Articles you may be interested inFloating-gate memory based on an organic metal-insulator-semiconductor capacitor Appl. Phys. Lett. 95, 093309 (2009); 10.1063/1.3223606Separating interface state response from parasitic effects in conductance measurements on organic metalinsulator-semiconductor capacitors

The admittance of polymer metal-insulator-semiconductor (MIS) capacitors has been measured as a function of frequency and applied voltage. The results reveal the presence of hole trapping states at the interface of the polysilsesquioxane insulator and the poly(3-hexylthiophene) semiconductor. The states appear to be distributed in two bands: one close to the equilibrium Fermi level at the semiconductor surface, the other ∼0.5eV above. Annealing the devices under vacuum for several hours at 90°C increases the concentration of the shallower traps to ∼3×1012cm−2eV−1, while decreasing the concentration of deep traps to ∼1×1010cm−2eV−1. Annealing improves the bulk hole mobility to ∼1×10−4cm2V−1s−1 while reducing the field-effect mobility in MIS field-effect transistors (FETs) slightly to ∼7×10−3cm2V−1s−1. Although the concentration of interface states is sufficiently great to account for gate-bias-induced threshold voltage instability in MISFETs, their associated time constants are much too short to explain the long term nature of the instability.

Photocapacitance measurements are reported on metal-insulator-semiconductor (MIS) capacitors employing polyimide (PI) or polysilsesquioxane (PSQ) as the gate insulator and poly(3-hexylthiophene) as the active semiconductor. By stressing devices into depletion while simultaneously irradiating with light of energy exceeding the semiconductor band gap, photogenerated electrons become trapped at the insulator/semiconductor interface or possibly in bulk insulator states. Additionally for the PSQ device, evidence is provided for the formation of a photogenerated inversion layer at the interface. The time dependence of electron detrapping in the PI case is similar to that observed for accumulation stress instability in organic MIS devices.

Polymer field effect transistors (FET) based on regio-regular poly(3-hexylthiophene) (P3HT) spin-coated onto a gate insulator formed from polyimide (PI) or polysilsesquioxane (PSQ) layers have been prepared and their electrical characteristics examined. The large threshold voltage, +25 V, obtained in PI-based FETs, which contrasts with the small threshold voltage of ∼0 V in PSQ-based devices, has been discussed in terms of charge exchange at the insulator/semiconductor interface. A combination of capacitance measurement as a function of biasing voltage or measured frequency and conventional surface potential measurements reveals a high density of electron trapping states, ∼1012 cm-2, at the PI/semiconductor interface. However, the high threshold voltage in the PI-FETs only partly explains the higher drain currents observed in these devices compared with the PSQ devices. A second factor is the higher hole mobility in PI-FETs (0.005–0.01 cm2/Vs) which is about 3 times greater than in the PSQ devices (0.002–0.004 cm2/Vs). We attribute this to differences in the microscopic structure of the insulator surface. Although the mobility of hole prepared on PSQ is at the low end of the range previously reported for P3HT-based FETs, the ON:OFF ratios (>104) and low threshold voltage (∼0 V) reported here are comparable to those of the FETs prepared on a SiO2 gate insulator. The subthreshold current behaviour suggests that interface states are active in both device types but the density is much lower in the PSQ devices.

We report a hybrid solar cell based on single walled carbon nanotubes (SWNTs) interfaced with amorphous silicon (a-Si). The high quality carbon nanotube network was dry transferred onto intrinsic a-Si forming Schottky junction for metallic SWNT bundles and heterojunctions for semiconducting SWNT bundles. The nanotube chemical doping and a-Si surface treatment minimized the hysteresis effect in current-voltage characteristics allowing an increase in the conversion efficiency to 1.5% under an air mass 1.5 solar spectrum simulator. We demonstrated that the thin SWNT film is able to replace a simultaneously p-doped a-Si layer and transparent conductive electrode in conventional amorphous silicon thin film photovoltaics.