Interface Engineering Emerges as the Decisive Battleground for Perovskite Solar Cells

Perovskite solar cells have staged one of the most dramatic efficiency climbs in the history of photovoltaics, surging past 25 percent power conversion efficiency in barely a decade of serious research attention. A new open-access review published in Advances in Industrial and Engineering Chemistry by Qurrotun Ayuni Khoirun Nisa and Joo Hyun Kim of Pukyong National University now argues that the next leap forward will not come from the light-absorbing perovskite layer itself, but from the thin, often invisible interfaces that sandwich it. The work systematically compares conventional n–i–p and inverted p–i–n device architectures and concludes that the choice and engineering of electron and hole transport layers now dictate efficiency, stability, and manufacturability in nearly equal measure.

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