Efficient Perovskite Solar Cells Based on a Solution Processable Nickel(II) Phthalocyanine and Vanadium Oxide Integrated Hole Transport Layer

An organic–inorganic integrated hole transport layer (HTL) composed of the solution‐processable nickel phthalocyanine (NiPc) abbreviated NiPc‐(OBu)₈ and vanadium(V) oxide (V₂O₅) is successfully incorporated into structured mesoporous perovskite solar cells (PSCs). The optimized PSCs show the highest stabilized power conversion efficiency of up to 16.8% and good stability under dark ambient conditions. These results highlight the potential application of organic–inorganic integrated HTLs in PSCs.     

Solution Processable Inorganic–Organic Double‐Layered Hole Transport Layer for Highly Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have reached their highest efficiency with 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). However, this material can cause problems with respect to reproducibility and stability. Herein, a solution‐processable inorganic–organic double layer based on tungsten oxide (WO₃) and spiro‐OMeTAD is reported as a hole transport layer in PSCs. The device equipped with a WO₃/spiro‐OMeTAD layer achieves the highest efficiency (21.44%) in the tin (IV) oxide planar structure. The electronic properties of the double layer are thoroughly analyzed using photoluminescence, space‐charge–limited current, and electrochemical impedance spectroscopy. The WO₃/spiro‐OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO₃ layer particularly improves the open‐circuit voltage (V_OC) by lowering the quasi‐Fermi energy level for holes and reducing charge recombination, resulting in high V_OC (1.17 V in the champion cell). In addition, the WO₃ layer as a scaffold layer promotes the formation of a uniform and pinhole‐free spiro‐OMeTAD overlayer in the WO₃/spiro‐OMeTAD layer. High stability under thermal and humid conditions stems from this property. The study presents a facile approach for improving the efficiency and stability of PSCs by stacking an organic layer on an inorganic layer.     

Efficient and Hysteresis‐Free Perovskite Solar Cells Based on a Solution Processable Polar Fullerene Electron Transport Layer

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl‐functionalized carbon buckyballs, C60 pyrrolidine tris‐acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide‐based ETLs in efficient and stable n‐i‐p‐structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA‐modified PSCs. Moreover, solution‐processable CPTA‐based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum‐performing device structures composed of ITO/CPTA/CH₃NH₃PbI₃/2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution‐processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications.     

Nonaromatic Green‐Solvent‐Processable,Dopant‐Free,and Lead‐Capturable Hole Transport Polymers in Perovskite Solar Cells with High Efficiency

With the recent developments in the efficiency of perovskite solar cells (PSCs), diverse functionalities are necessary for next‐generation charge‐transport layers. Specifically, the hole‐transport layer (HTL) in the various synthesized materials modified with functional groups is explored. A novel donor–acceptor type polymer, alkoxy‐PTEG, composed of benzo[1,2‐b:4,5:b′]dithiophene and tetraethylene glycol (TEG)‐substituted 2,1,3‐benzothiadiazole is reported. The alkoxy‐PTEG exhibits high solubility even in nonaromatic solvents, such as 3‐methylcyclohexanone (3‐MC), and can prevent possible lead leakage via chelation. The optical and electronic properties of alkoxy‐PTEG are thoroughly analyzed. Finally, a dopant‐free alkoxy‐PTEG device processed with 3‐MC exhibits 19.9% efficiency and a device with 2‐methyl anisole, which is a reported aromatic food additive, exhibits 21.2% efficiency in a tin oxide planar structure. The PSC device shows 88% stability after 30 d at ambient conditions (40–50% relative humidity and room temperature). In addition, nuclear magnetic resonance reveals that TEG groups can chelate lead ions with moderate strength (K_binding = 2.76), and this strength is considered to be nondestructive to the perovskite lattice to prevent lead leakage. This is the first report to consider lead leakage and provide solutions to reduce this problem.     

High‐Performance and Stable Perovskite Solar Cells Based on Dopant‐Free Arylamine‐Substituted Copper(II) Phthalocyanine Hole‐Transporting Materials

A power conversion efficiency (PCE) as high as 19.7% is achieved using a novel, low‐cost, dopant‐free hole transport material (HTM) in mixed‐ion solution‐processed perovskite solar cells (PSCs). Following a rational molecular design strategy, arylamine‐substituted copper(II) phthalocyanine (CuPc) derivatives are selected as HTMs, reaching the highest PCE ever reported for PSCs employing dopant‐free HTMs. The intrinsic thermal and chemical properties of dopant‐free CuPcs result in PSCs with a long‐term stability outperforming that of the benchmark doped 2,2′,7,7′‐Tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐Spirobifluorene (Spiro‐OMeTAD)‐based devices. The combination of molecular modeling, synthesis, and full experimental characterization sheds light on the nanostructure and molecular aggregation of arylamine‐substituted CuPc compounds, providing a link between molecular structure and device properties. These results reveal the potential of engineering CuPc derivatives as dopant‐free HTMs to fabricate cost‐effective and highly efficient PSCs with long‐term stability, and pave the way to their commercial‐scale manufacturing. More generally, this case demonstrates how an integrated approach based on rational design and computational modeling can guide and anticipate the synthesis of new classes of materials to achieve specific functions in complex device structures.     

Cesium Doped NiOx as an Efficient Hole Extraction Layer for Inverted Planar Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells have resulted in tremendous interest in developing next generation photovoltaics due to high record efficiency exceeding 22%. For inverted structure perovskite solar cells, the hole extraction layers play a significant role in achieving efficient and stable perovskite solar cell by modifying charge extraction, interfacial recombination losses, and band alignment. Here, cesium doped NiO_x is selected as a hole extraction layer to study the impact of Cs dopant on the optoelectronic properties of NiO_x and the photovoltaic performance. Cs doped NiO_x films are prepared by a simple solution‐based method. Both doped and undoped NiO_x films are smooth and highly transparent, while the Cs doped NiO_x exhibits better electron conductivity and higher work function. Therefore, Cs doping results in a significant improvement in the performance of NiO_x‐based inverted planar perovskite solar cells. The best efficiency of Cs doped NiO_x devices is 19.35%, and those devices show high stability as well. The improved efficiency in devices with Cs:NiO_x is attributed to a significant improvement in the hole extraction and better band alignment compared to undoped NiO_x. This work reveals that Cs doped NiO_x is very promising hole extraction material for high and stable inverted perovskite solar cells.     

Strategic Synthesis of Ultrasmall NiCo2O4 NPs as Hole Transport Layer for Highly Efficient Perovskite Solar Cells

This study proposes a novel strategy of controllable deamination of Co–NH₃ complexes in a system containing Ni(OH)₂ to synthesize ultrasmall ternary oxide nanoparticles (NPs), NiCo₂O₄. Through this approach, ultrasmall (5 nm on average) and well‐dispersed NiCo₂O₄ NPs without exotic ligands are obtained, which enables the formation of uniform and pin‐hole free films. The tightly covered NiCo₂O₄ films also facilitate the formation of large perovskite grains and thus reduce film defects. The results show that with the NiCo₂O₄ NPs as the hole transport layer (HTL), the perovskite solar cells reach a high power conversion efficiency (PCE) of 18.23% and a promising stability (maintained ≈90% PCE after 500 h light soaking). To the best of the author's knowledge, it is the first time that spinel NiCo₂O₄ NPs have been applied as hole transport layer in perovskite solar cells successfully. This work not only demonstrates the potential applications of ternary oxide NiCo₂O₄ as HTLs in hybrid perovskite solar cells but also provides an insight into the design and synthesis of ultrasmall and ligand‐free NPs HTLs to enable cost‐effective photovoltaic devices.     

Molecular Engineering of Copper Phthalocyanines: A Strategy in Developing Dopant‐Free Hole‐Transporting Materials for Efficient and Ambient‐Stable Perovskite Solar Cells

Copper (II) phthalocyanines (CuPcs) have attracted growing interest as promising hole‐transporting materials (HTMs) in perovskite solar cells (PSCs) due to their low‐cost and excellent stability. However, the most efficient PSCs using CuPc‐based HTMs reported thus far still rely on hygroscopic p‐type dopants, which notoriously deteriorate device stability. Herein, two new CuPc derivatives are designed, namely CuPc‐Bu and CuPc‐OBu, by molecular engineering of the non‐peripheral substituents of the Pc rings, and applied as dopant‐free HTMs in PSCs. Remarkably, a small structural change from butyl groups to butoxy groups in the substituents of the Pc rings significantly influences the molecular ordering and effectively improves the hole mobility and solar cell performance. As a consequence, PSCs based on dopant‐free CuPc‐OBu as HTMs deliver an impressive power conversion efficiency (PCE) of up to 17.6% under one sun illumination, which is considerably higher than that of devices with CuPc‐Bu (14.3%). Moreover, PSCs containing dopant‐free CuPc‐OBu HTMs show a markedly improved ambient stability when stored without encapsulation under ambient conditions with a relative humidity of 85% compared to devices containing doped Spiro‐OMeTAD. This work thus provides a fundamental strategy for the future design of cost‐effective and stable HTMs for PSCs and other optoelectronic devices.