Comparative study on charge photogeneration dynamics of Y small molecule and polymerized Y small molecule based polymer solar cells

Abstract

Understanding charge photogeneration processes in polymer solar cells utilizing polymerized Y-molecule acceptors (PYMAs) is of great importance for design and optimization of high-performance solar cells. In this work, excited state dynamics in PYMAs (PYT, PY-DT) and corresponding solar cells were comparably studied with those of Y small molecules (Y5, Y6) by using the steady state and time-resolved spectroscopies as well as time-dependent density functional theory calculation. We find that PYMA (PYT, PY-DT) films exhibit smaller Stokes shifts than that of Y small molecules, indicating a more rigid backbone of PYMAs. Temperature-dependent steady-state PL measurement reveals that compared to small molecule films, the energy barrier from radiative to non-radiative states is smaller in PYMA films. In addition, transient absorption spectroscopy demonstrates that the exciton diffusion process in PYT and PY-DT are mainly intra-chain exciton diffusion mechanism with exciton diffusion coefficients of 1.7 × 10−2 and 2.7 × 10−2 cm2 s−1, respectively, in contrast with the inter-molecular exciton diffusion in Y5 and Y6 films. For the blend films, the phase sizes of acceptors in PM6:PYT and PM6:PY-DT are determined as 2.3 and 3.3 nm, respectively, smaller than that of Y6 (4.7 nm) in the PM6:Y6 film. In addition, unlike bimolecular recombination in classical system PM6:Y6, the PYMA-based all-polymer solar cells exhibit geminate type recombination in ultrafast timescale. We find that carrier lifetime plays a critical role in the performance of PYMA-based polymer solar cells. This work provides a comprehensive understanding of the photophysical properties of PYMAs, which is pivotal for designing highly efficient all-polymer solar cells.