Trifluoromethylation in the Design and Synthesis of High-Performance Wide Bandgap Polymer Donors for Quasiplanar Heterojunction Organic Solar Cells.

New strategies for the molecular design to construct efficient electron-deficient units for D-A-type donor copolymers are urgently needed. Halogenation of electron-deficient units (A) has been shown to be the most effective strategy reported to date with which to produce high-performance donor polymers. Herein, we have constructed two different trifluoromethyl-substituted polymer donors, PBQP-CF3 and PBQ-CF3. The trifluoromethylation process typically involves complex protocols, which are not widely used in the synthesis of polymer donors. Accordingly, we have developed a single-step, one-pot synthesis of the new trifluoromethyl-substituted electron-deficient unit (A) of PBQ-CF3. The strong electron-withdrawing ability of the trifluoromethyl group ensures deeper highest occupied molecular orbital (HOMO) energy levels, and the non-covalent bonding interactions of the fluorine atoms are beneficial to the regulation of aggregation properties. Thus, both of the trifluoromethyl-substituted polymer donors obtained much higher power conversion efficiency (PCE) than PBDP-H (6.66%). PBQ-CF3 exhibits a deeper HOMO energy level, better aggregation behavior, and higher hole mobility than PBQP-CF3. PBQ-CF3-based quasiplanar heterojunction (Q-PHJ) devices therefore achieve simultaneously enhanced open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) and an impressive PCE (16.02%), which is much higher than that obtained by PBQP-CF3-based devices (12.57%). This work reveals a promising path to synthesis of the trifluoromethylation polymer donors and demonstrates that the trifluoromethylation strategy can be used to enhance the photovoltaic performance.

A Reversible Structural Phase Transition by Electrochemically-Driven Ion Injection into a Conjugated Polymer.

We find that conjugated polymers can undergo reversible structural phase transitions during electrochemical oxidation and ion injection. We study poly[2,5-bis(thiophenyl)-1,4-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG), a conjugated polymer with glycolated side chains. Using grazing incidence wide-angle X-ray scattering (GIWAXS), we show that, in contrast to previously known polymers, this polymer switches between two structurally distinct crystalline phases associated with electrochemical oxidation/reduction in an aqueous electrolyte. Importantly, we show that this unique phase change behavior has important physical consequences for ion-polaron pair transport. Notably, using moving front experiments visualized by both optical microscopy and super-resolution photoinduced force microscopy (PiFM), we show that a laterally propagating ion-polaron pair front in PB2T-TEG exhibits non-Fickian transport, retaining a sharp step-edge profile, in stark contrast to the Fickian diffusion more commonly observed in polymers like P3MEEMT. This structural phase transition is reminiscent of those accompanying ion uptake in inorganic materials like LiFePO4. We propose that the engineering of similar properties in future conjugated polymers may enable the realization of new materials with superior performance in electrochemical energy storage or neuromorphic memory applications.

Influence of Bridging Groups on the Photovoltaic Properties of Wide-Bandgap Poly(BDTT- alt-BDD)s.

To further advance polymer solar cells requires the fast evolution of π-conjugated materials as well as a better understanding of their structure-property relationships. Herein, we present three copolymers (PT1, PT2, PT3) made through tuning π-bridges (without any group, thiophene, and 3-hexylthieno[3,2- b]thiophene) between electron-rich (D: BDTT) and -deficient (A: BDD) units. The comparative studies reveal the unique correlation that the tune of π-bridge on the polymeric backbone governs the solid stacking and photovoltaic properties of resultant poly(BDTT- alt-BDD)s, which provide an effective way to deliver new and efficient polymer with feasible processability. That is, polymers with either twist zigzag backbone (PT1) or with linear coplanar backbone (PT2) result in inferior photovoltaic performance upon simple solution casting. Among them, PT3 with extended zigzag backbone and planar segments exhibits suitable processability and retains good efficiency in nonfullerene solar cells through a single-solvent cast without involving tedious treatments. This work illustrates that the tuning of the D-π-A polymer backbone facilitates efficient materials with feasible processability, promising for scale-up fabrication.

Optimal management of on-farm resources in small-scale dairy systems of Central Mexico: model development and evaluation.

This study evaluates the available on-farm resources of five case studies typified as small-scale dairy systems in central Mexico. A comprehensive mixed-integer linear programming model was developed and applied to two case studies. The optimal plan suggested the following: (1) instruction and utilization of maize silage, (2) alfalfa hay making that added US$140/ha/cut to the total net income, (3) allocation of land to cultivated pastures in a ratio of 27:41(cultivated pastures/maize crop) rather than at the current 14:69, and dairy cattle should graze 12 h/day, (4) to avoid grazing of communal pastures because this activity represented an opportunity cost of family labor that reduced the farm net income, and (5) that the highest farm net income was obtained when liquid milk and yogurt sales were included in the optimal plan. In the context of small-scale dairy systems of central Mexico, the optimal plan would need to be implemented gradually to enable farmers to develop required skills and to change management strategies from reliance on forage and purchased concentrate to pasture-based and conserved forage systems.

Farm and socio-economic characteristics of smallholder milk producers and their influence on technology adoption in Central Mexico.

In order to identify the factors influencing adoption of technologies promoted by government to small-scale dairy farmers in the highlands of central Mexico, a field survey was conducted. A total of 115 farmers were grouped through cluster analysis (CA) and divided into three wealth status categories (high, medium and low) using wealth ranking. Chi-square analysis was used to examine the association of wealth status with technology adoption. Four groups of farms were differentiated in terms of farms' dimensions, farmers' education, sources of incomes, wealth status, management of herd, monetary support by government and technological availability. Statistical differences (p < 0.05) were observed in the milk yield per herd per year among groups. Government organizations (GO) participated little in the promotion of the 17 technologies identified, six of which focused on crop or forage production and 11 of which were related to animal husbandry. Relatives and other farmers played an important role in knowledge diffusion and technology adoption. Although wealth status had a significant association (p < 0.05) with adoption, other factors including importance of the technology to farmers, usefulness and productive benefits of innovations together with farmers' knowledge of them, were important. It is concluded that the analysis of the information per group and wealth status was useful to identify suitable crop or forage related and animal husbandry technologies per group and wealth status of farmers. Therefore the characterizations of farmers could provide a useful starting point for the design and delivery of more appropriate and effective extension.