Molecularly Engineered Interfaces in Metal Halide Perovskite Solar Cells

Molecularly Engineered Multifunctional Bridging Layer Derived from Dithiafulavene Capped Spiroxanthene for Stable and Efficient Perovskite Solar Cells

This study introduces a novel approach centered around the design and synthesis of an interfacial passivating layer in perovskite solar cells (PSCs). This architectural innovation is realized through the development of a specialized material, termed dithiafulvene end-capped Spiro[fluorene-9,9'-xanthene], denoted by the acronym AF32. In this design architecture, dithiafulvene is thoughtfully attached to the spiroxanthene fluorene core with phenothiazine as the spacer unit, possessing multiple alkyl chains. AF32 passivates interfacial defects by coordinating the sulfur constituents of the phenothiazine and dithiafulvene frameworks to the uncoordinated Pb2+ cations on the surface of the perovskite film, and the alkyl chains construct a hydrophobic environment, preventing moisture from entering the hydrophilic perovskite layer and improving the long-term stability of PSCs. Furthermore, this conductive interlayer facilitates hole transport in PSCs due to its well-aligned molecular orbital levels. Such improvements translated into an enhanced power conversion efficiency (PCE) of 22.6% for the device employing 1.5 mg/mL AF32, and it maintained 85% of its initial PCE after more than 1800 h under ambient conditions [illumination and 45 ± 5% relative humidity (RH)]. This study not only marks progress in photovoltaic technology but also expands our understanding of manipulating interfacial properties for optimized device performance and stability.

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

Reducing Interfacial Losses in Solution-Processed Integrated Perovskite-Organic Solar Cells

Low bandgap organic semiconductors have been widely employed to broaden the light response range to utilize much more photons in the inverted perovskite solar cells (PSCs). However, the serious charge recombination at the heterointerface contact between perovskite and organic semiconductors usually leads to large energy loss and limits the device performance. In this work, a titanium chelate, bis(2,4-pentanedionato) titanium(IV) oxide (C10H14O5Ti), was directly used as an interlayer between the perovskite layer and organic bulk heterojunction layer for the first time. Impressively, it was found that C10H14O5Ti can not only increase the surface potential of perovskite films but also show a positive passivation effect toward the perovskite film surface. Drawing from the above function, a smoother perovskite active layer with a higher work function was realized upon the use of C10H14O5Ti. As a result, the C10H14O5Ti-modified integrated devices show lower interfacial loss and obtain the best power conversion efficiency (PCE) of up to 20.91% with a high voltage of 1.15 V. The research offers a promising strategy to minimize the interfacial loss for the preparation of high-performance perovskite solar cells.

Dopant-Free Bithiophene-Imide-Based Polymeric Hole-Transporting Materials for Efficient and Stable Perovskite Solar Cells

The development of hole-transport materials (HTMs) with high mobility, long-term stability, and comprehensive passivation is significant for simultaneously improving the efficiency and stability of perovskite solar cells (PVSCs). Herein, two donor-acceptor (D-A) conjugated polymers PBTI and PFBTI with alternating benzodithiophene (BDT) and bithiophene imide (BTI) units are successfully developed with desirable hole mobilities due to the good planarity and extended conjugation of molecular backbone. Both copolymers can be employed as HTMs with suitable energy levels and efficient defect passivation. Shortening the alkyl chain of the BTI unit and introducing fluorine atoms on the BDT moiety effectively enhances hole mobility and hydrophobicity of the HTMs, leading to improved efficiency and stability of PVSCs. As a result, the organic-inorganic hybrid PVSCs with PFBTI as the HTM deliver a power conversion efficiency (PCE) of 23.1% with enhanced long-term operational and ambient stability, which is one of the best efficiencies reported for PVSCs with dopant-free polymeric HTMs to date. Moreover, PFBTI can be applied in inorganic PVSCs and perovskite/organic tandem solar cells, achieving a high PCE of 17.4% and 22.2%, respectively. These results illustrate the great potential of PFBTI as an efficient and widely applicable HTM for cost-effective and stable PVSCs.

Molecularly Functionalized SnO2 Films by Carboxylic Acids for High-Performance Perovskite Solar Cells

Metal oxides are commonly employed as electron transport layers (ETLs) for n-i-p perovskite solar cells (PSCs), but the presence of surface traps and their mismatched energy alignment with perovskites limits the corresponding device performance. Therefore, the interfacial modification of ETLs by functional molecules becomes an important strategy for tailoring the interfacial properties and facilitating an efficient charge extraction and transport in PSCs. However, an in-depth understanding of the influences of their molecular structures on the surface chemistry and electronic properties of ETLs is rarely discussed. Herein, three carboxylic acid-based molecules with different chemical structures were employed to modify the SnO₂ ETL and their effects on the performance of PSCs were systematically investigated. We found that the alkyl-chain length and carboxyl number in molecular structures can dramatically alter their binding strength to SnO₂, providing a good strategy to fine-tune their film quality, electron mobility, and energy offset at the cathode interface. Benefiting from the optimal coordination ability of citric acid (CA) to SnO₂, the corresponding PSCs show better charge transport properties and suppressed nonradiative recombination, leading to a champion efficiency of 23.1% with much improved environmental stability, highlighting the potential of rational design of molecular modifiers for high-performance ETLs applied in PSCs.

Atomistic Mechanism of Surface-Defect Passivation: Toward Stable and Efficient Perovskite Solar Cells

Molecular engineering has been demonstrated to be a predominant strategy for augmenting the long-term stability and passivating adverse defects for perovskite solar cells (PSCs). Here, using density functional theory calculations combined with ab initio molecular dynamics (AIMD) simulations, the passivation effects of bidentate passivation molecules, 2-MP and 2-MDEP, on the iodine vacancy MAPbI₃ were comprehensively investigated. We demonstrate that 2-MDEP engenders stronger adsorption and localized charges on Pb atoms because the separated binding sites match with the MAPbI₃ lattice. Moreover, the activation barriers for ion migrations are improved by the passivation of 2-MP and 2-MDEP. Furthermore, AIMD simulations verify the improved structural stability and restrained nonradiative recombination after passivation. More importantly, the durable Pb-heteroatom interactions at the interface and stronger hydrophobicity endow 2-MDEP with more remarkable shielding effects against moisture compared to those of 2-MP. This work deepens our understanding of the passivation effects and paves the way for the design of passivation molecules toward the attainment of efficient and stable PSCs.

Enhancing the Performance of Quasi-2D Perovskite Light-Emitting Diodes Using Natural Cyclic Molecules with Distinct Phase Regulation Behaviors

In this study, two natural small molecules, α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD), are used as additives to improve the performance of quasi-2D PEA₂Cs_n-1Pb_nBr_3n+1 (n = 3, herein) PeLEDs. Both of them are shown to efficiently passivate the quasi-2D perovskite films to afford improved film quality and morphology, but they exhibit distinct phase regulation behaviors possibly due to their different pore sizes. It reveals that α-CD effectively suppresses the formation of the low-n phases (n ≤ 2), while β-CD better regulates the phase with a medium-n value (n = 3). Because of effectively suppressing the formation of low-n phases, the CD-assisted quasi-2D perovskite films possess facilitated exciton energy transfer and reduced nonradiative recombination. Consequently, the optimized α-CD-derived PeLED shows the highest luminance (L_max) of 37,825 cd/m² with an external quantum efficiency (EQE) of 3.81%, while the β-CD-derived PeLED delivers a lower L_max of 24,793 cd/m² with an EQE of 3.09%. Compared to the pristine device, L_max is enhanced by 6.3 and 3.8 times for α-CD- and β-CD-based PeLEDs, respectively, and EQE is enhanced by ∼4.8 times for both devices; besides, both CD-assisted devices also exhibit improved color purity and a lower bias dependency of electroluminescent intensity.

Interfaces and Interfacial Layers in Inorganic Perovskite Solar Cells

Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency (PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic-inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non-radiative recombination caused by traps and energy-level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.