Alkoxyphenylthiophene Linked Benzodithiophene Based Medium Band Gap Polymers for Organic Photovoltaics: Efficiency Improvement upon Methanol Treatment Depends on the Planarity of Backbone

Two donor–acceptor (D–A) medium band gap polymers, <b>P1</b> and <b>P2</b>, alkoxyphenylthiophene (APTh) linked benzodithiophene (BDT) as an electron-rich unit and 1,3-di­(2′-bromothien-5′-yl)-5-(2-ethylhexyl)­thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione (TPD) (<b>A1</b>) or [5,6-bis­(octyloxy)-4,7-di­(thiophen-2-yl)­benzo­[<i>c</i>]­[1,2,5]­thiadiazole] (BT) (<b>A2</b>) as an electron-deficient unit, have successfully been synthesized via microwave-assisted Stille polymerization and utilized for bulk heterojunction (BHJ) polymer solar cells (PSCs). <b>P1</b> shows a well-distinguished absorption shoulder between 590 and 620 nm attributed to the π–π stacking of a polymer backbone; such kind of absorption shoulder is not observed in <b>P2</b>, indicating that the <b>P1</b> has more planar structure than that of <b>P2</b>. This is due to the fact that the sulfur atom of thiophene spacer and the oxygen atom of carbonyl groups in TPD have more pronounced intramolecular noncovalent interactions (INCI) in <b>P1</b> than that of the sulfur atom of thiophene spacer and the oxygen atom of alkoxy groups of BT in <b>P2</b>. The bulk heterojunction polymer solar cells (BHJ PSCs) were fabricated with the configuration of ITO/PEDOT:PSS/polymer (<b>P1</b> or <b>P2</b>):PC<sub>71</sub>BM/LiF/Al. The <b>P1</b> device shows better photovoltaic performance with open-circuit voltage (<i>V</i><sub>oc</sub>) of 0.91 V and the power conversion efficiency (PCE) of 4.19% than the <b>P2</b> device (<i>V</i><sub>oc</sub>: 0.71 V; PCE: 1.88%) in neat blend films under the illumination of AM 1.5G (100 mW/cm<sup>2</sup>). Upon treating the active layers containing <b>P1</b> and <b>P2</b> with methanol, the PCE of the <b>P1</b> device is increased from 4.19 to 7.14%. In contrast, the PCE of the <b>P2</b> device is decreased from 1.88 to 1.82%. Space charge limited current mobility, atomic force microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and impedance spectroscopy studies strongly support the enhanced PCE for the <b>P1</b> device is attributed to the increased mobility, nanoscale morphology, and reduced resistance upon methanol treatment; these favorable properties for the <b>P1</b> polymer are highly correlated with the planarity of the backbone.