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In this report, we explore the underlying mechanisms by which doped organic thin films as a top hole-selective layer (HSL) improve the performance and stability of colloidal quantum dot (CQD)-based solar cells.

of two different materials. [7,[16][17][18][19] It also enables the hybrids to inherit the superior mechanical flexibility of organic materials. [20,21] Since the first CQD/ organic hybrid solar cells were proposed, [22] efforts have been underway to improve the PCE. Although many previous studies successfully demonstrated complementary absorption, the PCEs of hybrid solar cells are still affected by inefficient charge extraction, caused by 1) the short exciton diffusion length in polymers, [19] 2) poor nanomorphology of the CQD:polymer mix causing unfavorable charge transfer pathways, and 3) local trap sites at the interfaces of CQD and polymer. [18] Various strategies such as exploration of new materials for use in the hybrids, [23][24][25][26][27] tuning CQD size, [18,28] molecular modification, [18,29] and ligand exchange [30,31] have been proposed to enhance the charge extraction in CQD/polymer hybrids. Among them, tuning the CQD surface using organic/inorganic ligands has been widely investigated because it can modify the CQD surface that affects the electrical properties. [16,32,33] Recently, a small-molecule bridge was employed in CQD/ organic hybrid solar cells to overcome the short exciton diffusion length problem; [34,35] however, the hybrid cells still suffer from substantial trap sites at the CQD/organic interfaces, limiting charge extraction. The CQD/organic interfaces have many trap sites due to the large difference in surface energy between the CQD and polymer, [18] the presence of insulating ligands, [16] and defects on the CQD surfaces. [36] These CQD/polymer interfaces potentially hinder charge transport and induce bimolecular recombination.In this work, we systematically investigated how the CQD/ organic interfaces influenced the charge transport properties. First, we replaced the native ligand of CQD with halides to form conventional CQD solids. [37] Subsequently, a polymer layer was stacked onto the CQD solid to form a CQD/organic heterointerface. Additionally, an auxiliary interfacial layer passivated by various organic ligands was implemented at the CQD/polymer interface. The interfacial layer that provides the cascading conduction band offset (ΔE C ) between CQD and polymer relieved band bending that adversely hinders the charge transfer at CQD/polymer interfaces. The reduced band bending facilitated the charge transfer across the heterojunction by suppressing charge accumulation -as confirmed by transient photocurrent (TPC) spectroscopy -and by reducing bimolecular recombination at the CQD/polymer interfaces.Emerging semiconducting materials including colloidal quantum dots (CQDs) and organic molecules have unique photovoltaic properties, and their hybridization can result in synergistic effects for high performance. For realizing the full potential of CQD/organic hybrid devices, controlling interfacial properties between the CQD and organic matter is crucial. Here, the electronic band between the CQD and the polymer layers is carefully modulated by inserting an interfacial layer treated...

Environmentally friendly colloidal nanocrystals (NCs) are promising materials for next‐generation solar cells because of their low cost, solution processability, and facile bandgap tunability. Recently, silver bismuth disulfide (AgBiS2) has attracted considerable attention owing to its appreciable power conversion efficiency (PCE) of 6.4%. However, issues such as the low open‐circuit voltage (VOC) compared to the bandgap of the AgBiS2 NCs and the unoptimized energy level structure at the AgBiS2 NC/PTB7 hole‐transporting layer (HTL) interface should be resolved to enhance the performance of solar cells. In this study, a design strategy to obtain efficient energy level structure in AgBiS2 NC/organic hybrid solar cells is proposed. By selecting PBDB‐T‐2F as an HTL with a lower highest occupied molecular orbital level than that of PTB7, the VOC of the device is increased. Furthermore, iodide‐ and thiolate‐passivated AgBiS2 NC surfaces are generated using tetramethylammonium iodide (TMAI) and 2‐mercaptoethanol (2‐ME), which leads to the energy level optimization of NCs for efficient charge extraction. This improves the PCE from 3.3% to 7.1%. In addition, the polymer is replaced with a PBDB‐T‐2F:BTP‐4Cl blend to achieve a higher short‐circuit current density through complementary absorption. Accordingly, an AgBiS2 NC‐based solar cell with a PCE of 9.1% is fabricated.

Colloidal quantum dots (CQDs) have large surface-to-volume ratios; thus, surface control is critical, especially when CQDs are utilized in optoelectronic devices. Layer-bylayer solid-state ligand exchange is a facile and applicable process for the formation of conductive CQD solids through various ligands; however, achieving complete ligand exchange on the CQD surface without dangling bonds is challenging. Herein, we demonstrate that CQDs can be further passivated through two-step annealing; air annealing forms sulfonate bonding at (111) Pb-rich surfaces, and subsequent N 2 annealing removes insulating oxygen layers from the (100) surfaces of CQDs. By subsequently conducting annealing treatment in two different environments, traps on the surface of CQDs could be significantly reduced. We achieved a 40.8% enhancement of the power conversion efficiency by optimizing each two-step annealing process.

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.

A novel self-charging platform is proposed using colloidal-quantum-dot (CQD) photovoltaics (PVs) via the near-infrared (NIR) band for low-power electronics. Low-bandgap CQDs can convert invisible NIR light sources to electrical energy more efficiently than wider spectra because of reduced thermalization loss. This energy-conversion strategy via NIR photons ensures an enhanced photostability of the CQD devices. Furthermore, the NIR wireless charging system can be concealed using various colored and NIR-transparent fabric or films, providing aesthetic freedom. Finally, an NIR-driven wireless charging system is demonstrated for a wearable healthcare bracelet by integrating a CQD PVs receiver with a flexible lithium-ion battery and entirely embedding them into a flexible strap, enabling permanent self-charging without detachment.

Bandgap tunability and broadband absorption make quantum-dot (QD) photovoltaic cells (PVs) a promising candidate for future solar energy conversion systems. Approaches to improving the electrical properties of the active layer increase efficiency in part. The present study focuses on optical room for enhancement in QD PVs over wide spectrum in the near-infrared (NIR) region. We find that ray-optical light trapping schemes rather than the nanophotonics approach may be the best solution for enhancing broadband QD PVs by suppressing the escape probability of internal photons without spectral dependency. Based on the theoretical study of diverse schemes for various bandgaps, we apply a V-groove structure and a V-groove textured compound parabolic trapper (VCPT) to PbS-based QD PVs along with the measurement issues for PVs with a light scattering layer. The efficiency of the best device is improved from 10.3% to 11.0% (certified to 10.8%) by a V-groove structure despite the possibility of underestimation caused by light scattering in small-area devices (aperture area: 0.0625 cm2). By minimizing such underestimation, even greater enhancements of 13.6% and 15.6% in short circuit current are demonstrated for finger-type devices (0.167 cm2 without aperture) and large-area devices (2.10 cm2 with an aperture of 0.350 cm2), respectively, using VCPT.