P. Suresh

Efficient bulk heterojunction devices based on phenylenevinylene small molecule and perylene–pyrene bisimide

We report the fabrication and characterization of photovoltaic devices using a bulk heterojunction (BHJ) photoactive layer consisting of a small molecule (T), which contains a central p-phenylenevinylene unit, intermediate thiophene moieties, and terminal cyano-vinylene 4-nitrophenyls as the donor and a perylene–pyrene bisimide (PPI) as the acceptor. The difference in the LUMO levels (0.5 eV) of these materials is sufficient for the photoinduced charge transfer in the bulk heterojunction active layer. The optimum blend ratio (by weight) between T and PPI is 1 : 3.5 with a power conversion efficiency (PCE) of about 1.87%, beyond that the PCE starts to decrease. The incorporation of a thin ZnO layer between the organic BHJ layer and the top Al electrode increases the PCE to 2.46%, which is attributed to the enhanced light absorption due to the optical interference between incident light and reflected light from the Al electrode. It is also attributed to the improved electron transport in the device, since the conduction band of ZnO closely matches the work function of the Al electrode. The PCE is further increased to 3.17% when the device with the ZnO layer is annealed at a temperature of 100 °C for 5 min. This PCE is among the highest values reported to date for solar cells using solution processable small molecules.

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.

Novel p-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 approximately 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) cm(2)/(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 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 (J(sc)) and fill factor (FF) for solvent-treated and thermally annealed devices. These results indicate that the higher J(sc) 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 %.

Synthesis of perylene monoimide derivative and its use for quasi-solid-state dye-sensitized solar cells based on bare and modified nano-crystalline ZnO photoelectrodes

A novel perylene monoimide derivative, PCA, with a strongly electron-donating cyclohexylimide segment, bulky alkylphenoxy substituents at the 1,7-bay positions of the perylene core and an acid anhydride as the strong coupling group was synthesized and used as sensitizer for dye-sensitized solar cells (DSSCs). PCA was readily soluble in common organic solvents, decomposed above 390 °C and had glass transition temperature of 64 °C. The absorption curves had maximum at 505–512 nm, with thin film absorption onset at 643 nm corresponding to an optical band gap of 1.93 eV. We have fabricated the quasi-solid-state DSSCs with ZnO nano-crystalline films deposited from different values of pH. It is found that the power conversion efficiency (PCE) is high when the pH value of sol gel is about 9. The PCE of DSSC fabricated with the gold-coated ZnO photoanode is higher than that for bare ZnO photoanode. This enhancement is attributed to the formation of Schottky barrier at the Au/ZnO interface that blocks the electron transfer from ZnO to dye and electrolyte, increasing the electron density in the conduction band of ZnO. The incorporation of TiO2 nanoparticles in polymer gel electrolyte further increases the PCE of DSSC up to 3.0%. The enhancement is primarily explained by studying the dark reaction, diffusion coefficient of tri-iodide and exchange current density in the interface of electrolyte/Pt counter electrolyte, and lifetime of electrons in the anode film.