Achieving favorable band profile in low-temperature-grown Cu­(In,Ga)­Se2 thin films has been challenging due to the lack of thermal diffusion. Here, by employing a thin Ag precursor layer, we demonstrate a simple co-evaporation process that can effectively control the Ga depth profile in CIGS films at low temperature. By tuning the Ag precursor thickness (∼20 nm), typical V-shaped Ga gradient in the copper indium gallium diselenide (CIGS) film could be substantially mitigated along with increased grain sizes, which improved the overall solar cell performance. Structural and compositional analysis suggests that formation of liquid Ag–Se channels along the grain boundaries facilitates Ga diffusion and CIGS recrystallization at low temperatures. Formation of a fine columnar grain structure in the first evaporation stage was beneficial for subsequent Ga diffusion and grain coarsening. Compared to the modified co-evaporation process where the Ga evaporation profile has been directly tuned, the Ag precursor approach offers a convenient route for absorber engineering and is potentially more applicable for roll-to-roll fabrication system.

Memristors, or memristive devices, have attracted tremendous interest in neuromorphic hardware implementation. However, the high electric-field dependence in conventional filamentary memristors results in either digital-like conductance updates or gradual switching only in a limited dynamic range. Here, we address the switching parameter, the reduction probability of Ag cations in the switching medium, and ultimately demonstrate a cluster-type analogue memristor. Ti nanoclusters are embedded into densified amorphous Si for the following reasons: low standard reduction potential, thermodynamic miscibility with Si, and alloy formation with Ag. These Ti clusters effectively induce the electrochemical reduction activity of Ag cations and allow linear potentiation/depression in tandem with a large conductance range (~244) and long data retention (~99% at 1 hour). Moreover, according to the reduction potentials of incorporated metals (Pt, Ta, W, and Ti), the extent of linearity improvement is selectively tuneable. Image processing simulation proves that the Ti4.8%:a-Si device can fully function with high accuracy as an ideal synaptic model.

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A three-terminal top-gated flash device based on two-dimensional materials with a high coupling ratio exhibits highly linear synaptic weight updates.