# Photodiode

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Type Three Si and one Ge (top) photodiodes. Passive Converts light into current anode and cathode

A photodiode is a semiconductor device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as their surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode.

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the portion of the spectrum that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometers (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of roughly 430–750 terahertz (THz).

An electric current is the rate of flow of electric charge past a point or region. An electric current is said to exist when there is a net flow of electric charge through a region. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optical properties of filters are completely described by their frequency response, which specifies how the magnitude and phase of each frequency component of an incoming signal is modified by the filter.

## Contents

Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specially as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias. [1]

A semiconductor material has an electrical conductivity value falling between that of a conductor, such as metallic copper, and an insulator, such as glass. Its resistance decreases as its temperature increases, which is the behaviour opposite to that of a metal. Its conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the so-called "metalloid staircase" on the periodic table. After silicon, gallium arsenide is the second most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits and others. Silicon is a critical element for fabricating most electronic circuits.

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.

An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers.

## Principle of operation

A photodiode is a p–n junction or PIN structure. When a photon of sufficient energy strikes the diode, it creates an electronhole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. The total current through the photodiode is the sum of the dark current (current that is generated in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device. [2]

A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy. If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes.

A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.

The photon is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. The invariant mass of the photon is zero; it always moves at the speed of light in a vacuum.

To first order, for a given spectral distribution, the photocurrent is linearly proportional to the irradiance. [3]

In radiometry, irradiance is the radiant flux (power) received by a surface per unit area. The SI unit of irradiance is the watt per square metre. The CGS unit erg per square centimetre per second is often used in astronomy. Irradiance is often called intensity because it has the same physical dimensions, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity.

### Photovoltaic mode

When used in zero bias or photovoltaic mode, photocurrent flows out of the anode through a short circuit to the cathode. If the circuit is opened or has a load impedance, restricting the photocurrent out of the device, a voltage builds up in the direction that forward biases the diode, that is, anode positive with respect to cathode. If the circuit is open or the impedance is high, a forward current will consume all or some of the photocurrent. This mode exploits the photovoltaic effect, which is the basis for solar cells – a traditional solar cell is just a large area photodiode. For optimum power output, the photovoltaic cell will be operated at a voltage that causes only a small forward current compared to the photocurrent. [3]

The photovoltaic effect is the creation of voltage and electric current in a material upon exposure to light. It is a physical and chemical phenomenon.

A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Individual solar cell devices can be combined to form modules, otherwise known as solar panels. The common single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 to 0.6 volts.

### Photoconductive mode

In this mode the diode is reverse biased (with the cathode driven positive with respect to the anode). This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance and increases the region with an electric field that will cause electrons to be quickly collected. The reverse bias also reduces the dark current without much change in the photocurrent.

Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components.

In physics and in electronic engineering, dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons are entering the device; it consists of the charges generated in the detector when no outside radiation is entering the detector. It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device.

Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects. [4] The leakage current of a good PIN diode is so low (<1 nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.

### Other modes of operation

Avalanche photodiodes are photodiodes with structure optimized for operating with high reverse bias, approaching the reverse breakdown voltage. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.

A phototransistor is a light-sensitive transistor. A common type of phototransistor, called a photobipolar transistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base–collector junction . It was invented by Dr. John N. Shive (more famous for his wave machine) at Bell Labs in 1948, [5] :205 but it was not announced until 1950. [6] The electrons that are generated by photons in the base–collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfe). If the base and collector leads are used and the emitter is left unconnected, the phototransistor becomes a photodiode. While phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes.[ citation needed ] Phototransistors also have significantly longer response times. Field-effect phototransistors, also known as photoFETs, are light-sensitive field-effect transistors. Unlike photobipolar transistors, photoFETs control drain-source current by creating a gate voltage.

A Solaristor is a two-terminal gate-less phototransistor. A compact class of two-terminal phototransistors or solaristors have been demonstrated in 2018 by ICN2 researchers. The novel concept is a two-in-one power source plus transistor device that runs on solar energy by exploiting a memresistive effect in the flow of photogenerated carriers. [7]

## Materials

The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents.

Materials commonly used to produce photodiodes include: [8]

Material Electromagnetic spectrum
wavelength range (nm)
Silicon 190–1100
Germanium 400–1700
Indium gallium arsenide 800–2600

Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes.

### New materials

Like two dimensional materials such as MoS2, graphene etc. [9]

## Unwanted photodiode effects

Any p–n junction, if illuminated, is potentially a photodiode. Semiconductor devices such as diodes, transistors and ICs contain p–n junctions, and will not function correctly if they are illuminated by unwanted electromagnetic radiation (light) of wavelength suitable to produce a photocurrent; [10] [11] this is avoided by encapsulating devices in opaque housings. If these housings are not completely opaque to high-energy radiation (ultraviolet, X-rays, gamma rays), diodes, transistors and ICs can malfunction [12] due to induced photo-currents. Background radiation from the packaging is also significant. [13] Radiation hardening mitigates these effects.

In some cases, the effect is actually wanted, for example to use LEDs as light-sensitive devices (see LED as light sensor) or even for energy harvesting, then sometimes called light-emitting and -absorbing diodes (LEADs). [14]

## Features

Critical performance parameters of a photodiode include:

Responsivity
The Spectral responsivity is a ratio of the generated photocurrent to incident light power, expressed in A/W when used in photoconductive mode. The wavelength-dependence may also be expressed as a Quantum efficiency , or the ratio of the number of photogenerated carriers to incident photons, a unitless quantity.
Dark current
The current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.
Response time
A photon absorbed by the semiconducting material will generate an electron–hole pair which will in turn start moving in the material under the effect of the electric field and thus generate a current. The finite duration of this current is known as the transit-time spread and can be evaluated by using Ramo's theorem. One can also show with this theorem that the total charge generated in the external circuit is e and not 2e as one might expect by the presence of the two carriers. Indeed, the integral of the current due to both electron and hole over time must be equal to e. The resistance and capacitance of the photodiode and the external circuitry give rise to another response time known as RC time constant ${\displaystyle \tau =RC}$. This combination of R and C integrates the photoresponse over time and thus lengthens the impulse response of the photodiode. When used in an optical communication system, the response time determines the bandwidth available for signal modulation and thus data transmission.
Noise-equivalent power
(NEP) The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1  hertz bandwidth. NEP is essentially the minimum detectable power. The related characteristic detectivity (${\displaystyle D}$) is the inverse of NEP, 1/NEP. There is also the specific detectivity (${\displaystyle D^{\star }}$) which is the detectivity multiplied by the square root of the area (${\displaystyle A}$) of the photodetector, (${\displaystyle D^{\star }=D{\sqrt {A}}}$) for a 1 Hz bandwidth. The specific detectivity allows different systems to be compared independent of sensor area and system bandwidth; a higher detectivity value indicates a low-noise device or system. [15] Although it is traditional to give (${\displaystyle D^{\star }}$) in many catalogues as a measure of the diode's quality, in practice, it is hardly ever the key parameter.

When a photodiode is used in an optical communication system, all these parameters contribute to the sensitivity of the optical receiver, which is the minimum input power required for the receiver to achieve a specified bit error rate .

## Applications

P–n photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog; for measurement and the like), or to change the state of circuitry (digital; either for control and switching, or digital signal processing).

Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices [16] and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.

Photosensors of all types may be used to respond to incident light, or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a light-emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slotted optical switch), or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler). The combination of LED and photodiode is also used in many sensor systems to characterize different types of products based on their optical absorbance.

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators), instruments to analyze samples (immunoassay), and pulse oximeters.

PIN diodes are much faster and more sensitive than p–n junction diodes, and hence are often used for optical communications and in lighting regulation.

P–n photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding.

### Comparison with photomultipliers

1. Excellent linearity of output current as a function of incident light
2. Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other semiconductor materials
3. Low noise
4. Ruggedized to mechanical stress
5. Low cost
6. Compact and light weight
8. High quantum efficiency, typically 60–80% [18]
9. No high voltage required

1. Small area
2. No internal gain (except avalanche photodiodes, but their gain is typically 102–103 compared to 105-108 for the photomultiplier)
3. Much lower overall sensitivity
4. Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
5. Response time for many designs is slower
6. latent effect

### Pinned photodiode

The pinned photodiode (PPD) has p+/n/p regions in it. The PPD has a shallow P+ implant in N type diffusion layer over a P-type epitaxial substrate layer. It is not be confused with the PIN photodiode. The PPD is used in CMOS active-pixel sensors. [19]

Early charge-coupled device (CCD) image sensors suffered from shutter lag. This was largely resolved with the invention of the pinned photodiode (PPD). [20] It was invented by Nobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara at NEC in 1980. [20] [21] They recognized that lag can be eliminated if the signal carriers could be transferred from the photodiode to the CCD. This led to their invention of the pinned photodiode, a photodetector structure with low lag, low noise, high quantum efficiency and low dark current. [20] It was first publicly reported by Teranishi and Ishihara with A. Kohono, E. Oda and K. Arai in 1982, with the addition of an anti-blooming structure. [20] [22] The new photodetector structure invented at NEC was given the name "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, the PPD began to be incorporated into most CCD sensors, becoming a fixture in consumer electronic video cameras and then digital still cameras. [20]

In 1994, Eric Fossum, while working at NASA's Jet Propulsion Laboratory (JPL), proposed an improvement to the CMOS sensor: the integration of the pinned photodiode. A CMOS sensor with PPD technology was first fabricated in 1995 by a joint JPL and Kodak team that included Fossum along with P.P.K. Lee, R.C. Gee, R.M. Guidash and T.H. Lee. Since then, the PPD has been used in nearly all CMOS sensors. The CMOS sensor with PPD technology was further advanced and refined by R.M. Guidash in 1997, K. Yonemoto and H. Sumi in 2000, and I. Inoue in 2003. This led to CMOS sensors achieve imaging performance on par with CCD sensors, and later exceeding CCD sensors. [20]

## Photodiode array

A one-dimensional array of hundreds or thousands of photodiodes can be used as a position sensor, for example as part of an angle sensor. [23]

The photodiode array (PDA) was proposed as an early image sensor technology that predates both the charge-coupled device (CCD) and active-pixel sensor (APS) technologies. In an imaging PDA, pixels contain a p-n junction, integrated capacitor, and MOSFETs (metal-oxide-semiconductor field-effect transistors) as selection transistors. An imaging PDA was proposed by G. Weckler in 1968, two years before the CCD was first demonstrated in 1970. [24] The PDA the basis for the passive-pixel sensor (PPS), which in turn was the basis for the active-pixel sensor (APS). [20] However, early PDAs were complex and impractical, requiring selection transistors to be fabricated within each pixel, along with on-chip multiplexer circuits. The noise of photodiode arrays was also a limitation to performance, as the photodiode readout bus capacitance resulted in increased noise level. Correlated double sampling (CDS) could also not be used with a photodiode array without external memory. PDAs were thus not widely used, with CCDs and then APS CMOS sensors being more widely used. [24]

In recent years, one advantage of modern photodiode arrays (PDAs) is that they may allow for high speed parallel read out since the driving electronics may not be built in like a CMOS or CCD sensor.

## Related Research Articles

A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, such as conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins.

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

In electronics, an avalanche diode is a diode that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration and resulting hot spots, so that the diode is undamaged by the breakdown. The avalanche breakdown is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current when compared to a non-avalanche diode.

An avalanche photodiode (APD) is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. From a functional standpoint, they can be regarded as the semiconductor analog of photomultipliers. By applying a high reverse bias voltage, APDs show an internal current gain effect due to impact ionization. However, some silicon APDs employ alternative doping and beveling techniques compared to traditional APDs that allow greater voltage to be applied before breakdown is reached and hence a greater operating gain. In general, the higher the reverse voltage, the higher the gain. Among the various expressions for the APD multiplication factor (M), an instructive expression is given by the formula

An opto-isolator is an electronic component that transfers electrical signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecting the system receiving the signal. Commercially available opto-isolators withstand input-to-output voltages up to 10 kV and voltage transients with speeds up to 25 kV/μs.

A single-photon avalanche diode (SPAD) is a solid-state photodetector in which a photon-generated carrier can trigger a short-duration but relatively large avalanche current. This avalanche is created through a mechanism called impact ionization, whereby carriers are accelerated to high kinetic energies through a large potential gradient (voltage). If the kinetic energy of a carrier is sufficient further carriers are liberated from the atomic lattice. The number of carriers thus increases exponentially from, in some cases, as few as a single carrier. This mechanism was observed and modeled by John Townsend for trace-gas vacuum tubes, becoming known as a Townsend discharge, and later being attributed to solid-state breakdown by K. McAfee. This device is able to detect low-intensity ionizing radiation, including: gamma, X-ray, beta, and alpha-particle radiation along with electromagnetic signals in the UV, Visible and IR. SPADs are also able to distinguish the arrival times of events (photons) with a timing jitter of a few tens of picoseconds.

Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. A photo detector has a p–n junction that converts light photons into current. The absorbed photons make electron–hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components.

Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron with enough kinetic energy can knock a bound electron out of its bound state and promote it to a state in the conduction band, creating an electron-hole pair. For carriers to have sufficient kinetic energy a sufficiently large electric field must be applied, in essence requiring a sufficiently large voltage but not necessarily a large current.

An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves into signals, small bursts of current that convey the information. The waves can be light or other electromagnetic radiation. Image sensors are used in electronic imaging devices of both analog and digital types, which include digital cameras, camera modules, medical imaging equipment, night vision equipment such as thermal imaging devices, radar, sonar, and others. As technology changes, digital imaging tends to replace analog imaging.

An active-pixel sensor (APS) is an image sensor where each pixel sensor unit cell has a photodetector and one or more active MOSFET amplifiers. There are different types of integrated circuit active pixel sensors, including the complementary metal–oxide–semiconductor (CMOS) APS used most commonly in digital camera technologies such as cell phone cameras, web cameras, most modern digital pocket cameras, most digital single-lens reflex cameras (DSLRs), and mirrorless interchangeable-lens cameras (MILCs). Such an image sensor is produced using CMOS technology, which emerged as an alternative to charge-coupled device (CCD) image sensors and eventually outsold them by the mid-2000s.

A Hole accumulation diode (HAD) is an electronic noise reduction device in a charge-coupled device (CCD) or CMOS imaging sensor. HAD devices function by reducing dark current that occur in the absence of light falling on the imager for noise reduction and enhanced image quality.

A Position Sensitive Device and/or Position Sensitive Detector (PSD) is an optical position sensor (OPS), that can measure a position of a light spot in one or two-dimensions on a sensor surface.

Eric R. Fossum is an American physicist and engineer known for developing the CMOS image sensor. He is currently a professor at Thayer School of Engineering in Dartmouth College.

James R. "Bob" Biard is an American electrical engineer and inventor who holds 73 U.S. patents. Some of his more significant patents include the first infrared light-emitting diode (LED), the optical isolator, Schottky clamped logic circuits, silicon Metal Oxide Semiconductor Read Only Memory, a low bulk leakage current avalanche photodetector, and fiber-optic data links. He has been on the staff of Texas A&M University as an Adjunct Professor of Electrical Engineering since 1980.

A solaristor is a compact two terminal self-powered phototransistor. The two-in-one transistor plus solar cell achieves the high-low current modulation by a memresistive effect in the flow of photogenerated carriers. The term was coined by Dr Amador Perez-Tomas working in collaboration with other ICN2 researchers in 2018 when they demonstrated the concept in a ferroelectric-oxide/organic bulk heterojunction solar cell.

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