Synthesis of CulnS2, CulnSe2, and Cu(InxGa(1-x))Se2 (CIGS) nanocrystal "inks" for printable photovoltaics.

Chalcopyrite copper indium sulfide (CuInS2) and copper indium gallium selenide (Cu(InxGa(1-x))-Se2; CIGS) nanocrystals ranging from approximately 5 to approximately 25 nm in diameter were synthesized by arrested precipitation in solution. The In/Ga ratio in the CIGS nanocrystals could be controlled by varying the In/Ga reactant ratio in the reaction, and the optical properties of the CulnS2 and CIGS nanocrystals correspond to those of the respective bulk materials. Using methods developed to produce uniform, crack-free micrometer-thick films, CulnSe2 nanocrystals were tested in prototype photovoltaic devices. As a proof-of-concept, the nanocrystal-based devices exhibited a reproducible photovoltaic response.

Tuning interfacial charge-transfer excitons at polymer-polymer heterojunctions under hydrostatic pressure.

We present photoluminescence spectroscopy of blend thin films of poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) with electron-donating copolymers under hydrostatic pressure. The photoluminescence spectrum of F8BT redshifts by 530+/-60 meV over pressures of 0.1 MPa-8.8 GPa. That of the interfacial charge-transfer exciton redshifts by 270-370 meV over a similar pressure range. Despite the relatively small shift of the charge-transfer exciton, the excited state dynamics at the molecular heterojunction are strongly enhanced due to the decreasing intermolecular distance, leading to favored exciplex emission.

Optical spectroscopy of a polyfluorene copolymer at high pressure: intra- and intermolecular interactions.

We present optical spectroscopy studies of the conjugated polymer poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) at high pressure. The photoluminescence spectrum of F8BT in a dilute solid-state solution in polystyrene redshifts by 320 meV over 7.4 GPa, while that of a F8BT thin film redshifts 460 meV over a comparable pressure range. We attribute the redshift in solution to intrachain pressure effects, principally conformational planarization. The additional contribution from interchain pi-electron interactions accounts for the larger redshift of thin films.

Control of morphology in efficient photovoltaic diodes from discotic liquid crystals.

We report on the role of morphology in photovoltaic diodes with blend active layers composed of perylene tetracarboxdiimide (EPPTC) and hexabenzocoronene (HBC) derivatives as electron and hole acceptors. Controlled annealing of HBC:EPPTC films while in conformal contact with a flat elastomeric stamp improves photovoltaic response, leading to an external quantum efficiency of 29.5% at 460 nm and an open-circuit voltage of 0.70 V. The improved performance is attributed in part to larger crystalline domains following annealing. The elastomeric stamp restricts the top surface of the thin film during annealing, leading to low surface roughness, while also allowing for greater vertical stratification of the components in the bulk. Blended HBC:EPPTC films also exhibit an unique optical absorption feature near 590 nm, which we attribute to an altered crystalline packing of EPPTC in the presence of HBC. The significance of the local structure at molecular heterojunctions in blended active layer photovoltaic diodes is discussed.