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Matched energy level alignment is a key requirement for efficient organic devices such as organic light-emitting diodes, photovoltaics, and field-effect transistors. The effect of thermal stress/annealing on energy level alignment and related properties of the devices are less discussed compared to the extensively explored effect on morphology and corresponding device performance. Here all polymer solar cells (all-PSCs) are employed to study thermal annealing effects on energy level alignment and the corresponding effect on the device properties of the all-PSCs. It is found that optimized energy level alignment can be achieved by thermal annealing. An interface dipole layer at the donor/acceptor interface is introduced by energy level realignment that assists charge generation by reducing geminate recombination so that the voltage loss is dramatically reduced, improving the performance of the all-PSCs. Image 1 • Energy levels of organic semiconductors can be changed by thermal annealing (stress). • A change in energy levels can cause spontaneous integer charge transfer from polymer donor to acceptor. • Ground state integer charge transfer will introduce an interface dipole layer at the donor-acceptor heterojunction. • An interface dipole layer can enhance exciton-to-charge generation and decrease open circuit voltage losses.. • The design rule for energy level matching holds for state-of-the-art efficient organic solar cell. [ABSTRACT FROM AUTHOR]

Abstract Two-photon absorption (2 PA) with three-dimensional (3D) resolution and deep penetration of light into absorbing materials hold great promise for many applications such as 3D microscopy, 3D microfabrication, polymerization and optical data storage. We herein designed and synthesized two novel series of arylamine-coumarin based donor-acceptor dyads that showed good two-photon absorption activities. Tested by an open/close aperture Z-scan technique, the largest 2 PA cross-section value is 103.6 GM and the largest two-photon absorption coefficient (β) is 7.84 × 10−12 cm W−1. Fluorometric and electrochemical studies further reveal the formation of intramolecular charge transfer (ICT) among two dyads. Photoinduced singlet excited state intramolecular charge separation states were confirmed by ultrafast laser flash photolysis analyses coupled with TD-DFT excited state calculations. Surprisingly, the experimental measured lifetime of singlet excited state intramolecular charge separation state may be a diagnostic criterion for 2 PA cross-section of donor-acceptor systems. This work provides a new strategy for designing high-efficiency 2 PA materials. Graphical abstract Image 1 Highlights • Two novel series of arylamine-coumarin based donor-acceptor dyads have been designed and synthesized. • Both compounds showed good two-photon absorption activities. • The largest two-photon absorption coefficient (β) is 7.84 × 10−12 cm W−1 and the largest 2PA cross-section value is 103.6 GM. • Photoinduced singlet excited state intramolecular charge separation states were confirmed experimentally and theoretically. • The lifetime of singlet excited state intramolecular charge separation state may be a diagnostic criterion for 2PA. [ABSTRACT FROM AUTHOR]