Synthesis of Diketopyrrolopyrrole Containing Copolymers: A Study of Their Optical and Photovoltaic Properties

The diketopyrrolopyrrole-based copolymers PDPP-BBT and TDPP-BBT were synthesized and used as a donor for bulk heterojunction photovoltaic devices. The photophysical properties of these polymers showed absorption in the range 500−600 nm with a maximum peak around 563 nm, while TDPP-BBT showed broadband absorption in the range 620 − 800 nm with a peak around 656 nm. The power conversion efficiencies (PCE) of the polymer solar cells based on these copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were 0.68% (as cast PDPP-BBT:PCBM), 1.51% (annealed PDPP-BBT:PCBM), 1.57% (as cast TDPP-BBT:PCBM), and 2.78% (annealed TDPP-BBT:PCBM), under illumination of AM 1.5 (100 mW/cm2). The higher PCE for TDPP-BBT-based polymer solar cells has been attributed to the low band gap of this copolymer as compared to PDPP-BBT, which increases the numbers of photogenerated excitons and corresponding photocurrent of the device. These results indicate that PDPP-BBT and TDPP-BBT act as excellent electron donors for bulk heterojunction devices

Novel Low Band Gap Small Molecule and Phenylenevinylene Copolymer with Cyanovinylene 4-Nitrophenyl Segments: Synthesis and Application for Efficient Bulk Heterojunction Solar Cells

A novel star-shaped small monomer SM containing a 1,3,5-triazine core and arms with terminal cyanovinylene 4-nitrophenyls was synthesized. Moreover, an alternating p-phenylenevinylene copolymer P containing thiophene with cyanovinylene 4-nitrophenyl side segments was synthesized by Heck coupling. Both SM and P showed broad absorption spectra with long-wavelength maximum at 630−648 nm, which for P is attributable to an intramolecular charge transfer. The optical band gap was 1.57 eV for SM and 1.70 eV for P. Both SM and P were blended with PCBM to study the donor−acceptor interactions on the blend film morphology and device characteristics of organic bulk heterojunction solar cells. A combination of characterization techniques including X-ray diffraction and optical topographical images were used to investigate the film morphology. The HOMO and LUMO levels of both SM and P are well-aligned with those of the PCBM acceptor, allowing efficient electron transfer and suitable open circuit voltage, leading to overall power conversion efficiencies (PCEs) of 2.53 and 1.43% for SM:PCBM and P:PCBM-based devices, respectively. The thermal annealing leads to suitable phase separation due to the increase in crystallinity of donor material and material distribution so that highly effective bulk heterojunction morphologies are obtained which further increases the PCE up to 3.82% and 2.37% for SM:PCBM and P:PCBM-based devices, respectively. These results are preliminary based on the illumination without using a solar simulator

Effect of Solvent and Subsequent Thermal Annealing on the Performance of Phenylenevinylene Copolymer:PCBM Solar Cells

The morphology of the photoactive layer used in the bulk heterojunction photovoltaic devices is crucial for efficient charge generation and their collection at the electrodes. We investigated the solvent vapor annealing and thermal annealing effect of an alternating phenylenevinylene copolymer P:PCBM blend on its morphology and optical properties. The UV−visible absorption spectroscopy shows that both solvent and thermal annealing can result in self-assembling of copolymer P to form an ordered structure, leading to enhanced absorption in the red region and hole transport enhancement. By combining the solvent and thermal annealing of the devices, the power conversion efficiency is improved. This feature was attributed to the fact that the PCBM molecules begin to diffuse into aggregates and together with the ordered copolymer P phase form bicontinuous pathways in the entire layer for efficient charge separation and transport. Furthermore, the measured photocurrent also suggests that the space charges no longer limit the values of the short circuit current (Jsc) and fill factor (FF) for solvent-treated and thermally annealed devices. These results indicate that the higher Jsc and PCE for the solvent-treated and thermally annealed devices can be attributed to the phase separation of active layers, which leads to a balanced carrier mobility. The overall PCE of the device based on the combination of solvent annealing and thermal annealing is about 3.7 %

Novel <i>p</i>-Phenylenevinylene Compounds Containing Thiophene or Anthracene Moieties and Cyano−Vinylene Bonds for Photovoltaic Applications

Two novel soluble compounds T and A that contain a central dihexyloxy-p-phenylenevinylene unit, intermediate moieties of thiophene or anthracene, respectively, and terminal cyano−vinylene nitrophenyls were synthesized and characterized. They showed moderate thermal stability and relatively low glass transition temperatures. These compounds displayed similar optical properties. Their absorption was broad and extended up to about 750 nm with the longer-wavelength maximum around 640 nm and an optical band gap of ∼1.70 eV. From the current−voltage characteristics of the devices using both compounds T and A, it was concluded that both compounds behave as p-type organic semiconductors with hole mobility on the order of 10−5 cm2/(V s). The power conversion efficiency (PCE) of the devices based on these compounds was 0.019% and 0.013% for compounds A and T, respectively. When compounds A and T were blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the PCE dramatically increased up to 1.66% and 1.36% for devices with A:PCBM and T:PCBM, respectively. The efficiencies of the devices were further enhanced upon thermal annealing up to 2.49% and 2.33% for devices based on A:PCBM and T:PCBM, respectively

Effect of the Incorporation of a Low-Band-Gap Small Molecule in a Conjugated Vinylene Copolymer: PCBM Blend for Organic Photovoltaic Devices

The effect of the incorporation of a low-band-gap small-molecule BTD-TNP on the photovoltaic properties of vinylene copolymer P:PCBM bulk heterojunction solar cells has been investigated. The introduction of this small molecule increases both the short-circuit photocurrent and the overall power conversion efficiency of the photovoltaic device. The incident photon-to-current efficiency (IPCE) of the device based on P:PCBM:BTD-TNP shows two distinct bands, which correspond to the absorption bands of P:PCBM and BTD-TNP. Furthermore, it was found that the IPCE of the device has also been enhanced even at the wavelengths corresponding to the absorption band of P:PCBM, when the thermally annealed blend was used in the device. This indicates that the excitons that are generated in copolymer P are dissociated into charge carriers more effectively in the presence of the BTD-TNP small molecule at the copolymer P:PCBM interface by energy transfer from P to the small molecule. Therefore, we conclude that the BTD-TNP small molecule acts as light-harvesting photosensitizer and also provides a path for the generated exciton in copolymer P toward the P:PCBM interface for efficient charge separation. The overall power conversion efficiency for the P:PCBM:BTD-TNP photovoltaic device is about 1.27%, which has been further enhanced up to 2.6%, when a thermally annealed blend layer is used

New Triphenylamine-Based Organic Dyes with Different Numbers of Anchoring Groups for Dye-Sensitized Solar Cells

We synthesized two organic dyes (<b>TPA–CN1–R2</b> and <b>TPA–CN2–R1</b>) based on the TPA core unit having structure A–D–A, which contain the triphenylamine moiety as an electron donor and both cyanovinylene 4-nitrophenyl and carboxylic (anchoring) units as electron acceptors. Nanocrystalline TiO₂-based dye-sensitized solar cells (DSSCs) were fabricated using these dyes to investigate the effect of number of anchoring groups on their photovoltaic performance. The DSSCs based on <b>TPA–CN1–R2</b> and <b>TPA–CN2–R1</b> showed power conversion efficiency (PCE) of about 2.36% and 1.41%, respectively. The PCE has been significantly improved up to 4.37% and 2.8%, upon addition of 20 mM deoxycholic acid (DCA) to the dye solution for TiO₂ sensitization. Coadsorption of DCA decreased dye coverage but significantly improved the value of the short-circuit photocurrent (<i>J</i>_sc). The breakup of π-stacked aggregates might improve the electron injection yield and thus <i>J</i>_sc. Electrochemical impedance spectra and current–voltage characteristics in the dark indicate that the electron lifetime was improved by coadsorption of DCA, accounting for the significant improvement of open-circuit voltage (<i>V</i>_oc)