Zinc oxide is one of the most attractive semiconductors in nanotechnology. It combines a wide direct bandgap of about 3.37 electron volts with a high exciton binding energy of 60 milli-electron volts, excellent electron mobility, and strong transparency in the visible spectrum. It is also non-toxic, chemically stable, mechanically robust, and environmentally benign—qualities that have made it a staple candidate for ultraviolet photodetectors, transparent conductive coatings, gas sensors, photocatalysts, and solar energy converters. Yet pristine ZnO suffers from well-known shortcomings, including surface-related charge trapping, electron–hole recombination losses, and difficulty achieving uniform dopant incorporation. Conventional doping strategies rely on trivalent metals such as aluminum, gallium, and indium, which are expensive and raise environmental and health concerns.

The research team, led by Ezekiel Clinton Oroke of the Federal Polytechnic Ado-Ekiti with colleagues from Alex Ekwueme Federal University and Ebonyi State University, sidestepped metal dopants altogether. They prepared biochar by slow pyrolysis of fresh oil palm fingers collected from a plantation reserve in Ebonyi State, a material authenticated at the Ekiti State University herbarium. Half a gram of this biochar was added directly into each chemical bath during ZnO deposition, an approach that differs from the impregnation, co-precipitation, hydrothermal, and ball-milling routes that dominate the biochar–ZnO literature, which have mostly targeted adsorption, photocatalysis, and biomedical uses rather than semiconductor tailoring.

Structural analysis confirmed that the biochar does not destroy the crystal backbone of ZnO. X-ray diffraction showed the characteristic hexagonal wurtzite phase, indexed against the standard JCPDS card 36-1451, with an average crystallite size of just 22.1 nanometers. That small size highlights the biochar’s role as a growth-regulating matrix that suppresses excessive particle agglomeration during low-temperature synthesis. Refined lattice constants of 3.253 and 5.211 angstroms, a c/a ratio near 1.602, and a unit cell volume of roughly 47.8 cubic angstroms sat close to bulk ZnO values, with slight deviations pointing to lattice distortion and strain induced by interfacial interactions between ZnO nanocrystals and oxygen-containing functional groups on the carbonaceous scaffold. The team calculated an average microstrain of 3.41 × 10⁻³ and a dislocation density of 3.64 × 10⁻³ per square nanometer, alongside an estimated crystallinity of about 78.4 percent—a slight reduction attributed to amorphous carbon domains threading through the crystal lattice.

Raman spectroscopy told a complementary story about defects and interfaces. The spectra displayed well-defined ZnO vibrational modes, including the E1 (transverse optical) band at 412 wavenumbers and the defect-sensitive E1 (longitudinal optical) band at 634 wavenumbers, the latter signaling increased concentrations of oxygen vacancies and zinc interstitials created by the biochar interaction. The carbon D and G bands near 1347 and 1583 wavenumbers showed an intensity ratio of roughly one, indicating a balanced mix of ordered graphitic domains and defect or edge sites—a balance the authors describe as ideal for functional carbon–semiconductor hybrid systems, because it can enhance charge transfer, facilitate bandgap modulation, and reduce electron–hole recombination. Additional bands associated with carbonyl, aliphatic C–H, and surface hydroxyl groups revealed a high density of surface-active sites capable of inducing surface band bending and stronger light–matter interaction.

Electron microscopy reinforced the picture of a well-engineered hybrid. Scanning electron micrographs revealed a highly porous, interconnected surface morphology of densely packed quasi-spherical nanograins, with an average particle size of 59 ± 3 nanometers distributed across the biochar matrix. The raw biochar itself, by contrast, appeared as a collapsed fibrous network with a partially preserved plant framework, layered structures, micro-cavities, and an average particle size of 147 ± 27 nanometers. Those pores and defects, the researchers note, provide abundant nucleation sites for ZnO deposition and help disperse the nanoparticles, curb aggregation, and boost electron mobility. X-ray fluorescence further documented a rich elemental profile dominated by silicon, calcium, potassium, and aluminum oxides—multiple oxide species that the authors link to the improved optical conductivity of the modified films.

The optical results are where the practical promise sharpens. Across deposition temperatures from 30 to 100 degrees Celsius, the biochar-modified films showed blue-shifted absorption edges between 300 and 350 nanometers—shorter than the 365.5 nanometers at which ZnO normally absorbs—along with a general cutoff in the ultraviolet region suggesting potential as efficient UV detectors. Bandgap energies swung non-monotonically from 3.54 through 3.00, 3.10, and up to 3.70 electron volts depending on bath temperature, with the minimum at 50 degrees Celsius marking what the team interprets as optimal carbon–ZnO coupling, where π-conjugated pathways and defect states narrow the gap and extend absorption toward the visible. Films deposited at that temperature reached the highest transmittance, 65.3 percent, and the thinnest dimension, about 12.1 nanometers—close to quantum-dot scale. Refractive index values climbed from the published bulk figure of roughly 2.0 to as high as 2.64, a significant boost for optical switches, filters, and modulators.

Residence time in the deposition bath proved equally influential. As deposition stretched from 10 to 60 minutes, the modified nanoparticles showed non-monotonic bandgap evolution—3.7, 3.4, 2.51, 3.70, and 2.70 electron volts—while pristine ZnO followed a simpler, steadily narrowing path from 3.5 to 2.5 electron volts as crystallinity improved. The contrast, the authors argue, shows that biochar alters the electronic structure through interfacial interactions and defect-state modulation rather than plain crystal growth. Modified films absorbed progressively into the visible region at longer residence times and outperformed pristine ZnO in light harvesting, with the 60-minute sample delivering the strongest and broadest UV–visible absorption. Optical conductivity peaked in films prepared at 60 minutes, and the dielectric response—real part falling and imaginary part rising with photon energy—matched the inter-band transition behavior expected of semiconductors, with the real part proportionally higher than the imaginary throughout.

Not every metric moved upward, and the researchers are candid about the trade-off. Pristine ZnO, as crystallization progressed with residence time, achieved the highest optical conductivity overall, around 4.2 × 10⁶ siemens across the visible region, whereas the biochar-modified samples plateaued near or below 1.1 × 10⁶ siemens. The team attributes this suppression of carrier concentration to the biochar matrix, which promotes more controlled charge transfer through interfacial interactions and carrier trapping—an effect they argue is advantageous for curbing electron–hole recombination and improving photocatalytic performance, even at the cost of absolute conductivity. The modified films, they suggest, are strong candidates for UV and infrared photodetectors, photocatalytic devices, transparent optoelectronics requiring spectral selectivity, and solar energy conversion, where defect-rich, strained ZnO structures are known to offer enhanced charge-carrier mobility, more surface-active sites, and better adsorption behavior.

Beyond the numbers, the study carries a sustainability argument that aligns with several United Nations Sustainable Development Goals, including affordable clean energy, responsible consumption and production, and climate action. Oil palm bunch fingers are an agricultural residue with no known economic value, and valorizing them as a multi-element, eco-friendly dopant addresses two problems at once: it diverts waste from the environment while replacing toxic, costly metal dopants in semiconductor fabrication. Chemical bath deposition itself operates below 100 degrees Celsius, keeping the energy footprint low and making bath temperature a surprisingly effective, low-energy dial for band-structure engineering. The authors conclude that oil palm fingers biochar effectively tailors the optoelectronic properties of ZnO without compromising its wurtzite crystal integrity, and they frame the resulting nanoparticles as sustainable candidates for the energy-efficient devices the industry will need as demand for transparent, flexible, and low-power electronics continues to climb.

Subject of Research: Biochar-modified zinc oxide nanoparticles synthesized from oil palm agro-waste for enhanced optoelectronic properties

Article Title: Eco-friendly biochar-modified ZnO nanoparticles for enhanced optoelectronic properties

Article References: Oroke, E. C., Nwabue, F. I., Nworie, F. S., & Afuwape, O. M. (2026). Eco-friendly biochar-modified ZnO nanoparticles for enhanced optoelectronic properties. Discover Green Chemistry, 1(1), Article 34. https://doi.org/10.1007/s44509-026-00037-9

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00037-9

Keywords: biochar, zinc oxide nanoparticles, optoelectronics, Elaeis guineensis, chemical bath deposition, bandgap tuning, green chemistry, agro-waste valorization, semiconductor doping, Raman spectroscopy, X-ray diffraction, wurtzite ZnO

The importance of nickel oxide in inverted, or p-i-n, perovskite solar cells is difficult to overstate. In this architecture, the light-absorbing perovskite layer sits atop a hole-transporting layer, and every photon-generated hole must pass cleanly through that buried interface before it can be collected. Nickel oxide is prized for its chemical stability, wide bandgap, deep valence band and low cost, but depositing it uniformly over large areas has always involved a painful trade-off. Solution-based chemical bath deposition can cover large, even textured substrates conformally, but the rapid, disorderly hydrolysis of nickel salts tends to produce films riddled with pinholes and aggregates. Vacuum techniques such as sputtering or atomic layer deposition offer denser films but are slower, more expensive and harder to marry with high-throughput manufacturing.

The Nankai-led team attacked the problem at the level of crystallization kinetics rather than after the fact. Their insight was to introduce potassium tartrate, a ligand that binds nickel ions into exceptionally stable coordination complexes in the deposition bath. By sequestering a fraction of the free nickel ions, the ligand suppresses the chaotic, uncontrolled aggregation that normally plagues chemical bath growth and halves the hydrolysis rate constant, slowing it to 0.124 liters per mole per minute. That seemingly modest number is the heart of the result: slower, more orderly crystallization gives the film time to nucleate evenly across the substrate and to grow into a dense, continuous layer rather than a patchwork of islands and voids.

The kinetic control pays off in a striking way at the nanoscale. The resulting films are composed of remarkably refined crystallites measuring just 4.8 nanometers. For most materials, smaller grains would sound like a disadvantage, but here the fine nanostructure is precisely the point. Shrinking the crystallites maximizes the density of grain boundaries reaching the surface, and each boundary terminates in hydroxyl groups. These hydroxyl terminations serve as robust chemical anchoring sites for self-assembled monolayers, the carbazole-based phosphonic acid molecules that have become the workhorses of modern perovskite photovoltaics. A denser carpet of anchoring sites allows the monolayer to form a chemically cohesive, defect-suppressed buried interface, where charge extraction is fast and non-radiative recombination is minimized.

The team backed this picture with extensive interfacial characterization and charge-carrier dynamics measurements on nickel oxide monolayer stacks, supported by density functional theory calculations and molecular dynamics simulations that probed the coordination chemistry and the energetics of monolayer attachment. The calculations and experiments together paint a consistent story: the potassium tartrate-regulated surface is not merely smoother, it is chemically better matched to the self-assembled monolayer, so the molecules graft uniformly instead of clustering on reactive patches. Fewer clustered molecules means fewer shunting pathways and fewer deep traps, which translates directly into higher open-circuit voltage and fill factor.

The device numbers speak for themselves. Champion inverted cells built on the treated nickel oxide reached a power conversion efficiency of 27.40 percent, and an independent certified measurement returned a reverse-scan efficiency of 27.35 percent, placing these single-junction cells among the very best inverted devices ever reported. More importantly for commercialization, the advantage survived scale-up. A 17.7-square-centimeter mini-module achieved 23.42 percent efficiency, and a substantially larger 81.5-square-centimeter module still delivered 21.97 percent. In the perovskite field, where efficiency typically collapses as area grows because coating non-uniformities and interconnect losses compound, retaining better than 22 percent on a module the size of a postcard is a genuine milestone.

Efficiency alone, however, has never been the sole barrier to market entry. Stability under continuous illumination and heat has haunted perovskite devices since their inception, and the buried interface is one of the most common sites of degradation, where ion migration and interfacial reactions quietly erode performance. Here too the coordination-regulated films excelled. Small-area devices retained 90 percent of their initial efficiency after 2,656 hours of operation, and the 17.7-square-centimeter mini-modules held more than 90 percent of their initial performance after 1,200 hours under the demanding ISOS-L-1I protocol, an internationally recognized stress test combining continuous illumination with elevated temperature and electrical bias. Such endurance figures suggest that the defect-suppressed buried interface is not just a transient efficiency boost but a structural improvement in device physics.

What makes the result particularly compelling from a manufacturing standpoint is that the underlying technique is inherently scalable. Chemical bath deposition is a low-temperature, low-cost, solution-based process that requires no vacuum equipment and can, in principle, coat substrates of arbitrary size and shape, including textured surfaces that evaporative methods struggle to cover. By solving the coverage-versus-reactivity trade-off inside the bath chemistry itself, the researchers have essentially upgraded the method from a laboratory curiosity to a credible industrial candidate. The approach is compatible with subsequent self-assembled monolayer deposition and standard perovskite coating steps, meaning it slots into existing p-i-n fabrication flows without requiring an architectural rethink.

The work also carries a broader lesson for the field. Much of the recent progress in perovskite photovoltaics has come from interfacial engineering, and the Nankai study demonstrates that the substrate beneath a self-assembled monolayer is not a passive bystander but an active determinant of how well that monolayer performs. Controlling the crystallization kinetics of the inorganic layer, right down to the density of surface hydroxyl groups, is a form of interface design that operates one level deeper than most molecular engineering campaigns. It hints that similar coordination-regulated strategies could improve other metal oxide transport layers, from tin oxide electron contacts to zinc-based alternatives, across both single-junction and tandem devices.

Challenges remain before perovskite modules based on this chemistry can leave the laboratory for the factory floor. Scaling from 81.5 square centimeters to full-sized panels will require maintaining the same kinetic control across even larger baths and faster throughput, and long-term field testing beyond accelerated laboratory protocols will be essential. Lead content, encapsulation and recycling logistics also loom over the entire perovskite enterprise. But by demonstrating certified 27.35 percent cell efficiency, 23.42 percent module efficiency and more than a thousand hours of stable module operation from a single, industrially friendly deposition process, the team has decisively narrowed the gap between what perovskites can do in principle and what they can do in production. The sun delivers more energy to Earth in an hour than humanity uses in a year; chemistry like this brings the devices needed to harvest it one large step closer to being cheap, durable and genuinely scalable.

Subject of Research: Coordination-regulated chemical bath deposition of conformal nickel oxide hole-selective layers for efficient and stable perovskite solar modules

Article Title: Scalable and conformal nickel oxide for efficient perovskite solar modules

Article References: Yang, H., Wang, Y., Li, H., Wu, Y., Li, S., Han, X., Wang, D., Ding, Z., Han, Y., Zheng, Q., Chen, L., Du, Z., Alshahrani, T., Chen, C., Wang, X.-Y., Jiang, Y., & Yuan, M. (2026). Scalable and conformal nickel oxide for efficient perovskite solar modules. Nature Photonics. https://doi.org/10.1038/s41566-026-02012-z

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02012-z

Keywords: perovskite solar cells, nickel oxide, chemical bath deposition, self-assembled monolayers, hole transport layer, power conversion efficiency, solar modules, crystallization kinetics, buried interface, photovoltaic stability, potassium tartrate coordination, ISOS-L-1I protocol