Diketopyrrolopyrrole (DPP) is one of the most common building blocks for small molecules and conjugated polymers designed for organic electronic applications. By attaining a detailed understanding of the photophysical behaviour for a simple DPP-based molecule in fullerene blends, we establish a foundation for spectroscopic investigations into more complex DPP-based systems. Transient absorption spectroscopy (TAS) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy are used to examine bulk heterojunction blend films of a small diketopyrrolopyrrole-based molecule, 2,5-bis(2-hexyldecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TDPP) with the common fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). Following pulsed laser excitation, the spectral signatures of a fullerene anion and a TDPP triplet state are observed on the picosecond timescale by TAS. The presence of these species implies the formation of a TDPP:PCBM charge transfer state that subsequently undergoes ultra-fast spin-mixing and geminate recombination to produce a TDPP triplet state. The overall photophysical mechanism is confirmed by TR-EPR spectroscopy, which unambiguously shows that the TDPP triplet is formed via spin-mixing in the TDPP:PCBM charge transfer state, rather than direct intersystem crossing from the excited singlet state.

The strength of dielectric screening is one of the most intriguing yet least studied contributing factors to the operation and performance limit of organic solar cell devices. Increasing the dielectric constant of semiconducting polymers may close the performance gap between inorganic and organic solar cell devices. Here, a dielectric constant of 16.7 is reported for a DPP-based low bandgap polymer DT-PDPP2T-TT and 7 for its 1:3 blend with [60]PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) using frequency and voltage dependent capacitance and charge extraction by linearly increasing voltage (CELIV) techniques. The charge mobility within the blend device (1.8 × 10–3 cm2 V–1 s–1) is found to be among the highest reported by CELIV. Bimolecular recombination and charge carrier lifetime in efficient photovoltaic devices are measured and compared to poly(3-hexylthiophene) (P3HT):PCBM (1:1 w/w) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT):PCBM (1:2 w/w) devices. When normalized to mobility, the bimolecular recombination coefficient in DT-PDPP2T-TT:PCBM is a factor of 2 lower than in P3HT:PCBM and an order of magnitude lower than in PCPDTBT:PCBM. The recombination mechanism is found to be close to diffusion-controlled Langevin recombination. The reduced recombination is explained by a smaller Coulomb capture radius, which, together with higher charge mobility, leads to efficient charge extraction in photovoltaic devices with large active layer thicknesses approaching 300 nm.