Enhanced efficiency in polymer solar cells by incorporation of phenothiazine-based conjugated polymer electrolytes

• Conjugated polyelectrolytes (CPE) based on phenothiazine.
• The work function the cathode depends on the number of ionic groups of CPE.
• Arrangement of side chains affect the electron collection ability.

The power conversion efficiency (PCE) of bulk hetero-junction type polymer solar cells (PSCs) based on poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester can be enhanced by insertion of the thin layer of phenothiazine (PT)-based conjugated polymer electrolytes (CPEs), poly{10-[4-(N,N,N-trimethylammonium)butyl]-10H-phenothiazine} bromide (PHPT) and poly{10-butyl-10H-phenothiazine-alt-10-[4-(N,N,N-trimethylammonium)butyl]-10H-phenothiazine} bromide (PcoPT), at the cathode interface. The PHPT has quaternary ammonium salt on every side chains on PT rings whereas PcoPT has quaternary ammonium salt on the side chain on PT rings alternatively. Interestingly, regardless of high HOMO energy levels of PT-based CPEs (ca 5.0 eV), the thin layer of CPE at the cathode interface enhances the PCE by the formation of interface dipole which reduces the work function and a Schottky barrier at the cathode. It is also shown that the number of accumulated ionic groups of PcoPT on the photoactive active is larger than that of PHPT owing to the arrangement of side chain on the CPE backbone. In a similar way, the thin layer of PcoPT exhibits substantially better wetting ability on the photoactive layer than that of PHPT. Thus, the device with PcoPT exhibits higher short circuit current and fill factor than those of PSC with PHPT.

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