A Low-Cost and Lithium-Free Hole Transport Layer for Efficient and Stable Normal Perovskite Solar Cells

The most widely used material as a hole-transport layer (HTL) for effective normal perovskite solar cells (PSCs) is still 2,2′,7,7′-Tetrakis[N, N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD), which requires heavy doping with the hydroscopic Lithium bis(trifluoromethanesulfonyl)imide (Li-ΤFSI). However, the long-term stability and performance of PCSs are frequently hampered by the residual insoluble dopants in the HTL, Li+ diffusion throughout the device, dopant by-products, and the hygroscopic nature of Li-TFSI. Due to the high cost of Spiro-OMeTAD, alternative efficient low-cost HTLs, such as octakis(4-methoxyphenyl)spiro[fluorene-9,9′-xanthene]-2,2′,7,7′-tetraamine) (X60), have attracted attention. However, they require doping with Li-TFSI, and the devices develop the same Li-TFSI-derived problems. Here, we propose Li-free 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI) as an efficient p-type dopant of X60, resulting in a high-quality HTL with enhanced conductivity and deeper energy levels The optimized X60:EMIM-TFSI-enabled devices exhibit a higher efficiency of 21.85% and improved stability, compared to the Li-TFSI-doped X60 devices. The stability of the optimized EMIM-TFSI-doped PSCs is greatly improved, and after 1200 hr of storage under ambient conditions, the resulting PSCs maintain 85% of the initial PCE. These findings offer a fresh method for doping the cost effective X60 as the HTL with a Li-free alternative dopant for efficient, cheaper, and reliable planar PSCs

Mixed‐Halide Perovskite Memristors with Gate‐Tunable Functions Operating at Low‐Switching Electric Fields

Abstract Crossbar circuits based on two terminal (2T) memristors typically require an additional unit such as a transistor for individual node selection. A memristive device with gate‐tunable synaptic functionalities will not only integrate selection functionality at the cell level but can also lead to enriched on‐demand learning schemes. Here, a three‐terminal (3T) mixed‐halide perovskite memristive device with gate‐tunable synaptic functions operating at low potentials is demonstrated. The device operation is controlled by both the drain (VD) and gate (VG) potentials, with an extended endurance of >2000 cycles and a state retention of >5000 s. Applying a voltage (Vset) of 20 V across the 50 µm channel switches its conductance from a high‐resistance state (HRS) to a low‐resistance state (LRS). A memristive switching mechanism is proposed that is supported by current injection models through a Schottky barrier and Kelvin probe force microscopy data. The simultaneous application of a VG potential is found to further modulate the channel conductance and reduce the operating Vset to 2 V, thus requiring a low electric field of 400 V cm−1, which is by a factor of 50× less compared to state‐of‐the‐art literature reports. Gate‐tunable retention, endurance, and synaptic functionalities are demonstrated, further highlighting the beneficial effect of VG on device operation

Two-dimensional BiTeI as a novel perovskite additive for printable perovskite solar cells

Hybrid organic-inorganic perovskite solar cells (PSCs) are attractive printable, flexible, and cost-effective optoelectronic devices constituting an alternative technology to conventional Si-based ones. The incorporation of low-dimensional materials, such as two-dimensional (2D) materials, into the PSC structure is a promising route for interfacial and bulk perovskite engineering, paving the way for improved power conversion efficiency (PCE) and long-term stability. In this work, we investigate the incorporation of 2D bismuth telluride iodide (BiTeI) flakes as additives in the perovskite active layer, demonstrating their role in tuning the interfacial energy-level alignment for optimum device performance. By varying the concentration of BiTeI flakes in the perovskite precursor solution between 0.008 mg mL(-1) and 0.1 mg mL(-1), a downward shift in the energy levels of the perovskite results in an optimal alignment of the energy levels of the materials across the cell structure, as supported by device simulations. Thus, the cell fill factor (FF) increases with additive concentration, reaching values greater than 82%, although the suppression of open circuit voltage (V-oc) is reported beyond an additive concentration threshold of 0.03 mg mL(-1). The most performant devices delivered a PCE of 18.3%, with an average PCE showing a +8% increase compared to the reference devices. This work demonstrates the potential of 2D-material-based additives for the engineering of PSCs via energy level optimization at perovskite/charge transporting layer interfaces