Organic semiconductors have emerged as a fascinating class of materials with significant implications for numerous scientific disciplines, including electronics, photonics, and energy conversion. These materials, composed of carbon-based molecules or polymers, offer remarkable flexibility, tunability, and processability compared to their inorganic counterparts . Charge transport in organic semiconductors is a fundamental aspect that governs the performance and functionality of various organic semiconductor devices, such as organic solar cells (OSCs), organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and bio/chemo-sensing devices. The movement of charge carriers through these materials occurs via a complex interplay of electronic, structural, and energetic phenomena, presenting intriguing challenges and opportunities for scientific exploration .

Figure 1: Structures of some of the most versatile Qx scaffolds; dashed lines indicate the substitution sites for core expansion.

The utilization of Qxs as a charge transporting material, whether as a hole transport material (HTM) or an electron transport material (ETM), is largely dependent on its functionalization. Although the Qx material has primarily been recognized for its effectiveness in hole transport, several studies have unveiled its significant potential as an ETM , exhibiting desirable characteristics such as high electron mobility and efficient charge transfer. In particular, Qx derivatives find use as non-fullerene acceptors (NFAs) in OSCs and as essential building blocks in sensitizers for DSSCs. The significance of Qx extends beyond to thermally activated delayed fluorescence (TADF) emitters and chromophores in the development of organic light-emitting diodes (OLEDs), sensors, and electrochromic devices .

In this review, we have comprehensively examined the recent advancements of Qx-derived ETMs in various applications within the organic semiconductor device field over the past five to six years. Furthermore, we have briefly discussed the integration of Qx derivatives into relevant materials to enhance electron transport. The review also sheds light on future research directions and potential challenges in this area, emphasizing the importance of further exploration and innovation. Overall, this review presents a first detailed account of the electron transport properties of Qx derivatives in recent times, offering valuable insights into their potential as promising ETMs.

The development of efficient and high-performing polymer materials based on Qxs is of significant interest in the field of organic electronics. A compelling indication of this potential is the remarkable achievements made by a relatively simple polymer, poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)] (PTQ10). PTQ10 has demonstrated impressive power conversion efficiencies (PCEs) of over 12% in polymer solar cells (PCS) when paired with the IDT acceptor , over 16% with the Y6 acceptor and a champion PCE of 21.2% in perovskite solar cells . This outstanding performance is attributed to the versatile and tunable nature of the Qx moiety, wherein researchers substituted alkoxy chains to enhance solubility and difluoro groups to lower the highest occupied molecular orbital (HOMO) energy level.

Figure 2: Qx-derived polymer acceptors.

You and co-workers demonstrated the effectiveness of introducing electron-withdrawing cyanide (CN) groups at the 6- and 7-positions of the Qx moiety, named QxCN to address various concerns related to charge transport. The copolymers, Qx2, Qx3, and Qx4, formed by combining QxCN with different aryl groups showed down-shifting of lowest unoccupied molecular orbital (LUMO) levels and enhanced electron injection and transport. Furthermore, the dipole moment introduced by the CN groups improved charge separation by reducing Coulomb attraction and exciton binding energy. The resulting enhancement in exciton dissociation and reduced charge recombination contribute to the improved performance of polymer acceptors, especially Qx2 which achieved PCE of 5.32% with PBDB-T donor in an all-PSC device . In a recent study by Eedugurala et al., a terachlorobenzene ring were fused to the [1,2,5]thiadiazolo[3,4-g]quinoxaline unit in the polymer backbone of Qx6 which enhanced the stability of the polymer and led to its high-spin configuration compared to the analogous material Qx5 featuring 6,7-dimethyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline in its polymer backbone. The transition from a closed-shell aromatic state to a high-spin quinoidal form resulted in favorable changes in the bandgap, electron affinity, and delocalization of spin density. These changes have the potential to improve charge transport and efficient charge separation in all-PSCs such as switching from p-type dominated behaviour of polymer Qx5 to n-type dominated behaviour of Q6, with no off state due to presence of free charge carriers in the latter case .

Besides optimizing polymer structures, side chain engineering, introduction of electron-withdrawing terminal acceptor units, and careful selection of solvents and annealing processes have also been demonstrated as potential solutions to improve charge transport and refine the device fabrication processes. You et al. found that the position of alkoxy side chains on the pendant benzene rings significantly influenced the performance of Qx2 acceptors. Three variants of Qx2, i.e., Qx7 and Qx8 were synthesized with alkoxy side chains located at the meta and para positions of the pendant benzene rings. Qx7 exhibited efficient exciton dissociation, good electron-transporting ability, and a PCE of 5.07% in an all-PSC device with the PBDB-T donor, whereas Qx8 showed poor charge transport, severe charge recombination, and a PCE of 1.62%. This highlighted the significance of side chain engineering in achieving high-performance polymer acceptors .

In addition to the importance of side-chain modification, the study by Zhou et al. showcases the importance of solvent choice and annealing techniques in optimizing the performance of all-PSCs using Qx9 and Qx10. While both polymers served as donors in all-PSC devices, the study primarily focused on side chain engineering of the electron-deficient Qx unit. The combination of thermal annealing treatment and the use of THF as a non-halogenated solvent led to improvements in photovoltaic performance and charge carrier transport. Additionally, the impact of side chain modification on device characteristics, such as lower HOMO and higher circuit voltage (Voc), underscored the influence of the molecular structure .

A recent development by Liang et al. introduced Qx-derived double-cable conjugated polymers as a promising approach for improving the performance of single-component-OSCs (SCOSCs). They replaced the traditional benzothiadiazole core of Y-series acceptor with the Qx moiety (Y-O6) and copolymerized it with the PBDB-T donor in two ratios to give Qx11a and Qx11b. This approach enhanced charge transport and nanophase separation, resulting in a record PCE of 13.02% in an SCOSC device made of Qx11b with diluted YO6 component .

Table 1: Photovoltaic performance of Qx-derived polymer acceptors in PSCs.

Figure 3: Qx-derived small molecule NFAs.

Small molecule NFAs have significantly advanced the field of OSCs. Notably, the fused-ring electron acceptors (FREAs) have exhibited exceptional promise, heralding a new era of possibilities for OSC technology. In 2019, Yuan et al. reported on a new class of FREAs called Y6. Y6 utilizes a ladder-type electron-deficient core-based central fused ring (dithienothiophen[3.2-b]pyrrolobenzothiadiazole) and achieved a remarkable efficiency of 16% in OSCs . Building upon this breakthrough, Liu et al. blended Y6 with their polymer D18, resulting in an efficiency of 18% and marking a significant advancement in OSC research .

Zhou and co-workers synthesized Y6-type NFA acceptors, Qx12 and Qx13, by substituting Y6’s benzothiadiazole ring with a Qx moiety. Qx13 exhibited a stronger π–π interaction compared to Qx12, which facilitated enhanced electron hopping and reduced geminate recombination. Qx12 and Qx13 achieved remarkable PCEs of 13.31% and 16.64%, respectively, with PBDB-TF donor in OSC devices . Zhu et al. reported a modification in Qx13 by incorporating an imide-functionalized Qx moiety in its core and end-capping groups with fluorinated or chlorinated compounds, producing Qx14 and Qx15, respectively. This modification aimed to improve the device performance by enhancing aggregation control and optimizing the open-circuit voltage. The introduction of functional groups provided a strategic means to tailor the molecular structure, resulting in improved photovoltaic properties and overall PCEs (12.12–13.3%) . Researchers have explored Y6 derivatives for hydrogen production. Zhang and co-workers synthesized a two-dimensional polycyclic material by merging two Y6 molecules with a Qx unit. They blended the newly formed compound with the donor polymer PM6 to create BHJ nanoparticles and employed it in the hydrogen evolution reaction. This approach substantially reduced trap density, increasing the hydrogen evolution rate by 2–3 times compared to conventional inorganic/organic hybrid photocatalysts .

Computational chemistry offers a cost-effective and time-efficient means of screening and selecting promising candidates for experimental exploration. Bhattacharya et al. employed density functional theory (DFT) approach to explore structural modulation for tuning the optoelectronic properties of Qx13. Their designed molecule series, Qx16, featured a 1,4-dihydro-2,3-quinoxalinedione core and different terminal acceptor units. The modified NFAs demonstrated visible and near-infrared absorption as well as good electron mobility, suggesting their potential for experimental exploration in (OCSs) .

While FREAs are currently at the forefront and delivering great PCE, unfused electron acceptors have continued to garner considerable attention. Chang and co-workers focused on enhancing the aggregation and crystallinity of unfused Qx acceptors, Qx17 and Qx18, containing a Qx core and different halogenated end groups. Both compounds exhibited good coplanarity through intramolecular interactions, narrow bandgaps, broad absorption in the NIR region and PCEs above 10% . Ayub et al. decorated the central donor–acceptor–donor unit of Qx17 and Qx18 with five new terminal end groups, resulting in the Qx19 series, and predicted their optoelectronic properties to highlight the potential of their experimental exploration .

Figure 4: Qx-derived small molecule NFAs.

Xiao and fellows studied the impact of side chains on molecular packing and morphology on Qx26 and Qx27, employing indacenodithiophene, Qx and rhodanine as donor, acceptor and end group, respectively. The incorporation of specific side chains facilitated improved thermal stability, solubility, and broad absorption spectra (300–750 nm), narrow bandgaps (1.68–1.74 eV) and PCEs in the range of 4.03–4.81% in PSC devices . The team further explored side-chain engineering (phenyl groups) and end-group modification (2-(1,1-dicyanomethylene)thiazolidin-4-one) of new NFAs, i.e., Qx28–Qx30. While removal of all side chains supported a planar conformation of the molecule, it hampered phase separation, lowering short-circuit current (Jsc), fill factor (FF) and PCE. Contrarily, the presence of all groups also compromised crystallinity and electron mobility. The highest PCE of 6.37% was realized for Qx29 upon only taking away the phenyl side groups attached to the IDT units .

Qx29 was used to prove that employing the same acceptor unit for both donor and acceptor is an effective approach. This strategy has been shown to be successful in achieving high Voc for benzotriazole materials, and now it has been extended to quinoxaline materials . Ji et al. introduced fluorine atoms to the Qx moiety of Qx29, producing a new NFA, Qx31, as well as the thiophene side chains of the p-type polymer PE61 to fine-tune the optoelectronic properties. The PCE of PE62:Qx31-based solar cells improved from 4.19 to 9.78% with a relatively high Voc of 1.09 V, and the PE61:Qx31-based devices gave rise to the highest PCE of 10.45% .

Table 2: Photovoltaic performance of Qx-derived NFAs in OSCs.

Qx derivatives are highly attractive auxiliary acceptor and bridging materials for DSSCs. Their strong electron-accepting ability enables efficient electron injection and charge collection, while their extended conjugation enhances light absorption across a broad spectrum. Qx’s unique structure promotes effective incorporation into the dye-sensitized layer, ensuring good intermolecular connectivity and facilitating electron transport. In addition, they enable efficient electron transfer and increased conjugation when acting as efficient π-bridge.

Figure 5: Dyes and sensitizers based on Qx auxiliary acceptors or bridging units.

A careful choice of auxiliary acceptor is of great importance, as highlighted by Kumar and co-workers. Their study raises a concern regarding the performance of dyes with tert-butyl substituted DPQ acceptors, either containing benzene (Qx74) or thiophene (Qx75) as a π-conjugation linker and their benzotriazole analogue. While the incorporation of the Qx enhances the interaction between the donor and acceptor moieties, the resulting PCE falls slightly short. This observation highlights the importance of a careful choice of auxiliary acceptors to ensure optimal device performance . Godfroy and colleagues also emphasized the significance of molecular structure and backbone planarity in achieving efficient charge transport in DSSCs. Their work highlights the importance of molecular structure and backbone planarity in achieving efficient charge transport in DSSCs. The sensitizers, Qx36 and Qx37, employing Qx and dithieno[3,2-f:2',3'-h]quinoxaline acceptors, respectively, showed narrower absorption spectra, thus indicating well-matched energy levels, and exhibited superior performance. However, Qx38, featuring a thieno[3,4-b]pyrazine acceptor and more quinoidal backbone, suffered from reduced electron injection and increased recombination rates .

The challenge associated with the coplanarity of Qx-based dyes, Qx39–Qx42 and the resulting device performance were highlighted by Huang et al. The use of quinoxaline-dithienothiophene and phenazine-dithienothiophene as π-bridges with the benzothiadiazole moiety as an auxiliary group did not yield the expected improvement, potentially due to the non-coplanarity of the molecular framework. DSSCs devices exhibited PCE in the range of 5.23–7.77% with Qx41-based device . Jiang et al.'s research showcases the potential of choosing the right Qx derivatives as efficient electron-withdrawing acceptor in DSSCs. The utilization of phenanthrene-fused-quinoxaline (PFQ) in sensitizer Qx43 resulted in exceptional PCE of 12.5%, surpassing traditional acceptor materials such as benzothiadiazole. Additionally, the improved charge recombination and stability indicate the strategic advantage of employing the right Qx derivatives for enhanced DSSC durability .

Several computational studies have also been performed to design dyes for optimized performance in DSSCs . Shi and colleagues’ quantum modeling study sheds light on the optoelectronic properties of Qx-based dyes containing DPQ (Qx44a) or methoxy-substituted DPQ (Qx44b) as π units, highlighting the importance of specific substitutions. While Qx44b demonstrated favorable properties such as a superior dipole moment, narrow bandgap, and red-shifted absorption, the reduced charge transfer rate presented a challenge. This analysis emphasizes the need for a delicate balance between desirable electronic properties and efficient charge transfer dynamics . Arunkumar and colleagues demonstrated the utilization of indolocarbazole-Qx systems named ICZS4. The comprehensive investigation of ICZS4's optoelectronic properties highlighted its potential for high-performance DSSCs. The small energy gap, red-shifted absorption, good dye regeneration, and promising NLO properties underscored the multifunctional nature of Qx derivatives as auxiliary acceptors .

Table 3: Photovoltaic performance of Qx-containing dyes and sensitizers in DSSCs.