The appeal of perovskites begins with their crystal chemistry. The archetypal ABX₃ structure pairs an organic or inorganic cation such as methylammonium, formamidinium, or cesium in the A site with a divalent metal cation, usually lead, tin, or germanium, in the B site, and a halide anion such as iodide, bromide, or chloride in the X site. This arrangement delivers strong absorption across the visible spectrum, tunable bandgaps, long carrier diffusion lengths, low exciton binding energies, and a remarkable tolerance to defects. Crucially, these films can be deposited from solution at low temperature, opening the door to roll-to-roll printing and flexible substrates that rigid silicon panels cannot match.

A typical device stacks the perovskite absorber between an electron transport layer and a hole transport layer, all held between a transparent conductive oxide and a metal electrode. When light strikes the absorber, excitons dissociate almost effortlessly into free charges, which must then be swept out by the correct transport layer before they recombine. In the conventional n–i–p configuration the electron transport layer sits beneath the perovskite; in the inverted p–i–n design the hole transport layer does. Although both operate on the same photophysical principles, the review emphasizes that interfacial energetics, charge dynamics, and long-term stability diverge sharply between the two, and that interfaces are also the sites where the perovskite film nucleates and crystallizes, shaping grain size, morphology, and defect density.

One stubborn symptom of poor interfaces is current–voltage hysteresis, in which the measured efficiency depends on the direction and speed of the voltage scan. Mobile ions migrating through the lattice and charges trapped at interfaces are the chief culprits, which is why the review treats interface optimization as the central lever for reproducible performance. On the electron transport side, tin oxide has become the material of choice, processed at roughly 150 degrees Celsius and driving efficiencies from just over 17 percent in 2015 to 25.7 percent in recent years. Its rivals carry heavier baggage: zinc oxide degrades perovskites under thermal and ultraviolet stress, titanium dioxide demands sintering above 450 degrees Celsius and photocatalytically damages the absorber under UV light, and niobium pentoxide offers stability but lower conductivity and trickier processing.

Tin oxide, however, is not flawless. The review catalogs its limited carrier concentration, inherent surface defect density, and the hysteresis that follows, then details how elemental doping has rescued it. Alkali metal fluorides form coordination bonds with tin at oxygen vacancies, raising electron mobility while hydrogen bonding with amine groups suppresses organic cation diffusion. Zirconium doping delivered 19.54 percent efficiency through low-temperature solution processing, niobium doping reached 20.47 percent by cutting series resistance and balancing electron and hole flux, and magnesium doping controlled oxygen vacancy formation to lift efficiency from 6.62 to 17.25 percent. Even titanium dioxide has been revived: magnesium doping boosted efficiency by 16 percent with 91 percent retention after 30 days in ambient air, tin doping achieved 17.2 percent through improved charge collection, and rubidium chloride additives seeded dense, low-trap perovskite growth with negligible hysteresis and fill factors above 80 percent.

The frontier has since moved to molecular-scale interface modification. Water-dispersed tin oxide quantum dots spin-coated onto rough fluorine-doped tin oxide often agglomerate, leaving poor hole-blocking and depressed voltage and fill factor. Polyacrylic acid stabilization improves dispersion, but the breakthrough came when atomic layer deposition supplied a hydroxyl-rich underlayer for chemical anchoring, yielding 24.97 percent efficiency with strong stability even in larger-area devices. A thiazole-based molecule called TDA then demonstrated asymmetric dual-site passivation, with one nitrogen site mending tin-related defects and another addressing lead and iodide defects, reaching 24.96 percent efficiency and an open-circuit voltage of 1.20 volts. A π–π stacked bilayered molecular bridge built from an imidazolium salt pushed a certified efficiency to 25.27 percent, while sulfonyl diimidazole modification lifted efficiency from 21.61 to 23.31 percent by aligning energy levels and promoting uniform crystallization.

Cross-linking and gluing strategies round out the electron transport story. Bisphenol S cross-linked tin oxide films grew larger grains and better-oriented formamidinium lead iodide, delivering 24.87 percent efficiency against 23.55 percent for controls. Chitosan grafted with ethylenediaminetetraacetic acid stabilized the nanoparticle colloid and produced pinhole-free buried interfaces worth 25.12 percent. Most strikingly, a three-dimensional molecular glue formed from potassium tetrafluoroborate and trifluoromethane sulfonamide simultaneously tamed lattice mismatch, oxygen vacancies, and formamidinium cations, yielding 25.8 percent efficiency with negligible hysteresis and a certified 24.57 percent on a one-square-centimeter device, a scale that matters for commercialization.

Inverted devices tell a parallel tale on the hole transport side. The p–i–n architecture avoids high-temperature sintering, uses cheaper silver or aluminum electrodes instead of gold, and tolerates dopant-free transport layers that show less hysteresis and better stability. Yet its workhorse material, PEDOT:PSS, is acidic and hygroscopic, corroding the transparent electrode and inviting degradation, while its common dopant LiTFSI undermines longevity and the additive tert-butylpyridine can dissolve into the perovskite. Remedies include dedoping with sodium hydroxide, a self-woven polyionic complex deposition method that achieved 19.49 percent with a pinhole-free monolayer, and copper(II) counterions that raised efficiency to 19.44 percent by adjusting the work function. The hydrophobic polymer PTAA offers better energy alignment but resists coating; doping it with the π-conjugated molecule NPB reached 20.15 percent, and two-dimensional black phosphorus doping suppressed trap-assisted recombination while enhancing hydrophobicity.

The most consequential trend the review identifies is the rise of self-assembled monolayers, ultrathin molecules that chemisorb onto oxide electrodes and act as dopant-free hole selectors. Their minimal thickness cuts parasitic absorption, their energy levels are tunable, and they passivate defects while promoting high-quality perovskite growth. Dye-based monolayers such as N719 stabilized contacts at 24 percent efficiency; a thermally cross-linkable fluorinated carbazole monolayer exceeded 24 percent with superior solvent resistance; and an asymmetric design built on a fused thienoindole core with fluorine substitution achieved a certified 25.17 percent. Binary monolayer systems pairing a dipole-enhancing dibenzocarbazole molecule with the standard MeO-2PACz reached 24.52 percent with improved thermal stability, while perdeuterated carbazole lowered molecular vibrations to suppress non-radiative recombination, hitting 24.87 percent with added UV shielding. A spiro-type monolayer with a twisted, aggregation-resistant core outperformed the widely used 4PACz at 25.28 percent, and methylthio and thiophene substitutions pushed efficiencies to 25.13 percent and beyond by tuning dipoles and Lewis-basic passivation.

The authors close with a clear-eyed roadmap: scalable low-temperature electron transport deposition, real-time probing of interfacial degradation, hybrid inorganic–organic transport layers, uniform monolayer coverage over large areas, and roll-to-roll compatible SAM chemistries. The message for the field is that perovskite photovoltaics no longer hinge on discovering a better absorber but on mastering the few nanometers where absorber meets transport layer. If the molecular toolkit documented here continues its trajectory, the review suggests, the gap between laboratory champions and manufacturable modules may close faster than skeptics expect, positioning perovskites as a genuinely viable platform for next-generation solar energy.

Subject of Research: Interface engineering in conventional and inverted perovskite solar cells

Article Title: Emerging trends in interface processing: a comparative review of conventional and inverted perovskite solar cells

Article References: Nisa, Q. A. K., & Kim, J. H. (2025). Emerging trends in interface processing: a comparative review of conventional and inverted perovskite solar cells. Advances in Industrial and Engineering Chemistry, 1(1), Article 13. https://doi.org/10.1007/s44405-025-00013-0

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00013-0

Keywords: perovskite solar cells, interface engineering, electron transport layer, hole transport layer, tin oxide, self-assembled monolayers, PEDOT:PSS, PTAA, power conversion efficiency, dopant-free materials, photovoltaics, device stability

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