Unveiling and Modulating the Interfacial Reaction at the Metal-Hole Conductor Heterojunction toward Reliable Perovskite Solar Cells.

Interfaces between functional layers in perovskite solar cells (PSCs) are of paramount importance in determining their efficiency and stability, but the interaction and stability of metal-hole conductor (HC) interfaces have received less attention. Here, we discover an intriguing transient behavior in devices which induces a profound efficiency fluctuation from 9 to 20% during the initial performance testing. Air exposure (e.g., oxygen and moisture) can significantly accelerate this nonequilibrium process and simultaneously enhance the device maximal efficiency. Structural analysis reveals that the chemical reaction between Ag and HC occurred during the metal deposition by thermal evaporation, leading to the formation of an insulating barrier layer at their interfaces, which results in a high charge-transport barrier and poor device performance. Accordingly, we propose a metal diffusion-associated barrier evolution mechanism to understand the metal/HC interfaces. To mitigate these detrimental effects, we strategically develop an interlayer strategy by introducing an ultrathin layer of molybdenum oxide (MoO3) between Ag and HC, which is found to effectively suppress the interfacial reaction, yielding highly reliable PSCs with instant high efficiency. This work provides new insights into understanding the metal-organic interfaces, and the developed interlayer strategy can be generally applicable to engineer other interfaces in realizing efficient and stable contacts.

Thermally Crosslinked Hole Conductor Enables Stable Inverted Perovskite Solar Cells with 23.9% Efficiency.

Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) represents the state-of-the-art hole transport material (HTM) in inverted perovskite solar cells (PSCs). However, unsatisfied surface properties of PTAA and high energy disorder in the bulk film hinder the further enhancement of device performance. Herein, a simple small molecule 10-(4-(3,6-dimethoxy-9H-carbazol-9-yl)phenyl)-3,7-bis(4-vinylphenyl)-10H-phenoxazine (MCz-VPOZ) is strategically developed for in situ fabrication of polymer hole conductor (CL-MCz) via a facile and low-temperature cross-linking technology. The resulting polymer CL-MCz offers high energy ordering and improved electrical conductivity, as well as appropriate energy-level alignment, enabling efficient charge carrier collection in the devices. Meanwhile, CL-MCz synchronously provides satisfied surface wettability and interfacial functionalization, facilitating the formation of high-quality perovskite films with fewer bulk iodine vacancies and suppressed carrier recombination. Significantly, the device with CL-MCz yields a champion efficiency of 23.9% along with an extremely low energy loss down to 0.41 eV, which represents the highest reported efficiency for non-PTAA-based polymer HTMs in inverted PSCs. Furthermore, the corresponding unencapsulated devices exhibit competitive shelf-life stability under various operational stressors up to 2500 h, reflecting high promises of CL-MCz in the scalable PSC application. This work underscores the promising potential of the cross-linking approach in preparing low-cost, stable, and efficient polymer HTMs toward reliable PSCs.

Ambient Spray Coating of Organic-Inorganic Composite Thin Films for Perovskite Solar Cell Encapsulation.

Perovskite solar cells (PSCs) are developing rapidly in recent years, showing remarkable power conversion efficiency (PCE) of 25 %, which is comparable to crystalline silicon solar cells. However, since perovskite and other functional layers are very sensitive to the environment with high humidity, illumination, and heat, PSCs meet great challenges in device stability, which significantly limit their industrialization and commercialization. Encapsulation has become an effective strategy to enhance the stability of PSCs, and various encapsulation techniques have been developed, such as atomic layer deposition and glass-glass technology. Most of the current encapsulating methods are either time-consuming and sophisticated processes, or exhibit rigid configuration, which is unsuitable for flexible and curved devices. Here, an ambient spray coating method was developed to fabricate organic-inorganic composite film for direct encapsulation of PSCs. By systematical optimization of the film composition, thickness, and microstructures, a superhydrophobic encapsulating thin film with high compactness and homogeneity was achieved. As a result, the hybrid encapsulating film with polystyrene (PS)-4033/PS-4033-SiO2 significantly improved the stability of PSCs in humid environment (60-70 % relative humidity, 35 °C) by showing about 10 times longer lifetime than that of the unencapsulated devices, which was mainly attributed to complementary effects from the high compactness of PS and high hydrophobicity of SiO2 . This work suggests that direct deposition of organic-inorganic composite on devices as encapsulating films is an efficient strategy to enhance the device stability, and this method shows great promises of application in flexible and large-area devices.