Phenylethylammonium bromide-assisted solution-phase ligand exchange in CsPbBr3quantum dot solar cells.

CsPbBr3quantum dots (QDs) have excellent optical properties and good phase stability, but the long-chain ligands on their surfaces affect the charge transfer between QDs. Here, we propose a simple ligand exchange strategy: solution-phase ligand exchange. By adding an acetone solution of phenylethylammonium bromide to the purification process of CsPbBr3QDs, the long-chain ligands were effectively replaced and the electric coupling between QDs was improved. As a result, the power conversion efficiency of the solar cell was increased from 1.95% to 2.83%. Meanwhile, the stability of the devices was significantly improved in the unencapsulated case.

Acid-Assisted Ligand Exchange Enhances Coupling in Colloidal Quantum Dot Solids.

Colloidal quantum dots (CQDs) are promising solution-processed infrared-absorbing materials for optoelectronics. In these applications, it is crucial to replace the electrically insulating ligands used in synthesis to form strongly coupled quantum dot solids. Recently, solution-phase ligand-exchange strategies have been reported that minimize the density of defects and the polydispersity of CQDs; however, we find herein that the new ligands exhibit insufficient chemical reactivity to remove original oleic acid ligands completely. This leads to low CQD packing and correspondingly low electronic performance. Here we report an acid-assisted solution-phase ligand-exchange strategy that, by enabling efficient removal of the original ligands, enables the synthesis of densified CQD arrays. Our use of hydroiodic acid simultaneously facilitates high CQD packing via proton donation and CQD passivation through iodine. We demonstrate highly packed CQD films with a 2.5 times increased carrier mobility compared with prior exchanges. The resulting devices achieve the highest infrared photon-to-electron conversion efficiencies (>50%) reported in the spectral range of 0.8 to 1.1 eV.

Amino Acid Double-Passivation-Enhanced Quantum Dot Coupling for High-Efficiency FAPbI3 Perovskite Quantum Dot Solar Cells.

Formamidinium lead triiodide (FAPbI3) perovskite quantum dot has outstanding durability, reasonable carrier lifetime, and long carrier diffusion length for a new generation of highly efficient solar cells. However, ligand engineering is a dilemma because of the highly ionized and dynamic characteristics of quantum dots. To circumvent this issue, herein, we employed a mild solution-phase ligand-exchange approach through adding short-chain amino acids that contain amino and carboxyl groups to modify quantum dots and passivate their surface defects during the purification process. As a result, the photoelectric conversion efficiency of FAPbI3 perovskite quantum dot solar cells (PQDSCs) increased from 11.23 to 12.97% with an open-circuit voltage of 1.09 V, a short-circuit current density of 16.37 mA cm-2, and a filling factor of 72.13%. Furthermore, the stability of the device modified by amino acids retains over 80% of the initial efficiency upon being exposed to 20-30% relative humidity for 240 h of aging treatment. This work may offer an innovative concept and approach for surface ligand treatment to improve the photovoltaic performance of PQDSCs toward large-scale manufacture.

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.

Lead Selenide (PbSe) Colloidal Quantum Dot Solar Cells with >10% Efficiency.

Low-cost solution-processed lead chalcogenide colloidal quantum dots (CQDs) have garnered great attention in photovoltaic (PV) applications. In particular, lead selenide (PbSe) CQDs are regarded as attractive active absorbers in solar cells due to their high multiple-exciton generation and large exciton Bohr radius. However, their low air stability and occurrence of traps/defects during film formation restrict their further development. Air-stable PbSe CQDs are first synthesized through a cation exchange technique, followed by a solution-phase ligand exchange approach, and finally absorber films are prepared using a one-step spin-coating method. The best PV device fabricated using PbSe CQD inks exhibits a reproducible power conversion efficiency of 10.68%, 16% higher than the previous efficiency record (9.2%). Moreover, the device displays remarkably 40-day storage and 8 h illuminating stability. This novel strategy could provide an alternative route toward the use of PbSe CQDs in low-cost and high-performance infrared optoelectronic devices, such as infrared photodetectors and multijunction solar cells.

Colloidal Quantum Dot Inks for Single-Step-Fabricated Field-Effect Transistors: The Importance of Postdeposition Ligand Removal.

Colloidal quantum dots are a class of solution-processed semiconductors with good prospects for photovoltaic and optoelectronic applications. Removal of the surfactant, so-called ligand exchange, is a crucial step in making the solid films conductive, but performing it in solid state introduces surface defects and cracks in the films. Hence, the formation of thick, device-grade films have only been possible through layer-by-layer processing, limiting the technological interest for quantum dot solids. Solution-phase ligand exchange before the deposition allows for the direct deposition of thick, homogeneous films suitable for device applications. In this work, fabrication of field-effect transistors in a single step is reported using blade-coating, an upscalable, industrially relevant technique. Most importantly, a postdeposition washing step results in device properties comparable to the best layer-by-layer processed devices, opening the way for large-scale fabrication and further interest from the research community.

Enhanced Open-Circuit Voltage in Colloidal Quantum Dot Photovoltaics via Reactivity-Controlled Solution-Phase Ligand Exchange.

The energy disorder that arises from colloidal quantum dot (CQD) polydispersity limits the open-circuit voltage (VOC ) and efficiency of CQD photovoltaics. This energy broadening is significantly deteriorated today during CQD ligand exchange and film assembly. Here, a new solution-phase ligand exchange that, via judicious incorporation of reactivity-engineered additives, provides improved monodispersity in final CQD films is reported. It has been found that increasing the concentration of the less reactive species prevents CQD fusion and etching. As a result, CQD solar cells with a VOC of 0.7 V (vs 0.61 V for the control) for CQD films with exciton peak at 1.28 eV and a power conversion efficiency of 10.9% (vs 10.1% for the control) is achieved.