Temperature Activation of the Photoinduced Charge Carrier Generation Efficiency in Quaterthiophene:C₆₀ Mixed Films

We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C₆₀, as already employed in organic photovoltaics. By using DFT calculations and analyzing the recombination characteristics of the excited states revealed by photoinduced absorption (PIA) spectroscopy, the absorption peaks are assigned to triplet exciton, cation, and anion transitions. We determine the temperature dependent generation and recombination behavior of triplet excitons and cations in the mixed layer. At 10 K, we observe an enhanced triplet exciton generation rate compared to the pristine donor layer due to back recombination from a charge-transfer (CT) state at the donor–acceptor interface. With increasing temperature, the triplet generation rate first increases which is ascribed to an enhanced singlet exciton migration to this interface. Above 150 K, the triplet generation rate declines due to the beginning CT exciton separation, leading to the generation of free charge carriers. This temperature activated behavior is ascribed to a temperature activated increase of charge carrier mobility, facilitating CT exciton splitting

Comparative Study of Microscopic Charge Dynamics in Crystalline Acceptor-Substituted Oligothiophenes

By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes, we find that the internal acceptor–donor–acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields, substantial energetic disorder, and pronounced Poole–Frenkel behavior, which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder, which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition, the presence of a well-defined π-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes <b>1</b>–<b>3</b> led to highly efficient organic solar cells with power conversion efficiencies of 4.8–6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance