No abstract

A new tetracyclic lactam building block for polymer semiconductors is reported that was designed to combine the many favorable properties that larger fused and/or amide-containing building blocks can induce, including improved solid-state packing, high charge carrier mobility, and improved charge separation. Copolymerization with thiophene resulted in a semicrystalline conjugated polymer, PTNT, with a broad bandgap of 2.2 eV. Grazing incidence wide-angle X-ray scattering of PTNT thin films revealed a strong tendency for face-on πstacking of the polymer backbone, which was retained in PTNT:fullerene blends. Corresponding solar cells featured a high open-circuit voltage of 0.9 V, a fill factor around 0.6, and a power conversion efficiency as high as 5% for >200 nm thick active layers, regardless of variations in blend stoichiometry and nanostructure. Moreover, efficiencies of >4% could be retained when thick active layers of ∼400 nm were employed. Overall, these values are the highest reported for a conjugated polymer with such a broad bandgap and are unprecedented in materials for tandem and particularly ternary blend photovoltaics. Hence, the newly developed tetracyclic lactam unit has significant potential as a conjugated building block in future organic electronic materials.

light-harvesting active layer is composed of bulk-heterojunctions, i.e., blends of a polymeric electron-donor (hole-conductor) and an electron-acceptor (electron-conductor), with fullerene derivatives yielding particularly promising results. Thanks to recent advances in the synthesis of donor materials, power-conversion effi ciencies of 8-10% can now be achieved with lab-scale devices. [ 1 ] The precise bulk-heterojunction nanostructure, i.e., the distribution of donor and acceptor molecules, is crucial for maximizing the photovoltaic performance for a given blend composition. This is because a compromise has to be made between two critical aspects: i) a large contact area between donor and acceptor molecules aids charge generation and thus a fi nely intermixed blend is favored, and ii) percolation of separated, relatively phase-pure donor and acceptor domains to improve charge transport to the electrodes. Therefore, the ideal nanostructure features an intermediate degree of phase separation, which has to be carefully optimized through processing parameters such as the choice of solvent or solvent mixture, the blend stoichiometry and polymer molecular weight as well as post-deposition thermal or vapor annealing.The bulk-heterojunction nanostructure of non-crystalline polymer:fullerene blends has the tendency to rapidly coarsen when heated above its glass transition temperature, which represents an important degradation mechanism. We demonstrate that fullerene nucleating agents can be used to thermally arrest the nanostructure of photovoltaic blends that comprise a non-crystalline thiophene-quinoxaline copolymer and the widely used fullerene derivative [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). To this end, C 60 fullerene is employed to effi ciently nucleate PCBM crystallization. Sub-micrometersized fullerene crystals are formed when as little as 2 wt% C 60 with respect to PCBM is added to the blend. These reach an average size of only 200 nanometers upon introduction of more than 8 wt% C 60 . Solar cells based on C 60 -nucleated blends indicate signifi cantly improved thermal stability of the bulk-heterojunction nanostructure even after annealing at an elevated temperature of 130 °C, which lies above the glass transition temperature of the blend. Moreover, we fi nd that various other compounds, including C 70 fullerene, single-walled carbon nanotubes, and sodium benzoate, as well as a number of commercial nucleating agents-commonly used to clarify isotactic polypropylene-permit to control crystallization of the fullerene phase.

Thermal annealing of non-crystalline polymer:fullerene blends typically results in a drastic decrease in solar cell performance. In particular aggressive annealing above the glass transition temperature results in a detrimental coarsening of the blend nanostructure. We demonstrate that mild annealing below the glass transition temperature is a viable avenue to control the nanostructure of a non-crystalline thiophenequinoxaline copolymer:fullerene blend. Direct imaging methods indicate that coarsening of the blend nanostructure can be avoided. However, a combination of absorption and luminescence spectroscopy reveals that local changes in the polymer conformation as well as limited fullerene aggregation are permitted to occur. As a result, we are able to optimise the solar cell performance evenly across different positions of the coated area, which is a necessary criterion for large-scale, high throughput production.

Printing of polymer:fullerene solar cells at high speed requires annealing at temperatures up to 140 °C. However, bulk-heterojunction blends that comprise a non-crystalline donor polymer often suffer from insufficient thermal stability and hence rapidly coarsen upon annealing above the glass transition temperature of the blend. In addition, micrometer-sized fullerene crystals grow, which are detrimental for the solar cell performance. In this manuscript, we present a strategy to limit fullerene crystallization, which is based on the use of fullerene mixtures of the two most common derivatives, PC61BM and PC71BM, as the acceptor material. Blends of this fullerene mixture and a non-crystalline thiophene-quinoxaline copolymer display considerably enhanced thermal stability and largely retain their photovoltaic performance upon annealing at elevated temperatures as high as 170 °C.