For solar cells and photodetectors to convert light into an electrical signal, the electrons and holes generated by light energy must be separated and carried to different electrodes. When an electron absorbs a photon's energy and is excited to a higher energy state, it leaves behind a "hole" in its original state, which behaves as a positively charged carrier. If the two can be separated before they recombine, the energy produced by the light can be extracted as electric current. In graphene, however, a one-atom-thick material in which carbon atoms are arranged in a honeycomb lattice, achieving this charge separation efficiently has been difficult.
A research team at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany and the Technische Universität Dresden has published a new theoretical model that addresses this problem. The four researchers behind the study are Joris Josiek, Friedemann Queisser, Stephan Winnerl and Ralf Schützhold. Their results were published in volume 26 of the journal Physical Review Applied on August 21, 2026, as "Magnetophotoelectric effect in graphene via tailored potential landscapes" (article number 024061, DOI: 10.1103/k1y8-vr4y). On October 29, HZDR described the research and its planned experiments in more detail in an official announcement.
The team proposes applying a magnetic field perpendicular to the graphene and using electrostatic potential barriers created by electrodes to steer electrons and holes in different directions. Unlike previously studied approaches that rely on the edges or folds of graphene, this method can use a flat graphene sheet as it is. By changing the voltage applied to the electrodes, it may also be possible to adjust the charge transport paths dynamically.
The result, however, is a theoretical verification based on a mathematical model and calculations; no actual photoelectric device has been fabricated and its performance measured. Measured photocurrent values, energy conversion efficiency and practical power generation performance have not yet been shown. The team has also estimated the magnetic field and electrode layout scales needed when infrared light is the target, and the focus now is whether these conditions can be reproduced in experimental devices.
| Parameter | Scale indicated by the theoretical model | Technical significance |
|---|---|---|
| Applied magnetic field | About 0.1 tesla | A level that even a strong permanent magnet can generate |
| Electrode/barrier spacing | Micrometer scale | Corresponds to the cyclotron orbits of the excited carriers |
| Voltage applied to electrodes | Volt range | Voltage needed to form electrostatic potential barriers |
These are design conditions estimated by the HZDR team for infrared light. By suitably combining magnetic field strength and electrode spacing, it should be possible to form trajectories that guide electrons and holes in different directions. All of these are within the range of existing technology, but that alone does not guarantee that a photoelectric device will work.
Graphene Has No Bandgap, a Physical Constraint That Makes Charge Separation Hard
In a typical silicon solar cell, the semiconductor's bandgap and the pn junction make the conversion of light into electricity possible. The bandgap is the range of energies in which electrons cannot exist; when a photon with more energy than this is absorbed, an electron can be excited to a higher energy state. Joining a p-type semiconductor and an n-type semiconductor creates an internal electric field that drives the photogenerated electrons and holes in opposite directions. Combining light absorption with charge separation in this way allows current to flow through an external circuit.
Graphene, by contrast, has its conduction and valence bands touching at the "Dirac point," so the original single-layer structure has no bandgap. It has the advantage of high carrier mobility, allowing electrons to move quickly, but it is difficult to separate electrons and holes by the same mechanism used in ordinary semiconductors. Electric fields can be formed and photocurrents generated in graphene as well, but a mechanism different from conventional bandgap-based solar cells is needed.
The difficulty of photoelectric conversion in graphene is not that it cannot absorb light. On the contrary, because it has no bandgap, it may be able to use low-energy infrared light that silicon cannot absorb. But if the photogenerated electrons and holes cannot be transported in suitable directions, energy is lost through recombination and scattering before the current can be collected. Reconciling light absorption with charge separation is a key challenge in using graphene as a photoelectric material.
To address this, researchers have studied methods that use the edges or folds of graphene. When a magnetic field is applied perpendicular to the graphene plane, photoexcited electrons and holes experience the Lorentz force and trace circular orbits in opposite directions. A uniform magnetic field alone, however, makes carriers merely move in circles and does not lead to transport in a fixed direction. A method was therefore devised in which a cut edge or fold in the graphene changes the direction of the circling carriers.
In this approach, carriers repeatedly follow arc-shaped trajectories along a physical boundary and, as a result, drift in one direction. Because electrons and holes have opposite charge signs, a suitable structure can guide each in a different direction. But forming regular edges in one-atom-thick graphene, or creating uniform folds over a large area, is not easy. Edges are prone to defects and inhomogeneity, which becomes a manufacturing problem for solar cells that need a large light-receiving area.
Using Voltage-Created Barriers as "Guide Rails" Without Processing the Graphene
Instead of physical edges, the HZDR team proposes using electrostatic potential barriers created by external electrodes. The graphene itself is not cut or folded; voltage controls the movement of electrons and holes. This could allow charge transport paths to be designed more flexibly while avoiding problems caused by edge defects.
In the envisioned structure, an insulating layer is placed above or below a flat graphene sheet, with thin metal electrodes on the outside. When a voltage is applied to an electrode, the electrostatic potential inside the graphene changes according to its position, creating a barrier that affects the motion of electrons and holes. Fine fabrication of the electrodes is still required, but no cuts or folds need to be formed in the graphene itself.
When a uniform magnetic field perpendicular to the graphene plane is added, electrons and holes begin to trace circular orbits in opposite directions. When a circling carrier approaches an electrostatic potential barrier, it may be either reflected or transmitted, depending on the barrier's sign and height. The mechanism steers carriers in particular directions by combining the curvature of their orbits from the magnetic field with selective reflection by the electrodes.
For example, with a negative-potential barrier, under suitable energy conditions electrons are reflected while holes can pass through. The reflected electron again traces an arc under the magnetic field and, on reaching the next barrier, changes direction again. By repeating this motion, the electron travels in a fixed direction along the barriers in a series of arcs. The team proposes using this kind of motion to control the direction of charge transport.
With a positive-potential barrier, on the other hand, the behavior of electrons and holes is reversed. Holes are more likely to be reflected while electrons can pass, so combining barriers of opposite sign can give electrons and holes different paths. The paper examines a structure combining positive and negative potential barriers in a comb-like arrangement and shows how to guide electrons and holes to contacts in opposite directions.
Importantly, this separation mechanism does not work with an electric field alone or with a magnetic field alone. The team solved the effective Dirac equation describing carriers in graphene and analyzed the conditions under which a combination of electric and magnetic fields transports electrons and holes in different directions. They concluded theoretically that, with an appropriate potential distribution, electrons and holes can be guided selectively.
Another feature of this approach is that the transport paths can be adjusted by changing the electrode voltages. Physical edges and folds are hard to change once formed, whereas the height and distribution of electrostatic potential barriers can be tuned simply by changing the externally applied voltage. If multiple electrodes are controlled individually, it may even be possible to change the carrier guiding paths to match the energy of the incident light.
Conditions for Success: A 0.1-Tesla Field and Micrometer-Scale Electrode Layouts
This approach does not mean that electrons and holes can be separated under any conditions simply by arranging electrodes and applying a magnetic field. For carriers to follow the desired paths, the magnetic field strength, electrode layout, barrier height and width, and the energy of the photoexcited carriers must be suitably combined. The team studied these conditions theoretically and identified ranges of parameters thought to be achievable with existing technology.
First, the electrostatic potential barriers need sufficient height and width to reflect electrons or holes reliably. If a barrier is too low, it cannot change the carriers' direction, and if it is too narrow, carriers may pass through by quantum mechanical tunneling. Because quantum transmission cannot be ignored in graphene, the potential distribution formed by the electrodes must be designed carefully.
Second, the spacing of electrodes and barriers must be comparable to the size of the cyclotron orbits that carriers trace in the magnetic field. The radius of a cyclotron orbit depends on the carrier energy and the field strength: a stronger field makes the radius smaller, and a higher carrier energy makes it larger. If the barrier layout does not match the orbits, carriers cannot be guided in the intended direction.
As a concrete example, the team assumed carriers with an energy of about 0.1 electron volt (eV), which corresponds to the energy of carriers generated by absorbing infrared light. Under these conditions, applying a magnetic field of about 0.1 tesla gives a cyclotron orbit radius of about 1 micrometer, and the electrode and barrier spacing can be designed at a similar scale.
| Design item | Assumed value/condition | Feasibility and caveats |
|---|---|---|
| External magnetic field | About 0.1 T | A level that a strong permanent magnet can generate |
| Energy of excited carriers | About 0.1 eV | Assumes absorption of infrared light |
| Cyclotron orbit radius | About 1 μm | Varies with magnetic field and carrier energy |
| Potential barrier spacing | Micrometer scale | Designed to match the size of the cyclotron orbits |
| Voltage applied to electrodes | Volt range | Used to form barriers that reflect carriers |
These figures are representative design conditions in a theoretical model aimed at infrared light, not values optimized in an actual device. For example, if the energy of the incident light changes, the carrier orbit radius changes as well, so the magnetic field and electrode layout must be adjusted accordingly. The team says that weakening the magnetic field can accommodate lower-energy carriers, and that switching some electrodes on and off to change the effective barrier spacing is also conceivable.
Scattering, which impedes carrier movement, is also an important constraint. Although graphene has high carrier mobility, real samples contain impurities and defects, and scattering by lattice vibrations also occurs. If the carrier mean free path is much shorter than the size of the cyclotron orbit, carriers are scattered before they can move regularly along the barriers, weakening transport in the intended direction.
Because the potential barriers can be tuned by the voltage applied to the electrodes, the theory is thought to give device operating conditions some flexibility. Arranging many electrodes in two dimensions may also make it possible to guide charge over a wider area without cutting the graphene. However, the uniformity of electrodes, the quality of insulating layers and the effects of scattering at larger scales must be verified in real manufacturing processes, and for now these remain design advantages only.
How It Differs from Conventional Photoelectric Approaches, and Where Demonstration Stands
Research on using graphene for photoelectric conversion has examined a variety of charge separation mechanisms. In conventional semiconductors, the built-in field of a pn junction is standard, but given graphene's bandgap-free electronic structure, charge must be guided by other means. The new study differs from the previously studied edge- and fold-based approaches in that it uses voltage-controlled electrostatic potential barriers.
| Approach | How electrons and holes are separated | Main advantages/challenges | Demonstration status |
|---|---|---|---|
| Conventional silicon solar cells | Internal electric field from a pn junction separates charge | A mature technology, but has difficulty using infrared light at wavelengths beyond about 1,100 nm | Commercialized. Abundant measured data on conversion efficiency and reliability |
| Graphene approach using edges/folds | Combines circular motion in a magnetic field with physical boundaries | Charge separation has been confirmed in experiments, but edge defects and large-area processing are challenges | Basic experiments on the magnetophotoelectric effect have been reported |
| HZDR voltage-barrier approach | Magnetic field and electrostatic potential barriers guide electrons and holes in different directions | Can use flat graphene, and paths can be controlled by voltage | Theoretical model presented. Photocurrent measurements on actual devices not yet performed |
With the conventional edge-based approach, the magnetophotoelectric effect, which exploits the motion of charge along an edge when a magnetic field is applied perpendicular to the graphene, has been observed experimentally. The new study replaces physical edges with electrostatic potential barriers, and this method's operation has not yet been confirmed experimentally. Although both use magnetic-field-driven carrier motion, there is a clear difference in their stage of demonstration.
What HZDR expects from this approach is the manufacturing advantage of not relying on physical edges. Graphene edges are susceptible to defects and inhomogeneity, making it hard to reproduce the same properties over a wide area. Potential barriers formed by electrodes, on the other hand, could be laid out in a fixed arrangement with existing microfabrication techniques and have their operation adjusted by external voltage.
Even so, an electrode-based approach still requires fine electrode fabrication and insulating layer formation. Reducing the processing of the graphene itself is not the same as making the whole device easier to manufacture. Whether the theoretical advantages translate into manufacturing yield, stable performance and cost reductions from scaling up depends on future experiments and verification of manufacturing technology.
Promise for Infrared Conversion, but Single-Layer Graphene Absorbs Only About 2.3%
One application HZDR is particularly hopeful about is solar cells and photodetectors that use infrared light. The bandgap of silicon is about 1.1 eV at room temperature, and photons with less energy than this cannot generate electrons and holes through ordinary interband transitions. As a result, a typical single-junction silicon solar cell cannot effectively use infrared light with wavelengths beyond roughly 1,100 nm.
Graphene, which has no bandgap, may be able to excite electrons even with low-energy photons that silicon cannot absorb. In fact, graphene interacts with light over a wide frequency range from visible and infrared light to terahertz waves. This broadband optical response is an important advantage when considering applications such as infrared sensors and high-speed photodetectors.
But being able to absorb light and being able to convert that energy into electricity efficiently are separate matters. Single-layer graphene is only one carbon atom thick and therefore absorbs little light. A study published in Science in 2008 showed that single-layer graphene absorbs about 2.3% of incident visible light. Most of the rest is transmitted, so the amount of light usable for photoelectric conversion by a single graphene layer alone is limited.
Note that this figure of about 2.3% is a representative absorption rate mainly in frequency ranges such as visible light, and it does not stay the same across the entire infrared wavelength range. Actual absorption characteristics change with photon energy, graphene carrier density, temperature and surrounding materials. The figure of about 2.3% therefore cannot be taken as-is as the absorption rate of an infrared device.
To compensate for this low absorption, HZDR also envisions a structure using multiple graphene layers: graphene layers stacked and separated by transparent insulating layers, combined with a reflective mirror so that light passes through several active layers. Increasing the number of times light passes through graphene increases the chances of absorption, potentially allowing more light energy to be used than with a single layer.
This combination of multilayering and a reflective mirror, however, is still at the stage of feasibility study. Even if stacking graphene increases light absorption, power generation efficiency will not improve unless the electrons and holes produced in each layer can be separated efficiently and collected in an external circuit. Losses from forming the stacked structure and electrode placement for each layer must also be considered.
The paper and the HZDR announcement do not give measured values for power generation efficiency, output power, spectral sensitivity or manufacturing cost relative to silicon solar cells. Graphene's potential to use low-energy infrared light is important, but it does not by itself mean performance surpassing conventional silicon solar cells. The result should be seen as a proposal for a new physical mechanism for separating electrons and holes after light is absorbed.
The Path to Practical Use and the Experiments Still Needed
Whether this theoretical model holds up as a real photoelectric device depends on future experiments. HZDR plans experiments to build the proposed structure and verify its operation. The initial experimental device the team envisions sandwiches single-layer graphene between transparent insulating layers, with thin metal electrodes placed on top. Voltages in the volt range will be applied to the electrodes, and light will be shone under a magnetic field of about 0.1 tesla to check whether electrons and holes move in different directions as predicted.
In HZDR's announcement of October 29, Professor Ralf Schützhold expressed optimism about the approach. In earlier work using graphene edges, theoretical predictions were confirmed experimentally, and the observed signal was stronger than expected. Based on that experience, the team expects the new voltage-barrier approach to work as well.
However, the earlier experimental results mentioned do not indicate the performance of the proposed voltage-barrier device. The boundary conditions for charge movement differ between a method that guides carriers with edges and one that uses electrostatic potential barriers. Past success is a reason to pursue the new idea, but it does not guarantee the same results with the new approach.
The first thing experiments must confirm is whether the voltage barriers selectively reflect electrons and holes as calculated and guide them to different contacts. It will be necessary to measure whether the direction and magnitude of the photocurrent change as the theory predicts when the magnetic field direction or electrode voltages are altered. The team must also vary the wavelength and intensity of the light to find the range over which charge separation occurs.
Next in importance is the effect of scattering in real materials. The theoretical model shows that carrier trajectories can be controlled by the magnetic field and electrostatic potential. But real graphene contains impurities and defects, and at room temperature scattering by lattice vibrations also occurs. If these effects shorten the carrier mean free path, the directional transport assumed in the theory could weaken.
Furthermore, to be used as a solar cell, confirming charge separation is not enough. The current and voltage obtained under illumination must be measured to evaluate the extractable power and photoelectric conversion efficiency. The power needed to control the external electrodes and the equipment configuration for generating the magnetic field must also be considered. With permanent magnets, maintaining a static field does not itself require continuous power, but the magnet's size and placement, and the structure of the device as a whole, would be practical constraints.
Scaling up to large areas is another important issue. A method that forms potential barriers with electrodes may be better suited to deployment over wide areas than cutting or folding graphene. In practice, however, many electrodes must be formed uniformly so that charge is guided in the same way in every region. Nonuniform insulating layer quality or potential distribution between electrodes could prevent the expected photocurrent from being obtained, so manufacturing processes and electrical performance must be verified together.
The study shows that, even without artificially creating a bandgap in graphene, combining a magnetic field with electrostatic potential may guide photogenerated electrons and holes in different directions. By theoretically deriving a charge separation mechanism that does not depend on physical edges, it adds a new option for the design of graphene-based infrared sensors and photoelectric conversion devices.
How much photocurrent can actually be extracted, and how far conversion efficiency can be raised, are still unknown. If future experiments confirm charge separation as the theory predicts, it could also lead to graphene devices whose charge transport paths can be changed by voltage. The next important stage for the research will be whether voltage-barrier charge separation can be demonstrated in a real device and its performance evaluated quantitatively.