Thienopyrazine‐Based Low‐Bandgap Poly(heteroaryleneethynylene)s for Photovoltaic Devices

Summary: Low‐bandgap π‐conjugated polymers that consist of alkyl thiophene/alkoxy phenylene and 2,3‐diphenylthieno[3,4‐b]pyrazine units have been prepared in high yields by a Sonogashira polycondensation. The copolymers are characterized by NMR, IR, UV, GPC, and elemental analysis. Thin films of the polymers P1 , P2 , and P3 exhibit an optical bandgap of ≈1.57–1.60 eV. Under simulated AM 1.5 conditions P2/PCBM devices on polyester foil provide a short circuit current of I_SC = 10.72 mA · cm⁻², an open circuit voltage of V_oc = 0.67 V, and a power conversion efficiency of 2.37%.

Schematic of the photovoltaic device made from the polymers synthesized here.     

聚合物太阳能电池材料的研究进展

有机光伏技术为太阳能的有效利用提供了一条重要途径。有机太阳能电池因制造成本低廉、材料质量轻、加工性能好、易于携带等优势而备受关注。提高有机太阳能电池的光电转换效率是目前乃至未来的研究重点。设计和合成适合的窄带隙的共轭聚合物是提高有机太阳能电池光电转化效率的核心。综述了近年来基于窄带隙的共轭聚合物的太阳能电池材料的设计、制备和器件性能研究进展,探讨了目前存在的亟待解决的关键基础问题和未来发展方向。     

Charge carrier mobility,photovoltaic, and electroluminescent properties of anthracene‐based conjugated polymers bearing randomly distributed side chains

This article reports on the synthesis, characterization, and properties of various anthracene‐containing poly (p‐phenylene‐ethynylene)‐alt‐poly(p‐phenylene‐vinylene) (PPE‐PPV) polymers (AnE‐PVs) bearing statistical distributions of various side chains. Primarily, the ratio of linear octyloxy and branched 2‐ethylhexyloxy side chains at the poly(p‐phenylene vinylene) (PPV) parts was varied, leading to the polymers stat, stat1, and stat2. Furthermore, polymers also containing asymmetric substituted PPV and poly(p‐phenylene ethynylene) units (bearing methoxy and 2‐ethylhexyloxy side chains) were prepared yielding stat3, stat4, and stat5. These materials exhibit a broad variation in their photovoltaic properties. It is once more shown that side chains and their distribution can crucially affect the photovoltaic device performance. The introduction of units with asymmetric substitution into these systems seems to be harmful for their utilization in photovoltaic applications. Organic field‐effect transistors were fabricated to investigate hole mobilities in these new materials. Large variance was observed, falling in the range of almost two orders of magnitude, indicating rather different π–π stacking behavior of the polymer backbones owing to side‐chain modifications. Moreover, a selection of the new polymeric systems was investigated regarding their potential for light‐emitting diode (LED) applications. Polymer LEDs using the polymers AnE‐PVstat, ‐stat3, ‐stat4, and ‐stat5, as the active layer showed turn‐on voltage of ～2 V and exhibited red light emission. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

D–π–A–π–A Strategy to Design Benzothiadiazole–carbazole‐based Conjugated Polymer with High Solar Cell Voltage and Enhanced Photocurrent

The theoretical calculations are used to find that D–π–A–π–A style conjugated polymer PC‐TBTBT is more efficient for solar cells application than the D–π–A analog PC‐TBT because the D–π–A–π–A structure has a narrower band gap and higher molar absorption coefficient and redshift spectrum. Motivated by the theoretical prediction, 5,6‐bis(octyloxy)‐2,1,3‐benzothiadiazole and 2,7‐carbazole are adopted to synthesize the D–π–A–π–A style PC‐TBTBT (M_w = 31.1 kDa) and D–π–A analog PC‐TBT (M_w = 87.5 kDa) by Suzuki coupling reaction. Experimental results confirm that D–π–A–π–A PC‐TBTBT ‐based solar cell shows a power conversion efficiency (PCE) of 4.74% with high V_OC of 0.99 V and enhanced J_SC of 9.70 mA cm⁻². The PCE and J_SC achieve improvements of 17% and 26%, respectively, compared to the D–π–A PC‐TBT ‐based solar cell.

     

Crystalline Conjugated Polymers for Organic Solar Cells: From Donor,Acceptor to Single‐Component

Organic solar cells have made rapid progress in the last two decades due to the innovation of conjugated materials and photovoltaic devices. Microphase separation that connects with materials and devices plays a crucial role in the charge generation process. In this account, we summary our recent works of developing new crystalline conjugated polymers to control the microphase separation in thin films in order to realize high performance in solar cells, including crystalline diketopyrrolopyrrole‐based donor polymers, perylene bisimide‐based electron acceptors, and “double‐cable” conjugated polymers that contain covalently‐linked crystalline donor and acceptor in one material for single‐component organic solar cells.     

13.34 % Efficiency Non‐Fullerene All‐Small‐Molecule Organic Solar Cells Enabled by Modulating the Crystallinity of Donors via a Fluorination Strategy

Non‐fullerene all‐small‐molecule organic solar cells (NFSM‐OSCs) have shown potential as OSCs, owing to their high purity, easy synthesis and good reproducibility. However, challenges in the modulation of phase separation morphology have limited their development. Herein, two novel small molecular donors, BTEC‐1F and BTEC‐2F, derived from the small molecule DCAO3TBDTT, are synthesized. Using Y6 as the acceptor, devices based on non‐fluorinated DCAO3TBDTT showed an open circuit voltage (V_oc) of 0.804 V and a power conversion efficiency (PCE) of 10.64 %. Mono‐fluorinated BTEC‐1F showed an increased V_oc of 0.870 V and a PCE of 11.33 %. The fill factor (FF) of di‐fluorinated BTEC‐2F‐based NFSM‐OSC was improved to 72.35 % resulting in a PCE of 13.34 %, which is higher than that of BTEC‐1F (61.35 %) and DCAO3TBDTT (60.95 %). To our knowledge, this is the highest PCE for NFSM‐OSCs. BTEC‐2F had a more compact molecular stacking and a lower crystallinity which enhanced phase separation and carrier transport.     

Recent Progress in All‐Polymer Solar Cells Based on Wide‐Bandgap p‐Type Polymers

All‐polymer solar cells (all‐PSCs), with the photoactive layer exclusively composed of polymers as both donor and acceptor, have attracted growing attention due to their unique merits in optical, thermal and mechanical durability. Through the combined strategies in materials design and device engineering, recently the power conversion efficiencies of single‐junction all‐PSCs have been boosted up to 11 %. This review focuses on the recent progress of all‐PSCs comprising of wide band‐gap p‐type polymers, especially those based on the units of thieno[3,4‐c]pyrrole‐4,6(5H)‐dione], fluorinated benzotriazole, benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione, and pyrrolo[3,4‐f]benzotriazole‐5,7(6H)‐dione. Meantime, several categories of n‐type polymers used to match with these polymer donors are also reviewed. Finally, a brief summary of the strategies of molecular design and morphology optimization is given, and strategies toward further improving performance of all‐PSCs are outlined.     

Synthesis and characterization of a wide-bandgap polymer based on perfluorinated and alkylthiolated benzodithiophene with a deep highest occupied molecular orbital level for organic photovoltaics

A perfluorinated and alkylthiolated benzodithiophene (BDT)-ttTPD-based donor polymer (P2FS-ttTPD) was synthesized via a Stille polymerization, and found to have a number average molecular weight (M_n) of 13,000 g/mol (Đ = 2.3). P2FS-ttTPD has a wide bandgap (1.96 eV) and a deep highest occupied molecular orbital (HOMO) level (−5.70 eV). The perfluorination and alkylthiolation of the polymer backbone lower the polymer's HOMO level significantly. The hole and electron mobilities of P2FS-ttTPD were determined to be 1.12 × 10⁻⁴ and 9.38 × 10⁻⁷ cm²/V s, respectively. Polymer solar cell devices prepared with a P2FS-ttTPD:IT-4F (1:1) blend as the active layer were found to exhibit power conversion efficiencies of 4.15%, a short-circuit current density (J_SC) of 10.29 mA/cm², an open-circuit voltage (V_OC) of 0.97 V, and a fill factor of 41.6%. The (1:1) blend devices were found to exhibit high V_OC and low E_loss values.     

Ambipolar Blends of Cu-Phthalocyanine and Fullerene: Charge Carrier Mobility,Electronic Structure and their Implications for Solar Cell Applications

Summary: Ambipolar transport has been realised in blends of the molecular hole conductor Cu-phthalocyanine (CuPc) and the electron conducting fullerene C₆₀. Charge carrier mobilities and the occupied electronic levels have been analyzed as a function of the mixing ratio using field-effect transistor measurements and photoelectron spectroscopy. These results are discussed in the context of photovoltaic cells based on these materials.     

Effect of side‐chain end groups on the optical,electrochemical, and photovoltaic properties of side‐chain conjugated polythiophenes

Three new side‐chain conjugated polythiophene derivatives, poly{3‐[2‐(3‐methoxy‐4‐octyloxy‐phenyl)‐vinyl]‐thiophene} (P3MOPVT), poly{3‐[2‐(3,5‐dimethoxy‐4‐octyloxy‐phenyl)‐vinyl]‐thiophene} (P3DMOPVT), and poly{3‐[2‐(3,4‐dioctyloxy‐phenyl)‐vinyl]‐thiophene} (P3DOPVT), were synthesized by Wittig‐Hornor reaction and GRIM method and compared with poly{3‐[2‐(4‐octyloxy‐phenyl)‐vinyl]‐thiophene} (P3OPVT) for investigating the effect of the end groups of the conjugated side‐chain on the properties of the polymers. Owing to the electron‐donating ability of methoxy groups, the visible absorption peaks of P3MOPVT and P3DMOPVT solutions and films become stronger and red‐shifted compared with P3OPVT. The electrochemical bandgaps of the four polymers are 2.15 eV for P3OPVT, 1.99 eV for P3MOPVT, 1.85 eV for P3DMOPVT, and 2.36 eV for P3DOPVT, respectively, which indicate that the electron‐donating ability of the methoxy end group on the conjugated side chain of P3MOPVT and P3DMOPVT and the large steric hindrance of the two octyloxy end groups on the conjugated side chain of P3DOPVT have obvious influence on the electrochemical properties of the side‐chain conjugated polythiophenes. Polymer solar cells were fabricated with a structure of ITO/PEDOT:PSS/Polymer:PCBM/LiF/Al. The best device, based on P3DMOPVT, shows a power conversion efficiency of 1.63% under the illumination of AM1.5, 80 mW/cm². © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4916–4922, 2006     

Naphthalene‐Diimide‐Based Ionenes as Universal Interlayers for Efficient Organic Solar Cells

Self‐doping ionene polymers were efficiently synthesized by reacting functional naphthalene diimide (NDI) with 1,3‐dibromopropane ( NDI‐NI ) or trans‐1,4‐dibromo‐2‐butene ( NDI‐CI ) via quaternization polymerization. These NDI‐based ionene polymers are universal interlayers with random molecular orientation, boosting the efficiencies of fullerene‐based, non‐fullerene‐based, and ternary organic solar cells (OSCs) over a wide range of interlayer thicknesses, with a maximum efficiency of 16.9 %. NDI‐NI showed a higher interfacial dipole (Δ), conductivity, and electron mobility than NDI‐CI , affording solar cells with higher efficiencies. These polymers proved to efficiently lower the work function (WF) of air‐stable metals and optimize the contact between metal electrode and organic semiconductor, highlighting their power to overcome energy barriers of electron injection and extraction processes for efficient organic electronics.     

High‐Efficiency Large‐Bandgap Material for Polymer Solar Cells

High‐molecular‐weight conjugated polymer HD‐PDFC‐DTBT with N‐(2‐hexyldecyl)‐3,6‐difluorocarbazole as the donor unit, 5,6‐bis(octyloxy)benzothiadiazole as the acceptor unit, and thiophene as the spacer is synthesized by Suzuki polycondensation. HD‐PDFC‐DTBT shows a large bandgap of 1.96 eV and a high hole mobility of 0.16 cm² V⁻¹ s⁻¹. HD‐PDFC‐DTBT:PC₇₁BM‐based inverted polymer solar cells (PSCs) give a power conversion efficiency (PCE) of 7.39% with a V_oc of 0.93 V, a J_sc of 14.11 mA cm⁻², and an FF of 0.56.

     

Effect of Fluorination on the Photovoltaic Properties of Medium Bandgap Polymers for Polymer Solar Cells

Fluorination of conjugated polymers is one of the effective strategies to tune the molecular energy levels and morphology for high efficient polymer solar cells (PSCs). Herein, two novel donor‐acceptor conjugated polymers, PffBT and PBT, based on bis(3,5‐bis(hexyloxy)phenyl)benzo[1,2‐ b:4,5‐b']dithiophene and benzo[c][1,2,5]thiadiazole (BT) with or without fluorination, respectively, were synthesized, and their photovoltaic properties were compared. The polymer PffBT based on fluorinated BT showed lower frontier energy levels, improved polymer ordering, and a well‐developed fibril structure in the blend with PC₇₁BM. As a result, the PSCs based on PffBT/PC₇₁BM exhibit a superior power conversion efficiency (PCE) of 8.6% versus 4.4% for PBT‐based devices, due to a high space charge limit current (SCLC) hole mobility, mixed orientation of polymer crystals in the active layer, and low bimolecular recombination.     

Photophysics and morphology of a polyfluorene donor–acceptor triblock copolymer for solar cells

We present a study of the optical, structural and device properties of a polyfluorene (PFM)‐based (PFM‐F8BT‐PFM) donor–acceptor triblock copolymer for use in an organic solar cell. Neutron reflectivity is employed to probe the vertical composition profile before and after thermal annealing while the crystallinity was examined using grazing incidence wide‐angle X‐ray. The absorption spectra and photoluminescence emission for the triblock and analogous blend of PFM with F8BT reveal a greater degree of intermixing in the triblock. However, the triblock copolymer exhibits exciplex emission, which necessitates a geminate polar pair; long‐lived exciplex states are detrimental in organic photovoltaic devices. The triplet yield in the triblock and the blend is estimated using photoinduced absorption, with the triblock copolymer generating a triplet population 20 times that of the blend. This is far from ideal as triplets are wasted states in organic photovoltaic devices and they can also act as scavengers of polarons reducing the efficiency even more. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1705–1718     

4,9‐Dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione functionalized copolymers for organic photovoltaic devices

The synthesis of conjugated polymers 1 – 5 functionalized with 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione in the backbone is reported and their use in the construction of organic solar cells is demonstrated. Increasing the molar ratio of 2,7‐dibromo‐3,8‐dihexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione, relative to 4,4′‐dihexyl‐5,5′‐dibromo‐2,2′‐bithiophene, in the copolymer synthesis significantly lowers the solubility of these polymers. The incorporation of highly conjugated 3,8‐dihexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione unit into the polymer backbone has been confirmed by UV–vis absorption. The observation of decreasing quantum yield for the emission in the order of 1 , 2 , 3 is consistent with copolymers with different comonomer content. The power conversion efficiencies of solar cells using blends of these polymers with PCBM ([6,6]‐phenyl C₆₁‐butyric acid methyl ester) were determined to be 0.11% for polymer 1 , 0.33% for 2 , and 0.26% for 3 , respectively. Under identical white light illumination, the power conversion efficiency of the device based on polymer 2 /PCBM as the active layer was three times higher compared to that of device based on polymer 1 /PCBM. Owing to the limited solubility and poor film‐forming ability of polymer 3 , the power conversion efficiency of solar cell based on 3 /PCBM blend is lower than that of 2 /PCBM blend, but is still larger than that of 1 /PCBM blend. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2680–2688, 2008     

Stabilization of the film morphology in polymer: Fullerene heterojunction solar cells with photocrosslinkable bromine‐functionalized low‐bandgap copolymers

Novel bromine‐functionalized photocrosslinkable low‐bandgap copolymers, PBDTTT‐Br25 and PBDTTT‐Br50, are synthesized via Stille cross‐coupling polymerization for the purpose of stabilizing the film morphology in polymer solar cells (PSCs). Photocrosslinking of PBDTTT‐Br25 and PBDTTT‐Br50 copolymers dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport, which is confirmed by the insolubility of the films washed by organic solvents and by their thermal behavior. As a result, the formation of large aggregations of fullerene is suppressed in polymer:fullerene blend films even after prolonged thermal annealing, and the stability of the device is enhanced when compared with cells based on noncrosslinkable PBDTTT. The power conversion efficiency of the PSCs based on PBDTTT‐Br25 and PBDTTT‐Br50 reaches 5.17% and 4.48%, respectively, which is improved obviously in comparison with that (4.26%) of the PSCs based on the control polymer PBDTTT. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3123–3131     

Multifunctional Conjugated Polymers with Main‐Chain Donors and Side‐Chain Acceptors for Dye Sensitized Solar Cells (DSSCs) and Organic Photovoltaic Cells (OPVs)

A novel multifunctional conjugated polymer (RCP‐1) composed of an electron‐donating backbone (carbazole) and an electron‐accepting side chain (cyanoacetic acid) connected through conjugated vinylene and terthiophene has been synthesized and tested as a photosensitizer in two major molecule‐based solar cells, namely dye sensitized solar cells (DSSCs) and organic photovoltaic cells (OPVs). Promising initial results on overall power conversion efficiencies of 4.11% and 1.04% are obtained from the basic structure of DSSCs and OPVs based on RCP‐1, respectively. The well‐defined donor (D)‐acceptor (A) structure of RCP‐1 has made it possible, for the first time, to reach over 4% of power conversion efficiency in DSSCs with an organic polymer sensitizer and good operation stability.     

Effect of multiple adduct fullerenes on charge generation and transport in photovoltaic blends with poly(3‐hexylthiophene‐2,5‐diyl)

The effect of replacing [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) by its multiadduct analogs (bis‐PCBM and tris‐PCBM) in bulk heterojunction organic solar cells with poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) is studied in terms of blend film microstructure, photophysics, electron transport properties, and device performance. Although the power conversion efficiency of the blend with bis‐PCBM is similar to the blend with PCBM, the performance of the devices with tris‐PCBM is considerably lower as a result of small photocurrent. Despite the lower electron affinity of the fullerene multiadducts, μs‐ms transient absorption measurements show that the charge generation efficiency is similar for all three fullerenes. The annealed blend films with multiadducts show a lower degree of fullerene aggregation and lower P3HT crystallinity than the annealed blend films with PCBM. We conclude that the reduction in performance is due largely to poorer electron transport in the blend films from higher adducts, due to the poorer fullerene network formation as well as the slower electron transport within the fullerene phase, confirmed here by field effect transistor measurements. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

New Bithiazole‐Based Sensitizers for Efficient and Stable Dye‐Sensitized Solar Cells

A series of new push–pull organic dyes ( BT‐I – VI ), incorporating electron‐withdrawing bithiazole with a thiophene, furan, benzene, or cyano moiety, as π spacer have been synthesized, characterized, and used as the sensitizers for dye‐sensitized solar cells (DSSCs). In comparison with the model compound T1 , these dyes containing a thiophene moiety between triphenylamine and bithiazole display enhanced spectral responses in the red portion of the solar spectrum. Electrochemical measurement data indicate that the HOMO and LUMO energy levels can be tuned by introducing different π spacers between the bithiazole moiety and cyanoacrylic acid acceptor. The incorporation of bithiazole substituted with two hexyl groups is highly beneficial to prevent close π–π aggregation, thus favorably suppressing charge recombination and intermolecular interaction. The overall conversion efficiencies of DSSCs based on bithiazole dyes are in the range of 3.58 to 7.51 %, in which BT‐I ‐based DSSCs showed the best photovoltaic performance: a maximum monochromatic incident photon‐to‐current conversion efficiency (IPCE) of 81.1 %, a short‐circuit photocurrent density (J_sc) of 15.69 mA cm^?2, an open‐circuit photovoltage (V_oc) of 778 mV, and a fill factor (ff) of 0.61, which correspond to an overall conversion efficiency of 7.51 % under standard global AM 1.5 solar light conditions. Most importantly, long‐term stability of the BT‐I – III ‐based DSSCs with ionic‐liquid electrolytes under 1000 h of light soaking was demonstrated and BT‐II with a furan moiety exhibited better photovoltaic performance of up to 5.75 % power conversion efficiency.