Low band gap copolymer containing naphthalene-1,4,5,8-tetracarboxylic bisimide: Synthesis,properties and organic solar cell applications

A new alternating donor–acceptor copolymer, poly{[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(bis(2,2′-thiophene)-4,7-(2,1,3-benzothiadiazole)]-co-[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracarboxylic-N,N′-di(2-ethylhexyl)imide]} (PFTBTN), was synthesized for the use of photovoltaic cells. The copolymer containing fluorene, as the donor segment and naphthalene bisimide, dithienylbenzothiadiazole, as the acceptor segment was polymerized via Suzuki couplings to achieve a polymer with a narrow band gap. The band gap values of the copolymer film determined from optical and electrochemical measurements were 1.69 and 2.06 eV, respectively. The optical absorption spectrum revealed two broad bands in the range of 300–750 nm. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high open-circuit voltage when fullerene derivative ([6,6]-phenyl-C₆₁ butyric acid methyl ester) was used as an electron acceptor. Bulk heterojunction photovoltaic cells were fabricated in the device configuration of ITO/PEDOT:PSS/PFTBTN:PCBM/TiO_x/Al. Open-circuit voltage reached 0.74 V with the maximum energy conversion efficiency of 0.40% under the illumination of AM 1.5 (100 mW/cm²).     

Effect of thiophene donor units on the optical and photovoltaic behavior of fluorene-based copolymers

A series of novel conjugated polymers containing alternating electron-donating and electron-accepting units based on 9,9-dioctylfluorene, 4,7-dithienyl-2-yl-2,1,3-benzothiadiazole, and (oligo)thiophene were synthesized. The polymers were synthesized by the Suzuki cross-coupling polymerization of 9,9-dioctylfluorene-2,7-diboronic acid and 4,7-di(2-bromothien-5-yl)-2,1,3-benzothiadiazole with dibromo(oligo)thiophene (thiophene, bithiophene, and terthiophene). Optical properties of the copolymers were highly dependent on the length of the (oligo)thiophene unit. With the incorporation of three thiophene units in the polymer backbone, the copolymer absorption covers a broad range of the visible spectrum from 300 to 700 nm. The band gap energies derived from the absorption edge of the thin film spectra were in the range of 1.83-1.94 eV. The photovoltaic performance increases as the length of the (oligo)thiophene segments in the polymer backbone increases. The best performance of photovoltaic device was obtained by PFTBzTTT as the electron donor material with PCE of 1.25% under an AM 1.5 solar simulator.     

Synthesis and preliminary characterization of novel naphthalene bisimide based copolymers

Here we report the synthesis and preliminary characterization of three novel conjugated 2,6-(naphthalene-1,4,5,8-tetracarboxylic-N,N′-dialkyldiimide) based copolymers with π-conjugation throughout the main chain. Two (PNATF and PNTTOF) copolymers were polymerized through Suzuki type cross-coupling reactions, the other (PNATT3) via a Stille-type reaction. The modifications of the side chain on the naphthalene bisimide unit and the thiophene contents in the polymer backbone were investigated. The structures of these polymers were confirmed by NMR. PNATF and PNTTOF could be dissolved in common solvents such as toluene, CHCl₃ and 1,2-dichlorobenzene. The optical spectra of the three copolymers revealed that they had two strong absorption bands at 386–394 and 600–670 nm. The photoluminescence spectra of PNATF and PNTTOF showed a weak red emission. The band gap values of the copolymer films determined from electrochemical and optical measurements were in good agreement, ranging between 1.58–1.88 and 1.53–1.69 eV, respectively. From these data we suggest these three copolymers are worthy of further investigation as potential candidates for applications in electronic devices.     

Effect of trichlorobenzene additive on the performance and morphology of polyfluorene and fullerene derivative bulk heterojunction solar cells

The influence of additive, namely 1,3,5‐trichlorobenzene (TCB), on the morphology and performance of bulk heterojunction (BHJ) organic solar cells was studied based on a 1:2 (w/w) blend of benzothiadiazole/thiophene‐based copolymers (PFTBzTT) to [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM). The active layers were deposited by spin‐coating from solutions using chloroform, with different additive concentrations from 0 to 36 mg/ml. The addition of solvent additive into the polymer solution was able to improve the performance of BHJ solar cells. The maximum power conversion efficiency (%PCE) of 0.85% was obtained for a cell with the TCB concentration of 12 mg/ml after annealing at 180°C for 20 min. From atomic force microscopy (AFM) images, the films processed without TCB appear smoother than those with the TCB additive. A large extent of segregation was also observed in the films processed with a high TCB concentration. The optical images of the thin films suggest that the optimum concentrations of the additive cause the polymer self‐organisation, resulting in some aggregation of large PCBM crystals. © 2011 Canadian Society for Chemical Engineering     

Influence of SCN− moiety on CH3NH3PbI3 perovskite film properties and the performance of carbon-based hole-transport-layer-free perovskite solar cells

Journal of Materials Science: Materials in Electronics - CH3NH3PbI3 perovskite films were prepared via a hot-casting method using six different CH3NH3I, PbI2 and Pb(SCN)2 solutions. Surface...     

Effects of tetramethylene sulfone solvent additives on conductivity of PEDOT:PSS film and performance of polymer photovoltaic cells

A solvent additive in PEDOT:PSS solution is one of many methods to improve the conductivity of the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. We explore a new type of the solvent additive, namely tetramethylene sulfone (TMS), for the fabrication of the PEDOT:PSS conductive layer in the ITO/PEDOT:PSS/P3HT:PCBM/TiO_x/Al polymer photovoltaic cells, in comparison to a more common dimethyl sulfoxide (DMSO) solvent additive. At optimal conditions, the TMS additive at 10 wt.% has been found to enhance the conductivity of pristine PEDOT:PSS films from 0.04 S/cm up to approximately 189 S/cm, compared with the highest conductivity for the case of the DMSO additive at 15 wt.% of 117 S/cm. Possible mechanisms of this conductivity enhancement, relating to the polymer conformation and the film morphology, have been investigated by Raman spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The performance of the polymer photovoltaic cells fabricated with the solvent additives PEDOT:PSS films follows a similar trend to the conductivity of the films as a function of the additive concentration. The additives mainly lead to greater short circuit current density (J_sc) of the photovoltaic cells. The highest power conversion efficiency (PCE) of 2.24% of the device has been obtained with the 10 wt.% TMS additive of, compared to the PCE of 1.48% for the standard device without solvent additive.