Thermally stable high-efficiency polymer solar cells based on photocrosslinkable donor-acceptor conjugated polymers

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

Long-term Thermal Stability of High-Efficiency Polymer Solar Cells Based on Photocrosslinkable Donor-Acceptor Conjugated Polymers

Recently, the long-term stability of organic photovoltaic (OPV) devices has been recognized as an important area of research. Here, we report the first study on the thermal stability of OPV devices based on a donor-acceptor p-type copolymer. We have synthesized a photocrosslinkable version of a thieno[3,4-c]pyrrole-4,6-dione (TPD)-based polymer by incorporating TPD monomers with a bromide-functionalized side chain. By synthetically tuning the amount of Br-units in the polymer and by employing UV-light-mediated photocrosslinking, we were able to fabricate OPV devices with stable power conversion efficiencies (PCE) as high as 4.7% after 72 hours of thermal annealing at 150°C. In contrast, without complete photocrosslinking, the polymer device performances deteriorated upon annealing, showing no thermal stability. This study demonstrates the highest device PCE reported for OPV systems subjected to long-term thermal annealing at high temperatures

Quinacridone-Based Molecular Donors for Solution Processed Bulk-Heterojunction Organic Solar Cells

New soluble quinacridone-based molecules have been developed as electron donor materials for solution-processed organic solar cells. By functionalizing the pristine pigment core of quinacridone with solubilizing alkyl chains and light absorbing/charge transporting thiophene units, i.e., bithiophene (BT) and thienylbenzo[c][1,2,5]thiadiazolethienyl (BTD), we prepared a series of multifunctional quinacridone-based molecules. These molecular donors show intense absorption in the visible spectral region, and the absorption range and intensity are well-tuned by the interaction between the quinacridone core and the incorporated thiophene units. The thin film absorption edge extends with the expansion of molecular conjugation, i.e., 552 nm for N,N'-di(2-ethylhexyl)quinacridone (QA), 592 nm for 2,9-Bis(5'-hexy1-2,2'-bithiophene)-N,N'-di(2-ethylhexyl)quinacridone (QA-BT), and 637 nm for 4-(5hexylthiophen-2-y1)-7-(thiophen-2-Abenzo[c][1,2,5]chiadiazole (QA-BTD). The change of molecular structure also influences the electrochemical properties. Observed from cyclic voltammetry measurements, the oxidation and reduction potentials (vs ferrocene) are 0.7 and 1.83 V for QA, 0.54 and 1.76 V for QA-BT, and 0.45 and 1.68 V for QA-BTD. Uniform thin films can be generated from both single component molecular solutions and blend solutions of these molecules with [6,61-phenyl C70-butyric acid methyl ester (PC70BM). The blend films exhibit space-charge limited current (SCLC) hole mobilities on the order of 1 x 10(-4) cm(2) V-1 s(-1) Bulk heterojunction (BHJ) solar cells using these soluble molecules as donors and PC70BM as the acceptor were fabricated. Power conversion efficiencies (PCEs) of up to 2.22% under AM 1.5 G simulated 1 sun solar illumination have been achieved and external quantum efficiencies (EQEs) reach as high as similar to 45%