The characteristics of field effect transistors (FETs) fabricated using two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be modulated by surface treatment of the active layers. In this study, an ionic π-conjugated polyelectrolyte, poly(9,9-bis(4'-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was used for the surface treatment of MoSe2 and WS2 FETs. The photoluminescence (PL) intensities of monolayer (1L)-MoSe2 and 1L-WS2 clearly decreased, and the PL peaks were red-shifted after FPS-K treatment, suggesting a charge-transfer effect. In addition, the n-channel current of both the MoSe2 and WS2 FETs increased and the threshold voltage (Vth) shifted negatively after FPS-K treatment owing to the charge-transfer effect. The photoresponsivity of the MoSe2 FET under light irradiation (λex = 455 nm) increased considerably, from 5300 A W-1 to approximately 10 000 A W-1, after FPS-K treatment, and similar behavior was observed in the WS2 FET. The results can be explained in terms of the increase in electron concentration due to photogating. The external quantum efficiency and photodetectivity of both FETs were also enhanced by the charge-transfer effect resulting from surface treatment with FPS-K containing mobile cations (K+) and fixed anions (SO3-), as well as by the photogating effect. The variation in charge-carrier density due to the photogating and charge-transfer effects is estimated to be approximately 2 × 1012 cm-2. The results suggest that π-conjugated polyelectrolytes such as FPS-K can be a promising candidate for the passivation of TMDC-based FETs and obtaining enhanced photoresponsivity.
Keywords: charge transfer; conjugated polyelectrolyte; field effect transistor; photogating; photoresponsivity; transition-metal dichalcogenide.