Postdeposition Halide Exchange for Achieving Deep-Blue Perovskite Light-Emitting Diodes: The Role of the Organic Cations in the Chloride Source.

Postdeposition halide exchange has been a popular strategy for tuning the emission wavelength of metal halide perovskites and is particularly attractive in achieving deep-blue perovskite light-emitting diodes (PeLEDs), where the quality of the emissive layer is largely limited by the low solubility of chlorides in perovskite precursor solution. In this work, the halide exchange strategy is examined for deep-blue PeLEDs, with a focus on understanding the role of the organic cations of the halide salt (i.e., the chloride source for ion exchange) in modifying the properties of the perovskite films and consequently the PeLED performances. By comparatively investigating the treatment effects of two model systems, namely phenethylammonium chloride and 2,2-diphenylethylammonium chloride (DPEACl), it is found that although the two chlorides produce highly similar photoluminescence properties of the perovskite films, they create different landscapes for current flow in the PeLEDs. In particular, the bulky branch-structured DPEA cations exhibit minimal disturbance to the perovskite grains while providing highly effective inter-grain void filling and thus leakage current blocking, leading to 3D perovskite-based PeLEDs with a record high peak external quantum efficiency of 6.4% at 462 nm. The study highlights the importance of organic cation selection in the halide exchange processes for PeLEDs.

Addressing gain-bandwidth trade-off by a monolithically integrated photovoltaic transistor.

The gain-bandwidth trade-off limits the development of high-performance photodetectors; i.e., the mutual restraint between the response speed and gain has intrinsically limited performance optimization of photomultiplication phototransistors and photodiodes. Here, we show that a monolithically integrated photovoltaic transistor can solve this dilemma. In this structure, the photovoltage generated by the superimposed perovskite solar cell, acting as a float gate, is amplified by the underlying metal oxide field-effect transistor. By eliminating deep-trap defects through processing optimization, we achieved devices with a maximum responsivity close to 6 × 104 A/W, a specific detectivity (D*) of 1.06 × 1013 Jones, and an f3dB of 1.2 MHz at a low driving voltage of 3 V. As a result, a record gain-bandwidth product is achieved. The device further exhibits the advantage in photoplethysmography detection with weak illuminations, where our device accurately detects the detailed features that are out of the capability of conventional photodetectors.

Anisotropic Charge Transport Enabling High-Throughput and High-Aspect-Ratio Wet Etching of Silicon Carbide.

Wet etching of silicon carbide typically exhibits poor etching efficiency and low aspect ratio. In this study, an etching structure that exploits anisotropic charge carrier flow to enable high-throughput, external-bias-free wet etching of high-aspect-ratio SiC micro/nano-structures is demonstrated. Specifically, by applying a catalytic metal coating at the bottom surface of a SiC wafer while introducing patterned ultraviolet light illumination from its top surface, spatial charge separation across the wafer is achieved, i.e., photogenerated electrons are channeled to the bottom to participate in the reduction reaction of an oxidant in the etchant solution, while holes flow to the top to trigger oxidation of SiC and subsequent etching. Such design largely suppresses recombination-induced charge losses, and when used in combination with a top metal catalyst mask, the structure yields a remarkable vertical etching rate of 0.737 µm min-1 and an aspect ratio of 3.2, setting new records for wet-etching methods for SiC.

Efficient Slantwise Aligned Dion-Jacobson Phase Perovskite Solar Cells Based on Trans-1,4-Cyclohexanediamine.

The crystalline orientation and phase distribution are two important parameters for high-performance 2D perovskite solar cells. Therefore, it is essential to understand how the structure of spacer ligands influences the orientation and phase distribution of resulting 2D perovskite films. In this work, a new member of Dion-Jacobson (DJ) phase 2D perovskites based on trans-1,4-cyclohexanediamine (CHDA) is demonstrated and it is found that the crystalline orientation is self-aligned spontaneously, which is different from the well-known graded distribution in controlled sample with its isomer 1,6-diaminohexane (HDA) as spacer ligand. Grazing incident X-ray scattering suggests that the exact alignment is strongly slantwise to the substrate while it is still beneficial for charge transfer along the vertical structure of devices. The devices can achieve high efficiency up to 15.01% for (CHDA)MA3 Pb4 I13 (n = 4), one of the highest efficiencies reported by now. The encapsulated (CHDA)MA3 Pb4 I13 (n = 4) devices can retain 80.7% efficiency for 270 min under continuous maximum power point tracking. (CHDA)MA3 Pb4 I13 (n = 4) devices can retain 96.5% efficiency under 60 °C and 74.4% efficiency under 70 °C heating for 68 h. The results demonstrate the slantwise aligned DJ phase perovskite solar cells with excellent stability.

Identifying the functional groups effect on passivating perovskite solar cells.

Many organic molecules with various functional groups have been used to passivate the perovskite surface for improving the efficiency and stability of perovskite solar cell (PSCs). However, the intrinsic attributes of the passivation effect based on different chemical bonds are rarely studied. Here, we comparatively investigate the passivation effect among 12 types of functional groups on para-tert-butylbenzene for PSCs and find that the open circuit voltage (VOC) tends to increase with the chemical bonding strength between perovskite and these passivation additive molecules. Particularly, the para-tert-butylbenzoic acid (tB-COOH), with the extra intermolecular hydrogen bonding, can stabilize the surface passivation of perovskite films exceptionally well through formation of a crystalline interlayer with water-insoluble property and high melting point. As a result, the tB-COOH device achieves a champion power conversion efficiency (PCE) of 21.46%. More importantly, such devices, which were stored in ambient air with a relative humidity of ≃45%, can retain 88% of their initial performance after a testing period of more than 1 year (10,080 h). This work provides a case study to understand chemical bonding effects on passivation of perovskite.

Perovskite Bifunctional Device with Improved Electroluminescent and Photovoltaic Performance through Interfacial Energy-Band Engineering.

Currently, photovoltaic/electroluminescent (PV/EL) perovskite bifunctional devices (PBDs) exhibit poor performance due to defects and interfacial misalignment of the energy band. Interfacial energy-band engineering between the perovskite and hole-transport layer (HTL) is introduced to reduce energy loss, through adding corrosion-free 3,3'-(2,7-dibromo-9H-fluorene-9,9-diyl) bis(n,n-dimethylpropan-1-amine) (FN-Br) into a HTL free of lithium salt. This strategy can turn the n-type surface of perovskite into p-type and thus correct the misalignment to form a well-defined N-I-P heterojunction. The tailored PBD achieves a high PV efficiency of up to 21.54% (certified 20.24%) and 4.3% EL external quantum efficiency. Free of destructive additives, the unencapsulated devices maintain >92% of their initial PV performance for 500 h at maximum power point under standard air mass 1.5G illumination. This strategy can serve as a general guideline to enhance PV and EL performance of perovskite devices while ensuring excellent stability.

Bulk Heterojunction Quasi-Two-Dimensional Perovskite Solar Cell with 1.18 V High Photovoltage.

Multicomponent quasi-two-dimensional perovskites (Q-2DPs) have efficient luminescence and improved stability, which are highly desirable for light-emitting diode and perovskite solar cell (PSC). However, the lack of radiative recombination at room temperature is still not well understood and the performance of PSC is not good enough as well. The open-circuit voltage ( VOC) is even lower than that of three-dimensional (3D) PSC with a narrower band gap. In this work, we study the energy transfer of excitons between their multiple components by time-resolved photoluminescence and find that charge transfer from high-energy states to low-energy state is gradually suppressed during elevating temperature owing to trap-mediated recombination. This may reveal the bottleneck of luminescence at room temperature in Q-2DPs, leading to large photovoltage loss in 2D PSC. Therefore, we develop a p-i-n bulk heterojunction (BHJ) structure to reduce the nonradiative recombination and obtain high VOC of 1.18 V for (PMA)2MA4Pb5I15Cl (33.3% PMA) BHJ device, much higher than that of the planar counterparts. The enhanced efficiency is attributed to the improved exciton dissociation via BHJ interface. Our results provide an important step toward high VOC and stable 2D PSCs, which could be used for tandem solar cell and colorful photovoltaic windows.

General Nondestructive Passivation by 4-Fluoroaniline for Perovskite Solar Cells with Improved Performance and Stability.

Hybrid perovskite thin films are prone to producing surface vacancies during the film formation, which degrade the stability and photovoltaic performance. Passivation via post-treatment can heal these defects, but present methods are slightly destructive to the bulk of 3D perovskite due to the solvent effect, which hinders fabrication reproducibility. Herein, nondestructive surface/interface passivation using 4-fluoroaniline (FAL) is established. FAL is not only an effective antisolvent candidate for surface modification, but also a large dipole molecule (2.84 Debye) with directional field for charge separation. Density functional theory calculation reveals that the nondestructive properties are attributed to both the conjugated amine in aromatic ring and the para-fluoro-substituent. A hot vapor assisted colloidal process is employed for the post-treatment. The molecular passivation yields an ultrathin protection layer with a hydrophobic fluoro-substituent tail and thus enhances the stability and optoelectronic properties. FAL post-treated perovskite solar cell (PSC) delivers a 20.48% power conversion efficiency under ambient conditions. Micro-photoluminescence reveals that passivation activates the dark defective state at the surface and interface, delivering the impact picture of boundary on the local carriers. This work demonstrates a generic nondestructive chemical approach for improving the performance and stability of PSCs.