Abstract
Hybrid organic-inorganic heterointerfaces in solar cells suffer from inefficient charge separation yet the origin of performance limitations are widely unknown. In this work, we focus on the role of metal oxide-polymer interface energetics in a charge generation process. For this purpose, we present novel benzothiadiazole based thiophene oligomers that tailor the surface energetics of the inorganic acceptor TiO2 systematically. In a simple bilayer structure with the donor polymer poly(3-hexylthiophene) (P3HT), we are able to improve the charge generation process considerably. By means of an electronic characterization of solar cell devices in combination with ultrafast broadband transient absorption spectroscopy, we demonstrate that this remarkable improvement in performance originates from reduced recombination of localized charge transfer states. In this context, fundamental design rules for interlayers are revealed, which assist the charge separation at organic-inorganic interfaces. Beside acting as a physical spacer in between electrons and holes, interlayers should offer (1) a large energy offset to drive exciton dissociation, (2) a push-pull building block to reduce the Coulomb binding energy of charge transfer states and (3) an energy cascade to limit carrier back diffusion towards the interface.
Highlights
One promising approach to study the fundamental mechanism of charge separation at hybrid interfaces is a comparison between chemically and physically bound organic semiconductors to a metal oxide surface[37,38,39]
interfacial modifiers (IM) was achieved by a controlled Suzuki-Miyaura coupling polymerization (SMCP) protocol[42], based on prior works on phosphonate functionalized polyfluorenes[43]
Transient absorption spectroscopy was performed on samples without electrodes, i.e. the charge separation process is probed under quasi-open circuit conditions and only diffusive transport can occur
Summary
One promising approach to study the fundamental mechanism of charge separation at hybrid interfaces is a comparison between chemically and physically bound organic semiconductors to a metal oxide surface[37,38,39]. First investigations following this approach could show that chemically bound interfacial modifiers (IM), such as conjugated 3-hexylthiophene derivatives, inject electrons more efficiently and promote photocurrent www.nature.com/scientificreports/. The latter process is very sensitive to the chain length of the IMs, which influences frontier orbital energetics[39]. By means of ultrafast pump-probe spectroscopy and measurements on the electric field dependence of the charge separation process, we show compelling evidence that covalently bound push-pull systems improve the charge generation process at hybrid interfaces significantly
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