Electrochemical and optical studies of the band gaps of alternating polyfluorene copolymers

The electrochem. and optical properties of alternating polyfluorene copolymers with low band gaps were detd. These polymers incorporated fluorene units alternating with groups including electron-withdrawing (A) and electron-donating (D) groups in donor-acceptor-donor (DAD) sequence to achieve the lowering of band gaps. The polymers were solvent-casted on Pt disk electrode and the band gaps were estd. from cyclic voltammetry (CV). These values were compared with values obtained from optical absorption measurements. Although the electrochem. detd. band gaps are slightly higher than the optical band gap in most cases, values are well correlated. The values of the band gaps detd. range from 2.1 to 1.3 eV. [on SciFinder (R)

Theoretical models and experimental results on the temperature dependence of polyfluorene solar cells

The temp. dependence of device parameters of polymer solar cells based on alternating copolyfluorene incorporating dioctyl-fluorene and di-thienyl-benzothiadiazole (APFO-3) mixed with [6,6]-phenyl-C61-butyric acid methylester (PCBM) was modeled theor. by simulation and investigated exptl. under illumination of AM1.5 (100 mW/cm2). Both simulation and exptl. results show photocurrent, fill factor and power conversion efficiency all increase, and the open-circuit voltages monotonically decrease with temp. increase from room temp. (RT) to 120 DegC. These results can be explained by taking into account the temp. dependence of the mobility, and the thermal activation of the injection current from the electrodes. The increase of PCE with temp. is a distinguishing feature of polymer solar cells. [on SciFinder (R)

Theoretical models and experimental results on the temperature dependence of polyfluorene solar cells

The temp. dependence of device parameters of polymer solar cells based on alternating copolyfluorene incorporating dioctyl-fluorene and di-thienyl-benzothiadiazole (APFO-3) mixed with [6,6]-phenyl-C61-butyric acid methylester (PCBM) was modeled theor. by simulation and investigated exptl. under illumination of AM1.5 (100 mW/cm2). Both simulation and exptl. results show photocurrent, fill factor and power conversion efficiency all increase, and the open-circuit voltages monotonically decrease with temp. increase from room temp. (RT) to 120 DegC. These results can be explained by taking into account the temp. dependence of the mobility, and the thermal activation of the injection current from the electrodes. The increase of PCE with temp. is a distinguishing feature of polymer solar cells. [on SciFinder (R)

An alternating low band-gap polyfluorene for optoelectronic devices

An alternating polyfluorene (APFO) with low band-gap segments. APFO-Green1, was synthesized for use in optoelectronic devices. The low band-gap segment consists of an electron acceptor (A), fenced by electron donors (D). This D-A-D configuration leads to partial charge transfer in the polymer backbone and a low band-gap of 1.3 eV. Characterization of APFO-Green1 include measurement of light absorption and emission at extended wavelengths and high hole mobility was found. Blends of the polymer with different fullerene derivs. exhibit unusually high photovoltaic performance at long wavelengths, making this type of conjugated polymers promising for application in plastic solar cells. [on SciFinder (R)

New low band gap alternating polyfluorene copolymer-based photovoltaic cells

New low band gap alternating polyfluorene copolymers were synthesized for use in plastic solar cells and their optical, electrochem., and photovoltaic characteristics were detd. These polymers incorporated fluorene units alternating with groups including electron-withdrawing (A) and electron-donating (D) groups in donor-acceptor-donor (DAD) sequence to achieve the lowering of band gaps. The HOMO-LUMO values were estd. from electrochem. studies. By varying the donor and acceptor strength and position of the solubilizing substituents, similar HOMO values were obtained. These values were also found to correlate well with the open circuit voltage (VOC) values detd. from photovoltaic data of the polymers blended with the acceptor PCBM. Despite similar HOMO values, the absorption spectra of the polymers differ significantly. This prompted the prepn. of photovoltaic devices consisting of blends of two polymers with complementary absorptions in combination with PCBM to harvest more photons in the polymer solar cells. [on SciFinder (R)

An alternating low band-gap polyfluorene for optoelectronic devices

An alternating polyfluorene (APFO) with low band-gap segments. APFO-Green1, was synthesized for use in optoelectronic devices. The low band-gap segment consists of an electron acceptor (A), fenced by electron donors (D). This D-A-D configuration leads to partial charge transfer in the polymer backbone and a low band-gap of 1.3 eV. Characterization of APFO-Green1 include measurement of light absorption and emission at extended wavelengths and high hole mobility was found. Blends of the polymer with different fullerene derivs. exhibit unusually high photovoltaic performance at long wavelengths, making this type of conjugated polymers promising for application in plastic solar cells. [on SciFinder (R)

New low band gap alternating polyfluorene copolymer-based photovoltaic cells

New low band gap alternating polyfluorene copolymers were synthesized for use in plastic solar cells and their optical, electrochem., and photovoltaic characteristics were detd. These polymers incorporated fluorene units alternating with groups including electron-withdrawing (A) and electron-donating (D) groups in donor-acceptor-donor (DAD) sequence to achieve the lowering of band gaps. The HOMO-LUMO values were estd. from electrochem. studies. By varying the donor and acceptor strength and position of the solubilizing substituents, similar HOMO values were obtained. These values were also found to correlate well with the open circuit voltage (VOC) values detd. from photovoltaic data of the polymers blended with the acceptor PCBM. Despite similar HOMO values, the absorption spectra of the polymers differ significantly. This prompted the prepn. of photovoltaic devices consisting of blends of two polymers with complementary absorptions in combination with PCBM to harvest more photons in the polymer solar cells. [on SciFinder (R)

Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors

The photovoltaic characteristics of solar cells based on alternating polyfluorene copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4\u27,7\u27-di-2-thienyl-2\u27,1\u27,3\u27-benzothiadiazole)) (APFO-3), and poly(2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4\u27,7\u27-di-2-thienyl-2\u27,1\u27,3\u27-benzothiadiazole)) (APFO-4), blended with an electron acceptor fullerene mol. [6,6]-phenyl-C61-butyric acid Me ester (PCBM), have been investigated and compared. The two copolymers have the same arom. backbone structure but differ by the length of their alkyl side chain. The overall photovoltaic performance of the solar cells is comparable irresp. of the copolymer used in the active layer. However, the fill factor (FF) values of the devices are strongly affected by the copolymer type. Higher FF values were realized in solar cells with APFO-4 (with longer alkyl side chain)/PCBM bulk heterojunction active layer. On the other hand, devices with blends of APFO-3/APFO-4/PCBM were found to render fill factor values that are intermediate between the values obtained in solar cells with APFO-3/PCBM and APFO-4/PCBM active film. Upon using APFO-3/APFO-4 blends as electron donors, the cell efficiency can be enhanced by about 16% as compared to cells with either APFO-3 or APFO-4. The transport of holes in each polymer obeys the model of hopping transport in disordered media. However, the degree of energetic barrier against hopping was found to be larger in APFO-3. The tuning of the photovoltaic parameters will be discussed based on studies of hole transport in the pure polymer films, and morphol. of blend layers. The effect of bipolar transport in PCBM will also be discussed. [on SciFinder (R)

A polymer photodiode using vapour-phase polymerized PEDOT as an anode

We report the photovoltaic properties of devices made using a highly conducting polymer electrode, from vapor-phase polymd. poly (3,4-ethylenedioxy) thiophene (VPP PEDOT) on glass substrate as an anode and a polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4\u27,7\u27-di-2thienyl-2\u27,1\u273\u27-benzothiadiazole)] (APFO-3) mixed with [6,6]-phenyl-C61-butyric acid methylester (PCBM) in the ratio of 1:4 as the active layer. The device performance was compared with that of devices made with PEDOT-PSS on glass substrates. The surfaces of VPP PEDOT were imaged using at. force microscopy (AFM). [on SciFinder (R)

Polymer solar cells based on a low-bandgap fluorene copolymer and a fullerene derivative with photocurrent extended to 850 nm

Polymer solar cells were fabricated from a low band-gap alternating polyfluorene copolymer, APFO-Green2, combined with [6,6]-phenyl-C61-butyric acid Me ester (PCBM), from org. solns. External quantum efficiencies of the solar cells show an onset at 850 nm and a peak of >10% located at 650 nm, which corresponds to the extended absorption spectrum of the polymer. A photocurrent of 3.0 mA/cm2, photovoltage of 0.78 V, and power conversion efficiency of 0.9% were obtained with solar cells based on this new low-bandgap polymer under an illumination of AM 1.5 (1000 W/m2) from a solar simulator. [on SciFinder (R)