Bulk heterojunction photovoltaics using broadly absorbing small molecules based on 2-styryl-5-phenylazo-pyrrole.

Three new soluble small molecules (B, B6, and A) with a low band gap based on 2-styryl-5-phenylazo-pyrrole were synthesized. Molecules B and B6 contained pyrrole and N-hexylpyrrole, respectively, as the central unit, which was connected to N,N-dimethylphenyl-4-azo on one side of the pyrrole molecule. Molecule A contained N-hexylpyrrole as the central unit, which was connected to anthracenyl-9-azo on one side of the pyrrole molecule. The other side of the pyrrole molecule was connected to cyanovinylene 4-nitrophenyl for all molecules. The long-wavelength absorption maximum of the molecules was located at 601-637 nm, and their optical band gap was 1.62-1.67 eV. The photovoltaic properties have been investigated using blends of B, B6, or A with PCBM, and it was found that the device based on A:PCBM had a higher power conversion efficiency (PCE) (2.06%) than the devices based on B:PCBM (1.33%) and B6:PCBM (1.36%). This has been attributed to the higher hole mobility, the lower band gap of A relative to that of B or B6, and the higher energy difference between the LUMO of A and PCBM. The effect of solvent annealing and thermal-solvent annealing on the photovoltaic response of the device based on the A:PCBM blend has been investigated, and it was found that the devices based on solvent-treated and subsequent thermally annealed blends have PCEs of 2.56 and 2.83%, respectively. The increase in the PCE has been attributed to the enhanced crystallinity of the blend and the improvement in the charge transport due to a reduction in the difference between the electron and hole mobility in the blend.

Synthesis of a low-band-gap small molecule based on acenaphthoquinoxaline for efficient bulk heterojunction solar cells.

A novel small molecule (SM) with a low-band-gap based on acenaphthoquinoxaline was synthesized and characterized. It was soluble in polar solvents such as N,N-dimethylformamide and dimethylacetamide. SM showed broad absorption curves in both solution and thin films with a long-wavelength maximum at 642 nm. The thin film absorption onset was located at 783 nm, which corresponds to an optical band gap of 1.59 eV. SM was blended with PCBM to study the donor-acceptor interactions in the blended film morphology and the photovoltaic response of the bulk heterojunction (BHJ) devices. The cyclic voltammetry measurements of the materials revealed that the HOMO and LUMO levels of SM are well aligned with those of PCBM, allowing efficient photoinduced charge transfer and suitable open circuit voltage, leading to overall power conversion efficiencies (PCEs) of approximately 2.21 and 3.23% for devices with the as-cast and thermally annealed blended layer, respectively. The increase in the PCE with the thermally annealed blend is mainly attributed to the improvement in incident photon to current efficiency (IPCE) and short circuit photocurrent (J(sc)). Thermal annealing leads to an increase in both the crystallinity of the blend and hole mobility, which improves the PCE.

Single-walled carbon nanotubes decorated with a pyrene-fluorenevinylene conjugate.

Single-walled carbon nanotubes are noncovalently functionalized using a pyrene-fluorenevinylene dye and the resulting nanohybrids are isolated from the free molecules. The tubes modified by means of this noncovalent approach show enhanced solubility in organic media. The structure and morphology of this hybrid material are fully characterized using absorption, infrared and Raman spectroscopies as well as atomic force and scanning electron microscopies. Steady state fluorescence measurements reveal that significant quenching of the pyrene derivative excited state takes place through an energy transfer mechanism.