Mechanism of In-Plane and Out-of-Plane Tribovoltaic Direct-Current Transport with a Metal/Oxide/Metal Dynamic Heterojunction

Interfacial layer engineering has been demonstrated as an effective strategy for boosting power output in semiconductor-based dynamic direct-current (DC) generators, although the underlying mechanism of power enhancement remains obscure. Here, such ambiguity has been elucidated by comparing fundamental tribovoltaic DC output characteristics of prototypical metal–oxide–metal heterojunctions prepared by atomic-layer deposition (ALD) with a vertical (out-of-plane carrier transport through the interfacial layer) and a horizontal (in-plane carrier transport along the interfacial layer) configuration such that the influences from nonequilibrium electronic excitation and interfacial capacitive amplification can be individually tuned and investigated. It is found in the case of Al/TiO2/Ti vertical configurations that the open-circuit voltage (VOC) increases linearly from −0.03 to −0.52 V as the thickness of titanium oxide (tTiO2) increases from 0 to 200 nm with a linear amplification coefficient of −2.31 mV nm–1, which is validated by a parallel-capacitor theoretical model with tribovoltaic electronic excitation. In contrast, the VOC output with the horizontal configuration is ∼55 mV, where the potential difference is merely associated with the accumulation of surface charges and the subsequent charge rearrangement in the depletion region. Meanwhile, it is measured that the short-circuit current density (JSC) shows an initial increasing trend when tTiO2 increases, reaches its peak value at 0.21 A m–2 at tTiO2 = 20 nm, and then decreases as tTiO2 increases further. From current–voltage (I–V) characterization, it is proposed that such DC output variation with an optimal interfacial layer thickness stems from the competition of amplified voltage and increased resistance with increasing interfacial layer thickness, with the main charge transport mechanism switching from quantum tunneling to thermionic emission/trap-assisted transport. In contrast, tribovoltaic excitation is proven to be significantly weaker when a wide band-gap insulator (Al2O3) is involved. The elucidation of the fundamental mechanism of power enhancement by the interfacial layer in this work is of great significance in providing instructional direction for the development and optimization of high-performance DC nanogenerators.