Maxime Defour

The Influence of Conjugated Polymer Side Chain Manipulation on the Efficiency and Stability of Polymer Solar Cells

The stability of polymer solar cells (PSCs) can be influenced by the introduction of particular moieties on the conjugated polymer side chains. In this study, two series of donor-acceptor copolymers, based on bis(thienyl)dialkoxybenzene donor and benzo[c][1,2,5]thiadiazole (BT) or thiazolo[5,4-d]thiazole (TzTz) acceptor units, were selected toward effective device scalability by roll-coating. The influence of the partial exchange (5% or 10%) of the solubilizing 2-hexyldecyloxy by alternative 2-phenylethoxy groups on efficiency and stability was investigated. With an increasing 2-phenylethoxy ratio, a decrease in solar cell efficiency was observed for the BT-based series, whereas the efficiencies for the devices based on the TzTz polymers remained approximately the same. The photochemical degradation rate for PSCs based on the TzTz polymers decreased with an increasing 2-phenylethoxy ratio. Lifetime studies under constant sun irradiance showed a diminishing initial degradation rate for the BT-based devices upon including the alternative side chains, whereas the (more stable) TzTz-based devices degraded at a faster rate from the start of the experiment upon partly exchanging the side chains. No clear trends in the degradation behavior, linked to the copolymer structural changes, could be established at this point, evidencing the complex interplay of events determining PSCs’ lifetime.

Synthesis of ester side chain functionalized all-conjugated diblock copolythiophenes via the Rieke method

Ester-functionalized all-conjugated diblock copolythiophenes are synthesized by an optimized Rieke zinc protocol. These light-harvesting semiconducting electron donor polymers are of high interest in the field of organic photovoltaics as both the block architecture and the (functionalized) side chains play a crucial role in determining the bulk heterojunction active layer blend (nano)morphology and thereby the final polymer solar cell efficiency and lifetime.

Influence of the amorphous phase and preceding solution processing on the eutectic behaviour in the state diagram of P3HT : PC61BM determined by rapid heat–cool calorimetry

Rapid heat–cool calorimetry was used to determine the state diagram of P3HT:PC61BM mixtures. A particular focus was given to the determination of the eutectic composition and temperature. P3HT was fractionated in three different narrow molecular weight fractions (14.9 kg mol−1, 32.4 kg mol−1, 89.1 kg mol−1) in order to study its effect on the eutectic behaviour. Heating rates of 500 K min−1 were utilised to drastically decrease or suppress cold crystallization of the mixtures, especially around the eutectic composition. This led to a more accurate determination of the state diagram. An important decrease of crystallization kinetics was observed around the eutectic composition, making an accurate determination difficult. This was shown to be due to the melting (and crystallization) temperature depression around the eutectic composition coupled with the vitrification of PC61BM at 139 °C. These findings highlight the importance of the amorphous phase for the morphology development of the active layer in organic photovoltaics. By changing the preceding sample solution preparation and by using dedicated cooling and heating rates, the eutectic composition was found between 50–55 wt% PC61BM for 14.9 kg mol−1 P3HT, 55–60 wt% PC61BM for 32.4 kg mol−1 P3HT, and 55–65 wt% PC61BM for 89.1 kg mol−1 P3HT. The eutectic onset temperature of melting was found to be around 140 °C for the different P3HT molecular weight fractions, which is close to the glass transition of pure PC61BM.

A stability study of polymer solar cells using conjugated polymers with different donor or acceptor side chain patterns

Improvement of the power conversion efficiency and long term stability remains to be of crucial importance for the further development of polymer solar cells (PSCs). Herein, a donor–acceptor copolymer based on 4,8-di(thiophene-2′-yl)benzo[1,2-b:4,5-b′]dithiophene (DTBDT) and 4,7-di(thiophene-2′-yl)benzo[c][1,2,5]thiadiazole (DTBT), specifically selected because of its suitability for roll-coating in the ambient environment, is investigated in terms of operational stability via partial exchange (5 or 10%) of the alkyl side chain on either the donor or the acceptor monomer with a 2-hydroxyethyl or 2-phenylethyl group. It is shown that the exchange of the hexyl chain on the DTBT moiety has a negative impact on the stability of the polymer as well as on the performance of the resulting PSCs. On the other hand, partial exchange of the 2-hexyldecyl side chain of the BDT unit by a 2-hydroxyethyl group results in an improved photochemical stability of the polymer film and a higher efficiency of 5.6% for the spin-coated PSC. The stability of roll-coated devices also slightly increases with the incorporation of 10% of either the 2-hydroxyethyl or 2-phenylethyl side chain.