Recent Advances in Organic Photodetectors

Abstract

Organic photodetectors (OPDs) have garnered significant attention in fields such as image sensing, health monitoring, and wearable devices due to their exceptional performance. This review summarizes recent research advancements in materials, structures, performance, and applications of narrowband organic photodetectors, hybrid organic–inorganic perovskite photodetectors, flexible organic photodetectors (FOPDs), and photomultiplication type organic photodetectors (PM-OPDs). Organic semiconductors offer substantial potential in optoelectronic devices owing to their low cost, ease of processing, and tunable spectral response. Hybrid perovskite materials extend the spectral response range, FOPDs meet the demands of wearable devices, and PM-OPDs enhance sensitivity, allowing for the detection of weak light signals. Through innovations in materials, structural optimization, and improvements in manufacturing processes, the performance of OPDs has seen significant enhancement. This article also explores the application prospects of these technologies in medical monitoring, optical communications, and image sensing.

1. Introduction

Organic photodetectors (OPDs) have garnered considerable attention as a nascent photodetection technology since their inaugural design in the mid-20th century [1]. The initial research efforts were primarily directed towards elucidating the photoconversion performance of organic semiconductor materials. Despite the current dominance of inorganic photodetectors (IPDs) in the market, OPDs have exhibited a growing capacity to adapt to the increasing demand for lightweight, biocompatible, and cost-effective solutions. On the one hand, the inherent mechanical rigidity and high production costs of inorganic semiconductor materials present challenges in meeting the sensor requirements of the Internet of Things era. Conversely, the mechanical flexibility, semi-transparency, and low cost of organic semiconductor materials effectively address these emerging needs. With advancements in materials science and nanotechnology, the performance of OPDs has significantly improved, leading to an expanded application range that includes image sensing, environmental monitoring [2], health monitoring, wearable devices, and other domains.

Most inorganic semiconductors have broadband response characteristics (molybdenum oxide/tungsten oxide (MoO₃/WO_3-x, 400–1550 nm) [3], lead sulfide (PbS)/graphene (600–3000 nm) [4], antimony selenide (Sb₂Se₃, 370.6–2200 nm) [5]), and additional devices such as filters often need to be added in order to achieve detection of specific spectra. This reduces the intensity of light absorbed by sensors based on inorganic semiconductors, which not only raises the difficulty of detection, but also increases costs. Organic semiconductor materials are gradually replacing inorganic materials such as silicon by virtue of their excellent optoelectronic properties. In terms of spectral absorption, organic semiconductor materials can be designed to change the bandgap at the molecular level; in terms of the preparation process, organic semiconductor materials can be fabricated by solution method (printing, lamination [6]) and other low-cost, large-area manufacturing. These beneficial properties have contributed greatly to the development of OPDs, making the goals of lightweight, intelligent, and low cost no longer inconceivable [7]. With the rapid advancement in material technology and the introduction of solar cell design strategies [8,9], narrowband OPDs have also been developed [10].

Perovskite materials are an emerging class of optoelectronic semiconductors with a crystal structure denoted as ABX₃, where A represents an organic–inorganic cation, B is a metal cation, and X is a halide anion. This unique crystal structure endows perovskite materials with exceptional optoelectronic properties such as tunable bandgaps, low exciton binding energy, and long carrier diffusion lengths [11,12], making them highly attractive to researchers. Perovskite materials exist in three microscopic structures: polycrystalline thin films, single crystals, and low-dimensional nanostructures. These structures exhibit decreasing defect densities, increasing crystallographic order, and progressively superior performance. Moreover, hybridizing perovskite materials with organic compounds to form heterojunctions can extend the spectral response range of PDs. Another significant advantage of perovskite materials is their ease of fabrication. Compared to the complex manufacturing processes of traditional inorganic PDs, such as silicon-based materials [13], perovskite films can be produced using solution-based methods at low temperatures [14]. Inspired by these achievements, many researchers have made exciting advancements in the field of photodetectors, which will be discussed in subsequent sections.

Researchers have continually innovated and improved the materials and structures of flexible organic photodetectors (FOPDs) to ensure they possess excellent photoelectric performance and mechanical flexibility. Traditional glass substrates, despite their good light transmission, cannot conform to complex, curved surfaces. Therefore, polymer and fiber materials are often used to create flexible substrates [15]. These materials not only have stable chemical properties but also offer good light transmittance. Indium tin oxide (ITO) is the most widely used transparent electrode material [16], but its rigidity and high deposition temperature limit its further application. Emerging transparent electrode materials, such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) [17], carbon nanotubes (CNTs) [18], and silver nanowires (Ag NWs) are easier to fabricate and do not significantly compromise performance compared to ITO. The active layer is the crucial functional layer influencing the photoelectric performance of FOPDs. Traditional inorganic PDs are mostly based on silicon materials, which have poor mechanical flexibility and complex fabrication processes, making them unsuitable for applications like displays. Currently, organic materials and organic/perovskite hybrid materials are rapidly developing, gradually replacing traditional inorganic materials. FOPDs are gaining prominence in the field of wearable devices, particularly in medical health monitoring, where they serve as ideal devices for heart rate and blood oxygen saturation monitoring [19].

To meet the requirements for detecting weak light signals, enhancing the sensitivity of PDs is crucial [20]. Methods to improve sensitivity generally include increasing external quantum efficiency (EQE), reducing dark current, adjusting response speed, and broadening the spectral response range. The phenomenon of photomultiplication is an excellent choice for enhancing PD sensitivity [21]. Unlike inorganic materials that can utilize avalanche effects to achieve photomultiplication, organic materials have high exciton binding energies and less ordered microstructures compared to inorganic crystals, necessitating alternative approaches. Researchers have discovered that trap states exist at the interfaces between organic materials and metals [22], which facilitate exciton migration. Additionally, employing organic–inorganic heterostructures and diamond-based photoconductors [23] can also achieve photomultiplication effects.

Numerous studies have reported methods to improve the performance of OPDs, which are crucial for advancing OPD technology. However, to our knowledge, few reviews introduce and summarize the different types of OPDs, making it difficult for readers and researchers to gain a systematic and comprehensive understanding of the OPD technology landscape. In this review, we first present the performance parameters of OPDs to help readers better understand the advancements in OPD performance. Then, we provide a detailed introduction to the development and application fields of narrowband OPDs, hybrid organic–inorganic perovskite PDs, FOPDs, and photomultiplication type organic photodetectors (PM-OPDs) in terms of materials and device structures. The progress of OPDs is summarized in Figure 1. Finally, we discuss the current challenges and future prospects of OPDs.

2. Performance Parameters of Photodetectors

To better evaluate the performance of photodetectors, several commonly used parameters are employed.

On/off ratio reflects the sensitivity of photodetectors to light signals, which can be calculated as the ratio of the photocurrent to the dark current.

Responsivity (R) is defined as the ratio of the output current or voltage to the input optical power of the incident light signal. The calculation formula is as follows:

R = I/P

(1)

where I is the photocurrent or photovoltage and P is the input optical power of the incident light signal. R has units of A/W or V/W.

EQE is a primary performance metric for photodetectors. It is defined as the ratio of the number of collected electrons to the number of incident photons. The calculation formula is as follows:

EQE = (I_p/e)/(P/hν)

(2)

where I_p is the photocurrent, e is the elemental charge, P is the irradiated optical power intensity, h is Planck’s constant, and ν is the vibrational frequency of irradiated light.

Linear Dynamic Range (LDR) defines the range of optical power over which the output current or voltage of the photodetector maintains a linear relationship with the input light signal. The calculation formula is as follows:

LDR = 20log(P_MAX/P_MIN)

(3)

where P_MAX is the maximum value of the optical power range and P_MIN is the minimum value of the optical power range.

Noise Equivalent Power (NEP) refers to the minimum input optical power required for the output signal-to-noise ratio to reach 1. The calculation formula is as follows:

NEP = i_n/R

(4)

where i_n is the noise current and R is responsivity. NEP has a unit of W/Hz^1/2.

Detectivity is defined as the reciprocal of NEP, representing the ability of a photodetector to detect weak light signals. D* is the normalized detectivity used to measure the overall performance of the photodetector. The calculation formula is as follows:

$${\mathrm{D}}^{*}=\sqrt{\mathrm{A}\mathrm{B}}/\mathrm{N}\mathrm{E}\mathrm{P}$$

(5)

where A is the effective area and B is the noise bandwidth. D* has a unit of Jones.

3. Narrowband Organic Photodetectors

Organic semiconductor materials, due to their unique physical and chemical properties, have shown tremendous potential in the field of photodetectors. These materials offer advantages such as low cost, ease of processing, large-area fabrication, and excellent mechanical flexibility. Additionally, their molecular structures can be precisely tuned through chemical modifications, enabling spectral response coverage from ultraviolet to mid-infrared. These characteristics make organic photodetectors highly promising for applications in high-performance optoelectronic devices, wearable devices, and portable sensors.

However, organic semiconductors also have inherent drawbacks, such as high exciton binding energy and relatively low carrier mobility, which limit the performance of organic photodetectors to some extent. To overcome these challenges, researchers have been developing new organic semiconductor materials, optimizing device structures, and employing innovative fabrication techniques. These efforts aim to enhance performance parameters such as responsivity, detectivity, response speed, and photocurrent. This section will explore the selection of materials, structural innovations, and application areas in detail.

3.1. Innovations in Materials and Structures

Enhancing photocurrent and suppressing dark current are critical methods for improving photodetector performance. In 2018, Wei et al. designed and fabricated a photodetector based on an organic material p-type polymer (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)-benzo[1′,2′-c:4′,5′-c’]dithiophene-4,8-dione))]) (PBDB-T) and an n-type nonfullerene molecule (PBI-Por, which has four perylene bisimide (PBI) units connected at the meso-positions of a porphyrin ring through ethylnyl groups) (molecular structures shown in Figure 2a,b) combined with PbS quantum dots [28]. The fabrication method involved preparing a PbS-TBAI (tetrabutylammonium iodide) film on a glass substrate, then spin-coating a mixture of PBDB-T and PBI-Por to form a heterostructure, and finally depositing aluminum (Al) as the electrode. As shown in Figure 2c, the device exhibits a lateral bilayer structure. For comparison, the team also prepared single-layer PDs containing only the PBDB-T and PBI-Por mixture layer or only the PbS layer. Figure 2d,e demonstrate that the dark current of the OPD and PbS PD is one to two orders of magnitude higher than that of the bilayer PD, and the photocurrent of the bilayer PD is also larger. This can be explained by the fact that in the bilayer PD, the heterostructure promotes the dissociation of photogenerated excitons, whereas in the single-layer PD, the regions where photogenerated holes and electrons transport overlap, leading to easier recombination. Due to the enhanced photocurrent (Figure 2f) and significantly suppressed dark current, the responsivity and specific detectivity of the bilayer PD at 630 nm wavelength reach 1.89 A/W (Figure 2g) and 1.12 × 10¹³ Jones (Figure 2h), respectively. Additionally, under a 40 V bias and 630 nm wavelength, the signal-to-noise ratio and linear dynamic range of the bilayer PD reach 1690 and 65 dB, respectively, demonstrating the superiority of the bilayer structure.

There are various methods to achieve narrowband detection, such as self-filtering narrowband OPDs. As mentioned earlier, organic semiconductor materials have the drawbacks of high exciton binding energy and short diffusion distances, but these can be improved by adding functional layers. In self-filtering narrowband OPDs, there is a thick donor layer with a larger optical bandgap and a thin acceptor layer with a lower optical bandgap. High-energy photons in the donor layer generate excitons that cannot dissociate, while only low-energy photons with strong penetration can reach the interface of the donor and acceptor layers to generate free charges. Based on this principle, in 2020, Xing et al. developed a self-filtering narrowband OPD with a planar heterojunction (PHJ) structure [24], as shown in Figure 3a. By adjusting the ratio of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) in poly(3-hexylthiophene) (P3HT), the original peak responsivity wavelength shifted from 645 nm to 745 nm, as shown in Figure 3b. By further optimizing the doping ratio, the specific detectivity was improved to 10¹² Jones. This achievement demonstrates that thin-film OPDs can also selectively detect the red-light spectrum, potentially contributing to the development of high-performance image sensors. Recently, Yin et al. reported a molecule with a core structure of [1–3]triazolo[4,5-f]−2,1,3-benzothiadiazole (TBzC), which possesses an optical bandgap of 0.97 eV and effectively absorbs in the short-wave infrared range [29]. By precisely adjusting the energy levels between the electron donor and TBzC, it is possible to reduce energy disorder in the neutral state, facilitating effective absorption of short-wave infrared light. The device demonstrates excellent photonic performance, achieving an EQE of 26% at 0 V bias. By increasing the reverse bias voltage, the EQE can reach a maximum of 41%. Within the wavelength range of 0.50–1.21 μm, the D* exceeds 10¹³ Jones.

The response speed of OPDs is also an important performance parameter. In 2021, Luc et al. designed and fabricated a metal-organic semiconductor-metal structure (MSM)-based PD using single-crystal micro-rods of β-MPc crystals (β-CuPc and β-ZnPc) as the active layer [30], as shown in Figure 3c. The single crystals were prepared by first synthesizing nanostructured MPc materials using the phthalocyanine method, followed by crystallization through an improved physical vapor transport technique. Compared to β-CuPc-based PDs, β-ZnPc-based PDs achieved enhanced photocurrent and reduced dark current. Under a bias of 5 V and ultraviolet (UV) light irradiation (wavelength of 265 nm, power of 730 μW/cm²), the β-ZnPc PD exhibited the responsivity as high as 11.6 A/W and the EQE of 5.4 × 10³%. As shown in Figure 3d,e, the response speed of the β-ZnPc-based PD increases with the intensity of incident light. As the incident light intensity increases, the rise time (τ_r) and decay time (τ_d) decrease from 0.622 s and 0.420 s to 0.142 s and 0.186 s (Figure 3d,e), respectively. The team’s future plans include optimizing the on/off current ratio of the device.

Polarization-sensitive OPDs can detect the intensity and polarization of light in a specific wavelength range. Therefore, they can be used as a means of image enhancement in the field of remote sensing and target detection [31], and can also be applied to flexible and wearable electronics, but the related research interest is relatively low. In 2022, Wang et al. investigated the intrinsic linear dichroism of organic single-crystal materials and utilized this property to construct high-performance polarization-sensitive PDs [32]. They chose 2,6-diphenyl anthracene (DPA), which is easy to grow into high-quality organic single crystals (the molecular structure is shown in Figure 3f, and the real optical image of the crystal is shown in Figure 3g) as the organic semiconductor material and the active layer, and designed a two-terminal structure of the photodetector by evaporating gold electrodes connected to a pre-patterned electronic circuit. The final test achieved a polarization sensitivity of about 1.9, which provides valuable experience for the development of a new type of dense polarization-sensitive OPD. Near the ground, PDs with detection wavelengths in the range of 200–300 nm are subject to little interference due to the solar-blind effect. In 2024, Zhang et al. reported a polarization-sensitive solar-blind UV PD based on a single-crystal trans-1,2-bis(5-phenyldithieno[2,3-b:3′,2′-d]thiophen-2-yl)ethene (BPTTE), which achieves a high dichroic ratio (DR) of about 4.26 [33]. The BPTTE single crystal, with its two-dimensional structure and strong anisotropy, enables stable polarized light detection by voltage modulation. It is also mentioned in the paper that an image sensor designed based on this device achieves high-contrast polarized imaging with strong resistance to interference.

Color discrimination represents a fundamental prerequisite for the development of high-performance image sensors. In comparison to the accelerated advancement of green OPDs [34], the development of blue OPDs has been relatively limited. Recently, the team led by Zhang has made significant progress in enhancing the EQE of blue OPDs and suppressing dark current [35]. The active layer comprised donor and acceptor materials, namely Rubrene and C₆₀, respectively. Rubrene displays restricted absorption of blue light at approximately 500 nm, whereas C₆₀ is primarily responsible for absorbing the majority of this wavelength. The device structure is depicted in Figure 3. In PHJ, the thicknesses of C₆₀ and rubrene are 20 and 25 nm, respectively. In bulk heterojunctions (BHJ), C₆₀ and Rubrene are combined in two ratios, namely 10 wt% and 25 wt%. To optimize the absorption spectrum of blue light, the team devised a series of hole transport layer (HTL) combinations, including 4,4′-bis(3,6-diphenylcarbazole)benzophenone (BF-DPB) and 4,4′-bis(3-(4-(4-(3,6-diphenylcarbazole))phenyl)phenyl)fluorene (BPAPF). The team reached the conclusion that doping MoO₃ in the bottom layer facilitates carrier dissociation, thereby enhancing the EQE at 0 V, which was found to be 50%. However, in devices with the BHJ structure, doping has the effect of disrupting the internal dark current. In devices comprising the BPAPF HTL and a PHJ structure, the dark current density was observed to decrease to 2.46 × 10⁻¹² A/cm² at −0.1 V, while the D* value reached 6.35 × 10¹³ Jones, indicating an enhanced charge separation efficiency.

Figure 3. (a) Schematic illustration of the device structure [24]. (b) Responsivity spectrum tuning [24]. (c) Schematic illustration of the device structure [30]. (d,e) J-t relationships under different light intensities, with the slope reflecting the rise time (d) and decay time (e) [30]. (f) Schematic illustration of the DPA molecular structure [32]. (g) Optical image of DPA crystals on a quartz substrate [32]. (a,b): Reprinted from [24], Copyright (2020), with permission from American Chemical Society. (c–e): Reprinted from [30], Copyright (2021), with permission from Elsevier. (f,g): Reprinted from [32], Copyright (2021), with permission from John Wiley and Sons.

Figure 3. (a) Schematic illustration of the device structure [24]. (b) Responsivity spectrum tuning [24]. (c) Schematic illustration of the device structure [30]. (d,e) J-t relationships under different light intensities, with the slope reflecting the rise time (d) and decay time (e) [30]. (f) Schematic illustration of the DPA molecular structure [32]. (g) Optical image of DPA crystals on a quartz substrate [32]. (a,b): Reprinted from [24], Copyright (2020), with permission from American Chemical Society. (c–e): Reprinted from [30], Copyright (2021), with permission from Elsevier. (f,g): Reprinted from [32], Copyright (2021), with permission from John Wiley and Sons.

3.2. Applications

In the medical field, commercially available devices for detecting health parameters such as heart rate, blood pressure, and oxygen saturation are typically IPDs such as silicon-based photodiodes. However, with the advancement in the performance of OPDs, some simple-to-operate but reliable photodetection devices of emerging materials have emerged. In 2016, Yokota et al. fabricated an ultra-flexible reflective pulse oximeter [36]. The device can be covered on the finger to detect the blood oxygen concentration and display it with a seven-segment digital tube as shown in Figure 4a. Recently, Shahriar Kabir et al. designed a near infrared (NIR) PD and demonstrated its potential for photoplethysmography (PPG) applications [37]. Octaoctylphthalocyanine (8H₂Pc, molecular structure shown in Figure 4b), which has liquid crystal state properties, was used as the donor material to create a bulk heterojunction with phenyl-C₆₁-butyric-acid-methyl-ester (PC₆₁BM, molecular structure shown in Figure 4c) to form a bulk heterojunction, and the device structure is shown in Figure 4d. Efficient response to near-infrared light (maximum responsivity and specific detectivity of 0.7 A/W and 1.0 × 10¹³ Jones at 740 nm with a bias voltage of −1 V, respectively) was achieved. With a simple measurement setup (NIR light was emitted from above and the subject’s finger was placed on the OPD as shown in Figure 4e), a clear human PPG signal was successfully detected (the oscilloscope displayed the results as shown in Figure 4f). A moving average filter is applied to distinguish the systolic and diastolic peaks of the cardiac cycle, and finally the average heart rate is extracted, which is consistent with the real value, verifying the effectiveness of the device in practical medical applications.

In the field of optical sensing, OPDs also play a crucial role. In 2021, Yifei Wang et al. reported a PD based on a narrow-bandgap small molecule (hexane-1,6-diammonium pentaiodobismuth, HDA-BiI₅), which operates without an electron transport layer [38]. The device structure is shown in Figure 5a. Figure 5b presents the photocurrent response curve under 375 nm, 4.74 mW laser illumination. It can be seen that the rise and decay times are 61 ms and 62 ms, respectively. After 10 consecutive on-off cycles, the photocurrent remains stable, indicating that the device maintains good stability under frequent switching operations. As shown in Figure 5c and Figure 5d, the device achieves optimal responsivity and specific detectivity values of 5.37 × 10⁻⁴ A/W and 5.9 × 10¹⁰ Jones, respectively. It is also observed that as the optical power increases, the photoelectric performance decreases. This is explained by the reduced trap state density at the interface between the active layer and the electrode and the oxidation of the electrode, which leads to a longer electron transport distance. The authors also conducted control experiments, demonstrating that the performance under 375 nm illumination is better than that under 532 nm illumination. This is because short-wavelength light carries more energy, excites more electron–hole pairs, and promotes electron transition behavior. Although there is still a considerable gap in photoelectric performance compared to other similar PDs, its simple structure and fabrication process make it competitive. Due to its fast response speed, it has potential applications in target tracking and space exploration in the future.

In the field of image sensing, there are also many application scenarios. In 2020, Yokota et al. developed an image sensor integrated with a readout circuit and near-infrared OPD, with a thickness of only 15 µm, which can be used to capture images of veins and fingerprints [39], as shown in Figure 6a. This image sensor exhibited excellent performance, with a created image resolution of 508 pixels per inch and a readout speed of 41 frames per second. Additionally, by analyzing the area distribution, the optimal position for measuring the pulse can be selected. In 2024, Kim et al. designed a high-performance green selective material donor-linker-acceptor (D-π-A) merocyanine [40]. An OPD based on this material achieved an EQE of more than 65%, a full width at half maximum (FWHM) of less than 95 nm, and a thermal stability that maintained electrical properties at 180 °C. Integrating the device into OPD-on-silicon (OPDoS) hybrid image sensors can replace green color filters and increase the light sensing area per pixel to twice its original size. The layer structure of the pixels of the OPDoS hybrid image sensors, the pixel array, and the pixel array of the conventional image sensors are shown in Figure 6b, respectively.

4. Hybrid Organic–Inorganic Perovskite Photodetectors

In recent years, perovskite materials have garnered widespread attention in the field of photodetectors due to their unique optoelectronic properties. These materials not only have tunable band gaps, high absorption coefficients, and long carrier transport distances, but also possess good solution processability, making the fabrication of low-cost, large-area optoelectronic devices feasible. In 2021, Wang et al. used high-resolution electrohydrodynamic (EHD) printing technology to fabricate a full-color PD [41]. The device structure and the materials of each layer are shown in Figure 7a. The fabrication process, illustrated in Figure 7b, involves ejecting pre-processed high-quality ink droplets onto the substrate through a nozzle. Due to the presence of electrostatic forces, the surface tension of the droplets is neutralized, resulting in the formation of a smooth and uniform perovskite thin film after crystallization and post-annealing.

Perovskite materials typically have an ABX₃ crystal structure, where A is an organic or inorganic cation, B is a metal cation (such as Pb²⁺ or Sn²⁺), and X is a halide anion (such as Cl⁻, Br⁻, or I⁻). By adjusting the types and proportions of elements at the A, B, and X sites, the bandgap of perovskite materials can be precisely controlled to meet the photodetection requirements across different spectral ranges. Organic materials, through molecular design, can absorb light at various wavelengths and can be combined with perovskite materials to broaden the detectable spectral range [42]. For instance, forming heterojunctions by combining organic materials with perovskites can further enhance the optoelectronic performance of devices [11]. In the ultraviolet to near-infrared range, hybrid PDs exhibit higher spectral responsivity and specific detectivity.

4.1. Perovskite Structures

Perovskite materials primarily exist in three forms: polycrystalline thin films, single crystals, and low-dimensional nanostructures [43]. For polycrystalline thin films, parameters such as size and thickness can be adjusted by controlling the crystallization method, which in turn determines their optoelectronic performance [44]. This form of material has already been used in the fabrication of solar cells and photodetectors. However, the performance of optoelectronic devices based on polycrystalline perovskite films is significantly limited due to the presence of numerous internal defects.

In 2014, Yang et al. demonstrated a photodiode based on solution-processed organic–inorganic hybrid perovskite materials, using polycrystalline perovskite films as the active layer [45]. The device structure is shown in Figure 8a, where the PEDOT layer serves as the hole transport layer, the PC₆₁BM layer serves as the electron transport layer, and Al is the electrode. Under room temperature testing conditions, this detector exhibited extremely high detectivity for weak signals (10¹⁴ Jones) and a large linear dynamic range (over 100 dB). Yang et al.’s pioneering work has guided subsequent researchers.

In 2015, Wu et al. reported a phototransistor using solution-processed organolead triiodide (MAPbI₃, MA = CH₃NH₃) thin films [46]. The schematic structure is shown in Figure 8b, using a heavily n-doped silicon (Si) wafer as the substrate, with poly(methyl methacrylate) (PMMA) coated on top as a protective layer to block moisture and oxygen. This device had very low dark current (0.2 nA) and fast response speed, generally less than 10 µs, demonstrating the suitability of perovskite materials for applications requiring high response speed. The responsivity versus drain voltage (V_DS) and gate voltage (V_GS) curves of the device are shown in Figure 8c. The transfer curve shows a V-shape, with a maximum responsivity of 320 A/W, indicating the device’s bipolar characteristics and excellent optoelectronic performance.

Subsequently, Liang et al. constructed a self-powered flexible PD using MAPbI₃ as the active layer [47]. The structure is shown in Figure 8d. Notably, they fabricated two types of polymer films (ITO/polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS)/PET) and utilized the triboelectric effect between ITO and PDMS to detect finger taps. This device achieved extremely high mechanical toughness (as shown in Figure 8e, maintaining stable voltage response under bending at three angles) and high D* (1.22 × 10¹³ Jones). In 2022, Gu et al. reported an inkjet printing method capable of producing large-scale, high-quality perovskite films [48]. As shown in Figure 8f, by introducing a soluble polyethylene oxide (PEO) layer, the growth of perovskite crystals can be well controlled and high-resolution patterning can be achieved.

Compared to polycrystalline thin films, single-crystal perovskites have a lower defect density, resulting in stronger charge transport capabilities. Recent studies have found that formamidinium lead iodide (FAPbI₃) has a higher thermal decomposition temperature than MAPbI₃, indicating potentially better stability. Additionally, its detectable range can extend into the near-infrared spectrum. Yang et al. fabricated photoconductors based on single-crystal FAPbI₃ (two different single-crystal structures are shown in Figure 9a,b) [49]. Under 1 Hz conditions, the noise current was 0.13 pA/Hz^1/2, and the NEP under 380 nm illumination was 2.6 × 10⁻¹⁴ W. Subsequently, their team produced larger FAPbI₃ single crystals with a trap state density of only 1.34 × 10⁻¹⁰ cm⁻³. This significantly improved the device’s photocurrent, approximately 90 times that of devices based on polycrystalline perovskite films. Due to the unique physical properties of Pb (atomic number 82), it can even be used for X-ray detection. Huang and co-workers replaced polycrystalline films with single-crystalline methylammonium lead tribromide (MAPbBr₃) to fabricate a new type of X-ray detector [50]. The structure is shown in Figure 9c. Charges generated by visible light excitation are near the surface region of the MAPbBr₃ layer, while charges generated by X-ray excitation are within the MAPbBr₃ layer. This OPD achieved a sensitivity of 80 µCGy⁻¹cm⁻².

Low-dimensional nanostructures have a more defined crystal structure and exhibit characteristics such as lower trap density and unique light absorption ranges. These qualities enable photodetectors based on low-dimensional nanostructures to achieve higher optoelectronic performance. However, their highly disordered nature on a macroscopic scale and large surface area can lead to the formation of electron trap states, which can affect the stable operation of the devices. In 2016, Tang et al. demonstrated that the use of oleic acid (OA) passivation can improve the surface defects of nanowires [51]. The passivated PDs showed varying degrees of improvement in responsivity, detectivity, and response time.

4.2. Perovskite Heterostructures

The advantages of perovskite materials include long exciton diffusion distances and strong light-capturing abilities, allowing for full absorption of light with just a few hundred nanometers of thickness [52]. Organic materials, on the other hand, have the advantage of a wide and tunable spectral absorption range. Therefore, combining the advantages of both materials (e.g., forming heterostructures) is another important pathway for developing high-performance photodetectors in the future, besides altering the crystal structure of perovskites.

Although perovskite materials possess mechanical flexibility, they tend to develop cracks after bending cycles, which can severely impair the carrier transport process in devices. In 2021, Wang et al. addressed this issue by doping poly(vinyl alcohol) (PVA) microscale scaffolds into formamidinium lead iodide (FAPbI₃) films [53]. This photodetector acquired the ability to self-heal by absorbing moisture in humid environments, and the responsivity only decreased by 10% after self-healing. The repair process is shown in Figure 10a. At the crack locations, the intermolecular structure of PVA is disrupted, but in a moist environment, the hydroxyl groups of PVA get activated and spontaneously reform hydrogen bonds to repair the intermolecular structure.

Some researchers have combined perovskite materials with organic materials to form heterojunctions, thereby broadening the spectral response range of PDs. Organolead halide perovskites have a spectral response range from ultraviolet to the visible band. To extend it to the near-infrared band, Luo et al. designed a perovskite-based PD incorporating lead phthalocyanine (PbPc) [54]. The structural schematic is shown in Figure 10b. This PD significantly suppressed the dark current, reducing it to the pA level. As seen in Figure 10c, bending cycles had almost no effect on the photocurrent of the device, causing the dark current to rise by less than an order of magnitude. The device exhibited response times below 0.6 ms across the detectable full-spectrum range. This provides valuable insights for the future design of low-cost, fast-response PDs covering the ultraviolet to near-infrared spectrum.

Although research on lead-based hybrid organic–inorganic perovskite PDs has matured, the use of lead poses significant health and environmental risks, especially in practical applications like wearable devices. In 2020, Liu et al. reported a high-performance PD using a tin-based perovskite/PEDOT vertical heterojunction as the photosensitive layer (see Figure 10d for structure and material details) [25]. The response range of this device covers frequencies from ultraviolet to near-infrared. As shown in Figure 10e, the device’s maximum responsivity is 2.6 × 10⁸ A/W, and its outstanding optoelectronic performance is attributed to the grating effect formed by the perovskite and heterojunction. More importantly, the device maintains high sensitivity while exhibiting excellent bending stability.

To meet the demand for near-infrared detection, combining organic narrow-bandgap polymers is a viable approach. Constructing an organic polymer interface layer can block holes and facilitate electron transport, which is beneficial for photodetectors. To enhance photocurrent and suppress dark current, Meredith et al. added a bilayer fullerene structure (PC₆₀BM/C₆₀) [55]. The device structure is shown in Figure 11a. To compare the optoelectronic performance of four different active layers, the team plotted the current density versus applied voltage curves of the devices (Figure 11b), showing that the thick PC₆₀BM/C₆₀ layer had the lowest dark current density. To improve responsivity in the near-infrared range, Wu et al. introduced dual electron transport layers in the active layer (MAPbI₃/PC₆₁BM and PTB7-Th heterojunction; the molecular structures of 2,2′-((2Z,2′Z)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile (IEICO-4F) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]adithiophe-ne-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) are shown in Figure 11c,d). The highest responsivity in the near-infrared region reached 0.518 A/W [56], as shown in Figure 11e.

5. Flexible and Wearable Organic Photodetectors

With the rapid development of Internet of Things (IoT) technology, the demand for flexible and wearable electronic products is increasingly growing. PDs, as a crucial component of electronic devices, play a key role in image sensing, health monitoring, optical communication, and other fields. However, traditional inorganic PDs are rigid and have complex fabrication processes, making them unsuitable for flexible electronic products. In recent years, FOPDs have become a research hotspot due to their excellent flexibility, solution processability, low cost, and good optoelectronic performance. This section introduces the latest research advancements in materials, performance, and applications of FOPDs.

5.1. Flexible Materials

Organic semiconductor materials primarily rely on van der Waals forces for intermolecular interactions, making them less rigid compared to inorganic materials. As a result, traditional rigid substrates such as glass can be replaced with flexible substrates, enabling the development of flexible and even wearable OPDs. The material foundation of FOPDs includes the flexible substrate, the transparent conductive electrode (TCE), and the photoactive layer. To achieve effective FOPDs, certain requirements must be met for these functional layers. First, each layer must withstand bending without significant loss of optoelectronic properties. Second, specific layers must ensure that the intensity of incident light is not significantly affected. Finally, reliable interlayer connections must be maintained to ensure the overall device functionality.

5.1.1. Substrate

Flexible substrates can be manufactured using plastic film materials such as PET, polyethylene naphthalate (PEN), polyethersulfone (PES), polyimide (PI) [57,58], and PDMS [59], among others [60]. These plastic materials exhibit high transparency, excellent barrier properties against gas and liquid permeability, and outstanding flexibility. Yang et al. developed a vapor-solution method to deposit smooth and pinhole-free organometal halide perovskite films on PET substrates [61]. As shown in Figure 12a, the device demonstrates good mechanical flexibility. Figure 12b presents the responsivity data under different operating voltages. At 1 V, the responsivity reaches up to 81 A/W, significantly higher than previously reported values for other FOPDs.

Although PET and PEN materials are the most common substrate materials, their inability to stretch limits their further application. Research results on stretchable materials are still relatively scarce. Among stretchable materials, PDMS is commonly used by researchers, as it can withstand more than 100% tensile deformation. Leem et al. reported a FOPD deposited with metal halides on a flexible PDMS substrate, which could accommodate 5.72% tensile strain and more than 1000 stretching cycles [62]. High-performance FOPDs can also be fabricated using materials such as polyurethane (PU) [63] (as shown in Figure 12c), parylene C [64], paper-based materials, and cloth-based materials [65].

5.1.2. Transparent Conductive Electrode

ITO is known for its high optical transparency and excellent electrical conductivity, making it one of the most commonly used transparent conductive electrodes (TCEs) in OPDs. However, its rigidity and the high deposition temperature required limit its application in FOPDs. Alternative transparent electrode materials include Ag NWs [66,67], CNTs [68] and conductive polymers such as PEDOT [61]. CNTs offer outstanding electrical conductivity, mechanical flexibility, and high optical transparency. By optimizing the density and dispersion of CNTs, it is possible to create transparent electrodes with 90% visible light transmittance and low resistance (10–25 Ωsq⁻¹). A device using CNTs as flexible electrodes [69] is shown in Figure 12d. Ag NWs also possess high conductivity and good optical transparency, but they suffer from high surface roughness [70] and poor mechanical stability. Recent studies have improved these properties through surface modification and patterning techniques. Peeling processes and the addition of sacrificial layers can significantly reduce the risk of damage to PEDOT films during production.

Figure 12. (a) Photograph of a flexible perovskite PD at bending situation [61]. (b) R of the flexible PD at different voltages [61]. (c) A PD with a PU substrate [63]. (d) Schematic of the device with flexible CNT electrode [69]. (e) The normalized EQE of the optimal MAM based PM-OPD with respect to its initial value after different bending times under bending radius of 8.54 mm at −15 V bias [26]. (a,b): Reprinted from [61], Copyright (2017), with permission from John Wiley and Sons. (c): Reprinted from [63], Copyright (2020), with permission from American Chemical Society. (d): Reprinted from [69], Copyright (2021), with permission from Elsevier. (e): Reprinted from [26], Copyright (2020), with permission from IOP Publishing.

Figure 12. (a) Photograph of a flexible perovskite PD at bending situation [61]. (b) R of the flexible PD at different voltages [61]. (c) A PD with a PU substrate [63]. (d) Schematic of the device with flexible CNT electrode [69]. (e) The normalized EQE of the optimal MAM based PM-OPD with respect to its initial value after different bending times under bending radius of 8.54 mm at −15 V bias [26]. (a,b): Reprinted from [61], Copyright (2017), with permission from John Wiley and Sons. (c): Reprinted from [63], Copyright (2020), with permission from American Chemical Society. (d): Reprinted from [69], Copyright (2021), with permission from Elsevier. (e): Reprinted from [26], Copyright (2020), with permission from IOP Publishing.

In 2018, Rene Fischer et al. created an easily deposited dispersion by mixing single-walled carbon nanotubes with PEDOT [71], which lowered the specific resistance while maintaining good transparency and chemical stability. In 2020, Shi et al. [26] successfully fabricated flexible, highly sensitive PM-OPDs on a PET substrate using an ultrathin Ag film (9 nm) as the transparent electrode. The substrate (P3HT:PC₇₀BM, 100:1) exhibited excellent flexibility. Figure 12e illustrates the EQE values of this device after 1, 10, 100 and 1000 bending cycles. It can be seen that even after 1000 bending cycles, the EQE of the device remained above half of its initial value (51.4%). In 2024, Wang and his team developed a semitransparent near-infrared flexible organic photodetector (FOPD) with a large detection area (256 mm²) and a fast response speed (approximately 75.2 kHz) [72]. They selected silver (Ag) as the transparent anode and tested the photonic performance across thicknesses ranging from 6 nm to 100 nm. The results indicated that as the thickness of the Ag electrode increased, the EQE also increased, due to enhanced photon reflectivity that facilitated photon injection into the active layer. Additionally, variations in Ag thickness did not significantly alter the current density of the OPD; however, the dark current density was lowest at a thickness of 7 nm. Furthermore, when combined with a NIR organic light-emitting diode (OLED), this device can be utilized for PPG and optical communication.

5.1.3. Active Layer

Organic semiconductor materials, including polymers and small molecules, each have distinct characteristics. Polymers are usually easier to coat from solution, while small molecules have less batch-to-batch variation due to their monodisperse nature [73]. Conjugated polymers have the advantage of being van der Waals solids, which gives them superior mechanical properties (elasticity and toughness) compared to small molecules, making them commonly used in the construction of BHJs. Small molecules such as squaraine derivatives and acenes are often used as donor materials in BHJs, where their crystallinity significantly impacts the optoelectronic performance of the BHJ [74].

In the fabrication of organic phototransistors (OPTs), researchers often use small molecules due to their higher mobility. In 2021, Huang and his team developed a p-type copolymer (D18) [75], with a device image shown in Figure 13a. This small molecule exhibits excellent carrier generation capability and was used to optimize OPTs, as depicted in Figure 13b. The optimized OPT demonstrated significantly reduced dark current and noise, at 10 pA and approximately 10 fAHz^−1/2, respectively. The responsivity and detectivity curves are shown in Figure 13c, with the detectivity reaching a high level (4 × 10¹³ Jones). In 2024, Xia et al. reported a novel active layer material, which is a non-fullerene acceptor (NFA) with an acceptor-donor-acceptor (A-D-A) architecture [76]. This material features 9,14-dihydro-4H-dithieno[2′,3,:2,3;3′”,2′”:10,11]piceno[1,14,13,12-bcdefgh]carbazole (DTPC) as the core, with 2-(6,7-difluoro-3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (NINCN-2F) as the terminal group. The conjugated end of the molecular structure has been elongated, resulting in a broad light absorption range (300–1500 nm) and a low trap state density. This low trap state density and the ordered molecular packing contribute to a reduction in the recombination of thermally activated charges during transport, significantly lowering the dark current density to 0.81 nA/cm² in photovoltaic mode. The device achieves photonic performance in the NIR-II region (λ > 1000 nm) comparable to that of commercial silicon-based photodetectors, with optimal responsivity and the D* reaching 0.27 A/W and 9.24 × 10¹³ Jones, respectively.

Organic dyes exhibit high light absorption coefficients and allow for molecular structure design. Notably, they can be used to create flexible films through solution processing, making them popular among researchers. J. Kim et al. combined J-type dicyanovinyl-functionalized squaraine dye (SQ-H, molecular structure shown in Figure 13d) with PC₆₁BM to form a nanocrystalline BHJ active layer, resulting in a high-efficiency short-wave infrared (SWIR) OPD [77], with the device structure shown in Figure 13e. Figure 13f demonstrates the excellent mechanical flexibility of the device. The EQE maximum of this OPD can reach 12.3% at 0 V and 1050 nm. Furthermore, it has been successfully used for heart rate monitoring without requiring a reverse bias. Figure 13g shows the device in a real-world application scenario, and Figure 13h presents the heart rate waveform measured at 680 nm and 1050 nm with a working voltage of 0 V. Organic dye-based photodetectors have promising applications in low-power and wearable devices.

Elastic semiconductors, with their low Young’s modulus and high fracture strain, are suitable for stretchable optoelectronic devices. The e-BHJ films combining polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS), P3HT, and indene-C₆₀ bisadduct (ICBA) exhibit mechanical properties similar to human tissue and are used to fabricate stretchable OPDs [78]. The molecular formulas of P3HT, ICBA, and SEBS are shown in Figure 14a–c, and the fabrication process is illustrated in Figure 14d.

Three-dimensional graphene (3DG), with its high specific surface area and broad spectral absorption properties, is used in combination with organic materials to create high-performance flexible OPDs. In 2022, Zhen Ge reported an integrated FOPD with organic materials and 3DG [79], with the device structure illustrated in Figure 14e. This hybrid structure significantly enhances both light-responsive current and mechanical stability. The responsive current under 520 nm illumination can reach over 90% of that of rigid devices. As shown in Figure 14f, the responsive current decreases gradually with the number of bending cycles, and after 100 bending cycles, it only decreases by 20% from its initial value.

5.1.4. Encapsulation Layer

In practical applications, FOPDs are often exposed to environmental conditions. Factors such as water vapor and oxygen in the air, as well as mechanical stress (e.g., bending), can adversely affect FOPDs, diminishing their optoelectronic performance and lifespan [80]. Therefore, the implementation of appropriate encapsulation techniques is a crucial step toward enhancing the operational stability of FOPDs. Traditional sealing methods utilize rigid materials like glass or metal, which are inadequate for meeting the sealing requirements of flexible devices.

In 2023, Lee et al. developed a wearable quantum dot pulse oximeter (FQPO) for real-time health monitoring [81]. The team achieved breakthroughs in transferable encapsulation technology, enabling the large-scale fabrication of lightweight and thin FOPDs. The key to this transferable encapsulation technique lies in an inorganic/organic multilayer barrier, which consists of top and bottom layers of aluminum oxide/zinc oxide (Al₂O₃/ZnO, 3 nm/3 nm) with a SiO₂ composite polymer layer in between. The Al₂O₃/ZnO layer effectively prevents the permeation of water vapor and oxygen, while the SiO₂ composite polymer layer imparts excellent mechanical flexibility to the encapsulation. This sandwich structure design reduces processing time by half (as the top and bottom layers can be processed simultaneously) and lowers production costs. In terms of mechanical performance, the device retains good flexibility even after 1000 bending cycles at a strain of 1.3%. When measuring peripheral oxygen saturation levels (SpO₂), the narrow FWHM characteristic enhances sensitivity, allowing for more accurate data acquisition compared to traditional OLED measurement systems.

Specifically, if a material can serve simultaneously as a transparent electrode and a barrier layer, the encapsulation step can be omitted. For instance, CNTs were mentioned earlier [69]. Jang et al. utilized CNTs as the anode to fabricate a FOPD that achieved a D* of 2.07 × 10¹⁴ Jones while maintaining stability after 500 bending cycles. This dual-function material can simplify the device fabrication process; therefore, the search for and synthesis of such materials is a key direction for developing FOPDs with high operational stability in the future.

5.2. Application

5.2.1. Health Monitoring

In 2017, Xu et al. [82] innovatively combined flexible OPT with high-efficiency inorganic light-emitting diodes (LEDs) to develop a novel near-infrared PPG sensor, as illustrated in Figure 15a. The OPT exhibits a responsivity of up to 3.5 × 10⁵ A/W in the near-infrared range (e.g., 810 nm), significantly surpassing the performance of commercial silicon-based photodetectors. This ensures effective pulse signal detection even under low-light conditions. The NEP of the sensor is 1.2 × 10⁻¹⁵ WHz^−1/2, indicating an exceptionally high signal-to-noise ratio for precise pulse waveform analysis and improved measurement accuracy of physiological parameters. With an ultra-thin packaging design, the sensor achieves high flexibility and excellent skin conformity, allowing it to be comfortably adhered to the body for long-term, continuous, and non-invasive physiological monitoring. Experiments show that the sensor can continuously monitor heart rate variability (HRV) and accurately track changes in pulse pressure (PP) in different body positions. When used in conjunction with electrocardiograms (ECG), the sensor provides heart rate measurements highly consistent with ECG standards, with lower power consumption (only 4 mW peak power). In 2021, Chung et al. reported a skin-like health care patch (SHP) that displays real-time heart rate on an OLED screen [27], as shown in Figure 15b,c. Despite its thickness of only 15 μm, the device integrates a stretchable OLED and PPG heart rate sensor, offering a new approach for designing wearable medical monitoring devices. In 2024, Fang et al. [83] developed a photoluminescence bioimaging technology based on the NIR-IIb (1500–1700 nm) window for non-invasive in vivo quantitative assessment of tumor-associated vascular oxygenated hemoglobin saturation (sO₂). This technology enables high frame rate (33 Hz) real-time dynamic imaging, showcasing sO₂ levels in mouse brain vasculature. It is closely related to cancer cell metabolism and immune therapy responses, providing new insights into the tumor metabolic microenvironment and immune therapy efficacy.

5.2.2. Optical Communication

Optical communication is a technology that can enhance communication speed and data capacity. Although silicon-based photodetectors have matured in this field, their rigid structure and high-power consumption limit further advancements. Recently, Zhu et al. reported a high-speed NIR OPD [84]. The team synthesized a NFA with significant π-extension, referred to by the authors as CH17. This extension reduces the molecular spacing in the stacked structure, facilitating charge transport. The device not only features a small pixel size (0.1 × 0.1 cm²) and a rapid response time of less than 91 ns (λ = 880 nm), but also demonstrates a broad absorption range from visible light to near-infrared, along with a high LDR of up to 130 dB. In terms of signal transmission performance, the device exhibits data transfer rates of up to 80 Mbps and a bit–error rate of less than 3.5 × 10⁻⁴. Based on these exceptional characteristics, the researchers designed and assembled an optical communication system for traffic applications, achieving long-distance data transmission between two vehicles using near-infrared light, as shown in Figure 15d.

5.2.3. Image Sensors

Thanks to the rich surface functional groups and unique two-dimensional structure of Ti₃C₂T_x MXene, Hu et al. designed a near-infrared photodetector based on an organic–inorganic van der Waals heterostructure [85]. This photodetector uses Ti₃C₂T_x as the conductive electrode and reduced amorphous nitrogen-doped (RAN) film as the active material, showing significantly enhanced photoresponsivity. Specifically, under 1064 nm laser excitation, its on/off ratio is 6.25 times better than that of traditional devices using Au as the electrode. When infrared light passes through a deer-shaped mask onto the photodetector array, the captured image of the deer is shown in Figure 15e. Additionally, the detector exhibits excellent mechanical stability and high transparency, successfully applied in high-resolution image sensing, providing new solutions for biomimetic vision and flexible wearable devices.

6. Photomultiplication Type Organic Photodetectors

6.1. Photomultiplication Phenomenon and Preliminary Applications

The photomultiplication (PM) phenomenon is crucial for enhancing the sensitivity of photodetectors, enabling them to significantly improve the detection of weak light signals without using a preamplifier. Unlike the multiplication mechanism in inorganic materials, the PM phenomenon in organic materials is mainly achieved through the charge tunneling injection effect induced by interface traps.

In 1994, Hiramoto et al. first observed the PM phenomenon in organic materials at the interface between an n-type perylene pigment (N-methyl-3,4,9,10-perylenetetracarboxyl-diimide, Me-PTC) thin film and a gold (Au) electrode [86]. The cell structure and the molecular structure of Me-PTC are shown in Figure 16a,b. Similar phenomena were also observed in p-type quinacridone pigment (DQ) [87] and n-type naphthalene tetracarboxylic anhydride (NTCDA) [88]. They proposed a mechanism where, under illumination, photogenerated holes are trapped in the depletion layer formed within the bandgap, creating a strong electric field. Subsequently, under the influence of this strong electric field, electrons tunnel through the reverse-biased Au/pigment layer and the weakened depletion layer. When the Au electrode was replaced with a Pt electrode, the voltage required to achieve the PM effect increased from 5 V to 15 V. This is because the work function of Pt is greater (5.65 eV), necessitating a higher voltage for electrons to tunnel through the thicker depletion layer. Further studies on the impact of interface traps on the PM phenomenon revealed that the depth of the interface traps increases with the applied electric field [89].

In the late 20th century, researchers continued to explore the origins of the PM phenomenon, discovering that the formation of trap states at the organic/metal interface is crucial for the PM effect. Neher et al. reported the PM phenomenon observed in sandwich cells based on polyphenyleneimine derivatives (arylamino-PPV), which was attributed to the accumulation of trapped electrons in the vicinity of the Al electrodes [90]. While PM phenomena are typically introduced at the interface between organic materials and metal electrodes, Hiramoto et al. also discovered PM phenomena at the interface between organic materials. In a heterojunction formed by p-type phthalocyanine and n-type perylene pigment, the photocurrent was enhanced by more than 3000 times [91].

6.2. Photomultiplication Type Broadband and Narrowband OPDs

Due to the broadband response characteristics of organic polymers, research on PM-type broadband OPDs has progressed rapidly. By incorporating nanoparticles (NPs), Yang et al. designed a PM-type broadband OPD [92], with the device structure shown in Figure 17a. They added cadmium telluride (CdTe) NPs to the P3HT film, achieving the highest EQE value among similar devices at that time. As shown in Figure 17b, the device’s EQE increased with the reverse bias voltage, ranging from 0 V to 4.5 V. At a wavelength of 350 nm, the EQE reached a maximum value of 8000%, and at 700 nm (near the edge of the CdTe NPs’ absorption peak), the EQE also exceeded 100%. The remarkable gain is attributed to the CdTe nanoparticles at the interface between the active layer and the electrode, which assist in hole tunneling.

Chen et al. incorporated a near-infrared dye (4,5-benzoindotricarbocyanine, Ir-125) into the P3HT (1:1, wt/wt) active layer [93]. As shown in Figure 17c, the spectral response range extended from the original 650 nm to 1050 nm. The EQE increased with the reverse bias voltage, reaching a peak value of approximately 7200% at a wavelength of 510 nm. In 2012, Huang et al. improved the active layer material by doping ZnO NPs into the hole-conducting organic semiconductor polymer [94], with the device structure shown in Figure 17d. As seen in Figure 17e, as the reverse bias voltage increased from 0 V to 9 V, the EQE values in the UV to visible spectrum (300–680 nm) rose from 10⁰% to a high level of 10⁵%.

Additionally, although ternary strategies are generally used to design high-efficiency solar cells, they have also been applied to the design of PM-OPDs [95]. By adjusting the proportion of the polymer 2,6-bis(trimethylstannyl)-4,8-bis(5-(octylthio)-thiophen-2-yl)benzo[1,2-b:4,5-b′]-dithiophene (PBDT-TS1), the spectral detection range of the OPD can be extended from the ultraviolet to the near-infrared region. When the doping ratio of P3HT:PBDT-TS1 is 50:50:1, the overall EQE across the detection band is optimal. Under a bias of −10 V, the EQE can reach up to 830%, 720%, and 330% under illumination at 390 nm, 625 nm, and 760 nm, respectively. This demonstrates that the photomultiplication effect can effectively enhance the optoelectronic performance of OPDs.

Compared to broadband OPDs, achieving narrowband response OPDs is more challenging, especially when the target FWHM is less than 50 nm. The relatively wide absorption range of organic semiconductor materials makes it difficult to realize narrowband OPDs [96]. In 2015, Armin et al. introduced the charge collection narrowing (CCN) method, which can adjust the spectral response range by modulating the active layer thickness [8]. The use of thick junctions reduces the defect density during this process, resulting in a dark current of less than 10⁻¹⁰ A under a reverse bias of 1 V. The downside is the reduction in EQE values and the loss of narrowband detection capability under larger bias. In 2017, Wang et al. reported a vertically structured PM-OPD that achieved narrowband detection without adding filters [9]. When the operating voltage was reverse-biased to 60 V, the larger electric field facilitated hole transport in the active layer, resulting in maximum EQE and responsivity values of 53500% and 278 A/W, respectively. Increasing the thickness of the active layer gradually enabled the device to exhibit narrowband response characteristics, as shown in Figure 17f. Although increasing the active layer thickness causes the EQE to decrease, the PM effect is still present.

6.3. Performance Improvement

To enhance the optoelectronic performance of PM-OPDs, researchers have made significant efforts. Studies indicate that adjusting the active layer thickness, optimizing trap depth, and improving carrier transport properties can substantially increase the EQE and response speed of PM-OPDs. Hiramoto et al. achieved fast-response PM-OPDs using organic single crystals and co-deposited films [97]. The device structure, molecular structure, and multiplication rate curves are shown in Figure 18a. They used naphthalene tetracarboxylic anhydride (NTCDA) films deposited in a vacuum, achieving a photogenerated current gain of 10⁵ due to the accumulation of holes at the organic/metal interface. As shown in Figure 18b, the presence of numerous grain boundaries in polycrystalline films limits the photogenerated current gain and results in slower response speeds, with the current not reaching its maximum even after 60 s. In contrast, the device exhibited a response speed of less than 0.5 s compared to photodetectors based on polycrystalline films, providing insights for developing fast-response PM-OPDs, as shown in Figure 18c. In 2015, Wang et al. designed a PM broadband OPD with a P3HT active layer [98]. At a working voltage of −25 V, the optimal EQE reached approximately 3.8 × 10⁴%. This outstanding performance is attributed to the low dark current caused by the hole injection barrier and the efficient hole transport properties resulting from the low content of phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM).

Using small molecule acceptors as trap states, such as PC₇₁BM, Li et al. achieved high EQE in PM-OPDs with a P3HT:PC₇₁BM active layer [99]. By adjusting the ratio of P3HT, the team fabricated five devices (with ratios A to D being 100:1, 100:4, 100:15, 100:50, and 1:1, respectively). As shown in Figure 18d, Device A achieved the highest EQE (16,700%) at a wavelength of 380 nm and a working voltage of −19 V. Furthermore, it can be observed that different PC₇₁BM ratios resulted in two types of EQE curves: photodiodes (EQE below 100%) and PM-type (EQE above 100%).

The use of interface barrier layers and the introduction of trap states in the interface layer [100] can also enhance the PM effect and response speed. In 2010, Xue et al. developed a fast-response, high-multiplication-rate OPD by leveraging the principle of carrier confinement [101]. The device structure (with a CuPc and C₆₀ ratio of 7:3 in the active layer) and energy level schematic are shown in Figure 18e. CuPc contains a large number of holes, while C₆₀ acts as a hole-blocking material. By using NTCDA as a hole-blocking layer (HBL), holes accumulate to create a strong electric field, facilitating the tunneling of electrons from the ITO electrode into the active layer and forming a PM effect (with a photogenerated current gain of 500 in the visible spectrum). Additionally, this device only restricts hole transport and does not impede charge movement, resulting in a relatively fast response speed.

In 2020, Shi et al. developed a flexible PM-OPD by depositing an ultra-thin silver film on a PET substrate [26]. The Ag film is flanked by layers of MoO₃ on both sides, which serve to enhance Ag film penetration and act as a hole transport layer (referred to by the authors as the MAM layer, as shown in Figure 18f). While devices based on ITO demonstrate lower dark currents, those based on the MAM exhibit higher photocurrent. Furthermore, the team examined the influence of varying HTL layer thicknesses on device performance (Figure 18g,h). It was observed that a thinner HTL layer resulted in higher EQE and responsivity. At a bias voltage of −15 V, the optimal EQE and responsivity values were observed to be 1.3 × 10⁵% and 388.4 A/W, respectively, which is approximately twice the photodetector performance of devices that use ITO as the transparent anode. Moreover, these devices exhibited remarkable mechanical flexibility, which paves the way for the integration of PM-OPDs in flexible and wearable technology.

Figure 18. (a) Cell structure, NTCDA molecular structure, and multiplication rate as a function of electric field [97]. (b) Current response curve during switching of single–crystal thin film device [97]. (c) Current response curve during switching of polycrystalline thin film device [97]. (d) EQE curves of devices A–D at different reverse biases (−19 V and −1.5 V) [99]. (e) Device energy level schematic [101]. (f) Device structure schematic [26]. (g,h) EQE curve (g) and responsivity curve (h) at different HTL thicknesses [26]. (a–c): Reprinted from [97], Copyright (2002), with permission from AIP Publishing. (d): Reprinted from [99], Copyright (2015), with permission from Springer Nature. (e): Reprinted from [101], Copyright (2010), with permission from AIP Publishing. (f–h): Reprinted from [26], Copyright (2020), with permission from IOP Publishing.

Figure 18. (a) Cell structure, NTCDA molecular structure, and multiplication rate as a function of electric field [97]. (b) Current response curve during switching of single–crystal thin film device [97]. (c) Current response curve during switching of polycrystalline thin film device [97]. (d) EQE curves of devices A–D at different reverse biases (−19 V and −1.5 V) [99]. (e) Device energy level schematic [101]. (f) Device structure schematic [26]. (g,h) EQE curve (g) and responsivity curve (h) at different HTL thicknesses [26]. (a–c): Reprinted from [97], Copyright (2002), with permission from AIP Publishing. (d): Reprinted from [99], Copyright (2015), with permission from Springer Nature. (e): Reprinted from [101], Copyright (2010), with permission from AIP Publishing. (f–h): Reprinted from [26], Copyright (2020), with permission from IOP Publishing.

7. Challenges and Perspectives

In order to facilitate readers to better compare the effects of different materials on the performance of organic photodetectors, the materials and photoelectric properties of organic photodetectors that are presented in this paper are shown in

Table 1. Summarization of OPDs.

Active Layer	Dark Current (Density)	EQE (%)	R (A/W)	D* (Jones)	Applied Bias (V)	Response Time (ms)	Ref.
P3HT:PC61BM	1.94 × 10−8 A	5.35 × 104	278	1.3 × 1011	−60	-	[9]
PC71BM/PTB7:P3HT	1.98 × 10−10 A/cm2	~8	4.0 × 10−2	~1012	0	-	[24]
FASnI3/PEDOT:PSS	4.5 × 10−4 A	-	2.6 × 106	3.2 × 1012	-	-	[25]
P3HT:PC70BM	-	1.3 × 105	388.4	2.23 × 1013	−15	440	[26]
PBDB-T:PBI-Por/PbS-TBAI	5.2 × 10−11 A	-	1.89	1.12 × 1013	40	-	[28]
β-ZnPc	1.1 × 10−2 A/cm2	1.3 × 104	11.6	-	5	142	[30]
BPTTE	3.5 × 10−8 A	-	-	-	-	-	[33]
Rubrene/C60	2.46 × 10−12 A/cm2	60	-	6.35 × 1014	−1	~10−3	[35]
P3HT: PC61BM	-	~15	-	-	-	-	[36]
8H2Pc/PC61BM	4.4 × 10−9 A/cm2	-	0.7	1.0 × 1013	−1	-	[37]
HDA-Bil5	2 × 10−9 A	~0.1	5.37 × 10−4	5.9 × 1010	0	61	[38]
merocyanine:C60	2.9 × 10−9 A/cm2	66	-	-	−5	-	[40]
MAPbI3-xClx	1.0 × 10−7 A/cm2	80	-	1014	~0	-	[45]
MAPbI3	-	80	320	-	−30	10−2	[46]
MAPbI3	-	-	0.418	1.22 × 1013	10	-	[47]
MAPbBr3	1.3 × 10−8 A	-	1.036	-	3	-	[48]
FAPbI3	2.0 × 10−8 A	-	27.57	-	0.1	17	[49]
MAPbBr3	~3.0 × 10−8 A/cm2	43	-	6.6 × 1011	−8	0.216	[50]
MAPbI3	~2.0 × 10−12 A	-	4.95	2 × 1013	1	<0.1	[51]
FAPbI3:PVA	-	-	11.3	9.4 × 1011	5	-	[53]
MAPbI3-xClx:PbPc	8 × 10−11 A	-	0.147	-	± 50	<0.6	[54]
PC60BM/C60	5 × 10−10 A/cm2	70	-	-	−0.5	~1.67 × 10−4	[55]
MAPbI3/PC61BM	-	>70	0.518	>1010	−0.1	-	[56]
MAPbI3	3 × 10−5 A/cm2	1.5 × 104	81	1011	1	-	[61]
P3HT:PCBM	3 × 10−10 A/cm2	75	-	-	−5	-	[71]
D18	~10−11 A	-	>105	4 × 1013	1	-	[75]
DTPC	5.3 × 10−11 A/cm2	-	0.27	9.24 × 1013	0	9.76 × 10−4	[76]
SQ-H/PC61BM	4 × 10−8 A/cm2	12.3	-	-	0	-	[77]
PBDBT-2F:IT-4F	-	~85	0.298	7.05 × 1010	-	-	[81]
DPP-DTT:PC61BM	-	-	3.5 × 105	5.7 × 1013	<3	0.6	[82]
CH17	4.1 × 10−11 A/cm2	85	-	-	0	9.1 × 10−5	[84]
CdTe:P3HT:PC61BM	~4 × 10−9 A/cm2	8 × 103	-	-	24.5	-	[92]
Ir-125:P3HT:PC61BM	-	7.2 × 103	32.4	-	−4.5	-	[93]
P3HT:ZnO	10−7 A/cm2	105	1001	3.4 × 1015	9	-	[94]
P3HT:PBDT-TS1:PC71BM	-	830	~30	~2 × 1012	−10	-	[95]
P3HT:PTB7-Th	-	3.8 × 104	229.5	1.91 × 1013	−25	-	[98]
P3HT:PC71BM	~10−7 A/cm2	1.67 × 104	51.7	-	−19	-	[99]

.

This paper provides a detailed overview of the latest research advancements in narrowband OPDs, hybrid organic–inorganic perovskite PDs, flexible OPDs, and PM-OPDs. By thoroughly analyzing material innovations, structural optimizations, performance improvements, and application areas, it highlights the significant progress these four types of detectors have made in terms of photoelectric conversion efficiency, spectral response range, and environmental adaptability. Additionally, it identifies the current challenges faced in their development and offers personal insights into future research directions.

Narrowband OPDs offer substantial benefits in terms of achieving high sensitivity and adjustable spectral response ranges. Nevertheless, attaining exceptionally narrow spectral responses (such as a FWHM of less than 50 nm) remains a significant challenge, primarily due to the necessity for highly controllable light absorption properties in the materials. The synthesis of organic materials with low bandgaps and excellent charge transport capabilities remains a challenging endeavor [102]. In terms of the operation mechanism, the intricate microstructure inside the heterojunction is still poorly understood [103], making it difficult to realize devices with a variety of excellent optoelectronic properties. For instance, efforts to enhance the D* often result in increased noise current. Furthermore, the operational reliability of narrowband OPDs in diverse environmental conditions (e.g., humidity and temperature fluctuations) is a crucial factor influencing their practical applications. This underscores the necessity for further optimization of material composition and device structure to enhance environmental durability. With regard to fabrication techniques, the potential of high-resolution patterning technologies for the manufacture of narrowband OPDs has yet to be fully realized, thereby limiting their applicability in high-resolution imaging sensors and other applications. Future research should concentrate on the development of new materials, an understanding of the working mechanisms, an exploration of patterning technologies, and an improvement of stability strategies.

The potential of hybrid organic–inorganic perovskite PDs in the field of optoelectronics is immense, owing to the exceptional optoelectronic properties and tunable spectral response ranges exhibited by these devices. Nevertheless, the commercialization of these devices presents considerable challenges. The principal challenge is the stability of perovskite materials, which are susceptible to degradation in humid, oxygen-rich, and UV environments, negatively impacting device lifespan and reliability. The development of effective encapsulation technologies is paramount for enhancing device stability during application. Moreover, the uniformity, crystallinity, and interfacial characteristics of perovskite films during the manufacturing process directly influence device performance. While traditional solution-based methods can mitigate the formation of crystal defects caused by temperature fluctuations and compositional variations, they cannot produce large-sized crystals [104]. Expanding this methodology may result in a decline in crystal quality and an increase in costs, necessitating enhancements through optimized fabrication processes. Additionally, the urgent development of environmentally friendly perovskite materials is imperative, as current materials still rely on heavy metals such as lead. Future research should prioritize enhancing material stability, optimizing manufacturing processes, and investigating environmentally friendly alternatives.

FOPDs have the potential to revolutionize numerous applications, including wearable devices, flexible displays, and biomedical monitoring. Nevertheless, the advancement of these devices faces several obstacles. Firstly, it is paramount to ensure optimal contact and efficient charge transport between the transparent electrode and the active layer while maintaining the mechanical properties of the flexible substrate. Secondly, enhancing the performance, stability, and durability of flexible devices under complex deformations (such as bending and stretching) is essential to meet practical application demands. Additionally, the impact of complex mechanical deformations on device performance varies, necessitating the establishment of standards to accurately evaluate the performance of deformed devices [105]. Moreover, as FOPDs become more prevalent in wearable technologies, reducing heat generation and achieving self-powering capabilities to ensure user comfort and portability are crucial. Finally, integrating FOPDs with external readout circuits represents a significant avenue for future development.

The combination of FOPDs with graphene oxide (GO) materials may prove an effective approach to enhance their optoelectronic performance and mitigate the challenges associated with large-scale applications. GO exhibits a distinctive two-dimensional structure and a range of exceptional properties that have been empirically validated to enhance device performance. This is evidenced by studies involving Si-based materials [106] and poly 3-methyl aniline (P3MA) [107] with deposited GO films. GO has the potential to enhance photocurrent and optimize charge transport while maintaining mechanical flexibility, thereby making it a highly compatible addition to FOPDs. Furthermore, the low cost and simplicity of processing allow for the large-scale production of the devices.

PM-OPDs achieve high sensitivity to weak light signals through the photomultiplication effect, rendering them applicable in a multitude of fields, including optical communication and image sensing. Nevertheless, our comprehension of the photomultiplier mechanism remains imperfect, impeding our capacity to elucidate the functionality of specific devices. Moreover, some photomultiplier effects may not yet be viable for device fabrication. It is of great importance to precisely control the ratios of the materials used and to adjust the structures of the devices in order to achieve high-performance PM-OPDs. Further investigation is required to elucidate the nature of interface trap states, the underlying charge transport processes, and strategies for enhancing multiplication efficiency. The majority of reported PM-OPDs have response times in the millisecond range [108], which restricts their applicability in high-speed optical detection. Consequently, enhancing response speed represents a pivotal area of current research. Furthermore, the stability of PM-OPDs represents a significant challenge, particularly with regard to the retention of performance under prolonged operational conditions. It is therefore recommended that future research should concentrate on furthering our comprehension of the photomultiplier mechanism, the creation of new materials and the development of enhanced stability strategies, with a view to facilitating the continued advancement and application of PM-OPD technology.

For OPDs, thermal stability has a significant impact on both the fabrication difficulty and the operational stability. In the fabrication of flexible devices, the inability of the selected flexible substrate to withstand high temperatures precludes the use of high-temperature deposition techniques. The selection of optimal substrate materials and the optimization of device structures can assist in the resolution of this issue. Moreover, research indicates that increasing the thickness of the active layer can reduce device thermal stability [109], which presents a significant challenge for OPDs with thick active layers in practical applications. One potential strategy is to facilitate the formation of a glass phase during the synthesis of the active layer [110]. The methods for fabricating OPDs have undergone rapid evolution, with the advent of new techniques such as vacuum evaporation, vapor deposition, and solution processes. However, several challenges remain. For example, variations in the thickness of the active layer during roll-to-roll processing can have a detrimental impact on device performance. The manufacturing processes for fabric-based and paper-based materials may not be compatible with those of other substrates. Furthermore, the integration of manufacturing processes encounters challenges in complex systems and large-scale arrays due to the inherent variations among individual units. In the future, greater attention should be paid to ensuring compatibility and integration in the large-scale production of OPDs.

Metal-semiconductor-metal photodetectors (MSM-PDs) also play a pivotal role in optical detection and can serve as viable substitutes for OPDs in select applications. Inorganic semiconductor materials utilized in MSM-PDs exhibit superior environmental and thermal stability in comparison to their organic counterparts. Traditional devices based on gallium arsenide (GaAs) and Si are widely utilized in high-speed and precision-demanding fields such as fiber optic communication [111]. Third-generation wide-bandgap semiconductors, including GaN and ZnO, are capable of detecting ultraviolet light, thereby enhancing the ability of MSM-PDs to detect short wavelengths of light. However, their response speeds are slower than those of traditional materials, such as GaAs [112]. In regard to the fabrication of these devices, the purification of high-quality inorganic semiconductors presents a significant challenge, and the film deposition processes are inherently complex. However, recent advances in perovskite materials have enabled the use of solution-based techniques to prepare perovskite layers, thereby mitigating some of the fabrication challenges. With regard to encapsulation technology, the high stability of inorganic materials facilitates post-processing and packaging in comparison to the encapsulation of FOPDs.

Looking ahead, with continuous advancements in materials science, nanotechnology, and manufacturing processes, the technologies for narrowband OPDs, hybrid organic–inorganic perovskite PDs, FOPDs, and PM-OPDs are set to have even broader development prospects. Through interdisciplinary collaboration and technological innovation, we are likely to address the current challenges and drive ongoing progress and application expansion in optoelectronic detection technology. In the near future, these optoelectronic detectors will play crucial roles in various fields, contributing to the technological advancement and sustainable development of human society.