A single impurity in an atomically thin semiconductor can behave like a microscopic solar cell, according to a new study that visualizes photovoltaic activity at the scale of individual atoms. Researchers have shown that vanadium dopants embedded in bilayers of tungsten diselenide, or WSe₂, can create localized junctions that separate photoexcited charges within a region only about one nanometre wide. The finding challenges the traditional picture of a photovoltaic junction as an extended structure governed by depletion regions, diffusion lengths and smoothly varying electric fields. Instead, the work suggests that in two-dimensional materials, the elementary building block of a solar-energy device may be a single charged dopant capable of reshaping the fate of an exciton.
The study focuses on a van der Waals semiconductor homobilayer containing ionizing acceptors. WSe₂ is made of atomically thin sheets in which tungsten atoms are coordinated with selenium atoms, producing a layered semiconductor with strong optical responses. When two such layers are stacked, electrons and holes can occupy either the same layer or different layers, creating distinct excitonic configurations. In an intralayer exciton, the electron and hole remain in the same sheet and are strongly bound by their mutual electrostatic attraction. In an interlayer state, they are separated vertically between the two sheets. This separation can make the pair more useful for charge extraction, but converting a tightly bound intralayer exciton into a charge-separated state is difficult without an appropriate local driving force.
That driving force comes from vanadium atoms introduced into WSe₂ as acceptor dopants. An acceptor can capture electronic charge and become ionized, leaving behind a localized electrostatic potential. In conventional semiconductor devices, dopants are often treated statistically, with their collective effect described through bulk carrier densities and extended space-charge regions. At atomic thicknesses, however, the potential generated by an individual dopant can remain highly localized and can interact directly with nearby excitons. Because screening is reduced in two-dimensional crystals, Coulomb interactions are unusually strong, while excitons can possess binding energies far greater than those found in many conventional bulk semiconductors. The result is a material in which a nanoscale perturbation can influence an optical excitation over a comparable distance.
To observe this effect, the researchers used photoconductive atomic force microscopy, a technique that combines nanoscale imaging with local electrical measurements under illumination. In this approach, a conductive atomic-force-microscope probe scans across the sample while light generates mobile or partially mobile charge carriers. The probe records photocurrent as a function of position, allowing researchers to map where photoinduced electrical signals emerge. Rather than producing a uniform response across the bilayer, the measurements revealed sharply confined photocurrent hotspots centred on individual vanadium dopants. These features identify the dopants as active photovoltaic sites rather than passive chemical defects.
The most striking observation was that the polarity of the local photocurrent depended on which layer contained the dopant. Dopants positioned in the top WSe₂ layer generated a current direction opposite to that produced by dopants in the bottom layer. This reversal is a direct signature of the vertical structure of the process. A charged acceptor in one sheet modifies the local energy landscape differently from an equivalent acceptor in the other sheet. Under illumination, the resulting electric field can favour the movement of one charge carrier toward the probe or an electrode while directing the opposite carrier away from it. Switching the dopant from the upper layer to the lower layer effectively reverses the orientation of the local junction, and therefore reverses the measured current.
The local observations were supported by measurements on vertical WSe₂/V:WSe₂ homobilayer devices, in which an undoped WSe₂ layer was combined with a vanadium-doped WSe₂ layer. At the device scale, the macroscopic photocurrent increased linearly with dopant concentration. Such a relationship indicates that the active dopants contribute approximately as independent photovoltaic units over the measured range: doubling the number of ionizing acceptors approximately doubles the total photoresponse. This behaviour differs from the response expected when a conventional extended junction dominates the device. In an ordinary bulk homojunction, increasing dopant density can alter depletion widths, built-in fields, carrier transport and recombination in a strongly coupled way. Here, the data instead point to an additive collection of nanoscale junctions.
The devices also displayed a compensation voltage that remained independent of both illumination power and dopant density. A compensation voltage is the applied bias at which competing photocurrent contributions cancel, producing little or no net current. Its independence from optical intensity suggests that the voltage is not simply determined by the number of photocarriers generated. Its independence from dopant concentration likewise indicates that the underlying energy offset or local electrostatic configuration is not being substantially reshaped by the total population of dopants. In the researchers’ interpretation, this behaviour reflects the discrete nature of the junctions and the energetic alignment between the layers, rather than the broad, density-dependent electrostatics normally associated with bulk semiconductor junctions.
Photocurrent spectroscopy provided additional clues about what happens after light is absorbed. The measurements, together with quasi-classical modelling, indicate that charged vanadium dopants can locally convert tightly bound intralayer excitons into charge-separated interlayer states. Light first creates an exciton, an electrically neutral quasiparticle consisting of a bound electron and hole. Near an ionized acceptor, the local electrostatic potential perturbs the exciton and changes the relative energetic cost of keeping both carriers in the same layer. The electron and hole can then lower their energy by separating between the two sheets. The vertical displacement does not necessarily destroy the pair immediately; instead, it creates an interlayer exciton or related charge-separated configuration with a built-in dipole that can assist subsequent extraction.
The modelling places the effective exciton-dissociation region at approximately one nanometre, a length scale comparable to the thickness and interlayer spacing of the material rather than to the much larger depletion lengths common in bulk electronics. Within this tiny volume, the dopant’s electric field is sufficiently strong to compete with the exciton binding energy. This does not mean that every exciton generated anywhere in the crystal is automatically separated. Rather, excitons formed within the local capture or conversion region of a charged dopant have an enhanced probability of becoming charge separated. The photocurrent therefore emerges as a collection of point-like sources, with each source determined by the position, charge state and layer location of an individual impurity.
The discovery could reshape how researchers think about photovoltaic design in two-dimensional materials. Atomically thin homobilayers are often considered too symmetric to produce the strong internal fields required for efficient charge separation, especially when both sides of a junction are made from the same semiconductor. The study shows that chemical symmetry can be broken locally without creating a conventional interface between different materials. A sparse population of ionizing dopants can supply the required asymmetry, while the bilayer itself provides the vertical pathway for converting intralayer excitons into interlayer states. This strategy could enable nanoscale photodetectors, light-harvesting devices and optoelectronic circuits in which the active regions are positioned with atomic precision.
The work also offers a new way to connect microscopic defects with macroscopic device performance. Defects and dopants are frequently treated as sources of disorder, trapping or unwanted recombination. In this case, however, a carefully selected dopant becomes the functional centre of a photovoltaic process. The opposite signals from the two layers make it possible to identify not only where a dopant is located laterally, but also which side of the homobilayer contains it. Such layer-sensitive responses could support three-dimensional mapping of active sites in otherwise ultrathin materials. More broadly, the results suggest that future devices may be designed from individually addressable junctions rather than from uniform regions with averaged material properties.
By establishing dopant-defined point-like junctions as elementary photovoltaic units, the study brings atomic-scale electrostatics into the centre of solar-energy research. Its key message is that efficient exciton dissociation does not always require a large depletion region or a chemically distinct heterojunction. In a strongly interacting two-dimensional semiconductor, one ionized acceptor can generate a local field powerful enough to redirect an exciton within roughly a nanometre. The combination of nanoscale photocurrent imaging, layer-dependent polarity, concentration-dependent device measurements and theoretical modelling provides a coherent picture of how these atomic junctions operate. As researchers learn to control the number, position and charge state of dopants, atomically thin homobilayers could become platforms where photovoltaic functionality is engineered one microscopic junction at a time.
Subject of Research: Point-like photovoltaic junctions formed by ionizing vanadium acceptors in WSe₂ homobilayers
Article Title: Point-like photovoltaic junction in 2D semiconductor homobilayer
Article References: Vu, N.T.T., Chen, M., Ho, Y.W. et al. “Point-like photovoltaic junction in 2D semiconductor homobilayer.” Nature Nanotechnology (2026). https://doi.org/10.1038/s41565-026-02251-9
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41565-026-02251-9
Keywords: two-dimensional semiconductors, WSe₂, vanadium doping, homobilayers, excitons, interlayer charge separation, photovoltaic junctions, photocurrent, atomic force microscopy, nanoscale optoelectronics
