Retina organic photodiodes for event-driven response and methods of making the same
Patent Information
- Application Number
- CN202611274226.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-08-21
- Publication Date
- 2026-11-03
AI Technical Summary
Existing organic photodiodes output steady-state signals under illumination, making it difficult to achieve event-triggered responses to changes in brightness. This results in insufficient selective encoding of dynamic targets and complex lighting scenarios, and also leads to high manufacturing costs and limited flexibility and compatibility.
A LiF insulating layer is introduced between the metal electrode and the organic semiconductor layer of an organic photodiode to form a MIScap coupling structure. The interface barrier and capacitive coupling behavior are controlled by adjusting the LiF thickness, so that the device generates transient voltage signals when the illumination changes.
It achieves a highly sensitive event-driven response to changes in light intensity, simplifies the fabrication process, reduces power consumption, and is suitable for flexible electronics and low-cost manufacturing, as well as for perception and imaging in biomimetic vision systems.
Smart Images
Figure CN123035816A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the fields of organic optoelectronics and biomimetic visual sensing technology, and in particular to a retinal organic photodiode for event-driven response and its fabrication method. Background Technology
[0002] Most existing image acquisition systems are based on a frame-based operating mechanism, which synchronously samples the entire field of view at fixed time intervals and transmits the brightness information of all pixels to the subsequent processing unit. While this mode has significant advantages in static imaging and high-precision reconstruction, it often suffers from problems such as large data redundancy, heavy transmission burden, high processing latency, and high system power consumption when dealing with dynamic targets, rapid edge motion, and complex lighting changes. In contrast, the biological retina does not continuously encode a static background but prioritizes responses to brightness changes, moving edges, and spatiotemporal contrast, outputting electrical pulses only when an "event" occurs. Therefore, it has outstanding advantages such as high sensitivity, low redundancy, low latency, and low power consumption.
[0003] To mimic the information processing methods of biological retinas, event cameras, neuromorphic sensors, and various artificial retina devices have emerged in recent years. However, silicon-based event vision devices typically rely on complex pixel circuits and readout architectures, resulting in high fabrication costs and limited flexibility and compatibility, hindering their widespread application in wearable, curved surface integration, and large-area, low-cost manufacturing scenarios. In contrast, organic photodiodes (OPDs) offer advantages such as tunable absorption spectra, a rich variety of material systems, solution-processable characteristics, low-temperature fabrication, and the ability to be fabricated on flexible substrates, making them important candidate devices for constructing artificial retinas and neuromorphic visual front-ends.
[0004] Nevertheless, conventional organic photodiodes typically output a steady-state current or voltage signal corresponding to the light intensity under illumination, and their intrinsic response is more inclined towards "continuous readout" rather than "event triggering". Without special design of the interface structure, the device often produces a significant DC response to continuous illumination, thereby weakening the selectivity to the edge of brightness change, which is not conducive to directly realizing sparse encoding of moving targets and dynamic scenes.
[0005] Existing research has shown that the metal / insulator / semiconductor interface can significantly alter the dynamic output behavior of devices through mechanisms such as barrier modulation, charge accumulation, and capacitive coupling. If an ultrathin insulating layer is introduced on top of an organic photodiode, the device may generate transient signals similar to nerve impulses when stimulated by pulsed light due to the rapid accumulation, release, and redistribution of interface charges. However, an excessively thin insulating layer can lead to insufficient charge modulation or enhanced interface leakage; an excessively thick insulating layer can inhibit effective coupling, weaken the transport of photogenerated carriers, and reduce interface modulation. Therefore, selecting a suitable insulating material and its thickness window is crucial for realizing event-driven organic vision devices.
[0006] LiF is an inorganic thin-film material commonly used in organic electronic devices, characterized by its wide band gap, good insulation, mature thermal evaporation process, and easily controllable thickness. By placing LiF between a metal electrode and an organic semiconductor functional layer, a stable metal-insulator-semiconductor (MIScap) coupling interface can be constructed without significantly increasing process complexity. Furthermore, the interface barrier, capacitance effect, carrier accumulation, and release dynamics can be altered by adjusting the thickness. Therefore, there is a pressing need to develop a retinal organic photodiode that achieves event-driven output characteristics by controlling the LiF thickness, and to establish a matching fabrication method and parameter range to meet the application requirements of biomimetic vision and low-power sensing systems. Summary of the Invention
[0007] The purpose of this invention is to solve the above-mentioned problems. This invention designs a retinal organic photodiode for event-driven response and its fabrication method. This invention achieves the above objective through the following technical solution: The present invention provides a retinal organic photodiode for event-driven response, characterized in that it comprises, from bottom to top, a glass substrate, an ITO transparent electrode, a ZnO electron transport layer, an organic bulk heterojunction active layer, a MoO3 hole transport layer, a LiF insulating layer, and an Ag top electrode; wherein the Ag top electrode, the LiF insulating layer, and the lower semiconductor layer together form a MIScap coupling structure, and the device output terminal is disposed between the top electrode and the bottom electrode.
[0008] Preferably, the thickness of the LiF insulating layer is 2-8 nm, more preferably 8 nm, so as to achieve adjustment of the intensity and time constant of different event-driven responses.
[0009] In another aspect, the present invention provides a method for fabricating a retinal organic photodiode for event-driven response, comprising: S1. Place the ITO transparent electrode on the upper surface of the glass substrate to obtain the overlapping structure of ITO and glass substrate. Sonicate in cleaning agent, deionized water and ethanol for 30 minutes in sequence to clean the overlapping structure of ITO and glass substrate, and then blow dry with high-purity nitrogen. S2. All pre-cleaned ITO and glass substrate overlap structures are treated with oxygen plasma for 70 seconds. S3. Spin-coat the ZnO solution onto the overlap structure of the cleaned ITO and glass substrate at a speed of 4000 rpm for 30s, and then anneal in air at 150°C for 15min to form a ZnO electron transport layer. S4. Dissolve the donor and acceptor in chloroform at a mass ratio of D:A = 1:1.4, and heat and stir at 60°C for more than 3 hours to form a mixed solution with a total concentration of 16.5 mg / ml. S5. Transfer the overlapping structure of ITO and glass substrate coated with ZnO electron transport layer to a high-purity nitrogen-filled glove box. Spin-coat the mixed solution of organic bulk heterojunction active layer on the overlapping structure of ZnO electron transport layer, ITO and glass substrate at 6000 rpm for 40s. Then anneal in a glove box at 100°C for 5min. S6, Finally, in 7×10 -5 MoO3 with a thickness of 10 nm was deposited by thermal evaporation under vacuum conditions of Pa. S7. Continue to use the vacuum thermal evaporation method to deposit a LiF insulating layer on the surface of the MoO3 hole transport layer. By adjusting the evaporation time and rate, LiF insulating layers with thicknesses of 2nm, 4nm, 6nm and 8nm are prepared respectively. S8. Next, deposit 100nm Ag top electrodes on LiF insulating layers of different thicknesses to obtain stacked structure devices. S9. The fabricated stacked structure device is subjected to transient response testing under pulsed light illumination. The change of output voltage over time is recorded to evaluate the event-driven response characteristics of the device under different LiF thicknesses.
[0010] Further, the mixed solution in step S4, wherein the donor is D18 and the acceptor is L8-BO, has the following chemical structural formula: (C 76 H 92 F2N2S9) n C 84 H 90 F4N8O2S5.
[0011] Furthermore, the weight of LiF deposited by thermal evaporation in step S7 ranges from 2 to 8 mg.
[0012] The beneficial effects of this invention are as follows: (1) By introducing a LiF insulating layer and precisely controlling its thickness, the coupling strength of MIScap and the behavior of interface charge accumulation can be effectively changed, so that the device can be transformed from a conventional steady-state optical response to an event-driven response that is highly sensitive to changes in light intensity. (2) The device can output obvious transient voltage spike signals when the light pulse is turned on and off, which is beneficial to simulating the selective perception of dynamic edges, moving targets and brightness changes by the biological retina; (3) LiF thickness, as an adjustable process parameter, can be used to optimize the output peak value, recovery speed, repetitive pulse response consistency and power consumption characteristics of the device. The process is simple and has good repeatability. (4) The device has a simple hierarchical structure, is compatible with solution method and vacuum evaporation process, and is suitable for flexible electronics, low power machine vision, neuromorphic perception chip and intelligent imaging system. Attached Figure Description
[0013] Figure 1 This is a schematic diagram of the structure of the retinal organic photodiode used for event-driven response in this invention.
[0014] Figure 2 The figure shows the experimental results of the device output voltage changing with time when the LiF insulating layer thickness is 2nm, 4nm, 6nm and 8nm respectively, where Time represents time and V represents voltage.
[0015] Figure 1 The following are the markings: 1. Glass substrate; 2. ITO transparent electrode; 3. ZnO electron transport layer; 4. Organic bulk heterojunction active layer; 5. MoO3 hole transport layer; 6. LiF insulating layer; 7. Ag top electrode. Detailed Implementation
[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.
[0017] It should be understood that in the description of this invention, the terms "upper," "lower," "inner," "outer," etc., indicating orientations or positional relationships are based only on the orientations or positional relationships shown in the accompanying drawings, and are intended to facilitate the description and simplify the description of this invention, and are not intended to limit this invention. Unless otherwise expressly stated, terms such as "set" and "connect" should be interpreted broadly.
[0018] like Figure 1 As shown, the event-driven retinal organic photodiode of this embodiment includes, from bottom to top, a glass substrate 1, an ITO transparent electrode 2, a ZnO electron transport layer 3, an organic bulk heterojunction active layer 4 (BHJ active layer), a MoO3 hole transport layer 5, a LiF insulating layer 6, and an Ag top electrode 7. Since the Ag top electrode 7, the LiF insulating layer 6, and the MoO3 hole transport layer 5 overlap to form a MIScap coupling structure, when the external light illumination changes on / off, the carrier distribution and interface polarization state inside the device change rapidly, thereby generating positive and negative transient voltage spikes at the output terminal.
[0019] In this embodiment, the sheet resistance of the ITO glass substrate can be selected as 10–20 Ω / sq, and the light-transmitting area can be prepared as 0.01–10 mm² according to the pixel size requirements. If used for array-type artificial retina, the pixel area can be defined by photolithography, mask evaporation, or laser etching; if used for unit device verification, the effective area can also be directly defined by a metal cathode mask. To ensure device reproducibility, the substrate is preferably subjected to continuous solvent cleaning and surface activation treatment before use to reduce the impact of organic residues and surface particles on the uniformity of the film.
[0020] The ZnO electron transport layer can be prepared by spin-coating a precursor solution formed by dissolving zinc acetate dihydrate and ethanolamine in 2-methoxyethanol, or by solution-based film formation using a ZnO nanoparticle dispersion. The preferred thickness of the formed ZnO layer is 30–40 nm to balance surface coverage, charge extraction capability, and light transmittance. If the ZnO layer is too thin, pinholes and substrate exposure are likely to occur, leading to increased leakage current; if the ZnO layer is too thick, it may increase series resistance and weaken charge extraction efficiency.
[0021] The organic bulk heterojunction active layer is the core functional layer for photoelectric conversion in this invention. In one embodiment, the organic bulk heterojunction active layer can be formed by blending the polymer donor D18 with the non-fullerene acceptor L8-BO, resulting in a system with complementary absorption and suitable energy level matching. The BHJ solution can be prepared using chloroform solvent, and a small amount of additives such as DIO can be introduced to adjust the phase separation scale and film morphology. The thickness of the organic bulk heterojunction active layer is preferably controlled between 80 and 120 nm to achieve a balance between absorption capacity, carrier transport distance, and film integrity.
[0022] The MoO3 hole transport layer is mainly used to improve hole extraction efficiency and enhance energy level matching between the active layer and the top electrode of the organic bulk heterojunction. Preferably, the MoO3 layer thickness is 8–15 nm. If the MoO3 layer is too thin, the interface modulation effect is insufficient; if it is too thick, it may increase series resistance, affecting the device's dynamic response. The MoO3 thin layer formed by vacuum thermal evaporation exhibits good uniformity and repeatability, which is beneficial for the stable deposition of the subsequent LiF layer.
[0023] The LiF insulating layer is the key innovation of this invention. Unlike the traditional use of LiF merely as an electron-injection modification layer, this invention leverages LiF's insulating properties, barrier modulation capabilities, and controllable thickness within the ultrathin thickness range to introduce it as an event-driven functional layer between the top electrode and the organic semiconductor. The LiF layer thickness has a significant impact on the device's transient behavior: when the thickness is small, interface charges are more easily coupled, forming a larger spike response, but this may be accompanied by strong leakage and lower waveform stability; as the thickness increases, capacitive coupling, tunneling effects, and interface barriers all change, resulting in a systematic alteration of the device's peak value, voltage plateau, and recovery process. By adjusting the LiF thickness within the range of 2–8 nm, event response outputs with different intensities and time constants can be obtained.
[0024] The Ag top electrode is preferably formed by vacuum thermal evaporation, with a thickness set to 80–120 nm to ensure low sheet resistance and good coverage. To reduce thermal damage to the underlying organic film, the initial Ag evaporation rate is preferably controlled and gradually increased to the target value. For arrayed devices, the electrode shape can be defined using a metal mask; for flexible devices, low-temperature packaging processes can be used as needed to improve the device's environmental stability.
[0025] The present invention will be further illustrated below with examples of different LiF thicknesses. Except for the LiF thickness, all other preparation conditions remain consistent.
[0026] Example 1: Controlling the thickness of the LiF insulating layer to 2 nm. An ITO / ZnO / BHJ / MoO3 / LiF / Ag device was fabricated sequentially according to the above process, with the LiF thickness controlled to 2 nm. This device exhibited a relatively obvious transient spike response under light pulse stimulation, indicating that the thinner LiF layer could establish interface modulation. Under some test conditions, the 2 nm device showed strong edge sensitivity, but its background fluctuations and post-peak recovery behavior were more dependent on process uniformity and interface state.
[0027] Example 2: LiF thickness of 4 nm. All other fabrication conditions were the same as in Example 1, except the LiF thickness was adjusted to 4 nm. Compared to the 2 nm device, the 4 nm device still exhibits a significant transient response, while the interface blocking effect is further enhanced, improving waveform stability and repetitive response consistency. Devices with this thickness typically achieve a good trade-off between event response amplitude and baseline suppression.
[0028] Example 3: LiF thickness was 6 nm. All other fabrication conditions were the same as in Example 1, except the LiF thickness was adjusted to 6 nm. As the LiF thickness continued to increase, the interface capacitive coupling and charge release process of the device changed further, and the transient peak, recovery speed, and steady-state plateau all exhibited dynamic characteristics different from the previous examples. This example illustrates that LiF thickness does not simply affect signal strength, but rather systematically alters the overall shape of the event response waveform.
[0029] Example 4: LiF thickness of 8 nm. The remaining fabrication conditions were the same as in Example 1, except the LiF thickness was adjusted to 8 nm. A thicker LiF layer provides a stronger insulating barrier effect and further prolongs the establishment and relaxation process of interface charges. Under some test conditions, the 8 nm device exhibited more significant background suppression; however, further increasing the thickness could lead to weakened coupling, peak value reduction, or even response hysteresis. Therefore, controlling the LiF thickness within the range of 2–8 nm is more beneficial for obtaining an adjustable and stable event response. Under pulsed light illumination test conditions, the output voltage-time response of the above examples was tested, and the experimental results are as follows: Figure 2 As shown, when the LiF insulating layer thickness is 2nm, 4nm, 6nm and 8nm, the device exhibits obvious transient peak output during illumination on and off, while the output is relatively weak in the continuous illumination range, indicating that the device has significant event-driven characteristics.
[0030] As the thickness of the LiF insulating layer changes, the peak voltage, post-peak recovery rate, and consistency between repetitive pulses of the device all change significantly. This indicates that the LiF layer not only acts as an insulating barrier but also influences the dynamic output behavior of the device by regulating the interface charge storage and release process. By selecting an appropriate LiF thickness, the device can be better suited for applications such as motion edge enhancement, dynamic target detection, and low-redundancy visual information acquisition.
[0031] To verify the mechanism of this invention, devices with different LiF thicknesses can be compared with control devices without a LiF layer. Generally, conventional devices without a LiF layer are more likely to exhibit continuous output corresponding to the incident light intensity; however, after introducing a LiF insulating layer, the transient signal at the device's edge is significantly enhanced, while the output during continuous illumination is significantly weakened, indicating that the LiF layer plays a key role in suppressing steady-state background and highlighting event information. This phenomenon shows that this invention does not simply change the device resistance, but rather achieves selective encoding of dynamic light stimuli by constructing an tunable MIS interface.
[0032] In addition to the embodiments described above, this invention has a wide range of applicability in terms of material systems and structural parameters. For example, the substrate can be flexible polyimide, PET, or PEN substrates in addition to glass; the ZnO electron transport layer can be replaced with other n-type oxides or organic electron transport layers; the hole transport layer can be V2O5, WO3, or other transition metal oxides with similar functions in addition to MoO3; and the electrodes can be replaced accordingly based on the device polarity design. As long as it does not deviate from the basic concept of "achieving event-driven response by controlling the metal / insulator / organic semiconductor interface," it should be considered within the scope of protection of this invention.
[0033] The working mechanism of this invention is as follows: When the device is stimulated by a light pulse, the active layer of the organic heterojunction absorbs photons and generates electron-hole pairs. Under the influence of ZnO, MoO3, and the built-in electric field, charge carriers separate and migrate. Due to the presence of a LiF insulating layer on top, some photogenerated charges cannot be completely released immediately through the conventional conductive path, but instead accumulate and polarize near Ag / LiF / MoO3. When the illumination is turned on or off, the interface carrier distribution and local electric field undergo abrupt changes, which in turn induce significant transient voltage peaks in the external circuit through MIScap coupling. Changing the LiF thickness alters the interface equivalent capacitance, tunneling probability, barrier height, and charge relaxation time constant, thus allowing for the adjustment of the amplitude, width, and recovery speed of the event signal.
[0034] The device of this invention is particularly suitable as a sensing unit in a biomimetic vision system because it can directly output sparse event signals at the front end. It can be used in scenarios such as moving target detection, edge enhancement, brightness change monitoring, invalid data compression, neuromorphic preprocessing, and low-power image sensing. For array devices, it can be further integrated with thin-film transistors, memristors, or spiking neural network circuits to form an intelligent vision hardware platform with integrated "sensing-encoding-processing" characteristics.
[0035] In summary, this invention constructs a MIScap coupling structure by introducing LiF insulating layers of varying thicknesses between the metal electrode and the organic semiconductor layer. This allows for the manipulation of the interface barrier, charge accumulation, and capacitive coupling behavior, enabling the device to generate a significant transient voltage output in response to brightness changes such as illumination switching on and off, while maintaining a weak steady-state response to stable illumination. This invention establishes a simple and effective event-driven implementation path by introducing a precisely controllable LiF insulating layer at the top interface of the organic photodiode. The proposed solution features simple process, controllable parameters, and high compatibility with existing organic optoelectronic manufacturing processes. It is suitable not only for single-device verification but also for expansion to array-based, flexible, and system-level integration, demonstrating promising application prospects and widespread application value.
[0036] This invention is not limited to the specific embodiments described above. Without departing from the technical concept of this invention, those skilled in the art can make equivalent substitutions or modifications to the material system, layer thickness range, film formation method, electrode size, and testing conditions, and all such modifications should fall within the protection scope of this invention.
Claims
1. A retinal organic photodiode for event-driven response, characterized in that, The device comprises, from bottom to top, a glass substrate, an ITO transparent electrode, a ZnO electron transport layer, an organic bulk heterojunction active layer, a MoO3 hole transport layer, a LiF insulating layer, and an Ag top electrode; wherein, the Ag top electrode, the LiF insulating layer, and the lower semiconductor layer together form a MIScap coupling structure, and the device output terminal is located between the top electrode and the bottom electrode.
2. The retinal organic photodiode for event-driven response according to claim 1, characterized in that, The thickness of the LiF insulating layer is 2–8 nm, more preferably 8 nm, to achieve adjustment of the intensity and time constant of different event-driven responses.
3. A method for fabricating a retinal organic photodiode for event-driven response as described in any one of claims 1-2, characterized in that, Includes the following steps: S1. Place the ITO transparent electrode on the upper surface of the glass substrate to obtain the overlapping structure of ITO and glass substrate. Sonicate in cleaning agent, deionized water and ethanol for 30 minutes in sequence to clean the overlapping structure of ITO and glass substrate, and then blow dry with high-purity nitrogen. S2. All pre-cleaned ITO and glass substrate overlap structures are treated with oxygen plasma for 70 seconds. S3. Spin-coat the ZnO solution onto the overlap structure of the cleaned ITO and glass substrate at a speed of 4000 rpm for 30s, and then anneal in air at 150°C for 15min to form a ZnO electron transport layer. S4. Dissolve the donor and acceptor in chloroform at a mass ratio of D:A = 1:1.4, and heat and stir at 60°C for more than 3 hours to form a mixed solution with a total concentration of 16.5 mg / ml. S5. Transfer the overlapping structure of ITO and glass substrate coated with ZnO electron transport layer to a high-purity nitrogen-filled glove box. Spin-coat the mixed solution of organic bulk heterojunction active layer on the overlapping structure of ZnO electron transport layer, ITO and glass substrate at 6000 rpm for 40s. Then anneal in a glove box at 100°C for 5min. S6, Finally, in 7×10 -5 MoO3 with a thickness of 10 nm was deposited by thermal evaporation under vacuum conditions of Pa. S7. Continue to use the vacuum thermal evaporation method to deposit a LiF insulating layer on the surface of the MoO3 hole transport layer. By adjusting the evaporation time and rate, LiF insulating layers with thicknesses of 2nm, 4nm, 6nm and 8nm are prepared respectively. S8. Next, deposit 100nm Ag top electrodes on LiF insulating layers of different thicknesses to obtain stacked structure devices. S9. The fabricated stacked structure device is subjected to transient response testing under pulsed light illumination. The change of output voltage over time is recorded to evaluate the event-driven response characteristics of the device under different LiF thicknesses.
4. The method for fabricating a retinal organic photodiode for event-driven response according to claim 3, characterized in that, The mixed solution in step S4, wherein the donor is D18 and the acceptor is L8-BO, has the following chemical structural formula: (C 76 H 92 F2N2S9) n C 84 H 90 F4N8O2S5.
5. The method for fabricating a retinal organic photodiode for event-driven response according to claim 3, characterized in that, The weight of LiF deposited by thermal evaporation in step S7 ranges from 2 to 8 mg.