Role of single walled carbon nanotubes in improving the efficiency of P3HT:PCBM solar cells - impedance spectroscopy and morphology studies

Bulk heterojunction (BHJ) solar cells of P3HT:PCBM doped with SWNTs were fabricated which doubled the efficiency over the undoped devices. No surface modifications of SWNTs were done during fabrication. Absorption and photoluminescence spectra along with photocurrent and spectral response of the devices show that SWNTs do not result in any significant charge generation at the P3HT:SWNT interface indicating that possible type-II heterojunctions between s-SWNTs and P3HT were dominated by the effects due to metallic tubes. At an optimum concentration of 0.75 wt% SWNTs, a 10% improvement in effective mobility was observed. In the voltage range of solar cell operation, two orders increase in injected current density is observed which has an Ohmic behavior. From the peak voltage of the capacitance-voltage characteristics, it was inferred that SWNTs reduce the V_bi of the devices by only 60 mV. From the Cole-Cole in the diffusion transport regime, it was observed that the injected carrier life time gets lowered from 0.628 ms to 0.125 ms with SWNTs. A negative capacitance was observed in reverse bias in devices with SWNTs at low frequencies which has similar dependence on applied field as that in forward bias. This is attributed to the large reverse current injected through SWNT energy levels, making the effects of space charge, trapping, and recombination significant. The surface roughness and volume were more than doubled with SWNTs which resulted in increased cathode coverage area affecting the charge collection efficiency. Charge extraction efficiency is analyzed using the photocurrent loss normalized to the dark current where two orders of magnitude improvement is observed with SWNTs.