A highly flexible and durable transparent graphene electrode with thermal stability was developed via the direct integration of polyimide (PI) on graphene. Due to the high transparency of PI-integrated graphene electrode and intimate contact between graphene and PI substrate, high-efficiency flexible organic solar cell with a PCE of 15.2% and outstanding mechanical robustness was obtained.
Metal-based transparent conductive electrodes (TCEs) are attractive candidates for application in indium tin oxide (ITO)-free solar cells due to their excellent electrical conductivity and cost effectiveness. In perovskite solar cells (PSCs), metal-induced degradation with the perovskite layer leads to various detrimental effects, deteriorating the device performance and stability. Here, we introduce a novel flexible hybrid TCE consisting of a Cu grid-embedded polyimide film and a graphene capping layer, named GCEP, which exhibits excellent mechanical and chemical stability as well as desirable optoelectrical properties. We demonstrated the critical role of graphene as a protection layer to prevent metal-induced degradation and halide diffusion between the electrode and perovskite layer; the performance of the flexible PSCs fabricated with GCEP was comparable to that of their rigid ITO-based counterparts and also exhibited outstanding mechanical and chemical stability. This work provides an effective strategy to design mechanically and chemically robust ITO-free metal-assisted TCE platforms in PSCs.
Chemical vapor deposition (CVD) using liquid-phase precursors has emerged as a viable technique for synthesizing uniform large-area transition metal dichalcogenide (TMD) thin films. However, the liquid-phase precursor-assisted growth process typically suffers from small-sized grains and unreacted transition metal precursor remainders, resulting in lower-quality TMDs. Moreover, synthesizing large-area TMD films with a monolayer thickness is also quite challenging. Herein, we successfully synthesized high-quality large-area monolayer molybdenum diselenide (MoSe2) with good uniformity via promoter-assisted liquid-phase CVD process using the transition metal-containing precursor homogeneously modified with an alkali metal halide. The formation of a reactive transition metal oxyhalide and reduction of the energy barrier of chalcogenization by the alkali metal promoted the growth rate of the TMDs along the in-plane direction, enabling the full coverage of the monolayer MoSe2 film with negligible few-layer regions. Note that the fully selenized monolayer MoSe2 with high crystallinity exhibited superior electrical transport characteristics compared with those reported in previous works using liquid-phase precursors. We further synthesized various other monolayer TMD films, including molybdenum disulfide, tungsten disulfide, and tungsten diselenide, to demonstrate the broad applicability of the proposed approach.
exceeds 25% for the conventional structure. [5] The inverted structured PSCs also show great promises with advantages such as low-temperature processability and ionic dopant-free hole transport layer, which can contribute toward fabricating flexible devices or improving the operational stability. [6-14] Recently, Zheng et al. reported stable inverted PSCs exhibiting a PCE of 23.0%. [15] Meng et al. demonstrated a highly flexible inverted PSCs with a PCE of 19.9%. [16] Despite such rapid progresses in PSCs, the stability of perovskite still remains below commercialization standards, because perovskite is inherently unstable under ambient environmental conditions such as ultraviolet (UV) irradiation, and is particularly degraded by humidity and heat. Perovskite is hydrophilic, meaning that moistures can readily infiltrate along the grain boundaries of its crystals, triggering severe decomposition of the constituent elements and a consequent decline in device performance. [17,18] In addition, the constituent atomic species of perovskite materials are thermally decomposed at elevated temperatures, even under protective encapsulation layers. The decomposed organic and halide ions preferentially migrate along the grain boundaries of the perovskite crystals and into the adjacent charge transporting layers, or even into the metal electrodes of the solar cell. These behaviors readily lead to defect states (e.g., cations and halide vacancies) and metal halide formations, which severely degrade the device performance. [19,20] Moreover, the unfavorable volume expansion of the perovskite lattice under continuous thermal stresses accelerates the moisture diffusion, further deteriorating the quality of perovskite film. [21] Thus, improving the stability of PSCs in moist and thermally elevated environments is greatly demanded. Defects in perovskite crystals, such as the vacancies of perovskite constituent, grain boundaries, and uncoordinated Pb 2+ ions, become the source of nonradiative recombination centers, which can adversely affect the device performance of PSCs as well as the operational stability. However, pristine perovskite films are typically known to be quite vulnerable to intrinsic defect generation. [22,23] Therefore, to minimize the defect formation and thus achieve high-quality perovskite layers with large grain size and uniform surface morphology, researchers have added cations or anions, applied antisolvent treatments, and modified neighboring layers. [24,25] Chemical additives have Significant efforts have been devoted to modulating the grain size and improving the film quality of perovskite in perovskite solar cells (PSCs). Adding materials to the perovskite is especially promising for high-performance PSCs, because the additives effectively control the crystal structure. Although the additive engineering approach has substantially boosted the efficiency of PSCs, instability of the perovskite film has remained a primary bottleneck for the commercialization of PSCs. Herein, a newly conceived bithiophene-based n-...
Two-dimensional (2D) materials have been promoted as an ideal platform for surface-enhanced Raman spectroscopy (SERS), as they mitigate the drawbacks of noble metal-based SERS substrates. However, the inferior limit of detection has limited the practical applicability of 2D material-based SERS substrates. Here, we synthesize uniform large-area ReO x S y thin films via solution-phase deposition without post-treatments and demonstrate a graphene/ReO x S y vertical heterostructure as an ultrasensitive SERS platform. The electronic structure of ReO x S y can be modulated by changing the oxygen concentration in the lattice structure, obtaining efficient complementary resonance effects between ReO x S y and the probe molecule. In addition, the oxygen atoms in the ReO x S y lattice generate a dipole moment on the thin-film surface, which increases the electron transition probability. These synergistic effects outstandingly enhance the Raman effect in the ReO x S y thin film. When ReO x S y forms a vertical heterostructure on a graphene as the SERS substrate, the enhanced charge-transfer and exciton resonances improve the limit of detection to the femtomolar level, while achieving remarkable flexibility, reproducibility, and operational stability. Our results provide important insights into 2D material-based ultrasensitive SERS based on chemical mechanisms.
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