Abstract
The effect of C60 interlayer on charge transport and device performance in bulk heterojunction solar cells with active layer of poly3-hexylthiophene (P3HT) and l-3-methoxycarbonyl-propyl-l-phenyl-6,6 methanofullerene (PCBM) have been studied. The C60 layer of different thicknesses (5?nm, 10?nm, 15?nm, and 20?nm) was introduced between the cathode and the photoactive layer of the solar cell. The solar cell performance was found to be maximized at an optimum C60 thickness of about 5?nm. Subsequent increase in C60 interlayer thickness promotes charge transfer near the Al-C60 interface due to increased diffusion of Al atoms into the interstitials of C60. This results in the formation of s-shaped kink in JV spectra. To further investigate the cause of this detrimental effects, photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) and CELIV studies were performed on the real solar cell devices. The CELIV transients obtained from the device with 5?nm C60 interlayer shows no charge extraction peak whereas the devices with C60 layer of thicknesses from 10?nm to 20?nm shows characteristic maxima due to the transferred charge carriers from the Al-C60 interface. The PhotoCELIV studies performed on the devices showed characteristic single peak for the device with 5?nm?C60 interlayer whereas the other devices exhibited dual peaks due to charges generated from photo excitation and injection at the interface respectively. The charge mobility values calculated from the dual photoCELIV transients indicates the charge mobility imbalance between the carriers in the devices.