Significance of Dopant/Component Miscibility to Efficient N-Doping in Polymer Solar Cells.

The uncertain dopant location in the bulk heterojunction (BHJ) film hinders the wide application of molecular doping in polymer solar cells (PSCs) as is in other organic devices. It is known that the interaction between the dopant and component governs the dopant distribution in the BHJ film and thus largely controls the effectiveness of molecular doping. After excluding the strong dopant/component interaction by forming the charge-transfer complex in the solution, we estimate the dopant/component miscibility by calculating the difference of Hansen's total solubility parameters (△δ) and prove its correctness by contact angle measurements, and two model systems of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophe-2-yl)-benzo[1,2-:4,5-']dithiophene))--(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-:4',5'-']dithiophene-4,8-dione))] (PBDB-T)/poly{[,'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]--5,5'-(2,2'-bithiophene)} (N2200) and poly[4,8-bis(5-(2-ethylhexyl)-thiophene-2-yl)benzo[1,2-;4,5-']dithiophene-2,6-diyl--(4-(2-ethylhexyl)-3-fluorothieno[3,4-]thiophene-)-2-carboxylate-2-6-diyl]] (PCE10)/N2200 are selected to reveal the miscibility-photovoltaic performance relations. Only the material combination with large △δ between the n-dopant (4-(1,3-dimethyl-2,3-dihydro-1-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) and the donor polymer achieves enhanced photovoltaic performance. After that, we examine the doped morphology of polymer blends. Since the polymers' crystallizations are negatively affected by N-DMBI addition, we ensure the significance of n-doping on the enhanced device performance. Besides the dopant/polymer interaction, the solvent/polymer and solvent/dopant interactions are also considered to evaluate the kinetic effect on N-DMBI distribution by drawing the ternary phase diagram. We conclude that the kinetic morphological evolution does not change the miscibility-governed N-DMBI distribution in the BHJ film. Finally, we provide a direct relationship between the N-DMBI position and the device property by fabricating the bilayer devices. The enhancement of photovoltaic performances is observed in both material systems only if the N-DMBI distributes in N2200. Our work outlines a basis for using the dopant/component interaction and ternary phase diagram to predict the dopant distribution before extensive experiments. It significantly reduces the trial-and-error work and increases the reliability of molecularly doped PSCs.