Optical sensors are everywhere in modern life: applications like food quality monitoring, blood-oxygen measurement in conformal wristbands, or data-driven automated industrial production require sensitive optical detection at various wavelengths. Specifically, organic and hybrid photodetectors (OHPDs) promise excellent opportunities with beneficial properties such as large-area, flexibility or stretchability, transparency, biocompatibility, and low cost. Recent research has not only improved understanding of device physics and rules for material optimization, but also led to rapid improvement in OHPDs performance, which are now on-par or even better than their inorganic counterparts, such as silicon or indium gallium arsenide photodetectors. For instance, it is possible to directly design OHPDs for specific wavelengths, thus making bulky optical filters obsolete allowing miniature devices. As another example, intrinsically stretchable, biocompatible/resorbable detectors are possible, enabling completely new medical applications. Recently, OHPDs have been realized that reach or even surpass detectors based on inorganic crystalline materials.1,2 Therefore, research has recently significantly intensified and produced a variety of new detector approaches. This special collection summarizes this recent novel work and gives an outlook to what future topics should be addressed.
In this Special Issue, recent progress of OHPDs is systematically summarized by leading research groups in the field, covering all aspects from novel photo-absorbing organic and nanoparticle materials, device physics, and architecture. The recent challenges for OHPDs, like achieving high responsivity over tailored spectral regions of the visible and infrared spectral ranges, low electronic noise, high response speed, and high dynamic range, are addressed.
Fu et al.3 report organic–inorganic deep ultraviolet (DUV) photodetectors, based on PEDOT:PSS with an in situ transferred composite film PEDOT:PSS-nitrogen-doped graphene (NGr)-coated SnO2 microwire. The light-dark current ratio of the PEDOT:PSS-NGr devices was improved by three orders of magnitude by the NGr, showing that it is an effective way to modify detector performance.
HgTe nanocrystals are presented as platform for designing infrared optoelectronic devices by Alchaar et al.4 In particular, they show diode stacks combined with a readout integrated circuit whose back-end processing has been optimized to ensure compatibility diode deposition. Taking benefit from the full VGA format, high-resolution images are taken.
Significant progress in metal halide perovskites based x-ray detection is reported by Li et al. They introduce a heterovalent cation exchange strategy and achieve nanocrystals exhibiting relatively high photoluminescence quantum yields and improved thermal stability and water resistance. These Sb@CsPbBr3 nanocrystals also show great potential for x-ray detection as scintillators, with fast response, bright and radioluminescence, and excellent image quality.5
Bu et al.6 address polarization-sensitive photodetectors with a new approach: Using a van der Waals Schottky photodiode comprising the 2D semiconductor InSe and the semimetal 1T'-MoTe2, they demonstrate linearly polarized photodetection with a high polarization ratio of up to ∼22, useful for practical applications in angle-dependent photodetector and image sensors.
The dark current directly enters the detectivity and is thus a key issue of organic photodetectors. Liu et al.7 present a simple yet effective strategy to enhance detectivity by incorporating a non-conjugated polymer additive of polystyrene, suppressing the amorphous phase and tuning the phase separation in the bulk heterojunction blends. This generally applicable technique leads to significantly enhanced photodetector performance.
Novel infrared-to-visible upconverters combining infrared photodetectors with visible light-emitting diodes to directly visualize infrared images are presented by Rao et al.8 They integrate strong-short-wave-IR-response n-type germanium (Ge)/indium tin oxide (ITO) photodiodes with phosphorescent organic LEDs to realize efficient upconversion to green light through a simple fabrication process. The Ge-OLED upconverters show efficient upconversion of SWIR and a high efficiency of 7%.
Photomultiplication type quasi-planar all-polymer photodetectors (PM-QAPDs) are reported by Zhang et al.9 The response range of PM-QAPD was changed from broadband to narrowband by increasing the thickness of the donor layer. This is a smart strategy to adjust response range of PM-QAPDs by alerting the thickness of the donor layer.
The paper of Wu et al.10 presents cost-effective strategy to control the solution-state aggregation of hole transporting layers of quantum dot photodetectors by designing a dual polythiophene blend based on P3HT and its alkylthio-substituted analog named Poly(3-hexylthiothiophene) (P3HTT). They enhanced the performance of quantum dot photodetectors by incorporating a small amount of P3HTT into the dual polythiophene mixture.
Solution processed color-tunable upconversion devices integrating a near-infrared sensing photodetector and a color-tunable quantum dot (QD) light-emitting are addressed by Hu et al.11 They mixed the red and green QDs in a single emissive layer for multi-color emission. The image color can be modulated by bias voltage and driving current and shows a wide color-span range from red to green.
Xu et al.12 discuss the wavelength selection function of position-sensitive detectors by using PANI:PSS and organic–inorganic heterojunctions. This structure can effectively filter specific wavelengths of light by the conversion of the absorption layer, bistable switching, and enhanced light absorption by the self-powered voltage. This result provided the theoretical foundation for highly selective and tunable optoelectronic devices while helping to overcome the challenges of high manufacturing costs and customized application scenarios.
Jan et al.13 demonstrate NIR photodetectors based on an organic bulk heterojunction absorbing layer with state-of-the-art performance across critical response metrics. The external quantum efficiency spectrum of the devices presented extends from 500 nm to approximately 900 nm covering both visible and NIR wavelengths. The organic photodiodes (OPDs) show specific detectivities of around 7.4 × 1011 Jones and linear dynamic range (LDR) of 74 dB.
High-speed and sensitivity near-infrared organic photodetectors, based on PM6:CH-4Cl nonfullerene acceptors, are reported by Zhu et al.14 The OPD exhibits a short response time of 270 ns and detectivity of over 1013 Jones. This outstanding performance is attributed to the low trap states and energetic disorder of OPDs with CH-4Cl. They also demonstrated high-speed optical wireless communication using this high-speed OPD.
Kim et al.15 report an ultra-flexible energy harvesting system embedding thin, stretchable nanopillar arrays made of poly(dimethylsiloxane). This nanopillar arrays amplify the photogeneration of exciton pairs and increase the current density of PM6:Y6 based organic photovoltaic (OPV) from 25.46 to 28.01 mA/cm2, resulting in a PCEmax of 15.92%. This generally applicable new technique leads to significantly enhanced ultra-flexible organic photovoltaic performance.
High efficiency near-infrared (NIR) photomultiplication-type organic photodetectors (PM-OPDs) are presented by Guo et al.16 The performance of the PM-OPD was significantly improved by combining the energetic disorder and trap-assisted charge tunneling injection. In addition, they demonstrated heart rate detection using their NIR PM-OPD.
McGinn et al.17 address the use of parylene N (Pa-N), polymethyl methacrylate (PMMA), and polyvinylidene difluoride trifluoroethylene (PVDF-TrFE) as passivation layers on MoSO2 field-effect transistors. Using terahertz spectroscopy, Pa-N and PMMA are found to yield an increased photoconductivity through n doping, while PVDF-TrFE leads to much longer carrier lifetimes.
A solution-based approach is used by Al Mahfuz et al.18 to synthesize PbSe photoconductive films having mid-infrared photoconductive properties. They obtain photodetectors that achieve room-temperature specific detectivity values of 109 Jones at 3.5 μm.
Ma et al.19 employ a pressure bonding process to insert a two-dimensional perovskite into a three-dimensional perovskite, resulting in a two-order of magnitude reduction of the dark current in perovskite photodetectors. This process leads to perovskite photodetectors with specific detectivity values of 4.11 × 1012 Jones, a −3 dB cutoff frequency of 344 kHz, and a 160 dB linear dynamic range.
Perovskites are also used by Alphenaar et al.20 who report on the synthesis of carbazole-based organic cations and their use on two-dimensional layered hybrid perovskite photodiodes that achieve an unbiased specific detectivity of 6.95 × 1010 Jones at 485 nm.
Lv et al.21 discuss a human-eye inspired sensor consisting of quasi-two-dimensional perovskite and indium gallium zinc-oxide phototransistor arrays that achieve responsivity values of 5 × 105 A/W in the visible spectral range, with a low-light detection limit of 6.1 nW/cm2 and a dynamic range of 162 dB.
Filter-free visible-blind near-infrared organic photodetectors that use a double bulk heterojunction structure and a copper thiocyanate-based electron-blocking layer that suppresses visible-light photogenerated carriers and allows only near-infrared photogenerated carriers to contribute to the photocurrent are discussed by Wang et al.22 The double bulk heterojunction organic photodetectors achieve a responsivity of 0.38 A/W at 1050 nm and specific detectivity values >1013 Jones in the range from 800 to 1050 nm
Shao et al.23 report on polarization-sensitive organic photodetectors using anisotropic all-polymer bulk heterojunctions (BHJs) obtained by molecular fluorination engineering in the fused-ring backbone of the acceptor polymer. These photodetectors achieve a high photocurrent dichroic ratio of 3.73, a specific detectivity of 1.3 × 1011 Jones at 0 V, and a broad linear dynamic range of 120 dB.
Amorphous Ga2O3 metal–semiconductor–metal photodetectors passivated by an organosilicon layer are introduced by Liu et al.,24 who report on a. Ga2O3 photodetectors with organosilicon passivation layer exhibit a low dark current of 2.96 × 10−12 A, a responsivity of 11.82 A/W, and a specific detectivity of 9.01 × 1014 Jones.
Quantum well (QW) diodes, capable of dual functionality as light emitters and detectors, exhibit partial spectral overlap between emission and detection. Utilizing distributed Bragg reflection (DBR) technology, Fu et al. demonstrate wavelength-division multiplexing (WDM) visible light communication (VLC) via vertically stacked red, green, and blue (RGB) QW diodes.25 These identical diodes alternate as transmitters and receivers, enabling a single optical path for bidirectional communication. DBRs serve as optical filters, enhancing channel capacity by managing photon wavelengths. By integrating time-division multiplexing (TDM) with WDM, real-time multichannel bidirectional communication is achieved, showcasing significant potential for TDM-WDM VLC systems.
Consistent and standardized measurement techniques are essential for reliable comparisons across studies and different photodetector technologies. Thachoth Chandran et al. explore potential causes of inaccuracies in reporting linear dynamic range (LDR) values, focusing on the significance of unity slope in determining deviation points and highlighting risks of misinterpretation from varying definitions. Using a representative organic photodiode system, the authors propose criteria to standardize LDR representation, aiming to enable more meaningful comparisons and foster progress in this promising field.26
Choi et al. report on an organic phototransistor (OPT) with a photomultiplication mechanism, employing a C70-based bulk-heterojunction channel and Al Schottky contacts as source/drain electrodes.27 The photomultiplication effect is primarily driven by light absorption in the source-side region. To enhance light-sensing performance, Corbino source/drain geometries are proposed, featuring a larger source electrode area relative to the drain. These Corbino-type OPTs exhibit significantly improved photosensitivity, achieving a 1.7-fold performance increase over conventional lateral designs.
Amorphous oxide semiconductor photodetectors (PDs) show promising potential for ultraviolet (UV) sensing, offering low dark current and high photoresponse due to their superior UV absorption and carrier transport capabilities. Zhang et al. optimize UV-sensitive and power-efficient oxide phototransistors through precise engineering of indium-zinc oxide (InZnO) thickness and gate bias modulation.28 While ultrathin InZnO suffers from low photoresponse and thicker layers exhibit high dark current, an 8 nm thickness is found to achieve optimal performance. This configuration ensures high sensitivity (Smax = 1.25 × 106), low dark current (Idark ≈ 10−13 A), and reliable operation using constant gate bias for sensitivity and temporal bias to eliminate persistent photocurrents.
Keerthi report on a surface plasmon resonance-assisted hybrid photodetector (PD) utilizing a low bandgap covalent polymeric framework that demonstrates broadband detection from 350 to 1550 nm with subsecond response times.29 It achieves remarkable performance with a responsivity of 42.8 A/W at 410 nm. The PD maintains a responsivity >0.4 A/W at 1550 nm and records a peak detectivity of 7.4 × 1013 Jones at 400 nm. Rise and fall times of 0.31 and 0.22 s highlight its suitability for efficient broadband applications.
Traditional OPDs often require thick junctions or extra optical components, which can be costly and complex. In this study by Zhang et al., a low-donor content strategy in thin-film OPDs is used to achieve narrowband spectral features.30 This approach significantly reduces aggregation, improving shunt resistance and dark current while preserving photocurrent. The optimized device achieves an external quantum efficiency of 49.3% at 770 nm and a high specific detectivity of 1.0 × 1013 Jones at zero bias. It also shows fast NIR response times of 530 ns (rise) and 840 ns (fall) at 10 kHz, demonstrating the potential of low-donor content blends for narrowband photodetection.
We believe that the field of organic and hybrid photodetectors is a lively field that will open up many new application perspectives. We hope that this Special Issue will stimulate further research in that direction.
We thank all authors of the Special Issue for submitting their work. We thank Jaimee-Ian Rodriguez and Jessica Trudeau for the excellent collaboration and Maria Antonietta Loi for stimulating this project.