Oligo(ethylene glycol) Side Chain Architecture Enables Alcohol-Processable Conjugated Polymers for Organic Solar Cells

Abstract

Achieving green-solvent solubility of conjugated polymers in truly green solvents such as alcohols has proven to be a significant challenge. In this work, we report the synthesis and characterization of three conjugated polymers derived from poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)] (PTQ10) with the goal of developing derivates which are more green-solvent-processable. The traditional alkyl side chains are replaced by various oligo(ethylene glycol) (OEG) side chains of different architectures, including one linear and two branched, all of which contain six ethylene glycol repeating units. It is determined that the linear OEG side chain architecture, even when sufficiently long, will not give desired green-solvent solubility shown by a small solubility capacity (R 0). However, branched OEG side chains significantly improve solubility as R 0 was increased from 4.7 of PTQ10 to 11.9 of PTQ-6bO/6bO2. Although the solution states of the polymers were vastly different, the solid-state morphologies were more similar as all three OEG-based polymers retained a predominately face-on molecular orientation similar to PTQ10. It was demonstrated that PTQ-6O devices showed the most comparable power conversion efficiencies (PCEs) to PTQ10 in bulk heterojunction solar cells, while PTQ-6bO2 and PTQ-6bO showed poorer performances. With one extra carbon in the side chain, PTQ-6bO2 showed higher PCE than PTQ-6bO, attributed to the improved aggregation properties and solid-state morphology of PTQ-6bO2, highlighting the importance of OEG side chain architecture. This work serves to develop important guidelines for future alcohol-soluble materials for green-solvent-processed OPVs.

Aqueous Processed All-Polymer Solar Cells with High Open-Circuit Voltage Based on Low-Cost Thiophene–Quinoxaline Polymers

“…More specifically, the band gap of P­(Qx8O-T) was found to be much narrower than that of PTQ10 with up-shifted E HOMO and down-shifted E LUMO . This trend in energy level is commonly observed in the literature for many OEG-containing polymers and their alkylated analogues, but a clear explanation for these differences is still missing. , One possible explanation is that the electronegative oxygen atoms in the OEG-based materials draw some electron density through the inductive effect, leading to slightly deeper E LUMO but potentially contradicting the destabilization of E HOMO in these materials. To further understand this, we optimized the molecular geometry of trimeric donors and acceptor materials with both the OEG and alkyl side chains using DFT calculations at the HSE03/6-311G­(d,p) theory level.…”

“…Recently, another simple Qx-thiophene-based polymer, PTQ10, was further developed, which has become one of the best donors in this class of polymers and exhibited a comparable PCE in OSCs with that of the state-of-the-art donor PM6. , It is worth noting that PTQ10 has a lower-lying HOMO energy level (∼−5.5 eV) than most polymer donors, such as P3HT (∼−5.1 eV), PPDT2FBT (∼−5.4 eV), PTB7-Th (∼−5.2 eV), and PBDB-T (∼−5.3 eV), which is beneficial for producing high V OC . In addition, You et al . , reduced the cost of synthesizing PTQ10 further by employing the Mitsunobu reaction while maintaining the synthetic simplicity. Thus, we envisioned that employing the Qx-based backbone would provide a great opportunity for the development of new aqueous-soluble polymer donors.…”

The challenge with high permittivity acceptors in organic solar cells: a case study with Y-series derivatives

“…A promising example has been recently shown by Neu et al , who modified the popular polymer PTQ10 with glycol side chains to obtain an alcohol-processable donor. 51 An alternative solution is to focus on other strategies for the permittivity-modification – such as expanding the π-systems or incorporating functional groups polarizable at higher frequencies (listed in more detail in the Introduction) – which entail less complex system changes.…”

Green‐Processed Non‐Fullerene Organic Solar Cells Based on Y‐Series Acceptors

“…However, replacing alkyl side chains with alkyloxy units can greatly improve the δ P and δ H values, which allowed PTQ‐6bO and PTQ‐6bO2 polymers to be dissolved in green solvents like alcohol. [ 26 ] Despite this progress, only a few studies have explored the relationship between chemical structures and HSPs of conjugated materials. More studies in this area can aid in the development of new conjugated materials that are amenable to green processing.…”