In an atempt to meet the demand of energy producton from clean, renewable sources, organic materials and conjugated polymers have been investgated for use in organic photovoltaics (OPVs). The use of organic and polymeric materials is expected to provide a number of advantages in terms of low-cost fabricaton, fexible devices, and light-weight constructon.1,2 Together with power conversion efciency (PCE), long-term stability of OPV devices has been recently recognized as an important area of research, both in academia and industry.In partcular, chemical degradaton of the materials consttutng the actve layer has been shown to detrimentally afect device operaton. In additon to that, the formaton and preservaton of an optmized bulk heterojuncton (BHJ) morphology within the actve layer has proven to be critcal for sustaining high OPV performance. Indeed, most BHJ systems show poor stability and ofen undergo macrophase segregaton of the blend components, especially afer prolonged exposure to heat.3 Herein, we report a study on thermal stability of OPV devices based on donor-acceptor copolymers. Photocrosslinkable derivatves of a thieno[3,4-c]pyrrole-4,6-dione (TPD)-based polymer4,5 were developed, containing TPD repeat units with a terminal, primary bromide functonality appended to the octyl solubilizing group (TPD-Br), thereby allowing for photocrosslinking of the polymer in devices. By synthetcally tuning the amount of Br-units in the polymer and by employing UV-mediated photocrosslinking, it was possible to fabricate OPV devices with high PCE and excellent thermal stability. Devices employing copolymers with varying amounts of TPD-Br units were tested. In contrast to the sharp decrease in PCE observed for non-photocrosslinked devices afer annealing, devices containing optmal photocrosslinked polymer in the actve layer demonstrated remarkable long-term thermal stability. Afer 72 h of thermal annealing at 150 °C, an average PCE of 4.6% was obtained with a short circuit current density (JSC) of 10.1 mA/cm2, an open circuit voltage (VOC) of 0.85 V, and a fll factor (FF) of 54%. The peak PCE obtained was as high as 4.7%. In additon, the choice of fullerene acceptor in the BHJ was found to be critcal not only for achieving high PCE but also for stabilizing the morphology of the actve layer at high temperature. Atomic force microscopy (AFM) was employed to investgate the surface morphology of the actve layer in both crosslinked and non-crosslinked flms before and afer annealing. The AFM study well correlates with the photovoltaic behaviour of devices.
|Titolo:||Thermally stable high-efficiency polymer solar cells based on photocrosslinkable donor-acceptor conjugated polymers|
|Autori interni:||GRIFFINI, GIANMARCO ENRICO|
|Data di pubblicazione:||2011|
|Appare nelle tipologie:||04.2 Abstract in Atti di convegno|
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