Andreia G. Macedo

Hole mobility effect in the efficiency of bilayer heterojunction polymer/C60 photovoltaic cells

We report here bilayer heterojunction solar cells fabricated by using poly[9,9′-hexyl-fluorene-alt-bithiophene] (LaPPS43) polymer as active layer. The power conversion efficiency (η) displays a sevenfold increase upon annealing at 200 °C, reaching the value of 2.8%. This result is comparable to the highest η reported so far for bulk heterojunction solar cells using the same polymer. Simulation, absorbance spectra, and current versus voltage results indicate that the π–π stacking in solid state is enhanced after annealing with a reduction in traps and thus reflecting in higher hole mobility.

Annealing effect on donor-acceptor interface and its impact on the performance of organic photovoltaic devices based on PSiF-DBT copolymer and C60

In this work, poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) was used as active layer in bilayer solar cell with C60 as electron acceptor. As cast devices already show reasonable power conversion efficiency (PCE) that increases to 4% upon annealing at 100 °C. Space charge limited measurements of the hole mobility (μ) in PSiF-DBT give μ ∼ 1.0 × 10−4 cm2/(V s) which does not depend on the temperature of the annealing treatment. Moreover, positron annihilation spectroscopy experiments revealed that PSiF-DBT films are well stacked even without the thermal treatment. The variations in the transport of holes upon annealing are then small. As a consequence, the PCE rise was mainly induced by the increase of the polymer surface roughness that leads to a more effective interface for exciton dissociation at the PSiF-DBT/fullerene heterojunction.

Thickness Effect on F8T2/C60 Bilayer Photovoltaic Devices

Copolymers based on fluorene-thiophene units have presented promising efficiencies in photovoltaic devices applications. They present good transport properties mainly after thermal treatment of the polymer films. Here, we investigate the properties of bilayer devices formed by the heterojunction of the polymer F8T2 with variable thickness and the fullerene. The series resistance of the equivalent circuit associated with the device increases as the polymer film gets thicker. The current-voltage characteristics of the bilayer devices follow the Mott-Gurney law of SCLC. For the best performing device we measured 2.1% of power conversion efficiency.