Synergistic Surface Modification for High-Efficiency Perovskite Nanocrystal Light-Emitting Diodes: Divalent Metal Ion Doping and Halide-Based Ligand Passivation.

Surface defects of metal halide perovskite nanocrystals (PNCs) substantially compromise the optoelectronic performances of the materials and devices via undesired charge recombination. However, those defects, mainly the vacancies, are structurally entangled with each other in the PNC lattice, necessitating a delicately designed strategy for effective passivation. Here, a synergistic metal ion doping and surface ligand exchange strategy is proposed to passivate the surface defects of CsPbBr3 PNCs with various divalent metal (e.g., Cd2+ , Zn2+, and Hg2+ ) acetate salts and didodecyldimethylammonium (DDA+ ) via one-step post-treatment. The addition of metal acetate salts to PNCs is demonstrated to suppress the defect formation energy effectively via the ab initio calculations. The developed PNCs not only have near-unity photoluminescence quantum yield and excellent stability but also show luminance of 1175 cd m-2 , current efficiency of 65.48 cd A-1 , external quantum efficiency of 20.79%, wavelength of 514 nm in optimized PNC light-emitting diodes with Cd2+ passivator and DDA ligand. The "organic-inorganic" hybrid engineering approach is completely general and can be straightforwardly applied to any combination of quaternary ammonium ligands and source of metal, which will be useful in PNC-based optoelectronic devices such as solar cells, photodetectors, and transistors.

A Universal Perovskite Nanocrystal Ink for High-Performance Optoelectronic Devices.

Semiconducting lead halide perovskite nanocrystals (PNCs) are regarded as promising candidates for next-generation optoelectronic devices due to their solution processability and outstanding optoelectronic properties. While the field of light-emitting diodes (LEDs) and photovoltaics (PVs), two prime examples of optoelectronic devices, has recently seen a multitude of efforts toward high-performance PNC-based devices, realizing both devices with high efficiencies and stabilities through a single PNC processing strategy has remained a challenge.  In this work, diphenylpropylammonium (DPAI) surface ligands, found through a judicious ab-initio-based ligand search, are shown to provide a solution to this problem. The universal PNC ink with DPAI ligands presented here, prepared through a solution-phase ligand-exchange process, simultaneously allows single-step processed LED and PV devices with peak electroluminescence external quantum efficiency of 17.00% and power conversion efficiency of 14.92% (stabilized output 14.00%), respectively. It is revealed that a careful design of the aromatic rings such as in DPAI is the decisive factor in bestowing such high performances, ease of solution processing, and improved phase stability up to 120 days. This work illustrates the power of ligand design in producing PNC ink formulations for high-throughput production of optoelectronic devices; it also paves a path for "dual-mode" devices with both PV and LED functionalities.

Guanidinium-Pseudohalide Perovskite Interfaces Enable Surface Reconstruction of Colloidal Quantum Dots for Efficient and Stable Photovoltaics.

Complete surface passivation of colloidal quantum dots (CQDs) and their strong electronic coupling are key factors toward high-performance CQD-based photovoltaics (CQDPVs). Also, the CQD matrices must be protected from oxidative environments, such as ambient air and moisture, to guarantee air-stable operation of the CQDPVs. Herein, we devise a complementary and effective approach to reconstruct the oxidized CQD surface using guanidinium and pseudohalide. Unlike conventional halides, thiocyanate anions provide better surface passivation with effective replacement of surface oxygen species and additional filling of defective sites, whereas guanidinium cations promote the construction of epitaxial perovskite bridges within the CQD matrix and augment electronic coupling. Additionally, we replace a defective 1,2-ethanedithiol-treated CQD hole transport layer (HTL) with robust polymeric HTLs, based on a judicious consideration of the energy level alignment established at the CQD/HTL interface. These efforts collectively result in high-performance and stable CQDPVs with photocurrents over 30 mA cm-2, ∼80% quantum efficiency at excitonic peaks and stable operation under humid and ambient conditions. Elucidation of carrier dynamics further reveals that interfacial recombination associated with band alignment governs both the CQDPV performance and stability.

Solvent Engineering of Colloidal Quantum Dot Inks for Scalable Fabrication of Photovoltaics.

Development of colloidal quantum dot (CQD) inks enables single-step spin-coating of compact CQD films of appropriate thickness, enabling the promising performance of CQD photovoltaics (CQDPVs). Today's highest-performing CQD inks rely on volatile n-butylamine (BTA), but it is incompatible with scalable deposition methods since a rapid solvent evaporation results in irregular film thickness with an uneven surface. Here, we present a hybrid solvent system, consisting of BTA and N,N-dimethylformamide, which has a favorable acidity for colloidal stability as well as an appropriate vapor pressure, enabling a stable CQD ink that can be used to fabricate homogeneous, large-area CQD films via spray-coating. CQDPVs fabricated with the CQD ink exhibit suppressed charge recombination as well as fast charge extraction compared with conventional CQD ink-based PVs, achieving an improved power conversion efficiency (PCE) of 12.22% in spin-coated devices and the highest ever reported PCE of 8.84% among spray-coated CQDPVs.

Hybrid Surface Passivation for Retrieving Charge Collection Efficiency of Colloidal Quantum Dot Photovoltaics.

Efficient charge collection in photovoltaics is a key issue toward their high performance. Despite the promising performance of colloidal quantum dot (CQD)-based photovoltaics (CQDPVs), they suffer significant dissipation of photocurrent due to imperfect surface passivation of the CQD hole transport layer (HTL) by a single 1,2-ethaneditihol (EDT) ligand. To address the critical drawback of existing CQDPVs, we offer a hybrid passivation strategy, including both EDT and thiocyanate (SCN). The hybrid passivation leads to seamless surface passivation of CQDs, remarkably suppressing charge recombination. This strategy also augments the p-doping density of the CQD, resulting in a pronounced energy level bending at the active layer/HTL interface and facilitating efficient charge separation. Moreover, enhanced electronic coupling across the CQDs (originating from reduced inter-dot spacing) promotes rapid charge extraction. Consequently, the flawless charge collection by a hybrid-passivated HTL successfully retrieves the photocurrent, achieving an enhanced CQDPV power conversion efficiency of 12.70% compared with 11.49% for the control device.