Role of Oxygen in Two-Step Thermal Annealing Processes for Enhancing the Performance of Colloidal Quantum Dot Solar Cells

Suppression of Thermally Induced Surface Traps in Colloidal Quantum Dot Solids via Ultrafast Pulsed Light

Thermal annealing (TA) of colloidal quantum dot (CQD) films is considered an important process for recent high-performing CQD solar cells (SCs) due to its beneficial effects on CQD solids, including enhanced electrical conductivity, denser packing of CQD films, and the removal of organic residues and solvents. However, the conventional TA for CQDs, which requires several  minutes, leads to hydroxylation and oxidation on the CQD surface, resulting in the formation of trap states and a subsequent decline in SC performance. To address these challenges, this study introduces a flashlight annealing (FLA) technique that significantly reduces the annealing time to the millisecond scale. Through the FLA approach, it successfully suppressed hydroxylation and oxidation, resulting in decreased trap states within the CQD solids while simultaneously preserving their charge transport properties. As a result, CQD SCs treated with FLA exhibited a notable improvement, achieving an open-circuit voltage of 0.66 V compared to 0.63 V in TA-treated devices, leading to an increase in power conversion efficiency from 12.71% to 13.50%.

Quantum Dot-Siloxane Anchoring on Colloidal Quantum Dot Film for Flexible Photovoltaic Cell

Lead sulfide (PbS) colloidal quantum dots (CQDs) are promising materials for next-generation flexible solar cells because of near-infrared absorption, facile bandgap tunability, and superior air stability. However, CQD devices still lack enough flexibility to be applied to wearable devices owing to the poor mechanical properties of CQD films. In this study, a facile approach is proposed to improve the mechanical stability of CQDs solar cells without compromising the high power conversion efficiency (PCE) of the devices. (3-aminopropyl)triethoxysilane (APTS) is introduced on CQD films to strengthen the dot-to-dot bonding via QD-siloxane anchoring, and as a result, crack pattern analysis reveals that the treated devices become robust to mechanical stress. The device maintains 88% of the initial PCE under 12 000 cycles at a bending radius of 8.3 mm. In addition, APTS forms a dipole layer on CQD films, which improves the open circuit voltage (VOC ) of the device, achieving a PCE of 11.04%, one of the highest PCEs in flexible PbS CQD solar cells.

Unlocking the Potential of Colloidal Quantum Dot/Organic Hybrid Solar Cells: Band Tunable Interfacial Layer Approach

Hybrid colloidal quantum dot (CQD)/organic architectures are promising candidates for emerging optoelectronic devices having high performance and inexpensive fabrication. For unlocking the potential of CQD/organic hybrid devices, enhancing charge extraction properties at electron transport layer (ETL)/CQD interfaces is crucial. Hence, we carefully adjust the interface properties between the ETL and CQD layer by incorporating an interfacial layer for the ETL (EIL) using several types of cinnamic acid ligands. The EIL having a cascading band offset (ΔEC) between the ETL and CQD layer suppresses the potential barrier and the local charge accumulation at ETL/CQD interfaces, thereby reducing the bimolecular recombination. An optimal EIL effectively expands the depletion region that facilitates charge extraction between the ETL and CQD layer while preventing the formation of shallow traps. Representative devices with an EIL exhibit a maximum power conversion efficiency of 14.01% and retain over 80% of initial performances after 300 h under continuous maximum power point operation.

All-in-One Process for Color Tuning and Patterning of Perovskite Quantum Dot Light-Emitting Diodes

Although post-synthetic anion exchange allows halide perovskite quantum dots to easily change the optical bandgap of materials, additional exchange of shorter ligands is required to use them as active materials in optoelectronic devices. In this study, a novel all-in-one process exchanging ligands and halide anions in film-state for facile color tuning and patterning of cesium lead halide perovskite colloidal quantum dot (PeQD) light-emitting diodes (LEDs) is proposed. The proposed all-in-one process significantly enhances the performances of PeQD LEDs by passivating the PeQD with shorter ligands. In addition, the all-in-one process is repeated more stably in the film state. Red, green, and blue LEDs with extremely narrow emission spectra using cesium lead bromide PeQDs and appropriate butylammonium halide solutions are fabricated. Furthermore, the proposed all-in-one process in film-state facilitated rapid color change in localized areas, thereby aiding in realizing fine patterns of narrow widths (300 µm) using simple contact masks. Consequently, various paint-over red/green/blue patterns in PeQD LEDs by applying halide solutions additively are fabricated.

Solution Annealing Induces Surface Chemical Reconstruction for High-Efficiency PbS Quantum Dot Solar Cells

Colloidal quantum dots (CQDs) have a large specific surface area and a complex surface structure. Their properties in diverse optoelectronic applications are largely determined by their surface chemistry. Therefore, it is essential to investigate the surface chemistry of CQDs for improving device performance. Herein, we realized an efficient surface chemistry optimization of lead sulfide (PbS) CQDs for photovoltaics by annealing the CQD solution with concentrated lead halide ligands after the conventional solution-phase ligand exchange. During the annealing process, the colloidal solution was used to transfer heat and create a secondary reaction environment, promoting the desorption of electrically insulating oleate ligands as well as the trap-related surface groups (Pb-hydroxyl and oxidized Pb species). This was accompanied by the binding of more conductive lead halide ligands on the CQD surface, eventually achieving a more complete ligand exchange. Furthermore, this strategy also minimized CQD polydispersity and decreased aggregation caused by conventional solution-phase ligand exchange, thereby contributing to yielding CQD films with twofold enhanced carrier mobility and twofold reduced trap-state density compared with those of the control. Based on these merits, the fabricated PbS CQD solar cells showed high efficiency of 11% under ambient conditions. Our strategy opens a novel and effective avenue to obtain high-efficiency CQD solar cells with diverse band gaps, providing meaningful guidance for controlling ligand reactivity and realizing subtly purified CQDs.