The team, led by Zeinab PourMohammadi, Fatemeh Dehghan Nayeri, and Rouhollah Azimirad, focused on a heterojunction device that sandwiches the archetypal perovskite methylammonium lead iodide, known as MAPbI3, on top of p-type silicon. Silicon remains the backbone of the photovoltaic and optoelectronic markets thanks to its abundance, stability, and excellent electronic properties, but forming traditional silicon junctions requires high-temperature doping processes and sophisticated equipment. Pairing silicon with metal halide perovskites offers a cheaper, low-temperature alternative. MAPbI3, a crystalline material with the general ABX3 perovskite formula, brings direct-bandgap light absorption, low exciton binding energy, long carrier diffusion lengths, and simple solution processing to the partnership. In principle, the combination should harness the best of both materials.

In practice, however, the marriage has a persistent flaw. When silicon and perovskite are pressed directly against each other, mismatched energy band alignment creates a poor electrical contact. Photoexcited charge carriers recombine at the interface before they can be collected, squandering the very light the device is meant to detect. The standard remedy is a buffer layer, a thin film that bridges the two dissimilar materials and smooths out the energetic landscape. Previous studies have tested metal oxides such as tin dioxide, gallium oxide, and titanium dioxide in this role, each with its own trade-offs between dark current, recombination, and tunneling efficiency.

The new study takes a different route by turning to PEDOT:PSS, the workhorse hole-transport polymer of the perovskite world. This material is a polymer electrolyte with a split personality: positively charged PEDOT is highly conductive but water-insoluble, while negatively charged PSS is insulating but acts as a surfactant that lets PEDOT disperse in water. The two components naturally form a micelle-like structure, with conductive PEDOT cores wrapped in nonconductive PSS shells. Crucially for the new work, the arrangement of these components is not fixed. During spin-coating, the denser, hydrophobic PEDOT phase settles toward the bottom of the film while the hydrophilic PSS-rich phase accumulates at the surface, and a mild 120-degree-Celsius bake does not remix them.

The researchers exploited this segregation with an elegantly simple modification: instead of depositing a single PEDOT:PSS layer spun at 2000 rpm, they applied two consecutive coatings, first at 1500 rpm and then at 2000 rpm, with no chemical additives of any kind. Each new spin-coating cycle exposes the PSS-rich surface to water, which partially washes it away and replaces it with the conductive PEDOT-rich phase. With every additional layer, the stack becomes richer in PEDOT at the bottom and leaner in insulating PSS, driving down sheet resistance without a proportional increase in thickness. The resulting bilayer measured about 100 nanometers total, barely thicker than the roughly 90-nanometer single layer, yet its conductivity was inferred to be substantially higher.

The benefits rippled far beyond simple conductivity. Scanning electron microscopy revealed that perovskite films grown on the modified bilayer contained far fewer pinholes than films grown on pristine PEDOT:PSS, which itself performed worse than bare silicon in this respect. The explanation lies in surface physics: multiple coatings increase the roughness of the polymer layer, and according to the Wenzel equation, roughness enhances wettability. Better wetting lowers the energy barrier for perovskite nucleation, creating more nucleation sites and a more complete, pinhole-free film. Pinholes matter enormously because they introduce trap states and shunt paths that degrade carrier lifetime and cause leakage, so suppressing them directly improves device quality.

Structural analysis told a consistent story. X-ray diffraction confirmed the tetragonal MAPbI3 phase in all samples, with the characteristic (110) preferred orientation, but films grown on PEDOT:PSS interlayers showed sharper peaks and larger crystallites. Using the Williamson-Hall method, the team extracted crystallite sizes of about 99 nanometers for films on both pristine and modified PEDOT:PSS, compared with only 55 nanometers for perovskite grown directly on silicon, which also carried a compressive microstrain. Larger grains mean fewer grain boundaries, less carrier scattering, and less recombination. Notably, the telltale diffraction peaks of residual lead iodide, prominent in the silicon-only sample, were strongly suppressed by the interlayer, suggesting the acidic polymer promotes complete conversion of precursors into the perovskite phase, a factor linked to better device stability.

Optical and electrical measurements sealed the case. Photoluminescence from the perovskite was strongly quenched on the interlayer samples, indicating efficient extraction of photoexcited carriers by the built-in electric field at the heterojunction, with the modified layer outperforming all alternatives. A blue shift in the emission peak further hinted at reduced trap density near the band edges. Under 530-nanometer laser illumination at a modest intensity of 0.3 milliwatts per square centimeter, all devices showed rectifying behavior from the built-in field, but the champion device with the modified interlayer delivered a photocurrent roughly 170 times greater than the interlayer-free control at a reverse bias of 5 volts. The device achieved a responsivity of 0.78 amperes per watt and a detectivity of 4.9 times ten to the eleventh Jones, figures that compare competitively with recent perovskite-based photodetectors, many of which required far more elaborate fabrication.

The authors are careful to frame the work as a proof of concept. Direct carrier-lifetime measurements were not performed, and long-term stability testing remains a key direction for future investigation, particularly because the hygroscopic nature of PEDOT:PSS is a known degradation risk for MAPbI3 devices. Even so, the bilayer design offers two plausible stability advantages: the reduced PSS content should make the polymer less hydrophilic, and the denser perovskite film should limit moisture ingress into the bulk. If those predictions hold up, the implications are significant. A photodetector that combines silicon’s maturity with perovskite’s optical prowess, assembled from solution at room temperature with a modification as simple as spinning the same material twice, points toward scalable, low-cost hybrid optoelectronics in which the trade-off between performance and processability is decisively rebalanced.

Subject of Research: Enhancement of silicon/MAPbI3 heterojunction photodetectors using a modified bilayer PEDOT:PSS interlayer

Article Title: Improving photodetection ability of Si/MAPbI 3 heterojunction by using modified PEDOT:PSS interlayer

Article References: PourMohammadi, Z., Nayeri, F. D., & Azimirad, R. (2026). Improving photodetection ability of Si/MAPbI3 heterojunction by using modified PEDOT:PSS interlayer. Results in Optics, Article 101177. https://doi.org/10.1016/j.rio.2026.101177

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101177

Keywords: photodetector, perovskite, silicon, PEDOT:PSS, MAPbI3, heterojunction, interlayer, spin coating, responsivity, detectivity, thin films, optoelectronics

The heart of the device is a p-n heterojunction formed between two very different semiconductors. Copper oxide, or CuO, is a p-type material with a band gap ranging roughly from 1.2 to 2.1 electronvolts, prized for its structural stability and good conductivity. Tungsten disulfide, WS2, is an n-type transition metal dichalcogenide with a direct band gap of about 2.1 electronvolts and exceptional optical properties that make it a favorite for sensitive photodetection. When these two are layered together, a built-in electric field forms at their interface. Under illumination, that field sweeps photogenerated electrons and holes in opposite directions, separating charge carriers before they can recombine and driving a measurable current without any applied voltage. It is the same fundamental physics that powers a solar cell, here repurposed for light sensing.

What elevates this design from a simple two-layer diode is the insertion of an approximately 20-nanometer titanium dioxide buffer layer between the fluorine-doped tin oxide substrate and the WS2 film. TiO2 is a wide-band-gap, transparent n-type semiconductor, with a band gap in the range of 3 to 3.8 electronvolts, high optical transmittance across the visible region, and strong chemical stability. In this architecture it plays several roles at once: it optimizes the electron transport path, reduces charge recombination, and physically protects the WS2 layer from direct contact with the substrate, mitigating surface mismatch and instability. In effect, the buffer layer is the quiet architect of the device’s energy-band landscape, aligning the conduction and valence bands of the three semiconductors so that electrons flow toward the TiO2 side while holes migrate toward the CuO side.

The fabrication itself is a showcase of industrial-friendly vacuum deposition. Using a three-cathode magnetron sputtering system, the team deposited the layers sequentially: TiO2 from its own target at 200 watts at room temperature, WS2 at 75 watts with the substrate heated to 200 degrees Celsius, and CuO reactively sputtered from a copper target in a mixed argon-oxygen atmosphere at 300 degrees Celsius. The chamber was pumped down to a base pressure of 8.5 times ten to the minus five torr before each run, a detail that matters, because residual water vapor in a sputtering chamber is a notorious source of hydroxyl defects that degrade film quality. A gold top electrode completed the Glass/FTO/TiO2/WS2/CuO/Au stack. The deposition rates — 0.75, 8.56, and 6.93 nanometers per minute for TiO2, WS2, and CuO respectively — were tightly controlled, and cross-sectional electron microscopy confirmed a WS2 layer of about 250 nanometers and a CuO layer of about 100 nanometers with uniform, continuous coverage.

Structural characterization backed up the claim that the films grew as intended. X-ray diffraction revealed the hexagonal phase of WS2 with reflections indexed to its (104), (009), and (116) planes, and the monoclinic phase of CuO with peaks matching its standard reference card, alongside contributions from the substrate. Crystallite sizes calculated with the Scherrer equation came out at roughly 11.6 nanometers for WS2 and 10.2 nanometers for CuO, with correspondingly modest dislocation densities, indicating relatively clean lattices with few structural defects. Raman spectroscopy sealed the case: the characteristic WS2 vibrational signatures at 408 and 318 inverse centimeters and the CuO modes near 282 and 210 inverse centimeters all appeared, with no evidence of unwanted secondary crystalline phases. Notably, no TiO2 peaks were seen in the diffraction pattern — expected, given that the buffer layer is thinner than the instrument’s effective resolution limit.

Optical and electrical measurements of the individual layers explained why the stack works as a cohesive photodetector. Tauc analysis of the absorption spectra yielded direct band gaps of 3.24 electronvolts for TiO2, 2.43 for WS2, and 1.60 for CuO — a staggered ladder of energy levels that is critical for steering charge carriers in the right directions at each interface. Hall effect measurements in a four-probe configuration confirmed the intended doping characters: TiO2 and WS2 are n-type, while CuO is p-type with a high carrier concentration of about ten to the seventeenth per cubic centimeter and moderate mobility, well suited to collecting holes. Photoluminescence under 320-nanometer excitation revealed multiple emission bands tied to excitonic transitions and defect states in the layered structure, underscoring that both surface and defect-related electronic states shape the material’s optical response.

Under monochromatic illumination from ultraviolet to red LEDs at a fixed 1-watt output, the device behaved like a genuine photovoltaic diode. The dark current-voltage curve showed a rectification ratio of about 195, and analysis with the standard diode equation gave an ideality factor of 3.02 and a reverse saturation current density of 1.52 microamperes per square centimeter. In the fourth quadrant of the illuminated curves, the team extracted an open-circuit voltage near 0.008 volts and a short-circuit current density of roughly 23 microamperes per square centimeter under ultraviolet light — the telltale fingerprint of an internal built-in electric field capable of powering photodetection at zero bias. Quantitative band-alignment estimates, built from the measured gaps and reported electron affinities, support this picture: the offsets at the TiO2/WS2 and WS2/CuO interfaces channel electrons and holes along separate, energetically downhill paths while suppressing recombination.

The transient photoresponse told an equally interesting story. Switched on and off in ten-second cycles under zero bias, the device produced reproducible photocurrent over repeated cycles, with rise and decay times of 50 and 100 milliseconds under ultraviolet illumination — fast for an unbiased, multi-layer structure. Applying reverse bias did increase the raw photocurrent, from about 8 microamperes at zero volts to 34 microamperes at 1 volt, but it came at a cost: the photoresponse ratio collapsed from over 100 to around 1.1, and the switching times stretched to 130 and 170 milliseconds as trap states and defect-assisted charge transfer slowed the current dynamics. The lesson is counterintuitive but important — for this device, self-powered operation is not just the frugal option, it is the better-performing one, delivering the highest relative sensitivity and the fastest response. Under ultraviolet light the device achieved a responsivity of 0.25 milliamperes per watt, a specific detectivity of 3.9 times ten to the seventh Jones, and an external quantum efficiency of about 0.084 percent, with these figures declining toward the red end of the spectrum as longer-wavelength photons generate carriers deeper in the film, farther from the junction.

The authors are candid that the responsivity and detectivity trail those of some recently reported heterojunction photodetectors, many of which rely on aggressive plasmonic interface engineering or heavy external biasing to reach spectacular sensitivities. They attribute the shortfall to interfacial defects, trap-assisted recombination, carrier scattering across the multilayer stack, series resistance — including the relatively high resistivity of the TiO2 buffer — and the 250-nanometer thickness of the WS2 layer, which lengthens the journey carriers must make before extraction. Yet the comparison table in the paper makes the trade-off plain: among self-powered devices, this sputtered triple stack holds its own, and it does so with a fabrication route — scalable magnetron sputtering on inexpensive FTO glass — that is far closer to mass production than exfoliated van der Waals assemblies. As sensor networks proliferate and energy harvesting becomes a design requirement rather than an afterthought, this work suggests that sometimes the smartest way to boost a photodetector is not to push harder with power, but to arrange the energy bands so skillfully that the light does all the work.

Subject of Research: A self-powered broadband TiO2/WS2/CuO heterojunction photodetector fabricated by magnetron sputtering for zero-bias UV-to-visible light detection.

Article Title: Self-powered broadband WS 2 /CuO heterojunction photodetector with a TiO 2 buffer layer

Article References: Jafari, M., & Memarian, N. (2026). Self-powered broadband WS2/CuO heterojunction photodetector with a TiO2 buffer layer. Results in Physics, Article 108758. https://doi.org/10.1016/j.rinp.2026.108758

Image Credits: AI Generated

DOI: 10.1016/j.rinp.2026.108758

Keywords: photodetector, self-powered, heterojunction, tungsten disulfide, copper oxide, titanium dioxide, magnetron sputtering, band alignment, broadband photodetection, p-n junction, thin films, optoelectronics