Over 17.4% Efficiency of Layer-by-Layer All-Polymer Solar Cells by Improving Exciton Utilization in Acceptor Layer

Layer-by-layer all-polymer solar cells (LbL all-PSCs) are prepared with PM6 and PY-IT by using sequential spin coating method. The exciton dissociation efficiency in acceptor layer near electrode is rather low due to the limited exciton diffuse distance and impossible energy transfer from narrow bandgap acceptor to wide bandgap donor. In this study, less PM6 is incorporated into PY-IT layer to enhance exciton dissociation in PY-IT layer near electrode. A power conversion efficiency (PCE) of 17.45% is achieved in the LbL all-PSCs incorporating 10 wt% PM6 into PY-IT layer, which is much larger than 16.04% PCE of PM6/PY-IT-based LbL all-PSCs. Over 8% PCE enhancement can be realized by incorporating 10 wt% PM6 into PY-IT layer, which is attributed to the enhanced exciton utilization efficiency in PY-IT layers near electrode. The enhanced exciton utilization efficiency in PY-IT layer can be confirmed from the quenched photoluminescence (PL) emission in PY-IT:PM6 films. Meanwhile, charge transport in acceptor layers can be optimized by incorporating less PM6, as confirmed from the optimized molecular arrangement. This study indicates that the strategy of incorporating less donor into acceptor layer has great potential in fabricating efficient LbL all-PSCs by improving exciton utilization efficiency in acceptor layer near electrode.