High-Performance All-Polymer Photodetectors via a Thick Photoactive Layer Strategy

Multispectral and Circular Polarization-Sensitive Carbon Dot-Polydiacetylene Capacitive Photodetector

Multispectral photodetectors (MSPs) and circularly polarized light (CPL) sensors are important in opto-electronics, photonics, and imaging. A capacitive photodetector consisting of an interdigitated electrode coated with carbon dot/anthraquinone-polydiacetylene is constructed. Photoexcitation of the carbon dots induces transient electron transfer to the anthraquinone moieties, and concomitant change in the film dielectric constant and recorded capacitance. This unique photodetection mechanism furnishes wavelength selectivity that is solely determined by the absorbance of the carbon dots incorporated in the anthraquinone-polydiacetylene matrix. Accordingly, employing an array of polymerized-anthraquinone photodetector films comprising carbon dots (C-dots) exhibiting different excitation wavelengths yielded optical "capacitive fingerprints" in a broad spectral range (350-650 nm). Furthermore, circular light polarization selectivity is achieved through chiral polymerization of the polydiacetylene framework. The carbon dot/anthraquinone-polydiacetylene capacitive photodetector features rapid photo-response, high fidelity, and recyclability as the redox reactions of anthraquinone are fully reversible. The carbon dot/anthraquinone-polydiacetylene platform is inexpensive, easy to fabricate, and consists of environmentally friendly materials.

Design Principles for the Acceptor Units in Donor-Acceptor Conjugated Polymers

More than 50 different acceptor units from the experimental literature have been modeled, analyzed, and compared by using the computationally extracted data from the density functional theory (DFT) perspective for tetramer structures in the form of (D-B-A-B)₄ (D, donor; A, acceptor; B, bridge) with fixed donor and bridge units. Comparison of dihedral angle between acceptor, donor, and bridge units, bond order, and hyperpolarizability reveals that these three structural properties have a dominant effect on the frontier electronic energy levels of the acceptor units. Systematic investigation of the structural properties has demonstrated the band gap energy dependency of the acceptor units on the planarity, conjugation, and the electron delocalization. Substitution effect, morphological alternation, and insertion of π-electron deficient atoms in A unit have also an important role to determine physical properties of the donor-acceptor conjugated polymers. This benchmark study will be beneficial for the band gap engineering and molecular design of the donor-acceptor copolymers using different acceptor units for the organic electronic applications.

Novel Dark Current Reduction Strategy via Deep Bulk Traps for High-Performance Solution-Processed Organic Photodetectors

The performance improvement of the organic photodetectors (OPDs) focuses on suppressing the dark current density (J_d) to improve the specific detectivity. In this work, a dark current reduction strategy relying on constructing limited deep traps in the active layer to suppress charge injection rate was newly proposed. And an optimization method has been successfully demonstrated on the solution-processed OPDs accordingly. Compared with the J_d expressed by the OPD with the shallow trap system, the device with deep bulk traps exhibits a dramatically reduced dark current while ensuring high responsivity. At a bias of -2 V, the optimized photodiode with a J_d down to 1.4 × 10^-5 mA cm^-2 and a maximum responsivity of 0.42 A W^-1 @620 nm was realized, leading to a maximum detectivity calculated from shot noise of 6.23 × 10¹² Jones. This value is 49-fold higher than that of the original OPD with the same structure. The effects of deep traps inside the semiconductor film on injected carriers and photogenerated carriers are well explained by the relative positions of the initial hopping levels. A better understanding of charge transport regimes in OPD helps to open new approaches for constructing high-performance OPD toward practical applications.

High-Performance All-Polymer Photodetectors Enabled by New Random Terpolymer Acceptor with Fine-Tuned Molecular Weight

Reducing the dark current density and enhancing the overall performance of the device is the focal point in research for organic photodetectors. Two novel random terpolymers (P3 and P4) with different molecular weights are synthesized and evaluated as acceptors in bulk heterojunction (BHJ) polymer photodetectors. Compared with known acceptor materials, such as N2200 (P1) and F-N2200 (P2), polymer P4 has a lower lowest unoccupied molecular orbital (LUMO) energy level, favorable morphology, and good miscibility with a donor material J71, which leads to proper phase separation of the blend film and better dissociation of excitons and transport of carriers. Therefore, a considerably low dark current density (Jd) of 1.9 × 10-10 A/cm2 and a high specific detectivity (D*) of 1.8 × 1013 cm Hz1/2/W (also "Jones") at 580 nm under a -0.1 V bias are realized for the P4-based photodetector. More importantly, the device also exhibits a fast response speed (τr/τf = 1.24/1.87 μs) and a wide linear dynamic range (LDR) of 109.2 dB. This work demonstrates that high-performance all-polymer photodetectors with ideal morphology can be realized by random polymer acceptors with a fine-tuned molecular weight.

A New Dioxasilepine-Aryldiamine Hybrid Electron-Blocking Material for Wide Linear Dynamic Range and Fast Response Organic Photodetector

A new dioxasilepine and aryldiamine hybrid material DPSi-DBDTA is designed to act as the electron-blocking layer (EBL) for vacuum-processed organic photodetector (OPD). The O-Si-O-linked cyclic structure leads DPSi-DBDTA to have dipolar character, high LUMO, and good thermal and morphology stability suitable for vacuum deposition. An initial trial with C₆₀-based single active layer OPD device manifests the superior capability of DPSi-DBDTA for dark current suppression compared to the typical aryldiamines. Here, the bare and MoO₃-doped DPSi-DBDTA is further examined as EBLs for the visible light responsive OPD comprising DTDCPB/C₇₀ bulk heterojunction (BHJ) as the active layer. In sync with the result of C₆₀-based OPD, the low dark current density and high specific detectivity D* (7.085 × 10¹² cm Hz^1/2 W^-1) are achieved. The device with 5% MoO₃-doped EBL can exhibit a wide linear dynamic range (LDR) up to 154.166 dB, which is attributed to suppression of both dark current density and carrier recombination. Additionally, the devices also manifest fast time-resolved performance in both frequency and transient response measurements. Especially for the device with 20% MoO₃-doped EBL, a wide cutoff frequency response 692.047 kHz and record-high transient response demonstrating ≤0.683 μs for transient photovoltage (TPV) and ≤0.478 μs for transient photocurrent (TPC) have been realized, which is possibly owing to the balance of mobility that mitigates the damage from traps. Such submicrosecond response is comparable with the state-of-the-art perovskite-PDs and Si-PDs.

Narrowband organic photodetectors - towards miniaturized, spectroscopic sensing

Omnipresent quality monitoring in food products, blood-oxygen measurement in lightweight conformal wrist bands, or data-driven automated industrial production: Innovation in many fields is being empowered by sensor technology. Specifically, organic photodetectors (OPDs) promise great advances due to their beneficial properties and low-cost production. Recent research has led to rapid improvement in all performance parameters of OPDs, which are now on-par or better than their inorganic counterparts, such as silicon or indium gallium arsenide photodetectors, in several aspects. In particular, it is possible to directly design OPDs for specific wavelengths. This makes expensive and bulky optical filters obsolete and allows for miniature detector devices. In this review, recent progress of such narrowband OPDs is systematically summarized covering all aspects from narrow-photo-absorbing materials to device architecture engineering. The recent challenges for narrowband OPDs, like achieving high responsivity, low dark current, high response speed, and good dynamic range are carefully addressed. Finally, application demonstrations covering broadband and narrowband OPDs are discussed. Importantly, several exciting research perspectives, which will stimulate further research on organic-semiconductor-based photodetectors, are pointed out at the very end of this review.

Carrier Blocking Layer Materials and Application in Organic Photodetectors

As a promising candidate for next-generation photodetectors, organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using solution processes and a tunable spectral response range, making them particularly attractive for large area image sensors on lightweight flexible substrates. Carrier blocking layers engineering is very important to the high performance of OPDs that can select a certain charge carriers (holes or electrons) to be collected and suppress another carrier. Carrier blocking layers of OPDs play a critical role in reducing dark current, boosting their efficiency and long-time stability. This Review summarizes various materials for carrier blocking layers and some of the latest progress in OPDs. This provides the reader with guidelines to improve the OPD performance via carrier blocking layers engineering.

A Sensitive Visible Light Photodetector Using Cobalt-Doped Zinc Ferrite Oxide Thin Films

In this study, a highly sensitive trilayer photodetector using Co-doped ZnFe₂O₄ thin films annealed at 400 °C was synthesized successfully. Trilayer-photodetector devices with a film stack of 5 at % Co-doped-zinc-ferrite-thin-film/indium-tin-oxide on p⁺-Si substrates were fabricated by radio-frequency sputtering. The absorbance spectra, photoluminescence spectra, transmission electron microscopy images, and I-V characteristics under various conditions were comprehensively investigated. The outstanding performance of trilayer-photodector devices was measured, including a high photosensitivity of 181 and a fast photoresponse time with a rise time of 10.6 ms and fall time of 9.9 ms under 630 nm illumination. Therefore, the Co-doped ZnFe₂O₄ thin film is favorable for potential photodetector applications in visible light regions.

Broadband photomultiplication organic photodetectors

Broadband photomultiplication organic photodetectors (PMOPDs) can be achieved with a double-layered active layer prepared from IEICO-4F : PBDB-T blend solutions with different weight ratios (1 : 1 or 3 : 100, wt/wt). The response range of the double-layered PMOPDs covers from 310 nm to 930 nm, determined by the photon harvesting range of the IEICO-4F : PBDB-T (1 : 1, wt/wt) layer. The IEICO-4F : PBDB-T (3 : 100, wt/wt) layer was used as a PM layer in the double-layered PMOPDs, achieving external quantum efficiency (EQE) more than 100% based on the work mechanism of trap-assisted hole tunneling injection. The trapped electrons in PBDB-T/IEICO-4F/PBDB-T near the Al electrode will makeinterfacial-band-bending to narrow the injection barrier, resulting in hole-tunneling-injection from the external circuit. The polymer PBDB-T can provide an efficient charge transport channel for the injected hole from the external circuit. The specific detectivity (D*) and responsivity (R) of the double-layered PMOPDs are 1.05 ± 0.03 × 1012 Jones and 0.94 ± 0.03 A W-1 at 810 nm under a -10 V bias, respectively.

Copper Thiocyanate as an Anode Interfacial Layer for Efficient Near-Infrared Organic Photodetector

Interfacial modification between the electrode and the overlying organic layer has significant effects on the charge injection and collection and thus the device performance of organic photodetectors. Here, we used copper(I) thiocyanate (CuSCN) as the anode interfacial layer for organic photodetector, which was inserted between the anode and an organic light-sensitive layer. The CuSCN layer processed with ethyl sulfide solution presented similar optical properties to the extensively used anode interlayer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), while the relatively shallow conduction band of CuSCN resulted in a much higher electron-injection barrier from the anode and shunt resistance than those of PEDOT:PSS. Moreover, the CuSCN-based device also exhibited an increased depletion width for the PEDOT:PSS-based device, as indicated by the Mott-Schottky analysis. These features lead to the dramatically reduced dark current density of 2.7 × 10^-10 A cm^-2 and an impressively high specific detectivity of 4.4 × 10¹³ cm Hz^1/2 W^-1 under -0.1 V bias and a working wavelength of 870 nm. These findings demonstrated the great potential of using CuSCN as an anode interfacial layer for developing high-performance near-infrared organic photodetectors.

High-Detectivity Non-Fullerene Organic Photodetectors Enabled by a Cross-Linkable Electron Blocking Layer

The anode interlayer plays a critical role in the performance of organic photodetectors, which requires sufficient electron-blocking ability to simultaneously attain a high photocurrent and low dark current. Here, we developed two cross-linkable polymers, which can be deposited on the top of the widely used poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and form a robust layer that can effectively suppress the electron injection from the anode under reverse bias. The optimized device with the resulting cross-linkable XP2 exhibited the lowest dark current density of 5.81 × 10^-9 A cm^-2 at -0.1 V, which is about 2 orders of magnitude lower than the control devices. A remarkable responsivity of 0.5 A W^-1 and a detectivity of >1 × 10¹³ Jones at a near-infrared wavelength of 800 nm were achieved. Of particular importance is that the resulting device exhibited a linear dynamic range of >135 dB associated with a high working frequency that is shorter than typical commercial digital imagers. The planar heterojunction devices demonstrate that the dark current is closely correlated to the charge generation, which relied on the highest occupied molecular orbital energy levels of the developed cross-linked interlays. The Mott-Schottky analysis revealed that the optimized cross-linked interlayer increased the depletion width of the devices.

Bioinspired Multifunctional Organic Transistors Based on Natural Chlorophyll/Organic Semiconductors

Inspired by the photosynthesis process of natural plants, multifunctional transistors based on natural biomaterial chlorophyll and organic semiconductors (OSCs) are reported. Functions as photodetectors (PDs) and light-stimulated synaptic transistors (LSSTs) can be switched by gate voltage. As PDs, the devices exhibit ultrahigh photoresponsivity up to 2 × 10⁶ A W^-1 , detectivity of 6 × 10¹⁵ Jones, and I_photo /I_dark ratio of 2.7 × 10⁶ , which make them among the best reported organic PDs. As LSSTs, important synaptic functions similar to biological synapses are demonstrated, together with a dynamic learning and forgetting process and image-processing function. Significantly, benefiting from the ultrahigh photosensitivity of chlorophyll, the lowest operating voltage and energy consumption of the LSSTs can be 10^-5 V and 0.25 fJ, respectively. The devices also exhibit high flexibility and long-term air stability. This work provides a new guide for developing organic electronics based on natural biomaterials.

Photomultiplication Type Broad Response Organic Photodetectors with One Absorber Layer and One Multiplication Layer

Broad response organic photodetectors (OPDs) with a photomultiplication (PM) effect are achieved with one absorber layer and one multiplication layer. The response range of the PM-OPDs is primarily determined by materials in the absorber layer, and the external quantum efficiency (EQE) of the PM-OPDs is mainly controlled by the multiplication layer. Here, double-layered PM-OPDs were designed with an ITO/ZnO/PM6:Y6/PC₇₁BM:P3HT (100:5, w/w)/Au structure, where PM6:Y6 is employed as an absorber layer and PC₇₁BM:P3HT is used as a multiplication layer. The optimal PM-OPDs exhibit a broad response covering 350-950 nm. Meanwhile, the optimal PM-OPDs exhibit the largest EQE value of ∼1200% and a maximum specific detectivity (D*) of ∼6.8 × 10^-12 cm Hz^1/2 W^-1 under a 10 V bias. This double-layered approach may be a smart strategy for realizing PM-OPDs with an easily adjustable response range.