Nanoscale Control of Morphology in Fullerene-Based Electron-Conducting Buffers via Organic Vapor Phase Deposition

摘要

Small molecular weight organic thin film mixtures of the electron conducting C-60 in a wide energy gap matrix, 3,5,3',5'-tetra(M-pyrid-3-yl)phenyl[1,1']biphenyl (BP4mPy) forms, a high efficiency electron filtering buffer in organic photovoltaics (OPV). Electrons are conducted via percolating paths of C-60 whereas excitons are blocked by the BP4mPy We find that the conductivity and exciton blocking efficiency of the blends are strongly dependent oh film morphology that can be precisely controlled by the conditions used in the organic vapor,phase deposition (OVPD). Specifically,, we find that a background carrier gas presure of 0.28 Toni leads to extended and highly conductive crystalline C-60 domains. Furthermore; the structure is strongly influenced,by carrier gas pressure. Via a combination, of morphological measurements and molecular dynamics simulations, we find that this dependence is due to kinetically induced structural annealing at the growth interface. The highest electron mobility of (6.1 +/- 0.5) x 10(-3) (cm(2)/V.s) is obtained at 0.28 Torr, which is approximately 2 orders of Magnitude higher than for amorphous C-60 films. The: fill factors and power conversion efficiencies of vacuum deposited tetraphenyldibenzoperiflanthene (DBP):C-70 planar mixed heterojunction OPVs using an OVPD-grown buffer layer are (8.0 +/- 0.2)% compared to (6.6 +/- 0.2)% using amorphous buffers grown by vacuum thermal evaporation.