinated PT‐BT2F exhibit similar morphology features. In depth optical spectrum measurements demonstrate that molecular fluorination can further enhance charge transfer between donor and acceptor polymer. Moreover, all‐polymer blends exhibit improved hole mobilities and more balanced carriers transport when adopting fluorinated donor polymer PT‐BT2F. Therefore, although the PCE is relatively low, our findings may become important in understanding how subtle changes in molecular structure impact relevant optoelectronic properties and further improve the performance of all‐PSCSs.     

Progress of the key materials for organic solar cells

Organic solar cells have attracted academic and industrial interests due to the advantages like lightweight, flexibility and roll-to-roll fabrication. Nowadays, 18% power conversion efficiency has been achieved in the state-of-the-art organic solar cells. The recent rapid progress in organic solar cells relies on the continuously emerging new materials and device fabrication technologies, and the deep understanding on film morphology, molecular packing and device physics. Donor and acceptor materials are the key materials for organic solar cells since they determine the device performance. The past 25 years have witnessed an odyssey in developing high-performance donors and acceptors. In this review, we focus on those star materials and milestone work, and introduce the molecular structure evolution of key materials. These key materials include homopolymer donors, D-A copolymer donors, A-D-A small molecular donors, fullerene acceptors and nonfullerene acceptors. At last, we outlook the challenges and very important directions in key materials development.     

Modulating Benzothiadiazole‐Based Covalent Organic Frameworks via Halogenation for Enhanced Photocatalytic Water Splitting

Two‐dimensional covalent organic frameworks (2D COFs), an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar‐to‐hydrogen energy conversion owing to their pre‐designable structures and tailor‐made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py‐HTP‐BT‐COF) via chlorination (Py‐ClTP‐BT‐COF) and fluorination (Py‐FTP‐BT‐COF) can lead to dramatically enhanced photocatalytic H₂ evolution rates (HER=177.50 μmol h^?1 with a high apparent quantum efficiency (AQE) of 8.45 % for Py‐ClTP‐BT‐COF). Halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface. Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar‐to‐chemical energy conversion.     

Novel Small Four-armed Molecules with Triphenylamine-bridged Structure for Organic Solar Cells Featuring High Open-circuit Voltage

In view of few attention on star-shaped molecules containing triphenylamine(TPA) unit as π-linker, a series of small four-armed molecules, consisting of octyloxy-substituted 2,1,3-benzothiadiazole(DOBT) or 4-octyl-2-thienyl functionalized DOBT as the core, TPA as π-bridge and 4-methylphenyl or 4-methoxyphenyl groups as terminal units, was designed and synthesized. The effects of π-bridges and substitute groups on molecular photoelectric performance and photovoltaic performance were fully explored. With the help of the additional thiophene-linkers incorporation, 3-octylthienyl substituted molecule with end-capping 4-methylphenyl(T-BTTPAM) and 3-octylthienyl substituted molecule with end-capping 4-methoxyphenyl(T-BTTPAOM) showed stronger and broader absorption, as well as higher charge mobilities compared to the molecules without thiophene-linkers(BTTPAM and BTTPAOM). Additionally, changing substitute groups from methyl to methoxy helped BTTPAOM and T-BTTPAOM achieve better absorption properties than BTTPAM and T-BTTPAM, respectively. When paired with PC₆₁BM as the electron acceptor to fabricate solution-processed photovoltaic devices, the four materials gave high open-circuit voltage(V_oc) values over 0.90 V. These results demonstrate that our materials are promising candidates as donor materials for organic solar cells(OSCs), and further device optimization is in progress in our laboratory.     

基于非富勒烯受体的溶液加工型全小分子太阳能电池研究进展

有机太阳能电池(OPV),具有质量轻、可成本低制备等优势,是一种具有实际应用潜力的光伏技术。有机太阳能电池活性层可以由共轭聚合物或溶液可加工的小分子材料(给体与受体)共混组成。由于小分子材料具有明确的分子结构,纯度可控及无批次差别影响的特点;并结合近年来非富勒烯小分子受体的快速发展,使得非富勒烯全小分子(NF-SM-OPV)电池研究受到广泛关注。由于大部分A-D-A型非富勒烯受体分子具有各向异性的特点,这使激子解离和电荷传输,很大程度上受分子间堆积方式的影响,导致非富勒烯全小分子电池活性层形貌调控更加复杂。虽然非富勒烯小分子太阳能电池具有非富勒烯受体材料和小分子材料的双重优势,但高效率非富勒烯小分子太阳能电池的制备,仍具有很大挑战。因此,本文总结近年来高性能非富勒烯小分子太阳能电池的相关进展。着重介绍针对非富勒烯受体的给体小分子材料设计工作,并在此基础上近一步讨论非富勒烯小分子太阳能电池面临的挑战与展望。     

Influence of pyrimidine additives in electrolytic solution on dye-sensitized solar cell performance

The influence of pyrimidine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell with an I⁻/I₃⁻ redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 10 different pyrimidine derivatives under AM 1.5 (100 mW/cm²). The pyrimidine additives tested had varying effects on the performance of the cell. The additives drastically enhanced the open-circuit photovoltage (V_oc) and the solar energy conversion efficiency (η), but usually reduced the short circuit photocurrent density (J_sc) of the solar cell. Physical and chemical properties of the pyrimidines were computationally calculated in order to determine the reasons for the additive effects on cell performance. Consequently, the greater the calculated partial charge of the nitrogen atoms in the pyrimidine groups, the larger the V_oc but the smaller the J_sc values. The V_oc of the cell also increased as the ionization energy of the pyrimidine molecules decreased. Moreover, as the calculated dipole moment of the pyrimidine derivatives increased, the J_sc value was reduced, but the V_oc value was enhanced. These results suggest that the electron donicity of pyrimidine additives influenced the interaction with TiO₂ electrode and I⁻/I₃⁻ electrolyte, which lead to the changes in dye-sensitized solar cell performance.     

Molecular engineering on all ortho-linked carbazole/oxadiazole hybrids toward highly-efficient thermally activated delayed fluorescence materials in OLEDs

All ortho-linked D-A and D-A-D molecules exhibit non-TADF feature due to broad spatial overlap at triplet excited state for large △E_ST,while A-D-A compounds show strong TADF property owing to efficient spatial separation for small △E_ST.     

Novel alkoxyanthracene donor and benzothiadiazole acceptor for organic thin film transistor and bulk heterojunction organic photovoltaic cells

Novel alkoxy anthracene (ODA)‐based polymeric semiconductors were designed for polymer solar cell applications. Alkoxyanthracene, which contains many π electrons and electron donating group, was easily synthesized. The copolymers, poly(alkoxy anthracene‐alt‐thiophene benzothiadiazole thiophene) poly(ODA‐TBT) and poly(alkoxy anthracene‐alt‐benzothiadiazole) poly(ODA‐BT), have been obtained by Suzuki coupling polymerization. Both polymers have ODA unit as a donor and benzothiadiazole as an acceptor. ODA‐TBT has thiophene linkages between ODA and benzothiadiazole. The optical, thermal, and electrochemical properties have been investigated by UV–visible absorption, thermal gravimetric analysis, differential scanning calorimetry, and CV. Organic thin‐film transistor using polymers showed that the hole mobility of poly(ODA‐alt‐TBT) was around 3.6 × 10^?3 cm²/Vs with on/off ratio of 9.91 × 10⁵ while that of poly(ODA‐alt‐BT) was around 1.21 × 10^?2 cm²/Vs with on/off ratio of 2.64 × 10⁶. Organic photovoltaic performance based on polymers were evaluated with a configuration of ITO/PEDOT:PSS/active layer/LiF/Al. Poly(ODA‐TBT) exhibits a short circuit current (J_sc) of 3.9 mA/cm² and power conversion efficiency (PCE) of 1.4%, and poly(ODA‐BT) exhibits the J_sc of 6.4 mA/cm² and PCE of 2.2%. The better device performance of poly(ODA‐BT) is attributed to its charge transfer ability and enhanced mobility and crystallinity although poly(ODA‐BT) does not have extended π‐conjugation due to twisted structure compared with poly(ODA‐TBT). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1306–1314     

Synthesis and Photovoltaic Properties of Non-fullerene Solution Processable Small Molecule Acceptors

Two non-fullerene small molecules, BT-C6 and BT-C12, based on the vinylene-linked benzothiadiazole- thiophene(BT) moiety flanked with 2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)malononitrile have been synthesized and characterized by solution/thin film UV-Vis absorption, photoluminescence(PL), and cyclic voltammetry(CV) measurements. The two molecules show intense absorption bands in a wide range from 300 nm to 700 nm and low optical bandgaps for BT-C6(1.60 eV) and for BT-C12(1.67 eV). The lowest unoccupied molecular orbital(LUMO) levels of both the molecules are relatively higher than that of [6,6]-phenyl C61 butyric acid methyl ester(PCBM), promising high open circuit voltage(V_oc) for photovoltaic application. Bulk heterojunction(BHJ) solar cells with poly(3-hexylthiophene) (P3HT) as the electron donor and the two molecules as the acceptors were fabricated. Under 100 mW/cm² AM 1.5 G illumination, the devices based on P3HT:BT-C6(1:1, mass ratio) show a power conversion efficiency(PCE) of 0.67%, a short-circuit current(J_sc) of 1.63 mA/cm², an open circuit voltage(V_oc) of 0.74 V, and a fill factor(FF) of 0.56.     

电极界面缓冲层对P3HT:PC61BM太阳能电池热稳定性的影响

基于溶液法加工制备的聚合物太阳能电池的高温热稳定性是决定器件能否兼容后续高温热封装工艺, 如热压封装、高温原子层沉积(ALD)等的一个关键. 本文分别利用聚(3, 4-乙烯二氧噻吩)-聚苯乙烯磺酸(PEDOT:PSS)和MoO₃作为阳极缓冲层, 以及ZnO和LiF 作为阴极缓冲层, 制备了结构为氧化铟锡(ITO)/阳极缓冲层/3-己基取代聚噻吩:(6, 6)-苯基C₆₁-丁酸甲酯(P3HT:PC₆₁BM)/阴极缓冲层/Al 的太阳能电池, 系统地比较研究了不同界面缓冲材料对器件光电转换性能及稳定性的影响, 特别是在高温煺火条件下器件的性能稳定性差异. 结果表明, 聚合物太阳能电池的热稳定性同器件的结构以及所用的缓冲层材料有密切的相关性. 其中, 利用MoO₃及ZnO分别作为阳极与阴极界面修饰层的P3HT:PC₆₁BM器件在120-150 ℃的温度范围内能够较好地保持器件的光电转换性能. 这一结果为后续需要高温封装工艺的器件提供了有意义的结构优化指导. 此外, 研究结果还表明利用ZnO作为阴极缓冲层能够改善器件的长时间稳定性.     

Properties of chemical vapour deposited nanocrystalline TiO2 thin films and their use in dye-sensitized solar cells

Nanocrystalline titanium dioxide (TiO₂) thin films have been prepared using titanium(IV) isopropoxide as a precursor onto the glass and fluorine doped tin oxide coated glass substrates by chemical vapour deposition technique at 400 °C substrate temperature. X-ray diffraction study confirms the polycrystalline nature of TiO₂ with anatase phase having tetragonal crystal structure. The films are 975 nm thick and transparent having transmittance grater than 80%. Atomic force microscopy (AFM) images reveal the nanocrystalline morphology with grain size of 200 nm. The film shows a sharp absorption edge near 350 nm. Photoelectrochemical study shows that TiO₂ thin film sensitized with Brown Orange dye is found to exhibit relatively maximum I_sc and V_oc among the studied dyes. The values of fill factor (FF) and efficiency (η) for the dye-sensitized solar cell (Brown Orange dye-sensitized TiO₂) are 0.54 and 0.17%, respectively. Such films would serve as better prospects for dye-sensitized solar cells.