Efficient NDT small molecule solar cells with high fill factor using pendant group engineering†
Three naphthodithiophene (NDT)-based small molecules, bearing hexyl- (NDT-Hex), null (NDT) and fluorene (NDT-F) substituents at the β-position of the thiophene vertically conjugated to the NDT core, were designed and synthesized. The photovoltaic effect of engineered NDT molecules was investigated with a detailed characterization of optical absorption, electronic energy levels, blend morphology, carrier mobility and photovoltaic performance. It shows that the hexyl-substitution enhances the morphological tunability for NDT molecules, which contributes to achieving a promising power conversion efficiency (PCE) with high fill factor (FF). The optimized solar cells based on NDT-Hex:PC71BM blends provide a PCE of 6.7% with an FF of 0.717 and it reaches 8.6% with an FF of 0.638 if a non-fullerene molecular acceptor, NBDTP-Fout, is adopted to blend with NDT-Hex. Both FF are among the top values for the respective kinds of solar cells based on an NDT small molecule. The work demonstrates a feasible morphology-tuning method toward efficient organic solar cells with high FF.