Molecular structure–property engineering of low‐band‐gap copolymers,based on fluorene,for efficient bulk heterojunction solar cells: A density functional theory study

A comprehensive theoretical study of new donor (D)–acceptor (A) copolymers properties has been developed through quantum chemical calculations, on the basis of density functional theory (DFT). Fluorene and 2,1,3‐benzothiadiazole units were used as donor and acceptor units, respectively, in alternating D–A structures. The efficiency of D–A strength in the polymer was characterized, and their electronic properties are modulated through the insertion of thiophene, vinylene, or ethynylene as π‐spacer groups, in an attempt to reach the critical values, required for organic solar cells. Optoelectronic properties of these copolymers are rationalized on the basis of the patterns of their frontier orbitals. The analysis of the corresponding transition provides an efficient way for tracing the origin of various optical transitions of these copolymers and their photophysical properties. Bulk heterojunction photovoltaic cells, designed with the based fluorene‐copolymer as electron donor, blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester as an electron acceptor, were energetically elucidated. A model band diagram was established, simulating the energy behavior of this active layer. POLYM. ENG. SCI., 2013. © Society of Plastics Engineers     

Synthesis of a terpolymer containing fluorene,side chain conjugated thiophene and benzothiadiazole and its applications in photovoltaic devices

A terpolymer (POTVTh‐8FO‐DBT) containing fluorene, side chain conjugated thiophene and 4,7‐dithieny‐2,1,3‐benzothiadiazole was synthesized by palladium‐catalyzed Suzuki coupling method. The polymer is soluble in common organic solvents. The thermal, absorption, and electrochemical properties of the polymer were examined. Photovoltaic properties of POTVTh‐8FO‐DBT were studied by fabricating the polymer solar cells (PSCs) based on POTVTh‐8FO‐DBT as donor and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) as acceptor. With the weight ratio of POTVTh‐8FO‐DBT : PC₆₁BM of 1 : 1 and the active layer thickness of 80 nm, the power conversion efficiency (PCE) of the device reached 0.47% with V_oc = 0.61 V, J_sc = 1.61 mA/cm², and filled factor (FF) = 0.49 under the illumination of AM 1.5, 100 mW/cm². The results indicated that this polymer was promising donor candidates in the application of PSCs. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

New anthracene‐containing phenylene‐ or thienylene‐vinylene copolymers: Synthesis,characterization, photophysics,and photovoltaics

Four new conjugated alternating vinylene‐copolymers, PAP6 , PAT , PA , and TAT , incorporating anthracene rings along the backbone were synthesized by Heck coupling. They were very soluble in common organic solvents and absorbed at the range of 300–500 nm with optical band gaps of 2.38–2.47 eV. They behaved in solution as green emitters, with maximum photoluminescence at 455–518 nm. Finally, these soluble copolymers were used as donor material to realize bulk heterojunction solar cell with (6,6)‐C₆₁‐butyric acid methyl ester as the acceptor. More efficient photovoltaic cells were obtained from the copolymer that carried hexyloxy than dodecyloxy side groups. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thiophene π‐bridge effect on photovoltaic performances of dithienosilole and bithiazole backboned polymers

In this article, two dithienosilole (DTS) and bithiazole (BTz) backboned donor–acceptor (D‐A) copolymers with (poly{5‐(5‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b']dithiophen‐2‐yl)thiophen‐2‐yl)‐4,4'‐dinonyl‐5'‐(thiophen‐2‐yl)‐2,2'‐bithiazole} (PDTS‐DTBTz)) and without (poly{5‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b']dithiophen‐2‐yl)‐4,4'‐dinonyl‐2,2'‐bithiazole} (PDTS‐BTz)) thiophene π‐bridge were synthesized to study the influence of thiophene π‐bridge on their photovoltaic performances. Both polymers show similar band gap, but polymer with thiophene π‐bridge (PDTS‐DTBTz) has a higher molecular weight, narrower polydispersity index (PDI), more planar geometry, higher crystallinity, higher hole mobility, and better miscibility with fullerene (polymer solar cells (PSCs) acceptor). Although PDTS‐BTz polymer based PSCs devices show higher open circuit voltage (V_oc), PDTS‐DTBTz polymer does show higher power conversion efficiency (PCE) with improved short circuit current density (J_sc) and fill factor (FF). The present results indicate that thiophene π‐bridge does contribute to the PSCs performances of dithienosilole and bithiazole backboned copolymer. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42798.     

The influence of side chains on solubility and photovoltaic performance of dithiophene–thienopyrazine small band gap copolymers

Three small band gap copolymers based on alternating dithiophene and thienopyrazine units were synthesized via Yamamoto coupling and applied in bulk heterojunction solar cells as donor together with PCBM ([6,6]-phenyl C₆₁ butyric acid methyl ester) as acceptor. The polymers have an optical band gap of about 1.3 eV in the solid state and only vary by the chemical nature of the solubilizing side chains. The nature of the side chain has a major effect on solubility and processability of the polymer. Using n-butoxymethyl side chains a soluble, easy to process polymer was obtained that gave the best photovoltaic performance. With short-circuit currents up to 5.2 mA/cm² an efficiency of 0.8% was achieved under estimated standard solar light conditions (AM1.5G, 100 mW/cm²) with spectral response up to 950 nm.     

Design and synthesis of thieno[3,4‐c]pyrrole‐4,6‐dione based conjugated copolymers for organic solar cells

A new series of conjugated copolymers (PBDT‐TPD , PBDT ‐Th‐TPD , PBDT‐TT‐TPD ) containing donor–acceptor (D ? A) structure electron‐rich benzo[1,2‐b :4,5‐b ′]dithiophene (BDT ) units with branched alkyl thiophene side chains and electron‐deficient 5‐(2‐octyl)‐4H ‐thieno[3,4‐c ]pyrrole‐4,6(5H )‐dione (TPD) units was designed and synthesized. To tune the optical and electrochemical properties of the copolymers, the conjugation length of the copolymers was extended by introducing π‐conjugated spacers such as thiophene and thieno[3,2‐b ]thiophene units. It was observed that PBDT‐TPD showed broader absorption spectra in the longer wavelength region and the absorption maximum was red‐shifted compared to that of PBDT‐Th‐TPD, PBDT‐TT‐TPD. Stokes shifts were calculated to be 52 nm for PBDT‐TPD, 153 nm for PBDT‐Th‐TPD and 146 nm for PBDT‐TT‐TPD. Further, PBDT‐TPD exhibited a deeper highest occupied molecular orbital energy level of ?5.53 eV as calculated by cyclic voltammetry. Bulk heterojunction solar cells fabricated using PBDT‐TPD as donor material exhibited a power conversion efficiency of 1.92%. © 2017 Society of Chemical Industry     

Design and synthesis of carbonyl group modified conjugated polymers for photovoltaic application

In this article, a simple and common electron-withdrawing moiety, carbonyl group, is applied in the molecular design of conjugated polymers for high-performance polymer solar cells (PSCs). Two series of donor–acceptor (D–A) copolymers are synthesized through alternating copolymerization of the electron-donating (D) benzodithiophene and dithienopyrrole with various electron-accepting (A) units containing carbonyl groups. The absorption range and the band gap of copolymers can be tuned by changing the molecular structure of A unit and the number of carbonyl groups. Moreover, by introducing the carbonyl group, the highest occupied molecular orbital energy level of the copolymer is lowered efficiently, leading to the improvement of the open-circuit voltage (V _OC) of PSCs. The best photovoltaic performance is obtained while poly(benzodithiophene-alt-thiophene-3-carboxylate) is functioned as the electron donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester as electron acceptor in a bulk heterojunction solar cell with a power conversion efficiency of 4.13%, a V _OC of 0.80 V, a short-circuit current of 8.19 mA/cm², and a fill factor of 63.2%.     

Synthesis of two D‐π‐A polymers π‐bridged by different blocks and investigation of their photovoltaic property

The synthesis, characterization, photophysical and photovoltaic properties of two 5,6‐bis(octyloxy)benzo[c][1,2,5]thiadiazole‐containing wide‐band‐gap donor and acceptor D‐π‐A alternating conjugated polymers (HSD‐a and HSD‐b) have been reported. These two polymers absorb in the range of 300–700 nm with a band gap of about 1.88 and 1.97 eV. The HOMO energy levels were ?5.44 eV for HSD‐a and ?5.63 eV for HSD‐b. Polymer solar cells with HSD‐b :PC₇₁BM as the active layer demonstrated a power conversion efficiency (PCE) of 2.59% with a high V_oc of 0.93 V, a J_sc of 7.3 mA/cm², and a comparable fill factor (FF) of 0.38 under simulated solar illumination of AM 1.5G (100 mW/cm²) without annealing. In addition, HSD‐a :PC₇₁BM blend‐based solar cells exhibit a PCE of 2.15% with a comparable V_oc of 0.64 V, J_sc of 8.75 mA/cm^?2, and FF of 0.40. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41587.     

Synthesis and photovoltaic properties of polythiophene derivatives with side chains containing C60 end group

A donor–acceptor double‐cable polythiophene derivative ( PT‐F1 ) with side chain containing C₆₀ end group was synthesized, and characterized by infrared, UV‐vis absorption and photoluminescence (PL) spectroscopy, and electrochemical cyclic voltammetry. Cyclic voltammogram of PT‐F1 shows the oxidation peak of the polymer main chains and the reduction peaks of the C₆₀ end groups, indicating that there is no interaction between the polymer main chains and side chain C₆₀ groups on the ground state. The UV‐vis absorption spectrum of PT‐F1 film is red‐shifted in comparison with that of its chloroform solution. The PL spectrum of the polymer main chain was quenched by the C₆₀ pendant on the side chain. Polymer solar cell with the structure of ITO/PEDOT:PSS/ PT‐F1 /Ca/Al was fabricated. The power conversion efficiency of the device based on PT‐F1 reached 0.274% under the illumination of AM 1.5, 100 mW/cm². © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Molecular design and density functional theory investigation of novel low‐band‐gap benzobisthiadiazole‐based systems: from monomer to polymer

The electronic structure and properties of benzobisthiadiazole‐based alternating donor–acceptor conjugated oligomers and their periodic copolymers of donor and acceptor units with ratios of 1:1 and 2:1 were investigated systematically using the density functional theory method. The donors include thiophene, thieno[3,2‐b]thiophene and pyrrole. The ratio of donor to acceptor units (D:A ratio) plays a very important role in the geometric and electronic properties. The intramolecular charge transfer increases and the bond length alternation decreases with an increase in the D:A ratio for these oligomers and polymers. Moreover, an increase in D:A ratio can greatly reduce the band gap and effective mass of holes and electrons for these alternating donor‐acceptor conjugated copolymers. The unusually large intramolecular charge transfer caused by intramolecular hydrogen bonds reveals that pyrrole is not only a strong electron donor but also a potential hydrogen bond donor. The theoretical results suggest those copolymers possessing a D:A ratio of 2:1 are better candidates for conducting materials compared to those with a D:A ratio of 1:1. The almost zero band gap, large bandwidth and small effective mass of holes and electrons of poly(4,8‐bis(thieno[3,2‐b]thiophene‐2‐yl)benzo[1,2‐c:4,5‐c′]bis[1,2,5]thiadiazole) indicate that it is a very good candidate for an electrically conductive material. Copyright © 2010 Society of Chemical Industry     

Synthesis and photovoltaic properties of two side‐chain polymeric metal complexes containing 8‐hydroxyquinoline and fluorene units with Zn(II) and Cd(II)

Two novel side‐chain polymeric metal complexes (PF1 and PF2) containing 8‐hydroxyquinoline and fluorene units with Zn(II) and Cd(II) having donor‐acceptor π‐conjugated structure have been synthesized and characterized using Fourier transform infrared, ¹H NMR, UV‐visible and photoluminescence spectroscopies, thermogravimetric analysis, differential scanning calorimetry, elemental analysis and cyclic voltammetry. Dye‐sensitized solar cells (DSSCs) based on PF1 and PF2 as the dye sensitizers exhibit good device performance with solar‐to‐electricity conversion efficiency up to 0.32% (J_sc = 0.83 mA cm^?2, V_oc = 0.62 mV and FF = 0.62) and 0.24% (J_sc = 0.69 mA cm^?2, V_oc = 0.59 mV and FF = 0.60), respectively, under simulated AM 1.5 G solar irradiation (100 mW cm^?2). The data show that these novel polymeric metal complexes are suitable for DSSCs. Copyright © 2012 Society of Chemical Industry     

Impact of alkyl side chain on the photostability and optoelectronic properties of indacenodithieno[3,2‐b]thiophene‐alt‐naphtho[1,2‐c:5,6‐ c′]bis[1,2,5]thiadiazole medium bandgap copolymers

Three donor‐π‐acceptor (D‐π‐A) type alternating conjugated polymers, namely PIDTT‐DTNT‐C16, PIDTT‐DTNT‐HD and PIDTT‐DTNT‐OD bearing the same backbone of indacenodithieno[3,2‐b]thiophene (IDTT) as the D unit and naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole (NT) as the A moiety but with different flexible side chain (n‐hexadecyl (C16), 2‐hexyldecyl (HD) and 2‐octyldodecyl (OD)) substituted thiophene employed as π‐bridges, were synthesized and characterized. The effects of the side chain on absorption, photostability, energy levels, aggregation, backbone conformation, morphology and photovoltaic properties were systematically investigated. Because moderate D and strong A units were selected to construct the polymer backbone, a medium optical bandgap (ca. 1.66 eV) and low‐lying highest occupied molecular orbital energy level (E_HOMO ≈ ?5.36 V), thus resulting in a relatively higher open‐circuit voltage (V_OC) of 0.80–0.83 V, were achieved. It was found that the side chain gave rise to an insignificant impact on absorption, aggregation and photostability in chlorobenzene solution and energy levels but a non‐negligible influence on absorption, photostability and aggregation behavior in the film state. It was found that PIDTT‐DTNT‐C16 with the densest and most ordered packing structure exhibited the best photostability. Inverted bulk heterojunction polymer solar cells based on PIDTT‐DTNT‐HD:PC₆₁BM ([6,6]‐phenyl‐C₆₁‐butyric acid methyl ester) showed at least a 1.5‐fold increase in power conversion efficiency, chiefly originating from its slightly improved absorption, more balanced μ_h/μ_e ratio and favorable morphology of the active layer as a result of incorporating branched HD side chains into the IDTT‐alt‐DTNT backbone. © 2019 Society of Chemical Industry     

Pyrrole‐based narrow‐band‐gap copolymers for red light‐emitting diodes and bulk heterojunction photovoltaic cells

A series of narrow‐band‐gap conjugated copolymers (PFO‐DPT) derived from pyrrole, benzothiadiazole, and 9,9‐dioctylfluorene (DOF) is prepared by the palladium‐catalyzed Suzuki coupling reaction with the molar feed ratio of 4,7‐bis(N‐methylpyrrol‐2‐yl)‐2,1,3‐benzothiadiazole (DPT) around 1, 5, 15, 30, and 50%. The obtained polymers are readily soluble in common organic solvents. The solutions and the thin solid films of the copolymers absorb light from 300 nm to 600 nm with two absorbance peaks at around 380 nm and 505 nm. The PL emission consists mainly of DPT unit emission at around 624–686 nm depending on the DPT content in solid film. The EL emission peaks are red‐shifted from 630 nm for PFO‐DPT1 to 660 nm for PFO‐DPT50. Bulk heterojunction photovoltaic cells fabricated from composite films of copolymer and [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) as electron donor and electron acceptor, respectively, in device configuration: ITO/PEDOT : PSS/PFO‐DPT : PCBM/Ba/Al shows power conversion efficiencies 0.15% with open‐circuit voltage (V_oc) of 0.60 V and short‐circuit current density (J_sc) of 0.73 mA/cm² under AM1.5 solar simulator (100 mW/cm²). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis and characterization of 4‐dodecyloxyphenyl and (4′‐dodecyloxy‐4‐biphenyl)methylene‐substituted bispyrrolylvinylthiophene‐based polysquaraines having good solubility and very low bandgap for light absorption

In this work, two very low band‐gap (～1.0 eV) alternate conjugated copolymers bearing bispyrrolylvinylthiophene‐based polysquaraine backbone (PVTVPS) have been synthesized. In comparison with their analogous polymer with 2‐ethylhexyl side chain on the pyrrole segment, which possesses poor solubility after long‐time storage, the two target polymers with 4‐dodecyloxyphenyl (PVTVPS‐Ph) or (4′‐dodecyloxy‐4‐biphenyl)methylene (PVTVPS‐Ph₂) side chains exhibit dramatically improved solubility. Furthermore, PVTVPS‐Ph shows unexpected thermochromism in the Vis–near‐infrared (NIR) region of 600–1100 nm at 80–160°C in solution and thin film. This may be attributed to the presence of relatively rigid phenyl substituent restricting the free rotation between the D (pyrrole) and A (squaraine) segments of the main chain. To our knowledge, this is the first report on NIR thermochromic polysquaraines. Nevertheless, in the case of PVTVPS‐Ph₂, no thermochromism could be observed because of the existence of free‐rotating methylene linkage bridge between biphenyl unit and the conjugated polymer skeleton. When compared with PVTVPS‐Ph, PVTVPS‐Ph₂ has much improved thermostability and broader absorption property. Hence, PVTVPS‐Ph₂ is a more prospective candidate as photovoltaic materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

New fluorene–pyrazino[2,3‐g]quinoxaline‐conjugated copolymers: Synthesis,optoelectronic properties,and electroluminescence characteristics

New donor–acceptor conjugated copolymers called poly}2,7‐(9,9′‐dihexylfluorene)‐co‐5,10‐[pyrazino(2,3‐g)quinoxaline]{s or PFPQs [where F represents the 2,7‐(9,9′‐dihexylfluorene) moiety and PQ represents the 5,10‐(pyrazino[2,3‐g]quinoxaline) moiety], synthesized by the palladium‐catalyzed Suzuki coupling reaction, are reported. The PQ contents in the PFPQ copolymers were 0.3, 1, 5, and 50 mol %, and the resulting copolymers were named PFPQ0.3, PFPQ01, PFPQ05, and PFPQ50, respectively. Absorption spectra showed a progressive redshift as the PQ acceptor content increased. The relatively small optical band gap of 2.08 eV for PFPQ50 suggested strong intramolecular charge transfer (ICT) between the F and PQ moieties. The photoluminescence emission peaks of the PFPQ copolymer films also exhibited a large redshift with enhanced PQ contents, ranging from 551 nm for PFPQ0.3 to 592 nm for PFPQ50. However, the PFPQ copolymer based electroluminescence (EL) devices showed poor device performances probably due to the strong confinement of the electrons in the PQ moiety or significant ICT. This problem was resolved with a binary blend of poly[2,7‐(9,9‐dihexylfluorene)] (PF) and PFPQ with a volume ratio of 95/5 (BPQ05). Multiple emission peaks were observed at 421, 444, 480, 516, and 567 nm in the BPQ05‐based EL devices because the low PQ content led to incomplete energy transfer. The Commission Internationale de L'Eclairage 1931 coordinates of the BPQ05‐based EL device were (0.31, 0.32), which were very close to the standard white emission of (0.33, 0.33). Furthermore, the maximum luminescence intensity and luminescence yield were 524 cd/m² and 0.33 cd/A, respectively. This study suggested that a pure white light emission was achieved with the PFPQ copolymers or PF/PFPQ blends through the control of the energy transfer between F and PQ. Such PFPQ copolymers or PF/PFPQ blends would be interesting for electronic and optoelectronic devices. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis and solar cells applications of EO‐PF‐DTBT polymer

Poly{[2,7‐(9,9‐bis‐(1‐(2‐(2‐methoxyethoxy)ethoxy)ethyl)‐fluorene)]‐alt‐[5,5‐(4,7‐di‐2′‐thienyl‐2,1,3‐benzothiadiazole)]} (EO‐PF‐DTBT) was synthesized by Suzuki coupling reaction. The polymer is soluble in common organic solvent, such as toluene, THF, and chloroform, and it also shows solubility in polar solvent, such as cyclopentanone. Solar cells based on EO‐PF‐DTBT and PC₆₁BM show maximum power conversion efficiency of 2.65% with an open circuit voltage (V_OC) of 0.86 V, a short circuit current density (J_SC) of 6.10 mA/cm², and a fill factor of 51% under AM 1.5G illumination at 100 mW/cm², which is the best results for fluorene and 4,7‐di‐2‐thienyl‐2,1,3‐benzothiadiazole copolymers and PC₆₁BM blend. The 1,8‐diiodooctane can work well to reduce the over‐aggregated phase structure in polymer solar cells. Our results suggest that the introducing high hydrophilic side chain into conjugated polymer donor materials can tune the aggregation structure and improve the solar cells performances. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40478.     

Thiophene‐based copolymers synthesized by electropolymerization for application as hole transport layer in organic solar cells

Electrically conducting thiophene‐based copolymers were synthesized by electropolymerization. The potential range used has a strong influence on the film structure and properties. The extent of oxidation of the copolymers was determined from the ratio of the oxidation to reduction charge, Q_ox/Q_red. The use of wide potential range leads to reduced films, whereas the narrow range leads to partially oxidized films. The copolymers exhibit a characteristic band in UV–vis spectra at ～ 410 nm, which shifts to higher wavelengths for the more doped material. The electrical conductivity of the copolymers was correlated to their morphology and their structure. The copolymer with higher conductivity is partially reduced, has compact morphology and higher ratio of quinoid to benzenoid rings. The energy gap of the copolymers is reversely proportional to their electrical conductivity. The optical and electrical properties of the copolymers make them very well suited for use as hole transport layers (HTL) in organic opto‐electronic devices. We prepared polymer : fullerene solar cells with copolymer HTLs. The solar cell performance was tested with very encouraging initial results. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Donor copolymer with benzo[1,2‐b:4,5‐b′]dithiophene and quinoxaline derivative segments for photovoltaic applications

A donor copolymer Poly{2,6‐4,8‐bis(2‐ethylhexyl)benzo[1,2‐b:3,4‐b′]dithiophene‐5,8‐2,3‐bis(5‐octylthiophen‐2‐yl)quinoxaline} (PBDTThQx) with benzo[1,2‐b:4,5‐b′]dithiophene and quinoxaline derivatives was synthesized and characterized with NMR, ultraviolet–visible spectroscopy, thermogravimetric analyses, and cyclic voltammetry. Photovoltaic devices with the configuration indium tin oxide–poly(3,4‐ethylenedioxythiophene)–poly(styrene sulfonate)–PBDTThQx–[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM)–LiF–Al were fabricated, in which PBDTThQx performed as the electron donor and PC₆₁BM was the electron acceptor in the active layer. The device presented reasonable photovoltaic properties when the weight ratio of PBDTThQx:PC₆₁BM reached 1:3. The open‐circuit voltage, fill factor, and power conversion efficiency were gauged to be 0.75 V, 0.59, and 0.74%, respectively. The experimental data implied that PBDTThQx would be a promising donor candidate in the application of polymer solar cells. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40279.     

Tailoring side chains of low band gap polymers for high efficiency polymer solar cells

High efficiency organic solar cells (OSCs) require conjugated polymers with a low band gap, broad absorption in visible and IR region, high carrier mobility, and relatively high molecular weight as p-type donor materials. Flexible side chains on the rigid polymer backbone are crucial for the solubility of conjugated polymers. In this work, four polymers with the main chain structure of fluorene-thiophene-benzothiadiazole-thiophene and flexible side chains located on fluorene, thiophene, and benzothiadiazole moiety, respectively, have been synthesized by Suzuki-Miyaura-Schlüter polycondensation. Photovoltaic device measurements with a device configuration of ITO/polymer:PC₇₁BM blends/LiF/Al show that P1 carrying octyloxy chains on benzothiadiazole rings gives the best performance, with a power conversion efficiency of 3.1%.     

Bi-diketopyrrolopyrrole (Bi-DPP) as a novel electron accepting compound in low band gap π-conjugated donor–acceptor copolymers/oligomers

The synthesis and characterization of a novel 2,5-diketopyrrolo[3,4-c]pyrrole(DPP)-based accepting building block with the scheme DPP-neutral small linker-DPP (Bi-DPP) is presented, which was utilized as electron accepting moiety for low band gap π-conjugated donor–acceptor copolymers as well as for a donor–acceptor small molecule. The electron accepting moiety Bi-DPP was prepared via a novel synthetic pathway by building up two DPP moieties step by step simultaneously starting from a neutral phenyl core unit. Characterization of the synthesized oligomeric and polymeric materials via cyclic voltammetry afford LUMO energy levels from ?3.49 to ?3.59 eV as well as HOMO energy levels from ?5.07 to ?5.34 eV resulting in low energy band gaps from 1.52 to 1.81 eV. Spin coating of the prepared donor–acceptor oligomers/polymers resulted in well-defined films. Moreover, UV–vis measurements of the investigated donor–acceptor systems showed a broad absorption over the whole visible region. It is demonstrated that Bi-DPP as an electron accepting moiety in donor–acceptor systems offer potential properties for organic solar cell devices.