Light amplification in polymer field effect transistor structures

The amplified spontaneous emission (ASE) of optically pumped films of $\mathrm{poly}{\text{(2-(2}}^{\prime }{\text{,5}}^{\prime }-\mathrm{bis}(\mathrm{octyloxy})\mathrm{benzene}\text{)-1,4-}\mathrm{phenylenevinylene}$ (BOP-PPV) was studied in structures comprising a gate electrode, a thin film of gate insulator material $({\mathrm{SiO}}_2)$ and the polymer film as luminescent semiconducting layer (i.e. a field effect transistor without the source and drain electrodes). The influences of different gate electrodes on the threshold and the wavelength of the amplified emission were measured for variable thickness of the gate insulator. An exponential increase in ASE threshold ${\mathrm{(I}}_t)$ with decreasing separation between electrode and polymer layer was observed. In structures with 200 nm ${\mathrm{SiO}}_2$ gate insulator, $I_t\text{=300\hspace{0.17em}}{\mathrm{kW/cm}}^2$ with an $\mathrm{n-}\mathrm{Si}$ gate electrode and $\text{200\hspace{0.17em}}{\mathrm{kW/cm}}^2$ with Au electrode (100 nm thick). Compared to the same polymer film on pure ${\mathrm{SiO}}_2$ ${\mathrm{(I}}_t\text{=2\hspace{0.17em}}{\mathrm{kW/cm}}^2\mathrm{),}$ this increase results from waveguide losses in the nearby gate electrode. With an indium–tin–oxide (ITO) gate electrode (140 nm thick) on glass, again with a 200 nm ${\mathrm{SiO}}_2$ gate insulator, $I_t\text{=30\hspace{0.17em}}{\mathrm{kW/cm}}^2.$ The ITO electrode acts as a second waveguide, and the light is distributed into two modes. The observed wavelength shift and the increasing $I_t$ with decreasing ${\mathrm{SiO}}_2$ thickness result from this mode structure. When the thickness of the ITO electrode is less than 60 nm, the mode traveling mainly in the ITO is cutoff, and a single waveguide structure is formed with an associated reduction in $I_t.$ For an ITO thickness of 12 nm, $I_t\text{=4\hspace{0.17em}}{\mathrm{kW/cm}}^2,$ only two times bigger than that observed in a pure BOP-PPV film on fused silica.