Cascade surface modification of colloidal quantum dot inks enables efficient bulk homojunction photovoltaics

Colloidal quantum dots (CQDs) offer size- and surface-chemistry-tunable optical and electronic properties and have shown promise in a range of optoelectronic devices, including solar cells. Device performance has improved through advances in synthesis, passivation, and architectures, with single-junction CQD photovoltaics reaching certified efficiencies around 12%. A central performance lever is the minority-carrier diffusion length, governed by carrier lifetime and mobility; it can be extended by eliminating surface traps through ligand exchange and by designing device architectures that promote charge separation and extraction. Bulk heterojunction concepts have been realized using CQD/polymer blends or different CQD materials, but a homojunction using a single-bandgap CQD differentiated only by doping would simplify bandgap management while enabling effective charge separation. However, forming both p-type and n-type CQD inks with complete passivation and mutual miscibility has been challenging: conventional ligand-exchange routes to p-type behavior often leave surface defects due to steric hindrance and yield poor colloidal stability, inhibiting the formation of homogeneous bulk homojunction films. The research question addressed here is whether a new surface modification strategy can simultaneously deliver complete passivation, controlled p- and n-doping, and ink miscibility to enable homogeneous CQD bulk homojunction solids and, in turn, higher-efficiency CQD solar cells.

The paper reviews how CQD properties are tuned via size and surface chemistry and summarizes key developments that raised CQD solar cell efficiencies, including atomic-ligand passivation, air-stable n-type solids, and matrix engineering. Prior strategies to boost diffusion length include minimizing deep surface traps via improved ligand exchanges and using bulk heterojunctions to separate charge carriers spatially, implemented with CQD/polymer blends or combinations of different CQD materials. The possibility of bulk homojunctions in CQDs arises from ligand-driven control of band-edge positions and doping, as surface functionalization (electron-donating vs withdrawing ligands) modulates the density of states. Yet previous solution-phase exchanges for p-type inks led to incomplete passivation due to steric hindrance of functional ligands and to poor ink miscibility, causing aggregation and energetic disorder that hamper transport. This background motivates the need for a method that decouples passivation from doping control while ensuring colloidal stability and miscibility.

The authors developed a cascade surface modification (CSM) protocol to produce n-type and p-type PbS CQD inks that are mutually miscible and fully passivated. Step 1 (halogenation): oleic-acid-capped PbS CQDs are exchanged with lead halide anions (PbI2/PbBr2 with ammonium acetate) in DMF, infiltrating and passivating surface sites and yielding n-type CQDs, which are redispersed in butylamine (BTA). Step 2 (functionalization): the lead-halide-rich surface is re-exchanged with bifunctional thiol ligands to program p-type character and tune solubility; ligands explored include 1-thioglycerol (TG), 2-mercaptoethanol (ME), cysteamine (CTA), 4-hydroxythiophenol, 4-aminothiophenol, and malonic acid. X-ray photoelectron spectroscopy confirms thiolate binding and reduced halide signal after functionalization. Photoluminescence quantum yield (PLQY) is used to assess passivation quality. Ultraviolet photoelectron spectroscopy (UPS) quantifies Fermi level shifts (doping). Kelvin probe force microscopy (KPFM) maps surface potential differences across n/p interfaces in films. Space-charge-limited current devices assess electron and hole mobilities for n- and p-type films. To ensure miscibility in BTA, the team rationally selected ligands based on hydrogen-bonding complementarity with BTA (NH2-terminated CTA favored), screening colloidal stability of p-type inks (CTA stable, ME limited, MPA insoluble). Blend ink stability is monitored by absorption spectral evolution and PL stability; chemical orthogonality is assessed by tracking exciton peak shifts as a proxy for thiol migration. Film morphology and ordering are characterized by GISAXS. Carrier diffusion length is measured using a one-dimensional donor–acceptor PL quenching scheme: a 1.3 eV donor CQD layer (either n-type, p-type, or n/p blend bulk homojunction) is deposited atop a 1.0 eV acceptor CQD layer (type-I alignment), and acceptor PL is recorded versus donor thickness and fit to a 1D diffusion model. Ultrafast transient absorption spectroscopy probes inter-dot carrier transfer dynamics in blends of different bandgaps and dopings to distinguish type-I vs type-II offsets and identify hole transfer pathways. Solar cells are fabricated in the architecture ITO/ZnO/active CQD film (n-type, p-type, or bulk homojunction)/PbS-EDT HTL/Au. Active-layer thickness and p:n mass ratio are optimized; current–voltage under AM1.5G, EQE, and operational stability (MPP tracking in N2) are measured. Detailed synthesis, exchange, deposition, and characterization parameters are provided, including concentrations, solvents, spin-coating, annealing, and instrument settings.

• CSM achieves complete passivation and tunable doping: TG-based CSM inks show PLQY ~18% versus ~6% for conventional solution-exchanged TG inks; XPS confirms thiolate binding and reduced halide content after functionalization.
• Doping control: UPS shows the energy difference between valence band and Fermi level decreases from 0.77 eV (n-type, halogenated) to 0.60 eV (TG), 0.43 eV (ME), and 0.45 eV (CTA), evidencing p-type behavior after functionalization.
• Built-in potential in films: KPFM maps a ~−0.2 eV surface potential step at the n/p interface in thin films, indicating retention of distinct Fermi levels after film formation.
• Transport trends: SCLC mobility measurements show p-type films have higher hole mobility and lower electron mobility (μh ~1.3×10−3, μe ~1.5×10−3 V cm−1 s−1) than n-type films (μh ~8×10−4, μe ~3×10−3 V cm−1 s−1), consistent with doping.
• Ink miscibility and stability: Conventional n/p blend inks rapidly degrade (absorption peak intensity loss and HWHM increase within minutes), whereas CSM-based blends (CTA p-type + halogenated n-type) remain stable in BTA with preserved absorption features and PL over at least 1 h; no exciton peak shift indicates chemical orthogonality (no thiol migration).
• Film ordering: GISAXS reveals hexagonal in-plane ordering and denser, more uniform packing for CSM-derived bulk homojunction films; average inter-dot spacing ~3.32 nm (bulk homojunction) versus ~3.30 nm (n-type) and ~3.35 nm (p-type), supporting homogeneous mixing.
• Carrier diffusion length: One-dimensional donor–acceptor PL quenching analysis shows the bulk homojunction film attains Ld ~340 nm versus ~221 nm for the best n-type control, a ~1.5× increase; optical constants (n, k) are similar across films, ruling out optical artifacts.
• Charge transfer mechanism: TA spectroscopy on mixed-size, mixed-doping films shows rapid hole transfer from narrower-gap n-type to wider-gap p-type CQDs (type-II alignment), whereas blends of two n-type CQDs (different sizes) form type-I offsets with no appreciable inter-dot charge transfer.
• Device performance: Bulk homojunction devices enable thicker optimal active layers (~580 nm) than single-doped controls (~390–400 nm), consistent with longer Ld. Best device achieves AM1.5G PCE 13.3% with Voc 0.65 V, Jsc 30.2 mA cm−2, FF 68%. Certified PCE: 12.47 ± 0.33% (Newport), the highest certified CQD solar cell PCE at the time. EQE-integrated Jsc ~30 mA cm−2 vs 26.8 mA cm−2 for control. Larger-area (1.1 cm2) devices show similar Voc and Jsc with somewhat reduced FF due to series resistance. Operational stability: 87% PCE retained after 110 h MPP tracking under AM1.5G in N2.

The cascade surface modification strategy decouples passivation from doping control, overcoming the long-standing trade-off where achieving p-type behavior caused incomplete surface coverage and poor colloidal stability. By first halogenating to passivate inaccessible sites and then reprogramming with thiols chosen for both electronic (p-type) and solubility (NH2-terminated CTA) properties, the approach yields n- and p-type inks that are chemically orthogonal yet miscible in a common solvent. This enables homogeneous bulk homojunction CQD films with retained band offsets, verified by KPFM and spectroscopic methods. The resulting interpenetrating n/p network provides separate percolation paths for electrons and holes, suppressing recombination and extending the effective carrier diffusion length by ~1.5× relative to state-of-the-art controls. Ultrafast TA confirms type-II band alignment between p- and n-type domains with rapid hole transfer, underpinning the improved charge separation. In devices, the extended diffusion length permits thicker active layers without sacrificing voltage or fill factor, boosting Jsc and overall PCE to 13.3% (12.47% certified), a record for CQD photovoltaics. These results demonstrate that rational surface chemistry design can engineer both electronic structure and processing compatibility to unlock advanced CQD device architectures, with implications across optoelectronics.

This work introduces a cascade surface modification method that produces fully passivated, mutually miscible n- and p-type CQD inks. The approach enables homogeneous CQD bulk homojunction films with improved ordering, preserved band offsets, and significantly longer carrier diffusion lengths. Implemented in solar cells, the architecture supports thicker absorbers and delivers record performance for CQD photovoltaics (13.3% AM1.5G, 12.47% certified). Future research could extend CSM to other CQD compositions and ligand chemistries, further optimize domain morphology and percolation, investigate long-term operational stability under ambient conditions, integrate improved transparent conductors to enhance large-area device FF, and explore tandem or multi-junction implementations leveraging the homojunction strategy.

• p-type CQD inks could not be used directly as the hole transport layer due to similar solubility to n-type inks, necessitating a separate PbS-EDT HTL.
• Stability testing was limited to ~110 h of MPP operation in N2; long-term ambient and thermal stability were not reported.
• Large-area devices exhibited reduced fill factor attributed to series resistance of the transparent conductive oxide, indicating scaling challenges.
• The diffusion length study used a 1D donor–acceptor configuration; a fully 3D donor–acceptor test was avoided due to solubility and mixing concerns, leaving some uncertainty about 3D transport behavior.
• While several ligands were screened, optimal miscibility hinged on NH2-terminated CTA in BTA; generality across solvents and ligand families may require further validation.