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. 2026 Sep 25;16(45):57491–57498. doi: 10.1039/d6ra04404b

Enhanced photodetection via π-bridge tuning and intramolecular non-covalent interactions in non-fused electron acceptors

Ramkumar Kaliyaperumal a, Nizamuddin Shaik b, Venkata Trisanthi Orugu a, Akshay U Walke b, Kamatham Narayanaswamy b,✉, Sabyasachi Mukhopadhyay a,✉
PMCID: PMC13613684  PMID: 42799112

Abstract

We report a systematic study of three fully non-fused ring electron acceptors (NFREAs): SN-1 (furan), SN-2 (thiophene) and SN-3 (EDOT). These molecules were designed to probe how π-bridge identity and targeted intramolecular non-covalent interactions control optoelectronic properties and photodetector performance. Cyclic voltammetry and thin film absorption confirm a clear trend in frontier energy levels and optical bandgaps (SN-3 shows the deepest HOMO and LUMO). Devices fabricated in an ITO/Molecule/Al architecture reveal that SN-3 combines favourable electronic coupling and conformational locking to give the best performance: responsivity R ≈ 0.0789 A W−1 (532 nm), estimated specific detectivity under shot-noise-limited approximations (D* ≈ 6.03 × 1013 Jones, 532 nm, 0 V), and a photocurrent-to-dark ratio > 106 under laser excitation. Under 1 sun the SN-3 device maintains D* ≈ 1.6 × 108 Jones. Time-resolved measurements show a fast photocurrent rise (tr = 4.37 s) and a long decay (tf = 29.5 s), consistent with rapid electron extraction (near-ideal LUMO-Al alignment) and slower hole removal (deep HOMO relative to ITO), together with trap-assisted carrier retention. Intensity dependence follows a super-linear power law (α ≈ 1.26 ± 0.16), supporting progressive trap filling at low-moderate incident powers and eventually becoming saturated as the power increases. Together, the results show that π-bridge selection can dominate device performance while intramolecular non-covalent interactions improve backbone planarity, providing a practical, solution-processable route to low-noise NFREA photodetectors.

π-Bridge tuning and intramolecular non-covalent interactions improve the photodetection performance of NFREAs. SN-3 with an EDOT bridge shows favorable energy-level alignment and high detectivity.graphic file with name d6ra04404b-ga.webp

1. Introduction

Organic photodetectors (OPDs) are widely studied for next-generation optoelectronic applications owing to their inherent advantages, including mechanical flexibility, light weight, and compatibility with solution-based fabrication techniques.1–3 A key factor in improving OPD performance is the design of efficient acceptor materials.4 While fullerene-based acceptors have been used for many years, they suffer from drawbacks such as limited light absorption, fixed energy levels, and poor morphological stability.5–7 This has driven strong interest toward non-fullerene acceptors (NFAs). Among these, fused-ring electron acceptors (FREAs) have demonstrated impressive performance metrics owing to their rigid, planar structures that facilitate efficient charge transport.8,9 However, FREAs require complex multi-step synthesis, making them expensive and less attractive for large-scale production.10,11

In contrast, non-fused ring electron acceptors (NFREAs) offer simplified synthetic routes, better structural tunability, and improved solubility, making them attractive for cost-effective OPD fabrication.12–14 However, NFREAs typically exhibit increased conformational flexibility, leading to non-planar molecular geometries that can weaken π–π stacking and charge transport in the solid-state.14–16 To address these challenges, two main molecular design approaches have been widely reported: π-bridge optimization and non-covalent interaction engineering. π-Bridge optimization involves selecting suitable electron-donating or electron-accepting bridge units to tune the electronic structure and charge transport behavior.17 Many studies have shown that π-bridge engineering is a simple yet effective way to control molecular energy levels, photophysical properties, molecular aggregation, and ultimately photovoltaic performance.18

The flexibility of π-bridge engineering is further supported by studies showing that tuning both the small-molecule acceptor core and the π-bridge linker is an effective route to improve photovoltaic performance.17 In recent years, π-bridge modification has also enabled ultra-narrow bandgap NFREAs with extended near-infrared (NIR) absorption. These materials have delivered strong photodetector performance, including high responsivity (for example, 0.369 A W−1 at 1050 nm) and low noise currents, which are essential for sensitive detection.19

Along with π-bridge tuning, intramolecular non-covalent interactions such as O⋯H, S⋯N, S⋯H, and S⋯F have been widely used to restrict molecular conformations, promote backbone planarity, and enhance π-electron delocalization.10,20 A notable example is the fully NFREA bis-thieno[3,2-b:2′,3′-d] thiophene-fused diketopyrrolopyrrole (BTIC-2), which uses head-to-head alkoxy sidechain engineering to promote a more planar backbone through non-covalent locking. This design results in a narrow optical bandgap (1.35 eV) and broad NIR absorption extending to 900 nm.21–23 As a result, these engineered NFREAs can deliver low dark current, high detectivity, and fast photoresponse in OPDs, while also showing competitive power conversion efficiencies in organic solar cells.

Although both π-bridge engineering and intramolecular non-covalent interactions are recognized as effective molecular design strategies for NFREAs, their combined influence on photodetector performance remains insufficiently understood. To address this knowledge gap, we utilized three simple, low-cost fully NFREAs, namely SN-1, SN-2, and SN-3, sharing the same central alkoxybenzene and terminal hexyl dicyanorodanine units but differing in their π-bridges: furan, thiophene, and ethylenedioxythiophene (EDOT), respectively. This work therefore aims to establish the relationship between molecular design and photodetector performance. The three NFREAs were synthesized following our previously reported procedure.24 To promote a more rigid backbone, a non-covalent interaction strategy was employed, as shown in Scheme 1. Among the three synthesized NFREAs, SN-3, with its EDOT π-bridge, demonstrated superior π-electron delocalization, attributed to strong electronic coupling enabled by multiple O⋯S and O⋯H interactions. Furthermore, SN-3 showed better intramolecular charge transfer (ICT), which may be attributed to the electron-donating effect of multiple oxygen atoms.25,26

Scheme 1. Chemical structures of non-fused small-molecule acceptors SN-1, SN-2, and SN-3 were synthesized following our previously reported procedure,24 also see Scheme S1.

Scheme 1

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Through π-bridge optimization, a systematic comparison of furan, thiophene, and EDOT bridges reveals that π-bridge selection can outweigh the number of non-covalent interactions in determining performance. A synergistic design approach further shows that combining optimal π-bridges with non-covalent interactions dramatically enhances photodetection capabilities. The developed small molecules are solution-processable, enabling low-cost and large-area fabrication. Through precise π-bridge engineering and control of non-covalent interactions, we developed solution-processable small molecules that collectively enhance photodetector speed, sensitivity, and device scalability for next-generation OPDs.

2. Experimental

2.1. Synthesis

The non-fused ring electron acceptors (SN-1, SN-2, and SN-3) were synthesized following our previously reported multistep synthetic procedure,24 in which the final NFREAs were obtained through a Knoevenagel condensation reaction. The target molecules were prepared by reacting the corresponding core-linked dialdehyde intermediates containing furan, thiophene, or EDOT π-bridges with 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile in dry chloroform using piperidine as a catalyst. The reactions were carried out at 55 °C under an argon atmosphere for 14 h. Upon completion, the crude products were purified by flash column chromatography on silica gel using a dichloromethane/petroleum ether eluent system, affording SN-1, SN-2, and SN-3 as solid products in yields of 49%, 88%, and 82%, respectively. The molecular structures of all synthesized compounds were confirmed by 1H and 13C NMR spectroscopy together with MALDI-TOF mass spectrometry. Detailed synthetic procedures, spectral characterization data, and NMR spectra are provided in SI S5.

2.2. Device fabrication and characterization

2.2.1. Device fabrication

Photodetectors were fabricated using a vertical ITO/Molecule/Al device architecture. Indium tin oxide (ITO) coated glass substrates were cleaned by the solvent cleaning method. Active layers were prepared by dissolving 5 mg of SN-1, SN-2, or SN-3 in 1000 µL of chloroform, then drop-casting onto cleaned ITO substrates in a vacuum chamber. The drop-casting method was selected to evaluate the intrinsic photodetection behavior of the NFREAs under simple solution-processing conditions. The resulting film thicknesses were approximately 120 nm, as measured using a variable-angle ellipsometer (HOLMARC HO-ED-TH-02). Aluminum top electrodes (∼50 nm) were subsequently deposited by thermal evaporation under high vacuum (∼10−6 mbar) to complete device fabrication (see also SI S1.1).

2.2.2. Device characterization

Photocurrent density–voltage (J–V) measurements were carried out using a custom two-probe setup controlled by a Keithley 2636 B source meter under 532 nm laser illumination (∼8 mW cm−2). Temporal photoresponse measurements were performed using a class AAA 300 W solar simulator with periodic 30 s ON/OFF illumination cycles. At the same time, the transient current was monitored using a Keithley 6430 sub-femtoamp source meter. Dark J–V characteristics were recorded before illumination to confirm negligible leakage and injection currents (also see SI S1.2).

3. Results and discussion

3.1. Optical absorption and fluorescence measurements

The UV-vis absorption spectra of the SN-1, SN-2, and SN-3 acceptors are illustrated in Fig. 1. Although these fully NFREAs possess a similar molecular structure, slight variations in their π-bridge units lead to distinct variations in their optical absorption properties. In dilute chloroform solution, SN-1 and SN-2 exhibit maximum absorption peaks at 576 nm and 531 nm. Notably, SN-3 displays a red-shifted absorption maximum at 595 nm, ∼20 nm longer than SN-1 and SN-2. As shown in Fig. 1, all three molecules exhibited red-shifted absorption upon transitioning from solution to solid state, characterized by two distinct absorption peaks (0–0 and 0–1). This red shift is generally associated with enhanced intermolecular interactions in the solid state. Notably, both SN-1 and SN-3 showed a more pronounced red shift compared to SN-2, which is consistent with the proposed role of multiple intramolecular non-covalent interactions in promoting a more conformationally stabilized molecular structure. Notably, SN-3 showed a significantly enhanced 0–0 peak intensity compared with SN-1 and SN-2, suggesting a greater tendency toward J-aggregation in the solid state. While these optical features are consistent with the proposed structure–property relationship, direct thin film structural characterization was not carried out in the present study.

Fig. 1. (a) UV-vis spectra of SN-1, SN-2, and SN-3 in diluted chloroform, (b) in thin films.

Fig. 1

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3.2. Cyclic voltammetry (CV) measurements

The electrochemical properties of all three NFREAs were measured using cyclic voltammetry (CV) method, as previously reported,24 and the values are summarized in Table 1. The highest occupied molecular orbital (HOMO) energy levels were −5.79 eV, −5.64 eV, and −6.06 eV for SN-1, SN-2, and SN-3, respectively. The lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the optical band gap (Eg) and calculated to be −3.94 eV for SN-1, −3.68 eV for SN-2, and −4.21 eV for SN-3, as shown in the current density–voltage measurements (Fig. 2).

Table 1. Bandgap of SN-1, SN-2, and SN-3 from optical absorbance and cyclic voltammetry.

Sample name λ max (nm) solutiona λ max (nm) filmb λ onset (nm) filmc E g (optical)d E ox (eV)e HOMO (eV)e LUMO (eV)f
SN-1 545, 577 574, 625 669 1.85 1.48 −5.79 −3.94
SN-2 531, 556 544, 590 632 1.96 1.32 −5.64 −3.68
SN-3 556, 595 573, 630 669 1.85 1.74 −6.06 −4.21
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a

Absorption in chloroform solution.

b

Absorption in neat film.

c

Absorption onsets in neat film.

d

Optical bandgap was calculated through the onset of the thin-film absorption edge; Eg = 1240/λonset.

e

Using cyclic voltammetry (CV) method (EHOMO = −(4.80 − E1/2, Fc/Fc+ + Eox).

f

Calculated from equation: ELUMO = EHOMO + Eg.

Fig. 2. Photo- and dark current density (J–V) characteristics under 532 nm semiconductor laser ∼8 mW cm−2 (a) SN-1 (b) SN-2 and (c) SN-3.

Fig. 2

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3.3. Photocurrent density–voltage characteristics

To evaluate the influence of π-bridge variation and non-covalent interaction density on device performance, the three NFREAs SN-1 (furan), SN-2 (thiophene), and SN-3 (EDOT) were integrated into a photodetector using an ITO/Molecule/Al architecture (Fig. S1a, SI). Device fabrication and photocurrent density measurement details are provided in the SI Section S1. Dark J–V characteristics were recorded before illumination to confirm negligible leakage or injection current. All devices exhibited rectifying J–V characteristics and photoresponsivity under 532 nm illumination, confirming the materials' photoactivity.

Fig. 2 presents the current density–voltage (J–V) curves of SN-1, SN-2, and SN-3 under dark and 532 nm illumination. Under light, SN3 exhibits the highest photocurrent density (∼10−1 A cm−2), followed by SN-2 (∼10−2 A cm−2), while SN-1 shows a much weaker response (∼10−9 A cm−2). This performance trend suggests that the electronic nature of the π-bridge plays a more dominant role in governing charge-carrier mobility and photogeneration. This observation is consistent with UV-vis absorption results, in which SN-3 shows a ∼20 nm red shift relative to SN-1 and SN-2, suggesting stronger intermolecular interactions in the solid state, which may contribute to more efficient charge transport pathways (Fig. 1). SN-1 shows significantly lower performance, which can be attributed to the weaker electronic contribution of the furan bridge, leading to reduced π-conjugation and electron transport. Previous studies confirm that furan has reduced aromaticity and inferior charge-transport properties compared to thiophene and EDOT bridges.27,28

The dramatic performance improvement of SN-3 (Fig. 2c) confirms that the combination of the electron-rich π-bridge EDOT with stronger non-covalent interactions via its multiple oxygen atoms yields complementary effects that significantly enhance transport and photocurrent generation in these non-fullerene acceptors. Furthermore, SN-3 exhibits the largest photocurrent-to-dark-current density ratio, indicating superior light sensitivity and signal-to-noise characteristics essential for photodetection applications. These results support the idea that non-covalent interactions promote conformational locking, which may favor improved intermolecular organization, while the electronic properties of the π-bridge play the dominant role in determining device performance.

3.4. Light intensity dependent photocurrent density & trap-influenced dynamics

To probe the photogeneration and transport regime, we measured the photocurrent density (Jph) of the devices as a function of incident light intensity (Fig. S3, SI). The data were fitted using a power-law relationship:

Jph ∝ Pα 1

where P is the optical power density, and α is the power exponent.29Fig. 3 shows the log–log plot of Jphversus P for the SN-3 device. A linear fit yields α ≈ 1.26 ± 0.16, indicating a super-linear photocurrent density response. In ideal photodiodes dominated by direct photogeneration and field-driven extraction, α typically approaches unity, reflecting a proportional increase in photocurrent density with incident photon flux. The observed super-linear behavior (α > 1) points toward a trap-influenced photocurrent density. At low-to-moderate intensities, the progressive filling of these traps likely reduces the carrier capture probability, boosting the fraction of free carriers available for extraction and causing the apparent gain in Jph.30 As intensity increases, the response begins to saturate, a shift we attribute to the onset of bimolecular recombination and extraction-limited transport. This intensity dependence mirrors the temporal dynamics seen in SN-3, which exhibits a fast rise but a slow decay tail. Both observations point to a trap-mediated transport mechanism where carrier accumulation and subsequent delayed release drive both the intensity-dependent enhancement of photocurrent density and prolonged reset time after illumination is removed. While the power-law behavior suggests that trap-assisted carrier retention contributes to the photocurrent dynamics, the intensity-dependent measurements alone do not distinguish trap effects from interfacial extraction barriers. These results are interpreted together with the energy-level analysis discussed in Section 3.8.

Fig. 3. Light-intensity dependence of photocurrent density for SN-3 at 0 V under 532 nm illumination plotted on log-log scale axes; solid lines are the power-law fits Jph ∝ Pα over indicated ranges. (see more SI S3).

Fig. 3

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3.5. Temporal photoresponse behavior

To test the dynamic response, we recorded time-dependent photocurrent density under periodic illumination using a solar simulator (1 sun) with 30 s ON/OFF cycles at 0 V bias. While both SN-2 and SN-3 showed stable baselines and reproducible switching (Fig. 4a and b), their kinetics differed sharply. SN-2 exhibited a nearly symmetric transient response profile with a rise time (tr) of 11.68 s and a fall time (tf) of 13 s. SN-3, however, displayed a much faster rise (tr = 4.37 s) but a significantly prolonged decay (tf = 29.5 s). This asymmetry suggests that while electron extraction is efficient under illumination, carrier removal is bottlenecked once the light is cut. The energy-level alignment in Fig. 5 offers a clear explanation for this. SN-3's deeper LUMO (−4.21 eV) provides an ideal match for the Al electrode facilitating electron collection.

Fig. 4. Photoresponse at 0 V (black) and 0.5 V (blue) (a) SN-2 (b) SN-3, and rise time/fall time calculated from the photoresponse study under 1 sun illumination using solar simulator class AAA, 300 W, with 30 seconds light interval at 0 V (c) SN-2 (d) SN-3.

Fig. 4

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Fig. 5. (a) HOMO–LUMO from the cyclic voltammetry of (from left to right) SN-2 thiophene, SN-3 EDOT, and SN-1 furan (b) proposed photophysical schemes of photodetectors as prepared with SN-2 and SN-3 molecules under illumination (in reverse bias).

Fig. 5

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Conversely, its deep HOMO (−6.06 eV) creates a substantial barrier for hole extraction at the ITO interface. When the light is removed, the accumulated holes are extracted more slowly because of the large interfacial hole-extraction barrier, while trap-assisted carrier retention further contributes to the prolonged decay; a phenomenon well-documented in organic photodiodes with unbalanced extraction.31,32 Overall, the temporal response confirms that π-bridge tuning not only modulates the steady-state photocurrent density but also governs carrier-extraction dynamics. SN-2 provides balanced switching, while SN-3 offers faster turn-on at the expense of slower reset due to charge retention.

3.6. Responsivity and detectivity

Among the three devices, the SN-3-based photodiode achieved the highest responsivity (R): 0.0789 A W−1 under 532 nm illumination and 7.12 × 10−7 A W−1 under 1 sun, surpassing SN-2 (5.87 × 10−6 A W−1; 4.66 × 10−9 A W−1) by over an order of magnitude at both illuminations. The responsivity (R) was determined using:

Jph = Jl − Jd 2
3.6. 3

where Jl and Jd are the photo- and dark-current density, respectively, Jph is the total photocurrent density, and P is the incident power density (∼8 mW cm−2 for 532 nm laser & 100 mW cm−2 for 1 sun).33 The enhancement is driven by nearly ideal alignment between SN-3's LUMO level (−4.21 eV) and the Al electrode (−4.3 eV), facilitating efficient electron extraction (Fig. 4b). The deep HOMO level (−6.06 eV) of SN-3 further delays hole extraction at the ITO interface. This asymmetric alignment ensures efficient charge separation and high electron selectivity, resulting in larger photocurrent densities and prolonged photoresponse.

The specific detectivity (D*), which describes the sensitivity of a photodetector to weak optical signals, was estimated using the conventional shot-noise-limited approximation:

3.6. 4

where q is the elementary charge, and Jd is the dark current density.2 Among the devices, the SN-3-based photodiode exhibits the highest estimated detectivity 6.03 × 1013 Jones at 532 nm and 1.61 × 108 Jones under 1 sun at 0 V (Table 2). In comparison, SN-2-based photodiodes reached only 1.24 × 109 Jones and 3.04 × 107 Jones, respectively. The estimated detectivity values obtained for SN-3, 6.03 × 1013 Jones at 532 nm, are among the highest values reported for solution-processed NFA-based photodetectors. Recent studies on comparable systems show detectivities in the range of 1010–1013 Jones, such as fused-ring PDTTYM OPDs (∼1012 Jones),39 selenophene-based NFAs (∼3 × 1013 Jones),37 and non-fused acceptors (∼2 × 1011 Jones).19 The SN-3 device places the performance of SN-3 among the highest reported for solution-processed NFREA-based photodetectors while achieving this performance under self-powered (0 V) operation (Table 3). Although its responsivity (0.079 A W−1 at 532 nm) is moderate compared to certain high-gain OPDs, its extremely low dark current density ensures a far higher detectivity (Table 2). The superior detectivity of SN-3 stems from the deep HOMO level, which suppresses carrier injection and backbone planarity, and from efficient LUMO alignment and strong π–π stacking, which facilitate balanced charge extraction, increasing signal output. We also evaluated the noise-limited sensitivity using the photocurrent density-to-dark-current density ratio (Jph/Jd). SN-3 displays a Jph/Jd of approximately 106 under 532 nm illumination, confirming strong signal-to-noise discrimination. Because estimated detectivity scales as Inline graphic, the extremely low dark current density of SN-3 maintains high detectivity under 1 sun, where responsivity values (7.12 × 10−7 A W−1) appear reduced due to normalization by the large incident power density. In contrast, SN-2 has a shallower LUMO (−3.68 eV), producing an electron-extraction barrier with Al (≈0.67 eV). This unfavorable cathode alignment increases leakage current and reduces shot-noise-limited detectivity, even though SN-2 exhibits a slightly faster and more symmetric switching response due to improved hole extraction.

Table 2. Comparison between SN-3 and SN-2-based on photocurrent density, photoresponse, detectivity, and structural impact.

Sample π-bridge Source J l (A cm−2) J d (A cm−2) Responsivity (A W−1) Specific detectivity (Jones) Sensitivity
SN-3 EDOT 532 nm laser 4.68 × 10−5 5.35 × 10−12 0.0789 6.03 × 1013 106
1 sun 7.11 × 10−8 6.08 × 10−11 7.12 × 10−7 1.61 × 108 103
SN-2 Thiophene 532 nm laser 4.21 × 10−9 6.96 × 10−11 5.87 × 10−6 1.24 × 109 101
1 sun 4.66 × 10−10 7.33 × 10−14 4.66 × 10−9 3.04 × 107 103
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Table 3. Comparison of the present SN-3 photodetector with representative recently reported non-fullerene-based organic photodetectors. The P3HT:PC71BM:ITIC34 device is included as a representative ternary fullerene/non-fullerene OPD for comparison of broadband visible photodetection performance. The C8C8-4Cl/P3HT35 device operates in photomultiplication (PM) mode via engineered electron trapping and gain, whose gain mechanism differs fundamentally from the photovoltaic-mode operation of the remaining devices, including the present work.

Active layer Device architecture λ (nm) Bias (V) Responsivity (A W−1) Detectivity (Jones) Dark current density (A cm−2)
PM6:TPDC-4F36 Binary NFREA OPD 800 −0.1 — 1.27 × 1013 3.55 × 10−9
IDSe37 Binary NFREA OPD 790 −2 — >1012 1.65 × 10−9
NPB:SubPc38 Vacuum-deposited small-molecule OPD 530 −5 0.22 1.07 × 1013 1.32 × 10−9
C8C8-4Cl/P3HT35 Binary NFREA PM-type OPD 300–900 −7 — 4.92× 1013 —
P3HT:PC71BM:ITIC34 Ternary fullerene/non-fullerene OPD 560 — 0.21 2.12 × 1012 —
710 — 0.25 2.67 × 1012 —
SN-3 (this work) Binary fully NFREA OPD 532 0 0.0789 6.03 × 1013 5.35 × 10−12
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To evaluate the practical stability of the optimized device, the ambient shelf stability of the SN-3 photodetector was examined after three months of storage under ambient laboratory conditions without encapsulation (Fig S5, SI). The device retained stable rectifying J–V characteristics together with a pronounced photocurrent density response, indicating that its photodetection performance was largely preserved during storage. The observed stability was further considered in the context of representative non-fused acceptor systems reported in the literature. A completely non-fused electron acceptor retained approximately 84% of its initial power conversion efficiency after 1300 h of continuous simulated AM 1.5 G illumination,40 while another unfused-core non-fullerene acceptor retained approximately 70% of its initial power conversion efficiency after thermal treatment at 180 °C for 12 h.41 Although these studies were performed on photovoltaic devices under different stress conditions, they collectively demonstrate the intrinsic robustness of non-fused acceptor systems. The stable ambient-storage performance observed for the present SN-3 photodetector is consistent with this general trend and further supports the use of fully non-fused acceptors for stable optoelectronic devices.

3.7. Molecular structure–property correlation

Comparative data (Table 2) highlight the importance of interactive molecular design. In this series, π-bridge selection primarily governs charge transport and photogeneration efficiency, while targeted non-covalent interactions promote structural rigidity and favorable molecular packing. SN-3 exemplifies this synergy, combining the electron-rich EDOT bridge to achieve improved performance figures of merit for OPDs, including a photocurrent density of ∼10−1 A cm−2 and an estimated specific detectivity of 6.03 × 1013 Jones.

Importantly, detailed density functional theory (DFT) and noncovalent interaction (NCI) analyses for SN-1, SN-2, and SN-3 have been reported in our earlier work24 and support the conformational-locking interpretation used here. The NCI maps reveal multiple O⋯S and O⋯H contacts in SN-3 that restrict torsional freedom. Consistently, torsional scans show a larger rotational barrier (∼7.5 kcal mol−1) for SN-3 compared with ∼3 kcal mol−1 for SN-2, indicating greater conformational rigidity than SN-2. Although direct thin film structural characterization was not carried out in the present study, these molecular-level results are consistent with the experimentally observed thin film optical absorption behavior and support the proposed structure–property relationship.

Time-dependent density functional theory (TD-DFT) and natural orbital transition (NTO) results from the earlier work,24 further indicate greater hole/electron delocalization across the SN-3 framework, consistent with its superior photocurrent response and faster current rise (tr = 4.37 s) observed experimentally (Fig. 4d). SN-2, which has fewer conformational locks and a lower torsional barrier, exhibits faster and more symmetric switching (tr = 11.68 s and tf = 13.0 s). This reflects a clear trade-off between sensitivity and speed (Fig. 4c). SN-1, containing the furan bridge, exhibits the weakest performance, consistent with its weaker electronic coupling and reduced charge delocalization, reinforcing the need for electron-rich and conformationally constrained bridges in high-performance NFREA design.

3.8. Charge transport mechanism

To elucidate the observed variations in photoresponse, we analyzed the energy-level alignment and charge-transport characteristics of SN-1, SN-2, and SN-3 using HOMO–LUMO data derived from cyclic voltammetry. As shown in Fig. 5, the ITO/Molecule/Al architecture forms asymmetric energy barriers that govern carrier injection and extraction.

SN-1, containing a furan π-bridge, exhibits intermediate frontier orbital energies (LUMO = −3.94 eV; HOMO = −5.79 eV), resulting in moderate extraction barriers at both electrodes (≈0.36 eV at Al and ≈1.0 eV at ITO). However, SN-1 shows a negligible photocurrent density of ∼10−9 A cm−2 (Fig. 2a), which can be attributed to its weaker π-bridge. This limits charge delocalization and reduces effective intermolecular orbital overlap, leading to inefficient transport and poor photoresponse.28 SN-2, with a thiophene π-bridge, shows a shallower LUMO (−3.68 eV), which introduces a larger extraction barrier at the Al interface (≈0.63 eV). In contrast, its HOMO level (−5.59 eV) is more favorably aligned with that of ITO, reducing the hole-extraction barrier to approximately 0.89 eV. This relatively balanced alignment supports a measurable photocurrent density of ∼10−2 A cm−2 (Fig. 2b). It produces nearly symmetric rise and fall times of 11.68 s and 13.0 s, respectively (Fig. 4c), which is advantageous for stable real-time photodetection.

SN-3 with EDOT π-bridge exhibits the most favorable electron extraction conditions. Its LUMO (−4.21 eV) closely matches the Al work function (−4.3 eV), giving an ultralow electron extraction barrier (≈0.08 eV). This near-ideal alignment enables rapid collection of photogenerated electrons, leading to high photocurrent density (∼10−1 A cm−2) and a fast rise time (4.37 s). In contrast, the deep HOMO of SN-3 (−6.06 eV) is strongly mismatched with ITO (−4.7 eV), corresponding to a large hole extraction barrier (≈1.36 eV). This hinders hole removal after illumination and promotes carrier accumulation followed by slow trap-assisted release. As a result, SN-3 exhibits a prolonged decay time (29.5 s), consistent with carrier retention and delayed recombination, which explains the sharp rise, but slow fall observed in its transient response (Fig. 4d).

DFT/NCI results for the SN-3 further support these observations. SN-3 preferentially adopts more planar conformers with short O⋯S and O⋯H contacts and larger torsional barriers, suggesting a structural tendency that may facilitate efficient electron extraction and a tendency for interfacial hole accumulation that drives the slow decay.24 This comparative analysis clearly highlights that efficient photocurrent generation requires not only structural planarity but also optimal energy-level alignment, particularly at the cathode interface for electron transport and extraction. Together, the electrochemical, optical, and previously reported computational results mirror the photocurrent density trends and temporal response, providing a consistent mechanistic picture linking molecular design to device kinetics. Accordingly, the temporal response of SN-3 is interpreted as arising from the combined influence of contact-limited hole extraction at the ITO interface and trap-assisted carrier retention, rather than from either mechanism alone.

4. Conclusions

This work presents a systematic structure–property investigation into the dual role of π-bridge selection and non-covalent interaction engineering in fully NFREAs for high-performance OPDs. Among the examined materials, SN-3, featuring an EDOT π-bridge with multiple non-covalent interactions, delivers the best overall photodetector figures of merit, reaching an estimated specific detectivity of D* = 6.03 × 1013 Jones at 532 nm and a high photocurrent-to-dark-current density ratio (Jph/Jd ≈ 106), confirming excellent signal-to-noise ratio characteristics. SN-3 exhibits a fast rise (tr = 4.37 s) and long decay (tf = 29.5 s). This asymmetry is explained by energy-level alignment: a near-ideal LUMO-Al match enables rapid electron extraction. At the same time, a deep HOMO relative to ITO impedes hole removal, promoting carrier accumulation and trap-assisted release. The intensity dependence (α ≈ 1.26 ± 0.16) and onset of saturation at high intensity are consistent with progressive trap filling and extraction limits at elevated carrier densities. In ambient storage, SN-3 also shows superior shelf stability (≈3 months) compared with SN-2, which exhibits larger device-to-device variability and leakage-driven degradation. Overall, these results demonstrate that combining electronically favorable π-bridge tuning with conformational locking through intramolecular non-covalent interactions provides a simple and solution-processable molecular design strategy for developing low-noise, high-detectivity, and more stable NFREA-based organic photodetectors. Beyond the present device architecture, this strategy may also be applicable to future flexible and large-area organic photodetectors, as well as organic imaging technologies, where solution processability, low dark current, and self-powered operation are desirable for low-cost device fabrication.

Author contributions

S. M and K. N. S. conceptualized the project. N. S. carried out synthesis and data curation. R. K. carried out device fabrication, characterization, and data analyses at the device level. A. U. W. and V. T. O assisted with data curation. All authors contributed to preparing the manuscript.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-016-D6RA04404B-s001

Acknowledgments

SM acknowledges Anusandhan National Research Foundation for Core Research Grant (CRG/2023/000978). KNS acknowledges financial support from SRM University-AP through a seed grant (SRMAP/URG/SEED/2023-24/028). RK, NS, VTO, and AUW acknowledge the SRM University AP for their doctoral fellowship support.

Data availability

The data supporting this article have been included in the supplementary information (SI). The SI includes detailed experimental procedures for the synthesis and device fabrication, along with supporting characterization. Supplementary information: (1) device fabrication; (2) temporal photoresponse behavior of SN-1; (3) light intensity dependent photocurrent density of SN-3; (4) ambient shelf-stability of SN-2 and SN-3; (5) Material synthesis and NMR spectra. See DOI: https://doi.org/10.1039/d6ra04404b.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA04404B-s001
RA-016-D6RA04404B-s001.pdf (890.4KB, pdf)

Data Availability Statement

The data supporting this article have been included in the supplementary information (SI). The SI includes detailed experimental procedures for the synthesis and device fabrication, along with supporting characterization. Supplementary information: (1) device fabrication; (2) temporal photoresponse behavior of SN-1; (3) light intensity dependent photocurrent density of SN-3; (4) ambient shelf-stability of SN-2 and SN-3; (5) Material synthesis and NMR spectra. See DOI: https://doi.org/10.1039/d6ra04404b.

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