Synthesis and photovoltaic properties of two-dimensional conjugated polythiophene derivatives presenting conjugated triphenylamine/thiophene moieties

In this study we synthesized three two-dimensional (2-D) polythiophene derivatives (PTs), namely PTBPTPA, PTStTPA, and PTCNStTPA, featuring three different conjugated units—biphenyl (BP), stilbene (St), and cyanostilbene (CNSt), respectively—in the polymer backbones and presenting conjugated triphenylamine/thiophene (TPATh) moieties on the side chains. In addition, we also synthesized three conjugated BP-, St-, and CNSt-based main-chain-type conjugated polymers (PTBP, PTSt, and PTCNSt, respectively). Incorporating the St and CNSt moieties into the polymer backbones and appending TPATh units induced high degrees of intramolecular charge transfer within the conjugated frameworks of the polymers, thereby resulting in lower band gap energies and red-shifting of the maximal UV–Vis absorption wavelengths. Moreover, the energy levels of the highest occupied molecular orbitals of the BP-, St-, and CNSt-based main-chain-type and 2-D PTs were lower than that of P3HT, implying that they would be applicable for the preparation of polymer solar cells (PSC) with greater open-circuit voltages. The photovoltaic performances of PSCs fabricated from blends of the 2-D PTs and the fullerene derivative PC₆₁BM were superior to those of PSCs based on the main-chain-type polymer/PC₆₁BM blends.     

Enhanced Thermal Stability of Inverted Polymer Solar Cells with Pentacene

In this study, we focused on the thermal stability of organic solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM), fabricated by blends of P3HT : PCBM : pentacene. Enhanced thermal stability of organic solar cells was achieved by introducing pentacene (Pc) into blends of P3HT : PCBM in organic solar cells with the structure indium tin oxide/ZnO/P3HT : PCBM : Pc/poly(3,4-ethylenedioxythiophene) : polystyrene sulfonate/Ag (ITO/ZnO/P3HT : PCBM : Pc/PEDOT : PSS/Ag). The donor-acceptor interfaces of devices with Pc were more stable than those without Pc in the active layer. During the thermal annealing process, the Pc in the P3HT : PCBM blends suppressed the crystallization of P3HT and PCBM, which was confirmed by optical microscopic images and UV-visible absorption spectra. The power conversion efficiency (PCE) of the device with Pc was reduced to no less than 70 % of its original efficiency after keeping it at 120 °C for 24 hours, while that of the non-Pc device was reduced to 13 % of its original efficiency after 24 hours at the same temperature. Based on these results, we propose a new Pc-blended organic solar cell that has advantages in the thermal annealing process.     

A new method to disperse CdS quantum dot-sensitized TiO2 nanotube arrays into P3HT:PCBM layer for the improvement of efficiency of inverted polymer solar cells

We report that the efficiency of ITO/nc-TiO₂/P3HT:PCBM/MoO₃/Ag inverted polymer solar cells (PSCs) can be improved by dispersing CdS quantum dot (QD)-sensitized TiO₂ nanotube arrays (TNTs) in poly (3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) layer. The CdS QDs are deposited on the TNTs by a chemical bath deposition method. The experimental results show that the CdS QD-sensitized TNTs (CdS/TNTs) do not only increase the light absorption of the P3HT:PCBM layer but also reduce the charge recombination in the P3HT:PCBM layer. The dependence of device performances on cycles of CdS deposition on the TNTs was investigated. A high power conversion efficiency (PCE) of 3.52% was achieved for the inverted PSCs with 20 cyclic depositions of CdS on TNTs, which showed a 34% increase compared to the ITO/nc-TiO₂/P3HT:PCBM/MoO₃/Ag device without the CdS/TNTs. The improved efficiency is attributed to the improved light absorbance and the reduced charge recombination in the active layer.     

Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption

Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs. In this Account, I discuss the basic requirements and scientific issues in the molecular design of high efficiency photovoltaic molecules. I also summarize recent progress in electronic energy level engineering and absorption spectral broadening of the donor and acceptor photovoltaic materials by my research group and others. For high-efficiency conjugated polymer donors, key requirements are a narrower energy bandgap (E(g)) and broad absorption, relatively lower-lying HOMO (the highest occupied molecular orbital) level, and higher hole mobility. There are three strategies to meet these requirements: D-A copolymerization for narrower E(g) and lower-lying HOMO, substitution with electron-withdrawing groups for lower-lying HOMO, and two-dimensional conjugation for broad absorption and higher hole mobility. Moreover, better main chain planarity and less side chain steric hindrance could strengthen π-π stacking and increase hole mobility. Furthermore, the molecular weight of the polymers also influences their photovoltaic performance. To produce high efficiency photovoltaic polymers, researchers should attempt to increase molecular weight while maintaining solubility. High-efficiency D-A copolymers have been obtained by using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) donor unit and benzothiadiazole (BT), thienopyrrole-dione (TPD), or thiazolothiazole (TTz) acceptor units. The BDT unit with two thienyl conjugated side chains is a highly promising unit in constructing high-efficiency copolymer donor materials. The electron-withdrawing groups of ester, ketone, fluorine, or sulfonyl can effectively tune the HOMO energy levels downward. To improve the performance of fullerene derivative acceptors, researchers will need to strengthen absorption in the visible spectrum, upshift the LUMO (the lowest unoccupied molecular orbital) energy level, and increase the electron mobility. [6,6]-Phenyl-C(71)-butyric acid methyl ester (PC(70)BM) is superior to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) because C(70) absorbs visible light more efficiently. Indene-C(60) bisadduct (ICBA) and Indene-C(70) bisadduct (IC(70)BA) show 0.17 and 0.19 eV higher LUMO energy levels, respectively, than PCBM, due to the electron-rich character of indene and the effect of bisadduct. ICBA and IC(70)BA are excellent acceptors for the P3HT-based PSCs.     

Bulky side-chain density effect on the photophysical, electrochemical and photovoltaic properties of polythiophene derivatives

Low band-gap polythiophene (PT) derivatives, with bulky conjugated side-chains composed of the triphenylamine, thiophene, and vinylene groups (TPATh), are synthesized. The copolymers, synthesized by Grignard metathesis and Stille coupling with different copolymer configurations and side-chain densities, are regioregular-TPATh-PT (rr-TPATh-PT) and random-TPATh-PT (r-TPATh-PT), respectively. The incorporation of bulky conjugated moiety curtails the effective conjugation length in the main chain; thus, low HOMO levels are obtained for the copolymers. Moreover, r-TPATh-PT with less bulky side-chain content exhibits a better conjugation along the polymer backbone than rr-TPATh-PT. Higher absorption intensity in the vision region is observed for r-TPATh-PT in comparison with rr-TPATh-PT. In addition, polymer solar cells (PSCs) are fabricated based on an interpenetrating network of PT derivatives as the electron donor and the fullerene derivatives (PC₆₁BM and PC₇₁BM) as the electron acceptors. Better compatibility is observed for the r-TPATh-PT/PC₆₁BM-blend film as compared to the rr-TPATh-PT/PC₆₁BM-blend film. Higher photovoltaic (PV) performances of the r-TPATh-PT/PC₆₁BM-based PSCs are observed in comparison with the rr-TPATh-PT/PC₆₁BM-based PSCs. The power conversion efficiency (PCE) of the PSC based on the blend of r-TPATh-PT and PC₆₁BM (w/w = 1:1) reaches 0.94% under an illumination of AM 1.5G, 100 mW cm⁻², which is almost twice that of the cell based on rr-TPATh-PT. Further improvement of PV performance is achieved for the PSC fabricated from the blend of r-TPATh-PT and fullerene derivative PC₇₁BM (w/w = 1:3), with a short-circuit current of 6.83 mA cm⁻², an open-circuit voltage of 0.71 V and a PCE of 1.75%.     

Solvent vapor‐induced self assembly and its influence on optoelectronic conversion of poly(3‐hexylthiophene): Methanofullerene bulk heterojunction photovoltaic cells

Nanoscale‐phase separation of electron donor/acceptor blends is crucial for efficient charge generation and collection in polymer bulk heterojunction photovoltaic cells. We investigated solvent vapor annealing effect of poly(3‐hexylthiophene) (P3HT)/methanofullerene (PCBM) blend on its morphology and optoelectronic properties. The organic solvents of choice for the treatment have a major effect on the morphology of P3HT/PCBM blend and the device performance. Ultraviolet‐visible absorption spectroscopy shows that specific solvent vapor annealing can induce P3HT self‐assembling to form well‐ordered structure; and hence, the absorption in the red region and the hole transport are enhanced. The solvent that has a poor solubility to PCBM would cause large PCBM clusters and result in a rough blend film. By combining an appropriate solvent vapor treatment and post‐thermal annealing of the devices, the power conversion efficiency is enhanced. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Enhanced open-circuit voltage of two side chain conjugated polythiophene derivatives blended with indene-C60 bisadduct

Two side chain conjugated polythiophene copolymer: poly[3-(5′-octylthienylenevinyl) thiophene]-thiophene (P1) and poly{3-[4-(2-ethyl-hexyloxy)-phenylvinyl]-thiophene}-thiophene (P2) were synthesized. Polymer photovoltaic solar cells using P1 and P2 as donor and a high LUMO energy level fullerene derivative indene-C₆₀ bisadduct (IC₆₀BA) as acceptor were demonstrated. The polymer solar cells (PSCs) based P1/IC₆₀BA and P2/IC₆₀BA showed enhanced open circuit voltage (V_oc) of 0.89 and 0.88 V respectively, in comparison with that of P1/PC₆₁BM and P2/PC₆₁BM as 0.58 and 0.56 V, respectively. The improved V_oc of IC₆₀BA as acceptor in PSCs is benefited from the higher LUMO energy levels of the C₆₀ derivatives. The power conversion efficiency of the PSCs based on P1/IC₆₀BA and P2/IC₆₀BA were 0.32% and 0.35%, respectively.     

A comparative study on the influence of alkyl thiols on the structural transformations in P3HT/PCBM and P3OT/PCBM blends

Herein, we present a comparative study on the structural transformations in P3HT/PCBM and P3OT/PCBM blends using octane (C₈) and dodecane (C₁₂) thiols as solvent additives. Addition of dodecane thiol to the blends promotes needle-like PCBM crystal formation. These crystals are found abundant in P3OT/PCBM and less in P3HT/PCBM blend and are identified by SAXS, DSC and SEM measurements. Using polymer of two different tail lengths, the SAXS results provide the first experimental support on the recent findings on fullerene intercalation around the crystalline boundaries in a semi-crystalline polymer. The UV–Visible measurements reveal that the absorption maximum show an appreciable red-shift and photoluminescence spectra infers that the donor/acceptor interface is mostly retained, upon dodecane thiol addition. These needle-like PCBM crystals obtained by annealing free approach in the blends and their dependence on the alkyl tail length of the polymer, would pave way for more effective design of organic photovoltaic devices.     

A polythiophene derivative with dioctyloxyltriphenylamine‐vinylene conjugated side chain: Synthesis,hole mobility,and photovoltaic property

A new polythiophene derivative with dioctyloxyl triphenylamine‐vinylene ( DOTPAV ) conjugated side‐chain, DOTPAV‐PT , was synthesized by the Stille coupling method and characterized by ¹H‐NMR, ¹³C‐NMR, elemental analysis, gel permeation chromatography, thermogravimetric analysis, UV–vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. The polymer DOTPAV‐PT is soluble in common organic solvents and possesses good thermal stability with 5% weight loss temperature of 310°C. The weight‐average molecular weight of DOTPAV‐PT is 8.0 K with a polydispersity index of 1.24. The hole mobility of the polymer determined from space‐charge‐limited current model was 1.25 × 10^?4 cm² V^?1 s^?1. The bulk heterojunction polymer solar cell with the configuration of ITO/PEDOT : PSS/polymer : PCBM (1 : 1)/Ca/Al was fabricated, and the power conversion efficiency of the device was 0.16% under the illumination of AM1.5, 100 mW cm^?2. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mullite rod-enhanced porous SiC ceramics prepared at low temperature from photovoltaic waste

In this paper, porous SiC ceramics (PSCs) were fabricated from photovoltaic waste at low temperatures. The effects of different additives and sintering temperatures on PSCs were studied in detail. The temperature of PSCs preparation can be reduced to 850?°C by adding MoO₃ as catalyst. The PSCs are reinforced by mullite rods grown in-situ, they also have a high permeability coefficient due to their network structure. From 850?°C to 1200?°C, the open porosity of PSCs changed slightly, and was within 45.32?±?0.6%. The PSCs produced at 1000?°C had the highest gas permeability coefficient of 8.24?×?10^–11?m² and the highest flexural strength of 50.17?MPa. However, the same PSCs could not be fabricated at 850?°C when Y₂O₃ or CeO₂ were used as sintering aids. This study provides an environment-friendly method for reusing photovoltaic waste and reducing the cost of preparing PSCs.     

Using water‐soluble nickel acetate as hole collection layer for stable polymer solar cells

We report polymer solar cells (PSCs) based on poly(3‐hexylthiophene (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) using water‐soluble nickel acetate (Ni(CH₃COO)₂, NiAc) instead of acidic poly(3,4‐ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS) as hole collection layer (HCL) between the indium tin oxide (ITO) electrode and photoactive layer. The NiAc layer can effectively decrease R_s and increase R_p and shows effective hole collection property. Under the illumination of AM1.5G, 100 mW/cm², the short‐circuit current density (J_sc) of the NiAc based device (ITO/NiAc/P3HT : PCBM/Ca/Al) reach 11.36 mA/cm², which is increased by 11% in comparison with that (10.19 mA/cm²) of PEDOT : PSS based device (ITO/PEDOT : PSS/P3HT : PCBM/Ca/Al). The power conversion efficiency of the NiAc based devices reach 3.76%, which is comparable to that (3.77%) of the device with PEDOT : PSS HCL under the same experimental conditions. Moreover, NiAc based PSCs show superior long‐term stability than PEDOT : PSS based PSCs. Our work gives a new option for HCL selection in designing more stable PSCs. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Poly[(3-hexylthiophene)-block-(3-semifluoroalkylthiophene)] for Polymer Solar Cells

We report the synthesis of poly[(3-hexylthiophene)-block-(3-(4,4,5,5,6,6,7,7,7-nonafluoroheptyl)thiophene)], P(3HT-b-3SFT), carried out by the Grignard Metathesis Method (GRIM). The copolymers composition was determined by (1)H and (19)F NMR spectroscopies, and gel permeation chromatography (GPC). The thin films of P(3HT-b-3SFT) were investigated by ultraviolet-visible absorption spectroscopy and atomic force microscopy (AFM). We also fabricated bulk-hetero junction (BHJ) solar cells based on blends of P(3HT-b-3SFT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). Although the composition ratio of P3SFT in P(3HT-b-3SFT) was low, the influence of P3SFT on the morphology and properties of solar cells was significant. The annealing process for the BHJ solar cells induced the formation of large domains and led to poor solar cell performance. The BHJ solar cells, based on PCBM and P(3HT-b-3SFT), prepared by the non-annealing process, had a maximum power conversion efficiency of 0.84% under 100 mW/cm(2) (AM 1.5 solar illumination) in air.     

Synthesis of donor-acceptor poly(perylene diimide-altoligothiophene) copolymers as n-type materials for polymeric solar cells

Donor-acceptor alternated copolymers based on perylene diimide units linked in bay positions with oligothiophene units were prepared and used as acceptor material in polymeric solar cells. The copolymers have exhibited ambipolar electrochemical properties, high electronic affinities and wide electronic absorption in the visible spectrum. The spectroscopical characterization of the copolymer blends with poly-3-hexylthiophene showed evidences indicative of a charge transfer from the perylene-based copolymers to the polythiophene. All-polymeric solar cells were prepared with the blends and the photovoltaic characterization of these devices showed conversion efficiencies of 0.8% and 0.4%. The results demonstrate the potentialities of perylene-based copolymers as processable photoactive acceptor in organic solar cells.     

Semiconducting polymer-incorporated nanocrystalline TiO2 particles for photovoltaic applications

In this work, we study hybrid solar cells based on blends of the semiconducting polymer poly(3-octylthiophene-2,5-diyl)(P3OT) and [6,6]-phenyl C₆₁ butyric acid methyl (PCBM) coated titanium dioxide (TiO₂) nanocrystal film. The Fourier transform infrared spectra (FTIR), UV–vis absorption spectra and PL quenching researches show that the films had a stronger absorption in visible light range. The influence of the PCBM:P3OT ratio were researched and the optimized ratio of PCBM to P3OT (1:1.5) exhibit a short circuit current of 4.42 mA cm⁻², an open circuit voltage of 0.81 V, a fill factor of 0.73 and a light-to-electric conversion efficiency of 2.61% under a simulated solar light irradiation of 100 mW cm⁻².     

Perovskite Solar Cells Fabricated by Using an Environmental Friendly Aprotic Polar Additive of 1,3-Dimethyl-2-imidazolidinone

Perovskite solar cells (PSCs) have great potentials in photovoltaics due to their high power conversion efficiency and low processing cost. PSCs are usually fabricated from PbI₂/dimethylformamide solution with some toxic additives, such as N-methyl pyrrolidone and hexamethylphosphoramide. Here, we use an environmental friendly aprotic polar solvent, 1,3-dimethyl-2-imidazolidinone (DMI), to fabricate perovskite films. By adding 10 vol% DMI in the precursor solution, high-quality perovskite films with smooth surface are obtained. By increasing annealing temperature from 100 to 130 °C, the average grain size of the perovskite increases from ~?216 to 375 nm. As a result, the efficiency of the PSCs increases from 10.72 to 14.54%.     

Preparation and characterization of methanofullerenes for polymer–fullerene bulk heterojunction solar cells

A series of alkyl 4-benzoylbutyrate p-tosylhydrazones were synthesized and reacted with C₆₀ in the presence of sodium methoxide. Interestingly, n-butyl 4-benzoylbutyrate p-tosylhydrazone causes a high yield of the undesired compound-[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM). Photovoltaic cells with these derivatives as electron acceptors were fabricated. The surface morphology of poly(3-hexylthiophene)(P3HT)/C₆₀ derivatives was characterized by atomic force microscopy. The nano-scale phase separation was observed in P3HT/PCBM film through a slow-growth process. This phenomenon is indistinguishable in P3HT/PCBiB ([6,6]-phenyl C₆₁-butyric acid iso-butyl ester) film because of the higher solubility of PCBiB in P3HT. The power conversion efficiency of the device that was made of P3HT/PCBiB blend is 2.8%, which is lower than that of P3HT/PCBM-based device (4.0%).     

CdSe@ZnS胶体量子点的合成及其在发光二极管中的应用

以氧化镉、硒粉和二乙基二硫代氨基甲酸锌为原料,采用微流体法合成了稳定的CdSe@ZnS核-壳结构胶体量子点,利用X射线衍射仪(XRD)、透射电子显微电镜(TEM)、荧光光谱仪(PL)和紫外-可见吸收光谱(UV-vis)对量子点的结晶性能、微观形貌和发光性能进行了表征。将CdSe@ZnS核-壳结构量子点作为量子点发光二极管(QLED)的发光层材料,成功组装出ITO/PEDOT:PSS/Poly-TPD/Cd Se@ZnS/PCBM/Li F:Al三明治结构的量子点发光二极管,在2 V电压下成功点亮,器件发出黄色光,亮度达到4500 Cd/cm~2。     

Confocal ultrafast pump-probe spectroscopy: a new technique to explore nanoscale composites

This article is devoted to the exploration of the benefits of a new ultrafast confocal pump-probe technique, able to study the photophysics of different structured materials with nanoscale resolution. This tool offers many advantages over standard stationary microscopy techniques because it directly interrogates excited state dynamics in molecules, providing access to both radiative and non-radiative deactivation processes at a local scale. In this paper we present a few different examples of its application to organic semiconductor systems. The first two are focussed on the study of the photophysics of phase-separated polymer blends: (i) a blue-emitting polyfluorene (PFO) in an inert matrix of PMMA and (ii) an electron donor polythiophene (P3HT) mixed with an electron acceptor fullerene derivative (PCBM). The experimental results on these samples demonstrate the capability of the technique to unveil peculiar interfacial dynamics at the border region between phase-segregated domains, which would be otherwise averaged out using conventional pump-probe spectroscopy. The third example is the study of the photophysics of isolated mesoscopic crystals of the PCBM molecule. Our ultrafast microscope could evidence the presence of two distinctive regions within the crystals. In particular, we could pinpoint for the first time areas within the crystals showing photobleaching/stimulated emission signals from a charge-transfer state.     

石墨炔及其衍生物在钙钛矿太阳能电池的应用

钙钛矿太阳能电池(PSCs)因具有高效率、可溶液加工和低成本等优点受到了人们广泛的关注。然而，在PSCs的各个功能层及界面之间存在缺陷非辐射复合、界面接触不良和薄膜质量较差等问题，阻碍了PSCs光电转换效率和稳定性的提高。相较于石墨烯，含有sp杂化碳原子的石墨炔具有独特的三角微观结构、天然的带隙、超高的载流子迁移率以及优异的光电和机械性能，成为光电能源领域重要的候选材料。综述了石墨炔及其衍生物在PSCs的电子传输层、空穴传输层以及光吸收层中的应用，重点探讨了石墨炔及其衍生物在功能层及其界面中钝化缺陷、改善薄膜形貌和界面接触、提高载流子传输等方面的作用。最后，对石墨炔及其衍生物在PSCs领域中的发展提出了展望。     

Poly(thienylene-benzothiadiazole-thienylene-vinylene): A narrow bandgap polymer with broad absorption from visible to infrared region

Poly(thienylene-benzothiadiazole-thienylene-vinylene) (PTBTV) has been synthesized by Pd-catalyzed Stille-coupling method. The polymer shows broad absorption from visible to infrared region. The maximum absorption of PTBTV in solution and film state is at 600 nm and 614 nm, respectively. The absorption edge of PTBTV film is at 813 nm, indicating a narrow bandgap of 1.50 eV. The HOMO and LUMO energy levels of PTBTV are −4.99 eV and −3.49 eV, respectively. Polymer solar cell based on the blend of PTBTV as donor and [70] PCBM as acceptor was fabricated, the power conversion efficiency of the device is 0.51% under the illumination of AM1.5, 100 mW/cm².