Enhanced performance of poly-3(hexylthiophene) - single walled carbon nanotube bulk heterojunction solar cells using a poly-3(hexylthiophene) buffer layer and Ca electrode

An enhancement in the performance of P3HT:SWNT bulk heterojunction solar cell was achieved by using a buffer layer of P3HT between the anode and the bulk layer. The buffer layer improved the shunt resistance of the device (1 wt% SWNT) from 25 k¿ to 400 k¿. This resulted in the increase of open circuit voltage from 0.06 V to 0.45 V. Because of the increased thickness, the light absorbed by the device increased resulting in the short circuit current density, J_sc, to improve from 2.54 ¿A cm^-2 to 13.25 ¿A cm^-2. Variation of SWNT content in P3HT showed that the optimum concentration of SWNTs is near 0.25 wt% which gave the highest open circuit voltage, V_oc, of 0.53 V. Dark characteristics of these devices show a ohmic behavior at low voltages indicating that addition of SWNTs resulted in increased residual charges. Reverse photocurrent of the devices is found to increase considerably with higher concentration of SWNTs. Spectral response of the devices show that SWNTs do not contribute to generation of photocurrent. Thus, we conclude that SWNTs help only in charge transport in P3HT devices. Use of Ca cathode increased J_sc to 16.4 ¿A cm^-2 and V_oc to 0.65 V because of the higher built-in field.