Novel Bilayer SnO2 Electron Transport Layers with Atomic Layer Deposition for High-Performance α-FAPbI3 Perovskite Solar Cells

Orientation Manipulation and Defect Passivation for Perovskite Solar Cells by a Natural Compound

Different facets in perovskite crystals exhibit distinct atomic arrangements, influencing their electronic, physical, and chemical properties. Perovskite films incorporating tin oxide (SnO2) as the electron transport layer face challenges in facet regulation. This study reveals that tea saponin (TS), a natural compound serves as a SnO2 modifier, facilitates optimal growth of perovskite crystals on the (111) facet. The modification promotes preferential crystal orientation through hydrogen bond and Lewis coordination. TS forms a chelate with SnO2, resulting in a smoother film and n-type doping, leading to improved carrier extraction and reduced defects. The TS-modified perovskite solar cells achieve a champion efficiency of 24.2%, leveraging from an obvious enhancement of open-circuit voltage (Voc) of 1.18 V and fill factor (FF) of 82.8%. The devices also demonstrate enhanced humidity tolerance and storage stability, ensuring improved stability without encapsulation.

Surface modification of halide perovskite using EDTA-complexed SnO2 as electron transport layer in high performance solar cells

The long-term performance of metal halide perovskite solar cells (PSCs) can be significantly improved by tuning the surface characteristics of the perovskite layers. Herein, low-temperature-processed ethylenediaminetetraacetic acid (EDTA)-complexed SnO2 (E-SnO2) is successfully employed as an electron transport layer (ETL) in PSCs, enhancing the efficiency and stability of the devices. The effects of EDTA treatment on SnO2 are investigated for different concentrations: comparing the solar cells' response with 15%-2.5% SnO2 and E-SnO2 based ETLs, and it was found that 7.5% E-SnO2 provided the best results. The improved surface properties of the perovskite layer on E-SnO2 are attributed to the presence of small amount of PbI2 which contributes to passivate the defects at the grain boundaries and films' surface. However, for the excess PbI2 based devices, photocurrent dropped, which could be attributed to the generation of shallow traps due to excess PbI2. The better alignment between the Fermi level of E-SnO2 and the conduction band of perovskite is another favorable aspect that enables increased open-circuit potential (VOC), from 0.82 V to 1.015 V, yielding a stabilized power conversion efficiency of 15.51%. This complex ETL strategy presented here demonstrates the enormous potential of E-SnO2 as selective contact to enhance the perovskite layer properties and thereby allow stable and high-efficiency PSCs.

Emerging Trends in Electron Transport Layer Development for Stable and Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) stand at the forefront of photovoltaic research, with current efficiencies surpassing 26.1%. This review critically examines the role of electron transport materials (ETMs) in enhancing the performance and longevity of PSCs. It presents an integrated overview of recent advancements in ETMs, like TiO2, ZnO, SnO2, fullerenes, non-fullerene polymers, and small molecules. Critical challenges are regulated grain structure, defect passivation techniques, energy level alignment, and interfacial engineering. Furthermore, the review highlights innovative materials that promise to redefine charge transport in PSCs. A detailed comparison of state-of-the-art ETMs elucidates their effectiveness in different perovskite systems. This review endeavors to inform the strategic enhancement and development of n-type electron transport layers (ETLs), delineating a pathway toward the realization of PSCs with superior efficiency and stability for potential commercial deployment.

Improved eco-friendly CsSn0.5Ge0.5I3 perovskite photovoltaic efficiency beyond 20% with SMe-TATPyr hole-transporting layer

Perovskites composed of inorganic cesium (Cs) halide provide a route to thermally resistant solar cells. Nevertheless, the use of hole-transporting layers (HTLs) with hydrophobic additives is constrained by moisture-induced phase deterioration. Due to significant electrical loss, dopant-free HTLs are unable to produce practical solar cells. In this article, we designed a two-dimensional 1,3,6,8-tetrakis[5-(N,N-di(p-(methylthio)phenyl)amino-p-phenyl)-thiophen-2-yl]pyrene (termed SMe-TATPyr) molecule as a new HTL to regulate electrical loss in lead-free perovskite solar cells (PSCs). We optimized the power conversion efficiency (PCE) of PSCs based on mixed tin (Sn)/germanium (Ge) halide perovskite (CsSn0.5Ge0.5I3) by exploring different factors, such as the deep and shallow levels of defects, density of states at the valence band (NV), thickness of the perovskite film, p-type doping concentration (NA) of HTL, the series and shunt resistances, and so on. We carried out comparative research by employing the 1D-SCAPS (a solar cell capacitance simulator) analysis tool. Through optimization of the PSC, we obtained the highest parameters in the simulated solar cell structure of fluorine tin oxide (FTO)/titanium dioxide (TiO2)/CsSn0.5Ge0.5I3/SMe-TATPyr/gold (Au), and the PCE reached up to 20% with a fill factor (FF) of 81.89%.

Choline Derivative as a Multifunctional Interfacial Bridge through Synergistic Effects for Improving the Efficiency and Stability of Perovskite Solar Cells

The interfacial carrier non-radiative recombination caused by buried defects in electron transport layer (ETL) material and the energy barrier severely hinders further improvement in efficiency and stability of perovskite solar cells (PSCs). In this study, the effect of the SnO2 ETL doped with choline chloride (CC), acetylcholine chloride (AC), and phosphocholine chloride sodium salt (PCSS) are investigated. These dopants modify the interface between SnO2 ETL and perovskite layer, acting as a bridge through synergistic effects to form uniform ETL films, enhance the interface contact, and passivate defects. Ultimately, compared with CC (which with ─OH) and AC (which with C═O), the PCSS with P═O and sodium ions groups is more beneficial for improving performance. The device based on PCSS-doped SnO2 ETL achieves an efficiency of 23.06% with a high VOC of 1.2 V, which is considerably higher than the control device (20.55%). Moreover, after aging for 500 h at a temperature of 25 °C and relative humidity (RH) of 30-40%, the unsealed device based on SnO2 -PCSS ETL maintains 94% of its initial efficiency, while the control device only 80%. This study provides a meaningful reference for the design and selection of ideal pre-buried additive molecules.

Growth and Dispersion Control of SnO2 Nanocrystals Employing an Amino Acid Ester Hydrochloride in Solution Synthesis: Microstructures and Photovoltaic Applications

Tin oxide (SnO2) is a technologically important semiconductor with versatile applications. In particular, attention is being paid to nanostructured SnO2 materials for use as a part of the constituents in perovskite solar cells (PSCs), an emerging renewable energy technology. This is mainly because SnO2 has high electron mobility, making it favorable for use in the electron transport layer (ETL) in these devices, in which SnO2 thin films play a role in extracting electrons from the adjacent light-absorber, i.e., lead halide perovskite compounds. Investigation of SnO2 solution synthesis under diverse reaction conditions is crucial in order to lay the foundation for the cost-effective production of PSCs. This research focuses on the facile catalyst-free synthesis of single-nanometer-scale SnO2 nanocrystals employing an aromatic organic ligand (as the structure-directing agent) and Sn(IV) salt in an aqueous solution. Most notably, the use of an aromatic amino acid ester hydrochloride salt-i.e., phenylalanine methyl ester hydrochloride (denoted as L hereafter)-allowed us to obtain an aqueous precursor solution containing a higher concentration of ligand L, in addition to facilitating the growth of SnO2 nanoparticles as small as 3 nm with a narrow size distribution, which were analyzed by means of high-resolution transmission electron microscopy (HR-TEM). Moreover, the nanoparticles were proved to be crystallized and uniformly dispersed in the reaction mixture. The environmentally benign, ethanol-based SnO2 nanofluids stabilized with the capping agent L for the Sn(IV) ions were also successfully obtained and spin-coated to produce a SnO2 nanoparticle film to serve as an ETL for PSCs. Several SnO2 ETLs that were created by varying the temperature of nanoparticle synthesis were examined to gain insight into the performance of PSCs. It is thought that reaction conditions that utilize high concentrations of ligand L to control the growth and dispersion of SnO2 nanoparticles could serve as useful criteria for designing SnO2 ETLs, since hydrochloride salt L can offer significant potential as a functional compound by controlling the microstructures of individual SnO2 nanoparticles and the self-assembly process to form nanostructured SnO2 thin films.

High-Fill-Factor Perovskite Solar Cells via Pseudohalide Salt Modification of the Substrate to Mitigate Nonradiative Recombination at the Interface

The utilization of the sol-gel method for fabricating planar SnO2 as the electron transport layer (ETL) induces numerous defects on the SnO2 layer surface and perovskite film bottom, causing considerable deterioration of the device performance. Conventional inorganic salt-doped SnO2 precursor solutions used for passivation may cause incomplete substrate coverage due to the presence of inorganic salt crystals, further degrading the device performance. Here, a substrate modification approach involving the pretreatment of a fluorine-doped SnO2 (FTO) substrate with NH4PF6 is proposed. The interaction between PF6- ions and the FTO substrate enhances SnO2 film quality; excess PF6- ions decrease the number of defects on the film surface. NH4+ ions react with an -OH stabilizing agent in the SnO2 solution and are eliminated during annealing. The combined effects suppress nonradiative recombination and ion migration at the ETL-perovskite interface. The corresponding high-quality perovskite solar cells (PSCs) exhibit a fill factor of ∼0.825; PSC efficiency increases from 19.59% to 22.32%.