• Energy Research

For the first time, formamidinium–caesium perovskite thin films were successfully synthesised from a lead acetate-based precursor. Efficient perovskite solar cells (21.0%) and modules (18.8%) have been produced using blade coating techniques.

High charge carrier mobility in organic semiconductors is usually accomplished by its high crystallinity, yet, which, often leads to poor mechanical resilience, limiting their performance in flexible devices. In this work, a naphtho­[1,2- c :5,6- c ′]­bis­[1,2,5]­thiadiazole (NT)-based polymer PNztDTV containing multiple vinyl linkages was reported, exhibiting excellent ambipolar OFET performance, with hole and electron mobility up to 1.0 (μ h ) and 0.4 (μ e ) cm 2 V –1 s –1 , respectively. The GIWAXS analysis revealed overall a rather weak crystallinity in its thin film, yet with considerably strong localized π–π intermolecular interaction. The simultaneous low crystallinity and high performance of PNztDTV imply promising potential in flexible electronics, which is presumably resulted by the isomerization of the vinylene linkage and π-expanded NT core.

The charge transport–limiting factors in conjugated polymers without single-bond linkages in the backbone have been identified.

Enhanced scattering contrast afforded by resonant soft X-ray scattering (R-SoXS) is used to probe the nanomorphology of all-polymer solar cells based on blends of the donor polymer poly(3-hexylthiophene) (P3HT) with either the acceptor polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (F8TBT) or poly([N,N'-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-12-bithiophene)) (P(NDI2OD-T2)). Both P3HT:F8TBT and P3HT:P(NDI2OD-T2) blends processed from chloroform with subsequent annealing exhibit complicated morphologies with a hierarchy of phase separation. A bimodal distribution of domain sizes is observed for P3HT:P(NDI2OD-T2) blends with small domains of size ~5-10 nm that evolve with annealing and larger domains of size ~100 nm that are insensitive to annealing. P3HT:F8TBT blends in contrast show a broader distribution of domain size but with the majority of this blend structured on the 10 nm length scale. For both P3HT:P(NDI2OD-T2) and P3HT:F8TBT blends, an evolution in device performance is observed that is correlated with a coarsening and purification of domains on the 5-10 nm length scale. Grazing-incidence wide-angle X-ray scattering (GI-WAXS) is also employed to probe material crystallinity, revealing P(NDI2OD-T2) crystallites 25-40 nm in thickness that are embedded in the larger domains observed by R-SoXS. A higher degree of P3HT crystallinity is also observed in blends with P(NDI2OD-T2) compared to F8TBT with the propensity of the polymers to crystallize in P3HT:P(NDI2OD-T2) blends hindering the structuring of morphology on the sub-10 nm length scale. This work also underscores the complementarity of R-SoXS and GI-WAXS, with R-SoXS measuring the size of compositionally distinguishable domains and GI-WAXS providing information regarding crystallinity and crystallite thickness.

AbstractFilms of the fullerene derivatives [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) are patterned on silicon nitride membranes using photolithography to study, with X‐ray spectromicroscopy, the lateral, solid‐state diffusion of fullerene derivatives into conjugated polymer films. After patterning of the fullerene film, a film of conjugated polymer is laminated on top and the structure is annealed in order to study lateral intermixing and facilitate measurement of fullerene miscibility. Lateral intermixing of polymer and fullerene readily occurs for poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) and regiorandom poly(3‐hexylthiophene) (RRa‐P3HT). A 42 wt.% miscibility of PC61BM in PBTTT is measured, while miscibilities of 20 and 41 wt.% are measured for PC61BM and PC71BM, respectively, in RRa‐P3HT, thereby demonstrating a significant difference in the miscibilities of these two fullerene derivatives. For regioregular poly(3‐hexylthiophene) (RR‐P3HT), incomplete lateral intermixing of fullerene and RR‐P3HT is observed with PCBM crystallite formation competing with the lateral diffusion of PCBM molecules into the polymer film.

Control of domain purity in polymer/fullerene solar cells is realised through controlling the time that an anti-solvent treatment is applied.

A scalable, two-step synthesis facilitates the preparation of a polymer library of varying side chain and co-monomer, enabling rapid photovoltaic device screening. FO6-T emerged as the optimal donor achieving 15.4% PCE with L8BO as the acceptor.

The blending of two semiconducting polymers with offset energy levels enables efficient charge generation in thin-film ‘all-polymer’ solar cells. A key requirement for efficient charge separation and collection is the formation of interconnected phase-separated domains structured on the sub-20 nm length-scale. This review provides an overview of recent advances in the characterisation of conjugated polymer blend nanostructure and developments in the linking of blend structure and device performance. This review also provides a general introduction to the polymer physics behind phase separation, experimental techniques used for characterising blend structure and novel ways to control nanomorphology.