Effect of C 60 as an electron buffer layer in polythiophene-methanofullerene based bulk heterojunction solar cells

Abstract

The effect of C 60 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 C 60 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 C 60 thickness of about 5 nm. Subsequent increase in C 60 interlayer thickness promotes charge transfer near the Al-C 60 interface due to increased diffusion of Al atoms into the interstitials of C 60. This results in the formation of s-shaped kink in J-V 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 C 60 interlayer shows no charge extraction peak whereas the devices with C 60 layer of thicknesses from 10 nm to 20 nm shows characteristic maxima due to the transferred charge carriers from the Al-C 60 interface. The PhotoCELIV studies performed on the devices showed characteristic single peak for the device with 5 nm C 60 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. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.