Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by Embedding Molecular Dipoles within Their Hole-Blocking Layer

Impact of Dipole Effect on Perovskite Solar Cells

Interface modification and bulk doping are two major strategies to improve the photovoltaic performance of perovskite solar cells (PSCs). Dipolar molecules are highly favored due to their unique dipolarity. This review discusses the basic concepts and characteristics of dipoles. In addition, the role of dipoles in PSCs and the corresponding conventional characterization methods for dipoles are introduced. Then, we systematically summarize the latest progress in achieving efficient and stable PSCs in dipole materials at several key interfaces. Finally, we look forward to the future application directions of dipole molecules in PSCs, aiming at providing deep insight and inspiration for developing efficient and stable PSCs.

Synthesizing and formulating metal oxide nanoparticle inks for perovskite solar cells

The perovskite solar cell has commercial potential due to the low-cost of materials and manufacturing processes with cell efficiencies on par with traditional technologies. Nanomaterials have many properties that make them attractive for the perovskite devices, including low-cost inks, low temperature processing, stable material properties and good charge transport. In this feature article, the use of nanomaterials in the hole transport and electron transport layers are reviewed. Specifically, SnO2 and NiOx are the leading materials with the most promise for translation to large scale applications. The review includes a discussion of the synthesis, formulation, and processing of these nanoparticles and provides insights for their further deployment towards commercially viable perovskite solar cells.

Solvent-antisolvent interactions in metal halide perovskites

The fabrication of metal halide perovskite films using the solvent-engineering method is increasingly common. In this method, the crystallisation of the perovskite layer is triggered by the application of an antisolvent during the spin-coating of a perovskite precursor solution. Herein, we introduce the current state of understanding of the processes involved in the crystallisation of perovskite layers formed by solvent engineering, focusing in particular on the role of antisolvent properties and solvent-antisolvent interactions. By considering the impact of the Hansen solubility parameters, we propose guidelines for selecting the appropriate antisolvent and outline open questions and future research directions for the fabrication of perovskite films by this method.

Recent Progress in Interfacial Dipole Engineering for Perovskite Solar Cells

Design and modification of interfaces have been the main strategies in developing perovskite solar cells (PSCs). Among the interfacial treatments, dipole molecules have emerged as a practical approach to improve the efficiency and stability of PSCs due to their unique and versatile abilities to control the interfacial properties. Despite extensive applications in conventional semiconductors, working principles and design of interfacial dipoles in the performance/stability enhancement of PSCs are lacking an insightful elucidation. In this review, we first discuss the fundamental properties of electric dipoles and the specific roles of interfacial dipoles in PSCs. Then we systematically summarize the recent progress of dipole materials in several key interfaces to achieve efficient and stable PSCs. In addition to such discussions, we also dive into reliable analytical techniques to support the characterization of interfacial dipoles in PSCs. Finally, we highlight future directions and potential avenues for research in the development of dipolar materials through tailored molecular designs. Our review sheds light on the importance of continued efforts in this exciting emerging field, which holds great potential for the development of high-performance and stable PSCs as commercially demanded.

How Do Surface Polar Molecules Contribute to High Open-Circuit Voltage in Perovskite Solar Cells?

To date, the improvement of open-circuit voltage (V_OC ) offers a breakthrough for the performance of perovskite solar cells (PSCs) toward their theoretical limit. Surface modification through organic ammonium halide salts (e.g., phenethylammonium ions PEA⁺ and phenmethylammonium ions PMA⁺ ) is one of the most straightforward strategies to suppress defect density, thereby leading to improved V_OC . However, the mechanism underlying the high voltage remains unclear. Here, polar molecular PMA⁺ is applied at the interface between perovskite and hole transporting layer and a remarkably high V_OC of 1.175 V is obtained which corresponds to an increase of over 100 mV in comparison to the control device. It is revealed that the equivalent passivation effect of surface dipole effectively improves the splitting of the hole quasi-Fermi level. Ultimately the combined effect of defect suppression and surface dipole equivalent passivation effect leads to an overall increase in significantly enhanced V_OC . The resulted PSCs device reaches an efficiency of up to 24.10%. Contributions are identified here by the surface polar molecules to the high V_OC in PSCs. A fundamental mechanism is suggested by use of polar molecules which enables further high voltage, leading ways to highly efficient perovskite-based solar cells.

Persistent Ion Accumulation at Interfaces Improves the Performance of Perovskite Solar Cells

The mixed ionic-electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts-such as hysteresis or device degradation-leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼10²¹ cm^-3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

Construction of Charge Transport Channels at the NiOx/Perovskite Interface through Moderate Dipoles toward Highly Efficient Inverted Solar Cells

NiO_x-based perovskite solar cells (PSCs) have attracted much attention because of their low fabrication temperature, suppressed hysteresis, and superior stability. However, the poor interfacial contacts between NiO_x and perovskite layers always limit the progress of PSCs. Here, we applied 2-thiophenemethylamine (TPMA) as charge transport channels at the interface between NiO_x and perovskite layers. The introduction of TPMA provides moderate dipole moment pointing to the perovskite side and effectively promotes the charge transportation. Meanwhile, TPMA anchorage also passivates the defect states at the surfaces of both NiO_x and MAPbI₃, which compensates the voltage loss due to the change in NiO_x work function induced by the dipole. Thus, the device performance has been significantly enhanced in both electrochemical properties and power conversion efficiency. Our work has demonstrated a new way of improving current and voltage in the NiO_x-based PSCs simultaneously through a moderate interfacial dipole moment toward highly efficient PSCs.

An Effective Strategy of Combining Surface Passivation and Secondary Grain Growth for Highly Efficient and Stable Perovskite Solar Cells

Interfacial engineering methods have been developed to solve defect issues of perovskite solar cells (PSCs). However, traditional surface passivation has limited effects on eliminating defect-forming residuals, while secondary grain growth (SGG) is restricted by limited choices of additives and intrinsic properties of perovskites. Here, a pincer strategy of taking advantages of surface passivation and SGG is proposed to modify both exterior and interior of CH₃ NH₃ PbI₃ (MAPbI₃ ) perovskite, by employing cyanoacetate-containing donor-acceptor compounds (CA-D-A) including 2-cyano-3-(3,4,5-trimethoxyphenyl)acrylic acid (CA), methanaminium 2-cyano-3-(3,4,5-trimethoxyphenyl)acrylate (CAMA), and aminomethaniminium (Z)-2-cyano-3-(3,4,5-trimethoxyphenyl)acrylate (CAFA). In comparison to untreated perovskite, CA-D-A treated perovskites present better crystallinity because of SGG, lower trap densities due to the synergistic effect of surface passivation and SGG, and tuned energy levels induced by CA-D-A. Accordingly, the CA-D-A treated MAPbI₃ -based PSCs exhibit higher open-circuit voltage and fill factor than the control PSC without any treatment, leading to improved power conversion efficiency (PCE) and enhanced device stability, especially the CAMA treated PSCs with an average PCE promoted from 17.77 (control PSCs) to 18.71%, and importantly an excellent PCE of 19.71% through further optimization. This work provides an effective strategy for developing highly efficient and stable PSCs with the assistance of both surface passivation and SGG.

Enhanced Photovoltaic Properties of Perovskite Solar Cells by Employing Bathocuproine/Hydrophobic Polymer Films as Hole-Blocking/Electron-Transporting Interfacial Layers

In this study, we improved the photovoltaic (PV) properties and storage stabilities of inverted perovskite solar cells (PVSCs) based on methylammonium lead iodide (MAPbI3) by employing bathocuproine (BCP)/poly(methyl methacrylate) (PMMA) and BCP/polyvinylpyrrolidone (PVP) as hole-blocking and electron-transporting interfacial layers. The architecture of the PVSCs was indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/MAPbI3/[6,6]-phenyl-C61-butyric acid methyl ester/BCP based interfacial layer/Ag. The presence of PMMA and PVP affected the morphological stability of the BCP and MAPbI3 layers. The storage-stability of the BCP/PMMA-based PVSCs was enhanced significantly relative to that of the corresponding unmodified BCP-based PVSC. Moreover, the PV performance of the BCP/PVP-based PVSCs was enhanced when compared with that of the unmodified BCP-based PVSC. Thus, incorporating hydrophobic polymers into BCP-based hole-blocking/electron-transporting interfacial layers can improve the PV performance and storage stability of PVSCs.