Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor

To investigate the influence of donor molecule crystallinity on photovoltaic performance in all-small-molecule solar cells, two dithieno[2,3-d:2′,3′-d′]-benzo[1,2-b:4,5-b′]dithiophene (DTBDT)-based small molecules, denoted as DTBDT-Rho and DTBDT-S-Rho and incorporating different side chains, are synthesized and characterized. The photovoltaic properties of solar cells made of these DTBDT-based donor molecules are systemically studied with the [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) fullerene acceptor and the O-IDTBR nonfullerene acceptor to study the aggregation behavior and crystallinity of the donor molecules in both blends. Morphological analyses and a charge carrier dynamics study are carried out simultaneously to derive structure-property relationships and address the requirements of all-small-molecule solar cells. This study reveals exciton decay loss driven by large-scale phase separation of the DTBDT molecules to be a crucial factor limiting photocurrent generation in the all-small-molecule solar cells incorporating O-IDTBR. In the all-small-molecule blends, DTBDT domains with dimensions greater than 100 nm limit the exciton migration to the donor-acceptor interface, whereas blends with PC₇₁BM exhibit homogeneous phase separation with smaller domains than in the O-IDTBR blends. The significant energy losses in nonfullerene-based devices lead to decreased J_sc and fill factor values and unusual decrease in V_oc values. These results indicate the modulation of phase separation to be important for improving the photovoltaic performances of all-small-molecule blends. In addition, the enhanced molecular aggregation of DTBDT-S-Rho with the alkylthio side chain leads to higher degrees of phase separation and unfavorable charge transfer, which are mainly responsible for the relatively low photocurrent when using DTBDT-S-Rho compared with that when using DTBDT-Rho. On the other hand, this enhanced molecular aggregation improves the crystallinity of DTBDT-S-Rho and results in its increased hole mobility.