Natasha A.D. Yamamoto

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.

The current-voltage characteristics of polymer/C60 diodes in the dark: A direct way to assess photovoltaic devices efficiency parameters

A general description of dark transport properties in bi-layer organic photovoltaic devices formed by a heterojunction of a semiconducting co-polymer and fullerene (C60) is presented. The copolymers are composed of thiophene, phenylene, and fluorene units, where the thiophene content is kept constant while the fluorene/phenylene ratio is varied. Measurements show that the j × V characteristics display typical diode behavior (exponential increasing) at low and are space-charge limited at high voltages. Extending a theoretical analysis by Koehler et al. [J. Appl. Phys. 92, 5575 (2002)], a model which assumes a space-charge dependent inner series resistance—attributed to molecular and morphological aspects of the materials—is proposed. It turns out to be general and able to nicely fit the experimental curves for all the studied samples. Furthermore, the model quantifies relevant parameters (the effective mobility and the diode reverse saturation current j0) which will determine the systems efficiency. The fr...

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.

Space-Charge-Limited Bipolar Currents at High Fields in Polymer/C60 Diodes: A Simple Model Description

We investigate the dark transport properties of organic photovoltaic devices formed by a heterojunction of a semiconducting co-polymer and the fullerene (C60) [. The copolymers are composed of thiophene, phenylene and fluorene units, where the thiophene content was kept constant while the fluorene/phenylene ratio was varied. Measurements show that for low voltage, the current versus voltage (j x V) characteristics presents an exponential increasing, typical of a diode behavior. On the other hand, the j x V curves are space-charge limited at high voltages. This latter response follows the theoretical predictions proposed by Koehler et al in Ref. [ (see also [), which explains the dark current in polymer/fullerene photodiodes at high Vs. To describe the full j x V characteristics, we have extended the analysis in Ref. [ assuming a space-charge dependent inner series resistance. From such model we are able to nicely fit the experimental curves, to estimate the effective mobility in the whole device and to obtain the diode reverse saturation current j0. Finally, the results indicate that the copolymer with the highest effective total mobility is the PFT [(9,9dihexyl-9H-fluorene-2,7-diyl)-1,2-ethenediyl-2,5-thiophene-1,2ethenediy.