Photovoltaic Blend Microstructure for High Efficiency Post-Fullerene Solar Cells. To Tilt or Not To Tilt?

Achieving efficient polymer solar cells (PSCs) requires a structurally optimal donor-acceptor heterojunction morphology. Here we report the combined experimental and theoretical characterization of a benzodithiophene-benzothiadiazole donor polymer series (PBTZF4-R; R = alkyl substituent) blended with the non-fullerene acceptor ITIC-Th and analyze the effects of substituent dimensions on blend morphology, charge transport, carrier dynamics, and PSC metrics. Varying substituent dimensions has a pronounced effect on the blend morphology with a direct link between domain purity, to some extent domain dimensions, and charge generation and collection. The polymer with the smallest alkyl substituent yields the highest PSC power conversion efficiency (PCE, 11%), reflecting relatively small, high-purity domains and possibly benefiting from "matched" donor polymer-small molecule acceptor orientations. The distinctive morphologies arising from the substituents are investigated using molecular dynamics (MD) simulations which reveal that substituent dimensions dictate a well-defined set of polymer conformations, in turn driving chain aggregation and, ultimately, the various film morphologies and mixing with acceptor small molecules. A straightforward energetic parameter explains the experimental polymer domain morphological trends, hence PCE, and suggests strategies for substituent selection to optimize PSC materials morphologies.

Hole-Transfer Dependence on Blend Morphology and Energy Level Alignment in Polymer: ITIC Photovoltaic Materials

Bulk-heterojunction organic photovoltaic materials containing nonfullerene acceptors (NFAs) have seen remarkable advances in the past year, finally surpassing fullerenes in performance. Indeed, acceptors based on indacenodithiophene (IDT) have become synonymous with high power conversion efficiencies (PCEs). Nevertheless, NFAs have yet to achieve fill factors (FFs) comparable to those of the highest-performing fullerene-based materials. To address this seeming anomaly, this study examines a high efficiency IDT-based acceptor, ITIC, paired with three donor polymers known to achieve high FFs with fullerenes, PTPD3T, PBTI3T, and PBTSA3T. Excellent PCEs up to 8.43% are achieved from PTPD3T:ITIC blends, reflecting good charge transport, optimal morphology, and efficient ITIC to PTPD3T hole-transfer, as observed by femtosecond transient absorption spectroscopy. Hole-transfer is observed from ITIC to PBTI3T and PBTSA3T, but less efficiently, reflecting measurably inferior morphology and nonoptimal energy level alignment, resulting in PCEs of 5.34% and 4.65%, respectively. This work demonstrates the importance of proper morphology and kinetics of ITIC → donor polymer hole-transfer in boosting the performance of polymer:ITIC photovoltaic bulk heterojunction blends.

Tin-Free Direct C-H Arylation Polymerization for High Photovoltaic Efficiency Conjugated Copolymers

A new and highly regioselective direct C-H arylation polymerization (DARP) methodology enables the reproducible and sustainable synthesis of high-performance π-conjugated photovoltaic copolymers. Unlike traditional Stille polycondensation methods for producing photovoltaic copolymers, this DARP protocol eliminates the need for environmentally harmful, toxic organotin compounds. This DARP protocol employs low loadings of commercially available catalyst components, Pd₂(dba)₃·CHCl₃ (0.5 mol%) and P(2-MeOPh)₃ (2 mol%), sterically tuned carboxylic acid additives, and an environmentally friendly solvent, 2-methyltetrahydrofuran. Using this DARP protocol, several representative copolymers are synthesized in excellent yields and high molecular masses. The DARP-derived copolymers are benchmarked versus Stille-derived counterparts by close comparison of optical, NMR spectroscopic, and electrochemical properties, all of which indicate great chemical similarity and no significant detectable structural defects in the DARP copolymers. The DARP- and Stille-derived copolymer and fullerene blend microstructural properties and morphologies are characterized with AFM, TEM, and XRD and are found to be virtually indistinguishable. Likewise, the charge generation, recombination, and transport characteristics of the fullerene blend films are found to be identical. For the first time, polymer solar cells fabricated using DARP-derived copolymers exhibit solar cell performances rivalling or exceeding those achieved with Stille-derived materials. For the DARP copolymer PBDTT-FTTE, the power conversion efficiency of 8.4% is a record for a DARP copolymer.

Radicinin from Cochliobolus sp. inhibits Xylella fastidiosa, the causal agent of Pierce's Disease of grapevine

The fastidious phytopathogenic bacterium, Xylella fastidiosa, poses a substantial threat to many economically important crops, causing devastating diseases including Pierce's Disease of grapevine. Grapevines (Vitis vinifera L.) planted in an area under Pierce's Disease pressure often display differences in disease severity and symptom expression, with apparently healthy vines growing alongside the dying ones, despite the fact that all the vines are genetic clones of one another. Under the hypothesis that endophytic microbes might be responsible for this non-genetic resistance to X. fastidiosa, endophytic fungi were isolated from vineyard cvs. 'Chardonnay' and 'Cabernet Sauvignon' grown under high Pierce's Disease pressure. A Cochliobolus sp. isolated from a Cabernet Sauvignon grapevine inhibited the growth of X. fastidiosa in vitro. Bioassay-guided isolation of an organic extract of Cochliobolus sp. yielded the natural product radicinin as the major active compound. Radicinin also inhibited proteases isolated from the culture supernatant of X. fastidiosa. In order to assess structure-activity relationships, three semi-synthetic derivatives of radicinin were prepared and tested for activity against X. fastidiosa in vitro. Assay results of these derivatives are consistent with enzyme inactivation by conjugate addition to carbon-10 of radicinin, as proposed previously.