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(<i>m</i>-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 (cm²/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

Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures

We find that mixtures of C₆₀ with the wide energy gap, small molecular weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron conductivity and efficient exciton blocking when employed as buffer layers in organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C₆₀ mixed layer has an exciton blocking efficiency of 84 ± 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency is accompanied by a 100-fold increase in electron conductivity compared with neat BPhen. Transient photocurrent measurements show that charge transport through a neat BPhen buffer is dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Thus, we infer that C₆₀ undergoes nanoscale (<10 nm domain size) phase segregation even at very high (>80%) BPhen fractions