Morphology and energy transfer study between conjugated polymers thin films: experimental and theoretical approaches.

In this paper, the effect of a silafluorene derivative copolymer, the poly[2,7-(9,9-dioctyl-dibenzosilole)-alt-4,7-bis(thiophene-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) sensitized by a simpler homopolymer, the poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) were investigated in a bilayer and ternary blend configuration. The energy transfer between the polymers prior to electron transfer to the acceptors can be an efficient alternative to photocurrent improvement in photovoltaic devices. The interactions between the two donor polymer films were evaluated optically and morphologically with several experimental techniques and correlated to the photovoltaic performance. Improved photon to charge conversion was observed in the blend films at different device geometries-considering bilayer devices with fullerene and inverted flexible devices blade coated in air conditions with a non-fullerene small molecule acceptor. Resonant Auger spectroscopy using the core-hole clock method was employed to evaluate the ultrafast charge delocalization times of conjugated polymers in the low-femtosecond regime. Density functional theory and time-dependent DFT methods were used to help understand some experimental observations. The results show that the homopolymer can improve the absorption spectra and the nonradiative-energy transfer from MDMO-PPV to PSiF-DBT and act as a photosensitizer in the copolymer units. In addition, the PSiF-DBT blended with MDMO-PPV exhibits a more organized structure than the neat material resulting in better absorption stability of films kept under continuous illumination.

Molecular-shape-controlled photovoltaic performance probed via soluble π-conjugated arylacetylenic semiconductors.

The synthesis and characterization of a new series of anthracene-based derivatives and their use as donors in bulk-heterojunction solar cells is reported. It is found that when using well-defined building blocks in constructing the chromophore, the donor molecular shape dramatically affects organic photovoltaic (OPV) performance in a previously unrecognized way.

Marked alkyl- vs alkenyl-substitutent effects on squaraine dye solid-state structure, carrier mobility, and bulk-heterojunction solar cell efficiency.

We report two new squaraine dyes substituted at the pyrrolic rings with n-hexyl (squaraine 1) or n-hexenyl (squaraine 2) chains. Although internal molecular structure variations are minimal, the presence of the terminal double bond results in a much more compact solid-state structure, dramatically affecting charge transport in the thin films; the hole mobility of 2 is approximately 5x that of 1, and the BHJ OPV power conversion efficiency (PCE) of 2 is approximately 2x that of 1. PCEs surpassing 2% for ambient solution-processed devices are demonstrated, the largest so far achieved for squaraine-based organic solar cells.