Abstract
Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor–acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. Here, we use time-resolved optical spectroscopy to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron–hole encounters at later times, both charge-transfer states and emissive excitons are regenerated, thus setting up an equilibrium between excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination.
Highlights
Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor–acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole
In contrast to inorganic semiconductors, photoexcitation of organic semiconductors generates coulombically bound electron–hole pairs known as excitons[1]. These excitons have large binding energies and do not separate into free charges unless they are dissociated at a donor/acceptor (D/A) heterojunction to form charge-transfer excitons (CTEs), as shown in Fig. 1a, b, which must subsequently separate to free electrons and holes against their mutual Coulomb interaction in order to create photocurrent
Our study involves four model non-fullerene blends, namely P3TEA:SF-PDI224, P3TEA:FTTB-PDI437, P3TAE: SF-PDI238, and PffBT2T-TT:O-IDTBR39. Information about their chemical structures, absorption spectra, energy levels, blend morphologies, and photovoltaic performances can be found in Fig. 1c, d and in the Supplementary Information (Supplementary Figs. 2–4). Among these we focus on the P3TEA:SF-PDI2 blend
Summary
Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor–acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. In contrast to inorganic semiconductors, photoexcitation of organic semiconductors generates coulombically bound electron–hole pairs known as excitons[1] These excitons have large binding energies (typically ~0.5 eV) and do not separate into free charges unless they are dissociated at a donor/acceptor (D/A) heterojunction to form charge-transfer excitons (CTEs), as shown, b, which must subsequently separate to free electrons and holes against their mutual Coulomb interaction in order to create photocurrent. The third source of photovoltage loss is the offset between the energy of the photogenerated singlet excitons (ES1) and the interfacial CTEs (ECTE), see Fig. 1a This energy offset, ES1–ECTE, is widely believed to be required in order to drive efficient and rapid (within a few hundred femtoseconds) charge separation at organic heterojunctions[2,8,9]. We note that there exist several examples of fullerene OSCs that can achieve decent performance with small offsets[12,13,14]; most fullerene systems a Exciton CTE
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