An n-Type All-Fused-Ring Molecule with Photoresponse to 1000 nm for Highly Sensitive Near-Infrared Photodetector

Phenazine-Based Ultra-Narrow Bandgap Acceptors for Efficient Transparent Organic Photovoltaic

Developing ultra-narrow bandgap electron acceptors is an effective approach to achieving transparent organic photovoltaics (TOPVs). In this study, two phenazine based non-fullerene acceptors, namely PA-2H and PA-2Br, are synthesized by merging core bromination and extended conjugation. Applying extra ethylene double bonds as π bridge makes the absorption onsets of films red shift to 1021 and 1041 nm for PA-2H and PA-2Br, respectively. Single-crystal data illustrated that PA-2Br exhibits a tight and ordered 3D network packing with improved charge transport, different from the 2D step-like packing of PA-2H. Therefore, PA-2Br-based opaque organic photovoltaics achieves an excellent power conversion efficiency (PCE) of 13.7%. Notably, the TOPV attains a PCE of 4.60% with an average visible transmittance (AVT) of 70.2%, which exhibits promise in efficient TOPVs. This work demonstrates the importance of core bromination in the design of high-performance ultra-narrow bandgap electron acceptors and provides the opportunity to fabricate efficient TOPVs.

Fine-Tuning of Alkyl Side Chains in Ultra-Low Bandgap Polymers To Effectively Suppress the Dark Current for Short-Wavelength Infrared Organic Photodetectors

Short-wavelength infrared organic photodetectors (SWIR OPDs) have great potential for applications in health monitoring, night vision, optical communication, and image sensing. However, the development of SWIR OPDs is limited by challenges in achieving high responsivity (R) and detectivity (D*) due to the exponentially increased nonradiative recombination rate when decreasing the bandgap of conjugated polymers or molecules. In this study, we designed and synthesized a series of donor-acceptor (D-A) type ultralow bandgap (≤0.85 eV) polymers containing [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) units as the A unit and selenophene units as the D unit, respectively. The solubility, molecular stacking, charge transport properties, and film morphology of the polymers were finely tuned by varying the side chain lengths on the substituted phenyl groups of TQ units. It was found that the polymer PTQOD with 2-octyldodecyl alkyl chains has lower nonradiative recombination losses and lower trap state density. After device optimization, the PTQOD-based device achieved higher R and lower dark current density (Jd), resulting in a D* of 1.06 × 1010 Jones at 1300 nm under 0 V bias, representing the highest value for OPDs using TQ-based polymers. This work highlights the importance of optimizing the alkyl chains of ultralow-band gap polymer donor materials and provides a promising approach for developing highly sensitive SWIR OPDs.

Regiochemistry and Side-Chain Engineering Enable Efficient N-Type Mixed Conducting Polymers

Developing high-performance n-type organic mixed ionic-electronic conducting (OMIEC) polymers with simple structural motifs is still challenging. We show that high-performance, low-threshold-voltage n-type OMIEC polymers can be achieved using a simple diketopyrrolopyrrole unit flanked by thiazole groups, which is functionalized with glycolated side chains. Interestingly, the regiospecific sp2-N position in the repeating unit's thiazole governs the polymer chains' solvation and molecular packing. This specific backbone chemistry enhances conjugation efficiency, reduces trap density, and improves electrochemical doping efficiency. Moreover, systematic variation of glycolated side-chain lengths induces a sequential shift in molecular orientation-from edge-on through bimodal to face-on preferential alignment. This structural evolution achieves optimized ionic-electronic transport balance, resulting in exceptional device metrics: a geometrically normalized transconductance of 31.9 S cm-1, a figure-of-merit µC* of 96.3 F cm-1 V-1 s-1, and a threshold voltage of 0.31 V, positioning these materials among the highest-performing n-type OMIECs. An organic complementary inverter made from the optimized n-type OMIEC polymer and a reported p-type polymer exhibits a voltage gain of 198 V V-1, effectively amplifying the ECG signal and enhancing signal quality. This work establishes structure-property guidelines for designing bioelectronic n-type OMIECs.

Stable Open-Shell Conjugated Terpolymer with Extended NIR Absorption for Organic Photodetectors Detecting Beyond 1000 nm

Near-infrared (NIR) photodetectors play crucial roles in many scientific, industrial, and medicinal fields. However, conventional organic photodetectors (OPDs) often do not utilize the NIR region due to poor absorption beyond 1000 nm. In this study, an open-shell conjugated terpolymer is synthesized for NIR detection. This polymer contains diketopyrrolopyrrole (DPP), thiophene, and benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole (BBT); these components form the novel random terpolymer poly{2,5-bis(2-decyltetradecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione]-co-thiophene-co-benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole} (PDPPTBBT) via Stille coupling polymerization. The diradicals generated by the open-shell characteristics of PDPPTBBT become stronger as molecular packing is enhanced. This enhancement enables absorption at wavelengths beyond 1000 nm. PDPPTBBT exhibits temperature-independent Pauli paramagnetic properties. Additionally, electron paramagnetic resonance measurements reveal that compared with the singlet ground state, the polymer exhibits a higher stability in the triplet ground state and a high spin (S = 1). PDPPTBBT can act as an acceptor or a donor in films in which the material is blended with either poly(3-hexylthiophene-2,5-diyl) or Y6. OPDs prepared using the blended films display detection wavelengths exceeding 1000 nm with a maximum external quantum efficiency of 126% at 1050 nm and a specific detectivity (D*) of 7.5 × 1011 Jones.

Electron-Rich Heptacyclic S,N Heteroacene Enabling C-Shaped A-D-A-type Electron Acceptors With Photoelectric Response beyond 1000 Nm for Highly Sensitive Near-Infrared Photodetectors

A highly electron-rich S,N heteroacene building block is developed and condensed with FIC and Cl-IC acceptors to furnish CT-F and CT-Cl, which exhibit near-infrared (NIR) absorption beyond 1000 nm. The C-shaped CT-F and CT-Cl self-assemble into a highly ordered 3D intermolecular packing network via multiple π-π interactions in the single crystal structures. The CT-F-based organic photovoltaic (OPV) achieved an impressive efficiency of 14.30% with a broad external quantum efficiency response extending from the UV-vis to the NIR (300-1050 nm) regions, outperforming most binary OPVs employing NIR A-D-A-type acceptors. CT-Cl possesses a higher surface energy than CT-F, promoting vertical phase segregation and resulting in its preferential accumulation near the bottom interface of the blend. This arrangement, combined with the lower HOMO energy level of CT-Cl, effectively reduces undesired hole and electron injection under reverse voltage. The PM6:CT-Cl-based organic photodetectors (OPDs) devices achieved an ultra-high shot-noise-limited specific detectivity (Dsh*) values exceeding 1014 Jones in the NIR region from 620 to 1000 nm, reaching an unprecedentedly high value of 1.3 × 1014 Jones at 950 nm. When utilizing a 780 nm light source, the PM6:CT-Cl-based OPDs show record-high rise/fall times of 0.33/0.11 µs and an exceptional cut-off frequency (f-3dB) of 590 kHz at -1 V.

Quinoidal Semiconductor Nanoparticles for NIR-II Photoacoustic Imaging and Photoimmunotherapy of Cancer

Photoagents with ultra-high near-infrared II (NIR-II) light energy conversion efficiency hold great promise in tumor phototherapy due to their ability to penetrate deeper tissues and minimize damage to surrounding healthy cells. However, the development of NIR-II photoagents remain challenging. In this study, an all-fused-ring quinoidal acceptor-donor-acceptor (A-D-A) molecule, SKCN, with a BTP core is synthesized, and nanoparticles named FA-SNPs are prepared. The unique quinoidal structure enhances π-electron delocalization and bond length uniformity, significantly reducing the bandgap of SKCN, resulting in strong NIR-II absorption, a high molar extinction coefficient, and a photothermal conversion efficiency of 75.14%. Enhanced molecular rigidity also facilitates efficient energy transfer to oxygen, boosting reactive oxygen species generation. By incorporating the immunomodulator R848, FA-SRNPs nanoparticles are further developed, effectively modulating the tumor immune microenvironment by reducing Tregs and M-MDSCs infiltration, promoting dendritic cell maturation, M1 macrophage polarization, and activating CD8+ T cells and NK cells. Comprehensive studies using orthotopic ovarian cancer models demonstrated strong tumor targeting, photoacoustic imaging capabilities, and significant tumor suppression and metastasis inhibition, and also showing excellent therapeutic efficacy in an orthotopic breast cancer model. This study provides strong evidence for the potential application of quinoidal A-D-A molecules in cancer photoimmunotherapy.

Terminal Fluorination Modulates Crystallinity and Aggregation of Fully Non-Fused Ring Electron Acceptors for High-Performance and Durable Near-Infrared Organic Photodetectors

High dark current density (Jd) severely hinders further advancement of near-infrared organic photodetectors (NIR OPDs). Herein, we tackle this grand challenge by regulating molecular crystallinity and aggregation of fully non-fused ring electron acceptors (FNREAs). TBT-V-F, which features fluorinated terminals, notably demonstrates crystalline intensification and a higher prevalence predominance of J-aggregation compared to its chlorinated counterpart (TBT-V-Cl). The amalgamation of advantages confers TBT-V-F-based OPDs with lower nonradiative energy loss, improved charge transport, decreased energetic disorder, and reduced trap density. Consequently, the corresponding self-powered OPDs exhibit a 40-fold decrease in Jd, a remarkable increase in detectivity (D*sh), faster response time, and superior thermal stability compared to TBT-V-Cl-based OPDs. Further interfacial optimization results in an ultra-low Jd of 7.30×10-12 A cm-2 with D*sh over 1013 Jones in 320-920 nm wavelength and a climax of 2.2×1014 Jones at 800 nm for the TBT-V-F-based OPDs, representing one of the best results reported to date. This work paves a compelling material-based strategy to suppress Jd for highly sensitive NIR OPDs, while also illustrates the viability of FNREAs in construction of stable and affordable NIR OPDs for real-world applications.

Fast Near-Infrared Organic Photodetectors with Enhanced Detectivity by Molecular Engineering of Acceptor Materials

Organic photodetectors (OPDs) with a near-infrared (NIR) response beyond 900 nm are intriguing electronics for various applications. It is challenging to develop NIR OPDs with high sensitivity and fast response. Herein, the acceptor materials of OPDs are tuned to extend detection to ≈1100 nm with improved sensitivity. A new fused ring electron acceptor, ICS (2,2'-((2Z,2'Z)-(((4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)bis(4-((2-ethylhexyl)thio)thiophene-5,2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile), is developed with alkylthio thiophene as the bridge, achieving a small bandgap of 1.35 eV while decreasing dark current densities under reverse bias. By further introducing a secondary acceptor of PC61 BM, the doping compensation, and unfavored hole injection blocking enable further improvement of detectivity. The PTB7-Th (Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]): ICS: PC61 BM OPDs deliver a low dark current density of 1.23 × 10-9 A cm-2, a high peak specific detectivity of 1.09 × 1013 Jones at 950 nm under -0.2 V, and a fast response speed with a -3 dB bandwidth of 720 kHz biased under -2 V. The photoplethysmography system with the PTB7-Th: ICS: PC61 BM OPD can reliably monitor heartbeats under 980 nm NIR light. This study promises the development of organic NIR OPDs with high detectivity and fast response by tuning active materials.

Organic Bulk-Heterojunction Blends with Vertical Phase Separation for Enhanced Organic Photodetector Performance

The ternary blending strategy is a fundamental approach that is widely recognized in the field of organic optoelectronics. In our investigation, leveraging the inherent advantages of the ternary component blending methodology, we introduced an innovative design for organic photodetectors (OPDs) aimed at reducing the dark current density (Jd) under reverse bias. This pioneering effort involved combining two distinct conjugated molecules (IT-4F and IEICO-4F) with a conjugated polymer (PM7), resulting in a composite material characterized by a well-defined vertical phase separation. To thoroughly explore device performance variations, we utilized a comprehensive array of analytical techniques, including atomic force microscopy (AFM) cross-section methodologies and Kelvin probe force microscopy (KPFM). Through the optimization of the blend ratio (PM7:IT-4F: IEICO-4F at 1:0.8:0.2), we achieved significant advancements. The resulting OPD demonstrated an exceptional reduction in JD, reaching a remarkably low value of 4.95 × 10-10 A cm-2, coupled with an ultra-high detectivity of 4.95 × 1013 Jones and an outstanding linear dynamic range exceeding 100 dB at 780 nm under a bias of -1V. Furthermore, the attained cutoff frequency reached an impressive 220 kHz, highlighting substantial improvements in device performance metrics. Of particular significance is the successful translation of this technological breakthrough into real-world applications, such as in heart rate sensing, underscoring its tangible utility and expanding its potential across various fields. This demonstrates its practical relevance and underscores its versatility in diverse settings.

Sensitive SWIR Organic Photodetectors with Spectral Response Reaching 1.5 µm

The performance of organic photodetectors (OPDs) sensitive to the short-wavelength infrared (SWIR) light lags behind commercial indium gallium arsenide (InGaAs) photodetectors primarily due to the scarcity of organic semiconductors with efficient photoelectric responses exceeding 1.3 µm. Limited by the Energy-gap law, ultralow-bandgap organic semiconductors usually suffer from severe non-radiative transitions, resulting in low external quantum efficiency (EQE). Herein, a difluoro-substituted quinoid terminal group (QC-2F) with exceptionally strong electron-negativity is developed for constructing a new non-fullerene acceptor (NFA), Y-QC4F with an ultralow bandgap of 0.83 eV. This subtle structural modification significantly enhances intermolecular packing order and density, enabling an absorption onset up to 1.5 µm while suppressing non-radiation recombination in Y-QC4F films. SWIR OPDs based on Y-QC4F achieve an impressive detectivity (D*) over 1011 Jones from 0.4 to 1.5 µm under 0 V bias, with a maximum of 1.68 × 1012 Jones at 1.16 µm. Furthermore, the resulting OPDs demonstrate competitive performance with commercial photodetectors for high-quality SWIR imaging even under 1.4 µm irradiation.

Highly Stretchable and Oriented Wafer-Scale Semiconductor Films for Organic Phototransistor Arrays

Stretchable organic phototransistor arrays have potential applications in artificial visual systems due to their capacity to perceive ultraweak light across a broad spectrum. Ensuring uniform mechanical and electrical performance of individual devices within these arrays requires semiconductor films with large-area scale, well-defined orientation, and stretchability. However, the progress of stretchable phototransistors is primarily impeded by their limited electrical properties and photodetection capabilities. Herein, wafer-scale and well-oriented semiconductor films were successfully prepared using a solution shearing process. The electrical properties and photodetection capabilities were optimized by improving the polymer chain alignment. Furthermore, a stretchable 10 × 10 transistor array with high device uniformity was fabricated, demonstrating excellent mechanical robustness and photosensitive imaging ability. These arrays based on highly stretchable and well-oriented wafer-scale semiconductor films have great application potential in the field of electronic eye and artificial visual systems.

Superior End-Group Stacking Promotes Simultaneous Multiple Charge-Transfer Mechanisms in Organic Solar Cells with an All-Fused-Ring Nonfullerene Acceptor

The all-fused-ring acceptor (AFRA) is a success for nonfullerene materials and has attracted considerable attention as its high optical and chemical stability expected to reduce energy loss, and power conversion efficiency (PCE) approaching 15% in constructed all-small-molecule organic solar cells (OSCs). Herein, the intrinsic role of the structure of AFRA F13 and the reason for its high PCE were revealed by comparison with those of typical fused acceptors IDT-IC and Y6. An increased degree of conjugation in F13 leads to broader and red-shifted absorption peaks, facilitating enhancement of the short-circuit current. Multiple charge-transfer mechanisms are mainly attributed to the higher Frenkel exciton (FE) state due to the multiple transition ways for acceptors in the C1-CN:F13 system. The increased number of atoms contributing to the charge-transfer (CT) state facilitated the existence of more superior stacking patterns with easy formation of CT and FE/CT states and a high charge separation rate. It was found using the AFRA is an effective strategy to enhance end-group stacking, enhancing the borrowing of oscillator strength to promote multiple CT mechanisms in the complexes, explaining the high performance of this OSC device. This work is promising to guide designing an efficient AFRA in the future.

Near-Infrared Organic Photodetectors with Spectral Response over 1200 nm Adopting a Thieno[3,4-c]thiadiazole-Based Acceptor

The nonclassical ten-pi-electron 5,5-fused thieno[3,4-c]thiadiazole (TTD) unit is an excellent building block for constructing sub-silicon-band gap organic semiconductors. However, no small molecule acceptor (SMA) materials based on TTD have been reported despite the fact that high-sensitivity near-infrared organic photodetectors (OPDs) are generally achieved by using SMAs. In this work, we report a TTD-based narrow band gap (0.95 eV) SMA material TTD(DTC-2FIC)2 with strong near-infrared absorption. Employing PTB7-Th as a donor, OPDs based on TTD(DTC-2FIC)2 exhibit an optimized responsivity of 0.095 (±0.007) A W-1 at 1100 nm and sustain a decent responsivity of 0.074 (±0.008) A W-1 at 1200 nm. Moreover, a good specific detectivity over 1 × 1011 Jones is achieved at a wavelength of 1200 nm. Detailed characterizations imply that the performance of TTD(DTC-2FIC)2-based OPDs may be substantially improved by choosing lower-mixing donors with shallower energy levels. This work demonstrates that SMAs incorporating TTD as the core unit hold promise for constructing high-sensitivity sub-silicon-band gap OPDs.

Development of n-Type Small-Molecule Acceptors for Low Dark Current Density and Fast Response Organic Photodetectors

Suppressing the dark current density (Jd) while maintaining sufficient charge transport is important for improving the specific detectivity (D*) and dynamic characteristics of organic photodetectors (OPDs). In this study, we synthesized three novel small-molecule acceptors (SMAs) densely surrounded by insulating alkyl side chains to minimize the Jd in OPDs. Introducing trialkylated N-annulated perylene diimide as a terminal moiety to the alkylated π-conjugated core structure was highly efficient in suppressing Jd in the devices, resulting in an extremely low Jd of 4.60 × 10-11 A cm-2 and 10-100 times improved D* values in the devices. In addition, SMAs with a geometrically aligned backbone structure exhibited better intermolecular ordering in the blended films, resulting in 3-10 times as high responsivity (R) values in the OPDs. Outstanding OPD performances with a D* of 8.09 × 1012 Jones, -3 dB cutoff frequency of 205.2 kHz, and rising response time of 16 μs were achieved under a 530 nm illumination in photoconductive mode. Geometrically aligned core-terminal SMAs densely surrounded by insulating alkyl side chains are promising for improving the static and dynamic properties of OPDs.