Figure 1

(Color online) Experimental setup used to perform terahertz time-domain spectroscopy on polymer transistors, described in Sec. 2. One unit of the source and/or drain interdigitated array is shown. The hole and electron accumulation layers in the polymer and gate are represented by + and −. The polymer was illuminated with light in the visible frequency range from various LEDs. The monomer unit of the polymer F8T2 is also shown.Reuse & Permissions

Figure 2

(Color online) (a) Change in transmitted terahertz radiation pulse $\mid \Delta T∕T\mid$ induced by a gate-voltage modulation $V_{\mathrm{g}}=0\leftrightarrow -30\mathrm{V}$ at $40\mathrm{Hz}$. At zero time, a near-UV LED $(\lambda =396\mathrm{nm})$ illuminates the polymer, increasing $\mid \Delta T∕T\mid$ (squares, normalized to its preillumination value) until LED turn off $100\mathrm{s}$ later. The same experiment was repeated at comparable irradiances using a variety of LEDs, including a green LED (points, $\lambda =542\mathrm{nm}$, offset vertically by $-0.2$ for clarity) and a red LED (crosses, $\lambda =636\mathrm{nm}$, offset by $-0.4$). The solid lines are guides for the eyes. (b) Schematic of mobile (+) and trapped (⊕) holes in polymer and electrons (−) in the gate without (top) and with (bottom) an applied gate voltage in the dark (left) and light (right). (c) Source-drain current at $V_{\mathrm{g}}=-30\mathrm{V}$, $V_{\mathrm{sd}}=5\mathrm{V}$, under illumination with the $\lambda =542\mathrm{nm}$ LED from $0\text{to}100\mathrm{s}$. The nonzero source-drain current between $-50$ and $-40\mathrm{s}$ is a result of the removal of trapped holes in the dark (Ref. 24), which occurred during the period when the source measurement unit was connected, when no gate voltage was applied to the transistor.Reuse & Permissions

Figure 3

(Color online) Light-induced change in $\mid \Delta T∕T\mid$ at a gate-voltage modulation $V_{\mathrm{g}}=0\leftrightarrow -30\mathrm{V}$ (hollow points, left-hand $y$ axis), defined as $\Delta ={\mid \Delta T∕T\mid }_{\mathrm{LED}\text{on}}-{\mid \Delta T∕T\mid }_{\mathrm{LED}\text{off}}$, divided by the incident photon flux at various illumination wavelengths. $\Delta$ has been normalized with respect to its value at $2.78\mathrm{eV}$ excitation energy. Also shown are the absorption $\alpha$ of F8T2 (solid line, from thin-film transmission measurements) and the quantum efficiency $\eta$ (points, right-hand $y$ axis, obtained as described in the text). The measured LED emission spectra are plotted at the bottom.Reuse & Permissions

Figure 4

(Color online) The photoinduced carrier density $n_{\mathrm{ind}}$ (points, left-hand axis, $E_{\gamma }=2.29\mathrm{eV}$) during hole trapping for a pristine transistor normalized by the hole density in the absence of light, $n_0=3.2\times {10}^{12}{\mathrm{cm}}^{-2}$ $(V_{\mathrm{g}}=0\leftrightarrow -30\mathrm{V})$. The trapped hole density $n_{\text{off}}$ (solid line, right-hand axis) is also shown. The data shown in all other figures were obtained at times after $1.2\times {10}^5\mathrm{s}$, when the photoinduced carrier density is constant.Reuse & Permissions

Figure 5

(a) Gate-voltage dependence of the photoinduced carrier density $n_{\mathrm{ind}}$ (points), obtained as described in the text. The initial value of the mobile charge density at $V_{\mathrm{g}}=-30\mathrm{V}$ was $n_0=3.2\times {10}^{12}{\mathrm{cm}}^{-2}$. (b) Calculated electric-field distribution $F\left(x\right)$ (solid line, left $y$ axis) versus depth $x$ from the polymer-insulator boundary at $V_{\mathrm{g}}=-40\mathrm{V}$.Reuse & Permissions

Figure 6

(Color online) Experimental quantum yield $\eta$ against average field $⟨F⟩$ for photon excitation energies of (a) $2.78\mathrm{eV}$ and (b) $2.29\mathrm{eV}$ (points). Also shown are $\eta \left(⟨F⟩\right)$ calculated using the Onsager model (dashed lines) and the Arkhipov model (solid lines).Reuse & Permissions