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

Small molecular weight organic thin film mixtures of the electron-conducting C60 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 C60 whereas excitons are blocked by the BP4mPy. We find that the conductivity and exciton blocking efficiency of the blends are strongly dependent on 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 pressure of 0.28 Torr leads to extended and highly conductive crystalline C60 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) × 10(-3) (cm(2)/V·s) is obtained at 0.28 Torr, which is approximately 2 orders of magnitude higher than for amorphous C60 films. The fill factors and power conversion efficiencies of vacuum deposited tetraphenyldibenzoperiflanthene (DBP):C70 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.