Systems and methods for modulating memory of perovskite photosensors

Abstract

Apparatus, systems and methods for modulating memory of perovskite photosensors are disclosed. Apparatus, systems, devices and methods comprise a perovskite photocell configured to generate a photocurrent and / or a photovoltage in response to incident light, and a variable impedance module electrically coupled to the perovskite photocell and configured to apply a variable impedance load to the photocell. The variable impedance module is configured to modulate a decay rate of the photocurrent and / or the photovoltage such that the photocell retains a time-dependent electrical response or trace indicative of a prior illumination event.

Description

[0001] SYSTEMS AND METHODS FOR MODULATING MEMORY OF PEROVSKITE PHOTOSENSORS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to perovskite photosensors, and in particular to apparatus, systems, and methods for modulating memory of perovskite photosensors for energy efficient sensing.

[0004] BACKGROUND

[0005] A variety of vision sensor applications are currently being developed, including applications such as trajectory detection, motion detection, and fall detection. A typical vision sensor, such as a CMOS sensor, may generate a significant quantity of data via potentially millions of pixels of the sensor. However, the abovementioned applications do not necessarily require such detail, and such a large quantity of data, to function. In contrast, many vision sensor applications may need a lower quantity of data to function than the amount of data generated from a typical vision sensor’s pixels.

[0006] Software-hardware intelligent systems have been developed in response to this, which reduce the data quantity by transmitting or collecting only necessary data for a particular application. However, the selection of which data is necessary or important can be energy intensive in of itself.

[0007] Alternatively, application specific systems may generally be energy efficient or effective, but may be limited to specific applications or areas to which they are designed, meaning flexibility is reduced or lost.

[0008] In light of the abovementioned limitations in vision sensors, the design of devices and systems where one or more parameters can be tuned for specific applications becomes beneficial. For example, if a vision sensor is intended to monitor motion of humans, there is no need to rely on fast scanning or frames. However, if a vision sensor is intended to capture motions of a cheetah, it may be necessary to implement fast scanning or frames. Accordingly, the ability to tune a memory of a sensing element, may enable the same sensor to be used for various applications and in an energy efficient manner.

[0009] Currently, most optical sensors are CMOS or CCD based sensors, which are typically very fast and do not exhibit memory or a decay in photocurrent or photovoltage. In some embodiments, if memory is present in a sensor device, it may be possible to scan a pixel at reduced speed and not suffer data errors, leading to in-sensor or near sensor computation systems.

[0010] Therefore, in order to address or alleviate at least one of the aforementioned problems and / or disadvantages, there is a need to provide improved vision sensor systems and methods, particularly with improved energy efficiency characteristics.

[0011] SUMMARY OF THE INVENTION

[0012] According to a first aspect of the present disclosure a sensing device is provided. The sensing device comprises a perovskite photocell configured to generate a photocurrent and / or a photovoltage in response to incident light, and a variable impedance module electrically coupled to the perovskite photocell and configured to apply a variable impedance load to the photocell. The variable impedance module is configured to modulate a decay rate of the photocurrent and / or the photovoltage such that the photocell retains a time-dependent electrical response or trace indicative of a prior illumination event.

[0013] In an embodiment, the time-dependent electrical response provides information associated with an intensity, duration, and / or timing of a prior illumination event.

[0014] In an embodiment, the device is configured such that:

[0015] - if the variable impedance load is lower, the device discharges more of the photocurrent and / or photovoltage, thereby lengthening a time to attain a maximum saturated photocurrent and / or photovoltage when illuminated after an event, or

[0016] - if the variable impedance load is higher, the device discharges less of the photocurrent and / or photovoltage, or the photocurrent / photovoltage is not completely decayed or discharged, thereby shortening a time to reach a maximum saturated photocurrent / photovoltage when illuminated after an event.

[0017] In an embodiment, the variable impedance module is configured to modify a decay rate of the photocurrent and / or the photovoltage to enable detection of motion characteristics based on temporal changes in the photocurrent.

[0018] In an embodiment, the variable impedance module comprises a variable resistor, a memristor, and / or impedance control circuitry configured to apply a variable impedance load to the photocell.

[0019] In an embodiment, the variable impedance module comprises a pressure-sensitive impedance element configured to apply a variable impedance load to the photocell.

[0020] In an embodiment, the device further comprises a pressure-sensitive impedance element electrically coupled to the photocell, wherein a resistance of the impedance element varies with applied mechanical pressure, and processing circuitry configured to detect optical stimuli based on changes in the photocurrent / photovoltage, and detect mechanical pressure based on changes in the resistance of the pressuresensitive impedance element.

[0021] In an embodiment, the device further comprises a transimpedance amplifier (TIA) to isolate an operating point or bias of the sensor from an impedance of the variable impedance module.

[0022] In an embodiment, the device further comprises a readout circuit to detect optical stimuli based on changes in the photocurrent / photovoltage, and a transimpedance amplifier (TIA) configured to isolate the photocell and the variable impedance module from the readout circuit.

[0023] In an embodiment, the device further comprises an array of perovskite photocells, each photocell configured to generate a photocurrent in response to incident light, wherein the variable impedance module is electrically coupled to one or more of the photocells and configured to apply a variable impedance load to the one or more photocells.

[0024] In an embodiment, the variable impedance module comprises a plurality of variable impedance loads respectively coupled to each photocell in the array.

[0025] In an embodiment, the device further comprises processing circuitry configured to detect one or more characteristics of a change in illumination of one or more photocell in the array based on changes in the photocurrent / photovoltage of one or more photocells.

[0026] In an embodiment, a plurality of sensing devices are arranged in an array to form a sensor wall, each sensing device comprising a pixel size large enough to prevent identification of facial or fine structural features.

[0027] In an embodiment, the device further comprises a detection module configured to monitor a photocurrent decay profile and / or a photovoltage decay profile of one or more photocell and to output a movement indication signal based on one or more decay states of the photocell.

[0028] In an embodiment, the device further comprises a detection module configured to periodically sample a photocurrent and / or a photovoltage of the perovskite photocell and to output a movement indication signal based on one or more changes in sampled photocurrent and / or photovoltage of the photocell.

[0029] In an embodiment, each photocell in the array comprises a pixel, and wherein the array is configured to track a trajectory of a moving object by correlating time-stamped decay or rise signals across adjacent pixels.

[0030] In an embodiment, the array is configured to correlate time-stamped rise signals of each pixel post a decay occurred due to shadowing of the pixel during a motion event. In an embodiment, the device is configured such that the photocurrent from a portion of the photocells provides power to one or more of the variable impedance module and / or TIA and / or circuitry associated with the device.

[0031] In an embodiment, the device is configured as a solar cell such that the photocurrent provides power to one or more external devices.

[0032] In an embodiment, the device further comprises a solar inverter or controller.

[0033] In an embodiment, the device further comprises one or more perovskite photovoltaic cells configured to generate photocurrent in response to incident light, wherein the device is configured such that the photocurrent provides power to one or more of the variable impedance module and / or TIA and / or circuitry associated with the device and / or one or more external devices.

[0034] According to a second aspect of the present disclosure a sensing system is provided. The system comprises an array of perovskite photocells, each photocell configured to generate a photocurrent in response to incident light, a variable impedance module electrically coupled to one or more of the photocells and configured to apply a variable impedance load to the one or more photocells, wherein the variable impedance module is configured to modulate a decay rate of the photocurrent and / or photovoltage such that the photocell retains a time-dependent electrical response or trace indicative of a prior illumination event, and processing circuitry configured to detect one or more characteristics of a change in illumination on the array based on changes in the photocurrent and / or photovoltage of the photocells.

[0035] In an embodiment, the variable impedance module comprises a pressure-sensitive impedance element configured to apply a variable impedance load to one or more photocell.

[0036] In an embodiment, the system further comprises one or more pressure-sensitive impedance element electrically coupled to one or more respective photocells, wherein a resistance of the impedance element varies with applied mechanical pressure, and processing circuitry configured to detect optical stimuli based on changes in the photocurrent / photovoltage, and detect mechanical pressure based on changes in the resistance of the pressure-sensitive impedance element.

[0037] In an embodiment, the detection module is configured to periodically sample a photocurrent and / or a photovoltage of the perovskite photocell and output a movement indication signal based on one or more changes in sampled photocurrent and / or photovoltage of the photocell.

[0038] In an embodiment, the system is configured to discriminate between fast motion and slow motion by comparing a measured decay time constant of the photocurrent to a threshold value, and classifying motion as fast motion when the decay time constant is below the threshold value, and classifying motion as slow motion when the decay time constant is above the threshold value.

[0039] In an embodiment, the system further comprises adjusting the impedance load in response to detected motion characteristics to optimize detection sensitivity for subsequent motion events.

[0040] According to a third aspect of the present disclosure, a method of motion detection is provided. The method comprises illuminating a perovskite photocell or an array or perovskite photocells, applying, via control circuitry, a variable impedance load across the photocell or array of photocells, monitoring a decay profile of a resultant photocurrent of the photocell or one or more of the photocells in the array, and generating an output signal indicating movement trajectory, speed, and / or presence based on the monitored decay profile.

[0041] In an embodiment, the impedance load comprises a pressure-sensitive resistor, and the method further comprises simultaneously detecting applied pressure and light- induced photocurrent decay.

[0042] In an embodiment, the perovskite photocell is one of a plurality of photocells arranged in an array for motion tracking. In an embodiment, the array is configured as a low-resolution array for privacypreserving motion tracking.

[0043] According to a fourth aspect of the present disclosure, a sensing system is provided. The sensing system comprises an array of perovskite photocells, each photocell configured to generate a photocurrent and / or photovoltage in response to incident light, a variable impedance module electrically coupled to one or more of the photocells and configured to apply a variable impedance load to the one or more photocells, wherein the variable impedance module is configured to modulate a decay rate of the photocurrent and / or photovoltage such that the photocell retains a time-dependent electrical response or trace indicative of a prior illumination event, processing circuitry configured to detect one or more characteristics of a change in illumination of one or more photocell in the array based on changes in the photocurrent and / or photovoltage of one or more photocells, and a detection module configured to monitor a photocurrent decay profile and / or a photovoltage decay profile of one or more photocell and to output a movement indication signal based on one or more decay states of the photocell.

[0044] In an embodiment, the system further comprises one or more pressure-sensitive impedance element electrically coupled to one or more respective photocells, wherein a resistance of the impedance element varies with applied mechanical pressure, and processing circuitry configured to detect optical stimuli based on changes in the photocurrent / photovoltage, and detect mechanical pressure based on changes in the resistance of the pressure-sensitive impedance element.

[0045] In an embodiment, the detection module is configured to periodically sample a photocurrent and / or a photovoltage of the perovskite photocell and output a movement indication signal based on one or more changes in sampled photocurrent and / or photovoltage of the photocell.

[0046] In an embodiment, each photocell in the array comprises a pixel, and wherein the array is configured to track a trajectory of a moving object by correlating time-stamped decay or rise signals across adjacent pixels. In an embodiment, the array is configured to correlate time-stamped rise signals of each pixel post a decay occurred due to shadowing of the pixel during a motion event.

[0047] In an embodiment, the system is configured such that the photocurrent from a portion of the photocells (through the photovoltaic effect) provides power to one or more of the variable impedance module and / or TIA and / or circuitry associated with the device.

[0048] In an embodiment, the system is configured as a solar cell such that the photocurrent provides power to one or more external devices and / or an external grid or grid feed.

[0049] In an embodiment, the system is configured such that a central photocell is configured to detect a change in illumination, and one or more surrounding photocells are configured to provide power.

[0050] In an embodiment, the system further comprises a solar inverter or controller.

[0051] In an embodiment, the system further comprises one or more perovskite photovoltaic cells configured to generate photocurrent in response to incident light, wherein the device is configured such that the photocurrent provides power to one or more of the variable impedance module and / or TIA and / or circuitry associated with the device.

[0052] Any one or more of the abovementioned aspects or embodiments may be combined with one or more other or further aspects or embodiments as described herein.

[0053] Example embodiments disclosed herein demonstrate a sensor system to exploit the intrinsic photocurrent decay behavior of a perovskite-based photocell as a tunable, non-volatile memory element. In an example embodiment, by actively varying an impedance load to or across the photocell, systems and methods as disclosed herein may actively control both the magnitude and decay rate of the photocurrent. In an embodiment, the impedance may be provided as a fixed resistor, variable resistor or memristor. In an embodiment, the decay profiles may provide or encode detection of fast versus slow motion. In an embodiment, the resistive element may be replaced by a pressure-sensitive resistor to provide optical and force sensing in a single device.

[0054] In an embodiment, an array of photocells may form a privacy-preserving motiontracking array, for example on a wall, where large pixels may capture movement trajectories without imaging identifiable features.

[0055] It will be understood that any of the embodiments or aspects of the present disclosure may be combined with each other or any of the embodiments described in the other aspects of the present disclosure.

[0056] Systems and methods for modulating memory of perovskite photosensors according to the present disclosure are thus disclosed herein. Various features, aspects, and advantages of the present disclosure will become more apparent from the following detailed description of the embodiments of the present disclosure, by way of nonlimiting examples only, along with the accompanying drawings.

[0057] BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In the following, embodiments of the present invention will be described as non-limiting examples with reference to the accompanying drawings in which:

[0059] Fig. 1a illustrates an example of a read-out circuit for a photosensor according to an embodiment of the disclosure;

[0060] Fig. 1 b illustrates an example of a device structure of a perovskite photosensor according to an embodiment of the disclosure;

[0061] Figs. 1 c and 1 d are graphs of normalized photovoltage across a photosensor calculated under varied load resistance to the photosensor according to an embodiment of the disclosure;

[0062] Fig. 1 e is a graph showing an IV curve of a perovskite photosensor according to an embodiment of the disclosure; Figs. 2a and 2b show graphs comparing photovoltage (at load impedance: 5M ohms) and photocurrent (at load impedance: 0 ohms) in a photosensor using a SMU and the circuitry of Fig. 1a according to embodiments of the disclosure;

[0063] Fig. 3 is a graph of photocurrent decay of photosensors when subject to a resistor load and a memristor load according to embodiments of the disclosure;

[0064] Figs. 4a and 4b show graphs of photocurrent decay of photosensors under 0.1 second and 0.01 second of illumination, respectively, according to embodiments of the disclosure;

[0065] Fig. 5 illustrates an example of a read-out circuit incorporating a pressure sensor for a photosensor according to an embodiment of the disclosure;

[0066] Fig. 6a is a graph of a photosensor output signal of the circuitry from Fig. 5 when no pressure is applied according to an embodiment of the disclosure;

[0067] Fig. 6b is a graph of a photosensor output signal of the circuitry from Fig. 5 when pressure is applied under ambient room lighting according to an embodiment of the disclosure;

[0068] Fig. 6c is a graph of a photosensor output signal of the circuitry from Fig. 5 when pressure is applied and no light is provided according to an embodiment of the disclosure;

[0069] Fig. 7 illustrates a comparison between a trajectory tracking system formed with Si photodetectors and with Perovskite photosensors according to an embodiment of the disclosure;

[0070] Fig. 8 illustrates a gesture controlled robot arm according to an embodiment of the disclosure; Fig. 9 illustrates a halide perovskite solar cell array providing energy harvesting and sensing according to an embodiment of the disclosure;

[0071] Fig. 10a illustrates a power management circuit for a halide perovskite solar cell array according to an embodiment of the disclosure;

[0072] Fig. 10b illustrates a power management circuit for a halide perovskite photosensor array according to an embodiment of the disclosure;

[0073] Fig. 11a and 11 b illustrate motion tracking according to embodiments of the disclosure; and

[0074] Fig. 12 is a graph of normalized photocurrent against time for a sensor configured in short circuit mode according to embodiments of the disclosure.

[0075] DETAILED DESCRIPTION

[0076] Halide perovskite based photovoltaic cells are known to possess slow response, mainly due to ion migration, and this property of perovskite materials is utilised to design photosensors as disclosed herein that may address the problems outlined herein at device-circuit level.

[0077] Current vision systems may require various sensors, and therefore a significant quantity of data may be generated. Taking inspiration from the human body, which includes many sensors yet is comparatively energy efficient, it can be advantageous to develop devices and hardware which work similarly to human systems, i.e. possess various sensors and process the sensor outputs in an energy efficient manner.

[0078] Sensory fusion is one such concept which has recently been adopted. In this approach, data from various sensors may be integrated into a synaptic device which is connected to neurons that fire when an input crosses a threshold. Taking inspiration from this concept, systems and methods as disclosed herein may integrate visual (proximity sensing) and tactile (pressure sensing) information, and may not require or use any additional elements for integration. This approach may be applicable in / near-sensor computing where data is processed at the sensory node before being transmitted to a neuron, which may enable improved or higher energy efficiency.

[0079] The following describes tuneable photovoltage / photocurrent decay of photoexcited cells. Perovskite based photosensors have memory owing to ionic migration, and this concept may be exploited for reduced scan-rate implementations. However, modulating the memory range in these sensors may require consideration between the device and electronic performance.

[0080] Example embodiments may include the coupling of a photosensor cell with a circuit including a transimpedance amplifier (TIA). The photosensor cell may comprise a hal ide-perovskite photovoltaic cell, or any perovskite or other material system cell wherein a decay is determined by ionic and electronic transport within the system. The circuit including a transimpedance amplifier may be utilised as a buffer in example embodiments, however, the signal from the cell can also be amplified. Nevertheless, a primary function of the TIA in some example embodiments is to isolate the sensor or cell from readout circuit impedance.

[0081] In example embodiments, introducing a tuneable impedance component (e.g., a variable resistor, memristor, or pressure sensor) as a load to the cell can enable dynamically adjustable photovoltage / photocurrent decay, while keeping a rise time almost constant. Accordingly, systems and methods as disclosed herein may integrate a sensor (cell) with a memristor (as load to a cell) to achieve tuneable decays.

[0082] As described herein, the variable impedance can result in tuneable decay profiles of the sensor or cell. However, it will be understood that it is not just load resistance changing that leads to tuneable decay. Rather, the device dynamics of the systems and methods as disclosed herein change when the impedance is altered, leading to tuneable decays.

[0083] Advantageously, the adjustable decay aspect of the sensor or cell provides an insensor computing / memory concept or function. In some example embodiments, a rise curve (instead of a decay) may also be utilised to provide in-sensor computing / memory function. In example embodiments, the decay (or rise) results in the sensor holding a fading trace (or rising trace) of a prior light exposure or change in light condition (for example, a motion event as described herein).

[0084] In an example when an action occurs, such as a human walking by an array or wall of sensors, the speed of the human is not high relatively. Accordingly, a 60-100 Hz sampling method is not required (such as that of a conventional camera) nor is continuous pixel scanning required (e.g. DVS). Example embodiments may scan a frame at a start and end of an event, and determine motion (of an object, human etc.) with the difference in voltage values from each sensor. Example embodiments may also implement event-based system activation. Advantageously, this may help with temporal compression of data, thereby reducing the number of pixels data generated over time for motion detection as photocurrent and / or photovoltage decays.

[0085] Example embodiments may achieve the above by leveraging the performance of the systems and methods described herein, in which the rise time of the photocurrent / photovoltage is almost constant, and only the decay is modulated by varying the sensor's load. When a motion occurs, the load impedance determines the extent of photovoltage decay and time required by the cell to reach its saturated photovoltage. If the decay is higher, it may take a longer time for photovoltage to saturate, thus giving rise to memory effects. This memory can support implementations such as low-power movement and trajectory detection, dynamic vision sensing with configurable frame rates, reduced scan speeds, improved efficiency / self-powering via indoor photovoltaics, and enhanced privacy through minimal pixel resolution.

[0086] In an example embodiment, or example at the given set of range of resistances (55k and above as described herein), systems and methods as disclosed herein may be configured such that only a decay of the cell changes. In example embodiments, the rise time of the device or cell as described herein may be almost constant (for example, as observed in Figure 1 c and 1 d described below). No further modification may be necessary to obtain a constant rise rate and variable decay rate of the device as described herein, and this property of device may be exploited as described herein. In some example embodiments, systems and methods as described herein may be configured in photovoltage or photocurrent mode, depending on how the readout circuit is connected. In example embodiments, voltage may be generated at the buffer output, and photocurrent may be run only through the resistor or impedance as described herein. Accordingly, systems and methods as disclosed herein may measure photovoltage, photocurrent, or a combination of photocurrent and / or photovoltage.

[0087] In example embodiments, when in Jsc mode (i.e. V=0V) and Voc (i.e. I=OA), photocurrent and photovoltage may be measured respectively. In other cases, due to circuit configuration, systems and method as disclosed herein may obtain photovoltage from photocurrent and vice versa.

[0088] Photosensor transient response to light can be modulated by varying the load resistance with a circuit such as that shown in Figure 1 a, which illustrates a read-out circuit 100a for a photosensor according to an example embodiment. In this example embodiment, the circuit 100a may comprise photosensor 110a, which may comprise a Perovskite photocell as described herein, coupled to a load 120a. The load 120a may comprise a variable load as described herein.

[0089] Circuit 100a may further comprise a signal output 130a, and a transimpedance amplifier (TIA) 140a may be positioned between the signal output 130a and the load 120a. The circuit 100a may further comprise a resistor or further circuitry 145a positioned across the TIA 140a.

[0090] In an example embodiment, resistor 145a can comprise one or more capacitors and resistors formed in a network or arrangement or further circuitry to tune or improve the bandwidth of the TIA 140a as necessary.

[0091] Figure 1b illustrates an example embodiment of a device structure 100b of a photosensor according to embodiments of the present invention. Device structure 100b may comprise an FTO glass layer 110b with various layers fabricated on an upper surface of the FTO glass layer 110b.. In example embodiments, the fabricated layers may include one or more of a layer of compact TiO2 120b, mesoporous TiC 130b, insulating material such as ZrO2 140b, and carbon electrode 150b over ZrO2 140b for contact (e.g. to form electrical contacts to the device).. Photoactive material such as halide perovskite of various composition can be infiltrated to absorb light. It will be understood that the abovementioned list of fabricated layers is not exhaustive, and that Figure 1b provides for only one example embodiment of a structure, device, or photocell as described herein.

[0092] To obtain various decay constants (i.e. memory windows), in an example embodiment the load resistance to the cell or photosensor may be changed, and results of an example embodiment are plotted in Figure 1c and Figure 1d, which illustrates normalized photovoltage across a photosensor calculated under varied load resistance to the cell.

[0093] Advantageously, the systems and methods disclosed herein may provide for photosensors with improved characteristics. For example, photosensors of example embodiments disclosed herein may illustrates a reduction in an RC (resistancecapacitance) decay. In example embodiments disclosed herein, the photosensor load is reduced from 1 M to 110k ohms, i.e. approximately 10 times, however, the decay does not reduce by 10 times, as shown in Figure 1 d. Put another way, a non-linear decay is observed as photosensor load is reduced. This property of the systems and methods as disclosed herein may be exploited in various applications as described herein.

[0094] In an example embodiment, the load of the cell can be set to low resistance (e.g. 5-10 ohms) for transient photocurrent (TPC), and can be shifted to high resistance (e.g. 10M ohms) for transient photovoltage (TPV).

[0095] In an example embodiment, an important window of operation of the systems and methods as disclosed herein may be in the region on an IV curve where current changes with voltage as shown in Figure 1e. In this region, various decay states can be obtained, and thus various decay curves can be provided, which can be chosen by the user and therefore provide tunability. The decay obtained is not due to the circuitry, but is instead caused by device and load interaction. This is evident from Figures 2a and 2b, which illustrate a comparison between the photovoltage (at load impedance: 5M ohms) in Figure 2a and photocurrent (at load impedance: 0 ohms) in Figure 2b of a cell or photodetector using a source measuring unit (SMU) and the circuit of Figure 1 a. respectively, as described herein. It is evident that the SMU and the circuitry disclosed herein output substantially the same results. Accordingly, the circuit may enable biasing of the cell with load and avoid influencing any readout circuit impedance on the cell.

[0096] In an experimental example the resistance used as load to the cell may be replaced with memristors, and device response may be observed. In experimental examples, the memristor resistance is tuned to 560k ohms and tested as load to the cell. It is observed that cell response to a memristor is identical to its response to a resistor as shown in Figure 3. Therefore, example embodiments may implement an array of memristors as load to an array of photosensing cells. In such an example embodiment, each cell decay can be coded according to a resistance of a memristor in the array.

[0097] With a pulse illumination of 10 milliseconds, example cells are observed to have photocurrent decay in the 1~3 second range, and for 0.1 second of illumination, a photocurrent decay in the 1 ,5~4.5 second range may be observed, as shown in Figure 4a and 4b, respectively. In the legend of Figures 4a and 4b, D 2-2 is device number, R_x is fixed resistor of value X, and HL_M is memristor with high resistance (e.g. 500k ohms).

[0098] Table 1 illustrates results of experimental examples for various load resistances to a cell according to the systems and methods disclosed herein. In the experimental example, the decay time constants are obtained via a double exponential fit of the transient curve, to output two time constants, one fast and one slow. The fast decay time constant is attributed to electronic recombination, whilst the slow decay time constant is attributed to ionic migration. The slow time constant is illustrated in Table 1.

[0099] Table 1 : Photocurrent decay time constant with varied illumination time

[0100] Advantageously, when fast motion occurs, a system with low decay time provides for a trajectory that can be collected with any information loss at low scan speed. However, if memory is not desired, increasing a load to the cell helps in modulating the memory window as the cell would not have decayed to its initial state, thus taking a shorter time to reach its saturation levels as discussed herein. Put another way, low decay time may help in faster decay of the device as described herein, leading to prolonged times for a cell to reach its saturation. This can help in holding memory significantly longer as described herein. Therefore, systems and methods as disclosed herein may allow modulation of the memory of sensor pixels in accordance with a speed of a moving object and desired frame scan rate.

[0101] The following describes demonstration of sensory fusion, both visual and tactile. To achieve visual and tactile sensory fusion, systems and methods as disclosed herein may include the replacement of the load resistor in the circuitry described herein (for example 120a shown in Figure 1a) with a pressure sensor. An example embodiment of a circuit 500 including a pressure sensor is shown in Figure 5.

[0102] The circuit 500 in Figure 5 may comprise a photosensor 510, which may comprise a Perovskite cell as described herein, coupled to an output 530 via a TIA 540. In this example embodiment, a pressure sensor 520 may be provided, which may be configured to provide variable resistance and hence load to the photosensor 510 as described herein.

[0103] In an experimental example, the pressure sensor 520 may have a resistance of 5M ohms when no pressure is applied and when high pressure is applied, the resistance may reduce to around 8k ohms. In an experimental example, the independent response of each stimulus is recorded and is shown in Figures 6a-6c. Light of wavelength 623nm is used as an excitation source, and the photosensor is excited for 0.2 seconds. The decay is shown in Figure 6a.

[0104] The legend of Figure 6a indicates light intensity of the light source. An equivalent power and current is outlined in table 2 shown below. It is noted that with increase in the intensity of light, the output voltage increases when no pressure is applied.

[0105] Table 2: Light intensity of LED light source at varying current values.

[0106] Next, pressure is applied to the pressure sensor with constant ambient light, and Figure 6b shows that with increase in pressure, output increases. This demonstrates independent measuring of input stimulus.

[0107] However, when both stimulus change, as in case of Figure 6c, i.e. the photosensor is put to dark, and various pressures are applied, it is observed that for each different pressure, the output curves are different. This behaviour is exploited in the systems and methods as disclosed herein regarding sensory fusion.

[0108] In prior examples, an additional device may be utilized (generally a transistor) which integrates both the signal values and provides an output. The output may be collected using a readout circuit (TIA). However, the systems and methods as disclosed herein may provide for the output as described above without the need for additional components. Devices, systems and methods as disclosed herein may provide numerous advantages and improvements over existing methods, devices or materials. For example:

[0109] As described herein, in an example embodiment the decay of a perovskite cell can be modulated, which means the duration of memory can be changed according to various applications or needs.

[0110] In an example embodiment, a single readout can give both visual and tactile information, with no requirement for additional devices to integrate the information. This provides for reductions in hardware footprint.

[0111] In an example embodiment, a visual sensor, such as those disclosed herein, may operate as a solar cell. Accordingly, systems and methods as disclosed herein can be powered using the visual sensor, thereby reducing energy usage, leading to green systems. This advantage may be enhanced due to the reduced hardware footprint of the disclosed visual sensors, which can provide additional space for solar cells.

[0112] In an example embodiment, memristors may be integrated with the cells or visual sensors disclosed herein, and be used to modulate the memory of the visual sensor or cell. Other tuneable resistance elements may also be integrated as required.

[0113] Advantageously, in example embodiments the system design complexity and component count may be reduced significantly, because a single wire can be utilized for each visual and tactile pixel pair.

[0114] Advantageously, in example embodiments the hardware footprint may be reduced as fewer readouts are required, and there is no requirement for memory buffers to store pixel values.

[0115] Advantageously, in example embodiments energy consumption may be greatly reduced due to usage of power generating photosensors. In example embodiments, the photosensors described herein may be formed of perovskite material, which has volatile memory owing to the intrinsic ionic migration in the material.

[0116] Volatile memory can be useful as it may act as a buffer in a sensor, thus enabling a slow scan rate and thereby reducing hardware footprint and energy usage for dense pixel sensor applications. This memory can also be modulated due to the sensor device and electronic circuitry interaction as described herein.

[0117] In example embodiments, the memory of a sensor is modulated by exploiting the interaction described herein with circuitry that provides for energy efficient sensing in various applications including trajectory detection, sensory fusion, large area sensing or cognitive surfaces etc.

[0118] In example embodiments, the photosensor cell is connected to a load, and depending on the resistance of the load, the cell transient response changes. In example embodiments, when reading the voltage or current of a cell when connected to load, it may be necessary to ensure readout impedance does not affect the operating point of the cell. Accordingly, systems and methods provide for a circuit such that readout circuit impedance does not affect an operating point whilst providing accurate voltage or current of the cell. In example embodiments, this is obtained by exploiting the negative feedback and virtual short properties of a trans-impedance amplifier. By using this circuit architecture, systems and methods may provide advanced in-sensor computing and thus realise energy efficient computing.

[0119] In an example embodiment, there is provided a circuit arrangement comprising a photosensor, a transimpedance amplifier (TIA), and an electrical component configured to modulate a memory of the photosensor. The electrical component can include but is not limited to a variable resistor, a memristor and a pressure sensor. The photosensor is a halide perovskite based photovoltaic cell.

[0120] In example embodiments, tuneable photovoltage decay may be provided, leading to efficient trajectory detection. Tracking trajectory may be an important task in motion detection and dynamic vision sensing. In present systems, CMOS sensors are generally utilized, which are formed of silicon or lll-V semiconductor materials. However, there are many applications which may involve human tracking, for example, in a workspace, theft prevention, smart buildings, retail analytics, healthcare and aged care or elder-care monitoring, etc. where the motion of an object to be tracked is not fast. Accordingly, in such implementations a fast sensor, such as an existing CMOS sensor, is not required.

[0121] Additionally, in such implementations there is no need to read pixels immediately after motion. Accordingly, the scan speed can be reduced without loss in information, which can greatly reduce the energy consumption of such a system.

[0122] Systems and methods as disclosed herein may provide solutions to these problems as outlined above. Additionally, due to the tuneability of the systems and methods disclosed herein, if fast sensing is required, the load resistance can be modulated according to a speed required to capture trajectory with minimal frames.

[0123] Privacy benefits may also be provided utilising the systems and methods as disclosed herein. For example, a reduction in pixel count (compared to an existing CMOS sensor for example) may allow systems and methods as disclosed herein to capture the required data for motion tracking, without over capturing identifiable data or data that may enable identification of a subject beyond that required for motion tracking (e.g. facial features).

[0124] Figure 7 illustrates an example embodiment of trajectory tracking of an object as recorded from (a) a Si photodetector (b) a perovskite photosensor using the circuit disclosed herein. In this example embodiment the LEDs are connected to the output of the circuit, and correspond to respective photosensors.

[0125] As shown in Figure 7, a hand was moved rightside over an array (6x1 ) of silicone photodetector (Si detectors) and a perovskite based photosensor. For readout, the circuit described in Figure 1 a is used, and the output is connected to LEDs. In Figure 7a, since Si detectors are very fast in response, 7 scans or snapshots are required to determine a trajectory of the hand, as illustrated by the increasing lit LEDs in a clockwise direction.

[0126] In contrast, Figure 7b illustrates a perovskite photosensor and circuitry according to the systems and methods disclosed herein. Here, it is observed that just 1 scan after motion detection is sufficient to identify the trajectory of the hand.

[0127] In this example embodiment, the sensor with the longest exposure to light results in a bright LED, which means the object moved away from that sensor early.

[0128] Example embodiments may comprise an array, for example a 9x9 array of sensors, with an output of the array connected to a microcontroller. This may enable output data to be processed to form a trajectory image.

[0129] Current gesture and motion recognition systems may utilise cameras and machine learning algorithms, which can pose a privacy threat due to the capture of identifiable data, and are also typically energy intense. In contrast, systems and methods as disclosed herein may provide improved privacy due to a reduction in pixel quantity. Additionally, in photovoltaic mode the systems and methods as disclosed herein may generate energy, thereby providing energy efficient gesture and motion recognition capabilities.

[0130] Figure 8 is an example illustrating a gesture controlled robot arm. The top image illustrates the experimental example setup, and the lower images illustrate the displacing of a can using a robot arm commanded by a gesture over a sensor array according to example embodiments disclosed herein.

[0131] In the Figure 8, an example setup for a gesture controlled robot arm is shown, wherein a can is displaced using a robotic arm. The blue box includes an array (4x1 ) of perovskite photosensors according to example embodiments disclosed herein, and the system is trained to recognise gestures as shown in Figure 8 (Top). The set of gestures to which the system is trained to recognise include (a) clockwise motion to the right (b) anti-clockwise motion to the left (c) grab item if P1 or P2 are covered and (d) drop item if P3 or P4 are covered.

[0132] Figure 8 (Bottom) are photos showing a can being grabbed and handed over, where the commands were provided by gestures of a human. This experimental example provides a proof of concept that gesture recognition as described herein can be efficiently and securely performed utilising the systems and methods disclosed herein.

[0133] A robot arm with awareness grabbing capabilities and surroundings awareness may become important as robots become increasingly utilised and encountered in the near future.

[0134] In example embodiments, the sensor fusion concept described herein may contribute to awareness in grabbing and gripping of objects. For humans, hairs help in recognizing proximity to objects and provide awareness of our surrounds. Taking inspiration from this, example embodiments demonstrate grabbing and gripping capabilities via robot arms.

[0135] Example embodiments may comprise robot arms which have photosensors on a palm region and pressure sensors on a finger region. When an object is falling on the arm, the photosensor may be covered or shaded, and therefore the photosensor response may drop. This may actuate a grabbing action.

[0136] If the falling object is grabbed successfully, the circuit output may not rise, and may continue decaying. This may correspond to a scenario when the object is being gripped. The pressure sensors at the finger region may provide an output, and the output can determine the gripping of an object.

[0137] In an example embodiment, the action of gripping may be active until the pressure sensor output does not drop below a threshold. Once the output is below a threshold, gripping may be stopped, which can provide a system that is similar to the way in which humans grab and grip objects. An example embodiment may include indoor photovoltaic systems and methods. Halide perovskite solar cells can be fabricated, for example using scalable printing techniques such as screen printing and roll-to-roll printing. These methods may allow the production of electrodes with various shapes and sizes, enabling mosaic-style designs that can enhance the aesthetic appeal of indoor environments. Beyond their decorative potential, the infiltration of perovskite materials into printed electrode patterns may enable these structures to function as solar cells or motion sensors, for example as shown in Figure 9. Example embodiments may be capable of monitoring human activities while maintaining occupant privacy.

[0138] Figure 9 is an example embodiment comprising a substrate 900 of 25 halide perovskite solar cells 910, wherein in this example 24 cells 910a are used for energy harvesting, and 1 cell 91 Ob is used as a sensor. Multiple substrates 900 may be combined to form an array as described herein.

[0139] In an example embodiment, the size of the substrates 900 may comprise around 100 cm2. In an example embodiment the solar cells 910a provided for energy harvesting may substantially surround a central cell 910b comprising the sensor cell. It will be understood that any number or orientation of cells may be provided to provide for energy harvesting and / or sensor configurations falling within the scope of the present disclosure.

[0140] To efficiently harvest indoor light, systems and methods as disclosed herein may implement a power management circuit 1000a, as illustrated in Figure 10a. In an example embodiment, this circuit may comprise a maximum power point tracking (MPPT) circuit 1020a, that may ensure that an array of solar cells 1010a operate at their optimal power output. A DC-DC converter 1030a may be provided that regulates the harvested energy to provide the appropriate voltage and current required for battery charging, for example to battery 1040a. A power management unit (PMU) 1050a may be provided to distribute power to sensor readout circuits and store excess energy, for example for various home automation or loT applications. In example embodiments, the array of solar cells 1010a may comprise sensors as described herein, that can be used to monitor the motions of objects with minimal data points by leveraging the temporal compression capabilities of perovskite cells.

[0141] Figure 10b illustrates an example embodiment of a circuit 1000b as described herein for reading an array of photosensors 1010b utilising a variable impedance module 1020b and a circuit to modulate the impedance 1030b. The variable impedance module 1020b may comprise a memristor or an array of memristors as described herein, controllable by circuitry 1030b to modulate an impedance. The circuit 1010b may comprise a TIA 1040b as described herein, to a sensor readout 1060b.

[0142] The circuit in Figure 10b may comprise sensors as described herein, that can be used to monitor the motions of objects with minimal data points by leveraging the temporal compression capabilities of perovskite cells. To achieve this, the equilibrium saturated photocurrent / photovoltage may be measured for all cells to provide a first frame. When an event occurs, at the end of the event, all the cells may be scanned for a second frame. Both the frame values may then be subtracted, and the resulting array of photocurrent / photovoltage can provide information on object motion. In an example embodiment, the highest difference, corresponding to the darkest datapoint, comes from the pixel where the event occurred recently, and the lowest difference, corresponding to the white datapoint corresponds to no or little motion or event occurrence. Figures 11 a and 11 b illustrate example embodiments showing left to right and right to left motion, and diagonal motion, respectively.

[0143] In example embodiments, to detect the start and end of an event, a sensor as described herein can be configured in short-circuit mode, where the sensor may provide sudden current spikes when there is a change in the intensity of light falling onto the sensor. This may be used to trigger standard event detection circuits in an example embodiment.

[0144] Figure 12 illustrates an example embodiment of normalized photocurrent against time for a sensor such as those described herein configured in short circuit mode, where a rapid current spike is observed that can be used to as a trigger as described herein. In an example embodiment, an event (such as change in illumination) can be recognised using a cell as described herein when configured in load = 0 ohms mode.

[0145] As described herein, an event may be considered to occur when an object casts a shadow over the sensor. As the sensors are generally illuminated, systems and methods as disclosed herein may consider the photocurrent / photovoltage during the rise of photocurrent / photovoltage of the device. Accordingly, if the load impedance is lower, the device may discharge most of its photocurrent / photovoltage, thus making the time to attain maximum saturated photocurrent / photovoltage longer. If the load impedance is higher, then the photocurrent / photovoltage may not be completely decayed, thus taking a shorter time to reach the saturated photocurrent / photovoltage when illuminated after an event.

[0146] Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the art that many variations of the embodiments can be made within the scope and spirit of the present invention.

Claims

CLAIMS1 . A sensing device comprising: a perovskite photocell configured to generate a photocurrent and / or a photovoltage in response to incident light; and a variable impedance module electrically coupled to the perovskite photocell and configured to apply a variable impedance load to the photocell; wherein the variable impedance module is configured to modulate a decay rate of the photocurrent and / or the photovoltage such that the photocell retains a time-dependent electrical response or trace indicative of a prior illumination event.

2. The device of claim 1 , wherein the time-dependent electrical response provides information associated with an intensity, duration, and / or timing of a prior illumination event.

3. The device of claim 1 or claim 2, wherein the variable impedance module is configured to modify a decay rate of the photocurrent and / or the photovoltage to enable detection of motion characteristics based on temporal changes in the photocurrent.

4. The device of any preceding claim, wherein the variable impedance module comprises a variable resistor, a memristor, and / or impedance control circuitry configured to apply a variable impedance load to the photocell.

5. The device of any preceding claim, wherein the variable impedance module comprises a pressure-sensitive impedance element configured to apply a variable impedance load to the photocell.

6. The device of any preceding claim, further comprising: a pressure-sensitive impedance element electrically coupled to the photocell, wherein a resistance of the impedance element varies with applied mechanical pressure; and processing circuitry configured to:detect optical stimuli based on changes in the photocurrent / photovoltage; and detect mechanical pressure based on changes in the resistance of the pressure-sensitive impedance element.

7. The device of any preceding claim, further comprising a transimpedance amplifier (TIA) to isolate an operating point or bias of the sensor from an impedance of the variable impedance module.

8. The device of any preceding claim, further comprising an array of perovskite photocells, each photocell configured to generate a photocurrent in response to incident light; wherein the variable impedance module is electrically coupled to one or more of the photocells and configured to apply a variable impedance load to the one or more photocells.

9. The device of claim 8, wherein the variable impedance module comprises a plurality of variable impedance loads respectively coupled to each photocell in the array.

10. The device of claim 8 or claim 9, further comprising processing circuitry configured to detect one or more characteristics of a change in illumination of one or more photocell in the array based on changes in the photocurrent / photovoltage of one or more photocells.11 . The device of any one of claims 8 to 10, wherein each photocell in the array comprises a pixel, and wherein the array is configured to track a trajectory of a moving object by correlating time-stamped decay signals across adjacent pixels.

12. The device of claim 11 , wherein the array is configured to correlate time-stamped rise signals of each pixel post a decay occurred due to shadowing of the pixel during a motion event.

13. The device of any preceding claim, wherein the device is configured as a solar cell such that the photocurrent provides power to one or more external devices.

14. A method of motion detection, comprising: illuminating an array of perovskite photocells; applying, via control circuitry, a variable impedance load across the array of photocells; monitoring a decay profile of a resultant photocurrent of one or more photocells in the array; and generating an output signal indicating movement trajectory, speed, and / or presence based on the monitored decay profile.

15. The method of claim 14, wherein the impedance load comprises a pressuresensitive resistor, and the method further comprises simultaneously detecting applied pressure and light-induced photocurrent decay.

Images