Lead-based optoelectronics move toward monolithic integration with silicon chips

Lead-based optoelectronic materials, including lead sulfide colloidal quantum dots and lead-halide perovskites, are emerging as strong candidates for the next generation of light sensors and light-emitting diodes, and a new perspective article in Frontiers of Optoelectronics argues that their most transformative feature is compatibility with monolithic integration: fabricating the optoelectronic devices directly on top of silicon readout circuits rather than bonding separately manufactured chips together. Written by Shaoheng Xu, Jiajun Luo, Haisheng Song and Jiang Tang, the article reviews recent progress in photodetectors and active-matrix displays and highlights on-chip, in-situ fabrication as a route to high-performance optoelectronic systems.

The demand for such integration stems from the rise of optoelectronic integrated circuits, or OEICs, which combine light sources and photodetectors with silicon-based electronics to deliver high bandwidth, high functionality density and low-cost production using mature silicon technology. Complementary metal-oxide semiconductor image sensors and active displays are already integrated products of photodiodes and readout circuits: in a CMOS image sensor, a two-dimensional array of photodiodes converts incident light into charge carriers that are stored in a capacitor and read out under circuit control, while in an active display the process runs in reverse, with carriers injected into diodes that recombine to emit light under the drive of the circuits.

Conventionally, integrated optoelectronics has relied on heterogeneous techniques such as flip-chip bonding, in which III-V compound semiconductor devices, typically grown by molecular beam epitaxy or metal-organic chemical vapor deposition at temperatures above 500 degrees Celsius, are fabricated separately and connected pixel by pixel to CMOS circuits through pre-fabricated conductive bumps. That high processing temperature would otherwise damage low-k dielectrics, disconnect copper wiring and drive impurity diffusion, degrading the reliability and performance of CMOS devices. Flip-chip bonding offers design flexibility, functional expandability and high interconnect density, and has become a mainstream technology, but the authors note that significant challenges in alignment accuracy, hybridization yield and wafer bowing arise as bonding pitches shrink to sub-micron levels and wafer sizes grow to twelve inches. In micro-LED displays, the prevailing mass transfer printing technique can also introduce defects that degrade device performance.

Monolithic integration sidesteps these bottlenecks by building the optoelectronic devices directly on the silicon circuits, and lead-based materials make this feasible because they can be processed at low temperatures compatible with existing silicon technology. Lead sulfide is a narrow-bandgap semiconductor intrinsically sensitive to wavelengths up to 2.95 micrometers at room temperature, and its low Auger recombination coefficient, high absorption coefficient, long carrier lifetime and simple deposition have made it the dominant material for low-cost infrared applications. Because PbS has a large Bohr exciton radius, its bandgap and response band can be tuned simply by controlling nanocrystal size, which is attractive for full short-wavelength infrared detection. Lead-halide perovskites, ionic compounds with the general formula APbX3, combine defect tolerance with the ability to form nanometer-scale confined crystals at low temperatures, and have been exploited in sensors, displays and photovoltaics.

On the sensing side, the photodiode architecture is favored for imagers because its built-in electric field separates photogenerated carriers swiftly, yielding low noise and fast response without gain, and its vertically aligned structure couples easily to readout circuits. Fabricating a PbS CQD image sensor involves synthesizing the quantum dots, exchanging their surface ligands to prepare an ink, and depositing the film onto the readout circuit, followed by charge extraction and electrode layers. Trap states in the absorption layer and at hybrid interfaces are a critical challenge, because they capture flowing photogenerated carriers and slow the response while reducing collection efficiency. In 2022, Liu and colleagues demonstrated a monolithically integrated PbS CQD CMOS image sensor with an external quantum efficiency of 63 percent at 970 nanometers under -0.5 volt bias, a transient rising time of 0.49 microseconds and a -3 dB bandwidth of 140 kilohertz, using a NiOx/PbS CQDs/C60/ZnO photodiode stack in which a thermally evaporated C60 interfacial layer cut the interface state density by more than half to 2.3 x 10^16 per cubic centimeter. The 640 x 512 format imager achieved a resolution of 40 line pairs per millimeter, higher than that of an InxGa1-xAs imager.

Ligand and optical engineering extend these gains further. In 2024, Liu and colleagues reported a monolithically integrated PbS CQD thin-film transistor imager responsive up to 1.8 micrometers, in which a planar 1-ethyl-3-methylimidazolium cation added as a second ligand adsorbs on the (100) facets of the quantum dots, preventing nanocrystal fusion and doubling the photoluminescence lifetime; the resulting device showed a low dark current density of 14 nanoamperes per square centimeter and a peak external quantum efficiency of 62 percent at 1.65 micrometers, and could detect objects behind a silicon wafer and distinguish colorless tetrachloroethylene from water. Film thickness poses a trade-off between response speed and quantum efficiency, but an in-situ Fabry-Perot resonant cavity formed between the ITO and Au electrodes can compensate: with a 100-nanometer absorbing layer, Deng and colleagues achieved a record-fast response time of 4 nanoseconds and an external quantum efficiency of 42 percent at 1310 nanometers. Broadband imagers are also advancing, including a flexible PbS CQD sensor spanning X-ray to near-infrared that performs pixel-level multispectral image fusion, and a stacked PbS/mercury telluride CQD focal plane array with a 0.4 to 5.0 micrometer response, 6 percent photo-response non-uniformity and a noise equivalent temperature difference as low as 34 millikelvin.

Lead-halide perovskites are advancing displays in parallel. In 2024, Gao and colleagues reported microsecond-response active-matrix perovskite nanocrystal LED display panels on a 1.36 by 1.36 centimeter thin-film transistor backplane with a 16 by 16 array at 30 pixels per inch, using tetrabutylammonium tetrafluoroborate to inhibit ion migration through halide vacancies; the panel showed 5 percent deviation in brightness and efficiency uniformity, a peak external quantum efficiency of 23.3 percent at 1000 candela per square meter, and a half-life of roughly 10 hours at an initial brightness of 100 candela per square meter. In 2025, Lian and colleagues introduced a localized contact method that prevents non-radiative losses from etching, enabling pixel lengths from hundreds of micrometers down to about 90 nanometers, a record pixel density of 127,000 pixels per inch, and a TFT-powered active-matrix micro-PeLED display with 70 by 95 micrometer pixels. Vapor-phase deposition offers another commercial route: in 2024, Luo and colleagues integrated top-emitting PeLEDs directly onto a low-temperature polysilicon TFT backplane by thermally co-evaporating CsBr and PbBr2 with the Lewis base triphenylphosphine oxide, which constrained CsPbBr3 grain growth and passivated surface states, yielding a 1080 x 2400 resolution panel with 98.9 percent pixel yield over 6.88 by 11.62 centimeters and negligible brightness degradation during continuous operation.

Substantial obstacles remain before widespread commercialization. Scale-up faces barriers in large-area uniformity, batch-to-batch reproducibility and process scalability: reported solution-processed CQD imagers exhibit photo-response non-uniformity above 5 percent, with few below 2 percent, and thickness and performance variations across 8-inch or 12-inch wafers remain unreported, while spin-coated and blade-coated active-matrix PeLEDs show efficiency non-uniformity above 2 percent over several hundred square millimeters. Pixel downscaling is ultimately limited by the pixel pitch of the readout circuits, which is set by the physical limits and fabrication processes of individual transistors. Reliability is another concern: PbS CQDs can hydroxylate under high humidity, triggering nanocrystal fusion and inter-band traps, while perovskites degrade under humidity, oxygen, ultraviolet light, ion migration and heat, though passivation strategies and encapsulation schemes achieving water vapor transmission rates below 10^-4 grams per square meter per day offer partial remedies. Lead toxicity also demands encapsulation and recovery systems; cation-exchange resin encapsulants have captured over 90 percent of Pb2+ ions from degraded perovskite solar cells, and lead-free alternatives such as silver telluride quantum dots, with room-temperature detectivity near 10^12 Jones, and bismuth-based perovskite quantum dots with blue photoluminescence quantum yields up to 52 percent are emerging, though their performance still generally trails lead-based materials.

The perspective’s central conclusion is that the low-temperature, silicon-compatible processing of lead-based materials unlocks a new dimension of monolithic integration in which functional material synthesis and active layer fabrication happen synchronously on the driven circuits, with no pre-synthesized quantum dots needed, reducing process complexity while improving device performance. As spin-coated lead-based devices have already demonstrated the cost-effectiveness of monolithic integration, and PbS CQD image sensors have entered large-scale mass production as the only technology challenging III-V counterparts in the same wavelength range, the authors argue that directly growing patterned crystalline or nanocrystal films onto silicon circuits will further reduce integration complexity and open a promising pathway for the future of monolithic optoelectronic integrated circuits.

Subject of Research: Monolithic integration of lead-based optoelectronic materials with silicon circuits for photodetectors and displays

Article Title: Recent advances in monolithic-integrated lead-based optoelectronic devices

Article References: Xu, S., Luo, J., Song, H., & Tang, J. (2025). Recent advances in monolithic-integrated lead-based optoelectronic devices. Frontiers of Optoelectronics, 18(1), Article 13. https://doi.org/10.1007/s12200-025-00158-2

Image Credits: AI Generated

DOI: 10.1007/s12200-025-00158-2

Keywords: lead-halide perovskites, colloidal quantum dots, monolithic integration, optoelectronic integrated circuits, CMOS image sensors, infrared photodetectors, light-emitting diodes, thin-film transistors, vapor-phase deposition, active-matrix displays, lead toxicity, encapsulation