Study of photoluminescence quenching and DC conductivity measurements in polymer-SWNT composite films for various SWNT concentrations

Conducting polymer-SWNT composite films have a high potential in the area of Photovoltaic devices and Light emitting diodes. SWNTs have high electron affinity and high mobility for electrons. As a result of this, the separation of photoinduced charges in the solar cell is very fast and efficient. For LEDs, the polymer-SWNT composite films act as efficient electron transporting layer (ETL). In general the mobility for electrons in conducting polymers is much lower than that for holes due to efficient trapping of electrons by the impurities and traps in the polymers. Due to high mobility of electrons in SWNTs, using polymer-SWNT composite film as ETL is a good alternative. In the present work composite solutions of P3HT and SWNTs were prepared in 1,2-dichlorobenzene with nanotube to polymer mass ratios varied from 0 to 30%. To disperse the nanotube powder the solution was ultrasonically agitated for ~5 minutes. The solutions were left undisturbed for ~48 hrs to sediment out the present impurities. The upper half of the solution was decanted to obtain impurity free solution. The films from this solution were spin coated on a glass substrate with the help of spinner. The thickness of the film was measured with the help of Talystep and was ~100 plusmn 10 nm. The photoluminescence (PL) of the film was observed with the help of Shimadzu spectrophotometer. It was found that the PL intensity decreases by ~90% as the concentration of the SWNT is increased from 0 to 30%. This decrease is due to transfer of electrons from P3HT to SWNTs before the exciton in the polymer can decay radiatively to emit PL. For measuring do conductivity the composite films were spin coated on a glass substrate having Al coating. Six Al contacts having an area of 0.25 cm
²
were deposited on the upper surface of the film and conductivity was measured with the help of Keithley sourcemeter in sandwiched structures. It was found that conductivity increases by more than five orders of magnitude with the increase of SWNT concentration. The composite films can therefore act as good electron transporter for LEDs and efficient electron acceptor for solar cells.