A Medium Bandgap D–A Copolymer Based on 4‑Alkyl-3,5-difluorophenyl Substituted Quinoxaline Unit for High Performance Solar Cells

Development of high-performance donor–acceptor (D–A) copolymers has been indicated as a promising strategy to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In this work, a new medium bandgap conjugated D–A copolymer, HFAQx-T, based on 4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT-T) as donor unit, 4-alkyl-3,5-difluorophenyl substituted quinoxaline (HFAQx) as the acceptor unit, and thiophene as the spacer, was designed and synthesized. HFAQx-T is a well-compatible donor polymer; OSCs based on HFAQx-T exhibit excellent performance in both fullerene and fullerene-free based devices. The optimized conventional single junction bulk heterojunction (BHJ) OSCs of HFAQx-T:PC₇₁BM showed a PCE of 9.2%, with an open circut voltage (<i>V</i>_oc) of 0.9 V, a short circuit current (<i>J</i>_sc) of 14.0 mA cm^–2, and a fill factor (FF) of 0.74. Also, when blended with 3,9-bis­(2-methylene-(3-(1,1-dicyano­methylene)­indanone)-5,5,11,11-tetrakis­(4-hexylphenyl)­dithieno­[2,3-<i>d</i>:2′,3′-<i>d</i>′]-<i>s</i>-indaceno­[1,2-<i>b</i>:5,6-<i>b</i>′]-dithiophene (ITIC), the HFAQx-T-based device exhibited a PCE of 9.6%. HFAQx-T is among a few D–A copolymers that can deliver >9% efficiency in both fullerene and fullerene-free solar cells. This work demonstrates that the 4-alkyl-3,5-difluorophenyl substituted quinoxaline (Qx) is a promising electron-accepting building block in constructing ideal D–A copolymers for OSCs

A Ladder-type Heteroheptacene 12<i>H</i>‑Dithieno[2′,3′:4,5]thieno[3,2‑<i>b</i>:2′,3′‑<i>h</i>]fluorene Based D‑A Copolymer with Strong Intermolecular Interactions toward Efficient Polymer Solar Cells

Ladder-type electron-donating units for D-A copolymers applied in polymer solar cells usually comprise multiple tetrahedral carbon bridges bonded with out-of-plane alkyl chains for desirable solubility for device processing. However, molecular packing of resultant copolymers in the solid state and charge transport within devices are also impeded in spite of with multiple fused aromatic backbones. To mitigate this issue, a structurally well-defined ladder-type electron-donating heteroheptacene, 12<i>H</i>-dithieno­[2′,3′:4,5]­thieno­[3,2-<i>b</i>:2′,3′-<i>h</i>]­fluorene (<b>DTTF</b>) with an extended conjugated backbone and a single tetrahedral carbon bridge attached with two bulky alkyl chains was designed and synthesized. The copolymerization of <b>DTTF</b> with 4,7-bis­(4-hexyl­thio­phen-2-yl)­benzo­[<i>c</i>]­[1,2,5]­thia­diazole (<b>DTBT</b>) afforded a soluble D-A copolymer (<b>PDTTF-DTBT</b>) with a medium optical band gap of 1.72 eV and low-lying HOMO level at −5.36 eV. <b>PDTTF-DTBT</b> unprecedentedly exhibits strong intermolecular stacking ability and presents preferential face-on orientation on both ZnO and PEDOT:PSS layers. The improved packing order and appropriate phase separation of both the copolymer and PC₇₁BM in the bulk heterojunction blend on the ZnO layer over on the PEDOT:PSS layer lead to much improved power conversion efficiency of ∼8.2% in the inverted solar cell device, among the highest for reported ladder-type D-A copolymers. The research demonstrates that it is an effective method to incorporate a single tetrahedral carbon bridge to the molecular center of a ladder-type heteroacene with heavily extended π-conjugation to prepare D-A copolymers toward highly efficient PSCs

Understanding Morphology Compatibility for High-Performance Ternary Organic Solar Cells

Ternary organic solar cells are emerging as a promising strategy to enhance device power conversion efficiency by broadening the range of light absorption via the incorporation of additional light-absorbing components. However, how to find compatible materials that allow comparable loadings of each component remains a challenge. In this article, we focus on studying the donor polymer compatibilities in ternary systems from a morphological point of view. Four typical donor polymers with different chemical structures and absorption ranges were mutually combined to form six distinct ternary systems with fullerene derivative acceptors. Two compatible ternary systems were identified as showing significant improvements of efficiency from both binary control devices. Ternary morphologies were characterized by grazing incident X-ray scattering and correlated with device performance. We find that polymers that have strong lamellar interactions and relatively similar phase separation behaviors with the fullerene derivative are more likely to be compatible in ternary systems. This result provides guidance for polymer selection for future ternary organic solar cell research while relaxing the limitation of chemical structure similarity and greatly extends the donor candidate pool

Medium-Bandgap Small-Molecule Donors Compatible with Both Fullerene and Nonfullerene Acceptors

Much effort has been devoted to the development of new donor materials for small-molecule organic solar cells due to their inherent advantages of well-defined molecular weight, easy purification, and good reproducibility in photovoltaic performance. Herein, we report two small-molecule donors that are compatible with both fullerene and nonfullerene acceptors. Both molecules consist of an (E)-1,2-di­(thiophen-2-yl)­ethane-substituted (TVT-substituted) benzo­[1,2-b:4,5-b′]­dithiophene (BDT) as the central unit, and two rhodanine units as the terminal electron-withdrawing groups. The central units are modified with either alkyl side chains (DRBDT-TVT) or alkylthio side chains (DRBDT-STVT). Both molecules exhibit a medium bandgap with complementary absorption and proper energy level offset with typical acceptors like PC₇₁BM and IDIC. The optimized devices show a decent power conversion efficiency (PCE) of 6.87% for small-molecule organic solar cells and 6.63% for nonfullerene all small-molecule organic solar cells. Our results reveal that rationally designed medium-bandgap small-molecule donors can be applied in high-performance small-molecule organic solar cells with different types of acceptors