Preparation and Solar-Energy Applications of PbS Quantum Dots via In Situ Methods

Abstract

In situ preparation routes have become central to advancing lead sulfide (PbS) quantum dots (QDs) for solar-energy conversion, owing to their ability to create strongly coupled QD/oxide interfaces that are difficult to achieve with ex situ colloidal methods, along with their simplicity and potential for low-cost, scalable processing. This review systematically examines the fundamental mechanisms, processing levers, and device implications of the dominant in situ approaches successive ionic layer adsorption and reaction (SILAR), voltage-assisted SILAR (V-SILAR), and chemical bath deposition (CBD). These methods enable conformal QD nucleation within mesoporous scaffolds, improved electronic coupling, and scalable low-temperature fabrication, forming the materials foundation for high-performance PbS-based architectures. We further discuss how these in situ strategies translate into enhanced solar-energy applications, including quantum-dot-sensitized solar cells (QDSSCs) and photoelectrochemical (PEC) hydrogen production, highlighting recent advances in interfacial passivation, scaffold optimization, and bias-assisted growth that collectively suppress recombination and boost photocurrent utilization. Representative device metrics reported in recent studies indicate that in-situ-grown PbS quantum dots can deliver photocurrent densities on the order of ~5 mA cm⁻² at applied potentials around 1.23 V versus RHE in photoelectrochemical systems, while PbS-based quantum-dot-sensitized solar cells typically achieve power conversion efficiencies in the range of ~4–10%, depending on interface engineering and device architecture. These performances are commonly associated with conformal PbS loading within mesoporous scaffolds and quantum-dot sizes in the few-nanometer regime, underscoring the critical role of morphology and interfacial control in charge transport and recombination. Recent studies indicate that performance improvements in PbS-based solar-energy devices are primarily governed by interfacial charge-transfer kinetics and recombination suppression rather than QD loading alone, with hybrid heterostructures and inorganic passivation layers playing a key role in modifying band offsets and surface trap densities at the PbS/oxide interface. Remaining challenges are associated with defect-mediated recombination, transport limitations in densely loaded porous scaffolds, and long-term chemical stability, which must be addressed to enable scalable and durable PbS-based photovoltaic and photoelectrochemical technologies.

1. Introduction

Harnessing solar energy through materials capable of efficient light absorption and charge conversion has become a central priority in the development of carbon-neutral technologies. Semiconductor quantum dots (QDs), in particular, have attracted sustained attention owing to their size-dependent bandgap tunability, large absorption coefficients spanning the visible–near-infrared (NIR) region, and their potential for multiple exciton generation (MEG), which together enable optical and electronic functionalities highly desirable for both advanced display technologies and solar-energy conversion systems [1,2,3]. Among the diverse QD materials explored to date, lead sulfide (PbS) QDs stand out because their strongly tunable NIR bandgap and well-established colloidal chemistries allow seamless integration into device architectures requiring broadband photon harvesting and efficient charge transport [1,2,3].

From a materials perspective, semiconductor quantum dots can be broadly classified into II–VI compounds (e.g., CdS, CdSe), IV–VI chalcogenides (e.g., PbS, PbSe), metal halide perovskite quantum dots, and emerging ternary chalcogenide systems such as AgBiS₂. Among these, PbS quantum dots exhibit several distinctive advantages for solar-energy applications, including a narrow and continuously tunable bandgap extending into the near-infrared region, large absorption coefficients, and demonstrated multiple exciton generation. While perovskite quantum dots have achieved high efficiencies, their operational stability remains a challenge, and lead-free alternatives such as AgBiS₂ quantum dots, although promising from a sustainability perspective, generally show lower charge-transport performance and device efficiencies to date. Consequently, PbS QDs represent a well-balanced absorber system combining broadband light harvesting, materials robustness, and compatibility with in situ fabrication strategies for solar-energy conversion [4].

These advantages have positioned PbS QDs as promising absorbers across two representative solar-energy applications: quantum-dot-sensitized solar cells (QDSSCs) and photoelectrochemical (PEC) platforms for solar hydrogen production. In QDSSCs, PbS enables extended NIR absorption and improved interfacial charge injection, while in PEC photoanodes, PbS functions as a broadband sensitizer that enhances spectral response and charge utilization when coupled with wide-bandgap semiconductors such as BiVO₄ or TiO₂ [5,6,7,8]. Although PEC systems offer a direct route for solar-to-hydrogen conversion, and thus remain an important application of PbS QDs, the broader context of solar-energy utilization necessarily encompasses both photovoltaic and PEC pathways. Therefore, a balanced introduction of PbS QDs must first situate their optical and materials advantages before discussing specific device implementations.

A critical challenge in deploying PbS QDs in these devices lies in achieving strong electronic coupling and stable interfaces with oxide scaffolds. Two main assembly paradigms have been developed. Ex situ approaches rely on pre-synthesized colloidal QDs that are subsequently attached to oxide surfaces, often requiring ligand exchange or protective shell formation to alleviate recombination and electrolyte-induced degradation. In contrast, in situ fabrication routes, most notably successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD), nucleate PbS QDs directly on the electrode surface, forming chemically intimate and electronically well-coupled interfaces that are challenging to obtain via ex situ strategies [9]. Building upon these benefits, voltage-assisted SILAR (V-SILAR or VASILAR) accelerates ion transport and nucleation under an external bias, allowing denser and more uniform QD loading and yielding higher photocurrents in sensitized architectures [10,11,12].

In this context, this review deliberately focuses on in situ preparation strategies for PbS quantum dots, as these methods directly address the key interfacial limitations that govern device performance. By nucleating PbS QDs directly on oxide scaffolds, in situ routes eliminate long-chain insulating ligands, promote intimate electronic coupling, and enable efficient charge injection and extraction. Moreover, in situ techniques such as SILAR, voltage-assisted SILAR, and chemical bath deposition offer low-temperature, solution-processable, and scalable fabrication pathways, making them particularly attractive for practical photovoltaic and photoelectrochemical applications. Accordingly, understanding the mechanisms and processing levers of in situ PbS growth is essential for interpreting their impact on solar-energy conversion performance.

These interfacial advantages are essential for improving the performance of both QDSSCs and PEC photoanodes. In QDSSCs, recent studies have demonstrated that combining in-situ-grown PbS absorbers with optimized transport layers and advanced counter electrodes can significantly reduce recombination and series resistance, thereby enhancing current output and overall device efficiency [9,13,14]. From a device-physics standpoint, refined band alignment and recombination management continue to guide the development of high-efficiency PbS-based solar cells [15]. Similarly, in PEC systems, decorating wide-bandgap photoanodes with in-situ-grown PbS QDs broadens spectral absorption and enhances charge utilization, while additional passivation layers such as ZnS effectively suppress interfacial recombination and markedly increase photocurrent densities under AM 1.5G illumination [6,7].

Despite the well-known optical advantages of PbS quantum dots, device performance is frequently limited by interfacial recombination and inefficient charge extraction at the QD/oxide junction. In situ fabrication routes directly address these issues by eliminating long-chain ligands and promoting intimate electronic coupling during QD nucleation and growth. Understanding how different in situ methods control nucleation density, growth kinetics, and interface formation is therefore essential for interpreting their impact on photovoltaic and photoelectrochemical performance, which forms the focus of the following sections.

2. In Situ Preparation of PbS Quantum Dots

2.1. Successive Ionic Layer Adsorption and Reaction (SILAR)

SILAR is a low-temperature solution process in which a substrate is sequentially exposed to cationic and anionic precursors with rinse steps between, so that nuclei form and grow directly on the scaffold rather than in the bulk solution; this yields intimate interfacial contact and straightforward thickness/size control for PbS quantum dots. Recent studies depositing PbS by SILAR onto nanostructured scaffolds (e.g., Si nanowires, TiO₂, ZnO) confirm the method’s simplicity and robustness for device integration [16,17].

In a typical PbS SILAR cycle, adsorption of Pb²⁺ onto the oxide surface is followed by reaction with S²⁻ (often from Na₂S), forming PbS; repeating cycles increase QD loading and red-shift the effective bandgap as particles grow. Systematic experiments show that the number of SILAR cycles is a primary knob governing QD size and photovoltaic output (with an optimum before transport/recombination losses rise due to overgrowth or pore blocking) [18]. Beyond cycle count, precursor concentration, solvent, and complexing/pH conditions modulate ion adsorption/reaction kinetics and nucleation density; interfacial engineering surveys for QDSSCs detail these process–property relations and their impact on charge transfer and recombination [17].

Because PbS is chemically active in polysulfide or aqueous media, post-SILAR passivation is crucial. Depositing a thin ZnS overlayer by (p-)SILAR suppresses back electron transfer at the TiO₂/PbS/electrolyte interface and markedly improves photostability; raising the ZnS deposition temperature improves surface coverage and device durability [18]. Co-sensitization or synchronous SILAR of PbS with wider-gap sulfides (e.g., CdS) tailors band alignment and passivates PbS surface defects in situ, yielding sizable efficiency gains in solid-state QDSSCs compared with architectures where PbS is deposited first and CdS is subsequently introduced in a separate step [19].

Applying a small external bias during SILAR (V-SILAR) accelerates ionic transport into mesoporous films, promoting more uniform nucleation and denser QD loading; devices built with V-SILAR show reduced interfacial recombination (often combined with ZnS barrier layers) and higher photocurrents/PCEs compared to conventional SILAR. Although demonstrated widely with CdSe/CdS, the principle is directly applicable to PbS in the same electrode architectures [9].

Building on the promise of quantum dots for broad-spectrum light harvesting, Mao and co-workers provided the pivotal proof of concept that PbS quantum dots grown in situ by successive ionic layer adsorption and reaction on oxide scaffolds can operate as efficient sensitizers, an advance that effectively inaugurated the modern, in situ route for integrating PbS into solar-energy devices [20]. Following this lead, Sung et al. [21] consolidated a practical processing toolkit for PbS-QD-sensitized solar cells by coupling post-annealing/regeneration of SILAR-PbS with bilayer TiO₂ light-scattering photoanodes and Au/CuS/FTO counter electrodes, achieving record liquid-electrolyte device metrics for the period and establishing clear links between interfacial engineering, optical path management, and current extraction [21].

In parallel, Lee and co-workers demonstrated that tuning the electrode surface chemistry during SILAR via surface-charge-controlled deposition (triethanola-mine-assisted) dramatically increases QD nucleation density on TiO₂ and drives a six-fold photocurrent rise, thereby underscoring how controlled nucleation and coverage translate the intrinsically high absorption of PbS into device-level gains [22].

Extending the same interfacial logic beyond photovoltaics, Seo and co-workers decorated BiVO₄ photoanodes with SILAR-PbS to broaden visible/NIR response and then applied ZnS capping to suppress surface recombination, yielding substantial photocurrent enhancements for PEC H₂ generation evidence that the passivation and growth principles refined in QDSSCs generalize to PEC architectures [7].

This progression from early in situ demonstrations to process-level refinements indicates that device durability and performance are primarily governed by how PbS quantum dots are distributed and passivated at the oxide interface. In practice, high-density and conformal in situ growth must be coupled with targeted passivation layers (such as ZnS or CdS) to suppress interfacial recombination and preserve charge extraction, thereby translating the intrinsic optical advantages of PbS QDs into stable, high-performance solar-energy devices.

Figure 1 provides direct insight into how the SILAR cycle number governs PbS quantum-dot nucleation, growth, and scaffold coverage. As demonstrated in Figure 1a, the SILAR process enables sequential adsorption and reaction of Pb²⁺ and S²⁻ ions directly on the oxide scaffold, providing a straightforward route to control PbS nucleation through cycle number. The morphological evolution of ZnO nanowires before and after PbS deposition (Figure 1b–e) confirms that increasing SILAR cycles leads to progressively denser and more conformal PbS coverage along the nanowire surface. Optical evidence of this growth behavior is reflected in the photoluminescence spectra (Figure 1f), where the systematic red-shift with increasing cycle number indicates an increase in effective PbS domain size and reduced quantum confinement. Consistent with this trend, the XRF spectra (Figure 1g) show a monotonic increase in Pb signal intensity, confirming higher PbS loading, while the XRD patterns (Figure 1h) suggest changes in crystallinity associated with particle growth at higher cycle numbers. Collectively, these results illustrate that SILAR cycle number serves as a primary control parameter governing PbS loading, size evolution, and interfacial coverage, while also implying a practical trade-off between enhanced optical absorption and the risk of overgrowth-induced recombination in sensitized architectures [18].

Beyond precursor chemistry, the morphological outcomes of SILAR growth, QD size evolution, coverage uniformity, and pore accessibility directly govern charge injection and recombination at the PbS/oxide interface. Conformal infiltration within mesoporous scaffolds enhances electronic coupling and reduces interfacial trap density, whereas excessive growth can block pores, hinder electrolyte/ion transport, and increase recombination losses. This trade-off explains why an optimum cycle number is typically observed, and why morphology control is as critical as QD loading for achieving high device efficiency and stability.

2.2. Voltage-Assisted SILAR (V-SILAR)

In V-SILAR, a modest external bias is applied during one or both SILAR half-steps so the interfacial electric field accelerates migration of cations/anions through mesoporous scaffolds, strengthens electrostatic adsorption of the first monolayers, and stabilizes nascent nuclei. The outcome is a higher nucleation density, deeper pore infiltration, and more uniform quantum-dot (QD) coverage in fewer cycles, effects that mirror the earlier surface-charge-controlled SILAR concept, where tuning the electrode’s surface charge densified PbS-QD nucleation and markedly boosted photocurrent [10,11,12].

Using a ~2 V field during CdSe deposition into ZnS-passivated TiO₂, Jin et al. [10] formed ZnS/CdSe/ZnS stacks that achieved power conversion efficiency (PCE) ≈ 4.34%, outperforming passive SILAR under comparable conditions due to field accelerated infiltration and reduced interfacial recombination [10]. In a companion study, the same group reported room-temperature CdSe V-SILAR assembly with PCE ≈ 3.27%, again exceeding the conventional route at similar thickness/loadings [10]. Extending the idea to co-sensitization, Jin et al. [12] used VASILAR (CdS/CdSe) to overcome pore blocking by the seed layer and obtained more uniform deep loading with improved device figures versus passive co-SILAR [12].

A closely related electrostatic cue was shown earlier by Altieri et al. [23], where precursor solutions are continuously delivered onto a rotating substrate using a syringe pump rather than traditional dip–rinse cycles. This controlled hydrodynamic flow acts as a macroscopic bias, preventing concentration polarization and ensuring uniform precursor replenishment at the substrate surface. As a result, the “AutoDrop” method yields BiOI/TiO₂ heterostructures with highly homogeneous thicknesses (≈180–450 nm), tunable optical bandgaps (2.2–2.0 eV), and significantly enhanced photoelectrochemical performance compared with conventional SILAR. The evolution of film thickness and optical bandgap with controlled precursor delivery (Figure 2a–c) indicates that a hydrodynamic driving force can effectively replicate the role of external bias by enabling deeper precursor penetration and more uniform nucleation throughout the mesoporous architecture [23].

Further insight into bias-assisted nucleation is provided by Jemai et al. [24], who investigated PbS nanoparticle deposition on TiO₂ nanotube arrays. In this notation, {PbS NPs}_n/TiO₂ NTs denotes TiO₂ nanotube arrays decorated with PbS nanoparticles deposited through n successive SILAR cycles (n = 3, 5, 8), where each cycle consists of sequential Pb²⁺ and S²⁻ adsorption–reaction steps that incrementally increase the PbS loading. Although no external electric field was applied, the nanotube geometry itself introduces strong local electric field gradients and confinement effects that guide cation adsorption and chalcogenide reaction within the tubes. FESEM and TEM analyses reveal progressive filling of the nanotube interior with increasing SILAR cycles, demonstrating that geometric confinement can serve as a nanoscale bias favoring deep infiltration rather than surface accumulation. Optical measurements confirmed incremental PbS loading with increasing SILAR cycles; however, excessive deposition resulted in partial pore blockage and diminished performance. This transition from depth-resolved infiltration to surface-dominated accumulation is consistent with the progressive filling behavior observed along the nanotube interior (Figure 2d–f), underscoring the importance of balancing ion transport and nucleation density [24].

2.3. Chemical Bath Deposition (CBD)

Chemical bath deposition remains a widely adopted in situ route for forming uniform Pb-based nanocrystalline films directly on device substrates. In a typical CBD process, an aqueous alkaline solution containing a soluble lead precursor (e.g., Pb(CH₃COO)₂·3H₂O) and an oxidizing or sulfur-bearing source (such as H₂O₂ or thiourea) is prepared. Within this bath, Pb²⁺ ions form hydroxide complexes (e.g., Pb(OH)₄²⁻) while the anionic species is gradually released, enabling heterogeneous nucleation and subsequent growth on the immersed substrate surface rather than in the bulk solution. The schematic in Figure 3a of Erdem et al. [25] clearly depicts this sequence: adsorption of cations on the substrate, interaction with O²⁻ (or S²⁻) ions in the bath, and continuous film growth over a controlled deposition period [25].

Operating typically near ambient temperature (25–40 °C) for several hours, CBD achieves conformal, low-stress coatings using only simple glassware and without vacuum systems. Process levers such as bath pH, complexing agent concentration (e.g., triethylamine or Na₂S₂O₃), and deposition time regulate supersaturation and film morphology, thus tailoring crystallite size, coverage, and optical band gap. As demonstrated by Erdem et al. [25], CBD-grown PbO films exhibit tightly bound, porous morphologies composed of rod- or plate-like grains and an average crystallite size of ≈ 43 nm, larger than that obtained by the SILAR process, which produced finer and denser microspherical grains. The schematic and experimental comparison highlight how CBD’s slow, solution-mediated growth favors polycrystalline Pb-oxide (or PbS) frameworks with excellent substrate adhesion and scalable, low-cost processing [25].

Recent device-focused studies highlight how CBD-PbS is migrating from “materials demonstrations” to robust optoelectronics and solar-relevant photoelectrochemistry. Zayed et al. [26] constructed a p-PbS/p-CuO bilayer by combining CBD (PbS) with SILAR (CuO); under simulated sunlight the bilayer delivered a photocurrent density of ~–0.390 mA cm⁻², far above either single layer, evidencing improved charge separation and visible-light harvesting, an encouraging sign for PEC H₂ architectures that want narrow-gap sensitizers made in situ. The corresponding J–V characteristics under dark/illumination and the PEC H₂ evolution performance (Figure 3b–d) indicate improved charge separation and visible-light utilization relative to the single-layer counterparts [26]. Parallel materials-centric CBD studies, such as Gosavi et al. [27], map deposition-time effects (20–50 min) on structure, band gap (~1.48 eV), and even gas sensitivity (~80% to NH₃), providing tunability guidelines directly transferable to in situ PbS layers on solar/PEC electrodes, and report flower-like PbS nanostructures with distinct secondary features (Figure 3e,f), providing morphology–process guidelines that are transferable to in situ PbS layers on solar/PEC electrodes [27].

Architectures that marry CBD-PbS with nanosilicon and 2D conductors further illustrate the route’s device agility. Aggarwal et al. [28] fabricated a self-powered PbS/Si nanowire heterojunction by CBD (with trisodium citrate complexant), achieving ~0.21 A·W⁻¹ responsivity and ~6 × 10⁹ Jones detectivity at 1064 nm under 0 V bias, useful for on-chip near-IR sensing and as a template for solar-integrated photodiodes [28]. Pushing broadband response, Mu et al. [29] formed a graphene–PbS heterostructure where the PbS layer was grown by CBD at ~30 °C, then briefly annealed; the device reports 72 A·W⁻¹ at 792 nm and 5.8 A·W⁻¹ at 1550 nm with sub-20 ms response (265–2200 nm window), underscoring that low-temperature baths can deliver detector-grade films compatible with delicate conductors [29].

Process innovation is also accelerating. Zhang et al. [30] report a sensitization-free one-step CBD-PbS where an oxidant modulates intergrain chemistry during deposition; a monolayer PbS photoconductor reached D* ≈ 1.55 × 10¹¹ Jones at room temperature on par with commercial uncooled NIR detectors eliminating iodine/oxygen or high-T sensitization bottlenecks [30]. In photodetection, Lv et al. [31] showed that oxygen sensitization of CBD-PbS on quartz (400–700 °C) can raise responsivity to ~1.67 A·W⁻¹ and detectivity to ~1.22 × 10¹⁰ Jones at 650 °C, while proposing a 3D network model for the sensitization mechanism knowledge that feeds back into how we anneal in situ films on functional substrates [31]. For heterojunction formation via serial chemical routes, Encinas-Terán et al. [32] established a CBD PbS/CdS diode-like bilayer, demonstrating scalable sulfide–sulfide stacks that can be integrated above oxide scaffolds in QDSSCs or photoelectrodes [32].

These results indicate that CBD enables scalable formation of PbS-based layers through bath-controlled heterogeneous nucleation, although device performance remains sensitive to grain-boundary defects and surface recombination unless appropriate passivation strategies are employed. To provide a broader literature perspective,

Table 1. Representative studies on in situ chemical bath deposition of PbS thin films for optoelectronic and solar-energy-related applications.

Substrate	Bath and Key Conditions	Post-Treatment	Application	Performance Highlight	Ref.
Glass; PbS (CBD)	Pb(NO3)2/thiourea; inhibitor Na2S2O3 3.2–12.7 mM; 25–40 °C; 30–120 min	—	Process/structure	Inhibitor and mild heating tune grain size/coverage; alkaline mechanism clarified.	[25]
Glass; p-PbS (CBD)/p-CuO (SILAR)	Alkaline CBD; RT; ~1 h	—	PEC H2	Jph ≈ –0.390 mA·cm−2 (bilayer); band gap ~1.28 eV.	[26]
Glass; PbS (CBD)	20–50 min deposition series	—	Materials tuning/sensing	E9 ~ 1.48 eV; NH3 optical sensitivity up to ~80%.	[27]
Si-nanowire chip; PbS (CBD)	Trisodium-citrate complexant	—	Self-powered detector	R ≈ 0.21 A·W−1, D* ≈ 6 × 109 Jones @1064 nm, 0 V.	[28]
Graphene; PbS (CBD)	~30 °C bath; ~500 nm film	400 °C N2/O2 anneal	Broadband detector	R ≈ 72 A·W−1 (792 nm), 5.8 A·W−1 (1550 nm); <20 ms; 265–2200 nm.	[29]
Planar; PbS (CBD, monolayer)	Sensitization-free CBD with oxidant	None (no I/O2/thermal)	Uncooled NIR detector	D* ≈ 1.55 × 1011 Jones (RT); commercial-grade.	[30]
Quartz; PbS (CBD)	Standard CBD	O2 sensitization 400–700 °C	NIR photodetector	R ≈ 1.67 A·W−1; D* ≈ 1.22 × 1010 Jones (650 °C).	[31]
PbS/CdS bilayer (CBD)	Two CBD baths (CdS, PbS)	—	Junction/device physics	Diodic I–V; CBD scalability for sulfide bilayers.	[32]

summarizes representative reports on in situ chemical bath deposition of PbS thin films across different substrates, post-treatments, and optoelectronic applications.

Table 1 provides an overview of representative studies employing chemical bath deposition for the in situ growth of PbS thin films across different substrates and device configurations. However, PbS quantum dots for solar-energy applications are prepared using several distinct in situ strategies, each characterized by different growth mechanisms, degrees of interfacial control, and scalability. In particular, SILAR, V-SILAR, and CBD differ in terms of nucleation behavior, quantum-dot loading, and interface formation with oxide scaffolds, which in turn influence device performance. A comparative summary of these in situ preparation methods is therefore presented in

Table 2. Comparative summary of major in situ preparation strategies for PbS quantum dots used in solar-energy applications.

Method	Growth Principle	Interfacial Coupling	Control Over QD Size/Loading	Typical Device Impact	Scalability and Processing	Ref.
SILAR	Sequential adsorption of Pb2+ and S2− ions with rinsing steps	Strong chemical bonding at the QD/oxide interface	High (cycle-number-dependent)	Enhanced photocurrent and moderate PCE improvement with optimized passivation	Low-temperature, solution-based; compatible with large-area substrates	[18,19,20,21,22]
V-SILAR	SILAR assisted by an external electric field to enhance ion transport	Very strong and uniform interfacial coupling	Very high; improved nucleation density and depth infiltration	Higher photocurrent and PCE compared with conventional SILAR	Scalable; particularly suitable for thick or structured electrodes	[10,11,12]
CBD	Heterogeneous nucleation and growth from a supersaturated chemical bath	Moderate to strong, depending on surface chemistry	Moderate; governed by bath composition and deposition time	Stable photocurrent generation; moderate efficiencies without advanced passivation	Excellent scalability; industrially compatible bath processing	[25,26,27,28,29,30,31,32]

.

In situ preparation routes inherently link PbS quantum-dot formation with interface development at the oxide surface. In contrast to ex situ approaches, where quantum dots are synthesized and assembled in separate steps, in situ growth allows nucleation and interfacial bonding to proceed simultaneously. As a consequence, growth parameters such as precursor chemistry, deposition time, and applied electric fields directly affect quantum-dot density, size distribution, and the population of interfacial defect states. These interfacial characteristics strongly influence charge-injection efficiency and recombination pathways, providing a direct connection between growth conditions and the device performance trends discussed in the following sections.

3. Applications in Solar-Energy Conversion

Building on the in situ growth strategies established in Section 2 (SILAR, V-SILAR, and CBD), dense and conformal PbS quantum-dot loading within mesoporous scaffolds, together with interfacial passivation (e.g., ultrathin ZnS or CdS), plays a central role in suppressing recombination, sustaining J_SC and V_OC, and extending the incident photon-to-current efficiency (IPCE) into the near-infrared region. Across photovoltaic and photoelectrochemical platforms, device performance is closely linked to the morphological and structural characteristics defined during in situ growth, including quantum-dot size distribution, coverage uniformity, and depth of scaffold infiltration. Optimized morphologies promote intimate electronic coupling at the PbS/oxide interface, enabling efficient charge injection while mitigating trap-assisted recombination, whereas non-uniform coverage or excessive loading can introduce transport bottlenecks and recombination-active defects that limit photocurrent and photovoltage.

In photoelectrochemical photoanodes, PbS-sensitized oxides (e.g., TiO₂ and BiVO₄) benefit from broadened spectral response, while the combination of passivation and bias-assisted deposition lowers interfacial resistance and enhances photocurrent and operational stability under AM 1.5G illumination. In this section, we examine how PbS quantum dots translate their intrinsic materials advantages, near-infrared bandgap tunability, large absorption coefficients, and intimate semiconductor/oxide contact, into device-level gains across two technologically relevant platforms: (i) quantum-dot-sensitized solar cells and (ii) photoelectrochemical hydrogen production. Throughout, representative studies are compared against ex situ baselines and organized by key process levers (cycle number, bath chemistry/pH, bias assistance, and post-growth capping), enabling device trends to be interpreted through a common mechanistic framework.

3.1. Quantum-Dot-Sensitized Solar Cells (QDSSCs)

In PbS quantum-dot-sensitized solar cells (PbS-QDSSCs), size-tunable PbS absorbers (Eg ≈ 0.7–1.6 eV) enable efficient harvesting of visible-to-near-infrared photons. Upon illumination, photoexcited electrons are injected from PbS into the oxide scaffold, typically TiO₂ or ZnO, while photogenerated holes are extracted by the redox electrolyte, most commonly a polysulfide couple. Despite these advantages, device performance is frequently constrained by interfacial loss pathways, including trap-assisted recombination at the QD/oxide and QD/electrolyte interfaces, back electron transfer to Sn²⁻/S²⁻ species, and catalytic or ohmic losses at the counter electrode. Consequently, recent efficiency improvements have been driven primarily by three interconnected strategies: effective surface passivation of in-situ-grown PbS quantum dots, transport-layer and interface engineering to enhance charge injection and suppress recombination, and the development of highly active counter electrodes and electrolytes to reduce overpotentials and parasitic losses.

To elucidate these interfacial loss mechanisms and their impact on device performance, electrochemical impedance spectroscopy (EIS) is widely employed in quantum-dot-sensitized solar cells to probe interfacial charge-transfer kinetics and recombination processes that are not directly accessible from steady-state current–voltage measurements. By analyzing the frequency-dependent impedance response, EIS provides insight into key physical parameters such as series resistance (R_s), charge-transfer or recombination resistance (R_ct), and interfacial capacitance, which are closely related to carrier lifetime and recombination dynamics. In PbS-based devices, a reduced R_ct is commonly associated with suppressed interfacial recombination and improved charge extraction, while changes in capacitive elements reflect modifications in interface quality and trap-state density induced by passivation or heterostructure engineering.

Most studies interpret EIS data using equivalent-circuit models consisting of a series resistance in combination with one or more parallel resistor–capacitor (or constant phase element, CPE) units, representing charge transport through the photoanode and recombination at the semiconductor/electrolyte or semiconductor/transport-layer interface. Such models provide a quantitative framework for correlating interfacial properties with photovoltaic performance metrics, thereby complementing J–V and EQE analyses. To obtain a consistent picture of device performance, the EQE spectra should be interpreted together with the corresponding J–V characteristics. In PbS-QDSSCs, an extended EQE/IPCE response into the near-infrared region directly translates into additional photocurrent contribution, which is reflected by an increased Jsc in the J–V curves. At the same time, improvements in EQE magnitude across the visible range typically indicate more efficient charge injection and collection, consistent with reduced interfacial recombination enabled by passivation and interface engineering. Where reported, the J_SC values estimated by integrating the EQE spectrum under the AM 1.5G photon flux are generally in reasonable agreement with the Jsc measured from J–V curves, supporting the internal consistency of the performance analysis [14,33,34].

Seo et al. [33] addressed trap-assisted recombination in in situ PbS sensitizers by introducing an ultrathin metal halide perovskite (MAP) shell that preserves charge injection while passivating surface defects and systematically tuned shell thickness; they report that an ≈0.34 nm shell maximizes short-circuit current density (J_SC), open-circuit voltage (V_OC), and fill factor (FF) by suppressing surface recombination and improving carrier transfer, raising the QDSSC power conversion efficiency to ~4.1%, versus ~0.7% for bare PbS. The structural configuration of the PbS/MAP shell system, together with the corresponding EQE response and thickness-dependent J–V characteristics, directly supports this trend by linking shell-induced passivation to enhanced spectral response and optimized charge extraction (Figure 4a–c) [33]. These trends are consistent with their PL/XPS and device analyses and with the mechanistic picture that a soft, wide-gap shell can passivate Pb/S dangling bonds without impeding electron injection into TiO₂.

Recognizing that interfacial trap states at the oxide contact limit both injection and stability, Xiao et al. [34] passivated SnO₂ electron transport layers using acetonitrile treatment and assembled ITO/SnO₂/n-PbS/p-PbS/Au devices; their best cells improved from PCE = 8.53% to 10.49% with enhanced shelf stability, which they attribute to lower –OH-related trap density and reduced interfacial recombination resistance principles directly transferable to PbS-sensitized architectures where SnO₂/TiO₂ underlayers are used. Differences in charge-transfer resistance arising from ACN- and MeOH-washed devices are consistent with the separation of impedance arcs under illumination, indicating solvent-dependent modulation of interfacial recombination pathways (Figure 4d) [34].

Because high overpotential and sluggish charge transfer at the counter electrode can limit overall device efficiency, Dang et al. [14] designed a p-type PbS@rGO composite counter electrode; in full QDSSCs the optimized mass ratio delivered PCE = 5.358% with J_SC = 21.157 mA cm⁻², V_OC = 0.540 V, and FF = 0.516, supported by EIS evidence of improved catalytic kinetics for the Sn²⁻/S²⁻ couple and a percolating rGO network that lowers series resistance. This interpretation is reinforced by the coupled J–V behavior and impedance fitting results, which correlate changes in photovoltaic output with variations in transport and recombination elements extracted from the equivalent-circuit model (Figure 4e,f) [14].

From a broader perspective, the power conversion efficiency of PbS quantum-dot solar cells has shown a clear and steady evolution over the past decade, driven primarily by interfacial and device-level engineering rather than absorber replacement. Early PbS QD-sensitized solar cells typically exhibited PCEs below 1%, largely limited by severe interfacial recombination and inefficient charge extraction. Subsequent advances in in situ growth control, surface passivation (e.g., ZnS or metal halide shells), and transport-layer optimization have progressively increased PCE values into the 4–6% range. More recently, the introduction of cascade heterostructures, refined interface energetics, and improved electrode architectures has enabled PbS-based QD solar cells to approach or exceed PCEs of ~10%. This evolution highlights that continued efficiency improvements in PbS QD solar cells are governed by interfacial charge-transfer kinetics and recombination suppression, rather than light absorption alone.

Comparison across these studies shows that thin wide-bandgap shells primarily function by suppressing trap-assisted recombination at the PbS/oxide and PbS/electrolyte interfaces while maintaining efficient electron injection into the transport layer [14,33,34].

Table 3. Summary of representative photovoltaic performance parameters (Voc, Jsc, FF, and PCE) of PbS quantum-dot-sensitized solar cells employing in situ growth and interfacial modification strategies.

Interface	In Situ Modifications	Key Result(s)	Ref.
TiO2	PbS@rGO CE	PCE = 5.358%, JSC = 21.157 mA cm−2, VOC = 0.540 V, FF = 0.516; Rct ↓.	[14]
TiO2	PbS (in-situ) → ultrathin MAP shell	PCE ~4.1% vs. ~0.7% bare; optimal shell ≈0.34 nm; trap suppression validated by PL/XPS.	[33]
TiO2 EHMS	CdS/CdSe/PbS co-sensitization (SILAR)	Broader IPCE and higher JSC than CdS/CdSe alone.	[35]
TiO2 NR/PbS	PbS (SILAR)/TiO2	PCE = 5.47%; JSC = 13.71 mA cm−2; VOC = 0.62 V; FF = 0.643	[36]
TiO2/PbS–SnS	PbS/SnS (SILAR)	PCE = 9.95%; JSC = 19 mA cm−2; VOC = 0.77 V; FF = 0.68	[37]

summarizes selected examples of PbS QD-sensitized solar cells to illustrate how different in situ interfacial modification strategies translate into device-level performance.

As summarized in Table 3, representative PbS quantum-dot-sensitized solar cells exhibit clear performance trends across different in situ growth and interfacial engineering strategies. Improvements in “V_OC” are consistently associated with effective surface passivation and reduced interfacial recombination, while enhanced “J_SC” primarily originates from optimized quantum-dot loading and improved charge-injection efficiency. Higher “FF” and overall “PCE” are typically observed in device architectures that combine favorable band alignment with reduced series resistance. These trends highlight that efficiency gains in PbS-based solar cells are governed predominantly by interfacial quality and charge-transport management rather than absorber loading alone.

3.2. Photoelectrochemical Hydrogen Production

Photoelectrochemical conversion directly transforms sunlight into chemical fuels by driving oxidation–reduction reactions at illuminated semiconductor–electrolyte interfaces. In a water-splitting cell, the photoanode harvests light and catalyzes the oxygen evolution reaction (OER), but benchmark oxides such as TiO₂ and BiVO₄ suffer from wide bandgaps that limit visible/NIR absorption, leaving much of the solar spectrum unused. PbS quantum dots address this gap: quantum confinement enables bandgap tuning to the 0.7–1.6 eV window with large absorption coefficients, so PbS acts as a broadband “antenna” when integrated on wide-gap scaffolds (e.g., TiO₂, BiVO₄) and injects carriers into the host for transport and collection. Beyond spectral harvesting, PbS QDs can exhibit MEG, allowing >100% external quantum efficiency for H₂ evolution when high-energy photons create more than one e⁻/h⁺ pair, a capability demonstrated explicitly in PEC PbS systems and widely studied in lead–chalcogenide QD photovoltaics [38].

Depositing QDs in situ (e.g., SILAR and V-SILAR) nucleates PbS directly inside meso/porous oxide networks, eliminating long insulating ligands and creating atomically intimate heterojunctions that accelerate electron injection. Cycle number, ion concentration, and pore transport jointly control QD size/loading. Proper nucleation control is essential, as also demonstrated for CuInS₂ QDs [39]: more cycles red-shift absorption and raise J_ph until pore blocking and mass transport limitations emerge. For BiVO₄, Seo et al. [7] showed that a modest number of SILAR cycles plus a thin ZnS cap reduce interfacial recombination and lift J_ph from 2.92 to 5.19 mA cm⁻² at 1.23 V_RHE. Figure 5a,b provide the Nyquist impedance basis for this comparison, linking the reduced semicircle response to lower interfacial charge-transfer resistance after in situ PbS growth and overlayer/passivation treatment [7,9].

On mesoporous TiO₂, Kim et al. [40] mapped a clear “thickness window”: 11.9 µm films host more PbS without choking electrolyte transport, giving 15.19 mA cm⁻² at 0.60 V_RHE and the lowest charge-transfer resistance among the series. Figure 5c,d summarize the corresponding J–V and ABPE responses under the same set of conditions, connecting the photocurrent improvement to the device-level efficiency trend across the investigated PbS growth/thickness window [40]. Interfacial passivation layers can be co-engineered with in situ QDs: a TiO₂(B) surface passivation layer (SPL) placed under PbS/CdS-sensitized TiO₂ suppresses back recombination, improves surface energetics [41], and enables 14.43 mA cm⁻² at only 0.82 V_RHE, highlighting that interface quality can be as decisive as light harvesting [42].

Beyond photocurrent density, a more practical evaluation of photoelectrochemical performance requires consideration of efficiency metrics and hydrogen production rates. In PbS-sensitized photoanodes, reported applied-bias photon-to-current efficiencies typically reach the percent level, accompanied by hydrogen evolution rates ranging from tens to several hundreds of µmol h⁻¹ cm⁻², depending on the photoanode configuration and interfacial design. These values are consistent with the enhanced photocurrents observed after PbS sensitization and reflect how improvements in charge separation and recombination suppression translate into effective solar-to-hydrogen conversion.

The highest PEC performances are generally achieved when controlled in situ nucleation is coupled with rational electrode and interface design. In particular, optimized SILAR or S-SILAR growth enables uniform PbS deposition, which becomes most effective when integrated with transport-optimized scaffolds such as appropriately tuned thickness, porosity, or nanotubular architectures and complemented by interfacial engineering strategies including ZnS capping layers, surface passivation layers, or dopant incorporation. For instance, BiVO₄/PbS/ZnS photoanodes exhibit a pronounced increase in photocurrent density from 2.92 to 5.19 mA cm⁻² at 1.23 V_RHE as a result of suppressed surface recombination, while TiO₂/PbS photoanodes optimized at a thickness of ~11.9 µm deliver photocurrents up to 15.19 mA cm⁻² at 0.60 V_RHE, coinciding with the lowest reported charge-transfer resistance. Similarly, S-SILAR deposition on TiO₂ nanotube arrays enhances pore infiltration and lifts the photocurrent to ~4.14 mA cm⁻², underscoring the critical role of scaffold morphology in governing PEC performance [7,41]. To facilitate a direct comparison of photoelectrochemical performance,

Table 4. Representative photoelectrochemical hydrogen-evolution performance of in situ PbS-based photoanodes prepared by SILAR and related methods.

Photoanode	In Situ or Deposition	Photocurrent Density (Jph)	Applied Potential (VRHE)	Key Enhancement Strategy	Ref.
BiVO4/PbS/ZnS	SILAR + ZnS capping	5.19 mA cm−2	1.23 V	In situ PbS; ZnS surface passivation lowers Rct	[7]
TiO2/PbS	SILAR	15.19 mA cm−2	0.60 V	Optimize mesoporous TiO2 thickness (11.9 µm) to balance loading vs. transport; ZnS interlayer	[40]
TiO2/PbS-CdS with TiO2(B) SPL	SILAR on TiO2 + SPL	14.43 mA cm−2	0.82 V	TiO2(B) surface passivation layer suppresses recombination	[42]
WO3/BiVO4/Ni-PbS QDs	Deposited QDs (solution)	5.56 mA cm−2	1.23 V	Ni-doped PbS QDs on tandem WO3/BiVO4; synergistic catalysis	[43]
α-Fe2O3 (hematite)/PbS	SILAR	1.04 mA cm−2	1.79 V	PbS sensitization boosts α-Fe2O3 absorption/IPCE	[44]
ZnO/ZnFe2O4/PbS	SILAR (PbS on ternary)	0.577 mA cm−2	—	Ternary heterojunction (ZnO/ZnFe2O4) with PbS sensitizer	[45]

summarizes representative photocurrent densities, applied potentials, and key interfacial enhancement strategies reported for in situ PbS-based photoanodes prepared by SILAR and related methods.

In addition to efficiency, the operational stability of PbS quantum dots and related devices remains a critical consideration for practical solar-energy applications. PbS QDs are prone to surface oxidation, electrolyte-induced corrosion, and trap-state formation under prolonged illumination or applied bias, which can lead to performance degradation over time. Recent studies have demonstrated that interfacial passivation strategies, such as inorganic capping layers (e.g., ZnS), metal halide shells, and surface chemical treatments, can significantly enhance device stability by suppressing interfacial recombination and chemical degradation. In photoelectrochemical systems, improved stability is commonly evidenced by sustained photocurrent during chronoamperometric operation, highlighting the importance of interface protection in maintaining long-term performance. These results indicate that stability in PbS-based devices is closely linked to interface engineering rather than the intrinsic absorber properties alone.

Across the device configurations discussed above, the performance impact of PbS quantum dots prepared by in situ methods is consistently governed by interfacial processes rather than light absorption alone. In photovoltaic architectures, efficient carrier extraction relies on favorable band alignment and suppressed recombination at the PbS/oxide junction, both of which are strongly influenced by the growth pathway and surface chemistry of the quantum dots. In photoelectrochemical systems, these same interfacial factors additionally affect charge-transfer kinetics at the electrode–electrolyte interface and the operational stability under applied bias. The recurring role of interface-dominated phenomena across different application platforms underscores the importance of mechanistic control during in situ PbS growth when translating materials optimization into device-level functionality.

4. Recent Advances and Strategies

4.1. Hybrid and Heterostructure Engineering

Performance limitations in PbS-based devices frequently originate from inefficient charge separation and rapid recombination at semiconductor interfaces, issues that are also widely encountered in photocatalyst and nanomaterial systems [46]. Coupling PbS quantum dots with secondary semiconductors provides a means to redistribute band offsets and establish directional charge-transfer pathways, thereby mitigating recombination losses. Recent PbS-based heterostructures indicate that appropriate interfacial band alignment and coupling, rather than absorber thickness alone, govern carrier lifetime, charge extraction efficiency, and device stability. Through rational band offset tuning and lattice matching, these architectures enable directional charge transfer, enhanced carrier lifetimes, and improved device stability. Building upon the field-assisted growth concepts discussed in Section 2.2, recent works have demonstrated that heterostructures such as PbS/CdS, PbS/SnS, and PbS/graphene can leverage both band engineering and external-field effects to boost performance in solar and optoelectronic devices.

In a representative study, Agoro et al. [37] fabricated a TiO₂/PbS/SnS multilayer heterostructure via a two-step in situ deposition process and achieved a PCE of 10.05% in quantum-dot-sensitized solar cells [37]. In this work, the three devices prepared through this sequential PbS/SnS soaking process were labeled as P/S#1, P/S#2, and P/S#3, where “P” and “S” denote PbS and SnS precursor treatments, respectively, and the “#n” index corresponds to different soaking sequences. Electrochemical impedance spectroscopy revealed a significant reduction in R_ct, while J–V analyses showed an increase in J_SC and fill factor compared to single-component PbS or SnS films. The improved performance was attributed to cascade band alignment between PbS and SnS, forming a space-charge region that facilitates carrier separation and transport. The multilayer device also exhibited enhanced stability during prolonged illumination, highlighting the robustness of interfacial coupling within the heterostructure. As shown in Figure 6a–c, the Sn–thiolate precursor design is directly reflected in the resulting multilayer morphology and the PbS/CdS band-alignment scheme used to rationalize directional charge separation [37].

Complementary insight into the field-responsive nature of PbS/CdS systems was provided by Jellouli et al. [47], who performed a theoretical analysis of PbS/CdS core–shell quantum dots under external electric fields [47]. Their modeling demonstrated that increasing field strength induces a pronounced red-shift in the linear, non-linear, and total dielectric resonances, together with an enhancement in resonance amplitude. This dielectric modulation suggests that electric-field-assisted nucleation, such as in V-SILAR processes, could dynamically tune polarization and charge separation within PbS/CdS heterojunctions. Such tunability links structural design with bias control, offering a flexible lever for optimizing interfacial charge dynamics. As summarized in Figure 6d,e, increasing field strength shifts the dielectric resonances and amplifies their magnitude, providing quantitative support for the bias-sensitive polarization and charge-separation behavior discussed here [47].

Expanding beyond traditional semiconductor stacks, Mu et al. [29] developed a graphene–PbS heterostructure photodetector that exploits the high mobility of graphene to extract carriers generated in PbS quantum dots [29]. The built-in electric field at the interface suppresses recombination and prolongs carrier lifetime, yielding responsivities of 72 A·W⁻¹ at 792 nm and 5.8 A·W⁻¹ at 1550 nm, with a sub-20 ms response time and a broad detection window spanning 265–2200 nm. These results illustrate how hybrid integration with 2D conductors can extend the functional domain of PbS heterostructures from photovoltaics to high-speed optoelectronics while maintaining low processing temperatures and mechanical flexibility. Figure 6f–h then provide the structural context for this interpretation, combining the cross-section, optical micrograph, and band-diagram schematic that motivate the proposed graphene-assisted carrier-extraction pathway [29].

Analysis across recent PbS-based heterostructure studies shows that device stability and efficiency are enhanced when band alignment promotes directional charge separation rather than carrier accumulation at the interface. Graded or cascade junctions facilitate charge transport by establishing built-in potential gradients, while lattice compatibility and controlled interfacial bonding reduce the density of recombination-active trap states. From a processing perspective, integrating such heterostructures with bias-assisted or field-driven deposition methods offers a practical route to achieve uniform interfacial control over large areas, which is critical for scalable photovoltaic and photoelectrochemical device fabrication.

From a comparative perspective, hybrid heterostructure strategies in PbS-QD devices can be grouped into two functional categories. The first relies on high-mobility conductive interlayers (e.g., graphene-type carbon scaffolds) that mainly decrease series resistance and accelerate carrier extraction, which is typically evidenced by photoluminescence quenching and improved collection. The second category uses mixed-dimensional semiconductor heterojunctions (e.g., 2D/0D stacks) where the dominant benefit comes from engineered band offsets and directional charge transfer rather than purely improved conductivity. In practice, the performance gain is often limited not by absorber loading but by the interfacial quality: overly strong coupling or defective contacts can introduce new recombination pathways, whereas well-aligned interfaces act as selective extraction channels. This comparison clarifies why “hybridization” may improve devices through distinct mechanisms (transport acceleration vs. band-offset control) and highlights the need to report interface metrics (e.g., recombination resistance, lifetime proxies) when benchmarking heterostructures across studies [48,49].

4.2. Surface Passivation and Protection Layers

Surface passivation plays a decisive role in mitigating trap-assisted recombination and enhancing both efficiency and operational stability of PbS-based colloidal quantum-dot (CQD) architectures. Because the high surface to volume ratio of PbS QDs leads to abundant undercoordinated Pb²⁺ and S²⁻ sites, trap states can dominate charge dynamics, causing carrier loss and accelerated degradation under bias or illumination. Recent work has demonstrated that rational passivation implemented either in situ during synthesis or ex situ via targeted chemical treatments can effectively neutralize these trap states and stabilize interfacial energetics, thereby extending device lifetimes and improving photovoltaic performance.

A significant advance in this area was achieved by Ding et al. [50], who developed an in situ synergistic halogen passivation (MHP) strategy during PbS CQD synthesis [50]. In their approach, a controlled combination of halide ions (I⁻, Br⁻, Cl⁻) was introduced directly into the reaction medium, enabling dynamic coordination to Pb-rich surface sites throughout nucleation and growth. The resulting films exhibited substantially fewer surface defects and higher carrier mobility than those treated with a single halogen. Devices fabricated from the MHP-treated CQDs demonstrated a remarkable increase in power conversion efficiency (PCE) from 10.64% to 12.58%, accompanied by enhanced open-circuit voltage and fill factor. Photothermal deflection spectroscopy confirmed the near-elimination of deep trap states, while XPS analysis revealed robust Pb–X bonding stabilizing the QD surface. This in situ approach underscores that mixed halogen species can cooperatively passivate multiple defect types, Pb-vacancy, halide-vacancy, and oxide-related traps, without compromising electronic coupling between neighboring dots [50].

In a complementary study, Seo et al. [7] utilized a thin ZnS overlayer deposited by SILAR onto PbS-sensitized BiVO₄ photoanodes, observing a marked enhancement in photoelectrochemical performance [7]. The photocurrent density increased from 2.92 mA cm⁻² to 5.19 mA cm⁻² at 1.23 V_RHE, attributed to the suppression of interfacial recombination and reduced charge-transfer resistance. The ZnS shell acted as a wide-bandgap barrier, blocking back-electron transfer from the semiconductor to the electrolyte while still enabling hole transfer for oxidation. This improvement underscores the role of inorganic passivation layers in enhancing both photoactivity and long-term durability under illumination and bias, complementing chemical passivation strategies based on organic or halide ligands. As shown in Figure 7a,b, the J–V response and the chronoamperometric stability at 1.23 V_RHE track the same trend reported here, confirming that optimized PbS loading and ZnS passivation translate into higher photocurrent and improved operational retention [7].

Complementing this in situ chemical approach, Pinna et al. [51] introduced an ex situ PbI₂ post-passivation strategy to enhance the structural and electronic integrity of highly ordered PbS QD superlattices [51]. In this work, a thin PbI₂ layer was applied onto three-dimensional QD arrays, yielding uniform capping across superlattice domains. The PbI₂ treatment reduced mid-gap defect states, strengthened interdot coupling, and suppressed surface oxidation, collectively enhancing photoconductivity and charge delocalization throughout the lattice. The resulting metamaterial exhibited improved optical uniformity and superior carrier mobility, revealing that precisely engineered inorganic capping layers can simultaneously protect the QD lattice and preserve its electronic coherence. This structural passivation route demonstrates the value of inorganic–organic synergy, where crystalline overcoats act as both a chemical barrier and an electronic bridge between QDs. Figure 7c–e supports this discussion by linking the stepwise fabrication workflow to the resulting single-crystalline superlattice domain (STEM-HAADF) and the corresponding transport metric (subthreshold swing), which together substantiate the structural–electronic coherence of the assembled PbS superlattices [51].

Comparative analysis of recent passivation strategies indicates that suppressing non-radiative recombination in PbS quantum-dot assemblies requires simultaneous control over surface chemistry and interfacial structure. Mixed-halide treatments reduce a broad distribution of surface defect states by selectively coordinating undercoordinated Pb and chalcogen sites, consistent with the established role of interface engineering in high-performance photovoltaic systems [52]. In parallel, crystalline inorganic layers such as PbI₂ enhance mechanical integrity and electronic coupling across adjacent quantum dots. Combining chemical passivation with structurally defined interfacial encapsulation therefore provides an effective means to stabilize charge transport and maintain optoelectronic performance under operational conditions. When implemented alongside field-assisted growth approaches (e.g., V-SILAR), these passivation schemes enable more uniform interface control and improved scalability in PbS-based photovoltaic and photoelectrochemical devices.

Passivation in PbS-QD systems spans multiple “design philosophies” that can look similar in outcome (reduced recombination) but differ in trade-offs. Shell/overlayer approaches (wide-bandgap inorganic caps or interfacial passivation layers) primarily suppress surface states by chemically saturating traps, but excessive thickness can hinder injection/transfer by adding an energetic or tunneling barrier. In contrast, hybrid/ligand-based passivation can simultaneously improve electronic coupling while reducing trap density, offering a route to lower interfacial loss without necessarily blocking transport. This is well illustrated by hybrid passivation of PbS/CdS QDs for photoelectrochemical H₂ generation, where improved surface chemistry and interfacial charge transfer translate into higher PEC output, while systematic studies in PbS QD photovoltaics show that rigorous passivation is also decisive for photostability (mitigating trap-assisted recombination and degradation under illumination). Together, these reports suggest that “best passivation” must be benchmarked against both kinetic metrics (R_ct/recombination signatures) and durability rather than efficiency alone [53,54].

4.3. Doping and Ligand Engineering Strategies

In PbS quantum-dot solids, intentional compositional modification offers a means to influence band alignment, carrier density, and defect formation at both the dot surface and interdot junctions. Such control is particularly relevant because electronic losses in PbS assemblies often originate from trap-assisted recombination and inefficient charge extraction rather than from insufficient absorption. While aliovalent dopants can modulate Fermi-level pinning and facilitate charge extraction, coherent alloy/shell architectures can simultaneously passivate interfacial states and preserve electronic coupling. Recent studies illustrate how compositional engineering implemented either through epitaxial alloy-type shells or through dopant-compatible surface coordination translates into tangible device gains across photovoltaic and detector platforms.

A representative example is the coherent CsPbI₂Br epitaxial shell on PbS QDs, which functions as an alloy-like, lattice-matched overlayer to mitigate interfacial defects and stabilize energetics. Rahman et al. [55] demonstrated that a strain-free CsPbI₂Br shell (≈98% lattice match) on PbS enables longer carrier lifetimes and improved photostability; solar cells fabricated from these core–shell QDs reached 8.43% PCE, roughly twice that of pristine PbS devices under identical processing, indicating that compositionally engineered shells can serve as both structural stabilizers and electronic modulators for efficient carrier extraction [55].

Compositional tuning also intersects with dopant-compatible ligand/coordination chemistry in high-gain detectors. Zhan et al. [56] reported PbS-QD/IGZO phototransistors in which a long-chain dithiol exchange (1,10-decanedithiol) suppresses lateral leakage and electron trapping while enhancing vertical injection at the QD/oxide interface, yielding responsivity > 2 × 10⁴ A·W⁻¹ and detectivity > 10¹⁴ Jones under low-dose NIR illumination. Although not a lattice dopant per se, the ligand-controlled coordination environment plays a doping-analogous role by rebalancing trap occupation and interdot transport, thereby complementing conventional alloy/dopant strategies for performance and stability in PbS-based devices [56]. Figure 8a–c provide the device-level context for this point by linking the PbS QD/IGZO architecture to the detectivity–frequency benchmarking and the ligand-dependent transport picture that motivates the trap/transport interpretation discussed here.

Extending the concept of interfacial and compositional control, Liao et al. [57] demonstrated that in situ perovskite-like ligands, (BA)₂PbI₄, provide versatile surface passivation for both large- and small-sized PbS CQDs, suppressing trap-assisted recombination. With (BA)₂PbI₄ capping, infrared PbS CQD solar cells improved from 7.13% to 8.65% PCE for 1.0 eV CQDs and reached 13.1% PCE for 1.3 eV CQDs, accompanied by markedly enhanced ambient/thermal stability relative to PbI₂-based controls. These studies indicate that compositional modification is most effective when dopant incorporation preserves lattice coherence and does not disrupt electronic coupling between neighboring quantum dots. Under such conditions, alloying and ligand coordination reduce trap-assisted recombination while maintaining efficient charge transport, leading to measurable improvements in both optoelectronic performance and operational stability [57]. As summarized in Figure 8d–g, the accompanying device architecture, band-diagram rationale, and J–V behavior provide direct support for translating these composition-driven changes into measurable device performance trends under operation.

Doping and ligand engineering are often discussed together, but they influence devices through different levers. Intentional dopants mainly tune carrier density and band bending, which can improve extraction (e.g., strengthening the p-type functionality of hole-selective layers) when the limiting factor is depletion width or contact barriers. In parallel, atomic-ligand control tunes surface polarity and transport by altering inter-QD coupling and the balance of electron vs. hole mobility; comparative halide–ligand studies demonstrate that the choice of halide can shift transport behavior and thus the optimal layer role in device stacks. Importantly, both routes can backfire if they introduce mid-gap states or disrupt packing, so “performance-oriented” optimization should report not only PCE gains but also how the modification reshapes carrier density, mobility asymmetry, and recombination signatures. This comparative framing helps rationalize why some reports prioritize dopant-enabled extraction improvements whereas others achieve gains mainly by ligand-mediated transport and trap suppression [58,59].

5. Summary and Outlook

Over the past decade, research on PbS quantum dots has demonstrated that quantum confinement and strong near-infrared absorption can be effectively exploited in solution-processed optoelectronic systems. However, translating these intrinsic properties into stable device performance has required careful control over interfaces, defect chemistry, and charge-transport pathways. As reviewed throughout this article, these characteristics have enabled PbS QDs to become versatile building blocks for optoelectronic and photoelectrochemical systems, bridging the gap between low-cost processing and high-performance light harvesting. The early stages of research (Section 1 and Section 2) established the fundamental structure–property relationship of PbS QDs, where their size-dependent bandgap, MEG, and tunable surface states provided a flexible platform for device design. Subsequent material and device analyses (Section 3) confirmed that optimized ligand chemistry and controlled crystallinity are crucial for achieving balanced carrier transport, spectral response, and stability across different operating environments.

In the past few years, the research focus has shifted from material synthesis to interfacial and compositional engineering, as discussed in Section 4. The development of voltage-assisted SILAR (V-SILAR) and other field-assisted deposition techniques has allowed more uniform QD growth and improved carrier injection at semiconductor interfaces. Simultaneously, hybrid heterostructure architectures such as PbS coupled with CdS, BiVO₄, or perovskite shells have shown clear advantages in facilitating directional charge separation and minimizing interfacial recombination. Meanwhile, surface passivation using halides, oxides, or chalcogenides (e.g., ZnS, TiO₂ coatings) remains one of the most decisive strategies to stabilize QD surfaces and extend device lifetimes. More recently, doping and alloying approaches (e.g., incorporation of Hg, Mn, or Sn cations) have proven to be effective for modulating energy levels, enhancing carrier mobility, and strengthening chemical robustness. Across the studies discussed in this review, performance optimization in PbS-based systems is consistently linked to the interplay between surface chemistry, interfacial energetics, and nanostructural morphology. Improvements in one parameter alone rarely translate into sustained device gains unless accompanied by corresponding control over recombination pathways and charge transport.

Despite this progress, significant scientific and technological hurdles must still be addressed. A major bottleneck remains the control of defect dynamics: surface and interfacial traps continue to limit photovoltage and long-term durability, especially under operational bias or illumination. Developing covalently anchored, self-healing, or crosslinked ligand frameworks could provide more resilient passivation against oxidation and desorption. Another challenge lies in carrier transport anisotropy and structural disorder in QD solids. Even with ligand exchange and epitaxial-shell engineering, interdot coupling and electronic delocalization remain incomplete, restricting charge mobility. Achieving ordered or epitaxially fused superlattices could bridge the gap between nanocrystalline and bulk-like transport. Moreover, the environmental and sustainability aspects of PbS QD synthesis require increasing attention: while lead-based chemistry underpins their high performance, the toxicity of Pb and the complexity of its waste management remain concerns for large-scale deployment. Exploring Pb-reduced or Pb-free analogues (e.g., SnS, AgBiS₂) and adopting green-solvent, low-temperature processes will be essential for sustainable production.

Beyond performance and stability considerations, cost-related factors are also critical for assessing the practical viability of quantum-dot-based solar technologies. PbS quantum dots fabricated via in situ methods benefit from low-temperature, solution-based processing and do not require vacuum systems or extensive ligand-exchange steps, which can significantly reduce fabrication complexity and associated costs. In comparison, perovskite quantum dots often demand stricter processing control and encapsulation to maintain operational stability, while lead-free alternatives such as AgBiS₂ quantum dots, although attractive from a sustainability perspective, currently exhibit lower device efficiencies that can negatively impact cost-per-performance metrics. Consequently, PbS QDs offer a competitive balance between processing simplicity, scalability, and device performance for solar-energy applications.

In parallel with performance and cost considerations, the potential environmental and health risks associated with lead-based materials must be carefully addressed. The toxicity of Pb poses challenges for large-scale deployment of PbS quantum-dot-based devices, particularly in terms of material handling, disposal, and long-term environmental impact. To mitigate these risks, several strategies have been proposed, including robust encapsulation of PbS layers within solid-state device architectures, the use of stable inorganic passivation layers to suppress Pb leakage, and the development of recycling and containment protocols for end-of-life devices. In addition, ongoing research into Pb-reduced and Pb-free alternatives, such as AgBiS₂ and other chalcogenide quantum dots, provides complementary pathways toward more sustainable solar-energy technologies. Addressing Pb-related toxicity through both materials engineering and system-level strategies will be essential for the responsible advancement of PbS-based solar-energy applications.

Despite the rapid progress summarized above, several practical challenges continue to limit the transition of PbS quantum-dot-based solar-energy technologies toward commercialization. In particular, maintaining stable device performance under realistic operating conditions, including prolonged illumination, thermal stress, and electrochemical environments, remains a major obstacle. Equally important is the ability to achieve uniform quantum-dot deposition and reproducible device characteristics over large-area substrates, which is essential for scalable manufacturing. Addressing these challenges will require further advances in interfacial stabilization, process control, and in situ growth strategies that are compatible with large-area fabrication.

Further progress in PbS-based photovoltaic and photoelectrochemical technologies will depend on the ability to control defect formation, interfacial band alignment, and carrier transport under realistic operating conditions. In this context, combining interface-focused passivation strategies with scalable, field-assisted deposition methods represents a practical direction for improving both performance reproducibility and long-term operational stability. The synergy between chemical passivation and structural regulation will likely be complemented by machine-learning-assisted synthesis optimization, real-time spectroscopic monitoring, and data-driven defect modeling, which can accelerate discovery and improve reproducibility. Future work should also emphasize the coupling of PbS QDs with photocatalytic or PEC systems for solar fuel generation, as their broadband absorption and tunable band alignment offer unique advantages for visible-to-NIR-driven reactions.

From a future perspective, the development of large-area and scalable PbS quantum-dot-based devices represents a critical research direction toward practical deployment. In situ fabrication techniques, such as SILAR, voltage-assisted SILAR, and chemical bath deposition, are inherently compatible with low-temperature and solution-based processing, making them suitable for uniform coating over large substrates. However, achieving consistent quantum-dot loading, interfacial uniformity, and reproducible device performance across large areas remains a key challenge that requires further optimization of growth kinetics and process control.

Beyond scale-up, future research is expected to increasingly focus on interface-dominated performance limitations in PbS-based solar-energy devices. As highlighted throughout this review, charge-transfer kinetics, defect passivation, and recombination suppression at the quantum-dot/oxide interface play a more decisive role than light absorption alone. Advanced interfacial engineering strategies, including graded heterostructures, inorganic passivation layers, and field-assisted growth, are therefore anticipated to remain central themes in improving both efficiency and long-term operational stability.

Additional emerging research hotspots include the integration of PbS quantum dots with tandem or hybrid device architectures, the exploration of data-driven and machine-learning-assisted process optimization, and the development of environmentally responsible design strategies. Together, these directions are expected to further clarify the practical limits and opportunities of PbS quantum dots in next-generation solar-energy conversion technologies.

In summary, PbS QDs have evolved from a fundamental research curiosity to a promising technological platform for infrared photovoltaics, photodetectors, and PEC applications. Their continued advancement depends on balancing chemical robustness, electronic coherence, and environmental sustainability within manufacturable device architectures. Continued advances in interfacial chemistry and nanostructure control will further clarify the practical limits and opportunities of PbS quantum dots in optoelectronic and photoelectrochemical systems. A mechanism-guided approach that links materials processing with device operation will be essential for assessing their realistic potential in next-generation solar-energy technologies.